Howden North America’s Fan Engineering Handbook

Authors Note:  So, some good news on this.  For a while, some of the links below did not work and I could not get information back from Howden regarding how to get a copy of the handbook since my contacts there had moved on.  

But on a lark, I did a search today and discovered that Howden is now making the 9th edition available electronically at no cost.  Just submit this form and you should get an e-mail with a download link.

I don’t know if they will continue to make a printed, leather bound version like the one in the picture available.  But if you are old and sentimental about books (like I am) you can still find used copies of it and earlier editions out there, as I mention in the blog post.  Prices seem to vary all over the place.

Since the electronic version I illustrated in the post is the 9th edition, I suspect the version you will get by following the link will be very similar if not identical.   I submitted the form myself and will update this note if I notice any major differences.


I realized the other day that the Reading and Resource list I posted does not contain information on Howden North America’s  Fan Engineering Handbook, so I’ll use this post to draw your attention to that valuable resource, which is pictured below.

Fan Engineering

Incidentally, folks my age and older may know this book as the Buffalo Fan Engineering Manual.  I believe Howden North America aquired Buffalo Forge at one point, thus the name change.  If you look at the copyright page, it shows Howden Buffalo as the publisher.

Some have observed that the book looks a bit like a bible;  the fact is that it is kind of the “bible” of fan physics and application.  Howden North America’s Fan Engineering handbook is somewhat of a classic in the industry.  The first copy I owned is from the early 1970’s;  I picked it up in a used book store at some point after coveting the copy one of my mentors had, which they generously allowed me to borrow. The copy in the picture is the 9th edition, published in 1999.  However, the book has its roots in work by Dr. Willis Carrier, and was first published in 1914.

I bring all of this up because I recently discovered that Howden has made it much easier to obtain a copy of the book.   Specifically, if you follow the links on their web site, or simply click on this link, you will be take to an page where you can purchase new or used copies.


The only used copy offered on as I write this is going for $85, which is still a good deal considering the content.   But, I suspect you can find copies for less by shopping around a bit on the internet or used book stores.  For instance, I just found a copy of the 8th edition (1983) for $22.99 on the Alibris web site.  Since the principles behind Newtonian physics have not changed much since 1983, I suspect the used 1983 edition represents a good value for someone on a budget.

Having said that, read on as there are some advantages to getting the latest edition.  Interestingly enough, you will notice that the price range for a new copy, currently at least, is from $100 (directly from Howden) to $189 (from a reseller).  My thought is that unless the reseller has a copy signed by Willis Carrier, you’re probably better off ordering from Howden.

What’s not so obvious from the web site is that if you e-mail Howden at before ordering, you can specify your prefernce for an electronic copy of the handbook as a CD instead of the paper copy.  This alone may be worth the price of obtaining the latest copy for a number of reasons I discussed in a previous post, including portability, searchability, physical storage space and weight (especially when traveling), and sustainability.  But, the electronic version of the Fan Engineering Handbook complements all of these features by including interactive content.

Here is a screen shot of the table of contents, which is the starting point for using the electronic version and also illustrates the wealth of information available in the handbook  (both electronic and paper copies).


Clicking on the highlighted topic Fan Testing takes you to the content of the chapter.


Clicking on the hyperlink for the chapter takes you to a bookmarked summary of the chapter content, from which you can jump to the topic of interest.



You’re probably thinking “that’s all nice and everything, but every well structured electronic document does this sort of thing”.  And while that’s true, the electronic version of Fan Engineering has done a very thorough job of it by providing hyperlinks that take you to related relevant content elsewhere in the handbook.  For instance, in the actual handbook, clicking on the equation hyperlinks shown in blue above will take you to those equations, which are in a different chapter.

But better still, the manual includes spreadsheets pertinent to the various topics discussed.  For instance, if while working with the electronic version of the handbook, you click on the little calculator/spreadsheet icon illustrated in the screen shot above, the following Excel spreadsheet opens up.


This is a spreadsheet that assesses flow rate based field data from a pitot tube traverse.  Note that it is set up for both IP (U.S.) and SI (metric) units.  If you wonder exactly how to use it, you just page down a bit.


If you are wondering exactly how the calculations are done, you can simply highlight a cell, and view the formula, just like you can in any other Excel spreadsheet.


If you wonder what some of the “funny” symbols mean, then just use the electronic table of contents to jump to the Appendix of Symbols and Abbreviations


… or use the hyperlinks or .pdf search function to jump to one of the referenced equations (click on the binoculars or press“Ctrl” and “F” together to get a little search window where you can type in what you are looking for).


One word of caution;  as near as I can tell, the spreadsheets are not protected.  In other words, you should save a copy to your project directory using the “Save As” feature of Excel before you modify them or add values.  Otherwise, when you select “Save”, you will over-wright the original spreadsheets in the Fan Engineering directory.  That’s probably not the end of the world, but if you needed a clean copy, I suspect you would have to uninstall and then re-install the CD content.

For me, the electronic version of the CD is well worth the cost (it’s $149 vs. $100 for the paper copy).  And, truth be told, I really appreciate having both the electronic and paper copy.  Obviously, the electronic copy is handy for my field work and teaching.  But for me, there is some sentimental attachment associated with the paper copy; its like having a bit of a tangible connection with one of the founders of our industry.  And some of the affection for the paper copy is my age;  I grew up with paper books and while I have a Kindle Fire, which is wonderful for traveling and in low light conditions and for its functionality beyond just being an e-reader, I still like the feel of a book in my hand as I read it.

As you probably have noted, this is not a free resource, like many of the ones I list in the reading and resources list I posted previously.  But as I have indicated, it’s a classic reference and a valuable resource for anyone involved with HVAC as a profession and well worth the money.  And, there is some good news if you are a student.  Howden offers a $75 student discount on either the CD or the paper copy of the handbook.  To see if you qualify, simply send an e-mail to asking for more information.  I’m sure you’ll get a speedy reply.

So, many thanks to Howden North America for making this resource available and for making it easier for students to obtain a copy of the latest and greatest information on fan engineering from the pens of leaders in the industry.

David Sellers
Senior Engineer – Facility Dynamics Engineering

Posted in Air Handling Systems, Resource List and other Resources | 2 Comments

Pneumatic Controls Resources

Author’s Note: I originally published this post on March 3, 2013.  At that time, all of the resources I mention below were loaded up in “the cloud” on a Google Drive.  Since that time, I have created the commissioning resources website to complement the blog, which makes it much easier to distribute information like this and maintain it.  So, I have moved these resources to that location, specifically, to a page I have created for them under the General Resources page.

I have also added a little silent movie that starts with pictures of various types of pneumatic control hardware that I have taken on various existing building commissioning projects.  That is followed by a set of animations that illustrate how one and two pipe pneumatic controls work.   The video concludes with some slides summarizing the pros and cons of pneumatic control and actuation along with some bottom lines about pneumatic control.

The information in this blog post is still relevant and will provide additional insight into pneumatic control and what is included in the resources, so you if you are new to pneumatic controls, you may find it helpful, along with the post that preceded it titled Pneumatic Controls, First Cost Advantages, and Retrocommissioning Opportunities.


Since the post I recently put up on pneumatic controls (actually even before I did that), people occasionally ask me if there are resources out there that would help them better understand pneumatic controls.  Even though the industry is moving to direct digital control as “business as usual”, if you are working in existing buildings, you will still find yourself confronted with pneumatic control technology.  As a matter of fact, we just ran into a bunch of it during two field classes in the Existing Building Commissioning Workshop Series that I do with Ryan Stroupe  at the Pacific Energy Center (see page two of the linked brochure if you are interested).

Here is a picture of a pneumatic control panel that is currently operating a system in Post Montgomery Center, a large high-rise in downtown San Francisco.

Executive Floor Receiver Controller Panel

Erik Carlson, chief engineer for the facility, currently is working through the building and upgrading panels like this to DDC.  But meanwhile, the operating team needs to keep the existing pneumatics in top form to maintain their LEED Gold status.

Across the Bay on the Berkeley campus, we ran into this pneumatic control panel which had been retained and piped through manual valves to back-up the retrofitted DDC control system serving the facility.

Pneumatic Controls

In this picture, you can see the pneumatic lines from the old control panel coming together through small, manual, isolation valves with the pneumatic lines from the electro-pneumatic circuit boards in the panel to the left (the valves are the small “T” shaped devices right above the panel to the left and in the lines above the panel on the right side, which is the pneumatic control panel in the picture above with the door closed).

Pneumatic to Barrington Valves 01

Here is a close-up of the circuit boards in the panel to the left in the picture above. 

Pneumatic Interface

Basically, the boards respond to signals from the DDC control system to pulse air into or out of the line to the device they control. In the close-up of a typical board below, you can see the main air (nominal 25 psig supply air) coming into the board via the black tube and connecting to a manifold. 

Pneumatic Interface Close Up

Two small solenoid valves (the shiny cylinders mounted on the black cube, which is the manifold) are pulsed by signals from the DDC system. 

Pulsing one valve causes it to open and allow main air to flow into the output line (the yellowish tube with the gauge in it).  Pulsing the other one allows it to bleed air out of the output line.  These short, digital (on-off) signals work together to create the analog (continuously variable) air pressure output from the system that is used to position the final control element, which in this case, is a steam valve.

The bottom line is that to work on existing buildings (both now and for the foreseeable future near as I can tell) you probably need to have a working knowledge of pneumatic controls and some resources. 

Unfortunately, a lot of the really great resources I had available to me when I was learning this stuff  are no longer readily available.  That’s because many of them were mentors who have retired, or in some cases, passed on.   Others are printed materials that seem to have been supplanted by new information on the new technologies. 

So, in an effort to honor the knowledge that was shared with me by the people, and to make the printed information I have available for others, I decided to scan some of my more useful resources and post them on my Google Drive so you can access them if you would like.

This is likely a violation of some sort of copy-right law, and if someone from one of the companies whose literature I have scanned contacts me and tells me to “cease and desist” with my effort, I will have to pull them back off.  But I suspect that will not happen since much of this is from the 1960’s and 1970’s and most people consider the information it contains to be obsolete.

In fact, when I contacted one of the majors a while back to see if they would allow us to post their damper and valve sizing engineering bulletins as .pdfs on the California Commissioning Collaborative web site (I was with PECI back then) they not only said yes, the guy I was talking to asked me to forward copies to him too since they sounded really useful and he could no longer find the originals. 

I still had mine because I kept my application engineering manuals from when I had worked for them.  In the interim, the company had been sold several times and the various documentation and engineering resources were “cleaned up and made current” by the new owners, I guess.

Having said all of that, what follows is a listing of the documents I have put up along with a link to them and a description of what they are about and why I think they are useful.  There is also a link to the folder containing them in the resource links on the home page of the blog.

The Honeywell Gray Manual

This is the one resource on the list that is not on my Google Drive.  That is because you can download it from another location on the web.  I link to it and discuss the “Gray Manual” in a previous post on the topic.  The bottom line is that it contains useful information on a number of aspects of HVAC including chapters on all of the common control technologies, information on HVAC fundamentals, psychrometrics, and common HVAC system types, all presented in a very straightforward, understandable manner.   It’s a great way for somebody new to the industry to gain an introduction to it and a valuable resource for someone who has been in it for a while and needs a “refresher” on a particular topic.

Johnson Controls (JCI) Fundamentals of Pneumatic Control

To be honest, one of the things I like the most about this particular document is more nostalgic than technical, that being the neatly printed signature and date of Phil Sutherlin across the top of the first page.


That’s pretty much how Phil printed all of the time, be it on a drawing that was going out the door or on the sketches and illustrations he made to help me learn things when I asked him questions. 

As a young engineer, I strove to emulate it as well as and the free hand drawing style he used to lay out systems during schematics and design development.  I actually got kind of good at it, and one of the things that bugs me about where technology has taken me is that since I type and click more and print and draw on paper less, I have lost a bit of that.

But I’m sidetracking a bit here.   Nostalgia aside, the value of this particular document is that it contains a lot of very practical information about control theory (meaning that it applies to any control technology) as well as how that theory is implemented in a pneumatic instrument.


JCI Pneumatic Controls O&M Manual

Some of the information in this document is similar  to the Pneumatic Fundamentals documents.  But the focus is more towards helping operators understand how to adjust and maintain the instruments.


There is even a section on replacement parts.  Believe I t or not, we used to actually fix technology when it broke instead of throwing it away in a landfill and buying a new one.  Perhaps a little lesson in sustainable operations there for us today.

MCC Powers Fundamentals of Pneumatic Controls

MCC Powers is now Siemens after going through a number of other owners in the process.  Their Engineering bulletin on pneumatic controls fundamentals is a nice complement to the information in the JCI Document.  While the JCI document includes some diagrams and a lot of pictures of the hardware, the MCC Powers document includes a lot of diagrams with explanations that illustrate how mechanical systems of diaphragms, levers, and springs were used to accomplish control processes like proportional control or floating control or PID.


Now, of course, we do all of this digitally with math.  But for me at least, seeing  and understanding how a mechanical device would accomplish the function was key to understanding how things worked, both then and now. 

I think that is true for a lot of people actually because the “float in a tank” analogy I use when I teach PID  control really seems to connect the dots for a lot of folks.  I know it did for me and it has its roots in some of the discussions in documents like this one.

MCC Powers and JCI Pneumatic Main Sizing Data

This document describes the process used to size the air mains along with related reference tables and sizing charts.   One of the hardest things to find these days is some sort of documentation on the air consumption of various control devices.  That can be useful information in an existing building commissioning process if you are trying to understand if a compressor is running more than it should due to leaks or if the demand from the control instrumentation is the cause.

Eventually, you can find out what you need to know, but you have to dig around a lot in the catalogs from various manufacturers.  And often, the specific device you are looking for is no longer manufactured.  Back when I did a lot of pneumatic control design, I had a table that listed most of the common devices from the MCC Powers product line along with their air consumption rate and I have scanned that table and included it in this set  of documents.


I have found that it is reasonable to assume similar devices from other manufacturers use air at about the same rate in the general case, even though the specific values probably vary from manufacturer to manufacturer and device to device.

In addition, even thought pneumatic controls are receding from the scene, pneumatic actuation is alive and well.  So having the ability to size air mains and compressors is still quite handy out there in the field.

One  note with regard to sizing systems for actuators;  when I first got into this, I was puzzled about why the table above did not include consumption rates for actuators.   It finally dawned on me that it was because unlike a pneumatic controller ,which constantly bleeds air to operate (the link takes you to some slides that illustrate this), pneumatic actuators only use air when they move.  And the volume they require to go full stroke is basically the diameter of the actuator cylinder times its stroke.

That means if you had a system that was only serving devices like actuators, then the size of the compressor and mains is more a function of how many actuators you think might move concurrently and how quickly you need them to move.  That means that you can often get away with a smaller compressor, especially if you complement it with a large storage tank, which can act as a sort of pneumatic flywheel.

MCC Powers RC 195 Receiver Controller System

For me at least, one of the best ways to understand something is to immerse myself in the details of how it works.  This document will allow you to do that for an RC 195, which is the MCC Powers (now Siemens) receiver controller.   This device is still available and in use out there  In fact, the controllers in the Post Montgomery panel at the beginning of this post are RC 195s. 

RC 195 Close Up

The controllers in the UC Berkeley Facility control panel are similar devices but manufactured by Honeywell.

The fundamental difference between a receiver controller and a controller that has the sensor built into it, like the remote bulb controller illustrated below, is that instead of using a sensing element that measured a specific parameter, like temperature, which was an integral part of the controller, receiver controllers were set up to use a 3-15 psig pneumatic input.

image  image

Pneumatic Controller 01   Pneumatic controller 05

In the pictures, the upper row are taken from the JCI Operating Manual and show the parts and a cross section of a controller very similar to the one in the photographs below.  In the photo at the lower left, you can just see the bellows that is part of the temperature sensing assembly (the silver cap shape behind the plate with screws) that shows up connected to the thermal sensor in the illustration above.

Returning to the receiver controller discussion, by using a generic 3-15 psi pneumatic input, receiver controllers could be interfaced with pneumatic transmitters that were designed to measure a number of different HVAC parameters like temperature or humidity or pressure. 

The transmitters generated a signal that varied from 3-15 psi as the parameter they measured varied over a given span like 0-100°F.  They are identical in concept to the 4-20 ma, 1-5 vdc, and 2-10 vdc transmitters we use today with DDC systems.   They just used air pressure to transmit the information instead of electrons. 

If you understand one technology, you can easily understand the other, which is a topic I discuss in an article I wrote for HPAC magazine a while back called Analog Lessons for a Digital World, along with other things I learned working on pneumatic controls but apply every day while working with DDC controls. My bet is the “cross training” works the other way too; i.e. if you understand DDC control fundamentals, you can readily transition to pneumatics once you learn the jargon and hardware.

Here is a picture of a one-pipe temperature transmitter that is piped to one of the controllers in the panel at UC Berkeley, illustrated at the beginning of the post.

One pipe transmitter 03

The generic input capability meant that only one controller design was required and you adapted it to the HVAC process by virtue of the transmitter you selected.

Bottom line is that the RC195 documentation will allow you to get into the details of how one works and how to use it.  In addition, it has been my experience that once you understand a specific device, the knowledge is easily transferred to similar technologies from other manufacturers.

Fisher Process Grade Pneumatic Controllers and MCC Powers Series 200

If you really want to see how a remote bulb type controller works, the product bulletins for these process grade controllers will give you a pretty good picture of that.  Incidentally, the MCC Powers Series 200 controller is the controller that shows up in the pictures in the photo gallery in the post titled Retrocommissioning Findings: Make Up Air Handling System Simultaneous Heating and Cooling – The Clues – #2 – The Controls May be Pneumatic.  

The instructions are hard to find;  I lost my copy when I moved to Portland and the box the the binder with all of my MCC Powers manuals was lost.  The copy I ‘m sharing here was graciously shared with me by Adam Lebavitz of M&M Control Service Inc.  M&M now owns the remaining stock and manufacturing rights to the Series 200, so if you are looking for parts or service on this type of equipment, they are the folks to talk to.

JCI Damper Engineering

One of the advantages of going back into your files and looking for something is that sometimes, you discover something useful that you forgot you had.  This is an example of that and comes from the JCI engineering manual that Phil gave me.

Among other things, this document contains actuating force and stroke data for a number of different actuator types,   including a technical discussion of the topic and a really cool little nomograph that helps you figure out the relationship between lever arm, stroke and angular rotation for a given arrangement.


When I bring this topic up, some folks say its not very useful information given that the industry is moving to centerline drive actuators and eliminating the linkage systems between the actuator and the damper.  Personally, I’m not so sure about that as illustrated by these recent field photographs.

OA Damper - Centerline actuator with Crank 01

Zone Dampers - Opposite End Zone Dampers - Actuator End

The upper photograph is an actuator serving one of a number of outdoor air damper sections in a large (200,000 cfm) economizer equipped AHU.  The lower pictures are from a triple deck multizone unit.  The actuators on the left control all three deck dampers through a menagerie of lever arms, springs, and shafts.  Here is a system diagram of the unit which will help you understand.


Interesting study in kinematics if nothing else.

Another interesting thing is the damper performance curves included towards the end.


I haven’t tried it yet, but I suspect you could manipulate this data into a version of the damper performance data that shows linearity vs. pressure drop that can be found in a number of places like the Honeywell Gray Manual and the MCC Powers Damper Sizing Bulletin I link to in the resources on the blog’s home page.


Granted, the information in this document pertains to older models.  But my bet is that geometric similarity and some engineering judgment would allow you to extrapolate this information to a current application.  That can be particularly useful if you don’t have any other information to work from.

JCI Reference Information

This is one of those collections of useful facts and figures that are handy to have around.  This particular documents includes things like:

  • units conversion constants,
  • basic HVAC and physics equations,
  • a steam table, 
  • tables with the relationship between wet bulb, dry bulb, dew point, and relative humidity,
  • velocity vs. velocity pressure tables,
  • properties of things like glycol and other anti-freeze solutions,
  • standard pneumatic control drawing symbols (well, the JCI standard anyway, which was a pretty good one actually and similar to the other manufacturers),
  • design conditions for a number of places (bear in mind this is probably mid 1960’s data), and
  • summary psychrometric process diagrams for typical HVAC processes.

The latter is one of the reasons I have included it since it is a quick way for someone new to the industry to start to understand what happens in some of our HVAC processes and correlate it with the psych chart.  For example, here is the one for a cooling coil that is doing dehumidification and cooling.


Well, that’s it.

Hope this is useful to some of you.  I know I have found the information to be useful over the course of my career, including the current time frame.

David Sellers
Senior Engineer – Facility Dynamics Engineering

Click here for a recent index to previous posts

Posted in Operations and Maintenance, Pneumatic Controls, Resource List and other Resources, Retrocommissioning Findings | Leave a comment

A Steam Heating Resources

I recently helped to present a class titled Steam and Hot Water Systems;  Design, Performance, and Commissioning Issues;  one in a series of classes I am involved with at the Pacific Energy Center.  Ryan and I have been trying to make the classes more interactive and flexible in terms of being able to address the interests of the people attending them.  As a result, I tend to try to develop and expand content based on the feedback from the last class.

So I always have plenty to present, probably more content available than I will have time to deliver.  So what I actually end up talking about is driven by where the interactive exercises take us, student questions, and lately,  what the students say they are hoping to learn about when I ask them at the beginning of the class.

So, point being that for the latest round of the Steam and Hot Water Systems class, I had added some content about low pressure, one pipe and two pipe comfort heating systems; basically, the type of system you might find in an older residence,  older multi-family housing facilities, older hotels, and older schools.  I pulled that together because the last time I presented the class, a number of the students had specific comments or questions about that type of system and at the time, I did not have much content developed for it;  the class was more focused on hot water than steam and more focused on larger commercial installations rather than residential installations (although physics being physics, the principles are all the same).

But as luck would have it, for the most recent class, the majority of the interest was on other topics so I never got around to presenting the new material.   Having said that, since steam heat is rapidly becoming a lost art (more on that in a minute) and there are some really good resources out there for those who are just learning about it, I thought I would go ahead and write a short blog post to connect you with those resources and some of the content I put together but didn’t use in the class.

 The Lost Art of Steam Heating

One of the best, if not the best resource out there for you if you are trying to understand or otherwise work with steam heating systems is a book by Dan Holohan titled, appropriately enough The Lost Art of Steam Heating.   I have a paperback copy of the original version and a Kindle copy of the revised version.    The gallery below, which is made from screenshots from my Kindle version will give you a sense of what the book is like.  Click on any image and it will open a slide show with larger images.

The book starts out with a bit of history, including pictures of some of the early steam heating equipment, and then moves on to describe how one and two pipe low pressure steam systems work in an very easy to understand, engaging manner.  Generally, if you understand a few basic principles, like stuff will move from where the pressure is high to where the pressure is low and how a manometer works, principles that are nicely explained in layman’s terms in the book, then you can understand how steam systems work.

The author’s website itself is full of resources that complement the book, including pictures and technical articles dedicated to heating systems, steam and otherwise.  If you are involved in this business at all, you definitely should have it saved in your list of favorites.  And you probably will want and will enjoy a copy of the book.

Bill Coad on Steam

As you probably know if you follow my blog, one of my mentors was Bill Coad.   Bill did a lot of technical writing over the course of his career and one of the things he wrote about was steam.

There were a number of his Fundamentals to Frontier’s columns that covered the topic and several chapters in his energy engineering book that included content about steam.  In addition, he wrote a fairly extensive article for Heating, Piping, and Air Conditioning magazine on the fundamentals of steam heating.

All of those things are getting harder and harder to find on the internet since some would consider them dated, having been written in the 70’s, 80’s and 90’s and this being a different century and all of that.  But since Bill always wrote about things from a fundamentals level, and since Newtonian physics still seems to be a good model for what is going on around us (I just checked by dropping a pencil and it in fact accelerated towards the floor of my office and then stopped upon encountering the floor), the content is generally timeless and thus they are valuable resources.

To facilitate your ability to access them, I have created a Bill Coad’s Writings page on my website and have all of the resources I mentioned above accessible for download from that location.

A Typical One Pipe Steam Radiator

It just so happens that Hotel Carlton, the hotel I stay at when I am in San Francisco, has a one pipe steam heating system.

As an aside, I highly recommend it.   It’s a cool old building with really great people working in it in an interesting neighborhood;  you’ll really  feel at home right away I think.

That said, even if it didn’t have the steam heating system, it is a pretty cool place because it was one of the first buildings erected after the 1906 earthquake and its design was targeted at being earthquake and fire proof.

But, in the context of this blog post, it has a one pipe steam heating system and the system was recently upgraded to include self-contained radiator control valves.  Here is what one of the radiators looks like if you take the cover off of it (as I am sure most guests do once they realize they have an actual working one-pipe steam radiator in their room).

One pipe radiator with manual valve and self contained air vent type control valve 02

The pipe with the bronze valve is the “one pipe” serving the radiator, that allows steam to enter it and condensate to leave it.   If you think about that for a minute, you can start to see why the details of how you pipe and control a one pipe radiator might matter.

If you look at the other end of the radiator, you see this.

Self contained valve - air vent type

The silver gizmo is a type of air vent, and the gray thingy attached to it, along with the fitting it is mounted to, is a self contained control valve.  The little cable weaving away from it and up and to the right is the set point signal, which allows you to adjust the set point from a wall mounted controller, pictured below.

Self contained valve remote set point adjustment 02 This picture shows the entire control valve and set point adjustment in one picture.

Self contained valve remote set point adjustment 01

The general idea behind how this works is that steam, not air, even hot air, is what will warm up the room with a radiator in it.   If the radiator is full of air, it will not be full of steam and will not provide much if any heat.

The self contained valve basically allows air to exit the radiator, which means steam can enter it until the desired set point is achieved.   Then, it stops allowing the air out, which means it also stops letting the steam in.

Its a bit more complicated than that because of what happens to the pressure in the radiator as the steam condenses and things like that.  Dan Holohan does a great job of describing it in his book and this instructional PowerPoint from the Danfoss web site is also pretty good.

Danfoss is one of the manufacturers of self contained valves for radiators.  In fact a lot of the time, people in the industry will refer to any self contained radiator valve as a Danfoss valve even if it is one made by Watts or Honeywell or one of the other manufacturers in the bussiness.

Its sort of like calling all facial tissue Kleenex;  Kleenex is a brand of a type of product called “facial tissue”.  But they have been so successful in marketing their product that a lot of people equate “facial tissue” with “Kleenex”.  Same thing for “self contained radiator valves” and “Danfoss”.

Steam and the FDE Resource List

If you have a copy of the resource list on our website, you will find that there are a number of links to resources related to steam in it including links to the DOE tip sheets for steam systems, Wayne Kirsner’s web site, and the Armstrong International handbook to name a few.

I should mention that the links it contains to Bill Coad’s resources will no longer work.  But as I mentioned above, you can now find all of that information in the Bill Coad Writing’s page on our website.

So there you have it; a few resources for you to look into if you are trying to understand one of the oldest approaches out there for heating a building, an approach that can do a really nice job of it if you apply it properly.


David Sellers
Senior Engineer – Facility Dynamics Engineering

Posted in Boilers, Hot Water Systems, and Steam Systems, Resource List and other Resources | Leave a comment

Site versus Source Energy

Author’s Note:  I originally posted this in September of 2007 and used a report I had found at that time to develop some of the source energy factors I used in my illustration.   Since then, I have found a number of other resources on this topic which are more current and also provide more information.  I document the new resources in a footnote at the end of this post if you are interested in looking at them.

In my previous post, I mentioned that using an electric resistance coil to generate a btu of heat can be expensive relative to burning a fossil fuel on site for the same purpose because of the difference between site and source energy.

If one only considers energy crossing the site boundary and the conversion efficiency of the electric heating coil, electric resistance heat seems like a real winner. With a conversion efficiency of 100%, every kWh that runs through a resistance element provides 3.413 btu of heat to the facility.

In contrast, even the most efficient, current technology condensing boilers available with everything working just right can only deliver 95-98% of the energy that goes into them as heat to the facility. Conventional, non-condensing burners are lucky to achieve 85% efficiency and older equipment near the end of its useful life cycle may only have a combustion efficiency in the mid 60% range! So, why wouldn’t you want to use electric resistance heat and what makes it so expensive?

The answer to that question lies in the difference between site energy and source energy. There are a number of losses that occur between the point where a fossil fuel enters a power plant for conversion to electrical energy and the point where that electrical energy reaches the meter serving a given facility.

One of the most significant losses occurs in converting the fossil fuel to heat, the heat to shaft power and the shaft power to electricity. In general terms these losses are associated with combustion efficiency, losses through insulated piping systems, the irreversibility associated with the thermal cycle of a turbine, and mechanical and electrical losses in the turbine and generator (bearing friction, less than perfect conductors, less than perfect magnetic materials, aerodynamic losses associated with shafts and armatures spinning in air, etc.).

The Energy Information Administration tracks the heat rate for electric generation in a number of publications including their Monthly Energy Review. The graph below plots this data from 1946 through the current year (data for the current year is an estimate).

Note that in general terms, it takes about 3 btus into the generating system to produce 1 btu of electricity.

But wait, there’s more! It takes power to make power. Depending on the exact nature of the facility, the power plant will need to run forced draft fans, induced draft fans, feed water pumps, deaerators, cooling towers, condenser pumps, soot blowers, air compressors, control systems, cooling systems, lighting systems, and life safety systems, to name a few possibilities. All of these systems require power and impact the amount of energy that needs to go into the plant to make the electricity that shows up at your meter.

There are also losses in the transmission and distribution process. Generally, we think of copper and aluminum as being good conductors with virtually no losses associated with the current flowing through them. But if you run current through miles or even hundreds of miles of copper or aluminum wire the over-all resistance adds up.

If you do the math, power is lost as a function of the square of the current multiplied by the resistance it is flowing through. To minimize these “I squared R” losses, we transmit our electrical power at high voltages. But, since the electric and magnetic circuits in our transformers are not perfect, we also experience losses at that point in the distribution process. And since the high voltages used to distribute and transmit power make it very dangerous to be around, we step the voltage back down at the other end of the distribution system, incurring another loss.

The Department of Energy, Office of Energy Management estimates transmission and distribution losses to be in the range of 8%. In a 1995 report titled Measuring Energy Efficiency in the United States’ Economy, factors were developed to take all of these variables into effect on a regional basis (see footnote 1). These factors are depicted in the graph below.

A number of reporting agencies like CBECS apply these factors to their site energy consumption figures to assess the source or primary energy required at the power plant to produce the site energy.

To make this graph directly comparable to the heat rate graph, I multiplied the conversion factors by 3,413 btu/kWH.

The bottom line is that only 30-40% of the energy that enters a power plant as a fossil fuel shows up at your electric meter as electricity. Worse yet, short of resistance heaters, very few of the devices that use that electricity once it reaches the meter are 100% efficient.

For instance, a 1,200 gpm chilled water pump with a 25 hp motor might have 8-10% of its incoming power lost in the motor and another 15-20% lost in the pump itself.

If the site energy graph for the lab facility I have been discussing is converted to source energy using the 1992 Western Region conversion factor published in the 1995 report I mentioned, it reveals that while converting to boilers from heat pumps increased the site energy consumption, it decreased the source energy consumption.

So, the planet won, even though the Owner’s pocket book didn’t.  Had the original electric heat been resistance elements instead of heat pumps, the difference would have been even more significant.

The bottom line is that if you are going to burn fossil fuel to make heat, you are frequently a lot better off burning it on site rather than burning it at a power plant to make heat to generate electricity which you then convert back to heat, at least from the standpoint of the environment and conservation of non-renewable resources.

The big hit to your pocketbook typically associated with electric heat only begins to reflect the big hit to the non-renewable resources and environment that electric heat represents when it is served from a fossil fuel burning power plant.

David Sellers
Senior Engineer – Facility Dynamics Engineering

Footnote 1, 2017-06-30

Energy Star Portfolio Manager has a technical reference titled Source Energy, which publishes and discusses the factors they use in their projections.

In 2007, the National Renewable Energy Laboratory (NREL) updated a technical report NREL/TP-550-38617 titled Source Energy and Emission Factors for Energy Use in Buildings that documents the factors for electricity and fuels delivered to a facility and combustion of fuels at a facility at a regional level.  It also includes links to other references.

Emissions & Generation Resource Integrated Database (eGRID) periodically publishes a database in the form of a spreadsheet that covers just about all of the electrical power generated in the United States, including emissions, net generation and resource mix.  The spreadsheet lets you filter it in many ways.  The table below is an example of one I made to look at the resource mix on a state by state basis.


Posted in HVAC Calculations, HVAC Fundamentals, Resource List and other Resources | 2 Comments

Condenser Water Systems, Air Entrainment, and Pump Cavitation

This post is the result of a discussion I was having as the result of a comment made on a previous post about commissioning a condenser water system.  The initial question was to ask if the solution I had suggested to the problem that was the focus of the post – basically air venting problem – had worked.  The answer to that is “yes”.   The manual air vents were replaced with automatic air vents and, to the best of my knowledge the problem was resolved.


You can find some more details and close-up photos of the air vents in my reply comment in the original post if you are interested.

The person I was corresponding with thanked me and mentioned that they thought they had a similar issue in a system they were working with and that one of the symptoms was that there was a crackling noise in the pump suction when it was running at full speed/design conditions.  I started to reply because there are a number of things that can cause a noise like they were describing.  But as the reply grew in length, I realized it would probably make a good blog post or two. touching on a number of condenser water system topics, so here we are.

The links below will take you to the various topics that I ended up discussing.  The end of each section has a “Back to Contents” link that will bring you back here.

Air Entrainment

Air can definitely cause a noise like the one that was described in the comment. Condenser water pumps that are located very close to the tower they serve are particularly prone to this if the tower outlet is undersized or the flow rate is higher than the tower is designed to accommodate because a vortex forms in the tower basin at the suction connection and entrains air into the piping leading to the pump.

This exciting video clip is one I recently shot that shows a vortex starting to form at the connection to a sump in a cooling tower.

Back to Contents


Having said all of that, sometimes a crackling or banging sound on the suction side of a pump is cavitation, which is related to the velocity at the inlet of the pump and occurs because of the reduction in static pressure associated with the increase in velocity that occurs as the water accelerates into the eye of the impeller.

In general terms, the conversion of static pressure to velocity pressure that occurs as the water accelerates through the smaller cross-sectional area (relative to the pipe) of the pump impeller eye and impeller channels can cause the static pressure at those locations to approach and drop below the vapor pressure of the fluid that is being pumped.  The is especially true for condenser water systems where the pumps are only slightly below the elevation of the tower basin (meaning not much static pressure available to keep things positive at the pump suction) and especially if there are a lot of things in the pipe between the tower and the pump that can drop the pressure (like a plugged strainer).

The image below is from a pump manual that I have – The Durco Pump Engineering Manual – that shows the pressure gradient as a fluid approaches and moves through a pump impeller.  As a frame of reference, the upper image is a cross-section through the pipe and pump with the pipe to the left and the impeller, volute, and pump discharge just to the right of center.   The chart below the image shows the pressure gradient through the cross section;  the letters on the x axis of the pressure gradient diagram correspond to the locations referenced by those letters on the pump cross section.

Impeller pressure profile from Durco Pump Engineering Manual

If the absolute pressure at any point gets below the vapor pressure of the fluid being pumped, then the fluid will “flash”; i.e. change phase from liquid to vapor.

There is a tremendous volume change associated with a phase conversion from liquid to vapor and vice versa. For instance, if you convert a cubic inch of liquid water to water vapor at atmospheric pressure, the volume changes by a factor of about 1,600; i.e. 1 cubic inch of liquid water becomes 1,600 cubic inches of water vapor. When that happens in a confined area, like a pump impeller for instance, there is a force generated, which can make noise and also cause damage.

Of course, since pumps are designed to pump liquid, not vapor, as soon as the liquid flashes, the flow produced by the pump is reduced significantly. And, since it was the pressure drop due to flow that caused the low pressure that triggered the flashing, as soon as the flow drops, the pressure goes back up, and the vapor implodes back to a liquid, so another huge volume change and more noise and force.

When cavitation is triggered, the phase change phenomenon I described above is happening over and over again and that is what is causing the noise and what also can cause damage to the pump.

I should also note that while cavitation is most common in the pumps in our systems, it can also happen at the control valves, especially in situations where the valve imparts a large pressure drop to the fluid going through it.   The results are similar;

  • There is a lot of noise, and
  • The valve can be damaged, and
  • Since the valve is probably not as firmly anchored as a pump, the valve and pipe can actually start moving around.

The pipe movement can alarming; inches, not just fractions of an inch, and definitely will catch your attention.  At one point, Fisher Controls published a bulletin on the topic, which is how I learned about it.  If you are interested, I have a copy of it on my website and you can download it from there.

Back to Contents

Compound Gauges

The potential for sub-atmospheric pressures at the pump suction flange in situations like the one I just described and the ensuing potential for cavitation make it a good application for a compound gauge.  Most pressure gauges have the low end of their range at 0 psig.  The “g” in psig means “gauge” and indicates that the pressure indication is relative to atmospheric pressure.

But it is also possible to reference a gauge pressure reading to an absolute vacuum and those readings are termed psia, with the “a” indicating the reference is a vacuum.   For example, a reading of 0 psig at sea level is the same as a reading of 14.7 psia or 30 inches of mercury.

A compound gauge is a gauge that references a vacuum as the low end of its range.  Typically, the range will start at 30 in.hg, which stands for 30 inches of mercury vacuum (Hg is the chemical symbol for mercury if you look on a periodic table of the elements). The range will typically convert to psi at when it reaches atmospheric pressure, where it will have a zero, to indicate 0 psig.   From there, the range increases to full scale with the numbers representing psig.  Here is an example of a compound gauge along with a picture of what the internal workings of a typical gauge look like right below it.

DSCF5194 Gauge guts 01

This particular gauge has a handy feature called a “Maximum Indication Pointer”.  The red pointer is not directly connected to the internal mechanism.  Rather, it is arranged so that the black pointer (which is connected to the internal mechanism) can push it clockwise.   By turning the little brass knurled nut you can see sticking out of the center of the gauge, you can manually turn the red pointer counter clockwise so that it rests against the black pointer.

So, if you have the gauge connected to a manifold that allows you to take a number of readings using one gauge, like this ….

Preheat pump gauge piping

… then if you take the high reading first – say the pump discharge pressure – the red pointer will be pushed to that indication.  If you then take the suction pressure reading, the high reading is “retained” by the red pointer while the lower pressure reading is shown by the black pointer, which allows you to directly read the difference in pressure off of the gauge, which is usually the number you are actually interested in.

I should mention that one benefit of using one gauge to take multiple readings on a given piece of equipment is that it automatically will eliminate gauge accuracy as a source of error since you are subtracting readings taken with the same gauge.  And, you also don’t have to account for the impact of elevation differences on the gauge, which you need to do if you are using two gauges that are at different elevations to measure a pressure drop across something.

Back to Contents

Impeller Damage from Cavitation

Aside from making a pump under-perform, cavitation can do significant damage to the impeller.  Below are a couple of pictures that Jeff Mahin – a student in one of the classes I teach – shared with me. They were taken through an open suction diffuser associated with a condenser water pump.

Cavitated Impeller on Middle CW Pump Cavitated Impeller on Middle CW Pump Zoomed

As a part of a field exercise, his project team had run a pump test and the numbers were not adding up.  So we opened up the suction diffuser to see if there were any clues as to why. The second image is a close-up from the image to the left that zooms in on the impeller. In it, you can clearly see how the impeller has been chewed up by cavitation which is likely the reason the pump was not performing per the published curve associated with it.

The next string of pictures where shared with me by another student – Jay Cmiel – who has been trying to understand issues he has been having with a domestic water booster pump. At this point, he believes there are a number of things going on, including system configuration issues and possibly metallurgical issues. But he also suspects that cavitation is a major player behind the failures he is seeing.

These first two pictures illustrate what the impeller in one of his domestic booster pumps looked like back in June of 2016 after it was installed to replace a damaged impeller. The new and old impellers are shown side by side in the second picture.

Damaged Impeller 05 Damaged Impeller 02

This picture is what the impeller looked like in September of 2016 when they opened up the pump to replace a seal that had failed.

Damaged Impeller 06

These last two pictures are what the impeller looked like a couple of weeks ago (late April/Early May 2017) when they had to replace it again.

IMG_5214 IMG_5216 IMG_5217

So clearly, the message from the field is that cavitation is a thing to be avoided!

Back to Contents

Net Positive Suction Head (NPSH)

One way to understand if the potential exists for cavitation is to do a Net Positive Suction Head Available (NPSHa) calculation.   The basic idea is to do a calculation that tells you what the absolute pressure will be at the inlet flange to the pump in question at its design flow rate given the configuration of your system and its warmest operating temperature.   That number is is the Net Positive Suction Head Available or NPSHa.

You then compare that to the Net Positive Suction Head Required a.k.a. NPSHr for your pump.   That information shows up as a line on the pump curve.  For example, in the curve below, the design point for the pump is 1,000 gpm at 45 ft.w.c..  At that condition, it will be about 83.5% efficient, use about 13.25 bhp, and will require about 11.2 ft..w.c. of net positive suction head (or more if you want to be on the safe side) to avoid cavitation.


Notice how the NPSHr curve is read off of a secondary axis to the right and that it tends to rise as the flow increases because for a given cross-sectional area, an increase in flow will cause an increase in velocity.  And since the velocity pressure is  function of the square of the flow, the curve is not linear and really starts to take off as the flow increases.

The latter item is an important consideration in the field because it means that a pump that is just fine at it’s design condition can cavitate a lot if it runs out its curve.  That phenomenon is quite common in systems where large pumps are piped in parallel and serve common headers.  Consider the system illustrated below.


The three relatively large pumps (one is a back-up pump) serving the system are piped in parallel and deliver water to and from the chillers they serve via a fairly long piping run.  If both chillers are running, then the pumps need to deliver the design flow for the chiller at the head generated by that flow in the long piping run;  in this particular case, the total flow requirement was 2,400 gpm and the head requirement was 65 ft.w.c..

But if only one chiller is running, the required flow drops in half and as a result, due to the square law, the head required at the lower flow rate for the common piping that the pumps share is only one quarter of what it was (one half squared or half of a half).  As a result, in this particular case, when one pump was running, it ran out its curve and delivered significantly more flow that was required by a single chiller when it was in operation;  about 1,920 gpm at 47.5 ft.w.c. when only 1,200 gpm was required by the chiller.


That turned into an energy conservation opportunity that was worth anywhere from $1,100 to $15,000 a year depending on how you captured it.  If you want to know more about that, there is a PowerPoint case study on my website that you can look at which includes the cost/benefit metrics for the various options that were possible for capturing the savings.

My point in bringing this up here is that when system dynamics cause a pump operating point to shift out its curve significantly, the operating point on the NPSHr line can shift from a condition that was satisfied by the NPSHa to a condition where the NPSAa is not sufficient to prevent cavitation.   If you don’t fully comprehend the dynamic behind what is going on, then the cavitation problem can appear to mysteriously come and go.

For the system in the example above, this turned out not to be an issue.   But I have seen it be an issue on other systems with similar configurations and similar dynamics, which is why I bring it up.

The first time you do a NPSHa calculation, it can seem a little intimidating because you are working with absolute pressures and things like that;  it kind of seems like you are on a different planet or something. But Bell and Gossett publishes a handy little nomograph (chart that lets you graphically determine a solution) that makes it pretty easy.

B&G NPSH Nomograph

Us old folks have it in our Bell and Gossett Engineering Design Manual.

Bell and Gossett Manual

But you can now download electronic copies of the various bulletins that are in the manual from the Bell and Gossett website.  The nomograph shows up in the one on cooling tower pumping and piping, which also includes a lot of other very useful, practical information about cooling tower and condenser water system design and operation and how to apply pumps in them.

Back to Contents

So, I guess that ended up being a pretty long discussion for something that started out as answering a question about air venting.  But hopefully, the information is useful;  if it is, you can thank Kam for asking the question.


David Sellers
Senior Engineer – Facility Dynamics Engineering

Posted in Chillers and Chilled Water Systems, Operations and Maintenance, Retrocommissioning Findings, System Diagrams | Tagged , , | Leave a comment

A New Commissioning Resource Website

For a while now, I have had this idea for a commissioning resource website floating around in the back of my head.  It seemed like there would be things that I could do with the way a website is organized that are challenging or simply not possible on the blog.   So, I finally dipped my oar and tried to create one and the result is now “live” as they say.


The URL is and as you can see from the image above, the theme is basically the same as the blog’s theme;  i.e. we stand to learn a lot from the reality the buildings we work in are trying to communicate with us.

I am certainly not a web developer by any stretch of the imagination and what is up currently is something that seemed “good enough” as the result of the combination of my skill set and the tools Weebly provides.  Part of the impetus for doing this was a need to figure out a way to shift content from lecture to a “find the details out yourself” format, which is what I am trying to do with the “What’s That Thing” page, the “Tools” page, and the “Useful Formulas” page I have linked from my “Resources” page.

This is an evolving work in progress so you will currently find a lot of stuff “under construction” (which I admit may be an excuse for me to post pictures of cranes and construction sites).  But hopefully, I will be able to fill out that structure over the course of the next year or so as I go through a round of the classes I teach at the Pacific Energy Center, which are a primary driver behind my doing this.

Incidentally, I have included a training page, which I plan to keep current with the dates for the classes I help teach in public venues for the upcoming four to five months.  Many, but not all, of these classes are free of charge, so if you are new to the industry and trying to learn, you may find  value in them. 

I also plan to include links to the materials from past classes and presentations under the Resources tab.  That is still  under construction but you can also get to those resources from the blog using the link on the right side of the home page under the 03- Materials from Classes and Presentations topic.


The advantage of moving things like that to the new website is that it makes them easier to organize and find and allows the blog to be more focused on the articles I write vs. the resources I reference.

The website includes an entry point to the blog in the navigation buttons on the upper right, or you can simply continue to use the actual blog URL, which is and which you probably already know if you are reading this.


David Sellers
Senior Engineer – Facility Dynamics Engineering

Posted in Resource List and other Resources | Leave a comment

Learning about Relay Logic; What’s a Relay?

Author’s Note:  I edited this to put in pictures with better contrast for the relay images further down the post and for some reason, instead of just replacing the original post, the system put it up as a new post.   So, I am just going to let well enough be.  But bottom line, this is just a repost of the original of the same title with a bit better relay pictures.

In my last post, I talked about the Jeopardy game I had built that used relay logic and gave you a copy of my wiring diagrams in case you were interested in learning about relay logic by building your own version of the game.  In this post, I thought I would talk a bit about what a relay is since I am asked that question on occasion.  That will also set the stage for a discussion of Boolean algebra, which is the algebra of 1s and 0s and is the basis of doing logic with relays (excitement, excitement, excitement).

Relay logic has been with us for a long time, as you may have garnered from a post I did a while back titled Control Technology; a Glimpse Backwards and Some Thoughts on the Future where I discussed a legacy control system I had run into.

Old panel 01

What you are looking at represented the state of the art in the 1970s when I first started working in this business and is a far cry from the current approach to the control system implementation.   The control panel in the picture above originally sat behind where the blue chair is in the picture below.  All of its functionality, and then some, was replaced by a PC that is not visible in the picture below due to the camera angle, other than the display and keypad, which you can see sitting there on the desk.


If you looked inside the old panel, you would have seen what appears below;  hundreds of discrete electric and pneumatic control elements that were wired piped together so that they performed the desired control functions, some of which were analog (meaning the output of the devices was continuously variable) and some of which were digital (meaning the output of the device was in one of two states, like on or off, or open or closed).

Old panel 11 Sharpen

The yellow plastic cubes sitting on a gray base towards the bottom of the picture are electro-mechanical relays.  If you are wondering what the rest of the stuff is, I talk about it more in the post I mentioned previously.  For this post, I want to focus on the relays.

I have always, for what ever reason, kind of liked relays.  I didn’t know it at the time, but as a kid, I was making them out of nails with wire wrapped around them for an armature that moved pieces of tin can that connected other nails with wires on them to control my toy trains.  (Its probably a miracle that I did not electrocute myself).  In any case, I sort of have this weird natural curiosity and interest in them, so fair warning for what is to follow.

The picture below is a close-up of what one of the little cube relays in the picture above typically looks like.  The one after it labels all of the parts.  Incidentally, a common term used to refer to these relays is “ice cube relay” because in size and shape, its similar to an ice cube.



As you can see, its really a collection of levers and wires and the  “electro-mechanical relay” term I used previously is appropriate.  And as you might suspect, it depends on both electrical and mechanical principles to operate.

Fundamentally, a relay is simply an electrical switch that is operated by another  electric circuit instead of a lever that someone moves manually.  Thus, they provide a way to automate the operation of equipment using circuits that are electrically isolated from each other.  We use them a lot in building systems for that reason.  The isolation feature is important because it allows a low power, low voltage control signal to operate a high voltage, high amperage piece of machinery.  this makes things safer and keeps costs because the low voltage wires can be smaller and used for the long runs to the high voltage/amperage equipment point of use.  That is their other big application in building systems with prime examples being the interface a DDC control system output and a motor starter or lighting contactor. 

And, the motor starter or lighting contactor themselves are also relays, allowing high amperage, high voltage loads like 1,000 ton chillers or a 277 volt lighting distribution panel to be controlled by a 120 vac control circuit that is in turn, controlled by a 24 vdc DDC control system output.  So, ultimately, a  little tiny transistor or op-amp ends up controlling a 4 kV chiller motor.

The relay in the picture is a control relay like you apply as the interface between a DDC control system and a starter (or in a Jeopardy game).  Its not intended to handle heavy currents like three phase motor or lighting loads,  But having said that, the operating principle is fairly straight-forward an very similar to that associated with a motor starter or a lighting contactor:

  1. The armature is connected to a moving contact and swivels on a pivot, just like the see-saw you used to play on when you were a kid.  The contact is called the “common” contact since it will be in the circuit no matter what the state of the relay is (On or Off).
  2. When the relay is “off” (the coil has no power applied, which is termed “de-energized), a spring on the left side of the armature pulls that side down, causing the armature to rotate counter-clockwise. which moves the contact it is attached to up until it hits the fixed contact above it, which is termed the “normally closed” contact.  This completes a circuit through the common contact to the normally closed contact.  So wires that were hooked to the blades associated with these two contacts could be connected to light a light or sound a buzzer or start a motor if the relay was off.
  3. When the relay is “on” (the coil has power applied, in other words, it’s energized) a magnetic field is created.  This attracts the right side of the armature towards the coil and the armature rotates clockwise on the pivot until the common contact hits the fixed contact located below it, which is termed the “normally open” contact.  This opens up the circuit from common to normally closed and completes the circuit from common to normally open.  And, as was the case for the normally closed contact, wires could be attached to the blades associated with these contacts to perform useful functions.

The blades sticking out of the relay are designed so that you can solder wires to them in many cases.  But in most applications, they will also plug into a base that allows wires to be terminated under screws.  You can see these bases in the picture at the beginning of the post;  they are the gray squares with all of the wires hooked to them that are under the yellow relay cubes. 

This arrangement allows a failed relay to be quickly replaced with out having to re-wire anything.  In addition to saving time, this arrangement ensures the persistence of the logic created by the relay circuit because you never lift a wire to change a relay.  You just unplug it and plug in a new one.

Note that this “plug in” feature is something generally found with low voltage control relays vs. motor starters and lighting contactors.  But, in a motor control center, the entire enclosure containing the motor starter is often designed to be removable and simply plugs into the buss bars inside the motor control center, so again, the concept is similar.

The relay in the pictures also has a couple of handy features that are options for most manufacturers.  One is the LED that lights up when the coil is energized.   This is a good news/bad news thing.  The good news is that it tells you there is power to the coil.  But, the bad news is that it doesn’t really tell you if the coil actually moved the armature.

That is the benefit of the orange colored plastic lever.  If you look closely, you will notice that it is connected to the armature.  It turns out that the connection is arranged in a way that causes a little window on the top of the relay to turn orange if the armature has moved and keep it clear if the armature has not moved, as illustrated in this picture.

Slide Show 12162011 123148 PM

The blue lever allows you to manually move the contacts with out energizing the coil, a handy feature for verifying your wiring and trouble shooting problems.  Its also handy in an emergency when the control system is down and you want to force the output that it would have controlled had it been up and running to do something.

The terms “normally closed” and “normally open” are used because this is the state the relay will be in when no power is applied to the coil;  thus, the circuit through the “normally closed” contact would allow current to flow through that path as shown below.

Slide Show 12162011 123204 PM

Meanwhile, no current could flow through the “normally open” contact since it is not touching the common contact.

As you can surmise, when the relay is energized, the contacts switch and the normally closed contact opens and the normally open contact closes and current goes the other way as illustrated by the red line in the figure below, when contrasted with the yellow line in the figure above.

Slide 12162011 52943 PM

If you think about that for a minute, you might (correctly) conclude that in a way, the relay can “think”.  If it has power it reaches one conclusion represented by the normally closed contact being open and the normally open contact being closed.  If its not powered, then it reaches a different conclusion represented by the contacts being in their “normal” state.  Its this simple decision making capability that is the basis of relay logic, another major application for relays in buildings, especially in legacy control systems.

Its also important to note that the coil can control more than one set of contacts.  For instance, in a three phase motor starter or a three phase lighting contactor, the coil controls a set of contacts for each phase.  And in the control logic world, as you will notice if you study the wiring diagrams in my previous post, one coil controls three or four different contacts, meaning it can influence three of four different streams of relay “thought”.

Each set of contacts that can serve an independent circuit is called a pole.  So, in the drawings in the previous post for the Jeopardy game, the relays are three pole and four pole relays.

And while most control relays have a contact that can have two states as shown in the pictures above and in the wiring diagrams for the Jeopardy game, it is also possible to have a contact that is only “normally open” or only “normally closed”. Motor starters and lighting contactors are common examples, containing three normally open contacts (one for each phase) that close when the contactor coil is energized and open, but do not (typically) complete a different circuit, when the contactor coil is de-energized.

A set of contacts that completes one circuit when it is energized and a different circuit when it is de-energized, like in the pictures above and in the Jeopardy game is called a “double throw” contact.  A contact that completes or opens up only one circuit when it is energized and does not directly impact a second circuit when it is de-energized is called a “single throw” contact.

Relay engineers have combined these two terms and will say, for instance, that a relay is a four pole double throw relay, meaning it is a relay that can control four independent circuits and that it can connect each circuit in a different way depending on if the relay is energized or de-energized.  The relays in the Jeopardy game wiring diagram are three pole double throw and four pole double throw relays. In contrast contacts like those in motor starters and lighting contactors are usually what is termed “single throw”.

And, being engineers, relay engineers have created a set of acronyms to describe all of this.  A Double Pole Double Throw relay is abbreviated as DPDT.  A 4 Pole Single Throw relay would be called a 4PST relay.

If you really are interested in this (and who wouldn’t be), then you may want to poke around and see if you can find a copy of the Engineer’s Relay Handbook.  I, as you may have guessed (perhaps with some alarm) happen to have a copy and I can tell you that it contains everything you want to know and more about relays.  

In the course of developing this post, I looked around to see how easy it was to find a current copy and it seems to be not that easy.  My guess is its not published anymore so you will probably only find one at a used book store or a garage sale.

The good news is that the most relevant chapters seem to have been posted on line as HTML files on the Easterline Power Systems web site.  This includes the chapters on terminology and on principles of operation.  The files can be downloaded to your hard drive for off-line reference.

So there you have it;  probably more than you care to know about relays.  The next post will look at how we can use them to “think” just like I did in the Jeopardy game.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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