System Diagrams: Applying My System Diagram Concepts to Air Handling Systems

A while back, I started a string of posts on the topic of system diagrams to support a workshop I was doing at the Pacific Energy Center on the topic.  In the last post of the string, I mentioned that I was going to devote a few posts to applying the concepts to air systems.  Then, as frequently happens with me, I got side-tracked and never actually put those posts together.    So, my goal for the next few posts is to remedy that oversight on my part.

A System Diagram Learning Opportunity

If you find this topic to be of interest, we are repeating the System Diagram workshop at the Pacific Energy Center (PEC) next month on October 9th.  If you happen to live in the Bay Area, you could attend the class in person. It’s an all day class starting with lecture in the morning,  followed by a lab session during which we will apply the lecture concepts interactively. 

But, even if you don’t live in the Bay Area, you can still tune in via the internet for the lecture portion of the class, which is scheduled to run from approximately 8:30 am to 12:30 pm. If you follow the link above, you will find a description of the class and also a link that lets you register for either the webinar or the on-site version. 

The PEC classes are offered free of charge and many of the classes at the PEC these days are offered as webinars.  So, you may find that the Energy Center is a valuable resource for you, even if you don’t live in California.  They publish an online calendar twice a year for the fall and spring semesters which shows up under the PEC website’s “Classes and Seminar’s” link

Once you are at the web page, you can browse the class list by selecting your market sector, topic of interest, location where you would like to attend, and your professional affiliation (designer, inspector, operator, etc.).  If you are interested in classes offered as webinars, select “Internet” as the class location.

Air Handling System Diagrams

When I get to the topic of air system diagrams in the workshop, I usually start with this picture and ask the following question:

Can you identify the HVAC system  components in this picture?

Training room 02 Enhanced

Some of you may recognize that this is the classroom we typically use for many of the classes at the PEC.  And I bet many of you can identify the diffusers and grills in the ceiling through which air enters and leaves the space.  People sitting in the class room often note other items like the thermostats and CO2 sensors. 

The Building Envelope;  It’s Part of the Air Handling System

These items are all valid observations, but the answer I am looking for and the point I am trying to make is that the room and building structural elements defining it are a part of the HVAC system.  Specifically, in the case of the class room:

  • The interior walls, which you see in the picture above, separate the class room from adjacent areas in the facility, which are separate zones of control with potentially different set points, schedules of use, and occupancy loads.
  • The ceiling, also visible in the picture above, separates the occupied space from a return plenum used to direct air from this zone and others back to a return air shaft.
  • The exterior wall, illustrated in the picture below, separates the interior of the building, which is the environment we are trying to control, from the exterior of the building, which is the uncontrolled (by us) environment.

Rear 01

The two windows at the bottom center of the picture along with the wall areas immediately adjacent to them, constitute a significant portion of boundary of the classroom illustrated in the previous picture.

The preceding concepts lead to one the key differences between how I draw an air system diagram versus a water system diagram.  For an air system diagram, the envelope becomes a part of the system I am trying to document.

Sample system diagram v2 r1a

Some of you may recognize this drawing from the Functional Testing Guide, where it is used to support the chapter on Integrated Operation and Control , illustrating a fictitious system that goes through a commissioning process.  If you are new to the field, you may find the chapter to be enlightening since it is sort of a nerdy novel describing the commissioning process for a fictional building in the Midwest.  The building doesn’t really exist, but everything that happens in the course of the process actually happened to the authors at one point or another in their careers.

For those who would like to take a closer  look at the image above, I have uploaded it as .pdf file to my Google Documents account (which I just learned is now going to be a Google Drive account;  I think it doesn’t change the links but if you have trouble getting to stuff, let me know). 

I have also put the AutoCAD file there so those with AutoCAD (or any software that can open .dwg files) can pull it back and use any of the symbols it contains.  Finally, I also provided a  copy of the file as a .dxf file (drawing exchange format) which should let you open it with other programs like Google Drawing  or Visio.

Air Handling Systems;  Some Significant Differences from Piping Systems

While the rules and concepts that I use to develop air handling system diagrams are very similar to what I use when I develop piping system diagrams, there are a few features associated with air handling systems that make them unique.

  • Mass (air and water vapor) are actively moved across the system boundary by the operation of the system.  In a piping system, we would generally consider mass (fluid) actively (intentionally) moving across the system boundary (the pipe wall) to be a leak (with the evaporation that occurs in a cooling tower and other open systems being the exception).   With air handling systems, its more along the lines of “business as usual” given our desire to use them to ventilate the area they serve and perhaps, cool it via an economizer process.
  • Water often changes state in the system.  We condense it out of the air stream if a cooling coil is below the dew point of the air stream moving through it. And we evaporate water into the air stream in evaporative cooling and humidification processes.  In addition, the loads we serve typically add water;  people give off moisture in addition to heat.  And open water surfaces like water features, pools, hydrotherapy baths, and aquariums can also add water to the air stream.
  • People move around inside the system.   This seldom if ever happens in piping systems (unless you are watching a James Bond movie).   But its ‘business as usual” for most air handling processes.  The same is true for the production and  other processes that occur in our buildings.

So, I hope this can serve as a starting point for my discussion of air handling system diagrams.   In the next post, I take a closer look at the envelope as a part of the air handling system and the implications that can have in terms of energy, efficiency, and operability.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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