Saturated, Multi-Phase Systems and Proof that a Watched Pot Does Actually Boil – Part 1

Well, I’m behind on where I had hoped to be on my blog posts, but its been a busy couple of months.  And since this is more of a hobby than not, it tends to run lower on the proverbial totem pole than work and family stuff. Having said that, in preparing for an upcoming class and for the string of posts on refrigeration systems, I had a few insights and came across a few resources I wanted to share.

Overview

As is often the case for me, this post ended up being a bit long.  So, I broke it up into several posts and link to them from the bullets below.  That will give you a way to jump to a topic of interest if you don’t want to read the whole thing.

In this post, which, after a great deal of consideration, I have given the sub-title of “Part 1”, I will write about creating original content to prevent copyright problems when using data from ASHRAE in a training class, which is what triggered all of this.  This includes information on a resource found that allowed me to create my own Mollier charts.

In subsequent posts, discuss:

  • Part 2 – A brief history of the evolution of the ideal gas equation and some resources if you want to learn more.
  • Part 3 – A discussion about saturated systems and why they should matter to you if you are working in the HVAC industry.
  • Part 4 – An exiting experiment where I demonstrate how a saturated systems works and concurrently prove that a watched pot will actually boil.

Creating Original Content

All of this evolved via a number of drivers, one of them being that when you prepare slides for classes and posts for a blog like I do, one of the things you have to be aware of is copyright issues.   If you are talking about engineering topics, this has become more and more challenging as the years have gone by because it turns out that the charts and tables we use every day in our practice all are copyrighted.   Meaning that while I can go my merry way and size pipes, plot cycles, read properties, etc. on or from the charts and tables in my ASHRAE handbooks, I start flirting with copyright issues if I take a screen shot of one of them and use it in a slide for a class or maybe even for an image on this blog.

In the context of my current state of affairs, that is an issue because I wanted to show refrigeration cycles on a pressure-enthalpy diagram for the purposes of this blog and a commissioning class, and I wanted to talk about saturated systems in general for an upcoming class on steam and hot water systems.  That meant I wanted to be able to reference a number of different thermodynamic diagrams and tables.  But, as indicated above, simply making a scan of them and using that was a “no-no”, at least in the eyes of my clients and the organization who’s handbook I was going to scan.

For example, I wanted to show the refrigeration cycle for the Le Conte Hall chiller on a p-h diagram like this.

image

Before a bunch of you point out that the copyright law basically says that it is O.K. to use certain things like charts and tables for purposes of education, I should tell you that I have heard that before and thought it was my salvation.  But my client’s legal department sees it another way, and thus, my dilemma.  However, like a lot of dilemmas, this one turned into a blessing because it lead me to new discoveries and I learned a few things as a result.

Re-creating Information from a Resource

So far, the way around the copyright issue for my clients who were concerned about this is to simply re-create the charts and tables on your own from the tables and charts in the original resource, citing the source somewhere in your creation.  But, for something like a p-h (pressure-enthalpy) diagram, that is more complicated than it sounds.

My first effort at developing the chart I showed above involved trying to trace over a scan of the chart of interest – specifically, a p-h diagram for R-134a –  on a PowerPoint slide after which I intended to delete the original chart, an extremely time consuming process.  Fairly early on in that process, I got to thinking that there must be some sort of resource like the Akton Psych chart, but for other types of Mollier diagrams.

It turns out that there is, courtesy of NIST (the National Institute of Standards).  Specifically, NIST publishes a software package called REFPROP that has the equations of state for a whole bunch of fluids of interest.  Here is a screen shot of the current list.

nist23 - Windows Internet Explorer 722013 74658 PM

The bad news is that obtaining it costs about $250, something that I could justify given what I do, but probably not something everyone can afford.  The good news inside the bad news is that there is a student version with the properties for water, CO2, R134a, nitrogen, methane, propane, hydrogen, and dodecane (not sure what dodecane is) that is free.

So, I down-loaded the software and was able to use it to create the p-h chart for R134a in the illustration above by generating the appropriate tables and then loading them into Excel and plotting the results.  REFPROP will also make charts, but the formatting options are limited and for my purposes, there was a lot more flexibility available by using the Excel based approach.

So bottom line, REFPROP solved my original problem.  But working with it got me curious about where all the numbers come from, which is what I discuss next.

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David Sellers
Senior Engineer – Facility Dynamics Engineering
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