Developing a System Diagram: Getting Started

When I first started doing system diagrams for existing systems, one of the hardest things about the process was trying to figure out where to start.  For one thing, we humans don’t have the luxury of  the “super duper, selective, HVAC, X-ray vision” I mentioned previously to provide the over-all perspective I illustrated in my last post.  In most instances, you can never see the entire system you are targeting at one time, only bits and pieces, like a jig saw puzzle.

For another, it may be your first time on a particular site so you may not even be sure exactly which piece of equipment is which.  And in fact, if you are new to the industry, you may not even know what the equipment you are looking for looks like.  For instance, the first time Chuck McClure (one of my mentor’s) took me out on a site with him, he walked me into a mechanical room and told me to see if I could figure out why the chiller and chilled water system weren’t working while he went to a meeting.

At that point in time, I had never seen nor heard of a chiller (as I recall, it was my 2nd day), at least not that I knew of.  But, I figured from the name, it must be something that made things cold, probably water since Chuck mentioned it in the context of the chilled water system.  And I figured it was pretty likely that it was in the room that Chuck had dropped me off in.  So, I started poking around looking at things and found a machine that was making noise kind of like the air compressor at the hanger where I had worked as an A&P during college.  And it had pipes coming out of it, two of  which were cold.

Turns out I had found the 25 ton  reciprocating chiller and associated system that Chuck had wanted me to look at just by being a bit logical about things.  The problem with it, and the solution to the problem will have to wait for a different time.  My point here is that even if you are new to the game, you can probably make some deductions that will help get you started on your system diagram.

For instance, if you had been assigned the task of making the system diagram for the Pacific Energy Center’s ice storage system that I have been using as an example in this string of posts,  it’s not out of the question that you would have been sent up to the roof by someone at the facility because they new that was where the chiller was.  When you walked out on the roof, you would see something like this.

And, when you walked around to the other side of the equipment you would see something like this.

Even if you had never seen a chiller before, its quite possible that as  you stood there and thought about it, you would realize that the square cube in the middle of the collection of machinery kind of looks like the air conditioner at your Mom’s house but with bigger pipes coming out of it.  Maybe, if its warm warm outside, it even sounds like the air conditioner at your Mom’s  house.  So you take a closer look.

The pipes have labels on them that say “Chilled Water” so you start to feel like you are on the right track.  But you have also heard of labels on being wrong, so you’re a little skeptical.

Recognizing the thermometers, you take a look at them and discover that what ever is in the pipe is cold;  specifically, the pipe on the left is at about 45°F and the pipe on the right is at about 52°F.  Putting the clues together, you can conclude that:

  • You’ve probably found a chiller and it’s probably the one serving the ice storage system since its the only one around.
  • The flow through the chiller is probably from the right to the left since the pipe on the right has warmer fluid in it than the pipe on the left.
  • The pipe on the left is probably the supply pipe serving the loads. That matches what the labels say, so your confidence builds a bit because things are cross-checking.

The other thing you have done is found a starting point for your diagram.  You can take out your clip board and note paper, draw a square on it and label it “chiller” or “evaporator” or some term that has meaning to you.

Incidentally, as a frame of reference, here is what is represented by our field sketch in the context of the final system diagram and the context of the piece of equipment we are talking about.  (The highlighted areas are the areas we have documented in our field sketch).

You may even want to take a picture as a memory jogger and to remind you of the details when you are back in the office putting your notes together into  a more formal diagram.  That’s what my note “see picture for trim”  is about.

If you are really curious, you may even take off an access panel to see what’s inside.

Note that its a good idea to get permission from the Owner or operator before you start taking things apart.  Usually, they will be fine with it and a lot of times, your curiosity will breed some curiosity on the part of the operator and they will help you do it.  That’s a good thing on a number of fronts, including some non-technical and non-labor related ones like team building and persistence.  The more involved the Owner and their staff are in the work you are doing, the more likely it is that any benefits you realize will persist, which is really the over-arching goal.

The next step is to start following one of the pipes to see where it takes you. Its really that simple.  There is no right or wrong starting point, just a starting point, something you can identify as being in the system you are working with that can be the first point of reference in your diagram.

In the next post, I’ll show you how I moved on from the square labeled “evaporator” on my note pad when I started to develop the my system diagram for the PEC ice storage system.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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