Developing a System Diagram; Moving Forward from Your Starting Point

Before I got side-tracked talking about organizational issues, we had started our system diagram by picking out a piece of equipment, identifying it, and putting a box that was associated with it on our note paper; something like this.

The equipment we were looking at was the chiller serving the ice storage system …

… and if we could step back and look at it from a slightly different angle, sort of to the right and above the angle in the photo, see through the insulation, and only see the piping and equipment related to the ice storage system, we would see something like this.

For those who are new to HVAC system drawings, the perspective view shown above is often referred to as an isometric view or a piping isometric.  I generated it by selecting an appropriate viewing angle for the three dimensional model I made of the system, but before there were computers and 3D modeling software, you would draw the view by projecting the horizontal lines on a 30° angle.

Viewing the evaporator and related piping from directly above is the more familiar plan view …

… and viewing it “straight on” from the side is called an elevation …

… and these views, along with the isometric, say a lot about the physical arrangement of the equipment and piping.  I present them here partly for familiarization for those who have not been exposed to construction documents, and partly so I can contrast them with the system diagram, which is our focus.

To move forward from our starting point, we just need to pick a pipe and start following it to see where it takes us.  Since we are interested in what the chiller might serve, we might pick the supply pipe, which is the pipe with the colder water and the pipe on the left in the picture.  Its the dark blue line in all of the drawings.  To follow it, we need to look under the supporting steel since that is where it heads.

If you did that at the Energy Center, you would see something like this.

As a frame of reference, you are looking at the piping from the left side of the chiller as viewed in the photo at the beginning of the post.  In the picture above, the structural steel running at an angle across the top of the picture is the chiller frame and the structural steel on the right side of the picture is the supporting steel you see in the photo at the beginning.

All of that said, the maze of pipe and conduit you are looking at can seem pretty intimidating;  I know it can be for me, even at this point in my career.  But I have found that the key to that is to realize you are only interested in one of those items at a time.  In our case, we are interested in the supply line leaving the chiller.

Here is the picture above with everything but the line of interest grayed out.  The blue line and arrow show the direction of flow.

If you continued to follow the line, looking under the steel as you went, you would come to a point where the line had a tee in it as illustrated below.

As a frame of reference, in the picture above, the pipe continues on to the left to connect to the evaporator.  The point where the pipe turns, to the right of the photo is what we will explore next.   At the point where the tee is, the flow splits and part of it continues on to the right (continuation of the dark blue line).  The rest of it turns, flows out the branch of the tee (which is towards the “back” of the picture and hidden by the run of the tee) and disappears through the roof (the teal colored line).

At this point in your development process, you need to make a decision.  You can continue to follow the piping on the roof, or you can try to find the point on the floor below where the pipe from the branch shows up and continue your effort from there.  Ultimately, you need to follow both paths.

Usually, I elect to continue on in the location where I am at and follow the pipe that disappears as the next step.  So, I will simply mark the branch I am not going to follow with a letter or number or some form of designation on my notes and move on.  The reason for labeling the branch I did not follow is that I will probably end up documenting what it does on a different page of my notes.  On that page, I can start my line with the same label, allowing me to match things up when I piece my notes together into a first draft of the diagram later on.

Moving on from the tee, you would discover that the pipe turns and re-emerges from under the steel support where there is another tee.

For orientation purposes, the tee referenced in the preceding paragraph is where the dark blue line and teal line split.  The cyan colored line is a chilled water return line crossing under the line we are following, which we will discuss in a bit.  The light gray object going through the roof on the left side of the photo is the supply duct from the air handling unit that is sitting on the structural steel directly above where we are looking at the piping.

If you step back from the tee, here is what you see.

The line colors are the same as in the previous picture.  The orange square is the electric actuator for a three-way valve that serves the cooling coil in the air handling unit (AHU), which is the gray box behind it.

The large gray box to the left with the red box mounted on it is the supply duct with smoke detector (the red box).  Taking a step or two backward provides this perspective.

As you can see, the supply line runs up the side of the unit, makes  U-turn, and drops back down and seems to end. The small tube you see leaving the top of the U-turn is piped to an automatic air vent.  What you can’t see is that there are three tees in the supply line that penetrate the AHU casing near the top, middle, and bottom of the supply line as it drops back down from the top of the unit.

Adjacent to it, the return line also has three connections that are collected and then piped to the three way valve.  If you had X-ray vision, here is what you would see.

As you can see from the piping isometric, the three way valve must control the discharge temperature from the cooling coil by regulating how much water goes through the coil vs. around the coil in the bypass connection.   The two water streams mix in the valve body and then head back under the steel supporting the AHU, as can be seen from the three pictures that preceded the piping isometric.

If you kept following the return line, you would come to a point where it also penetrates the roof, very close to the branch from the supply line that we discussed several paragraphs ago.  There is also a third line penetrating the roof in about the same area.  You can see all three of them in the preceding photo that is looking at the piping below the valve where it emerges from under the steel.

If you went  to the other side of the supply duct to get a better look at the three roof penetrations, here is what you would see.

If you followed the line labeled “yet to be explored” you would find that it connects to the other side of the evaporator, our starting point.

Here is the magic 3D, X-ray view of what we have been talking about in the course of this post …

… and here is my sketch of the roof level piping;  the beginning of my system diagram.

So at this point:

  • We have followed the supply line to the AHU cooling coil, noting that it branches to something on a floor below (point “B” on my notes), before getting to the AHU ; and then;
  • From the cooling coil we have followed the line (as chilled water return) to the point where it penetrates the roof (point “C” on my notes); and
  • We have noticed that a line comes through the roof near where the other two lines we have been following come through (point “A” on my notes)  and connects to the chiller evaporator.

In the next post, I will show you the details of the coil connections.   Then I will explore how I moved on from my diagram of the roof level piping.  Meanwhile, I hope everyone is having a good weekend.  Kathy and I just got back from happy hour in the garden at the St. Johns Pub, which was a great way to spend a pretty evening in Portland.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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