11 - Create Custom Chiller Curves

Script Preview

In this lesson, we’re going to illustrate the impact of the chiller curve. The chiller curve is defined under the cooling equipment. It’s not something that you see when you define the plants, but it’s in the library and it’s defined in the library. We can look at the chiller curves, as we did before, under the Graph. We’re just going to be looking at the “Primary Power Consumed” curve. The “Ambient Relief” curve is modeled in a similar fashion, but the typical curve that we can change a lot is the primary power consumed. The reason being that the ambient relief curves really don’t change a whole lot when we’re dealing in the same realm of cooling applications, so chillers to have pretty similar ambient relief curves if you’re in the same class. Of course, when we start adding VFDs and things like that, that can change.

The one that’s more difficult to model is usually the primary power consumed chiller curve. There’s a TOPSS Import, that’s Trane Official Product Selection Software. You can actually import chiller curves if it’s a Trane chiller, from the TOPSS program. It’s not on every chiller, so far as I know, but it’s a pretty neat feature. 

The first thing we’re going to do — we’re going to note, we’re on the 3-stage centrifugal chiller that we used, and it has a CVHE curve. What we want to do, before we do anything else, we’re just going to copy this. We use a name; this is a fake chiller. What we’re going to test is, basically, we have a sort of inflection point on this curve. We saved the changes, there. Make sure that we select the fake chiller there. 

There’s an inflection point, basically, at around 70. We are more efficient in this region, as we mentioned, and then the chiller actually loses a little bit of efficiency in this region. We’re going to change that so that it’s less efficient in this region, and more efficient in this region. We have our fake chiller here. We made a note of the primary power consumed as CVHE. We copied the chiller. When we copy the chiller, it copies all of these little details, the kilowatts per ton, the pumps associated with the chiller, the pump types, and of course, the curves are all built in as well. The options are also copied from here as well.

So, if we build this from scratch, we’d inevitably vary some other variables, and we just want to change the single graph to see the impact of the single chiller curve. So, what I did here is, I looked at the CVHE curve, and this has coefficients. This curve was defined by coefficients. Part of the overlap here on the screen, that’s just because I’m recording in lower resolution than I have in standard operation mode. The cycle point is 15%, basically this chiller, if it was 100 tons, could only operate down to 15 tons, at which point it would cycle. It can’t operate at say, 5 tons on a 100-ton chiller. It’s typically a cycle point, and that’s why we just have some linear interpolated lines, here. 

Coefficients. These you can get from your manufacturer. You can’t always get these. If you can get these though, that makes modeling the curve really easy, because you just make a curve and enter the coefficients. There are other ways of doing this, so that’s what we’re going to focus on. So what I did is, I went in to Excel, and I took the coefficient, and it was .1111, 1.0144, -0.5137, .3882, and that’s just the polynomial here. I entered that X vs Y. X in this case is the x-coordinate, percent of full load, and Y is percent of full power. What I’m doing here is, I’m essentially trying to make the inverse of this other curve, so what I did is, you’ll see here, Y-Fake is basically X + (X - Y). Of course, at this point here, there’s no difference in the two, so Y-Fake should be the same. 

You can see right here that Y was .94 vs X, which was .95. We have the .01 in the opposite direction. So, instead of gaining a little efficiency on this curve, we’re going to lose a little efficiency on this curve. Same goes here, .7, we’re at the inflection point, as I call it. And then, at .6, we are gaining efficiency in Y-Fake, while we were losing this efficiency in the original chiller curve. So now we’re going to make this chiller curve. What we want to do is, we’re just going to copy this. I’m just going to call it CVHE - Fake, and we don’t have load units in the curve. We could probably generate a polynomial, but what fun is there in that when there’s a different way for us to do things? 

So we’re just going to go with percent, and percent. Pardon the overlapping figures here. We’ll have 100 and 100. We’re just going to go with 95 and 96, since we’re working with percents. The cycle point remains at 15. We’re just punching in these points. We compare the two curves. You can see in this low range we have quite a gain in efficiency in this fake curve. So actually, the best way to superimpose them is to just go to Excel, Insert, and here we have them. The red is our fake curve and blue is our real curve. You can see they start to deviate from the straight line at .7 and at .1, the major differences become substantial at our lower tonnages.

Minimize that, go back to our cooling equipment, our fake chiller. We’re just going to go to CVHE Fake. We’ll go back into Trace, and let’s go to save. What we can simply do is swap out our plants. We have one chiller here, so that’s quite simple. We have our 3-Stage Centrifugal Fake, we’re going to select this, and it’s going to prompt us, and we’re just going to change the equipment type. All that does is change the things in the background, primarily the chiller curve, so the tower stays the same. Any primers we had entered here will stay the same. Change the type — there’s two chillers. 

All we’re really doing is changing the chiller curve. We didn’t change it by a whole lot, when it’s operating at its peak load, but when it’s operating at low loads, we did change it quite a bit. I’m just going to look at these two chillers again. I didn’t change any other parameters. Now we can calculate this, and we’ll calculate. If we look at our results here, you’ll notice that actually, every single one of these alternatives improved in performance. The reason is that the curve was only made worse very slightly near its peak load, and it was made much, much better during the times of low load. So, of course the chiller over the integration period of time, anytime it might run at low load, it saves a lot more than any time that it lost efficiency at high load. So these should all be lower in their life cycle cost than they all are. Parallel still is the best option, and that’s because both chillers tend to run at the lower load more often, as opposed to a chiller running at the high load. 

Again, in this curve we saved a lot at the points around 20% and so on, and thus, as a result we’ll see that the Series option is very close to the Parallel option. That’s the key difference. So, just by changing that chiller curve, which was not a substantial difference outside of the operating ranges between 20% and maybe 50%, it was pretty similar. We actually ended up changing our life cycle cost on this by about 10%, so you can see the impact that chiller curves have, and why they’re important to enter.