8 - Chiller Unloading Explained

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Like I mentioned earlier, one of the larger factors in all of this is the chiller that we select. So we go to chiller plants. We had selected a water-cooled chiller and a 3-stage centrifugal chiller. There’s a bunch of stuff going on in the background when we do that, and that’s what we’re gonna take a look at. It’s 3-stage centrifugal, and we just go to Libraries > Equipment > Cooling, and now we have to select that it’s a water-cooled chiller. There’s other categories, but it had defaulted to water-cooled chiller. We just select the 3-stage centrifugal. We want to make sure that’s the exact name. Sometimes they’re pretty similar. 

What we want to look at is this graph. We have the ambient unloading, and we have an example that you can look at for how unloading works, but we won’t consider the ambient unloading right now because we know that’s the same for all the chillers, because we used identical chillers. The math is the same whether we have one chiller or one hundred chillers, regarding the ambient unloading. 

We look at this unloading curve here, and since we have our chillers oversized by 20%, we know that our peak load is somewhere around 80% of the 1,200. When we divide that by two, that’s around 40%. When we look, we see 40% here. We’re operating at about 45% of the full power. If we double that, if we look at 80, we’re at 80. So what that means is, when we have one chiller running full bore, it’s more efficient than two chillers running at 50%. However, the number of hours that we spend in this range is more when we have one large chiller. So for instance, if we were at 500 tons, we’re at somewhere between 40 and 50%, and at this line, at 50%, we’re operating at greater than 50% of the full power, which means we’ve lost some efficiency.

At 90% of the full load, which would be 90% of 1,200 tons, we are operating at less than 90% of the full power, so the chiller is actually more efficient than that .52 kilowatts per ton here. However, if we have one large chiller, we’re going to spend most of our time in this region. In fact, any time we’re below 70% of the full load, we’re operating at a lower efficiency or a worse efficiency than we are when we are above 70%. By taking two chillers, two smaller chillers, and running them in Parallel, we actually operate more efficiently because the first chiller, whenever we’re at 500 tons or so, is operating in this range. If we’re at 600 tons, it’s operating in this range. 

If we take the large chiller, and we run it at 600 tons, we’re operating in this range, where we lose some efficiency. We’re going to copy this Alternative. Let’s go Two Chillers ARI in Series. What we’re going to do here is go back to the plants, and cooling equipment at 600. So we have our one chiller here, and we’re going to make that 600 tons. We’re going to put that in Series, and copy this. Now we have two chillers in Series at 600 tons apiece. 

If we compare our numbers here actually, by putting them in Series we did lose a little efficiency. That’s probably because we made the chiller operate on and off the second chiller by putting that in Series. You can see the loss is actually very slight, since our life cycle cost is very close to what it would be for the two chillers in Parallel. Of course, this is something that would be mathematically impossible to predict, unless you actually did the full analysis. Which you can see was quite simple to do.