Variable Primary Chilled Water Systems Part 4: Compensate for Low Delta-T!

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By Chad Edmondson

One of the greatest incentives for designing a chilled water system to be variable primary is reducing the likelihood and problems associated with low Delta-T syndrome. It is, after all, one of the most common and costly problems in standard primary secondary chilled water design. But how exactly does variable primary reduce the potential for low Delta-T?

Going back to Part 1 of this series, we learned that in any system, BTUH = 500 x GPM x ΔT. And if we want to convert this equation to chiller tons, we simply divide by 12,000 because there are 12,000 BTUs in one cooling ton:

Cooling-Tons-Conversion.jpg

Hold that thought as we take a look at the variable primary system in Figure 1.

Figure 1

In this example we are operating at a lower ΔT then design. Our system is demanding 1200 gpm of flow at a 10 degree ΔT whereas our design was for a 12 degree ΔT. Our variable speed primary pumps are sized for 1000 gpm each. If this were a standard primary secondary pump, we would have no choice but to operate both pumps, both chillers, both condenser water pumps and both cooling towers, otherwise we would not be able to satisfy the load (See Part 2). We will use a lot more energy running all of that additional equipment!

However, with variable primary, we can operate both our primary pumps at 600 gpm each and meet our flow demand while still achieving our rated 500 tons of cooling through just one chiller. And, of course, we avoid having to operate an extra cooling tower and condenser water pump.

Our chiller is fully loaded but we are operating now at a 10 degree ΔT instead of our design of 12 degree ΔT. But that’s okay, because if we go back to our equation and plug those numbers in, we can see that we are still able to get 500 Tons of cooling from our chiller:

Increasing-flow-to-maintain-chiller-tonnage-with-one chiller.jpg

We also have the option of sizing our pumps up to 1200 gpm and operating only one pump under this condition. Either way, we are saving a tremendous amount of energy and wear and tear on chilled and condenser water equipment!