Chill water resets

Do you have a chiller plant? Is your air handling equipment operating at some level below 100%? You can probably benefit greatly from a chill water reset, especially if you operate with a largely sensible load (like a data center!). Even if you have latent cooling, the latent capacity is not impacted as much as you think by higher chill water temperatures. The coils leaving air temperature is what determines how well the air handler removes humidity as that air is typically at saturation. So as long as you are meeting your air temperatures set point, latent capacity is not impacted.

So what of the chill water reset? Well, ASHRAE recommends, in a series of articles co- authored with Taylor Engineering, the use of a chill water temperature reset followed by a chill water d/p reset.

ASHRAE chiller control article
Since combining the d/p reset and chill water reset typically achieves very little d/p reset, ensure the d/p sensor is located at the most remote point in the system and set to the absolute minimum value to minimize pump energy. The reason the d/p reset rarely makes an appearance is that the chill water reset will cause the chill water valves to run out of headroom before the d/p reset can kick in.

To set a chill water reset, you need to monitor your chill water valve. So long as the valves are not at 100%, you can reset the chill water supply temp upward. This reset could move fairly slowly, at only a few degrees an hour to allow the system feedback to occur and influence the algorithm. And should your demand increase, chill water valves positions will move further open, and the chill water can reset back downward as necessary. A decent thumb rule to use is that for every degree of temperature reset (in degrees f) you will observe a 1.5% increase in capacity of the chiller and chiller efficiency. Raising chill water set points by ten degrees may actually achieve a near 20% reduction in chiller energy consumption. A chiller using 1,000,000 kWh a year could save 200,000 kWh, and at ten cents a kWh, save $20,000 annually. Any competent BMS contractor should be able to program a reset for no more than a few thousand dollars, resulting in a fantastic project payback.

A 5MW data center, using economizer for approximately half the year and chill water cooling for the other half, with consumption of approximately .4 kWh consumed for kWh consumed, would use around 9,000,000 kWh in chiller load each year, and with a 15 degree reset might save $225,000 a year on energy! If your chiller plant has a water side economizer, the hours of availability increase remarkably with increased chill water temperatures, and these savings could be several times greater!

I highly recommend evaluating your systems and consider using a chill water reset based on chill water valve positions!

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4 thoughts on “Chill water resets”

  1. In the ASHRAE article it seems it is comparing the energy consumption of chilled water reset with DP reset based on variable speed chiller. Is there any data on how does it compare on constant speed chillers? I think the energy saving by resetting chilled water temperature upwards may not be as high in constant speed chiller compare with variable speed chiller. What I have seen mostly is critical valve reset thorugh DP reset on constant speed chiller. Variable speed chiller is not common here yet (probably due to cost and people are not familiar with how it works). Chilled water temperature reset is something new and interesting to me.

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    1. With a fixed speed chiller there are always other challenges but if you end up stepping off a chiller, the the huge benefit is obvious. Do the Singapore chillers really have no capacity control at all? I thought they at least had the slide valve on the screw, and those units can still save with the reset schedule.

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      1. Yes the constant speed chiller here have slide valve on screw or IGV on centrifugal to throttle refrigerant to modulate capacity. The VFD chiller modulate capacity by combination of modulating compressor speed and IGV position. The reason I feel constant speed chiller may benefit less from chilled water reset is because a lot of energy is wasted at the throttling process, while VFD chiller can benefit more on reducing compressor lift by lowering compressor speed. I agree that even on constant speed chiller chilled water reset will definitely be beneficial. But when comparing chilled water reset and DP reset it’s down to which one can reap more savings. In the case of constant speed chiller, savings on reducing chiller compressor lift may or may not outweigh savings on reducing chilled water pump speed.. I don’t know the answer but it is something worth exploring.

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  2. Well we have th efficiency curves for the rtac chillers. They do improve efficiency quite well with reduced loads, which is why our systecon system stages chillers the way it does. And our pump energy is pretty much optimized. Send me a work email and I will forward you the documentation on the systecon system, it provides a lot of insight into the chiller operation in general.

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