Variable Primary Flow CHW and CHW temperature reset
Variable Primary Flow CHW and CHW temperature reset
(OP)
Im installing 2 Trane Screw Chillers with variable primary flow chilled water pumps on Variable speed drives (there is no secondary loop). The pumps are being controled by a Trane Summit Controller. Is it a good idea or not in a variable flow system to reset the CHW temperature for energy savings or is it too complex. I was thinking of resetting the temperature depending on AHU valve positionins once the chiller pumps are running at a minimum for say 10 minutes continuously. Any ideas??





RE: Variable Primary Flow CHW and CHW temperature reset
If the chiller system already allows a variation of primary flow [indicating to me that the chillers can vary load (refrigerant flow or TXV position) to meet the discharge temperature under varying flow], further energy-saving controls might not be necessary. Maintaining a set discharge temperature, but one that could be varied based on outside air conditions, would be preferable to modifying temperature to accommodate control valve position.
RE: Variable Primary Flow CHW and CHW temperature reset
To avoid control problems, you should not vary the flow rate and re-set the temperature at the same time.
RE: Variable Primary Flow CHW and CHW temperature reset
RE: Variable Primary Flow CHW and CHW temperature reset
RE: Variable Primary Flow CHW and CHW temperature reset
RE: Variable Primary Flow CHW and CHW temperature reset
With modern DDC controls on chillers today, which are capable of very close control to set point, the amount of energy savings you are talking about is miniscule compared to the possible headaches you may have. As was mentioned, the real secret to successful operation of any plant is in the knowledge of the operating personnel. You can design the best design scheme in the world, but if the operation personnel find it too difficult, believe me, they will simplify it as fast as you can say “danker shain” the secret for successful Engineering …Keep it simple!
RE: Variable Primary Flow CHW and CHW temperature reset
1) The chiller manufacturers like to recommend CHW reset because it makes their individual chillers appear to operate more efficiently. In many cases this comes at the expense of SYSTEM energy consumption.
2) As stated earier pump energy consumption rises when CHW supply temperature rises. This is because delta T decreases as the difference between average air temperature and the entering water temperature decreases.
3) Fan energy can also rise as a result of CHW supply temperature reset. Higher CHW supply temperature reduces the capacity of the coil (latent and sensible) and it is possible that the leaving air temperature setpoint will become tough to meet. If it rises the fan speed may need to increase to sustain comfort conditions, especially if chilled water balancing valves are installed.
4) Make sure to monitor a CHW supply temperature leaving the central chiller plant. In my project we found that the system was behaving as a "virtual" primary/secondary system. As much as 1/3 of the design flow was being pumped through non-operating chillers at low load. While the operating chiller appeared to be delivering 42 degree water, after blending temperature would rise to as high as 52 degrees. We recommended that the chiller be sequenced to maintain the distribution CHW supply temperature, not just the temperature at each individual chiller.
5) If you do reset the CHW supply temperature, I'd recommend that it never causes the delta T entering and leaving the plant to drop below design. If it does, the second chiller and its auxiliaries will need to run before it is necessary once the chiller evaporator flow limit is reached. Furthermore, the temperature should not ever exceed the design CHW supply temperature for the cooling coils.
Hope this is useful for you given the date of your original post.