Variable Primary Flow CHW and CHW temperature reset 4

[remp]

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??

 

[ChasBean1]

Certainly not a bad idea, but I think it might add unnecessary complexity. If you target the chiller discharge temperature to keep your control valves 70% open (as an example), your control valves could as well be eliminated and you can control chiller discharge temperature to maintain AHU discharge temperature. If you do this, you eliminate the advantages of controlling at individual points in a moving fluid system, which could result in excess flow through some AHUs while not enough through others under varying load conditions.

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.

 

[Cooky]

I agree with chasbean1 when he says you are better to re-set the chilled water temperature based on the outside air temperature. You should be carefull not to change the flow rate too rapidly (Trane will advise you the maximum rate of change).

To avoid control problems, you should not vary the flow rate and re-set the temperature at the same time.

 

[DouginMB]

I tried to incorporate a variable CHW temp control sequence for a hospital retrofit I designed. I quickly learned that the complexity of the control sequence should not exceed the understanding of the people having to work on the system. When I tried to explain to the maintenance staff that I wanted to re-set the chilled water temperature I saw their eyes glaze over. All they wanted was one water temperature to know if the system was functioning poperly or not. If you are dealing with smart maintenance technicians, then I say go ahead. If not you are wasting your time and will have to deal with major headaches in the future.

 

[abb001]

IT has been my experience that Trane has minimal flow conditions which should be watched- and while the original description did not state so- it is my belief that the intent is to control the pump flow using pressure monitor through the Summit sytem- which then should also include a 'lead-lag' and shut off staging sequence. All of this is adding complexity which many operators may find difficult- but if the systems are 'Commissioned' in good form- and if the operators are well trained- it all could be a good time. The one difficulty I have is believing that the CHW reset will result in a truly net positive Energy Savings if you only enable it when reaching a lower flow condition for some time- as if you are allowing the flow to vary you are gaining - but the reset should demand increased flow- (which is a good thing for the Trane units). Trane probably has an article on this on their web.

 

[moonpe]

The goal of a primary variable flow system is just like a variable air volume system: provide a constant temperature of supply air and let the pump/fan energy savings be realized without sacrificing comfort. If you start to reset temperatures, you will eliminate any transport energy savings and most likely will impact comfort. Anytime the leaving temperature is raised in proportion to outside air, return temperature(s) or average temperatures, you could have equipment/zones that need the water/air at design temperatures to meet its load that won't be getting it. I have seen a number of comfort and humidity (AKA MOLD) problems created in buildings created by control sequences that reset based upon some arbitary conditions without regard for what the impact it is causing. I prefer to leave the supply temperatures at the design setpoint and only reset when you need to maintain minimum flow rates for chillers (or use a bypass) or minimum ventilation rates (assuming humidity is okay or you will need reheat).

 

[imok2]

Billyg
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!

 

[Rhonda2]

There is a lot of very good input in the above responses to your original post. I agree in particular with the comments to keep the operation simple (moonpe, imok2). I'm working with a large university campus variable primary flow system. There are a couple of observations we have made that you may wish to keep an eye on.

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.