Effects of rasing chilled water temperature
Effects of rasing chilled water temperature
(OP)
We all know that reducing energy will reduce greenhouse gases emitted into our atmosphere. I've been assigned to explain to building mangers that rasing their chill water will reduce energy consumption. Here are the questions I have, in order to have ammunition; 1. what type of chiller systems can this application be implimented on? (I heard only centrifugal and screw type). 2. How do you maintain dew point? How do you monitor? 3. Payback? 4. Where can I find real numbers, real facts? 5. What about everyone's concern of MOLD and IAQ..





RE: Effects of rasing chilled water temperature
Raising the cold side air temp. will force the system to pump more gpm to obtain the same net cooling tons. How will that save power? Chillers are designed to have best efficiency at their design point delta-T or cold setpoint, operating off-design will reduce their efficiency...
So how can you save any power reducing the delta-T?
RE: Effects of rasing chilled water temperature
RE: Effects of rasing chilled water temperature
RE: Effects of rasing chilled water temperature
I'm starting off with the supposition that what you want to do is to raise the temperature coming out of the chiller or, in other words, to reduce the amount of chilling that is done. Otherwise, as btrueblood comments, it makes no sense. In response to your questions:
1) the more important question is: What type of chillers do you have? The rationale for replacing major equipment has to be a lot better that "you're going to notice it's a bit hotter in here.."
2) you maintain dewpoint by keeping your relative humidity low. And you monitor it by monitoring the humidity.
3) there isn't enough information to determine this.
4) I'd suggest starting with the facility maintenance department.
5) I don't know what IAQ means... but in regard to mold, a lot depends on what exactly you're trying to do, what your process is, what temperature changes are being made.
I believe you need to do a lot more work to define what you're trying to do. If it's only "give a presentation" then someone else should have answers for you. If it's to present a plan, then you need to gather a whole bunch more information. Time to go back to your boss and get clarification.
Patricia Lougheed
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RE: Effects of rasing chilled water temperature
The system should have been designed such that sum of the energy used in the chiller, plus the pumps, plus the fans would be almost at a minimu. The reason they won't be at the minimum is because of the capital to build each componet.
RE: Effects of rasing chilled water temperature
I would suggest that the power savings are based on the reduction in gas compressor power required. By increasing the chilled water temperature (evaporator temperature and therefore the compressor suction pressure) and maintaining the condensor temperature constant (compressor discharge pressure) the compressor absorbed power is reduced. this is more so with centrifugal or screw compressors. The refrigerant mass flow will also increase to maintain the same cooling capacity. As the other responses poinit out there will be other alterations that need to be made to the system to compensate such as, increased chilled water flow, potentially larger coil areas to obtain equivalent cooling.
If you want to be convinced do the thermodynamics, and get some real data from compressor suppliers in relation to compressor efficiencies.
Great to see someone asking the questions!
Mark Hutton
RE: Effects of rasing chilled water temperature
So can anyone point me in the right direction? I’ve researched ASHRAE, EPA and TRANE. Are there other websites or companies that discuss CHW reset, and the possible reduction of energy consumption?
RE: Effects of rasing chilled water temperature
Chilled water at 55F requires less energy to produce, end of story, and you don't need more of it provided you can get the same delta T out of your coils (which means more coil). More coil = more copper = more cost. The air and water pressure drops of the coil can potentially be increased, but again, larger coils can fix this problem.
The balance has always worked around 45/55 chilled water for a balance of coil cost and chiller cost. With raising energy prices it is time to start looking at those numbers again.
The issue is that chilled water at 55F has a reduced ability to dehumidify an incoming outdoor air stream, or recirculated air stream if you have a large latent load.
If you plan on using an alternative dehumidification strategy: active dessicant, DX, dessicant wheels, you won't have any issues.
Quick question, what is your 'EPA 608'?
RE: Effects of rasing chilled water temperature
Uh. Delta-T is temperature change. Bigger coils don't give you more delta-T. Raising the CHW temp. from 45 to 55 drops your available delta-T by 10, requiring an increase in flow (and cooling surface area) to deliver the same cooling.
RE: Effects of rasing chilled water temperature
Traditional chilled water temperature - 45/55 (delta T = 10) through the coil.
Warm chilled water temperature - 55/65 (delta T = 10)
Q = 500*gpm*deltaT
If the deltaT is the same, the same flow will produce the same Q.
The approach has decreased from 55F to 75F, down to 65F to 75F, a closer approach means we need bigger coils (which is increased cooling surface area).
Uh.
RE: Effects of rasing chilled water temperature
When your 65F circulating air enters a room intended to be kept at 70F, it will take a lot more mixing/circulating to cool the room, since the TOTAL delta-T is only 5F. Air entering the room at 55F can do the job more quickly, with less circulation. Add outside air to the calculation? Or is that the "end of the story"?
The real uh is your tacit assumption in your first post that changing a setpoint will reduce power; that is only true, as you mentioned rather glibly, if the coils are grossly oversized for the original application, and thus have ample margin to handle the load at 1/3 of the original water-to-air delta.
It's possible that the approach can work in a scenario with less than design cooling load, but it's easier just to throttle back the flow and leave the setpoint alone, and probably works out cheaper on power, depending on the system design.
Bottom line - a chilled water system and chiller designed and optmized for one setpoint will not necessarily perform more efficiently by just changing the setpoint; often such "simple" approaches end up costing more (more pumping, more air side circulation, chillers operating off-design at lower efficiency).
Uh.
RE: Effects of rasing chilled water temperature
I'll try to be less glib for you in the future, but I don't understand the criticism. I stated, as you mention, correctly that a closer approach will require more heat exchanger...
More copper means more capital, not more operating cost necessarily.
My other statement you disagree with is:
What part is incorrect? Warmer chilled water costs less energy to produce. No?
Total system design for energy is going to take into account all aspects of design. Warmer chilled water used in a radiant cooled facility with an alternative method for dehumidification and outdoor air control is more energy efficient... and more costly.
My point was, and will remain, the traditional 45/55 design parameter of a chiller should be reexamined as those numbers are based back when energy was cheaper than materials, and VAV systems with 55F air were the only option for comfort cooling.
RE: Effects of rasing chilled water temperature
RE: Effects of rasing chilled water temperature
RE: Effects of rasing chilled water temperature
RE: Effects of rasing chilled water temperature
RE: Effects of rasing chilled water temperature