Effects of rasing chilled water temperature 4

[tigerpause]

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..

 

[btrueblood]

???

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?

 

[willard3]

Resetting chilled water temperatures with outside wb/db data is a good strategy. Delta t from the coil may stay the same, but the cooling load is less.

 

[btrueblood]

Why not just pump less water? Throttle back the flow via a control valve to supply a lower cooling load.

 

[vpl]

tigerpause (cute name, especially if read out loud)

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

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[dcasto]

If you want to cool a building to 70 F, you could circulate 69.999999 F water, you just need more water circulation and bigger fans.


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.

 

[HEC]

Tigerpause,
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

 

[tigerpause]

Thank you all for your input. It sounds to me that I need to do a little more research and determine what type of chiller system I'm going to be talking about. BTW, Patricia, IAQ - is indoor air quality. Please excuse my lack of knowledge on how chillers work. All I've achieved so far is my EPA 608. I've got lots more to learn, and I'm glad you all didn't take my question lightly. It's good to see such enthusiasm.

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?

 

[ChrisConley]

A big item to watch is the ability of your chilled water to dehumidify.

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'?

 

[btrueblood]

"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 "...

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.

 

[ChrisConley]

Air temp is 75F.

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.

 

[btrueblood]

Fine, if you are only looking at water side deltaT. What about your system deltaT?

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.

 

[ChrisConley]

If you read all of my posts, rather than just the parts that contradict one of your statements you'll see that we aren't actually disagreeing.

as you mentioned rather glibly, if the coils are grossly oversized for the original application

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 coil = more copper = more cost.

More copper means more capital, not more operating cost necessarily.

My other statement you disagree with is:

Chilled water at 55F requires less energy to produce, end of story,

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.

 

[imok2]

It seems to me that the only time you can raise the set point on a chiller system is when the total load goes down The problem I have with btrueblood's analyses is he is interjecting situations where the resetting of the set point on the chilled water would not take place. For example:"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" That shouldn't be the case as the probability of that taking place seems to be slim and none. So in conclusion, raising the set point is a good stragity given that all conditions are right

 

[btrueblood]

Ok, I agree Chris, we agree on some points; my original reply to your post was somewhat tongue-in-cheek. The point I'm trying to make, and I think you are too, is that a cooling system needs to be evaluated as a system, considering fixed as well as operating costs, and decisions based upon what can give you the best "bang for the buck". I deal a lot more with retrofit situations where the system was designed for a high delta-T water system, and has trouble keeping that delta-T across part loads. Raising the chiller setpoint often just exacerbates that problem, or at least makes the problems with other parts of the system more apparent.

 

[zerosum]

If the ASHRAE Systems Handbook didn't clarify the above discussions satisfactorly, try "Hydronic System Design and Operation" Erwin G. Hansen, McGraw Hill, 1985, (probably later editions in print), for analyzing & optimizing the water side. However, it doesn't offer much coverage of the air side, also important as noted above.

 

[zerosum]

Another source for case studies: HPAC.com (trade publication)

 

[tigerpause]

Thanks Zerosum, I'll check out those two resources.