Over capacity chiller

[Mechya]

Hi,
We have a 500 Tons capacity centrifugal water cooled chiller serving air conditioning to a special function auditorium. This building is rarely used and if use it is used for a short period of time, say 3-5 hours for exams and seminars. In addition to this during winter the chiller loads to merely 10 % of the capacity. In Both the above mentioned cases, chiller recycles frequently or Too many ON /OFF's are needed to maintain the comfort condition in the building. My question is what can be done to avoid this problem?
Please note: 1. Hot Gas Bypass is always ON, though it is not a good practice considering the energy conservation.
2.For sure, this is an over designed case.

Thanks...

 

[mfi2000]

What options can you use? Can you downsize, sell the chiller and put in a different one? What about the air side. What type of air distribution do you use. The constant start stop isn't good for the motor. Need a little more info. In my building we can go to outside air when it's cold enough.

 

[SAK9]

Add a thermal storage tank and charge it up for use before the auditorium ia about to be used.You may not need to turn the chiller on when the auditorium is in use.
But this solution is not cost effective.

The best option for this kind of application is a very small chiller(say 50 tons) with a thermal storage tank.So take MFI 2000's advice and sell it off!

 

[TXiceman]

You can add a theral storage volume, but this will take up a lot of room. Have you considered an inerter or speed control on the compressor.

Most chillers can only turn down so far (maybe 40 to 50% on a centrifugal). Once you are turned down to the minimum capcity, you will need to go on hot gas or some form of capacity reduction. If energy reduction is the controlling factor, you may need to look at a smaller chiller to parallel the current machine and just run it in the low load conditions.

Ken

TXiceman

http://www.rae-corp.com

 

[Mechya]

Thanks mfi2000 and SAK9 for the advices. I am planning to install a new 100 Ton chiller with a plate heat exchanger between the new chiller flow and the existing chilled water network (as I cannot reduce the chilled water flow on the building side because of the distributed AHU's inside the building and all are needed to be in operation with lesser cooling load but with constant flow). This seems to me a bit expensive to start with, My question is; is this a good solution, I need advice from all those experienced guys in the forum.

Thanks

 

[quark]

Centrifugal chillers with capacity control by guide vanes and variable speed device together can be operated at such low capacities without having to loose much efficiency. No problems, also, with surge. This may cost you less than what you have to pay for a new chiller of smaller capacity. Speak to your manufacturer about the possibility.

 

[MintJulep]

Or, could you tackle the problem from the other direction?

Is this auditorium a stand-alone building, or part of a larger building or campus?

You could use your big chiller for other spaces, and decomission a bunch of smaller units.

 

[tombmech]

I believe I like MintJulep's suggestion best. See if there's some DX capacity you can turn into chilled water. Adding coils and pipe can have an easy payback if you've got some of your load tied up into compressors and refrigerant elsewhere.

I love chilled water storage tanks, but I believe you have to get into 10,000 ton-hours or more before you quit wasting money.

BTW, no offense to the suggestions, I agree in principle. However, retrofitting an existing 500 ton centrifugal with a VFD would be problemmatic. I agree that's the direction for the future, but its something that you'd want the mfr to build-in, unless you're into fabricating your own component process systems, compressors, heat exchangers, controls, etc. Guide vanes are most likely already in there. Nothing will help at 10% or below. If you're getting that, you're already doing great.

 

[MedicineEng]

With energy prices going sky high, using the hot gas option you are just trowing money into the street. Make your assessment in terms of energy savings and you will see that your new chiller has a shorter payback time that you would expect.
Or follow mintjulep's suggestion.