What are the advantages of Using DX Coils?

[scottbalan]

Hi all,

I am curious as to why use DX COils when chilled water is available? What are the advantages of Using DX Coils?
Lower initial cost? Low Cost of Maintenance?

Please advice.

Thank you.

[madhat]

If the chilled water is piped to where the coils are, the only advantage I can see is a system that is independent of the chiller. Critical Data rooms that need year round cooling, could benefit from a DX application.

[hvacpope]

Also if you look at the thermodynamics properties of a given refrigerant like R-22 for example and compare it to chilled water, you’ll realized that water is not a very good refrigerant and the size of the coil and the piping most be increased in order to obtain a refrigeration effect comparable to R-22. There are also possibilities related to the initial engineering of the chilled water system, if it was designed to handle a certain load you just cant add a few more coils here and there without throwing the whole system out of balance.

[madhat]

Good point hvacpope. We had a church that had to have better humidity control, we just repiped the existing chilled water coils, and reprogrammed the controls to ramp down the supply fan. Not something we could have easily done with DX.

[jayguy]

generally speaking, when possible, chilled water will do a better job of cooling and dehumidification. however, with that said, chilled water freezes up easier than refrigerants. if the coil is exposed to cold outside air (computer room ac, some schools and auditoriums, etc.) for free cooling purposes then you have to use glycols instead of water to prevent freeze ups. glycols do not transfer heat as well as water and can cause you to use more energy than you might with a DX unit.

then there is the part of having to have a chilled water unit. typically these are going to be larger units (50 tons and up). if you do not have the load for a larger unit, then DX might be a cheaper installation and operation choice.

i have a hopital where the hospital joins a community center that they serve. the chilled water piping is not large enough to supply that hospital wing and the community center. to use chilled water, they would have to rip into the ceiling and walls of the hospital to tear out the old piping and install the new piping. the dx unit costs only a fraction of the labor to install the chilled water piping. in this case, first costs was the determining factor.

[just_opinion]

Chilled water coil has more advantages the DX coil.

[socal]

Copper vs. steel piping = dollars
Refrigerant leak vs.water leak = dollars
Plan ahead for possible expansion of facilities,i.e; sell a higher capacity chiller and make avalible fittings to connect to new ares of service.

[hvac_two]

I beg to differ with hvacpope. Nothing, or very little has the capacity to transfer heat as good as water. The problem is that "they" haven't figured out how to effeciently compress water and boil it away within a closed loop as "they" have with refrigerant. In a large building chilled water is way more cost effective than a gazillion tin-can compressors eating up voltage. Plus, with a chilled water system two main pieces take care of your heat transfer(chiller & tower).

[gsxrsquid]

cost is always a factor but the application is imporant as well. I have seen surgery suites that they wanted to pull down to 50f. The original chill water coils could do it over night but after the first case it was over. This was running the chillers at 38f out. not a good thing. We added DX coilds with glycol and used existing chill water system for pre-cool. froze their a$$es off.

I have had buildings with both and CW and DX. It just depends on what your needs are.

[gsxrsquid]

I have to agree with HVAC_two as far as water bing a good refrigerant. That is what makes an absorber work so sweet.
in a chill water loop it is not being used as refrigerant but just as a heat transfer medium.
I always did like absorbers better. LOL

[hvacpope]

Originally posted by hvac_two
I beg to differ with hvacpope. Nothing, or very little has the capacity to transfer heat as good as water. The problem is that "they" haven't figured out how to effeciently compress water and boil it away within a closed loop as "they" have with refrigerant. In a large building chilled water is way more cost effective than a gazillion tin-can compressors eating up voltage. Plus, with a chilled water system two main pieces take care of your heat transfer(chiller & tower).

I can’t really see your point, you cant prove that water with a boiling point of 212 F at atmospheric pressure is a better refrigerant than R-22 with a boiling point of -40F, water may have a greater enthalpy but that's different story. Absorbers will not work with water only the correct term is brine and you all know what that is.
A chilled water system will do wonders no doubt, but they are expensive to buy, operate and maintain.

[RichardL]

I would respond to that last post if I had the slightest inkling of what in the hell he was talking about.....

[hvacker]

A DX coil has one large disavantage ie: the need to reserve a portion of the coil to superheat the refrigerant to protect the compressor. This is all a loss. I remember a question I was once asked, "What is the ideal superheat" The answer is 0. It's 0 because any heat added to the refrigerant = a net loss. Your using coil capacity to heat the refrigerant rather than doing what you intended, cool air.

DX coils operate at a net loss unless a device such as a expansion valve with a microprosser type of controller can achive 0 deg or close superheat.

A water coil has none of these problems. It's simple, just temp drop. As long as pure water is used ( no glycol) it's all net effect cooling.