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.

[heatingman]

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.

This is not true. THe main reason mechanical cooling works as well as it does is because of the heat required to change the state of a liquid to a gas. this is also true for water. 1 btu to heat one lb. of water 1 degree in a deffinate state of matter. Take the same lb of water to heat from 212 liquid to 212 gas it take 944 btu/lb. I don't the exact numbers for refrigerant, but the concept is still true. And while adding superheat, your still below roomtemp. Go ahead run a 0 degree superheat evap and add in a desubcooling txv at the inlet of the compressor for comfort cooling, and watch the call backs keep on a commin. Another thing, water is very limited, it cant deliver to close to 32 without asking fo trouble waterside, while the refrigerant itself will go un affected at very cold temps which may be required to drop RH on milder days

[wonder_boy]

Another advantage of a chilled water coil is you have more control of temp.It can be controlled to within 1/2 degree with a 3 way valve.

[emcoasthvacr]

along with better temperature control and more stability, water has a superior specific heat compared to refrigerants (4.2 Joules/gram centigrade compared to R22 which has 0.31 Joules/gram centigrade); in other words, it's a superior energy resevoir compared to the "quick pop" of refrigerants.

It's also cheaper and safer on both the tech and the environment.

[dan sw fl]

Originally posted by hvacpope

Originally posted by hvac_two
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).

A chilled water system will do wonders no doubt, but they are expensive to buy, operate and maintain.


CW is definitely less expensive to operate.

Maintenance must be performed regularly by experienced personnel.

[beenthere]

A 45° coil is a 45° coil weather DX or CW.

A chiller can do that for lees cost per btu then DX.

And with modulating valves, you can maintain a better temp control with less wear and tear then you would have on a DX system.

[hvacpope]

Originally posted by beenthere
A 45° coil is a 45° coil weather DX or CW.

A chiller can do that for lees cost per btu then DX.

And with modulating valves, you can maintain a better temp control with less wear and tear then you would have on a DX system.

I‘m not so sure about it, factor in the energy that the chiller uses, plus the water pumps, the cooling tower, the cost of the water treatment,the quarterly and annual PM’s, and the elaborated control systems needed to sync every piece together and you are looking at a nice wad of cash.
All rooftop unit needs is a thermostat and a clean set of filters, any decent maintenance man with a bit of oil and coil cleaner could keep the unit running for a very long time, chillers are a bit harder to figure out and most of the time the maintenance and repairs services are performed by contractors.

[beenthere]

It takes alot to keep 4000 tons of split units, or package units going also.

Each has its place.

[esornivram]

peak load demand for larger equipment brings in higher utility bills. each for its application has costs associated with them, wheter its water coolant loop chemical treatment, cooling tower,cleaning, rodding tubes costs, chiller costs,motor starter costs as well as size,
dx equipment used in smaller sizing has smaller unit cost for maintence,

each works for its enginnered applications, most design enginers dont take into acount the maintenance costs,
CW still has air filters, belts. motors, valves, and on and on and on.
I havent seen many small chilled water system that was easy to keep up with, worked on an liebert once was for cooling an MRI liquide helium units,
however i will have to think about the statements that superheat the dx is an loss, my numbers says it is still absorbing heat, cause 32 degree refrigerant is still cooling the air, even the plus 6 to 8 degrees of superheats is still cooling the air, and yes the heat of compression is an load on the system, however so is pumping water up 6 stories,
i would say that usualy its about cost of install versus the size of the load, as well as ease of access of the space to the equipment,
process cooling has higher lift calculations and reduced tonnage versus engery consumed, where dx equipment is simplier to achieve the desire condition. we had an 32 degree process cooling using 500 ton cvhe's however due to the design we were only getting 338 tons from each 500 ton machine, and had to run well water( that was required cause we had to supply 68 degree condenser water at all times,) so cooling tower wasnt feaseable to use, well water at 1,000,000 gallon wells arent cheap to run either, and they filled the tubes with mud, we were rodding tubes once an week, this was leading to excess wear on the insides of the tubes, the TDS in the water was excessive, the plant had to bring in reverse osmosses equipment, built with SS equipment, to condition 1,000,000 gallons that aint cheap guys,
so some of the previous statments even tho said in general dont always applied as the actual application is considered mbr

[heatingman]

beenthere
Professional Member

Registered: Jan 2004
Posts: 9433
A 45° coil is a 45° coil weather DX or CW.

This is not true

A water coil at 45 will not and can not remove as much heat as a refrigerant coil. Why? Water has a specific heat of 1 degree while r-22 has a specific heat of .3, so how can r-22 remove more heat? Because with mechanical compression air conditioning you are forcing a change of state from liquid to gas and in order to do that the r-22 must take on 101 btu/lb to flash from a liquid to a gas. That is known as LATENT HEAT OF VAPORIZATION. While that water coil may hold 5 gallons or 25 lbs. Example. return air temp of 90 and water temp of 45 is a 45 diffence and the 25 lbs of water at a given time will only absorb 1125 btu before the water is 90, but a dx coil with liquid changing to gas even with only 1 lb r-22 will absorb 4545 btu before becoming 90 degrees. And multiply that by constant water and refrigerant flow, and refrigerant still wins. There is no arguing with fact.

[beenthere]

Originally posted by heatingman
[B
A 45° coil is a 45° coil weather DX or CW.

This is not true


[/B]

First, 5 gals of water weighs 41 pounds.
Second, r22 returning to a compressor at 90° would wipe out the compressor.


To make the kind of comparision your trying to make, you have to use real flow rates.

[mikeswear]

Water is no doubt the most effecient ref. known to man. But the only way to use it as a ref. is in an absorbtion machine which uses vacuum and the affinity salt has for water to make large amount change state (latent heat) to go to the "salt bed" (heavy brine). One in a million work anywhere close to trouble free. That is why almost all of them have been changed to centrifical units (which, by the way do run with a "0" superheat. To add signif amount of tonnage one must do a load study to see if there is any available.
Chilled water systems are by far the most desirable for large cooling applications for a couple of reasons. One main one is diversity factor...it use to be called the "follow the sun" principle. In the am most cooling is required in the east and pm in the west. Another reason is the larger the motor the more efficent it runs. And they are built to last thirty to fifty years. Water coils are far supperior for dehumidification because there is no limit to how many passes (thick) the coil is. The air that pass through the average dx coil only half of it is changed in temp or humid. I hate absorbtion but I love chilled water. Dirty filters do not effect the water system like it does dx equipment either. In the chiller the water is not a refrigerant but a cooling medium.
Happy Thanksgiving

[Edited by mikeswear on 11-26-2006 at 11:02 PM]

[mikeswear]

oooops..where you are missing the fact about a dx or water coil in your discussion of 45 is 45. That is correct. The difference is that a chilled wather coil may have six or eight single passe rows. The water comes in at 45 and leaves at 55. Where as freon coils have to have multiple passes to have room for superheating to prevent flood back. In extreem cases like a lab etc or computer mfg the coils may be a foot thick with fifteen single pass design. The water may come in at 38 and leave at 44 resulting in very, very dry and cold air. You will never find any thing but multi chiller application with the temp controlled down to 100/th of a degree. You just cant get the reliablity or perfection from dx.

[hvacker]

Originally posted by heatingman
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.

This is not true. THe main reason mechanical cooling works as well as it does is because of the heat required to change the state of a liquid to a gas. this is also true for water. 1 btu to heat one lb. of water 1 degree in a deffinate state of matter. Take the same lb of water to heat from 212 liquid to 212 gas it take 944 btu/lb. I don't the exact numbers for refrigerant, but the concept is still true. And while adding superheat, your still below roomtemp. Go ahead run a 0 degree superheat evap and add in a desubcooling txv at the inlet of the compressor for comfort cooling, and watch the call backs keep on a commin. Another thing, water is very limited, it cant deliver to close to 32 without asking fo trouble waterside, while the refrigerant itself will go un affected at very cold temps which may be required to drop RH on milder days

I don't know what part you're referring to that's not true.
If you think i'm sugesting running 0 superheat I'm not. What I'm saying is 0 is the ideal but not obtainable today w/o damage.
And I've taken water w/glycol down to 22 deg for injection molding w/o problems other than a drop in tonage.

The exact numbers you refer to is called refrigeration effect and they are different for every refrigerant.

And I'll stand by what I said any part of a cooling coil used to superheat adds to a system's inefficency.

[tex~n~oz]

It's all very simple people...
It has nothing to do with enthalpy, coil sizes, specific heats,...........

Economies of scale..

For small to medium installations it's cheaper to install and operate Direct Expansion equipment.
For larger installations, the chiller wins.
It really boils down to cost per KWh (or whatever unit of cooling you are using).
This cost is considered for installation, maintenance and operation. In some cases redundancy, precision, space, and environment (and many other things) are taken into consideration.

I've seen many installations where the "love child" of them both wins. The infamous water source condensed DX unit.

Some locations can't handle aluminum coils while evaporative cooling towers are unfavorable in others.

If one was "better than the other" there would only be one type. The engineers and bean counters would see to that.

Cheers