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Air Cooled Chillers vs Water cooled Chillers

thegg1guy (Mechanical) (OP)
1 Aug 07 23:22
What are the advantages of Water cooled chillers over air cooled chillers ?
RossABQ (Mechanical)
2 Aug 07 0:02
KW/ton is the chief advantage.  But I have become acutely aware in recent years of how expensive maintaining cooling towers is.  There is no such thing as "free cooling".
willard3 (Mechanical)
2 Aug 07 8:35
The primary advantage of water is that the film coefficient is 10-100 times better with water than air.

Film coefficient controls heat transfer given constant area and delta t.
NESLABman (Mechanical)
3 Aug 07 10:49
As a general rule, water-cooled units make less noise, offer more cooling per square foot, and depending upon environment, usually require a little less routine maintenance....which is always good

The NESLAB specialists in the UK

http://www.weekstechnic.com/index.htm

krb (Mechanical)
3 Aug 07 12:16
Size is also a consideration, smaller units are usually air cooled, larger are water cooled.
HVAC68 (Mechanical)
18 Sep 07 22:49
Do you have good quality water available ?  Does it cost money to get water ?  In many parts of the world, water is a scarce commodity.

When you compare air and water cooled chilles, an overall system cost (both running and installed) has to be worked out.  Also, the type of application, the ambient conditions (wet bulb and dry bulb), number of hours of operation, part load pattern, cost of maintenance, etc., play a role in this decision.

There is no single answer to this question - Water-cooled is better or air-cooled is better.  It depends.

HVAC68

gepman (Electrical)
18 Sep 07 23:26
Water-cooled chillers will normally condense at an average lower pressure than air-cooled chillers.  This is because the water temperature will usually be less than the air temperature.  Unless a condenser is a one pass through water source, which is rare these days with the cost of water, the water temperature will be at or close to the wet-bulb temperature which is always at or below the dry-bulb temperature.  Lower condensing pressures translate into lower operating costs (or kW/TR as mentioned by RossABQ.  willard3 is correct that the film coefficient is greater for water-cooled units however the ultimate advantage is the lower condensing temperature which is not a function of the film coefficient but of the temperature of the heat sink.  The difference between the wet-bulb temperature and the dry-bulb temperature gives the available difference between the condensing temperatures of water-cooled and air-cooled condensers respectively.  

I would disagree with NESLABman that water-cooled units require less maintenance.  Maintenance and water treatment costs are the main disadvantage of the cooling towers and/or evaporative condensers.

There are a few locations (usually coastal climates) where the difference between the wet-bulb temperature and the dry-bulb temperature does not offset the required pumping costs for cooling towers.  

krb's statement is true in my experience.  Small systems are almost always air-cooled since the extra requirements of water treatment and maintenance are capital intensive and will not be offset quickly by the lower operating costs.  Larger systems are almost always water-cooled at least in my industrial experience.  
ChrisConley (Mechanical)
19 Sep 07 18:34
You'd be astounded how large of an air-cooled system some of our clients will install to avoid the additional maintenance costs of an evaporative cooling tower.

Evapco is pushing a new 'chemical free' maintenance system that looks promising.

http://www.evapco.com/whatsnew.asp?WN_ID=13
HVAC68 (Mechanical)
20 Sep 07 8:53
Despite the advantages of lower IKW/TR of a water-cooled system, sometimes, air-cooled systems are preferred for various applications.  

For example - 24 hour application.

The night time dry bulb temperature are generally much lower than the day time dry bulb temperature, as compared to the difference in wet bulb temperature between day and night.  This advantge is best capitalised by an air-cooled system.

Other instances could be where the wet bulb temperatures are very high, especially coastal areas.  

Water scarcity and hence cost of water is another issue which is a point to be considered while deciding on the system.  This happens in many countries - not necessarily in the US or the European countries.

HVAC68

boaterbill (Electrical)
20 Sep 07 10:33
If you have the potential for dirty water, then why are you considering a chilled water system?  If that is such a problem, DX will work.
And the chilled water lines are sealed anyway, you always treat the water on installation, so again, why is the cost /quality of water a problem in less than ideal situation?

Common sense should be the deciding factor on the chiller selection.  Air cooled are smaller and noisy, neighbors don't like then on ice make mode.  water cooled are quiter and more costly to install maintain.
ChrisConley (Mechanical)
20 Sep 07 10:42
Hi Boaterbill,

The cost/quality of the water is an issue due to the water that is lost in an evaporative cooling tower. The chilled water lines are sealed and treated, condenser water is lost to evaporation and exposed to the outdoors.
BIPVguy (Mechanical)
22 Sep 07 5:08
What is  criteria for sizing pipes for chilled water?
What speed (m/s) and unit pressure drop (Pa/m) are recommended?

Thanks
TBP (Mechanical)
23 Sep 07 12:58
ChrisConley - regarding the Evapco "Pulse-Pure" - I could be wrong, but it sure looks & sounds like all of the other water treatment chemical-eliminating snake oil gadgets that hit the market every couple of years for the last 20 or so. There was another big HVAC equipment maker that had a similar device on the market a few years ago. I haven't heard or seen anything on it for a while. I suspect it was quietly dropped from their product line.

I dunno, maybe this one works. I'd let somebody else try it for couple of years first, though.
Plexed (Specifier/Regulator)
16 Oct 07 21:55
Believe it or not!  We have a home with a 7.5 ton 1954 Carrier water chilled tower and a Raypak boiler.  At the age of 53 years, should I consider a DX system?  Carrier still makes this type of unit and I can simply replace it and the water tower.  The operating costs seem to be more  favorable than a DX system.  It admittedly takes more maintenance than a DX system.  I also have 3-phase electric service, another preceived advantage.  Any suggestions?

gepman (Electrical)
17 Oct 07 21:36
Plexed
Where do you live (affects WB temperature distribution)?  What is the minimum condensing temperature for the water cooled unit?  What is the minimum condensing temperature for the DX system?  There are BIN analyzers for refrigeration (and A/C) equipment that can tell you what the difference in energy usage will be.  Don't forget that you need to take into account water pumping costs and water treatment costs.  If it lasted 53 years then I guess the water can't be that bad.  There are some fiberglass cooling towers available now that may be better.  Yours is the first residence that I have heard of with a cooling tower.
Plexed (Specifier/Regulator)
18 Oct 07 12:12
Thank you gepman.

Not sure of how to figure the minimum condensing temperature.  Typically, here Dallas, Texas  experiences a great many summer days in the 90-100 degree range, and we keep the thermostat at 78 during the day while we are away, and lower it to 75 in the evenings, and 74 at night.  It can get uncomforably cool if we go much lower. The duct outlet temperature when in cooling mode can give be 40 degrees cooler than the outside air.  

The cooling tower is located in a shady spot on the North side of the house. It uses a 1/2 hp single phase recirculating pump, and a 1 1/2 hp 3 phase for the squirrel cage blower.  I plan to replace it with one of the newer fiberglass cooling toweres you mentioned, so the water and energy costs are reasonable for cooling a 3900 SF residence.

My dilemma is whether or not to replace the water cooled equipment or junk it and put in DX equipmnet taking into consideration operating costs and comfort.  Eithr system will be in the $14-16,000 price range, which is why I am so concerned about making the right decision.

Where could I find BIN analyzer information that you mentioned to compare operating costs?

Your comments are appreciated.
gepman (Electrical)
19 Oct 07 12:11
Plexed

See my post in "residential water-cooled air conditioning", part of which I have copied below.

If you want to actually calculate how much water (evaporative) cooled air conditioners will save you compared to air-cooled systems you can download the Carlyle (a division of Carrier) compressor rating software at http://www.carlylecompressor.com/corp/details/0,,CLI1_DIV24_ETI1240,00.html
In this software there is a BIN analyzer which allows you to specify the degree of approach of the condensing temperature to the heat sink temperature.  The heat sink is selectable as either dry-bulb (air) temperature or the wet-bulb temperature.  You also put in the minimum condensing temperature (to take care of the problem of the TEV minimum condensing temperature).  Select the location (or the location with a similar climate) and the system will go through the weather data hour by hour and calculate the energy used.  The compressors that they have are more small commercial compressors but the COP's or EER's are basically the same as residential.  After you do this then you need to add the additional water pumping energy, water treatment energy, and water usage energy to the energy used by a water-cooled air conditioner.

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