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COP air cooled evaporator vs. water cooled (incl. CT)

abcmex (Mechanical) (OP)
23 Oct 06 3:58
Do you know any internet reference comparing COP of air cooled chilled water unit vs. water cooled chiller (including the cooling tower pumps/ventilators)?

Kind Regards
AbbyNormal (Mechanical)
23 Oct 06 13:19
No, I can't give you a link but I feel the sales reps could.

Water here is made by RO, we have a salt corrosion problem and deisel generated electricity.

Trane Latin America asked me for some input on water and power costs here on a large project some one else was designing. The job eneded up with cooling towers, but I cannot say if it was an operating cost alone, most likely factoring in condenser coils corroding driving up head pressures on the operating costs, then replacing coils etc in on the operating cycle.

Take the "V" out of HVAC and you are left with a HAC(k) job.

imok2 (Mechanical)
23 Oct 06 16:10
The term Coefficient of Performance (COP) is simply the ratio of the cooling effect produced expressed in Btu/hr divided by the energy input expressed on the same basis. For an electric chiller at 0.6 kW/ton, this ratio is 12,000 Btu for a ton of cooling divided by the corresponding 0.6 kW energy input. The units of kW can best be thought of as kW hours per hour. Each kW is equivalent to 3,413 Btu, therefore 0.6 kW is 2,048 Btu. Therefore, a 0.6 kW/ton chiller is equivalent to a COP of 12,000 Btu/2,048 Btu, about 5.9 COP. Notice the term COP is dimensionless.

There is a short-cut formula to compute COP directly from any given kW/ton. Simply divide 3.516 by the chiller's kW/ton to derive COP. (3.516 comes from dividing 12,000 Btu by 3413 Btu per kWh). For example, 3.516/0.6 is the same ~ 5.9 COP as before. Be careful when comparing cooling system COPs to be sure exactly what is being included. A fair comparison includes the full heat rejection circuit as well.
All mfg's can give you their kw/ton on their equipment.
ChrisConley (Mechanical)
23 Oct 06 18:20
One item that makes it tricky to 'eyeball' a comparison between the COP of a water-cooled system and an air-cooled one is that some factors are installation dependant. For instance if the chiller is located in the basement, and the cooling tower is on the roof you might have significant pumping energy to add to the plant efficiency.

One possibly useful resource is ASHRAE's 90.1 standard. In it it sets the minimum performance of water and air-cooled equipment. You'll notice that the minimum COP for water-cooled equipment is much higher than air cooled for the same tonnage; this is due to the additional energy costs related to water-cooled systems.

One final factor to consider is maintenance. While many don't have to use RO for water (I can't imagine the cost!) a cooling tower does require maintenance and chemicals to keep bacterial growth in check.

www.ashrae.org
abcmex (Mechanical) (OP)
23 Oct 06 20:57
Chris,
you got exactly my question. How to compare both systems. I think that the COP of the Air cooled units is the "total" COP because there are no more motors to feed. the COP of a water cooled chiller is only a "partial" COP because the pump and the ventilator motors are not included. That is why I am looking for a general comparison. Yeah, I know the installation configuration will affect the results.

PD. regarding you cooling tower in the roof: the pump dynamical height of such installation is not very high because the pump suction pressure equalize the pump static discharge sad


imok2,
COP (BTU/h)/(BTU/h) equals COP (kW/h) / (kW/h) because it is unitless; I suppose you agree. The only problem starts by mixing unit like kW/ton ref.
HVAC68 (Mechanical)
23 Oct 06 23:44
The comparison should NOT be restricted to only the COPs of both water-cooled and air-cooled chillers.  Even if you were to add the other power consuming items like condenser water pumps, cooling tower fans, it will still not be a complete comparison.  The best way forward would be to do a Life Cycle Cost analysis.  

Also, in some countries, water is expensive simply because of its scarcity or the quality of water could be bad, which will necessitate water treatment plants and the associated costs.  Pumping of water from an underground sump (usually) to an overhead tank for make-up for the cooling tower also is to be added to the water-cooled system life cycle cost.  Even the water losses on account of bleed-off and evaporation (usually close to 1% of the circulation water) can end up in significant costs depending on the location.

HVAC68

imok2 (Mechanical)
24 Oct 06 1:36
quark (Mechanical)
25 Oct 06 7:42
The main advantage of water cooled systems come from the lower discharge pressure of the compressor. The saturation temperature of water cooled liquid refrigerant is lower than the ambient DBT where as it is higher than ambient DBT for air cooled liquid refrigerant.

The COP considers the refrigerating effect and compressor power only. This doesn't include other power consumption. However, if you want to calculate specific power consumption of entire system, you should work out on the following details(for operating cost).

Air cooled chiller
Compressor power consumption
Condenser fan power consumption

Water cooled chiller
Compressor power consumption
Cooling water pump power consumption
Cooling tower fan power consumption
Cost of make up water
Cost of chemical treatment

mak687 (Mechanical)
3 Mar 07 7:59
mak687:how does a reverse return cycle work and what are its advantages comparing that to a direct return system
KiwiMace (Mechanical)
3 Mar 07 12:02
This is a scale dependent exercise.  

The COP of the air-cooled chiller is typically much worse than the water cooled.  In a small installation, the operating cost of the additional pumps, fans, water loss, installation cost, space usage etc may outweigh the difference in the chillers and make the air-cooled a better deal over the life of the plant.

Where the crossover point is will differ in every climate zone and with labour cost, electricity cost... the list goes on. But i'd roughly throw down 300-500RT to start an argument.
marcoh (Mechanical)
3 Mar 07 21:34
If you are comparing for a specific project, get some equipment selections for both air cooled and water cooled solutions.  A water cooled system will probably used half the electricity compared to an air cooled system as average loads (NPLV values)if not less.

As said before taking into account additional costs for water, chemicals, water cooled systems here are generally used for >1000kWr system.

Water cooled systems are more energy efficient because heat the rejection temperature (and resultant head pressure) is relative to ambient wet bulb, air cooled systems reject heat to the higher dry bulb temperature.

Have a look at the Trane/Carrier/York/McQuay websites.  They have a lot of information, technical guides, case studies etc and I am sure one will have this covered somewhere
HVAC68 (Mechanical)
4 Mar 07 12:45
Well, the answer is "it depends".  What is your application ?  24hours running ? If yes, the advantage one gets due to lower night time ambient dry bulb temperatures (for air-cooled) as compared to lower wet bulb temperatures (for water-cooled) is significant.

You should do a "SPLV" calculation for your specific project factoring in scheduling of various internal loads as well.  Carrier has a good software for the same.  Not sure whether Trane or others have.

HVAC68

quark (Mechanical)
4 Mar 07 23:55
mak687,

You should start a new thread for new topic.

freefallingbody (Electrical)
5 Mar 07 6:00
A comparison of two types of systems are given below. These are DX units. Not chilled water systems.

Type                         DX chiller     DX chiller    
                     (air cooled cond.)(water cooled Cond.)    
Capacity, TR                         100    100    
Saturated Suction temp, C            6.1    6.1    
Saturated Discharge temp, C         52.7    37.8
Compressor power, kW  (a)          104.5    62.0    
Chilled water pump, kW(b)            0.0    0.0    
Condenser cooling fan/pump(c)        7.8    13.5    
Total power, kW   (a) + (b) +(c)   112.3    75.5    
Total Specific power, kW/TR        1.12    0.76

Regards

Dinesh

Sorry for the misalignment of columns.


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