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Hello everybody,
A problem confused me long time: the inlet and outlet temperature of the chilled water for air condition is normally 55F and 45F, but the evaporating temperature is usually not a constant, the range is from 35F to 42F. so what's is the evaporating temperature to rating the standard cooling capacity , different manufacturer means different evaporating temperature?
Waiting for positive reply.
Hi trace,
We learn in school and most books as if life happens as a picture. In reality life is more like a movie
In real life the evaporating temperature is normaly always changing and the nominal values of specifications are, at the best, mean (or medium) values.
In different equipments the difference between the water temperatures and the evaporating temperature can be different, but normaly not much. It is established by the equilibrium between the price of the compressor and the price of the evaporator as when one reduces the other increases. Well, there is also the energy consuption, when that difference increases the electrical consumption also increases, so to be compared in terms of EER the different manufacturers use values quite close to each other.
:)
Hi,cduque,
Thanks for your reply. What a good metaphor :)
I have ever read ARI Standard 480 utilizing R–22, which gave the condition of 54° F water in, 44° F water out and 35° F refrigerant temperature. but I also learn from other chillers which gave the evaporating temperature of 40 or 42° F. actually, the cooling capacity difference is large at different evaporating temperature to a certain chiller, because of the fix evaporator heat exchange area. so the cooling capacity that the manufacturers give must at a certain evaporating temperature.
Now I think I get you, different manufacturer means different evaporating temperature, is it true?
The temperature of the refrigerant in the chiller is always lower than the temperature of the circulating coolant (ie: the chilled water). The coolant will never get below the refrigerant temperature. How close the coolant temperature gets to the refrigerant temperature is governed by the engineering of the chiller. If the evaporator is really large and expensive, the coolant temperature can get close to the evaporating refrigerant temperature in the chiller. If the chiller is built to a poverty pack budget, and there is not much surface area for good contact between the refrigerant and the chilled water at the pipe interface in the evaporator, the approach temperature (as it is called - the difference between the evaporating refrigerant temperature and the chilled water temperature) can be substantial.
In a very efficient chiller with plenty of evaporating surface area, the approach temperature can be as low as 2°C. (ie: Evaporating temperature is 4°C and the water temperature is 6°C - a difference or approach temperature of 2°C). In a cheap machine, with much less copper in the evaporator, it can be as high as 10°C ( Evap temp of 2°C and chilled water temperature of 12°C).
So yes, depending on the manufacturer, and the balance they've drawn between machine cost and efficiency, the approach temperatures will be different.
Hi trace,
generally apply the ten ten rule for majority of water chillers.
10 ' F change across water chiller evaporator, and 10'F difference between leaving water temp and compressor SST at 100 % load capacity, anti freeze stat in leaving chilled water set at minimum 34 ' F dependent on design water flow rates. Basically the same same for water cooled condensers. Some have a dedicated sub cooling bundle to add additional using coolest water at entry point from cooling tower.
Hi, Scramjetman.
Thanks for your detailed and definite reply. the relationship between temperature approach and the area is so brief and clearly to solve the problem.
Hi,Megoo. I don't understand the meaning of "10'F difference between leaving water temp and compressor SST at 100 % load capacity," do you mean if the leaving water temp is 45 'F, then the SST is 35 'F ? if so, there will be no or not sufficient evaporator superheat.Quote:
Originally Posted by Magoo
.
Are you only ever working with R22?
Other refrigerants are blends and therefore they have "glide".
R407c for example has a glide of about 7K which is excelent
for water chillers, but makes it complicated when callculating
one evaporating temperature.
Rob
.
Hi, Rob.
thanks for your remind. I only consider about azeotropic blends. for non-azeotropic blends, the evaporating temperature glides, it's true that the calculating is complicated and hard to definition the evaporating temperature.