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Abe
23-03-2003, 10:44 AM
If i have worked out say a total heat load for a space to be 4kw for -20 c environment.

Do I select a coil and cu that is rated at 4kw evaporating pressure -20c from their printed charts.

I ask this because I was taught to always add another 10 degrees, ie: If I am evaporating at -20C then I read off -30C from the chart

All this stuff is mighty confusing. I need an education how to correctly select equipement

herefishy
25-03-2003, 01:55 PM
Hi Abe,

You need to determine what the highest ambient temperature the condenser will see. In texas it's about 110degf (for rating purposes). You would want to select the condenser according to it's capacity at that ambient, so that there is enough capacity - when you need it most. :) (rule of thumb, is you lose 6% capacity for every 10degF increase in condensing temperature, which would apply if you can only refer to the C.U. capacity at say 90degF. But if you have a table that gives other ratings at different ambients.... much the better.

The unit cooler (evaporator) doesn't care. select it according to your load, or the minimum capacity of the C.U., according to the desired TD


C.U. capacity @ design S.S.T. = TD
unit cooler capacity @ 1degF td

Prof Sporlan
25-03-2003, 04:53 PM
Do I select a coil and cu that is rated at 4kw evaporating pressure -20c from their printed charts.

I ask this because I was taught to always add another 10 degrees, ie: If I am evaporating at -20C then I read off -30C from the chart

You do not want to confuse design evaporating temperature with desired box temperature. Desired box temperature will be your design evaporator temperature plus the TD of the evaporator coil. Rating charts will note the coil TD, and for medium temp applications, sizing the evaporator at a 15F TD is fairly common. For low temp applications, a 10F TD, is more coomon.

The smaller the TD, the more efficient the condensing unit will run. But obtaining a smaller TD requires a larger evaporator coil for the same rating. For example, an evaporator having a 4 kW rating at a 20F TD will only have about a 2 kW rating at a 10F TD.

herefishy
25-03-2003, 09:01 PM
What is the application Abe?

If you want a -20C room, use the C.U. capacity for -30C evaporating temperature. 10degF TD is the most common application.

If you have a condenser rated 1,000 btuh, and service an evaporator rated 1,000 btuh @ 10degF TD (100btuh per degF TD), your TD will be 10. At a 35degF box, your SST will be 25degF. the C.U. selection is made at an evaporating temperature of 25degF.

If you applied a C.U. rated at 1,500 btuh to service the same evap. (100btuh/degF), your TD would be 15degF. for a 35degF box, the evaporating temperature would be 20degF. The C.U. capacity would need to be referenced at the 20degF rating.

Determine your desired TD:

7degF - 9degF (approx humidity 90%) least amount of dehydration, vegetable rooms, flowers, chill rooms, unpackaged ice (more sensible load, less latent)

10degF - 12degF (approx humidity 80%-85%) General storage, packaged foods.

12degF - 16degF (approx humidity 65%-80%) beer, wine, potatoes onions, pharmaceuticals

17degF - 22degF (approx humidity 50%-65%) prep rooms, beer warehouses, candy and film storage, loading docks.

...choose one. :)

herefishy
03-04-2003, 05:45 PM
Originally posted by herefishy
What is the application Abe?

If you want a -20C room, use the C.U. capacity for -30C evaporating temperature. 10degF TD is the most common application.


...Wait a minute... I'm thinking in terms of Fahrenheit. Would you convert the degF TD (10degF) to a Cenitgrade TD?....

Abe
03-04-2003, 06:16 PM
Sorry Mark for not replying
Ive just been a little busy
But Ill come back soon with details

Prof Sporlan
03-04-2003, 11:52 PM
Would you convert the degF TD (10degF) to a Cenitgrade TD?....

Actually, the correct metric unit of temperature is degrees Celsius (or Kelvin if you prefer absolute temperature). The Centigrade scale was dropped in favor of the Celsius scale in 1948. Many think the Celsius and Centigrade scales are the same. Amusingly, they are not.

On the Celsius scale, the boiling point of water at standard atmospheric pressure is 99.975 degrees, whereas it is 100 degrees defined by the Centigrade scale.

A temperature difference of 10F converts to: 10 / 1.8 = 5.555555555C. As a result, the Prof finds it a bit more difficult to size low temp evaporator coils at a 10F TD using metric units..... :D :D :D :D

herefishy
04-04-2003, 03:20 PM
DOH!!!

Difficult.... yes. However one could perhaps simplify the "translation" ... for instance

4 - 5degC TD (approx humidity 90%) least amount of dehydration, vegetable rooms, flowers, chill rooms, unpackaged ice (more sensible load, less latent)


5 - 6degC TD (approx humidity 80%-85%) General storage, packaged foods.

7 - 9degC TD (approx humidity 65%-80%) beer, wine, potatoes onions, pharmaceuticals

10 - 12degC (approx humidity 50%-65%) prep rooms, beer warehouses, candy and film storage, loading docks.


:)