I am trying to size a split system air conditioning unit to cool a telecoms equipment room built in an un-insulated attic space. The equipment room is to be kept at a temp. of 21 degrees Celsius. My heat load Q = heat transfer thru walls + heat transfer thru ceiling + heat transfer thru floor + heat transfer thru leakage + heat transfer from radio equipment = 5.9kw. my question is do I spec a 5.9 kw rated unit or do I have to allow for a safety margin and a correction factor for the evaporator?
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Telecoms Equipment room Cooling
- Thread starter PhoenixP
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I should add heat from people who are working there and I think it is a good idea to do calculations with peak temperatures (during summer and winter). Yes, you should add a safety margin to final result and also a correction factor for the evaporator.
Not knowing the geographic location this unit is to be installed in, keep in mind that your completely uninsulated space more than likely does not have a vapor barrier. With the possibility of large temp and vapor pressure differentials, the possibility of moisture problems are real.
PhoenixP
In addition to the actual heat loads that you have described. You MUST also add heat of compression (watts) of your chosen compressor, and you must also size the condensor for the condensing "temperature" in you're area. If the area your in is hot in the summer months, you can be sure you'll be buying a larger condensor to be able to maintain the required capacity during that hot period. Then again you might also want to add extra capacity in case they want to upgrade in the next 2 or 3 years. Thinking Forward so to say. Keep good records of what you deside.
Best regards
pennpoint
In addition to the actual heat loads that you have described. You MUST also add heat of compression (watts) of your chosen compressor, and you must also size the condensor for the condensing "temperature" in you're area. If the area your in is hot in the summer months, you can be sure you'll be buying a larger condensor to be able to maintain the required capacity during that hot period. Then again you might also want to add extra capacity in case they want to upgrade in the next 2 or 3 years. Thinking Forward so to say. Keep good records of what you deside.
Best regards
pennpoint
Haneyrm and HVAC68 are exactly right. Large telephone equipment rooms are very little different than computer rooms: high sensible loads and a critical need to maintain 50% relative humidity. Unless a coil or A/C device is specifically designed for high-sensible cooling, though, its Sensible Heat Ratio will be about 65%. That means the A/C will only cool a sensible load to 65% of its rated capacity. The other 35% is latent cooling (moisture removal/dehumidifying).
For instance, if the unit is sized for total load - 5.9KW - the reality is that only 0.65 x 5.9KW = 3.8KW is available to cool your "radio equipment" load. If you size the unit according to sensible load, the total unit capacity will be something like 5.9KW/0.65 = 9.1KW. Typically, this capacity has to be balanced with available sizes, so you may end up with a 3 ton unit (10.6KW) before it's all said and done.
In typical applications, an oversized unit will result in runaway humidity, as the unit constantly short-cycles due to the load/capacity imbalance, and the resulting failure of the coil to ever dehumidify. This doesn't happen if you've sized the unit to match the sensible load. However, you are wasting a huge amount of energy and equipment expense.
If you size the unit just for total capacity - without regard to the sensible load - it will be undersized in sensible capacity. The result will be that the unit will almost always dehumidify, as it fails to ever adequately cool. The result will be a hot and super-dry environment: not ideal for electronic equipment.
That is why there is an entire A/C unit market devoted to computer room type cooling.
All that being said, though, your room seems quite small (5.9KW=1.68 tons), and all of these issues are tempered somewhat by that small size. Moreover, the additional loads and moisture leakage contribute to "tempering" the load's sensible heat ratio. Below 2-3 tons, I have had reasonable success with Mitsubishi ductless split systems in small telco/network closets.
Follow the advice by the other posters about accounting for the other loads - they are real.
For instance, if the unit is sized for total load - 5.9KW - the reality is that only 0.65 x 5.9KW = 3.8KW is available to cool your "radio equipment" load. If you size the unit according to sensible load, the total unit capacity will be something like 5.9KW/0.65 = 9.1KW. Typically, this capacity has to be balanced with available sizes, so you may end up with a 3 ton unit (10.6KW) before it's all said and done.
In typical applications, an oversized unit will result in runaway humidity, as the unit constantly short-cycles due to the load/capacity imbalance, and the resulting failure of the coil to ever dehumidify. This doesn't happen if you've sized the unit to match the sensible load. However, you are wasting a huge amount of energy and equipment expense.
If you size the unit just for total capacity - without regard to the sensible load - it will be undersized in sensible capacity. The result will be that the unit will almost always dehumidify, as it fails to ever adequately cool. The result will be a hot and super-dry environment: not ideal for electronic equipment.
That is why there is an entire A/C unit market devoted to computer room type cooling.
All that being said, though, your room seems quite small (5.9KW=1.68 tons), and all of these issues are tempered somewhat by that small size. Moreover, the additional loads and moisture leakage contribute to "tempering" the load's sensible heat ratio. Below 2-3 tons, I have had reasonable success with Mitsubishi ductless split systems in small telco/network closets.
Follow the advice by the other posters about accounting for the other loads - they are real.
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