Originally Posted by
suemarkp
You're assuming there is a 1:1 conversion between cooling tons and applied energy. If an AC worked like a resistance heater, that would be a good assumption. But they don't work that way.
The EER is a factor that tells you how to divide the BTU usage to get the actual consumed power (but look out, the 3.412 BTU conversion factor is in there too). EERs vary by temperature and the design of the equipment (newer units being more efficient than old ones, mostly because they have larger coils). The KW value above needs to be divided by a factor between 2 and 4. Power use is roughly TONS * 12000 / EER. Note that this method is not a good way to derive amps or MCA -- need the nameplate or datasheet for that.
EER is a measurement of the thermal efficiency (not electrical efficiency, so don't mixed it up with the electrical efficiency formula), while tons is a measurement of size. One ton of air conditioning is equivalent to 12,000 BTU. A BTU is a British Thermal Unit, and it is simply a measurement of heat. One BTU is roughly equivalent to the amount of heat generated by burning one wooden kitchen match. Simply put, this means that a one-ton air conditioner can remove 12,000 BTU from a space per hour.
Also, the EER rating simply refers to how much it will cost to operate the unit when it is running, and varies with every model or manufacturer. The size (tons) of the system is the determining factor on how well the A/C will cool your room.
The Energy Efficiency Ratio (EER for air conditioners, SEER for central air conditioning systems)tells the user how much energy is consumed compared to the heat energy removed from the space. See the formula.
BTU heat removed from a space
EER= --------------------------------
Watts input to the equipment
Back to the OP, he wants to know the correct power needs for a 5 ton A/C unit. IMO, 18kW is correct for a 5 ton ACU.
Rules of thumbs are based on experiences; say 2kVA per ton (10kVA on your case) can accommodate your 5 tons ACU if this 5-ton size is a design bigger than the actual cooling requirement of the installation. The temp settings will correct the cycle time of the refrigerant pump and the cooling mode selector will be selectable to a lower refrigeration pump/condenser fan speed; all leading to a smaller demand factor.
Hope this clears all doubts.
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