Air Conditioning Efficiency (Coefficient of Performance) Questions
June 21, 2014 7:37 PM Subscribe
I have come to understand that a typical modern window or residential-central air conditioner (which I believe is referred to in the industry as a "Direct Expansion" or DX unit) has a coefficient of performance which typically ranges between 3.0 and 3.5. That is to say, for every unit of energy consumed, the air conditioner removes between 3 and 3.5 units of heat from the conditioned space. I have a few questions regarding this, inside.
posted by Juffo-Wup to Science & Nature (3 answers total) 3 users marked this as a favorite
1) For a standard window/central residential type unit, what (if any) factors determine how much power the compressor draws (assuming steady-state operation; I don't mean "if it's cooler out, less heat will enter the conditioned space, so the thermostat will call for cooling less frequently")? For example, does the difference between the indoor and outdoor (evaporator and condenser) temperature make a difference somehow? Does the compressor motor draw more power if its input pressure is higher / lower / etc? I would be very interested to hear about this at as much length as you are willing to entertain.
1a) Is it true that the type of air conditioning system that is similar to a window or home central unit (in which there is a compressor, condenser, and evaporator, and the condenser and evaporator exchange heat, via air, directly with the conditioned and heat-rejection spaces) is equivalent to the term "Direct Expansion System"? Or, does Direct Expansion/DX have some other connotations that I don't know about?
2) If the CoP of a typical home system is 3.0 - 3.5, what kind of CoP do you see in commercial/industrial units, the kind that you see next to or on the roof of commercial or academic buildings, which often appear to be emitting some form of steam or water vapor?
2a) Is the CoP of those units significantly higher than 3.0-3.5?
2b) If not, why are those units used instead of "regular" residential type units?
2c) How do those types of units work, anyway? What are they doing -- using the enthalpy of vaporization of (for instance) tap water to their advantage? Or is there something else to it?
In case it matters, the climate I am thinking of here is that of southeast Michigan. The relative humidity here is high enough that a residential "swamp cooler" would be largely useless, but not so constantly high that there is never any advantage to be had from evaporating a liquid.
For what it's worth, I have an educational background in engineering, but no formal education concerning HVAC systems. So, please feel free to go into as much technical/physical/mathematical detail as needed to make a good explanation.
Thanks in advance!