why do we cool cold air?

[MKimagin]

hat might be trivial question.

I wonder why in air conditioning system, in most cases, it cool the colder air from inside the room, even thought the air conditioner (the heat pump) have the highest efficiency if the air feed ed in to it has as high as possible temperature, or the differential between the cold side and the hot side of heat pump is as close as possible.

Why do we not pull the hot air from out side, cool down and then push in to the room? It look like logical looking on the heat pump characteristic.

I try to analyze my room windows air conditioner characteristic, and I get to conclusion that the highest efficiency it achieve when you just start the unit, when the room is really hot. Once the room cool down the efficiency of that air conditioner significantly drop down.

 

[IRstuff]

You are somehow confused. Efficiency is simply the ratio of an output power to an input power.

90% efficiency with a 50º delta is worse power consumption than 75% efficiency with a 20º delta.

TTFN

FAQ731-376

 

[MKimagin]

IRstuff

Efficiency of heat pump are measure in terms of ration of power use to "shovel" heat power from one place to another (usually from cold to worm space).
It little difficult express that in percentage because it will have to be 200% efficient for refrigerator and 350% and more for most air conditioners, which have no sense, heat pump is not over unity device if I am correct. So better stick with absolute ration.

Now the question is: do you "shovel" the heat down heel or up heel, if up heel what the slop grade is, steep or not. Your logic is correct between input of cold side and output of cold side, however it look different if you look the problem between input of cold side and output of worm side.
The "heel" have less gradient between that two sides of heat pump when temperatures are closer to each other.

 

[lilliput1]

The heat pump is just a reversible air conditioning unit. They don't "cool" or "heat" by themselves. They just transfer heat. In the air conditioning mode the heat indoors is absorbebed by the evaporator (refrigerant boiling) and transfered to the outdoors by the condenser (refrigerant condensing.) In the heat pump mode, a reversing valves makes the indoor unit be the condenser and the outdoor unit the evaporator. Heat from outside is transfered indoor but at some point it needs supplemental electric heating. Heat pumps may be efficient but their service life is only 15 years. They are high maintenance if used in a big building. There will be lots of small units requiring maintenace and usually they are located where they are hard to service. They also could get very noisy particularly when operating at low loads.

 

[IRstuff]

There is no such thing as "absolute ratio" The bottom line is that you are either removing heat or adding heat. In either case, the question is how much absolute heat you are moving. If you are trying to cool a room, the starting with cool air means less heat to remove, therefore, while the system might be more "efficient" with hot air, the total amount of heat to be removed will be so much larger that the total amount of heat moved and therefore, the amount of power consumed will be higher with the higher temperature inlet air.

The opposite is true of heating. You want to start with the warmest air possible to minimize the amount of power consumed. The fact that you have a lower efficiency is less relevant than the fact that you'll consume less power overall.

Qconsumed = ht*deltaT/eff

Start with a lower deltaT, you consume less energy.

TTFN

FAQ731-376

 

[krb]

I think you are still confused, but it sounds as if you are asking about coefficient of performance. This document addresses the basics of the cycle:
coef

http://www.ecr.unimelb.edu.au/~mbrear/436-352/chapter 3 - refrigeration.pdfficient.

The higher the delta t, the more energy required for cooling.

 

[lilliput1]

Heat exchanger wize it may be more efficient, but system wize it is not. If you bring in additional heat to improve heat exchanger efficiency, you system actually comes up defficient because now it has all that added heat to get rid off. Instead HVAC of large units use enthalphy economizer and outdoor air economizer controls. Enthalphy economizer is used when outdoor air temperature at or above the design cooling coil discharge temperature, usually 55 deg F. The enhalphy of outdoor air is compared with that if the return. If it is less, the return air is relieved out and the system takes in 100% outdoor air. A return/relief fan is required. When outdoor air is below 55 deg F, the compressor does not have to run. Instead the OA, return air and relief air dampers are simultaneously adjusted to come up with a mixed air (RA+OA) temperature equal to 55 deg F.