Efficiency of Heating Air vs Cooling

[gruder]

If i have this correct, it takes 1BTU of heat to raise the temp of 1lb of water by 1 deg.
Is it the same for cooling, is the efficiency still the same, 1BTU to lower the Temp by 1 deg?

I am specifically looking for the equation for how many BTU's of heat required to heat 1 cubic meter of Air by 5deg.

Also then how to work the equation correctly for lowering the temp--cooling.

Thanks

 

[speco]

Gruder,

Your definition of a BTU is essentially correct. Water at around 60 F has a specific heat of 1.0, but it's a bit higher at higher temperatures. However the specific heat of air is around .24 (.242 is probably more accurate) at temperatures below about 150 F.

To answer your question, you need to know the density of the air, which really depends on both the pressure and the temperature. Once you know the density, you can calculate the mass of air (in pounds mass) in a cubic meter. The amount of heat added or removed for a 5 degree F range is the same, as long as there is no water condensing in the cooling range. If your degrees are in Celcius, then the answer is 1.8 times as large.

Regards,

Speco

 

[25362]

Although the heats gained and lost are the same, the COP values may differ. See, for example,

http://www.newton.dep.anl.gov/askasci/eng99/eng99577.htm

 

[dcasto]

for pure air, the energy to heat 1 pound 5 degrees is the same as cooling 1 pound of air. In HVAC, the issue is the water content. When heating air, the water in the air is a vapour and it requires sensible heat to warm, just like air. Typically, AC systems must content with lot's of moisture in the air. The heat of condensing water is a couple of thousand times as great as just changing the temperature of water vapour 1 degree.

 

[IRstuff]

Your question posed appears to not be about efficiency, but about specific heat. For a given state of air and water vapor, the specific heat is relatively constant over a narrow range.

The thermomechanical process by which you heat or cool the air may have drastically different efficiencies, since they are often accomplished with different technologies. In the case of a residential home, the heating is done with burning natural gas, while cooling is done with compression and expansion of a refrigerant.

TTFN

FAQ731-376

 

[chicopee]

I think that gruder is looking to size an automobile air conditioner with his question.

 

[IRstuff]

In a similar vein, heating the passenger compartment of a car uses waste heat from the engine, while cooling requires loading of the engine with the compressor.

TTFN

FAQ731-376