thermodynamics

[POLAND_SPUD]

I am wondering how much heat one can get from compressing air...
The way I understand it all - air at, let say 20C, has some heat in it and compressing it 'squeezes' that heat...
so for example 10 L of air at 20C turns into 1L of air at 10 bar and X temperature... if there is enough time to dissipate that heat

Now let's imagine that a 1kW compressor is mounted inside a building but the inlet air is taken from the outside... the tank is mounted inside and has large enough surface that the hot air can freely dissipate heat to the air inside the building...

how much heat enters the building ? is it equal to 1kW (power of the motor) or is it higher than that?

I was reading about heat pumps and I am wondering whether something similar to the COP applies in this case?

[jimmy101]

How much heat from compressed air? Not much. How high a temperature? Very high. Since heat is the temperature times the heat capacity times the mass, and since air has very little mass, you can get high temperature air but it contains relatively little heat (or energy if you prefer).

According to GasEq, compressing air 14:1 (roughly what a diesel engine does) heats air from 70F to 650F.

As to your 1KW generator question, it depends on the temperature inside and outside the building. If the temperatures are the same then 1KW (minus any losses in the motor, compressor etc.) of heat is pumped into the building.

Heat pumps operate a bit differently. First of all, they don't pump air. By clever use of the heat of vaporization of the coolant they can pump heat against a temperature gradient. The heat pump creates little heat of it's own, instead its energy is simply used to move heat from one place to another.

[Technician1002]

This heating process is reversible as the pressure is later reduced as the pressure is dropped back down. To get an idea of the heat gain, simply empty a full compressor in the building and note the temperature drop. It is about the same as the resulting temperature increase. The energy of compression is also released, so work is also performed and heat generated by work must be subtracted. For example a die grinder in use will generate heat on the item being worked on, but the exhaust air from the die grinder will be cold.

[POLAND_SPUD]

As to your 1KW generator question, it depends on the temperature inside and outside the building. If the temperatures are the same then 1KW (minus any losses in the motor, compressor etc.) of heat is pumped into the building.

That's what I found in other sources but I don't know whether this fact

This heating process is reversible as the pressure is later reduced as the pressure is dropped back down

is taken into account in your response?


what if the compressed air is allowed to exhaust outside of the building (so the cooling effect of expanding does not take place inside the building but outside )?

wouldn't that work as a sort of a heat pump ?
If the temperature of air increases then the heat of air can be pumped from ambient air against heat gradient into the room... Isn't it that you can get somethign for 'free' here ?

[jimmy101]

For the simple example of an exterior compressor compressing air into a reservoir inside a building then the "1KW consumed = 1KW heat" is true, minus various unavoidable losses.

"Isn't it that you can get something for 'free' here ?"

I know you know that is not true. A heat pump just moves heat from one place to another, and that costs money. I have a huge pile of old PVC you can have for free. You just have to pay the shipping.

In heat pumps, you spend your money on the "shipping". Heat pumps are often more cost efficient than conventional heaters, though they typically have their own unique set of problems. They don't transfer heat as fast as a conventional system can create heat. They generally have a maximum temperature gradient against which they can work (which is why many home heat pumps have backup resistive heating elements). Because they are slow to move heat they don't work as well with temperature set back thermostats. etc

[Technician1002]

For the simple example of an exterior compressor compressing air into a reservoir inside a building then the "1KW consumed = 1KW heat" is true, minus various unavoidable losses.

"Isn't it that you can get something for 'free' here ?"

Using compressed air, as the heat is taken from the compressed air, the pressure drops. Now the compressed air when used for work (IE impact wrench removing lug nuts) the energy delivered is LESS than what it took to compress the air. As air is used in the tool, the pressure drop in the tool causes additional cooling, resulting in even more pressure drop, and even less energy delivered to the job. Only by constantly adding back the heat to the air as it is used can you retain the compressed air energy.

I know you know that is not true. A heat pump just moves heat from one place to another, and that costs money.

A heat pump gets it's efficiency by pumping more heat from outside to inside using the same energy. When the outside is slightly cool, lots of heat can be pumped inside using little energy.

In heat pumps, you spend your money on the "shipping". Heat pumps are often more cost efficient than conventional heaters, though they typically have their own unique set of problems. They don't transfer heat as fast as a conventional system can create heat.

Actually a heat pump can transfer LOTS of heat. Think of pumping water uphill. The greater the height the water has to be lifted the greater the energy required to lift the same amount of water. A heat pump is the same way. A heat pump can heat a large volume of air with lots of heat, or a small amount of air to higher temperature with the same energy. If you want HOT from your heater, you are best to simply use resistive heat. If you want a large area warmed up a few degrees from the outside temperature a heat pump can be very efficient. The colder it is outside the more work is required to pump outside heat inside. There is point where it takes more energy to pump heat from outside to inside than the energy to simply heat the inside and not pump heat in from outside. This is why heat pumps have supplemental electric or gas heat.

Heating from 50 to 70 uses little energy. Heating from 15 to 70 uses lots of energy to pump in the heat. you get more heat for the same energy with the supplemental heat. (I'm in the US, use metric if elsewhere)

[POLAND_SPUD]

I know how heat pumps work and I am familiar with the concept... actually my grandfather uses a heat pump for heating

But I am wondering whether the compressor works (to some extent) as a heat pump or not ?

Well the way I see it it moves air that already has some heat into the building... compressing air also 'compresses' heat of it into smaller volume so the resulting temperature of 1L of air @ 10 bar is higher than the temperature of input air...
as the temperature is higher heat can be transferred from the tank to the building...

so is the increase of temperature equal to the work done or is it higher than that, as the compressor is merely moving air that already has some heat ?

[Technician1002]

Compressing air does work as a heat pump. The capacity is very small. A commercial refrigeration system (heat pump) relies on most of the heat transfer through phase change of the gas to a liquid and back. Compression of the gas does heat it, but in BTU's this is very small compared to the BTU's transfered changing state from liquid to gas and gas to liquid.