Pressure and AC/Heat pumps

Pressure and AC/Heat pumps

PostBy: Richard S. On: Thu Mar 17, 2016 10:16 am

I understand the basic concept, you compress the refrigerant which also "compresses" the heat energy. You run it though a coil and then decompress on the other side.

Just so it's clear I'm trying to get firmer grasp on the physics of it, for example if we have a normal air compressor and fill it with air it's going to be hot. If you let it sit for hours the air inside the tank will equalize with the ambient temperature. You're still left with the compressed air that is storing the mechanical energy. We can let the air out releasing the mechanical energy.

On an AC/Heat pump where does that mechanical energy stored in the refrigerant go? Is it vented somehow?

If it's not vented then all that mechanical energy is transformed to heat too?
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite


Re: Pressure and AC/Heat pumps

PostBy: McGiever On: Thu Mar 17, 2016 12:22 pm

I'm no refrigeration tech, But I did stay at a Holiday Inn Express last night...

A Heat Pump is a closed loop. And there is a "phase change" involved...liquid to gas, then back to a liquid.
At atmospheric pressure refrigerants are gases...when compressed they become a liquid.
There when motor/compressor runs there are 2 different pressure zones, a high and a low, and refrigerant volume differs from running to resting/equalized.
Air compressors deal in all gases and are not a closed loop...there is a refrigerant like effect/cycle as even air will release heat when compressed. Then there is a lot of motor and piston heat involved in the air compressor not all being removed away by the compressor.

Heat Pumps use super refrigerants that are very efficient at absorbing and releasing heat.

Nothing is wasted, even the heat generated by the motor current is moved to the high side to be useful heat or to be dumped depending on which cycle it is in.

HTH :)
Last edited by McGiever on Thu Mar 17, 2016 12:26 pm, edited 1 time in total.
McGiever
 
Stoker Coal Boiler: AXEMAN-ANDERSON 130 "1959"
Coal Size/Type: PEA / ANTHRACITE
Other Heating: Ground Source Heat Pump
Stove/Furnace Make: Hydro Heat /Mega Tek

Re: Pressure and AC/Heat pumps

PostBy: franco b On: Thu Mar 17, 2016 12:22 pm

Thinking in terms of air alone; if you compress 100 cubic feet of air at 70 degrees into an area of 10 cubic feet, the heat contained in the 100 cubic feet is now contained in 10. It is concentrated in the same way that a magnifying glass can concentrate sunlight into a small hot spot. The container of air gets much hotter. There is only potential mechanical energy, as in a compressed spring.

If you let the container of compressed air cool to room temperature and then release it, the reverse happens. Ten cubic feet of compressed air at 70 degrees suddenly wants to become 100 cubic feet, so has to absorb heat from somewhere, and the container become colder as the air expands. there might even be frost at the outlet.

Heat pumps and AC use a refrigerant which is just a liquid that evaporates at low temperatures and becomes a gas, but the principle is the same.
franco b
 
Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

Re: Pressure and AC/Heat pumps

PostBy: Richard S. On: Thu Mar 17, 2016 3:10 pm

franco b wrote:Thinking in terms of air alone; if you compress 100 cubic feet of air at 70 degrees into an area of 10 cubic feet, the heat contained in the 100 cubic feet is now contained in 10. It is concentrated in the same way that a magnifying glass can concentrate sunlight into a small hot spot. The container of air gets much hotter. There is only potential mechanical energy, as in a compressed spring.


I understand all that but when the heat in the air dissipates in the compressor you are still left with the potential mechanical energy. We can use it to drive an air wrench....

Where is that potential mechanical energy going if the heat pump is closed system? It has to go somewhere because the system would continually gain pressure. It has to be transformed to heat if it's closed system, yes?
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite

Re: Pressure and AC/Heat pumps

PostBy: franco b On: Thu Mar 17, 2016 3:48 pm

A heat pump or AC is closed, but it has two coils with a restriction between them, so that one side can build up pressure higher than the other. If the system is shut off the high pressure bleeds off through the restriction to equal pressure in both coils.
franco b
 
Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

Re: Pressure and AC/Heat pumps

PostBy: Richard S. On: Thu Mar 17, 2016 6:27 pm

I think I have my head wrapped around it. What I was overlooking was the loss of potential energy on the low side as the refrigerant is compressed into the high side. Unlike a compressor the mechanical energy is never stored to begin with and essentially wasted.

Correct?
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite

Re: Pressure and AC/Heat pumps

PostBy: coaledsweat On: Thu Mar 17, 2016 7:13 pm

That's it.
coaledsweat
 
Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

Re: Pressure and AC/Heat pumps

PostBy: grumpy On: Thu Mar 17, 2016 7:24 pm

I have a question, why is it bad to restart a compressor before waiting 3 minutes ? Is it because starting it up with the already built up pressure is hard on it ? So thats why you wait for it to bleed off..
grumpy
 


Re: Pressure and AC/Heat pumps

PostBy: McGiever On: Thu Mar 17, 2016 10:20 pm

grumpy wrote:I have a question, why is it bad to restart a compressor before waiting 3 minutes ? Is it because starting it up with the already built up pressure is hard on it ? So thats why you wait for it to bleed off..


Exactly.
Very hard on the electric motor and the associated wiring with near deadhead pressure. For real home owner efficiency to operate then the motor needs designed not be any larger than what can keep the cycle flowing after a equalized pressure start.
When running... the low side pressure is lower than when not running and with the equalized pressure...
while the high side pressure is higher than when not running and with the equalized pressure.

All this happens due to what *francoB* mentioned, there is a narrow orifice to hold back most of the liquid refrigerant and only meters out just enough liquid to instantly flash into a gas on the low side which quickly absorbs all the heat energy it can reach until becoming fully saturated. :)
McGiever
 
Stoker Coal Boiler: AXEMAN-ANDERSON 130 "1959"
Coal Size/Type: PEA / ANTHRACITE
Other Heating: Ground Source Heat Pump
Stove/Furnace Make: Hydro Heat /Mega Tek

Re: Pressure and AC/Heat pumps

PostBy: franco b On: Thu Mar 17, 2016 10:22 pm

grumpy wrote:I have a question, why is it bad to restart a compressor before waiting 3 minutes ? Is it because starting it up with the already built up pressure is hard on it ? So thats why you wait for it to bleed off..


Yes that is correct. In a car the restriction is a valve controlled by electricity which opens wider to equalize pressure quickly.
franco b
 
Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

Re: Pressure and AC/Heat pumps

PostBy: davidmcbeth3 On: Thu Mar 17, 2016 10:53 pm

https://en.wikipedia.org/wiki/James_Prescott_Joule

To understand fully I hope you have some diffyQ and calculus knowledge.
davidmcbeth3
 
Coal Size/Type: nut/pea/anthra
Stove/Furnace Make: Hitzer
Stove/Furnace Model: 503

Re: Pressure and AC/Heat pumps

PostBy: grumpy On: Fri Mar 18, 2016 2:10 pm

Thanks for the reply's..
grumpy
 

Re: Pressure and AC/Heat pumps

PostBy: Richard S. On: Fri Mar 18, 2016 2:49 pm

coaledsweat wrote:That's it.


If it's not stored as potential energy as you would with regular air compressor and not transformed to heat, where does it go?
Richard S.
 
Stoker Coal Boiler: Van Wert VA1200
Coal Size/Type: Buckwheat/Anthracite

Re: Pressure and AC/Heat pumps

PostBy: franco b On: Fri Mar 18, 2016 3:47 pm

Richard S. wrote:
coaledsweat wrote:That's it.


If it's not stored as potential energy as you would with regular air compressor and not transformed to heat, where does it go?


The cycle continues lesser and lesser until pressure equalizes.
franco b
 
Hand Fed Coal Stove: V ermont Castings 2310, Franco Belge 262
Baseburners & Antiques: Glenwood Modern Oak 114
Coal Size/Type: nut and pea

Re: Pressure and AC/Heat pumps

PostBy: McGiever On: Fri Mar 18, 2016 5:24 pm

The term "Heat Pump" may add to the confusion, maybe?

It is NOT the HP compressor that is doing the pumping of heat...but the refrigerant cycling from the high side into the low side and then compressor pumps it all over again back into the high side to repeat the cycle again and again.

Remember the fixed metering orifice. The pressure never reaches a point to cycle off. At best, when the compressor does stop the orifice then will bleed the pressure so it will equalize both sides once again.
Likewise it takes several minutes for the compressor to settle in to get the high and low sides adjusted before the cycle is on cruising control.;)
Weight of the refrigerant charge to be added to a new empty system or after a repaired leak loss is very precisely calculated/measured for every different system design. Too much or too little and the system will not run optimized and be inefficient.

;) We've been intentionally leaving out the role of the condenser and evaporator coils for simplicity of the discussion.
McGiever
 
Stoker Coal Boiler: AXEMAN-ANDERSON 130 "1959"
Coal Size/Type: PEA / ANTHRACITE
Other Heating: Ground Source Heat Pump
Stove/Furnace Make: Hydro Heat /Mega Tek