March 4, 2010
In refrigeration and air conditioning, why does heat need be dissipated before going through expansion valve?
Raymond R asked:
the compressor heats up the cooled liquid (which has gotten warmer already by absorbing heat) into a hot pressurized gas. with the release of pressure by going through the expansion valve the gas is cooled abruptly from the change in pressure, so why do all diagrams for it show coils dissipating the heat before reaching the expansion valve? is it really necessary? won't it cool to the same temperature after the change from very high to very low pressure?
in response to the car AC analogy, how can heat be dissipated (lost to environment) while keeping the pressure the same? according to the ideal gas law: PV = nRT
pressure is inversely proportional to temperature, therefore, a decrease in temperature should net an increase in pressure….AH! there's the answer, you're absolutely right, by increasing pressure, it will allow an even larger pressure reduction at the expansion valve making it easier for the gas to be forced through. thank you so much for the help, it makes much more sense now. but now i have to ask, does my reasoning even make sense or did i miss something else?
Desert Tripper, just to get things straight. after the gas goes thru the Expansion Valve, it is turned to liquid, and as it absorbs heat it is slowly returning back to a gas as it reaches the compressor, which is the reason it is able to be compressed (cuz it's a gas now). so now it's under very high pressure as a hot gas, the heat is dissipated by going through the condenser coil, which brings the temperature down to switch it back to a liquid. it passes through the expansion valve where the pressure is greatly reduced forcing a higher boiling point, which forces all/most of the gas into a liquid so it can absorb heat again. but i think i'm still missing something, why does it need to be partially turned into liquid before hitting the expansion valve by going through the condenser coil when the real phase change occurs due to the change in pressure at the expansion valve?
HVAC Georgia
the compressor heats up the cooled liquid (which has gotten warmer already by absorbing heat) into a hot pressurized gas. with the release of pressure by going through the expansion valve the gas is cooled abruptly from the change in pressure, so why do all diagrams for it show coils dissipating the heat before reaching the expansion valve? is it really necessary? won't it cool to the same temperature after the change from very high to very low pressure?
in response to the car AC analogy, how can heat be dissipated (lost to environment) while keeping the pressure the same? according to the ideal gas law: PV = nRT
pressure is inversely proportional to temperature, therefore, a decrease in temperature should net an increase in pressure….AH! there's the answer, you're absolutely right, by increasing pressure, it will allow an even larger pressure reduction at the expansion valve making it easier for the gas to be forced through. thank you so much for the help, it makes much more sense now. but now i have to ask, does my reasoning even make sense or did i miss something else?
Desert Tripper, just to get things straight. after the gas goes thru the Expansion Valve, it is turned to liquid, and as it absorbs heat it is slowly returning back to a gas as it reaches the compressor, which is the reason it is able to be compressed (cuz it's a gas now). so now it's under very high pressure as a hot gas, the heat is dissipated by going through the condenser coil, which brings the temperature down to switch it back to a liquid. it passes through the expansion valve where the pressure is greatly reduced forcing a higher boiling point, which forces all/most of the gas into a liquid so it can absorb heat again. but i think i'm still missing something, why does it need to be partially turned into liquid before hitting the expansion valve by going through the condenser coil when the real phase change occurs due to the change in pressure at the expansion valve?
HVAC Georgia
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Comments on In refrigeration and air conditioning, why does heat need be dissipated before going through expansion valve?
10:10 pm
HVAC Georgia
Cold is the absence of heat, hence in order to decrease a temperature, one "removes heat", rather than "adding cold." In order to satisfy the Second Law of Thermodynamics, some form of work must be performed to accomplish this.
This work is traditionally done by mechanical work but can also be done by magnetism, laser or other means. However, all refrigeration uses the three basic methods of heat transfer: convection, conduction, or radiation.
10:00 am
HVAC Georgia
No the heat loss through dissipation while dramatic is limited.
You need to remove the waste heat in the evaporator(I think) outside then push it through the expansion valve.
Otherwise you would still get a temperature drop but the base temperature would increase with each cycle, so you need away of removing the excess heat to maintain a constant base.
9:34 am
HVAC Georgia
This might help….CAR AC…
How does it work? Where does the cold air come from? Modern refrigeration uses refrigerant to absorb heat and transfer heat and shed heat. Absorb, Transfer, Shed,- Absorb, Transfer Shed. Got it? It is a cycle of heat, or a heat cycle. The refrigerant absorbs heat from the interior of the vehicle in a component called the evaporator. Air is pushed (sometimes pulled) across the evaporator and since the evaporator is colder than the air coming across it, heat (from the air) is absorbed by the evaporator, and the resulting action is "cold" air being blown into the passenger compartment. However, this "cold" air is really just the same old air, but with the heat removed, or absorbed. Remember from science class-"heat moves from hot to cold". Now the refrigerant must move along to the next part of the A/C system, because the compressor is pulling it toward itself. Once inside the compressor, a tremendous amount of compression occurs, and the refrigerant travels at high pressure to the condenser. Remember all the heat we absorbed in the evaporator? The condenser is going to help us get rid of it. Since the compressor has increased our pressure, the result is an enormous amount of heat loss across the face of the condenser. As the refrigerant pushes on it's way out of the condenser, it has shed all the heat that was absorbed inside the vehicle. Still under high pressure, and with all it's heat removed, the refrigerant is now "hungry" for heat, so let's send it back inside to absorb more heat from inside the vehicle. This is accomplished simply by dropping the pressure with a small valve or fixed orifice. Once the pressure is reduced, the refrigerant enters the evaporator where it readily absorbs more heat. The heat cycle is complete. Of course there are more complexities involved with this heat cycle, such as the change of state that occurs in the refrigerant. It's bit larger than the scope of this article, as this is intended for the consumer, not the scientist.
7:04 pm
HVAC Georgia
Ojaibrad, I think the question IS dealing with state change. State change is the basis of modern refrigeration and has everything to do with why the heat has to be dissipated before going through the XV. Understanding state change is vital to grasping the idea of how refrigeration works.
Simply described, a refrigeration system has four devices: a compressor, a condensor coil, an expansion valve (or a capillary tube in small devices like home refrigerators), and an evaporator coil.
The refrigerant is a special, usually synthetic, substance with a very low boiling point (often way below zero degrees F at atmospheric pressure.)
Obviously, for a compressor to work, the refrigerant must be a gas when it reaches it – a liquid cannot be compressed. Yet, to perform its work, the refrigerant must become a liquid. How does it do this? By dissipating the latent heat stored in it.
Let's start at the expansion valve, where the refrigerant is a high-pressure liquid. Moving through the valve, it enters the low-pressure area of the evaporator (which is also the suction for the compressor.) Lowering the pressure lowers the boiling point, so the liquid refrigerant boils to a gas as it flows through the coil, absorbing heat from the coils, and the area to be cooled, in the process. The heat is now "stored" in the refrigerant as something called "latent heat of vaporization."
That heat has to go somewhere! The only way to release the heat is to change the gas's pressure. The compressor takes in the gas and pressurizes it, raising its boiling point to a very high temperature. It is sent to the condenser coil, where it is condensed back to a liquid, releasing the latent heat that it absorbed from the evaporator coil.The heat is dissipated to the outside. In effect, the refrigerant has "moved" the heat from one place to another!
So, in brief, the heat must be dissipated prior to going through the expansion valve because the gaseous refrigerant has to change state back to a liquid in order to absorb more heat.