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Pressure changes within a refrigeration cycle

  1. Oct 7, 2012 #1
    So I get that when the VP is equal to atmospheric pressure of 14.7 psia, if you will be at a saturated liquid and when more heat is added, it is latent head changing it to a gas. If however you are at a closed container where the can physically alter the "atmospheric pressure" in the container to 10 psia, the vapor pressure and the "atmospheric pressure" of 10 psia will be equal at a lower temperature and the liquid will boil.

    In a refrigeration cycle however, I am confused about where the "atmospheric pressure" comes into play. After a compressor compresses a superheated vapor gas, because the molecules move faster in a smaller volume, you get a higher pressure/temperature gas. This rule applies anywhere. What I don't get is when it turns into a liquid in the condensor. If you remove 'x' amount of heat from the superheated vapor once it leaves the compressor, you will reach a point where it will condense into a liquid.

    In the closed container above at 10 psia "atmospheric pressure", I would know at what temperature that once I reach and remove further heat, condensation will occur because the VP of the evaporated water and the 10 psia would match. In the condensor though, what is the "atmospheric pressure" here? Where does it comes from and how would I change it? The above can also be applied to the evaporator, low-pressure side.

    Is it the size of the tubing that controls the "atmospheric pressure" here in both the evaporator and condensor coils or something else?
     
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  3. Oct 8, 2012 #2

    Andrew Mason

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    The modern refrigeration cycle uses Joule-Thomson cooling operating between a cold reservoir (the cooled space) and a warm reservoir - the surroundings.

    When certain substances expand rapidly due to a sudden drop in pressure, the temperature drops. Refrigerants are selected based on how well they cool when expanded rapidly through a throttle valve.

    The cooled expanded refrigerant absorbs heat from the cooled space. It is then compressed back to its original volume which increases its temperature above that of the warm reservoir (when work is done on a gas by compressing it, the gas will get hotter). That heat is then removed by putting it in contact with the surroundings, cooling it to the temperature of the surroundings. The cycle is then repeated.

    AM
     
  4. Oct 8, 2012 #3
    Thanks, I got all that but am still fuzzy on what I asked. At the exit of the evaporator coil, a refrigerant must be all vapor. In other words, a saturated temperature must be reached so that all the liquid refrigerant can turn into a vapor at the end of the coil when latent heat is added. The only way to achieve this is to reach a saturated temperature where the vapor pressure, due to the temperature of the liquid, is equal to some external pressure just like water vapor pressure being equal to an atmospheric pressure of 14.7 psia where boiling will occur. The root of my question was, where does this external pressure come from and how is it modified. The latent heat comes from the inside of the room. It adds enough heat so that the liquid becomes completely vapor in the end. Thus, there must be some external pressure in equilibrium with the vapor pressure from the liquid as it heats up. Otherwise, an increase in heat would always be sensible heat and the liquid would just keep heating up and the vapor pressure would keep increasing.

    In terms of the compressor, all that's doing is just decreasing voluming and thus increasing the average KE along with the TXV. However, what I stated above must have some sort of external pressure coming from somewhere in order for the liquid to become vapor.
     
    Last edited: Oct 8, 2012
  5. Oct 8, 2012 #4

    russ_watters

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    The term "external pressure" doesn't make sense.

    The pressure in the two halves of the cucle are pre-selected based on the performance needs and achieved by using the right amount of refrigerant and a properly selected compressor and expansion valve.

    So your evaporator and condenser are at optimal pressure because you've designed the AC unit to achieve the right pressure.
     
  6. Oct 8, 2012 #5

    russ_watters

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    ....er, unless you are thinking the vapor pressure is only defined with respect to 14.7 psi? It isn't.
     
  7. Oct 8, 2012 #6
    No, that was used as an example of external pressure being the atmosphere at sea level and water getting to 212F because now its VP is equal to the external pressure of the atmosphere so boiling is occurring. The question had to do with the refrigerant in a closed container being able to boil only when its VP is equal to some other pressure. My question was, where does this other pressure come from?
     
  8. Oct 8, 2012 #7
    Ok, so then how is this optimal pressure met? The liquid in the evaporator boils into vapor. The hot air passing over the coil transfers heat into the liquid cause a phase change. Phase changes only occur once a saturation temperature is reached which means that the VP produced by the evaporation of the liquid at a certain temperature is at equilibirum with some other pressure and thus, an increase in thermal energy will cause the state change rather than more VP/temperature increase.
     
  9. Oct 9, 2012 #8
    The refrigerant gas exits the compressor at a certain pressure and temperature, and enters the tubes of the condenser. The pressure of the gas stays about the same, as it flows through the tubes, but its temperature drops due to the cooling air being blown across the outside of the tubing. Finally, at a certain point along the condenser tube, the temperature has dropped enough for the gas to start condensing out. The temperature at which this happens is the so-called dew point of the gas, and is the temperature at which the gas pressure is equal to the equilibrium vapor pressure at that temperature. Beyond that point in the tubing, the refrigerant pressure along the tubing does not change, and neither does the temperature, but the fraction of liquid refrigerant is increasing, and the fraction of refrigerant vapor is decreasing. Finally, at some point along the condenser tube, the cross section of the tube is filled with liquid refrigerant, and these is no vapor left. Beyond that point, the temperature of the liquid starts dropping again, as more cooling by the outside air occurs. The pressure is still not that much different from the entering pressure.
     
  10. Oct 9, 2012 #9

    russ_watters

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    The "other pressure" is the pressure of the refrigerant, just like for water in air. That's why "other pressure" is odd: a fluid only has one pressure at a time.
    Again, you said "other pressure". There is only one other pressure: the pressure of the refrigerant in the pipes.

    How does it work with a pot of water on a stove? The pressure of the water is atmospheric pressure, always. That's your "other pressure". It doesn't change. The only thing that changes is the vapor pressure, which increases as the temperature of the water increases. Once the temperature is high enough that the vapor pressure equals the pressure of the water, it starts to boil.

    But that doesn't really explain how the "optimal pressure is met". The optimal pressure is met by selecting a fluid, pump and expansion valve that when turned-on result in the pressure you calculated that you want.

    How do you get the pressure you want in a tank of air? Connect it to a compressor until the compressor pumps enough air into it until the pressure is the pressure you want. Same idea here. It's just odd that you're calling this "other pressure". Makes it sound like you think there is something more complicated going on than there is.
     
    Last edited: Oct 9, 2012
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