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    Pressure changes and temperature through the cycle



    I pretty much want to make sure that all my assumptions about what occurs through the basic refrigeration cycle are fine based on thermodynamic principles.

    Q1: A saturated temperature is where our an equal amount of evaporation and condensation can occur. If latent heat is applied to a saturated liquid however, more and more is turned to a vapor. Based on this fact, how can there be an equality when more and more of the liquid turns to vapor and less as a liquid?

    Q2: A high pressure/high temperature subcooled liquid at 110F/278 psig enters the expansion valve. When it exits, it is 40F/69 psig. Now, the boiling point for the refrigerant at 69 psig is much lower than 110F. Thus, boiling will occur since 110F is much higher than the boiling point of 40F. However, what I am confused about is two things.

    Q2a: How do we know how much of the subcooled refrigerant boils and turns into vapor? In other words, with evaporation/boiling, when does this process stop?

    I understand the opposite more clearly in that if we match the vapor pressure with the external pressure, boiling will occur but when thinking about dropping the pressure and the states, I have less of an idea.

    To me, if a temperature is really high and boiling occurs due to the external pressure dropping, we would have boiling occur and energy would be lost in the liquid (making it colder) but the pressure might be higher (40F is reached but the corresponding pressure is not 69 psig but rather maybe 100 psig). My reasoning is that there is so much vapor already boiled that no more can be produced within the volume and we reach some sort of equilibrium. In other words, we reach an equilibrium due to a closed container but not the same 40F/69 psig temperature/pressure relationship.


    Q3: The high pressure on the high pressure side is due to the compressor's result and the low pressure is a result of the valve correct? It's not like these two parts are set before hand to contain a particular amount of pressure right?



  2. #2
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    Re: Pressure changes and temperature through the cycle

    Consider your thoughts on Q3 first in simple terms.

    The compressor provides the low pressure also by means of it's intake stroke therefore the evaporator is never filled with an equalised pressure.

    The high pressure side pushes the liquid and the low pressure side pulls the vapour.

    Yes, the expansion device, valve, does lower the pressure but without the compressor maintaining that low pressure it would soon equalise.

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  3. #3
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    Re: Pressure changes and temperature through the cycle

    Quote Originally Posted by Brian_UK View Post
    Consider your thoughts on Q3 first in simple terms.

    The compressor provides the low pressure also by means of it's intake stroke therefore the evaporator is never filled with an equalised pressure.

    The high pressure side pushes the liquid and the low pressure side pulls the vapour.

    Yes, the expansion device, valve, does lower the pressure but without the compressor maintaining that low pressure it would soon equalise.

    College question??
    Nope, heh, in fact I completed my masters in CS a couple of years ago but the last time I touched thermodynamics was during my BS degree. So, after reading 70 or so pages of material related to HVAC I have a couple questions. For Q3 that makes sense though, what about the others?

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    Re: Pressure changes and temperature through the cycle

    Hi ZaxHex
    The direction for answers to question 1 & 2 can be seen in a pressure enthalpy chart for the refrigerant diagram will be helpful. It is a dynamic process and when it goes through the metering device there is an amount of energy from the refrigerant required to cool its self to the change in pressure. Briefly flash gas occurs with the pressure dropping below the normal P/T relationship. This process exaggerated would look like cavitation on a graph. The pressure in an actual process going through the metering device drops below like a P Trap on the graph then it rises slightly and continues on a downward slope to the compressor.

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    Re: Pressure changes and temperature through the cycle

    Quote Originally Posted by Tesla View Post
    Hi ZaxHex
    The direction for answers to question 1 & 2 can be seen in a pressure enthalpy chart for the refrigerant diagram will be helpful. It is a dynamic process and when it goes through the metering device there is an amount of energy from the refrigerant required to cool its self to the change in pressure. Briefly flash gas occurs with the pressure dropping below the normal P/T relationship.
    I understand the energy loss bit resulting in cooling but not the pressure drop. Usually when something is evaporating we have more molecules/atoms in the air and have an increase in the VP. Thus, why does the opposite occur here? Forgive me if this is an ignorant question heh, still piecing it all together from what I learned thus far.

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    Re: Pressure changes and temperature through the cycle

    Ahh interesting point - there is not enough heat in the short time and distance the refrigerant can suck up to correspond with its new pressure. This is where the pressure drops further to maintain an energy equilibrium and flash gas occurs. It might be easier to check out cavitation where there is a high pressure drop through a small orifice. It all appears quite magical when thinking about it.

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