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  1. #14
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,350
    Or may be I am missing something.
    I don't think you are. I was focusing on the action of the TXV in response to conditions across the coil, which airflow obviously is a major condition.

    It strikes me as a combination of total heat load, airflow, and mass flow rate that ultimately dictate the outcome of evap superheat. Vary any one of these three factors and your superheat result will vary. Total heat load obviously varies throughout the day. Mass flow rate can vary by TXV adjustment, compressor pumping efficiency, cleanliness of condenser coils, cleanliness of liquid or suction line driers, length of lineset, restrictions, etc. Airflow is obvious...blower speed or return/supply restriction of whatever cause.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  2. #15
    Join Date
    Sep 2003
    Posts
    1,311
    I appreciate the reply and ask that you be patient with me a little more for calrification. Let's say that the suction temperature is 40 which corresponds to a 68 PSIG pressure in the sensing bulb. So on the TXV power head there is a 68 PSI downward prssure attempting to shut the refrigerant flow off. If the superheat is right at 0 then the suction pressure would also be 68 PSI. On an externally equalized TXV that would mean that there is 68 PSI pushing up on the power head thus offsetting the sensing bulb pressure. Now the only force to monitor the refrigerant flow is the spring which is also pushing the valve open from the bottom of the TXV. Thus the spring pressure will determine exactly how much pressure difference between the suction pressure and the suction temperature that the valve will modulate at. That pressure difference will remain the same at different suction pressures after the TXV ceases hunting. Since the pressure difference remains the same (speaking of the pressures acting internally on the TXV) the resulting temperature difference will remain relatively the same over the operating range. That temperature difference, of course, is the superheat.

    I see what you have said regarding the latent temp/evap pressure correspondence but don't see how that comes in to play since the internal pressure difference remains the same. It could be said that the spring inside the TXV will have a varying force according to its temperature but that force difference, I would presume, is very minimal. Mathematically the forces inside the TXV is Suction Pressure + Spring Pressure - Suction Saturation Pressure.

    Correct my logic if you have a different understanding of the TXV operation or if there is a factor I have overlooked in the TXV operation. Although your explanation gives food for thought (thanks) I still don't see how the TXV adjusts the superheat for different temperatures and/or pressures.

    [Edited by sadlier on 06-06-2005 at 12:19 PM]

  3. #16
    Join Date
    Jul 2004
    Posts
    322
    This always confuses me. Would a bulb that is poorly insulated or not tight against the vapor line result in higher or lower supply air temp?

  4. #17
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Originally posted by sadlier
    I appreciate the reply and ask that you be patient with me a little more for calrification. Let's say that the suction temperature is 40 which corresponds to a 68 PSIG pressure in the sensing bulb. So on the TXV power head there is a 68 PSI downward prssure attempting to shut the refrigerant flow off. If the superheat is right at 0 then the suction pressure would also be 68 PSI. On an externally equalized TXV that would mean that there is 68 PSI pushing up on the power head thus offsetting the sensing bulb pressure. Now the only force to monitor the refrigerant flow is the spring which is also pushing the valve open from the bottom of the TXV. Thus the spring pressure will determine exactly how much pressure difference between the suction pressure and the suction temperature that the valve will modulate at. That pressure difference will remain the same at different suction pressures after the TXV ceases hunting. Since the pressure difference remains the same (speaking of the pressures acting internally on the TXV) the resulting temperature difference will remain relatively the same over the operating range. That temperature difference, of course, is the superheat.
    Okay, I think I know where you might be having some difficulty. Consider this:

    P1 = P2 + P3

    Whereas:

    P1 = bulb pressure
    P2 = spring pressure
    P3 = evap (equalizer line) pressure

    Remember:

    P1 = opening pressure
    P2 = closing pressure
    P3 = closing pressure

    In conclusion:

    The two closing forces of the spring and evap pressure must equal the opening pressure of the thermostatic bulb. This is how the system maintains equilibrium under variant conditions.

    Application:

    Let's take an evaporator where the saturated vapor temperature is 40 degrees F. This equates to 68 psig. Keep in mind this 40 degrees in my example is saturated vapor temperature, or SVT, not evaporator superheat temperature, or EST.
    From what I will call "Point X", the exact point in the evaporator where all the liquid refrigerant is changed to a saturated vapor, to the outlet of the evaporator where EST is measured, let's give it a 10 degree rise in temperature.

    Now, from Point X to the evap outlet, did the refrigerant rise in pressure (for the moment we are setting aside pressure drop through the evap, which is why expansion valves under our discussion are externally equalized)? No, as once a vapor is heated past its saturation point, there is very little rise in pressure for a considerable rise in temperature.

    However, the temperature of the refrigerant picked up 10 degrees from Point X to the evap outlet.

    Here's the key. What state is the charge inside the thermostatic bulb? It is at saturation. Its own saturation, reacting to the heat of the suction line it is strapped to. It is not reflective of the SVT at Point X of the evap. It is responding to the temperature of the suction line downstream of Point X, which is 10 degrees warmer than Point X. So, if the charge in the bulb is R22 at saturation is 10 degrees warmer than Point X - 50 degrees - what is the corresponding pressure exerted on the power head by the thermostatic bulb according to a pressure/temperature chart?

    40 degrees SVT = 68 psig inside evap
    50 degrees EST = 84 psig inside thermostatic bulb

    So, the bulb with a charge of R22 will exert 84 psig on the power head to offset the combined closing pressures of both the spring and the evaporator pressure through the equalizer line.

    P1 = P2 + P3

    Please let me know if this helps.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #18
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    Never mind...I can't get what I want posted to work. I'll try again later.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #19
    Join Date
    Sep 2003
    Posts
    1,311
    Thanks for the time taken. I realized after I left for the day that the pressures which open/close the valve were flopped from what I posted; I figured you'd catch that which you did.

    In the example given we have a 10 superheat at 16 PSIG differential pressure (84 PSIG - 68 PSIG = 16 PSIG). The 16 PSIG differential is directly caused by the force of the spring; Thus the 16 PSIG should stay the same since the spring isn't being adjusted. Now let's say that the airflow changes in the evap and we now have a 74 PSIG suction pressure which is equivalent to a 44 SVT. Since the differential pressure is set for 16 PSIG the bulb would then be exerting a 90 PSIG pressure on the power head (74 PSIG + 16 PSIG). 90 PSIG gives a 54 EST. 54 EST - 44 SVT = 10 Superheat which is the same as when the suction pressure was at 68 PSIG.

    Let's say the suction pressure dropped to 60 PSIG. 60 PSIG gives a 34 SVT. 60 PSIG + 16 PSIG = 76 PSIG being exerted on the power head by the bulb. 76 PSIG = 45 EST. 45 EST - 34 SVT = 11 Superheat. So we see here that the superheat only changed one degree from an operating range of 60 PSIG suction to 74 PSIG suction.

    The way that I understood the original statement is that the TXV varies the superheat to adjust for ambient conditions. Perhaps there is more that can be accounted for, such as double metering (TXV is usually followed by cap tubes which feed the larger coil tubing) which is not being added to the equation. Did I misunderstand the statement altogether or is there more to consider which would affect the TXV more drastically?

    [Edited by sadlier on 06-07-2005 at 12:21 AM]

  7. #20
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,350
    The way that I understood the original statement is that the TXV varies the superheat to adjust for ambient conditions. Perhaps there is more that can be accounted for, such as double metering (TXV is usually followed by cap tubes which feed the larger coil tubing) which is not being added to the equation. Did I misunderstand the statement altogether or is there more to consider which would affect the TXV more drastically?
    I would say the TXV reacts to changes in superheat in order to maintain a consistent superheat. Airflow and heat load would be the main things to affect a change in evaporator superheat. Using what we've discussed above, the valve reacts to these changes to maintain a consistent superheat temperature, which translates to a consistently loaded evaporator.
    With fixed metering devices, the "Point X" I referred to above will move significantly with variation of load, airflow, and outdoor ambient temperature. With a TXV, "Point X" will be consistent throughout the range of temperatures and conditions the system is expected to operate under.

    Take a spin by Sporlan's website at http://www.sporlan.com and download the Bulletin 10-9 pdf file. It is chock full of good info on TXV operation and application. It helped me to open my eyes to how TXV's really work vs. how I thought they worked.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

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