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  1. #1
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    Pressure Balancing in TXV



    @http://www.ac-heatingconnect.com/how-thermostatic-expansion-valves-txv-work/

    Above link indicates that condenser pressure or the inlet pressure to TXV also is a parameter in pressure balancing equation

    TXV Pressure Balance EquationTXV
    P1+P4 = P2+P3
    P1 = Bulb Pressure (Opening Force)
    P2 = Evaporator Pressure (Closing Force)
    P3 = Superheat Spring Pressure (Closing Force)
    P4 = Liquid Pressure (Opening Force)
    However in all other literature including WIKI & Danfoss educational media in the above equation P4 is totally absent



    ...My question is this equation universal or is design dependent. By the look of the valve operation I beileve the former is true but why many others aren't involving condenser pressure



  2. #2
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    Hi, the difference may be in the two types of TXV. Internally or externally equalised, so the equation would be more relevant on an externally equalised TXV as its measuring the pressure of the suction (P4) as well as the temperature of the suction pipe (P1) and from this it can calculate the superheat and therefore can open and close accordingly.
    Martin

  3. #3
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    Re: Pressure Balancing in TXV

    To me the condensing pressure would be a force that the bulb pressure has to overcome.
    This would be taken into account in the designer of the valve, no matter who it is.
    The surface area on the liquid in side of the valve would have an opposing force, but very small.
    Also velocity over needle & seat may have some opposing forces as well.
    A lot of the information is simple as TX valves & superheat is largely misunderstood, without adding extra information.
    I could be wrong though!

    In real terms that is irrelevant except to understand that when a valve is selected, it is normally on pressure difference across valve to gives valves different capacity.
    So if discharge pressure to low valve may be fully open but cannot let enough refrigerant in.

    That & other scenarios is why adjustments could be made for the wrong reasons.

  4. #4
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    Re: Pressure Balancing in TXV

    Hi ioncube
    You are talking about what we term as Single Port valves and Balanced Port Valves.
    In theory normal single port expansion valves are designed so that only 3 forces act on the expansion valve .
    P1 the thermal bulb pressure (opening force)
    P2 the evaporator pressure (closing force )
    P3 the orifice or superheat spring (closing force )
    in theory there is a 4th force P4 the condensing pressure or liquid pressure (closing force)

    But in the design of the expansion valve orifice the nozzle cone is machined to equalise the pressure around the nozzle cone thereby removing the effect of the P4 force.
    On large capacity valves where the orifice nozzle is large in diameter, it is more difficult to design this orifice cone to remove the effect of P4.
    So the use of the balanced port orifice expansion valve becomes necessary
    A very good example of this type of valve is the Danfoss TE 55 expansion valve. The orifices for this valve have 2 cones fitted to the push pin. The liquid enters the orifice area between the 2 cones. The liquid tries to push the lower cone open and the liquid tries to push the upper cone closed , so the liquid pressure is equalised by the 2 cones opposing each other. This removes the P4 effect and also removes the need to have a very large diameter power assembly diaphragm to increase its opening power.

    Balanced port valves are designed to remove the P4 effect and are also an advantage on reverse cycle heat pumps where these balanced port valves allow the expansion valve to be used in a bi directional mode for Heating and cooling.

    Please go to the Danfoss webb site and look at the TGE range of Balanced port valves expansion valves.

  5. #5
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    Re: Pressure Balancing in TXV

    Hi Glenn
    The liquid enters the orifice area between the 2 cones. The liquid tries to push the lower cone open and the liquid tries to push the upper cone closed , so the liquid pressure is equalised by the 2 cones opposing each other.
    I got your point crystal clear ..but do tell me P4 though can be altogether eliminated via fabrication symmetry ..however if you look at the figure I posted ..once the valve starts to open P4 starts to play role ..by opposing the opening (downward coming) stem

  6. #6
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    Re: Pressure Balancing in TXV

    Hi
    Danfoss carried out tests in comparison with a large U.S. Manufacturer who claimed that balance port valves gave better control and capacity but there was no difference in operation as P4 had no influence due to the cone design and the larger diameter power head more than compensate for the small influence that P4 potentially could exert

  7. #7
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    Re: Pressure Balancing in TXV

    The diagram acts like springs exert pressure. Springs can only exert force.

    The right way to look at a valve's internals is as a force balance, not a pressure balance. So the Bulb Pressure Pb is to the top of the diaphragm and wants to open the flow port. Force is Pb x Ad

    The liquid line pressure (near condensing) acts on the area of the valve plug Apl and tends to close the valve....Obviously the flow port is much smaller than the diagphragm so the force from that end is much smaller...But still there.

    The Evaporating pressure Pe acts on both the underside of the diaphragm and the top side of the plug....

    The Spring acts on the bottom side of the diaphragm but knows nothing about area: Its simply a force trying to close the valve....

    So the right balance equation looks like:

    Tend to Open.............................Tend to Close
    Pb x Ad + Pe x Apl = Fsp + Pe x Ad + Pc x Apl

    And: If Ad is a hole lot larger than Apl: both Pe x Apl and Pc x Apl become negligible and...

    More or less:

    Pb x Ad = Fsp + Pe x Ad

    As the plug area become significant compared to the diaphragm area: the valve becomes responsive to (Pc-Pe) so for lower temperature duties or greatly variable head pressures on high capacity valves: go tto Balanced port arrangement. One advantage: it nominally lets the circuit operate at pretty low condensing pressures if the ambient conditions will permit......

  8. #8
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    Re: Pressure Balancing in TXV

    The diagram acts like springs exert pressure. Springs can only exert force.
    great point sterl

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