Thermostatic Expansion Valve TXV - Block Type

[AastroGuru]

In my system, TXV is normally operating at 12-19 bar inlet pressure and providing good amount of cooling in the system. But if i increase the pressures to 25 bar the cooling effect reduces drastically.

What is the correlation of the pressure here? What really will happen with TXV if I increase the pressure?

If with 25 bar i need to operate, whether i should change the TXV spring setting by rotating the screw?

System:
Automobile HVAC, Receiver Drier/TXV system, 2 ton Evaporator, R134a, 5K Subcooled Condenser, 170cc Swash Plate reciprocating compressor

TXV:
7 kW, cross charge - C3,Superheat - 10 deg C, Block Type

[icemakerman]

My first question is
WHY would u want to operate at different pressures to normal operating conditions??
Suction Pressure around 16PSI @

[US Iceman]

I think you need to spend some time reading how expansion valves work before you do any adjusting.

[icemakerman]

sorry @1500 rpm engine speed = aprox 110Kpa ( 1.1 Bar)
Next inlet pressure on the valve is determined primarily by the condensor and ambient conditions and checking on p/t chart shows condensing temp of 50C @ 12bar through to 65C @ 19bar...dont think r134a will operate much above these pressures and they seem very high for an automotive system in australia with extreme summers & i still dont see many vehicle running much higher than 10 bar condensing pressure much less 12 bar. 25 bar (not on my p/t chart) would equate to over 75C . Once again.. why would you want to operate at this pressure. Next thing is the adjustment on the valve is to adjust the superheat... no I would not adjust this (most are factory set)

[AastroGuru]

Hats off icemakerman!!

Assume i just removed the condenser fan from the system, then i observe that due to less heat transfer in condenser (worst in case when the vehicle is in idle and parked in front of a wall) the operating pressures tend to increase... In such a case the inlet to TXV, temperature and pressure builds up and i dont know whether TXV will be capable to produce desired effect at TXV outlet therby it can prevent the overall cycle stacking into higher pressures.

Coming to the basic point why lower pressures are better for R134a from your viewpoint?

I will be eager to see your answer......

[AastroGuru]

Can u quote some material so that i can go through the TXV principles..... I know the basic operation, working principle and even i have stripped and cut many TXVs for study... But i dono whether any direct formaulae available that if we give input parameters we get to know the TXV output parameters.

[Reflexive]

Not necesarily better for the r134a, but for the compressor! Your subjecting it to alot more work and pain than it should have to live with.

[icemakerman]

Why Remove the Condensor fan... the system is designed to run with the fan operating ....to move air across the condensor whilst vehicle is stationary, If u must remove the fan fit an idle cut out to the system so that any time the vehicle is at idle (stationary) the air conditioner will not run otherwise as reflexive posted u will put the compressor under unneccesary load and risk failure of the compressor....also some engines use the a/c fan for engine cooling and you also risk overheating the engine

[AastroGuru]

Thanks!!

I got your point... U mean if condenser cooling is not adequate, then pressure and temperature increase in inevitable which may lead to higher compressor load.

But what happens if gaseous refrigerant enters TXV (I suppose only liquid should be given, subcooled from condenser). What might be the probable effect on evaporator backpressure and compressor discharges if vapor is fed to TXV.

Can anyone suggest some TXV related theory material.... Like formulae or some methodology which gives approximate output parameters provided Evaporator out and TXV inlet properties known....

[US Iceman]

The reason the TXV appears to be not providing adequate cooling capacity is because the liquid enthalpy is much higher at higher pressures and temperatures (because the condenser fan was not working).

If the liquid enthalpy is higher, you loose a large part of the refrigeration effect for the refrigerant being circulated by the compressor.

You cannot adjust the TXV to compensate for this!

[AastroGuru]

Thanking u all...