Actual Expansion Valve Process

[AastroGuru]

I observe that the expansion process in Thermostatic Expansion Valve is not even close to isenthalpic process. The inlet to EV is around 16 bar 45 deg C and output of EV is around 3.5 bar 2 deg C both lying outside vapour dome in liquid region and in P-h chart the deviation from straight line is quite high.

Is there any parameter for defining effectivess of expansion valve or efficiency kind of?

Usually in what range this efficiency might lie in actual air conditioning processes?

Expansion Valve: Cross Charge - C3 Block type with 10 deg superheat design

[icemakerman]

what type of vehicle ?
whixh refrygerant ? (r134a)
is this the same one with the condensor fan not working?
get the g#$damn fan working and the valve will operate normally

[icemakerman]

I observe that the expansion process in Thermostatic Expansion Valve is not even close to isenthalpic process. The inlet to EV is around 16 bar 45 deg C and output of EV is around 3.5 bar 2 deg C both lying outside vapour dome in liquid region and in P-h chart the deviation from straight line is quite high.

Is there any parameter for defining effectivess of expansion valve or efficiency kind of?

Usually in what range this efficiency might lie in actual air conditioning processes?

Expansion Valve: Cross Charge - C3 Block type with 10 deg superheat design

1: 16 bar = 160kpa too high reduce refrigerant charge maybee
2: or is the fan still not running

[Chef]

I observe that the expansion process in Thermostatic Expansion Valve is not even close to isenthalpic process. The inlet to EV is around 16 bar 45 deg C and output of EV is around 3.5 bar 2 deg C both lying outside vapour dome in liquid region and in P-h chart the deviation from straight line is quite high.

Well observed, the expansion process for a refrigerant is never isenthalpic, as it turns partially into a gas it therefore increases in velocity the enthalpy is reduced by vel^2 (with correction for units you use) and so generally h4=h3-v^2 (again corrected for units).

However without the gas type it is difficult to follow this on a P-H diagram.

Can you you post the gas and other system details?

Chef

[AastroGuru]

Dear icemakerman, as per ur suggestion i have already fitted the fan and its working fine. Anyhow thanks for ur response. Here i just wanna know how much enthalpy loss will be there in TXVs in general, since i find enthalpy loss is very high in my system.

[AastroGuru]

Hats off Chef,
Its R134a.
automobile ac.
7 kW flat tube evaporator.
90cc scroll compressor.
2 ton txv.
u can see from ph diagram both inlet and outlet points lie in liq zone.
compressor gives 10-17 bar varying output as per veh speed.

i understand u mean whatever kinetic energy gained will be the enthalpy loss. if so can i use mass_flow x hout = mass_flow x hin - mass_flow x (v^2) / 2


should i divide v^2 term by 2 or not. if i understood something wrong, kindly crrect me. Also i cant find the velocity term by any means. kindly suggest some general thumb how much enthalpy loss will be there?

[Chef]

Hats off Chef,
Its R134a.
automobile ac.
7 kW flat tube evaporator.
90cc scroll compressor.
2 ton txv.
u can see from ph diagram both inlet and outlet points lie in liq zone.
compressor gives 10-17 bar varying output as per veh speed.

i understand u mean whatever kinetic energy gained will be the enthalpy loss. if so can i use mass_flow x hout = mass_flow x hin - mass_flow x (v^2) / 2


should i divide v^2 term by 2 or not. if i understood something wrong, kindly crrect me. Also i cant find the velocity term by any means. kindly suggest some general thumb how much enthalpy loss will be there?

The change in enthalpy for SI units will be v^2/2000 and that is never likely to be more than about 10KJ/Kg and for a txv it is likely to be more like 2 up to 5KJ/Kg. This is certainly not enough to keep h4 in the liquid zone of the PH diagram.
Maybe you could recheck your plot. h3 should be at p=10 and h=255 (approx) and h4 should be at p=3.2 and h=250 (approx) so the line will be almost vertical.

The quality will be about 0.3 so about 30% of the refrigerant will be gas. You may be able to calculate the velocity if you can determine the dimensions of the internals of your txv but I dont think its worth the effort.

Maybe you should enter your data into 'coolpack' - it will give you a very accurate PH plot but it wont give you the change in h due to kinetic energy. Its probably too small in a txv system to worry about.

The question that needs to be answered is? Does the refrigerant get back its lost enthalpy due to any increase in kinetic energy when the gas slows down in the larger bore pipes of the evaporator?

Chef

[AastroGuru]

Gr8, but whatever values i had mentioned are actual measured values and not design values or design plot. To be in fact i had measured the pressure and temperature before and after TXV and found the mentioned values. As per you, there will not be an enthaply drop more than 5 kJ/kg in normal TXVs. Thanks for that. I dont know i had measured in 4 systems with a 150cc compressor, 170cc compressor 110cc compressor and 90cc Compressor. Only changing the compressor in the system. But to my surprise all 4 sytems are showing measured values in liquid zone after TXV. Thats the chaos. Thanks for ur support. Anyhow, when i consider in future i think i have to consider 5 kJ/kg roughly. Comment pls.

[AastroGuru]

Also, the manufacturer is mentioning something like subcooled TXV or subcooling in TXV of 2 - 5 deg C. Is there anything like subcooling setting in TXV. I think TXV is mostly related to evaporator superheating!!! If the TXV outlet goes to liq. zone what are the consequences?

[AastroGuru]

Thanks Chef,

I called up the mfr and confirmed. Sorry, the values i have given are a bit interspersed or mixed up. The actual values are OK as per you. I will come back if i have any doubt. There is no subcooling in TXV outlet, its a mixture. Subcooling is what before TXV.

[Peter_1]

what type of vehicle ?
whixh refrygerant ? (r134a)
is this the same one with the condensor fan not working?
get the g#$damn fan working and the valve will operate normally

Seems you don't understand this very good question.
Your reply was not relevant.
Sort or refrigerant doesn't matter, even the 'vehicle' doesn't matter nor the condenser fans.

[Peter_1]

1: 16 bar = 160kpa too high reduce refrigerant charge maybee
2: or is the fan still not running

1. What has pressure to do with charge?
2. You're back here with your fans. I suggest you read once Dossat 'Principles of Refrigeration'