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weighman2210
05-02-2009, 06:27 PM
Hi all,

I'm a marine engineering student currently working on a refrigeration lab. After plotting cycles on p-h diagrams for various evaporator pressures it is clear that in practice the flow through the expansion valves is not isenthalpic. Aside from heat transfer through the expansion valve to the surroundings and the change in velocity of the fluid what other reasons are there for a reduction in enthalpy as the fluid passes through the expansion valve?

Thanks

rajbunny
05-02-2009, 07:02 PM
Hi weighman I been reading this site for a few weeks now and this my first attempt at assisting any one ,I hope it helps.
As you will see from your PH chart that the point at which the refrigerant enters the expansion valve can vary from right to left. On an ideal cycle the refrigerant enters at a point corresponding to saturated liq, but in reallaty if you could subcool the liquid to a greater degree than would occur normally in a system thru heat loss in the reciever and liquid line then what you will notice is that the temp liquid enters exp valve would also be lower and hence the line followed by constant enthaly would also be reduced. This over all makes the whole sytem more efficient. Hope this makes sense.

Chef
06-02-2009, 06:43 AM
The three things are change in kinetic energy, heat loss or gain and work done. You already have the heat exchanged and the kinetic energy but is there any work done in an expansion valve - probably not. There will be friction but that should manifest itself as heat.
Seems you have all the bases covered.
Chef

Tesla
06-02-2009, 07:24 AM
FLASH GAS is the term used here, in order to cool the refrigerant (after pressure drop to corrosponding saturated pressure - sometimes refeered to refrigerant quality on PH chart

Chef
06-02-2009, 08:25 AM
weighman, looking at this further I would be interested to know how you managed to plot this on a P-H diagram and know its non isenthalpic flow?

Do you have a way to measure the quality coming out of the TXV or did you measure the heat rejected and calculate h4 from that and known conditions at at h1?

It is always assumed that h3 is equal to h4 on all P-H diagrams which assumes the phase change is isenthalpic - I can only see a way to get h4 differant to h3 is actually calculate the differance?

Tell us your method!

Chef

weighman2210
06-02-2009, 11:50 AM
Hi Chef,

I've been having a problem with obtaining the enthalpy value at evaportaor inlet. It became clear when plotting that we had been mislead in the method a bit. Our lecturer wanted us to explain in our report why the process was not isenthalpic but it has become clear that we are unable to actually obtain the value of enthalpy at that point without knowing the mixture quality. I have emailed my lecturer to see if he has the quality or enthalpy values at this point. Until your message i had been tearing my hair out wondering how the hell we were expected to work out the enthalpy change with just the given data.

Thanks

Peter_1
07-02-2009, 08:57 AM
There's no heat lost nor gained during the expansion process, so it is for me isenthalpic isn't it?

hamidpia
08-02-2009, 04:39 AM
Refrigerant enters the evaporator as a boiling low pressure liquid, which consists of 70% to 80% liquid and 20% to 30% vapor. The vapor released in the expansion device is called flash gas or flash vaporisation.There is no addaition or removal of heat. The vapor and liquid mixture mixture has same total energy before and after expansion device.By exchaniging liquid for vapor, there is change of density, that is what causes change in temperature but there is no change in energy.

Chef
11-02-2009, 08:07 AM
There's no heat lost nor gained during the expansion process, so it is for me isenthalpic isn't it?

Hi Peter

Isenthalpic is, by definition, no heat loss or gain, no work done and no change in velocity. As the liquid flashes to gas during its passage through the valve there is an increase in velocity. This increase in velocity reduces the enthalpy and so its not really an isenthalpic process.

But it is so close and as there seems to be no way to measure the enthalpy everyone assumes it is isenthalpic. Normally it could be anywhere in the range from 2 to 10 KJ/Kg but in extreme circumstances it may be over 30KJ/Kg which becomes significant but how can we find out?:mad:

Chef

Peter_1
11-02-2009, 01:17 PM
...
Isenthalpic is, by definition, no heat loss or gain, no work done and no change in velocity. As the liquid flashes to gas during its passage through the valve there is an increase in velocity.

Isn't it no change in kinetic velocity (EK) and isn't then the Ek not the same before and after the expansion valve?

And http://www.refrigeration-engineer.com/forums/archive/index.php/t-133.html

http://www.answers.com/topic/isenthalpic-process

Chef
11-02-2009, 02:49 PM
Isn't it no change in kinetic velocity (EK) and isn't then the Ek not the same before and after the expansion valve?



It is correct that 'isenthalpic' is "no change in the kinetic velocity".

However the TXV sees liquid entering it and a gas/liquid mixture exiting it. As the gas has a bigger volume the only way to get it out fast enough is for the velocity to increase.

You will probably find the typical liquid velocity entering the TXV is about 2 or 3 m/s but leaving the mixture will be about 50 to 120 m/s depending on the system specs. That is a change in velocity and so it is not isenthalpic.

The Wikepedia link says no change in velocity and as the laws of mass conservation apply it is the same mass going in as coming out of the TXV - it's just faster!

Prof Sporlans thread is a work of art but maybe he calls the process isenthalpic because the thread is already as complex as it can get.

Chef