Expansion

[alesindo]

Dear Forum....

I am a newbie in refrigeration.

During the expansion the refrigerant come from the higher pressusre and temperature in condenser to the lower pressure and temperature in evaporator.

I want to know... which expansion is used to expanse the refrigerant form the condenser to the evaporator .... is it isotherm ? adiabatic(isentrope) ? Which one is ideal which one real ?

How is the calculation of the outlet temperature after expansion... if the inlet temperature and pressure is known... and the pressure after expansion is also known ?

with isenthalp ? dh=0=cp dT means dT=0 ????

or adiabatic with isentropic coefficient ? how do I get this coefficient ???



Sincerely,

Yen Sindo Boentoro

[Vincent Yu]

It is reasonable to regard it as adiabatic. Enthopy is constant crossing the expansion valve. But temperature is not constant. The fluid cannot keep the same temperature at such a low pressure. So part of the fluid vaporizes until the temperature reaches to the saturation temperature at that pressure.

dh=0 is right. Since there exists phase changing, it does not equal to cp dT.

[frank]

It is reasonable to regard it as adiabatic. Entropy is constant crossing the expansion valve. But temperature is not constant. The fluid cannot keep the same temperature at such a low pressure. So part of the fluid vaporizes until the temperature reaches to the saturation temperature at that pressure.

dh=0 is right. Since there exists phase changing, it does not equal to cp dT.

Hi Vincent

An Adiabatic process is one that does not involve the loss or gain of heat energy, therefore the the expansion of refrigerant is not Adiabatic.

Isothermal processes are processes where the temperature stays constant.

Isobaric processes are processes where the pressure stays constant..

As the metering device causes the refrigerant to lose both heat and pressure the process is called ?? - com'on then folks - time for a little research. What do you think the process is called ?

Entropy is the term for unusable energy.

[wambat]

Polytropic, but falls closer to adiabatic then isothermal because most of the work of expansion is supplied by the gas then from the surroundings. thats why we normally say that expansion is an adiabatic process

[wambat]

Marc is correct, the expansion process experences no change in enthalpy and is adiabatic What went wrong?

[bersaga]

All throttling processes are ideally Adiabatic and all compression processes are ideally Isentropic.

[Vincent Yu]

Theoretically, throttling processes are adiabatic. In practice, mornally we regard an expansion process as adiabatic. If a thermal expansion valve is used, it must be well insulated. Otherwise, heat exchange exists. If a cap tube is used as metering device, usually, the process is not adiabatic.

Frank, I hope it answers your question.

Bersaga, I do not agree that all compression processes are ideally isentropic. When you buy a compressor, you can find the “isentropic efficiency” from the datasheet. Usually it is between 50% and 70%. This means that the compression process is not isentropic. When you design a refrigeratin system, you also need to concern the isentropic efficiency of the compressor.

[frank]

Isentropic - see page 11 for proof http://www.andrew.cmu.edu/course/24-221/summary.pdf

[bersaga]

Vincent, please note that I specifically mentioned " IDEALLY ISENTROPIC" - hope you understand that.

What you're saying about Isentropic Efficiencies is basic and can be be found when you plot an actual working refrigeration vapor compression cycle on a pH chart. The compression line will deviate from the constant ' s ' lines , which are constant entropy lines !

[Vincent Yu]

Bersaga, by my understanding, " IDEALLY ISENTROPIC" means "perfectly isentropic", "exactly isentropic" or "absolutely isentropic". Maybe you want to say "all ideal throttling processes are Adiabatic and all ideal compression processes are Isentropic". Is it correct?

[bersaga]

Vincent - I think we're both on the same page on this-Thanks

[rbartlett]

Vincent - I think we're both on the same page on this-Thanks

bersaga, did you get my email??

cheers

richard