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Hi,
i am just going into the refrigerator science, the thing confused me is the state of refrigerant before and after Expansion device.. theoretically its a free expansion (throttling) process, but for real gases enthalpy changes?? i want to know what governs the final state of the refrigerant?
No, it's not free. The final state is the temperature and pressure to which the heat is exchanged, and it's governed by the energy balance in the heat exchanger itself.
Hi,Quote:
Originally Posted by NoNickName
Expansion process, is the intermediate between the high pressure and low pressures of the system, the thing i am not understanding is how the hot liquid before TXV gets cooled to a very low temperature on expansion in a valve???
Hi there,
When pressure of a liquid is decreased so is its boiling point. Water evaporates at 100°C at 1 bar pressure. If you go to the top of a mountain where the atmospheric pressure is less than 1 bar then water boils at lower temperature say 80°C.
NOW expansion process decreases the liquid pressure (regardless of its temperature). By doing so, the boiling point of the refrigerant decreases as well.
Expansion process is not completely enthalpy = const. but for our purposes, it is enough to assume it is.
For the proof of this you should refer to thermodynamic books.
Cheers
Quote:
Originally Posted by ragu
Exactly as Lana explained, the TXV does not evaporate the liquid. It just reduces its pressure, and depresses its evaporating temperature consequently.
The evaporation takes place in.... the evaporator.
http://www.refrigeration-engineer.co...read.php?t=888Quote:
Originally Posted by ragu
thank you all..is there any relationship for joule thompson effect for this expansion process?? and how do i can find out the final temperature if i am using R134a??
Quote:
Originally Posted by ragu
The Joule Thompson effect is not realy applicable in a basic refrigeration cycle because it refers to vapour gases.
The refrigeration process is about the transfer of heat.
Heat can be measured in 2 ways
One is tempreature and the other is heat content (the amount of work it can do) and one term for that is called enthalpy (heat content per Kg).
To understand the process you need to understand that energy can't be destroyed it can only be converted or moved.
So to answer your original question.
Liquid refrigerant before the expansion device is pure liquid at quite a high pressure after the expansion device it is at a lower pressure.
This presure differance is caused by the compressor circulating refrigerant through the expansion device.
The refrigerant after the expansion device is at a lower presure so the refrigerant has a lower saturation temp (boiling point) and the suction created by the compressor cause's the refrigerant to evaporate (boil) and the heat that boils it comes from the product being cooled (or giving up its heat) around the evaporator coil.
The expansion device does nothing but restrict the flow of the refrigerant and it causes a pressure drop in the refrigerant.
It is the decrease in the pressure that changes the state of the refrigerant from liquid to vapour because the refrigerant follows basic laws of thermodynamics.
One is energy can't be destroyed it can only be converted or moved.
another is heat will always flow from high to low and
a third is that by affecting the pressure of a liquid you affect the temp it boils at.
All the best
coolrunning
.
thanks sir, now i am clear... i have a general doubt, if we are expanding a refrigerant, it is getting cooled. on the other side if a liquid is pressurized will its temperature will go up???
Expanding the refrigerant does not cool it, expanding the refrigerant causes it to boil at a lower temperature / pressure.Quote:
Originally Posted by ragu
To boil, the refrigerant removes energy from the product being refrigerated (cooling it).
Try not thinking in terms of temperature and cooling.
Think more about pressure and state.
Pressure effects the temp that a fluid will boil or the temp a Vapour will condense.
State will change as the fluid boils (from liquid to vapour) or when vapour condenses.
It is all about the relationship between pressure and temerature.
We have very little control over temperature but we can manipulate the temperature by
adjusting the pressure and that is what we do with vapour compression refrigeration.
coolrunnings
.
Coolrunnings, Thanks a lot
Interesting thread guys :cool:
Well actually expanding the refrigerant ie dropping its pressure does cool it.Quote:
Originally Posted by cool runings
When the liquid refrigerant passes through the TXV it emerges as liquid and gas - the value is typically a quality = 0.3 but that is system dependent. The energy to convert the liquid to a gas comes from the liquid itself as it boils of or flashes off. Also as some of the two phase refrigerant passes through the TXV the gas part is expanded and has cooling effect from the Joule Thompson effect.
There have been thousands of comments on this site that by feeling the liquid line one can deduce if there is an obstruction by feeling a temperature drop. This is cooling as the pressure drops.
Otherwise just feel the outlet of a TXV - its cold!
Also if you look at the PH diagram for the point h4 after expansion it will be at the temperature of the downstream pressure of the TXV - this did not come from the evaporator or heat being collected as it passes through the valve but from the internal energy of the liquid/gas.
Chef