PDA

View Full Version : Throttling Effect





gasgas77
11-09-2001, 01:07 PM
what is "Throttling Effect" and how is it related to the mechanics(working) of a refridgerator??:confused:

Prof Sporlan
11-09-2001, 04:57 PM
Throttling effect as it pertains to a refrigeration system is simply the process of reducing the pressure of liquid refrigerant as it passes thru the expansion device. By reducing pressure, we cause the liquid refrigerant to flash into vapor which creates a cooling effect.

To understand why reduced pressures are associated with lower temperatures, one can look at the ideal gas law: PV = RT, where P pressure, V = volume, R = gas constant, and T = temperature. We can equate two different states of an ideal gas as follows: P1*V1/T1 = P2*V2/T2, since R is constant in both cases. If we hold volume constant, we get: P1/T1 = P2/T2, i.e., pressure is proportional to temperature. So if we have an ideal gas at 200 psia and 530R (70F), and we wish to reduce this pressure to 100 psia while holding volume constant, we must lower the gas temperature to: T2 = 100*530/200 = 265R (-195F).

One can observe this cooling effect by simply opening a nitrogen bottle to atmosphere. As the compressed nitogren is expelled to atmosphere, its temperature is reduced.

gasgas77
12-09-2001, 12:43 PM
so which means that if taking any suitable gas..compress it substantially to a certain pressure and thus changing it to liquid in the compression chamber, afterwhich is expanded rapidly. As it is expanded, the gas loses heat to the surrounding and thus the cooling effect.

So is it possible to explain the phenomenon in molecular term?:confused:

Prof Sporlan
12-09-2001, 03:15 PM
so which means that if taking any suitable gas..compress it substantially to a certain pressure and thus changing it to liquid in the compression chamber, afterwhich is expanded rapidly.
In fact, it is not necessary for the gas to condense to liquid to produce the cooling effect. Taking high pressure gas to low pressure is sufficient to produce cooling. If we allow the gas to change phase from liquid to vapor in the expansion process, however, less volumetric flow is needed to produce the same cooling.

As it is expanded, the gas loses heat to the surrounding and thus the cooling effect.
Actually, the gas absorbs heat from the surrounding area when it is expanded which produces the cooling effect. But the Prof understood what you meant.. :)

So is it possible to explain the phenomenon in molecular term?
Probably. You and Marc O'Brien must think alike... :)

Bharath Rajes
14-02-2009, 12:16 PM
I have a doubt Prof. For an Ideal gas undergoing throttling process 1-2, is this relationship P1/P2=T1/T2 valid.. If not why?

Bharath Rajes
14-02-2009, 12:19 PM
what happens to the volume during throttling?

Bharath Rajes
14-02-2009, 12:24 PM
I have a doubt Prof. For an Ideal gas undergoing throttling process 1-2, is this relationship P1/P2=T1/T2 valid.. If not why?

nike123
14-02-2009, 12:45 PM
I have a doubt Prof. For an Ideal gas undergoing throttling process 1-2, is this relationship P1/P2=T1/T2 valid.. If not why?
His last activity on RE is 4 years ago.
I doubt that you will get answer from him here!

Bharath Rajes
14-02-2009, 01:11 PM
anyone who knows the answer can help

bad-engineer
16-04-2009, 08:05 AM
Thermodynamically, through throttling process, no heat transfered (adiabatic), no work have been done, very small potential energy changes, and insignificant change in kinetic energy as well. Therefore, enthalpy of inlet and outlet are the same (isenthalpic). And enthalpy includes: internal energy (u)+ flow energy (Pv). Actually, these 2 kinds of energy has changed, one is increased and the other decreased. So, if internal energy decreases, temperature decreases, and vice vesa.
In the case of ideal gas, enthalpy is a function of temperature h=h(T), temperature will be unchanged.

Hope this helpful.

P.S.: this is referenced from "Fundamentals of Thermal-Fluid Sciences"-Cengel & Turner, PP. 178-180