Chapter2, Lesson D - Dew Point Calculations for R-134a

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2D-2 :	Dew Point Calculations for R-134a						4 pts

R-134a is contained in a sealed glass vessel at 50^oC. As it is cooled, droplets are noted condensing on the sidewalls at 20^oC. Find the original pressure in the vessel.


Read :	We know the initial temperature of the R-134a. If we knew the intial temperature, we could look up the specific volume in the Superheated Vapor Tables for R-134a. There is only one pressure that yields this value of the specific volume when the system is at 20^oC. Therefore, we could also look at the problem in the following way. We know the intitial temperature of the R-134a and IF we also knew the specific volume of the R-134a, we could use the Superheated Vapor Tables to work backwards and determine the initial pressure ! That is what we are going to need to do in this problem.








	When the 1st droplet of liquid appears on the wall of the glass vessel, the vapor inside the vessel is a satuated vapor. We can look up the porperties of this saturated vapor in the R-134a Tables. Since the vapor is saturated at 20^oC, it must be superheated at 50^oC. But at both the initial and final state the specific volume must be the same because neither the mass nor the volume of the system changed ! This is the key to the problem. Because we know the values of 2 intensive variables at the initial state, specific volume and temperature, and the initial state is a pure substance in a single phase, we can determine the values of ALL other properties ! In this case we need to determine the pressure.










Given :	T₁	50	^oC	Find :	P₁	???	kPA
	T₂	20	^oC
	x₂	1

Solution :	The specific volume of the system is equal to the specific volume of saturated R-134a vapor at T₂.


	We can look up this value in the Saturated Temperature table of the R-134a Tables at 20^oC :

					V_{sat vap}	0.035997	m³/kg

	Next, we scan the Superheated R-134a Tables to determine the 2 pressures between which this value of specific volume falls, at the given temperature of 50^oC.



	Here are the data at 50^oC:			P (kPa)	V (m³/kg)
				100	0.25938
				200	0.127660
				400	0.061724
				500	0.048499
				600	0.039659
				700	0.033322

	We need to interpolate between 600 and 700 kPa to determine the system pressure that corresponds to our value of specific volume at a temperature of 50^oC.



							Eqn 1

							Eqn 2

	slope	-15780.34	kPa/(m³/kg)
	P	657.8	kPa		P₁	658	kPa

Example Problem with Complete Solution