# Thermodynamic

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Unit Two Homework Solutions, September 9, 2010
1 A frictionless piston-cylinder device initially contains 200 L of saturated liquid refrigerant-134a. The piston is free to move, and its mass is such that it maintains a pressure of 900 kPa on the refrigerant. The refrigerant is now heated until its temperature rises to 70oC. Calculate the work done during this process.
The freely-moving piston can be interpreted as giving a constant pressure process such that P1 = P2 = P = 900 kPa. For a constant pressure process, the concept that the work is the area under the path is particularly simple. That area is a rectangle whose area is P (V2 – V1). We know that P is 900 kPa, and the initial volume is 200 L = 0.2 m3, but we have to find the final volume.
Because this is a constant pressure process, the final pressure equals the initial pressure of 900 kPa (0.9 MPa) and we are given that the final state has a temperature of 70oC. From the superheat tables for refrigerant-134a in Table A-13 on page 930, we find that the specific volume at this temperature and pressure is 0.027413 m3/kg.
In order to find the volume (V in m3 as opposed to the specific volume, v, from the property tables in m3/kg), we have to know the mass. We can find the mass from the initial volume and the value of the specific volume at the initial state of saturated liquid at 900 kPa. At this pressure, we use the saturation table, A-12, on page 928, to find the specific volume of the saturated liquid, vf = 0.0008580 m3/kg at 900 kPa. We can then find the mass as follows. We can now compute V2 = mv2 and find the work done. W = 5,571 kJ
2 A mass of 2.4 kg of air at 150 kPa and 12oC is contained in a gas-tight, frictionless piston-cylinder device. The air is now compressed to a final pressure of 600 kPa. During the process, heat is transferred from the air such that the temperature inside...

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