A piston-device contains steam at 500 kPa and 50 percent quality. The initial vo
ID: 1717176 • Letter: A
Question
A piston-device contains steam at 500 kPa and 50 percent quality. The initial volume is 0.04 m^3. Determine the amount of heat addition required to increase the volume to 0.1554 m^3 at constant pressure. 0.75 kg of Refrigerant 134a a ta pressure of 400 kPa and quality of 60 percent is heated in a piston-cylinder device at constant pressure until the volume is doubled. The refrigerant is then compressed isothermally until the vapor has turned to saturated vapor. Show the activities on a T-v diagram, and determine: the temperature of the refrigerant at the end of the expansion process (degree C), the final pressure of the refrigerant (kPa), and the total heat involved (kJ).Explanation / Answer
a) Given : Pressure of steam inlet P1 = 500 kPa = 500 x 10 ^3 Pa
1 Pa = 1N/m^2
1 bar = 1 x 10^5 N/m^2
= 0.5 bar
Dryness fraction x = 0.5
Volume V1 = 0.04 m^3
Volume V2 = 0.1554 m^3
Heat Addition = change in enthalpy
= h1 = h2
h1 at saturated steam and h2 at supersaturated steam
From steam tables at prerssure 500 KPa the specific enthlapy od steam hs = 2747.64 Kj/kg
dry ness fraction (x) = 0.5
latent heaat L = 2225.31 Kj/kg
Enthalpy h1 = h + XxL
2747.64 + 0.5 x 2225.31
= 3860.3 kj /kg
Enthalpy at 2
h2 = 2780 kj/kg at 500 kpa and temperature 151 C
Heat addition = h1 - h2
3860 - 2780
= 1080 kj / kg
b) Given ; Mass = m = 0.75 kg
Refregerant = R134a
Pressure P1 = 400 kpa = 0.4 MPa
Dryness fraction x = 0.6
find : Temperature of the refregerant at end of expansion = ?
Final pressure of the refregerant
total heat involved
enthalpy h1 = hf1 + Xx hfg1
from Refregerant tables of R134a
hf1 = 63 kj/kg, hfg1 = 252 kj/kg at pressure 0.4 MPa
h1 = 63 + 0.6 x 252
= 214.2 kj /kg
h1 = m x Cp (T2 - T1 )
214.2= 0.75 x 1.34 (10 - T2 )
T2 = 203 C
Total heat involved = h1 - h2
h2 = 252kj/ kg
h = 252.- 214.2
= 37 .8 kj /kg
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