Respuesta :
Answer:
a) COPactual=4.33
b) COPmaximum=26.91
c) vminimum=0.00021 m^3/s
Explanation:
The properties of saturated refrigerant R-134a can be obtained from tables at 400 kPa
h1=enthalpy=255.61 kJ/kg
v1=specific volume=0.05127 m^3/kg
The properties of superheated refrigerant R-134a can be obtained from tables at 1.2 MPa and 70°C
h2=300.61 kJ/kg
a) the mass flow rate is:
[tex]m_{R} =\frac{v_{11} }{v_{1} }[/tex]
where v11 is the volume flow rate at inlet 1= 80 L/min=0.00133 m^3/s. Replacing values:
[tex]m_{R} =\frac{0.00133}{0.05127} =0.02601 kg/s[/tex]
The power consumption of the compressor is:
[tex]W_{in} =m_{R} (h_{2} -h_{1} )=0.02601(300.63-255.61)=1.17kW[/tex]
The refrigeration load is:
[tex]Q_{L} =Q_{heat} +Q_{equipment}[/tex]
Qheat=250 kJ/min=4.17 kJ/s
[tex]Q_{L} =4.17+0.9=5.07kW[/tex]
The coefficient of performance is:
[tex]COP_{actual} =\frac{Q_{L} }{W_{in} } =\frac{5.07}{1.17} =4.33[/tex]
b) The maximum coefficient of performance of a reversible refrigerator is:
[tex]COP_{maximum} =\frac{1}{\frac{T_{H} }{T_{L} }-1 }[/tex]
TL=23°C=296 K
TH=34°C=307 K
Replacing:
[tex]COP_{maximum} =\frac{1}{\frac{307}{296}-1 } =26.91[/tex]
c) The minimum power input to the condenser is:
[tex]W_{minimum} =\frac{Q_{L} }{COPx_{maximum} } =\frac{5.07}{26.91} =0.188kW[/tex]
The minimum mass flow rate is:
[tex]m_{minumum} =\frac{W_{minimum} }{h_{2}-h_{1} } =\frac{0.188}{300.63-255.61} =0.00418 kg/s[/tex]
The minimum volume flow rate is:
[tex]v_{minimun} =m_{minimum} v_{1} =0.00418*0.05127=0.00021m^{3}/s[/tex]
