370: ME Practice Exam 1. A single stage industrial reciprocating-piston air compressor operating between ambient pressure (Pamb = 14.5 psia) and 100 psig at a speed of 750 rpm is found to produce an air flow rate of 12.56 scfm. The ambient temperature is 90 F. Dimensions of the single cylinder compressor are as follows: Cylinder diameter = 2.5 inches Piston stroke = 10 inches Clearance volume = 1 cubic inch a) (10 pts) Find the volumetric efficiency. vol = __________________
b) (10 pts) When the piston has moved 8.05 inches from bottom-dead-center, the compressor begins to exhaust the air. Find the value of the polytropic exponent describing this compression process.
n = ___________________
c) (8 pts) Assuming that this compressor has the same type of valves as the one used in experiment #13, sketch both the actual and ideal-adiabatic cycles on the PV diagram below. Be sure to label the inlet and exhaust pressures, as well as the clearance, swept, and total volumes.
P
V
2. A heat pump identical to the one operated during experiment 8 displays an initial mass flow rate for the refrigerant of 7.62 g/s. Information regarding the states around the cycle are given in the table below. Here the state numbers refer to those given in figure 4 in your lab manual on page 8-8. T (C) 15 82 39.4 24.4 -10 (kg/m3) 39.07 9.06 1150 1210 43.75 10.17
State 8 2 3 4 5 6
P (kPa) 202 1000 202
h (kJ/kg) 262.4 313 104 82.74 241
X superheated superheated 0 Sub-cooled 0.23 1
Other known information at the initial condition: Rate of swept volume in compressor = 0.456 liters/minute Mass of water in the condenser tank = 50 kg Initial temperature of water in the condenser tank = 26.7 C Mechanical efficiency = 0.5 Cooling capacity = 1.21 kW Sub-cooler effectiveness = .3039 a) Fill in the information in the blank spaces in the table above
b) Determine the COP for the heat pump, based on the shaft power delivered by the compressor.
c) Assuming that the heat transferred in the condenser is constant in time and that the heat transfer from the condenser tank to the surroundings is negligible, determine h...