Analysis of Thermal Cycles MEC3002
Laboratory Experiments
M Farrugia 14/11/2005
Air Compressor, Single Stage
Object:
To carry out various tests on a single stage, water cooled, air compressor at constant
speed. The changes in the following list of parameters is to be studied when the delivery
pressure is varied
1. Volumetric efficiency
2. Difference between adiabatic and actual temperatures
3. Heat rejected in the cooling water
4. The value of the polytropic index of expansion, n
5. Motor power, motor efficiency
Apparatus:
Compressor
Broom and Wade Type N3 Ser No 1/790447
Bore 4in, Stroke 4in, clearance volume 1.573in3
Max Speed 720 rev/min
Free air delivered 14ft3/min
Max pressure 200psig
Dynamometer
Swinging field type, motor No 28517/1
Rated power output 7.5hp @ 720 rev/min
Length of brake arm 12.605in
WN
hp 
Formula,
1 hp = 746 W
5000
W is the weight on spring balance in lbs
N is the rpm of the motor
Intake Air Box
Size 30in x 20in x 24in
Orifice diameter 0.8115in
Coefficient of discharge 0.60
h460  t 
Q  25.9d 2
Formula
B
d is the orifice diameter in feet
h is the head of water across the orifice in feet
B is the barometric pressure in inches of Mercury
t is the ambient temperature in F
ft3/s
1
Cooling water flow meter
Number R183
Orifice size No0, Capacity 0-40gall/hr
Formula, Q  9.23H 0.512
Q in gall/hr
H is the head of water in inches
Air Receiver
Broom and Wade No Y 22264/1
Size: 60in long x 24in diameter
Max working pressure 205psig
Test pressure 308psig
Safety valve, Broom and Wade, set to 205psig
Additional instruments
Tachometer 0 - 1000rpm
Cooling water thermometers
Inlet and Delivery air thermometers
Procedure
Run the compressor as close to a fixed speed as possible by adjusting the voltage control.
A speed between 300 and 600 rpm is suggested. The amount of cooling water is adjusted
so that the level in the measuring level is higher than half the maximum. By adjusting the
amount of air discharged from the air vessel, the delivery pressure is varied between 0 bar
and maximum 10bar. Record the readings listed in the reading table, allowing enough
time for the parameters to settle.
Theory
From the area under the pv diagram for a single stage reciprocating compressor it can be
shown that the indicated power required, i.p., is given by
 n 
 T2  T1 
i. p.  
 mR
 n  1
n 1


 p2  n
 n 

 1    1
i. p.  
or
 mRT
 p1 

 n  1


where n is the polytropic index of expansion and compression
m is the mass induced per unit time
R is the specific gas constant (287 J/kgK for air)
T1 is the absolute temp at the end of the induction stroke
T2 is the absolute temp at the end of the compression process
p1 is the pressure at the induction stroke, the free air pressure
p2 is the pressure of the receiver
2
From the second equation for i.p., it is noted that the power required to drive the
compressor increases with higher pressure ratios.
The mass flow rate can be calculate by
p
m  . Q 
Q
RT
The volumetric efficiency v is defined as the volume of air delivered measured at the
free air pressure and temperature, divided by the swept volume of the cylinder.
The adiabatic exit temperature would be the temperature if no heat was lost from the air
being compressed to the surroundings.
T2 adiabatic
p 
 T1  2 
 p1 
 1

The heat lost to the cooling water = m water . Cwater . Twater Watts
m is the cooling water flow rate in kg/s
where C is the specific heat capacity of water, 4200 J/kgK
Twater is the temperature rise of the cooling water
n 1
p  n
 T  n  1  p2 
T
Since 2   2 
then ln  2  
ln 
T1
n
 p1 
 T1 
 p1 
Hence this equation can be used to find a value for the polytropic index of compression n,
by plotting ln[T2/T1] against ln[p2/p1].
Results
Calculate and put down in table form for each delivery pressure.
the free air flow rate, and its uncertainty.
the volumetric efficiency, and its uncertainty.
the cooling power dissipated in the cooling water
the adiabatic exit temperature
the motor electrical power (I V)
the shaft power (measured by the dynamometer)
the motor efficiency
In the calculations of uncertainty, limit your analysis to uncertainties due to parameters
measured during the experiment, i.e. assume all constants and given data (e.g. bore and
stoke) to be known accurately.
Plot curves for volumetric efficiency and shaft power against delivery pressure. Also plot
the actual exit temperature (T2) and the adiabatic exit temperature on the same sheet
versus the delivery pressure. Plot ln[T2/T1] against ln[p2/p1] and obtain a value for n.
Conclusions
Draw your own conclusions on the experiment and results obtained.
3
Readings
0
Ambient temperature:
Atmospheric pressure:
Delivery
Pressure
bar
Motor
Voltage
Volts
C
cmHg
Current
Amps
Rotational
Speed
rpm
Dynamo
meter
Load lbs
Air
Head H2O Air Temp Air Temp
mm
in 0C
out 0C
Cooling water
Head
Temp in
0
cm
C
Temp out
0
C
4