Compressors raise the pressure of a flowing fluid by mechanical work. There are two main types: reciprocating for high pressure/low flow, and rotative for low pressure/high flow. Compressed air has many industrial applications. Compressor performance is evaluated based on factors like motor efficiency, mechanical efficiency, compression efficiency, volumetric efficiency, and overall efficiency. Problems involve calculating mass flow rate, power requirements, and other parameters for given compressor specifications and operating conditions.

1. LECTURE UNIT NO. 10
Compressors
Compressors are machines in which mechanical work is done on a flowing fluid, raising its pressure.
General Types of Gas Compressors
1. Reciprocating Compressors – for high pressure and low volume flow rate.
2. Rotative compressor – for low pressure and high volume flow rate.
a. Centrifugal Compressor
b. Rotary Compressor
Application of compressed air
1. Driving and starting engines (diesel and gas turbines)
2. Driving pneumatic tools
3. Spraying paints
4. Cleaning by air blast (Sandblasting)
5. Operating air hoist
6. Pump water by air lift
7. Air supply/ventilation in mine tunnel
8. Food processing
9. Liquefying gases
10. Conveying materials
11. Refrigeration
Air compressors
ma ma
Fresh Air
s d
Wc
IP
Motor
Pressure gage Pressure regulator
Storage tank
To application
Water drain
PV Diagram
P
V

2. Theoretical Work of Compression
1. Isothermal Compression (T=C)
- Water cooled (ideal process)
No Pressure Drop
With Pressure Drop
2. Isentropic (S=C) Theoretical Process PVk = C, k = 1.4
- Perfectly insulated (theoretical process)
No Pressure Drop
With Pressure Drop
3. Polytropic Compression PVn = C, n = 1.33 – 1.35
- Air cooled (Actual process)
No Pressure Drop
With Pressure Drop
Note:
No Pressure Drop
ΔPs = 0 Ps = P1 (no pressure drop or valve resistance)
ΔPd = 0 Pd = P2 (no pressure drop or valve resistance)
ΔTs = 0 Ts = T1 (no temperature change)
Vs = V’1
With Pressure Drop
ΔPs = P1 = Ps - ΔPs (no pressure drop or valve resistance)
ΔPd = P2 = Pd + ΔPd (no pressure drop or valve resistance)
ΔTs = 0 Ts = T1 (no temperature change)
Compressor Performance
1. Motor efficiency
2. Mechanical efficiency

3. Where: BP = IP + FP
FP = Friction power
Use: FP = 0.105 Vd 3/4 , hp if not given.
Vd = actual volumetric or piston displacement, ft3/min
Compression Efficiency
1. Isothermal Compression Efficiency
2. Isentropic Compression Efficiency
3. Polytropic Efficiency
4. Overall – compression Efficiency
5. Actual Volumetric Efficiency
VF, Free air or the actual quantity of gas delivered can be obtained using general gas equation
PF = Ps = 101.325 kPa
TF = Ts = 21°C if not given
Hence;
VD, Actual piston displacement
Single Acting Compressor
- suction at the end of the piston
Where: AHE = area of head stroke
L = length of stroke
Ns = cycle per sec, cycle/ sec
c = number of cylinders
a = number of acting, 1 for single acting , 2 for double
acting
n = rotative speed, rpm

4. Double Acting Compressor
- suction on both ends of piston
Double acting compressor, diameter of rod considered
Where: AT = total area for head end and compressor end
D = diameter of piston
d = diameter of piston rod
Note: a = 1 since area where already combined
6. Conventional (Clearance) Volumetric Efficiency
7. Piston Speed
vp = 2Ln
Problems:
1. A single-acting reciprocating air compressor has a clearance volume of 10%. Air is received at 90
kPa and 29.3 °C and is discharged at 600 kPa. The compression and expansion are polytropic with
n = 1.28. The pressure drop is 5 kPa at the suction port and 10 kPa at the discharge port. The
compressor piston displacement is 500 cm3 when operating at 900 rpm. Determine the following:
a. Mass of air compressed in kg/hr.
b. The required compressor power.
2. A single-acting air compressor operates at 150 rpm with an initial condition of air at 97.9 kPa and
27 °C and discharges the air at 379 kPa to a cylindrical tank. The bore and stroke are 355 mm and
381 mm, respectively, with a 5% clearance. If the surrounding air is at 100 kPa and 20 °C while
the compression and expansion processes are PV1.3 = c, determine:
a. Free air capacity in m3/sec
b. Power of compressor
3. A single stage air compressor handles 0.454 m 3/sec of atmospheric pressure, 27 °C air, and
delivers it to the receiver at 652.75 kPa. Its volumetric efficiency is 0.72, its compression efficiency
on an isothermal basis is 0.85 and its mechanical efficiency is 0.90. If it rotates at 350 rpm, what
power in kW is required to drive it?