Compressed Air System

Theoretical compressor diagram

Real compressor diagram for a piston compressor.

Analysis of Compressor Diagram

• The stroke volume is the cylinder volume that the piston travels during the suction stage.

• The clearance volume is the area that must remain at the piston’s turning point for mechanical reasons, together with the area required for the valves, etc.

• The difference between the stroke volume and the suction volume is due to the expansion of the air remaining in the clearance volume before suction can start.

• The difference between the theoretical p/V diagram and the real diagram is due to the practical design of a compressor, e.g. a piston compressor. – The valves are never fully sealed and there is always a degree of

leakage between the piston and the cylinder wall.– In addition, the valves can not open and close without a delay, which

results in a pressure drop when the gas flows through the channels. – Due to reasons of design the gas is also heated when it flows into the

cylinder.

c

b

e

V2 V1

a

P

d

ADIABATIC

ab suction stroke at constant pressure

bc adiabatic compression

cd discharge stroke at constant pressure

Work done = abcd

ISOTHERMAL

be isothermal compression

Work done = abed

abed < abcd

Isothermal vs Adiabatic compression

Work done on air is considerably less with isothermal compression

Shaded area is work saved by compressing isothermally

Compression in several stages• Theoretically a gas can be compressed isentropically or isothermally. • Compressed air is rarely used immediately at its final temperature after

compression-Isentrophic • In practice, gas can be rarely used without being cooled –isothermal process • Isothermal process is preferred, as this requires less work.• In practice attempts are made to realise isothermal process by cooling the gas

during compression. • For example, with an effective working pressure of 7 bar that theoretically

requires 37% higher output for isentropic compression compared with isothermal compression.

• A practical method to reduce the heating of the gas is to divide the compression into several stages and cooling the gas after each stage (The gas is cooled after each stage, to then be compressed further).

• This also increases the efficiency, as the pressure ratio in the first stage is reduced. The power requirement is at its lowest if each stage has the same pressure ratio.

• The more stages the compression is divided into the closer the entire process gets to be isothermal compression.

• However there is an economic limit for how many stages a real installation can be designed with.

Why Isothermal is not Practical?

• Isothermal compression would mean that compressor would need to run extremely slow

• But in practice, compressors run at fairly high speeds and hence compression tend to be adiabatic

How to approach Isothermal Compression

Multi-stage compression• Air is compressed in several stages

instead of full compression in a single cylinder

• Same as no of compressor in series• pv diagram for 4-stage compression• Doted line bf is isothermal• To keep compression near

isothermal, air is compressed and cooled to initial temperature

• Each stage increases the pressure while initial temperature is maintained at the end

• If compression is done in single stage, compression line would have followed be

• Shaded area is work saved• If intercooling is imperfect, point d

would not be reached

V

P

a b

c

e

d

f

Adiabatic

Isothermal

A typical compressed Air System

Energy cost of compressed air

A 100 kw compressor working for 300 days a year could consume Rs.32 lacs

Inefficient power source even if well maintained

Assessing the compressed air requirements of a site

• What Pressure is required ?

• What air quality is required ?

• What is the pattern of demand ?

• What compressor capacity is required ?

• Centralised or decentralised ?

• Can the waste heat be used ?

Compressor Selection

Tool (a) Air

Consumption (cfm)

(b) No.of Tools

(c) Air

Required (Cfm)

(d) Load

Factor

(e) Probable Air

Demand

Grinding Wheel 6” 50 5 250 0.3 75

Rotary Sandor 9” 55 2 110 0.5 55

Chipping Hammers 30 8 240 0.4 96

Nut Setters 20 10 200 0.6 120

Paint Spray 10 1 10 0.1 1

Plug Drills 40 3 120 0.2 24

Rivetters 35 5 175 0.4 70

Steel Drills 80 5 400 0.4 160

Total demand 601

Determine Air Quantity and top pressure Allowance for expansion and leakage Allow for 10 percent leak in the system

What pressure is required ?

• Most of the equipment need 6 – 6.3 bar• Add a pressure drop of 0.7 bar to the farthest

end• Generation pressure is 7 bar• For lower pressure requirements pressure

regulators can be used.• For example equipment operating at a

fluctuating supply of 6-7 bar which could operate at 5.5 bar would save 14 % of energy thro local pressure regulation

Pressure vs Power

Sizing the Compressors

Pattern of Demand

Compressor Capacity vs Power

Types of compressors

Reciprocating

Double acting two stage

Centrifugal

Selecting a compressor

Types of capacity controls

• Start/stop

• Cylinder unloading

• Modulating

Waste heat recovery

Case study: Energy Savings in Screw Compressor

Before:

• For catering demand of compressed air, site uses one 500 Nm3 Atlas Copco make screw compressor.

• The KW rating of this compressor is 75 KW.

• Oil free pneumatic air is required for pneumatic valves and other pneumatic devices installed at site

Study on compressor loading pattern

• Unloading as a percentage: 40% • Loading as a percentage : 60%

• Power consumed during unloading cycle = 32 KW • Power consumed during loading cycle = 72 kW• Total power consumed per day = 780 KWH

After installing VFD

• With the new VFD fit compressor in place the average power consumption of the compressor has gone down by 180 KWH

• Daily consumption of power is now: 600 KWH

• Savings of 180 units per day

Alternatives to compressed air