6.1 Reciprocating Compressor | unit 6 compressor

Unit – 6

Compressor

6.1 Reciprocating Compressor

A reciprocating compressor or piston compressor is a positive-displacement compressor that uses pistons driven by a crankshaft to deliver gases at high pressure.

The intake gas enters the suction manifold, then flows into the compression cylinder where it gets compressed by a piston driven in a reciprocating motion via a crankshaft,and is then discharged.

Applications include oil refineries, gas pipelines, chemical plants, natural gas processing plants and refrigeration plants

6.2 Applications of compressed air

For operating pneumatic tools such as drills, screw drivers, hammers, chiessels.

For pneumatic cranes.

For pneumatic brakes of automobiles, railways and presses.

For agricultural accessories such as dusters and sprayers.

For drive of CNC machine tools.

For pneumatic conveying of materials.

For pneumatic gauging, inspection and low-cost automation systems.

6.3 Single stage compressor (without clearance and with clearance volume)

Working Principle:

Here, pressure is increased by means of variation in the volume of cylinder obtained by a moving piston.

Construction:

Single stage single acting air compressor consists of a piston, which reciprocates inside a cylinder having connecting rod and crank mechanism.

There are inlet and delivery valves mounted in the head of cylinder.

The inlet and delivery valves are of pressure differential type i.e., they operate as a result of pressure difference across the valves.

Fig. Single stage Reciprocating Air Compressor

Working:

Working of single stage reciprocating air compressor is completed in two strokes.

a). Suction Stroke:

When the piston moves in the downward direction, from (T.D.C.) to (B.D.C), the air compressed in the previous compressions and entrapped in the clearance space begins to expand.

Due to expansion, the pressure of air inside the cylinder starts to decrease.

After some travel of piston from T.D.C. towards B.D.C, the pressure of air inside the cylinder falls below the atmospheric pressure.

Thus, due to pressure difference, the inlet valve gets opened. Now, the atmospheric air is sucked into the cylinder.

b). Compression Stroke:

When the piston starts moving from BDC to TDC, the pressure of sucked air inside the cylinder begins to increase.

When the pressure of air inside the cylinder increases above the atmospheric pressure, the inlet valve gets closed.

Further movement of piston towards TDC causes compression of air sucked during suction stroke.

Due to this, pressure of air inside the cylinder goes on increasing.

As soon as, the pressure reaches upto desired discharge pressure, the delivery valve gets opened and compressed air is delivered into the receiver.

Now again the piston starts moving from TDC to BDC, and the cycle is repeated again and again.

Computation of work done

Expression for work done on air in a reciprocating compressor without clearance.

A P-V diagram of a reciprocating compressor without clearance has been shown in fig.

Process 0-1 Constant pressure suction

Process 1-2 Polytrophic compression of air

Process 2-3 Constant pressure delivery.

Fig. P-V diagram of a reciprocating compressor without clearance

Work transfer will be given by area under the curve on P-V diagram.

Work transfer per cycle = ---- (1)

For process 1-2

PVn = C

Vn = C/P

V = where K = C1/n

:. Put in eqn (1)

Work transfer per cycle = -

= - K [ P-1/(n + 1) / -1/n + 1 ]P2P1

= - K * [ P2-1/(n + 1) – P1-1/(n + 1) ]

= [ V2P21/n. P2-1/n. P2 – V1P11/n. P1-1/n. P1]

= [ P2V2 – P1V1]

Hence,

Work done on the airper cycle= [ P2V2 – P1V1]

= P1V1[]

= P1V1[ ()-1/n– 1. . . () = ()-1/n

=P1V1[- 1]

= mRT1[- 1]

Expression for work done on air in a reciprocating compressor with clearance.

The P-V diagram of reciprocating compressor with clearance is shown fig.

Fig. P-V diagram of reciprocating compressor with clearance

Work transfer = area (a – 1 – 2 – b - a) – area (a – 4 – 3 – b - a)

per cycle

= { x P1V1 x [ ()(n-1)/n - 1]} – { x P4V4 x [ ()(n-1)/n - 1]}

= { P1V1[ ()(n-1)/n - 1]} – { P1V4 [ ()(n-1)/n - 1]}

= P1 (V1 – V4) [ ()(n-1)/n - 1]

= P1Va [ ()(n-1)/n - 1]

= maRT [ ()(n-1)/n - 1]

6.4 Volumetric efficiency:

It is defined as the ratio of actual volume of air sucked in per cycle to the swept volume per cycle.

ɳv =

= 1-

= 1- ……… k = clearance ratio.

=1- k[()1/n -1] () = ()1/n

6.5 Isothermal efficiency:

It is defined as ratio of isothermal power to the indicated power. It is also called as compressor efficiency.

Isothermal efficiency =

ɳiso = ..Single stage compressor

ɳiso = .. Doublestage compressor

6.6 Effect of clearance volume:

1). If clearance volume is large, it will delay the opening of inlet valve. Therefore, amount of air sucked per stroke will be reduced.

2). Due to clearance volume, the actual volume of air sucked per stroke is less than swept volume. Therefore, volumetric efficiency decreases.

3). More power is required to drive the compressor for same pressure ratio, due to increase in volume to be handled.

4). High compression pressure value is limited due to clearance volume & we obtain low compression ratio.

6.7 Free air delivery (FAD):

It is the actual volume of air delivered by compressor, when reduced to normal temperature & pressure condition.

The capacity of compressor generally expressed in terms of free air delivery.

It SI unit is m3/min.

6.8 Actual indicator diagram for air compressor:

During suction stroke, the charge of air is drawn along 4-1 at constant pressure P1, which is slightly below Patm.

At point 1 piston completes suction stroke and starts compression stroke.

During starting both the valves being closed air will be compressed along 1-2. At point 2 pressure P2 is reached which is slightly higher than the pressure in the receiver.

The delivery valve at this point opens and the air is delivered along 2-3 into the receiver.

The network input required for compression and delivery of air per cycle is given by the area 1-2-3-4 on the P-V diagram.

Theoretical Indicator Diagram Actual Indicator Diagram

In actual air compressor, some air is present in the clearance volume. This air expands during suction stroke.

At point 4, the inlet valve will not open in actual practice because of valve inertia and pressure differential required to open the valve.

Thus, the pressure drops away until the valve is forced to open. Some valve bounce will take place and slowly intake will become nearly steady.

This negative pressure is known as Intake depression.

A similar situation occurs at point 2 i.e., at point 2 pressure has to increase otherwise valve will not open.

Actual work required to compress and deliver the air will be greater than theoretical work.

6.9 Multi staging of compressor:

Fig. Multi staging of compressor

Fig shows schematic arrangement of two stage reciprocating air compressor with water-cooled intercooler.

It consists of a low-pressure cylinder & a high pressure cylinder.

Fresh air is sucked from the atmosphere in low pressure cylinder during suction stroke at intake pressure P1& temperature T1.

After compression in low pressure cylinder air is delivered to the intercooler at pressure P2& temperature T2.

In intercooler air is cooled to temperature T3 at constant pressure P2.

After intercooling air is admitted to high pressure cylinder, where it is compressed to pressure P3& temperature T3.

Computation of work done of Multi- stage compressor:

Work required for two stage reciprocating air compressor with inter cooler.

Work done in L.P. Cylinder

W1 = P1V1 [ ()(n-1)/n - 1 ]

Work done in H.P. cylinder

W2 = P2V2 [ ()(n-1)/n - 1 ]

Total work done,

W = W1 + W2

= { P1V1 [ ()(n-1)/n - 1 ] } + { P2V2 [ ()(n-1)/n - 1] }

= { P1V1 [ ( )(n-1)/n - 1 ] + P2V2 [ ( )(n-1)/n - 1 ] }

When intercooling is complete,

P1V1 = P2V2

W = . P1V1 {( )(n-1)/n + ( )(n-1)/n - 2 }

6.10 Optimum intermediate pressure:

Consider a two-stage compressor, with intercooler incorporated in between.

When intercooling is complete

Work required by compressor,

W = P1V1 [( )(n-1)/n + ( )(n-1)/n– 2]……….. (1)

Least value of intermediate pressure P2 may be obtained by differentiating the above equation with respect to P2.

Value of P2 obtained denotes the pressure of the intercooler at which work required to drive the compressor is minimum.

Thus work required is minimum when

= 0

[ . P1V1 [ ( )(n-1)/n + ( )(n-1)/n - 2] ] = 0

put = a (constant)

[ aP1V1 [ (P2/P1)a + (P3/P2)a - 2 ] ] = 0

aP1V1 [ (1/P1)aa. P2a-1 + P3 (-a)( P2 )-a-1 ] = 0

aP1-a P2a-1 – aP3a P2-a-1 = 0

a(P1)-a (P2)a-1 = a P3a . P2-a-1

P2a-1 . P2a+1 = P3a . P1a

P22a = (P3P1)a

P22 = P3P1

P2 =

P2/P1 = P3/P2 = (P3/P1)1/2

Put this value in eqn (1)

W = P1V1 [ ( )(n-1)/n + ( )(n-1)/n - 2]

= P1V1 [ ()(n-1)/n - 1 ]

= P1V1 [ ()(n-1)/2n - 1 ]

6.11 Intercooler

Types of Inter-cooling of air in a two-stage reciprocating air compressor.

a). Complete Inter-cooling

When the temperature of air leaving the intercooler(T3) is equal to the temperature of original atmospheric air(T1), then the inter-cooling is said to be complete or perfect inter-cooling.

In the process 2-3, the intercooling is carried out at constant pressure (P2 = P3).

In this case, point 3 lies on the isothermal curve as shown in Fig.

The compression process in two stages is carried out as 1-2-3-4 from P1 to P2 and then P3, where P2 is intermediate pressure.

The path 1-2-5 shows single stage polytrophic compression process.

Area under the curve 1-2-3-4 indicates work required for two stage compression with perfect or complete inter-cooling.

Area under the curve 1-2-5 indicates the work required for single stage compression.

It is clear that work required during two stage compression is less than single stage compression.

The shaded area 2-3-4-5 shows the work saved due to perfect inter-cooling.

b). Incomplete or Imperfect Inter-cooling:

When the temperature of air leaving the inter-cooler (T3) is more than the original atmospheric air temperature (T1), then the inter-cooling is said to be incomplete or imperfect inter-cooling.

In this case, point 3 lies on the right-hand side of isothermal curve

(PV = C) as shown in Fig.

The shaded area 2-3-4-5 shows the work saved due to incomplete inter-cooling.

Also, it is very clear from Fig, and that, amount of work saved due to perfect inter-cooling is more than work saved due to imperfect inter-cooling.

6.12 After cooling

After-cooling is the process of removing heat from compressed air.

This is done by use of heat exchangers known as ‘After-coolers’.

After coolers remove the moisture or water vapour present in the compressed air by cooling the air below its dew point temperature, causing the water vapour to condense into liquid form.

After-coolers control the amount of water vapour in the compressed air.

In a distribution system or a process manufacturing system, liquid water can cause significant damage to the equipment, which uses compressed air.

Thus, after-cooler is necessary to ensure the proper functionally of pneumatic or air handling devices.

After-coolers can use either water cooled or air-cooled mechanism.

Functions of After-cooler:

a). To cool the discharged air leaving the compressor.

b). To reduce the risk of fire. (For example: hot compressed air pipes can be a source of ignition).

c). To reduce moisture level or amount of water vapour in the compressed air.

d). To protect the downstream equipment’s (i.e., equipment’s installed after compressor in a process plant) from excessive heat.

6.13 Capacity Control

There are two main reasons why compressor capacity regulation is used.

The most important reason is to adjust the suction flow to match the process demand.

Second reason is to save energy.

Capacity control is determined by the compressor discharge pressure.

Following methods are used to control the capacity of compressor.

a). By pass control: -

This control method uses external bypass around the compressor to recycle gas from the compressor discharge to the inlet, or to the atmosphere in the case of an air compressor.

This method is simple, smooth & less costly as compared to other methods. It is however inefficient because excess compressor capacity is expanded across the control valve.

b). Inlet Valve Unloaders:

Valve unloader are mechanisms that are held open or bypass one or more cylinders inlet valves at each end of double acting cylinders.

c). Clearance pocket: -

Capacity can be reduced by increasing the cylinders clearance volume, this is done by clearance pocket. There are two types of pockets fixed & variable.

Opening the pocket reduces the cylinders inlet volumetric flow by trapping additional gas in the larger clearance volume at the end of the stroke.

d). Stepless Capacity Control: -

In this system an unloading device is fitted to each suction valve.

At partial loads, the unloading device does not allow the inlet valve to close when the piston is at its BDC.

e). Variable Speed drive: -

When variable speed drivers are used, all equipment should be designed to run safely throughout the operating speed range upto& including the trip speed.

6.14 Rotary Compressors:

In rotary air compressor, the air is entrapped between two sets of engaging surfaces and the pressure of air is increased either by reducing the volume or using back flow of the air.

Some compressors involve both methods. For eg, Vane type rotary compressor.

Whenever large quantities of air or gas are required at relatively low pressure, rotary compressors are employed.

6.15 Roots blower:

Working Principle:

Here, the air is compressed irreversibly using backflow of air at constant volume.

Construction:

Lobe type or Roots blower type of rotary compressor consists of two rotor lobes rotating in an air-tight casing, which has inlet and outlet ports.

The lobes are so designed that, they provide an air-tight sealing at the point of their contact.

Fig. Lobe type or Roots blower Type Compressor

Working:

Mechanical energy is provided to one of the rotors from external source (an electric motor) and the second rotor is gear driven from the first. Thus, both rotors rotate at the same speed.

The rotation of rotors creates space in the casing at the entry port.

The air is drawn into the casing to fill the space.

With further movement of lobed rotor, the air is trapped between casing and one rotor.

At this condition, we can observe that tip of rotor touches the casing.

This part of blower is not open to suction port.

But, the air flows into the space created by rotation of the other rotor.

The second rotor also carries out the same cycle as the first rotor after 900.

The trapped volume of air is not internally compressed, it is only displaced at high speed from suction side to delivery side.

The continued rotation of lobes opens the discharge port.

The compressed air at higher pressure is present at the delivery side.

Therefore, when the rotor lobe uncovers the discharge port, some pressurised air enters into the space between the rotor and casing of the compressor.

This flow of air is called as backflow of air.

This backflow of air continues until the pressure in the blower gets equalized.

After the backflow, the air is compressed irreversibly at constant volume.

Finally, at higher pressure, the air is delivered from the blower to receiver.

Applications of Lobe type or Roots Blower Compressor:

a). For scavenging and supercharging of two strokes I.C. engines.

b). For low pressure supply of air to steel furnaces, sewage disposal plants, low pressure gas boosters, etc.

6.16 Vane type Rotary Compressor: -

Working Principle:

The air is compressed first by decreasing the volume and then by using backflow of air.

Construction:

A vane type rotary compressor consists of a rotor rotating eccentrically in an air-tight casing with inlet and outlet ports.

The rotor has a number of slots containing vanes, which are spring loaded.

The disc rotates in anticlockwise direction. During this rotation, the vane’s remain in contact with the casing, due to spring forces.

Working:

When the rotor rotates, the vanes are pressed against the casing, due to centrifugal force and form air– tight pockets.

The mechanical energy is provided to the rotor from some external source like electric motor.

As the rotor rotates, the air is trapped in the pockets formed between the vanes and casing.

As the rotor further rotates, compression takes place due to decrease in volume provided for the trapped air.

Further compression is obtained by backflow of air from the receiver, i.e. when the rotating vane uncovers the exit port and compressed air enters into the receiver, then at the same time, some air flows back into the pocket.

Thus, the pressure of air, entrapped in the pocket, is increased first by decreasing the volume and then by back flow of air as shown in Fig.

Now, the compressed air is delivered to the receiver by the rotation of the vanes. Finally, the air at a high pressure gets stored inside the receiver.

Fig. Vane type rotary compressor

Applications of Vane type compressor:

For super charging of I.C. Engines.

For construction purpose due to ease of portability i.e., easily portable.

6.17 Screw compressor and Scroll compressor:

Screw Compressor:

Construction:

Screw compressor consists of two mutually engaged helically grooved rotors suitable housed in a casing.

The male rotor consists of four helical lobes and is normally the driven rotor.

The female rotor has six flutes or gullies and is the driven rotor.

The four helical lobes of male rotor are engaged in corresponding gullies (flutes) of female rotor.

Screw compressor is directly coupled to the prime mover (electric motor) and require low starting torque.

Fig. Screw compressor

Working:

When the driving rotor rotates, an inter-lobe space between a pair of lobes of male-female rotors and the housing nearest to suction end gets opened and is filled with air.

Now, the air so trapped is moved both axially and radially with the rotation of rotors and gets compressed due to reduction in volume, until it reaches the discharge end.

This compressed air is finally delivered through discharge port.

As the number of lobes of male rotor and number of flutes of female rotor are different, therefore the male rotor and female rotor rotate at different speed.

A four-lobe male rotor will drive six gully (flutes) female rotor at two third of its speed.

Advantages of Screw Compressor: -

Continuous and uniform flow of air.

High volumetric efficiency.

Less power consumption even at part load operation.

Can be directly coupled to prime mover like electric motor.

Low maintenance cost due to a smaller number of moving parts.

Scroll Compressor:

A scroll compressor is a device for compressing air or refrigerant. In rolling stocks, scroll compressor is used for HVAC and brake systems.

The orbiting scroll is coupled to the crankshaft and orbits, rather than rotates. The orbiting motion creates a series of gas pockets traveling between the two scrolls. On the outer portion of the scrolls, the pockets draw in gas, and then move into the center of the scroll, where the gas is discharged. As the gas moves into the increasingly smaller inner pockets, the temperature and pressure increase to the desired discharge pressure.

Advantages:

Very quiet.

It is very small

Simple design, not so many parts

Low maintenance (hardly any)

Oil-free design

Disadvantages:

Low capacity (flow, liters/minute or cfpm).

Relatively expensive

When the compressor-element fails, there’s a very big chance you just have to buy a whole new element.

The compressed air gets very hot! Much hotter than compared to other types of compressors.

Reference:

1) V. Ganesan: Internal Combustion Engines, Tata McGraw-Hill

2) M.L. Mathur and R.P. Sharma: A course in Internal combustion engines, Dhanpat Rai

3) H.N. Gupta, Fundamentals of Internal Combustion Engines, PHI Learning Pvt. Ltd

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