Thermal Engineering-II

Banoj Kumar Behera

4^th Semester

Mechanical Engineering

Topic: Air Compressor

Air Compressors

COMPRESSOR – A device which takes a definite quantity of fluid ( usually gas, and most often air ) and deliver it at a required pressure.

Air Compressor – 1) Takes in atmospheric air,

2) Compresses it, and

3) Delivers it to a storage vessel ( i.e. Reservoir ).

How they are different from pumps?

• Major difference is that compressors handles the gases and pumps handles the liquids.

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• As gases are compressible, the compressor also reduces the volume of gas.

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• Liquids are relatively incompressible; while some can be compressed

Applications

Compressors have many everyday uses, such as in :

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• Air conditioners, (car, home)

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• Pneumatic devices

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• Home and industrial refrigeration

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• Hydraulic compressors for industrial machines

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• Air compressors for industrial manufacturing

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Classification of Compressor

Compressor classification can be described by following flow chart:

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Centrifugal Compressors

• Achieves compression by applying inertial forces to the gas by means of rotating impellers.

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• It is multiple stage ; each stage consists of an impeller as the rotating element and the stationary element, i.e. diffuser

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• Fluid flow enters the impeller axially and discharged radially

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• The gas next flows through a circular chamber (diffuser), where it loses velocity and increases pressure.

Positive displacement Compressor

Air Compressors

Single – acting

Double - Acting

Rotary Compressor

• The gas is compressed by the rotating action of a roller inside a cylinder.

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• The roller rotates off-centre around a shaft so that part of the roller is always in contact with the cylinder. 

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• Volume of the gas occupies is reduced and the refrigerant is compressed.

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• High efficient as sucking and compressing refrigerant occur simultaneously.

Reciprocating Compressor

It is a positive-displacement compressor that

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• Uses pistons driven by a crankshaft to deliver gases at high pressure.

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• The intake gas enters the suction manifold, then flows into the compression cylinder

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• It gets compressed by a piston driven in a reciprocating motion via a crankshaft,

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• Discharged at higher pressure

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Reciprocating Compressor - Detailed Analysis

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Principle of Operation

• Fig. shows single-acting piston actions in the cylinder of a reciprocating compressor.
• The piston is driven by a crank shaft via a connecting rod.
• At the top of the cylinder are a suction valve and a discharge valve.
• A reciprocating compressor usually has two, three, four, or six cylinders in it.

Reciprocating Compressor - Working

Reciprocating Compressor – Equation for Work

Operations : 4 – 1 : Volume V₁ of air aspirated into Compressor, at P₁ and T_1.

1 – 2 : Air compressed according to PVⁿ = Const. from P₁ to P₂.

→ Temp increase from T₁ to T₂.

2 – 3 : Compressed air at P₂ and V₂ with temperature T₂ is delivered.

Reciprocating Compressor – Equation for Work

During Compression, due to the excess temperature above surrounding, the air will exchange the heat to the surrounding.

As Compressor is a work consuming device, every effort is desired to reduce the work.

Work done = Area under P-V curve

Reciprocating Compressor – Equation for Work

Thus, comparison between the Isothermal Work and the Actual Work is important.

Thus, more the Isothermal Efficiency, more the actual compression approaches to the Isothermal Compression.

Actual Work = W_act = Area 4-1-2-3-4

W_act = Area (4-1) – Area (1-2) – Area (2-3)

Reciprocating Compressor – Equation for Work

Now,

Reciprocating Compressor – Equation for Work

The solution of this equation is always negative.

This shows that Work is done ON the Compressor.

Reciprocating Compressor – Equation for Work

Clearance Volume :

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Volume that remains inside the cylinder after the piston reaches the end of its inward stroke.

Thus, Effective Stroke Volume = V₁ – V₄

Actual Work = W_act = Area 1-2-3-4

W_act = Area (5-1-2-6) – Area (5-4-3-6)

Reciprocating Compressor – Equation for Work

But, P₄ = P₁ and P₃ = P₂

Reciprocating Compressor – Volumetric Efficiency

Volumetric Efficiency :

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Ratio of free air delivered to the displacement of the compressor.

Ratio of Effective Swept Volume to Swept Volume.

Presence of Clearance Volume

Volumetric Efficiency less than 1. ( 60 – 85 % )

( 4 – 10 % )

Reciprocating Compressor – Volumetric Efficiency

Reciprocating Compressor – Volumetric Efficiency

Reciprocating Compressor – Actual P-V Diagram

1-2-3-4-1 : Theoretical P-V Diagram.

At 4, inlet valve does not open due to :

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1. There must be a pressure difference across the valve to open.
2. Inlet valve inertia.

Pr. Drop continues till sufficient level for valve to force its seat.

Some valve bounce is set (wavy line).

Eventually, the pressure sets down at a level lower than atmospheric pressure. This negative pressure difference is known as Intake Depression.

Similar situation appears at 2, i.e. at the start of the delivery.

Pressure rise, followed by valve bounce and then pressure settles at a level higher than the delivery pressure level.

Air delivery to a tank / receiver, hence, generally known as Receiver Pressure.

Reciprocating Compressor – F.A.D.

Free Air Delivery (F.A.D.) : If the volume of the air compressor is reduced to atmospheric temperature and pressure, this volume of air is called FAD (m³/min)

Delivered mass of air = intake mass of air

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If clearance volume is neglected

Where

Reciprocating Compressor – Multistage

High Pressure required by Single – Stage :

This demands for MULTI – STAGING…!!

Why multistage compressor?

• High temp rise leads into limitation for the maximum achievable pressure rise.

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• Discharge temperature shall not exceed 150ºC and should not exceed 135⁰C for hydrogen rich services

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• A multistage compressor compresses air to the required pressure in multiple stages.

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• Intercoolers are used in between each stage to removes heat and decrease the temperature of gas so that gas could be compressed to higher pressure without much rise in temperature

Reciprocating Compressor – Multistage

Series arrangement of cylinders, in which the compressed air from earlier cylinder (i.e. discharge) becomes the intake air for the next cylinder (i.e. inlet).

Reciprocating Compressor – Multistage

Overall Pr. Range : P₁ – P₃

Single – stage cycle : 8-1-5-6

Without Intercooling :

L.P. : 8-1-4-7

H.P. : 7-4-5-6

With Intercooling :

L.P. : 8-1-4-7

H.P. : 7-2-3-6

Perfect Intercooling : After initial compression in L.P. cylinder, air is cooled in the

Intercooler to its original temperature, before entering H.P. cylinder

i.e. T₂ = T₁ OR

Points 1 and 2 are on SAME Isothermal line.

Reciprocating Compressor – Multistage

Ideal Conditions for Multi – Stage Compressors :

Single – stage cycle : 8-1-5-6

Reciprocating Compressor – Multistage

Without Intercooling :

L.P. : 8-1-4-7

H.P. : 7-4-5-6

Reciprocating Compressor – Multistage

With Intercooling :

L.P. : 8-1-4-7-8

H.P. : 7-2-3-6-7

Reciprocating Compressor – Multistage

Now, T₂ = T₁

P₂V₂ = P₁V₁

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Also P₄ = P₂

Shaded Area 2-4-5-3-2 : Work Saving due to Intercooler…!!

Reciprocating Compressor – Multistage

Intermediate Pr. P₂ → P₁ : Area 2-4-5-3-2 → 0

Intermediate Pr. P₂ → P₃ : Area 2-4-5-3-2 → 0

Reciprocating Compressor – Multistage

Condition for Min. Work :

Reciprocating Compressor – Multistage

P₂ obtained with this condition (Pr. Ratio per stage is equal) is the Ideal Intermediate Pr. Which, with Perfect Intercooling, gives Minimum Work, W_min.

Reciprocating Compressor – Efficiency

Isothermal work done / cycle = Area of P – V Diagram

= P₁V₁ log_e(P₂/P₁)

Indicated Power : Power obtained from the actual indicator card taken during a

test on the compressor.

NOTE : Shaft Power = Brake Power required to drive the Compressor.

Reciprocating Compressor – Efficiency

Adiabatic Efficiency : Ratio of Power required to drive the Compressor; compared

with the area of the hypothetical Indicator Diagram; assuming

Adiabatic Compression.

Mechanical Efficiency : Ratio of mechanical output to mechanical input.

Reciprocating Compressor – Efficiency

How to Increase Isothermal Efficiency ?

1. Spray Injection : Assimilation of water into the compressor cylinder towards the

compression stroke.

Object is to cool the air for next operation.

Demerits : 1. Requires special gear for injection.

2. Injected water interferes with the cylinder lubrication.

3. Damage to cylinder walls and valves.

4. Water must be separated before delivery of air.

2. Water Jacketing : Circulating water around the cylinder to help for cooling the

air during compression.

Reciprocating Compressor – Efficiency

How to Increase Isothermal Efficiency ?

3. Inter – Cooling : For high speed and high Pr. Ratio compressors.

Compressed air from earlier stage is cooled to its original

temperature before passing it to the next stage.

D. External Fins : For small capacity compressors, fins on external surfaces are useful.

5. Cylinder Proportions : Short stroke and large bore provides much greater surface

for cooling.

Cylinder head surface is far more effective than barrel surface.

Reciprocating Compressor – Efficiency

Clearance Volume : Consists of two spaces.

1. Space between cylinder end & the piston to allow for wear.

2. Space for reception of valves.

High – class H.P. compressors : Clearance Vol. = 3 % of Swept Vol.

: Lead (Pb) fuse wire used to measure the gap between

cylinder end and piston.

Low – grade L.P. compressors : Clearance Vol. = 6 % of Swept Vol.

: Flattened ball of putty used to measure the gap

between cylinder end and piston.

Reciprocating Compressor – Work Done

Assumption : Compression and Expansion follow same Law.

Work / cycle = Area 1-2-3-4-1

P₃ = P₂ and P₄ = P₁

Reciprocating Compressor – Work Done

How to select a particular type of compressor ?

Graph showing operating regions of various compressors

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Table showing operating conditions of various compressors

Advantages and Disadvantages of Dynamic compressors

Advantages and Disadvantages of Positive displacement compressors

Thank You