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UNIT - III

Chapter – 5

Thermal Engineering 2:

Thermal Systems Applications

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�Thermal Systems Applications �

Refrigeration system
Air conditioning system
Pumps
Compressors, and
Blowers

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Session – 2

Contents:

Pumps – Introduction, Classification
Working principle of Pumps
Compressors - Introduction
Major components of Compressors
Classification of Compressors
Working principle of Compressors
Blowers - Introduction, Classification

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Hydraulic Pumps

Definition:

Water pumps are devices designed to convert mechanical energy to hydraulic energy.

They are used to move water from lower points to higher points with a required discharge and pressure head.

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BME _ Thermal Systems Applications _ Session 2

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The purpose of a pump is to add energy to a fluid, resulting in an increase in fluid pressure, not necessarily an increase of fluid speed across the pump.

The purpose of a turbine is to extract energy from a fluid, resulting in a decrease of fluid pressure, not necessarily a decrease of fluid speed across the turbine.

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Comparison

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Description	Fan	Blower	Compressor
Type	Gas pump	Gas pump	Gas pump
Pressure of air/gas	Low	Moderate to high	Very high
Flow rate	High	Moderate to high	Low to moderate
Applications	Ceiling fans, Propellers	Ventilation system, Furnaces	Pneumatic tools, Refrigerators, Air-conditioners.

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Classification of Pumps

Pumps are broadly classified as follows.

Dynamic or Non-positive displacement pump

Primarily used for transporting fluid
Low pressure

(2) Positive displacement pump

Primarily used for fluid power application
Fixed amount of discharge
Higher pressure

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Dynamic or Non-positive displacement pump

No Fixed discharge per revolution.
Large clearance between blades and casing.
Output will be reduced as resistance increases because fluid slips back into clearance space i.e discharge depends upon system pressure.
Not self priming, due to clearance - no suction.
Pressure relief valve not required.
Used for high volume flow rate and low pressure applications.

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Positive displacement pumps

Fixed discharge per revolution.
Least clearance between blades and casing.
Discharge does not depends upon system pressure.
Pressure relief valve required.
Used for low volume flow rate and any pressure range applications.

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Detailed classification of Pumps

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Non-positive displacement pumps	Positive displacement pumps
Centrifugal pump	1. Gear Pumps: External, Internal, Lobe, Screw & Gerotor pumps
Axial pump
	2. Vane pumps: Balanced and unbalanced pumps

	3. Piston pumps: Axial and Radial pumps

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Centrifugal Pump

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Centrifugal Pump

Impeller:

It is the rotating part of the centrifugal pump.

It consists of a series of backwards curved vanes (blades).

The impeller is driven by a shaft which is connected to the shaft of an electric motor.

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Centrifugal Pump

Casing:

It is an air-tight passage surrounding the impeller .

Designed to direct the liquid to the impeller and lead it away.

Volute casing: It is of spiral type in which the area of the flow increases gradually.

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Centrifugal Pump

Suction Pipe:

Delivery Pipe:

Shaft: which is the bar by which the power is transmitted from the motor drive to the impeller.

Driving motor:

which is responsible for rotating the shaft.
It can be mounted directly on the pump, above it, or adjacent to it.

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Centrifugal Pump

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Centrifugal Pump

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Centrifugal Pump

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Reciprocating Pump

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Reciprocating Pump (Single-acting)

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Reciprocating Pump (Single-acting)

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Reciprocating Pump (Double-acting)

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External Gear Pump

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External Gear Pump

Two mating gears, run in a pump casing.
Only one gear is driven by the prime mover.
Meshing gears 🡪 Form a seal between inlet & outlet.
Pressure of Oil in sump 🡪 Atm. Pressure (as it is vented to atmosphere)

When the gears rotate,

Suction side volume expands
Partial vacuum is created, so pressure decreases
Fluid is drawn in through inlet
Fluid is forced out from outlet at high pressure

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External Gear Pump

Gears Used:

Spur Gears 🡪 Simplest

🡪 Common type

🡪 Noisy at high pump speeds

(2) Helical Gears 🡪 Less noise,

🡪 Experiences high end thrust,

Hence are used for low pressure applications (10 – 20 Bar)

(3) Herringbone Gears 🡪 Eliminates end thrust

🡪 Less noisy,

🡪 Used for high pressure applications.

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Internal Gear Pump

Consists of,

A ring gear (with internal teeth)
A pinion gear (with external teeth)

Both gears are mounted eccentrically in a pump casing.

A stationary crescent, acts as a seal between the inlet & outlet parts.

A motor drives the pinion 🡪 In turn, pinion drives the ring gear.

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Internal Gear Pump

Advantages:

Similar to external gear pumps in many respects

Quieter as gear slap is reduced

Disadvantages:

Somewhat more difficult to manufacture

Same issues of unbalanced forces

Fixed displacement

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Lobe Pump

Similar to external gear pump.
Two gears are replaced by lobes.
Both lobes are driven externally.
They do not engage physically with each other.

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Vane Pumps

Disadvantages of gear pumps:

High leakage rates through the gap between teeth & casing.
Hence, results in low volumetric efficiency.

Vane pumps 🡪 Minimizes leakage

🡪 Improves volumetric efficiency

Classification of Vane pumps:

Unbalanced vane pump
Balanced vane pump
Variable displacement pump
Pressure compensated vane pump

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Compressors��

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Compressors

A Compressor is a mechanical device, that compresses the gases.
Compressor increases the pressure of a gas by reducing its volume.

Difference between a pump and a Compressor:

Major difference is that compressors handles the gases and pumps handles the liquids.
As gases are compressible, the compressor reduces the volume of gas.
Liquids are relatively incompressible; while some can be compressed.

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Compressors

Introduction:

It is a machine which takes in air or any other gas at low pressure, and

🡪 Compresses it to high pressure.

A compressor used for increasing the pressure of air is called as Air- compressor.

Compressors are power consuming machines in which,

🡪 Mechanical work is converted into the pressure head of air or gas.

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Compressors

Air Compressor :

Draws atmospheric air
Compresses it, and
Delivers it to a reservoir.

Generally, the compressors are driven by Prime movers

(Electric motors, IC Engines or Gas Turbines)

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Applications of Air Compressors

The main uses of compressed air are:

For inflating automobile tyres.
To clean workshop machines, generators etc.
To operate air operated drills, hammers (Pneumatic tools).
To inject fuel in the diesel engine cylinder.
In spray painting.
To operate air brakes in automobiles.
To operate compressed air engines/air motors in mines.

(In mines IC engines and electricity are not used because of fire risks. So high pressure air operated machines are used).

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Components of Air-Compressor system

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Major components
Intake Air Filter	Air-Dryers
Compressor Cooling	Pressure Regulators/Flow Controller
Inter-Stage Coolers	Lubrication system
After-Coolers	Air Receivers
Air Separators	Air Transport and Distribution System

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Components of Air-Compressor system

Intake Air Filter:

Ambient air enters the compressor via inlet air filters.
Filters particulate matter present in air.
Allows the entry of clean and dry air into the compressor.

Compressor Cooling:

A substantial amount of heat is generated during compressor process.
Hence, cooling is necessary to reduce the wear & damage to the parts.
This in turn, increases the compressor efficiency.
Compressors are normally cooled using air, water or oil (lubricant).

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Components of Air-Compressor system

Inter-Stage Coolers:

Inter coolers act as heat exchangers to lower the temperature of air before it enters into successive stages of compression.
They reduce the overall compression work and increase compressor efficiency.

After-Coolers:

These are employed at the end of compression process to lower the discharge temperature of air.
During this final stage of operation, as the temperature is lowered, moisture present in air condenses and separated from the discharge air.

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Components of Air-Compressor system

Air Separators:

They are installed either after an inter-cooler or after-cooler,
to dry the discharge air.

Air-Dryers:

High pressure air discharges into the air-dryer before storage in the reservoir.
Air that enters the dryer is saturated (contains moisture).
If the saturated air enters the piping, it can lead to corrosion and damage to the transport equipment.
Hence, air is dried in an air-dryer to ensure that moisture-free air reaches the reservoir.

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Components of Air-Compressor system

Pressure Regulators/Flow Controller:

They minimize pressure drops in the system by stabilizing system pressure.

Lubrication:

Lubrication is necessary for effective compressed air operation.

A lubricator is used to lubricate and cool the moving parts and prevent overheating.

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Components of Air-Compressor system

Air Receiver/Storage tank:

High pressure compressed air from the system is discharged to a storage tank.

Air Transport and Distribution System:

Final stage is, the proper distribution of compressed air to the demand points.
Distribution system includes piping, valves and hoses.
To minimize the pressure drop during distribution, the pipe length is kept minimum and diameter to maximum.

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Classification of Compressors

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Dynamic Compressors

It is a continuous flow compressor.
It is characterized by rotating impeller to add velocity and pressure to the fluid.
It is widely used in chemical and petroleum refinery industry for specific services.

Types of Dynamic Compressors:

Centrifugal Compressor
Axial flow Compressor

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Centrifugal Compressor

Major components:

Impeller
Diffuser
Volute / Collector
Casing

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Centrifugal Compressor

Impeller:

Impellers are mounted on a shaft.
This assembly is known as rotor.
It moves the gas in the compressor.
As the impeller rotates, gas moves towards outer rim of the impeller and velocity of the gas increases.

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Centrifugal Compressor

Diffuser:

As the gas leaves the impeller, it flows into a passage-way called the diffuser.

Diffuser being larger in volume, the velocity of the gas decreases and its pressure increases.

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Centrifugal Compressor

Volute / Collector:

From diffuser gas passes into the volute.

In the volute, conversion of velocity energy to pressure energy continues.

Casing:

It is the outer metallic cover of the compressor, which contains inlet and discharge nozzles.

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Working principle of Centrifugal Compressor

Achieves compression by applying inertial forces to the gas by means of rotating impellers.
It is a multiple stage compressor.
Each stage consists of an impeller as the rotating element and diffuser as the stationery element.
Fluid flow enters the impeller axially and discharges radially.
Gas next flows through a circular chamber (diffuser).
In diffuser, gas loses its velocity with increase in its pressure.

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Centrifugal Compressor

Classification based on number of stages:

Single stage Centrifugal compressor:
Consists of only one impeller.
Advantage of it over the multistage compressor is that,

🡪 it provides high efficiency and the delivered gas is totally oil free.

Multistage Centrifugal compressor:
Consists of 1-10 impellers.
Throughout each and every stage,

🡪 the temperature and the compression ratio are assumed to be constant.

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Centrifugal Compressor

Single stage Centrifugal compressor:

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A typical single-stage centrifugal compressor delivers large amounts of compressed air.

Centrifugal Compressor

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Centrifugal Compressor

Advantages:

Compared to other compressors, it is relatively easy to manufacture.
Highly energy efficient and reliable.
Consists of a small number of rubbing parts and are absolutely oil free in nature.
Generates a higher pressure ratio per stage than the axial flow compressor.

Disadvantages:

Produces a limited amount of pressure and are not suitable for very high compression.
Sensitive towards problems such as stalling and choking.

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Positive Displacement Compressors

These Compressors causes movement by trapping a fixed amount of air.
Then, they force (displacing) the trapped volume into discharge pipe.

Classification of Positive Displacement Compressors:

According to the mechanism used to move the air,

Reciprocating Compressor
Rotary Compressor

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�Reciprocating Compressor�

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Reciprocating Compressor

Introduction:

In this, a volume of gas is drawn into a cylinder.
Gas is trapped and compressed by piston.
Then the compressed gas with high pressure discharges through discharge lane.
Gas flow is controlled by the valves.

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Reciprocating Compressor

Parts:

Suction valve
Suction chamber
Piston & Piston rod
Cylinder
Discharge valve
Discharge chamber

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Reciprocating Compressor

Piston is driven by a crank shaft through a connecting rod.
Piston rings prevents leakage through piston and liner.
Suction & delivery valves are located at the top.
They open due to the effect of the differential pressure between cylinder and the suction or discharge chambers, and

🡪 Close due to the force of springs acting on the plates.

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Working of Reciprocating Compressor

Downward stroke:

As the piston travels from TDC to BDC, the volume of the cylinder increases.
Due to the pressure difference, the suction valve opens.
Gas / air enters into the cylinder through suction valve.
Volume of gas/air increases inside the cylinder.
Suction valve closes when the piston reaches BDC.

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Working of Reciprocating Compressor

Upward stroke:

As the crank shaft rotates, piston moves from BDC to TDC.
Gas/air gets compressed to high pressure, inside the cylinder.
Again due to pressure difference between delivery line and inside of the cylinder,
Delivery valve opens and allows the high pressure gas/air flow through discharge lane.
Cycle repeats.

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Reciprocating Compressor

Classification based on number of stages:

Single stage Reciprocating compressor:

Compression is done in single stage or

🡪 by single cylinder only.

Used to generate low pressure air.

Multistage Reciprocating compressor:

Produces high pressure air.
Used in heavy duty mechanical devices.

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A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders.

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Reciprocating Compressor

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A large Reciprocating compressor in gas service

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Rotary Compressors

These compressors are not of reciprocating nature, therefore does not have any pistons and crankshaft.

Instead, these compressors have screws, vanes, scrolls, and other devices which rotate and thus compress air.

Classification:

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1	Root Blower
2	Vane type
3	Screw type
4	Scroll type

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Screw type Rotary Compressor

In a casing , two large helical screws (rotors) rotate against each other.
Air is sucked inside the casing through an inlet port, located at one end of the screws.
Then, air flows between the two screws.
As air moves down the length of the screws, the air gap gets tighter, compressing the air towards the outlet.

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Screw type Rotary Compressor

Air gets squeezed and its pressure increases.
High pressure air goes out through delivery valve, located at the other end.

Advatages:

Quiet in operation compared to reciprocating compressor.

Produce large volumes of air, with great consistency.

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Screw type Rotary Compressor

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�Applications of Compressors�

Compressors have many everyday uses, such as in:

Air conditioners (car, home)
Home and Industrial refrigeration
Hydraulic compressors for industrial machines
Air compressors for industrial manufacturing

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Blowers��

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Blowers

It is a machine used for generating flow of air at substantial pressure.

Blower increases the velocity of air or gas when it is passed through equipped impellers.

The air flow generated is used for different purposes.

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Blowers

Blowers are mainly used for flow of air/gas required for:

Exhausting,
Aspirating,
Cooling,
Ventilating,
Conveying etc.

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Blowers

In a blower, the inlet pressure is low and is higher at the outlet.
The kinetic energy of the blades increases the pressure of the air at the outlet.
Blowers are mainly used in industries for moderate pressure requirements where

🡪 the pressure is more than the fan and less than the compressor.

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Blowers

Types of Blowers:

Centrifugal blower
Positive displacement blower

Blowers use blades in various designs such as backward curved, forward curved and radial.
They are driven by an electric motor.
They can be single or multistage units and use high speed impellers to create velocity to air/gas.

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Positive displacement Air Blower

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Blowers

Applications:

Blowers, in other words vacuum pumps are used in,

Industrial dust extraction,
Bottle filling machines,
Automatic filling machines,
Paper cutting industry,
Printing paper transportation process,
Car washing,
Vegetable/ fruit washing

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