bme notes (module 3 & 4)
DESCRIPTION
Class notes for Basic Mechanical Engineering, KTU, first year.TRANSCRIPT
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BASICS OF MECHANICAL ENGINEERING KAILAS S C
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MODULE III
REFRIGERATION AND AIR CONDITIONING
VAPOUR COMPRESSION SYSTEM
A simple Vapour Compression system of refrigeration consists of the following basic
components: Compressor, Condenser, Expansion valve and Evaporator.
The function of the compressor is to compress the refrigerant to high pressure. At the time of
compression, the condition of refrigerant is dry vapour. This vapour enters the condenser
where cooling water is circulated. Heat from the refrigerant is removed and it becomes liquid.
This high pressure liquid refrigerant is expanded suddenly when it flows through the
expansion value. The pressure is decreased at this time and some portion of the refrigerant
becomes vapour. The condition of the refrigerant coming out of the expansion valve is wet
vapour. This enters the evaporator and heat is absorbed by the refrigerant and it becomes
dry vapour. This dry vapour enters the compressor and cycle continues.
COP = Heat extracted from the evaporator
Work done by the compressor
VAPOUR ABSORPTION SYSTEM
The working of Vapour Absorption system is similar to vapour compression system. The
difference lies in the compression of refrigerant. In vapour compression, a compressor is used
to compress the dry refrigerant to high pressure whereas in vapour absorption, the
compressor is replaced by a combination of three devices; An absorber, a pump and a
generator.
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The dry vapour refrigerant from the Evaporator enters the absorber and is dissolved in cold
water. This solution of water and refrigerant is called strong solution. This strong solution is
pumped into generator using a liquid pump. In the generator, this strong solution is heated
using heating coils. As a result of heating, the refrigerant vapor is driven out of the solution
as a high pressure vapour. The remaining water in the generator flows back to the absorber.
From the generator, the refrigerant vapour flows to the condenser where cooling water is
circulated. Heat from the refrigerant is removed and it becomes liquid. This high pressure
liquid refrigerant is expanded suddenly when it flows through the expansion value. Thepressure is decreased at this time and some portion of the refrigerant becomes vapour. The
condition of the refrigerant coming out of the expansion valve is wet vapour. This enters the
evaporator and heat is absorbed by the refrigerant and it becomes dry vapour. This dry vapour
enters the absorber and cycle continues.
COP = Heat extracted in the evaporator
Work done by pump + Heat supplied to the generator
DOMESTIC REFRIGERATOR
The domestic refrigerator works under the principle of vapour compression system. It consists
of the following four basic components:
Evaporator: It is the equipment in which liquid refrigerant evaporates. In a typical
refrigerator, it is located in the freezer compartment of the refrigerator. The refrigerant flows
through the tubes surrounding the evaporator absorbs heat and becomes dry vapour.
Compressor: The function of the compressor is to compress and increase the pressure of dry
vapour refrigerant coming out of the evaporator.
Condenser: The equipment used to convert the refrigerant vapour to liquid is calledcondenser. It is usually a wire and tube mounted at the back of the refrigerator. The heat
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absorbed by the refrigerant while passing through the evaporator is released in the
condenser.
Capillary Tube (Expansion device): The liquid refrigerant leaving the condenser enters the
capillary tube. It is a pressure reducing device. When liquid refrigerant flows through the
capillary tube, pressure is reduced suddenly and as a result some part of the refrigerant
converts to liquid. This wet vapour coming out of the capillary tube is fed into the evaporator
and the cycle is repeated.
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MODULE IV
MULTI POINT FUEL INJECTION (MPFI)
MPFI is a new technology used in Petrol engines. In ordinary petrol vehicles, a carburetor is
used to mix fuel and air in required ratio. This fuel-air mixture is fed to the engine cylindersthrough inlet valves. In MPFI engines, the use of carburetor is eliminated. The fuel is mixed
with fresh air just before reaching the inlet valve using a fuel injector. Fuel injectors are placed
near the inlet valves of each cylinders. Fuel in required quantity is injected using this injectors
before the air enters the engine cylinders. The quantity of fuel injected and the timing of
injection is controlled by an Engine Control Unit (ECU). It is a microprocessor used in modern
vehicles. Using different sensors, the information regarding the engine temperature, engine
speed, coolant temperature, inlet air temperature, exhaust air temperature, oxygen content
in the inlet air, speed and acceleration of the vehicle etc. are collected and this data is
processed by the ECU for deciding the quantity and timing of fuel injection.
Advantages:
• More power is produced
• Complete combustion of fuel
• Better Efficiency
• Better Mileage
• Less Pollution
COMMON RAIL DIRECTION INJECTION (CRDI)
CRDI is a new technology used in Diesel engines. In ordinary diesel engines, a fuel injecton pump is
used to distribute fuel to fuel injectors. In CRDI engines, a closed tube known as Common Rail is usedand fuel is maintained at a pressure of 200 MPa inside this rail using a high pressure pump. This rail
has a number of solenoid or piezoelectric valves which are controlled by ECU. From this valve, fuel at
high pressure is distributed to fuel injectors. The quantity of fuel and the timing of injection of fuel is
controlled using the solenoid values by the ECU. Using different sensors, the information regarding
the engine temperature, engine speed, coolant temperature, inlet air temperature, exhaust air
temperature, oxygen content in the inlet air, speed and acceleration of the vehicle etc. are collected
and this data is processed by the ECU for controlling the solenoid valve.
Advantages:
• More power is produced
•
Complete combustion of fuel
• Better Efficiency
• Better Mileage
• Less Pollution
• Less vibration of engine
TYPES OF CHAINS
ROLLER CHAIN
Roller chain is the most commonly used type of chain in chain drives. Compared to the other type,
Roller chains are less cost and maintenance is easy. But these type requires more lubrication and noisy
also.
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SILENT OR INVERTED TOOTH CHAIN
These types of chains are used in special equipment where more power has to be transmitted which
is not possible using ordinary chains. These chains are silent in operation but more costly.
SINGLE PLATE CLUTCH
Clutch is a device for connecting and disconnecting the engine power from reaching the gear
box. It is situated between the engine and the gear box. When the clutch is engaged, power
from the engine shaft reaches the gear box. When clutch is disengaged, the connection
between the engine shaft (driver shaft) and the gear box shaft (driven shaft) is separated.
Single plate clutch is a friction clutch, which transmits power by friction. There are mainly 3
components in a single plate clutch. A flywheel which is attached to the end of the driver
shaft, a pressure plate which is actuated with a spring and a clutch plate which has a friction
lining on both the sides. The clutch plate is free to slide over the driven shaft, in between the
flywheel and the pressure plate. When the clutch plate rotates, the driven shaft is alsorotated.
When the clutch engages, the spring in the pressure plate is expanded and the pressure plate
is moved towards the clutch plate. The pressure plate pushes the clutch plate to the flywheel
and due to the friction between the surface of the flywheel and clutch plate, the rotation of
the flywheel is transferred to the clutch plate and consequently the driven shaft starts
rotating.
When the clutch disengages, the spring in the pressure plate compresses and the pressure
plate is pulled back. This separates the flywheel and the clutch plate. And then the flywheelrotates without driving the clutch plate and the driven shaft.
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TYPES OF BRAKES
DRUM BRAKE
A Drum brake contains a brake shoe with a friction lining on their surface. It has lever and
spring mechanisms for actuating the brake. These components are housed in a casing calleddrum which is fixed to the wheel. When brake is applied, the brake shoe is expanded outwards
and it touches the inside surface of the brake drum. Due to friction, the rotation of drum is
reduced causing the vehicle to slow down or stop.
DISK BRAKE
Disk brakes are more efficient than drum brakes and the maintenance is less. These usually
works by means of hydraulic mechanisms. It contains a disk which is fixed to the wheel of the
vehicle. A brake pad with friction lining is equipped to stop the rotation of the disk when the
brake is applied. The brake is actuated by a small piston cylinder arrangement which is
controlled using fluid pressure.