COMPRESSORS AND COMPRESSED AIR

COMPRESSORS:

Q.1) What is a Compressor? What is the difference between a Compressor and a Pump? What are the practical uses of Compressed Air?

A.1) A compressor is a device which is extensively used to raise the pressure of a compressible fluid like pure air. In a compressor, the pressure is increased at the expense of work done on the fluid, which is generally provided by an electric motor, IC engines or Gas Turbines. In a compressor, fluids are compressed by reducing the specific volumes of the working fluids. Due to compression, the temperature of the fluid is also increased.

If air is used as the working fluid in a compressor and air is compressed into a high pressure by the application of work on the fluid, then it is known as Air Compressor.

PRACTICAL USES OF COMPRESSED AIR

In industry, compressed air is so widely used that it is often regarded as the fourth utility, after electricity, natural gas and water. Compressed air, commonly called Industry's Fourth Utility, is air that is condensed and contained at a pressure that is greater than the atmosphere. The process takes a given mass of air, which occupies a given volume of space, and reduces it into a smaller space. In that space, greater air mass produces greater pressure. The pressure comes from this air trying to return to its original volume. It is used in many different manufacturing operations.

Compressed air is extensively used in industrial applications like pneumatic machines, as well as in the refrigeration and air-conditioning systems or supercharging CI engines to boost the output of the engine.
01) Compressed air is extensively used to operate pneumatic tools like drills, hammers, rivetting machines etc.
02) It is used to drive Compressed Air Engine.
03) Compressed air is used to spray painting.
04) Compressor is a vital component of Air-conditioning and Refrigeration industry.
05) Very often, compressed air can be used as a means of energy storage.
06) It is used in Gas Turbine power plants.
07) It is used to Super-charging an IC engines.
08) It is used to convey or pump to flow the materials like sand or concrete slurries along a pipe line.
09) It can be used as a means to pump water through the pipe lines.
10) It is used to drive minning machineries in a fire risky zone.
11) It is also used in blast furnaces.

Q.2) Classify the compressors
(i) On the basis of operations employed
(ii) On the basis of pressure achieved
(iii) On the basis of pressure ratio
(iv) On the basis of capacity of compressors.

A.2) Depending upon different parameters, compressors can be classified on the basis of operations employed, the delivery pressure achieved, pressure ratio and capacity of compressors as follows.

On the basis of operations employed, compressors are classified into two groups
i) Reciprocating compressors : It uses piston cylinder arrangement and due to positive displacement of air in the cylinder, the air is compressed and delivered to a vessel called Receiver. These are capable to produce high delivery pressure with low volume flow rate.
ii) Rotary Compressors : These compressors operate at high speeds, therefore, can handle large volume flow rates compared to reciprocating compressors.

In rotary compressors, the dynamic head is imparted to the gas with the help of very high speed impeller rotating at a confined space so that the air is compressed due to centrifugal action.

On the basis of delivery pressure, compressors are classified into three categories
i) Low Pressure Compressors : Delivery pressure upto 1.1 bar
ii) Medium Pressure Compressors : Delivery pressure upto 7 bar
iii) High Pressure Compressors : Delivery pressure between 7 to 10 bar.

On the basis of pressure ratio, we can classify the devices as follows,
Fans : Pressure ratio upto 1.1
Blower : Pressure ratio upto 1.1 to 4.0
Compressors : Pressure ratio above 4.0

On the basis of capacity, compressors can be classified as follows,
Low capacity compressors : Volume flow rate upto 10 m³/min, or less
Medium capacity compressors : Volume flow rate 10 m³/min to 300 m³/min
High capacity compressors : Volume flow rate above 300 m³/min

QUESTION PAPER: EME-505, IC ENGINES & COMPRESSORS

FIRST MINOR TEST: IC ENGINES IN SGIT

Shree Ganpati Institute of Technology; Ghaziabad
From 23rd September, 2013 to 26th September first minor test has been organised. This semester, I am teaching IC Engines and Compressors (EME-505) and Thermodynamics (ME-301).
Here is the Question paper of EME-505

  

snapshot of the question paper
ME-301; Thermodynamics
3rd Semester; Mechanical Engg

THE CONCEPT OF VAPOUR LOCK IN IC ENGINES

VAPOUR LOCK

Vapour lock is a problem that mostly affects " Gasoline-fuelled internal combustion engine. " It occurs when liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburettor or fuel injection system resulting in transient loss of power or even complete stalling.

REASONS OF VAPOUR LOCK

The fuel can vapourise due to being heated by the hot engine or by the local hot climate or due to a low boiling point at high altitude.

In regions where higher volatility fuels are used during winter to improve the cold starting, the use of winter fuels during summer can cause vapour lock more easily.

Vapour lock occurs in older type gasoline fuel systems where a low pressure mechanical fuel pump driven by the engine is located in the engine compartment and feeding a carburettor. These pumps are typically located higher than the fuel tank, are directly heated by the engine, and feed fuel. directly to the float bowl or float chamber of carburettor. As in these pumps fuel is drawn from the feedline and fed into the fuel pump under negative pressure, it lowers the boiling temperature of the liquid fuel. As a result fuel gets evaporated fast and totally invades the fuel pump system and carburettor. As the carburettor becomes devoids of liquid fuel, the mixture it prepares will have less amount of fuel as the volume of vapour of fuel is larger than the equal amount of liquid fuel.

The automotive fuel pump is designed to handle a mixture of liquid and vapour phases of fuel, hence it should handle both the phases of fuel. But, if the amount of fuel evaporated in the fuel system is critically high, the fuel pump stops functioning as per the design and started to pump more vapours than liquid fuel and hence, less amount of liquid fuel will go to the engine. The vapours of fuel will invade the fuel pump delivery system which stops the flow of liquid fuel into the engine.

Most carburettors are designed to run at a fixed level of fuel in the flat bowl of carburettor and reducing the level will reduce the fuel to air mixture and hence, will deliver a lean mixture to the combustion chamber which translates into uneven running of the engine or even stalling while idling or sometimes momentary stalling when running.

VAPOUR LOCK AND (V/L) RATIO

The vapour liquid ratio or (V/L) ratio of a gasoline, defined as the amount of vapour released from a gasoline to the amount of liquid remaining at a given temperature directly correlates with the degree of vapour lock likely to be experienced with this gasoline in the fuel system of a car. At V/L ratio = 24, vapour lock may start and at V/L ratio = 36, vapour lock may be severe. Therefore, the volatility of the gasoline should be maintained as low as practical to prevent vapour lock.