You are here

Question and answers

Examination:
Que.No Marks
Q 6 b )

4

Question:

Define : i) Free air delivered ii) Compressor capacity iii) Swept volume iv) Pressure ratio, w.r.to compressor.


Answer:

Define i) Free air delivered (FAD) – It is volume of air delivered under the condition of temperature and pressure existing at compressor intake, i.e. volume of air delivered at surrounding air temperature & pressure. In absence of any given free air conditions these are generally taken as 1.101325 bar and 150 c. ii) Compressor capacity – It is quantity of free air actually delivered by compressor in m3 /min. iii) Swept volume – It is the volume of air taken during sanction stroke. It is expressed in m3 . iv) Pressure ratio – It is defined as delivery pressure to suction pressure.

-----------------------------------------------------------------------------------------------------
Examination: 2017 SUMMER
Que.No Marks
Q 1a)(a)

4

Question:

     Enlist uses of compressed air (any four).              


Answer:

Following are the applications of compressed air 1) To drive air motors in coal mines. 2) To inject fuel in air injection diesel engines. 3) To operate pneumatic drills, hammers, hoists, sand blasters. 4) For cleaning purposes. 5) To cool large buildings. 6) In the processing of food and farm maintenance. 7) In vehicle to operate air brake. 8) For spray painting in paint industry.

-----------------------------------------------------------------------------------------------------
Q 1a)(b)

4

Question:

What are the advantages of multistaging ?


Answer:

1. The air can be cooled in between two cylinders 2. The power required is less 3. Mechanical balance is good 4. Reduced leakage losses 5. More volumetric efficiency 6. High pressure range 7. Comparatively lighter in construction

-----------------------------------------------------------------------------------------------------
Q 2 c )

8

Question:

Differentiate between reciprocating air compressor and rotary air compressor.........


Answer:

-----------------------------------------------------------------------------------------------------
Q 4a)(b)

4

Question:

Explain with sketch working of screw compressor.


Answer:

A rotary-screw compressor is a type of gas compressor that uses a rotary-type positivedisplacement mechanism. They are commonly used to replace piston compressors where large volumes of high-pressure air are needed, either for large industrial applications or to operate highpower air tools. Rotary-screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a dry-running rotary-screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary-screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threadsas the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws.

 

-----------------------------------------------------------------------------------------------------
Q 4b)(ii)

6

Question:

Explain the working of two stage reciprocating compressor. Show work saved on PV diagram.


Answer:

-----------------------------------------------------------------------------------------------------
Examination: 2017 WINTER
Que.No Marks
Q 1a)(c)

4

Question:

Classify air compressors


Answer:

Classification of Air compressors:

1. According to principle: a) Reciprocating air compressors b) Rotary air compressors 2. According to the capacity a. Low capacity air compressors b. Medium capacity air compressors c. High capacity air compressors 3. According to pressure limits a. Low pressure air compressors b. Medium pressure air compressors c. High pressure air compressors 4. According to method of connection a. Direct drive air compressors b. Belt drive air compressors c. Chain drive air compressors

 

-----------------------------------------------------------------------------------------------------
Q 2 a )

8

Question:

Reciprocating air compressor draws 6 kg of air per minute at 25°C. It compresses the air polytropically and delivers it at 105°C. Find the work done by the compressor and air power. Also find mechanical efficiency if shaft power is 14 kW. Assume R = 0.287 kJ/kg°K and n = 1.3.


Answer:

-----------------------------------------------------------------------------------------------------
Q 5 a )

8

Question:

State the methods to improve efficiency of air compressor. Explain two stage air compressor with perfect intercooling with neat sketch.


Answer:

Following are the methods to improve efficiency of air compressor 1. Cooling cylinder by spraying water during compression stroke. 2. Circulation of water surrounding to cylinder by providing jackets 3. Installing inter cooler between two cylinders 4. Providing greater fins on cylinder 5. By selecting suitable material for cylinder 6. By providing suitable choice of cylinder proportions i.e. short stroke and large bore in construction with sleeve valve Two stage reciprocating air compressor :

 

Two stage reciprocating compressor

Multistage compression refers to the compression process completed in more than one stage i.e. a part of compression occurs in one cylinder ( L.P. cylinder) and subsequently compressed air is sent to subsequent cylinders ( H.P. cylinder) for further compression.  Figure shows the schematic of two stage compressor with intercooler between stages. The total work requirement for running this shall be algebraic summation of work required for low pressure (LP) and high pressure (HP) stages. The size of HP cylinder is smaller than LP cylinder as HP cylinder handles high pressure air having smaller specific volume. Intake temp of air at LP =intake temp of air at HP for perfect intercooling.

-----------------------------------------------------------------------------------------------------
Q 5 b )

8

Question:

State the applications of reciprocating compressor and rotary compressor (4 each).


Answer:

Applications of Reciprocating Compressor  1. In spray painting shop. 2. In workshop for cleaning machines. 3. For operation of pneumatic tool like rock drill, vibrator etc. 4. In automobile service station to clean vehicle. 5. To drive air motors in coal mines. 6. Food and beverage industry

Applications of Centrifugal Compressor  1. In gas turbines and auxiliary power units. 2. In automotive engine and diesel engine turbochargers and superchargers. 3. In pipeline compressors of natural gas to move the gas from the production site to the consumer. 4. In oil refineries, natural gas processing, petrochemical and chemical plants. 5. Air-conditioning and refrigeration and HVAC: Centrifugal compressors quite often supply the compression in water chillers cycles. 6. In air separation plants to manufacture purified end product gases. 7. In oil field re-injection of high pressure natural gas to improve oil recovery.

 

-----------------------------------------------------------------------------------------------------
Q 6 d )

4

Question:

Enlist different uses of compressed air.


Answer:

Following are the applications of compressed air

1) To drive air motors in coal mines.

2) To inject fuel in air injection diesel engines.

3) To operate pneumatic drills, hammers, hoists, sand blasters.

4) For cleaning purposes.

5) To cool large buildings.

6) In the processing of food and farm maintenance.

7) For spray painting in paint industry.

8) In automobile & railway braking systems.

9) To operate air tools like air guns.

10) To hold & index cutting tools on machines like milling

-----------------------------------------------------------------------------------------------------
Examination: 2016 SUMMER
Que.No Marks
Q 1a)(b)

4

Question:

Define following terms w.r.t. air compressor. i) FAD ii) Compression ratio.


Answer:

i) FAD – It is the volume of air delivered under the intake conditions of temperature and pressure.  ii) Compression ratio – It is defined as the ratio of absolute discharge pressure to the absolute inlet pressure.

-----------------------------------------------------------------------------------------------------
Q 1a)(c)

4

Question:

Enlist different uses of compressed air.


Answer:

1) To drive air motors in coal mines. 2) To inject fuel in air injection diesel engines. 3) To operate pneumatic drills, hammers, hoists, sand blasters. 4) For cleaning purposes. 5) To cool large buildings. 6) In the processing of food and farm maintenance. 7) For spray painting in paint industry. 8) In automobile & railway braking systems. 9) To operate air tools like air guns. 10) To hold & index cutting tools on machines like milling.

-----------------------------------------------------------------------------------------------------
Q 2 a )

8

Question:

 It is desired to compress 15 m3 of air per minute from 1.0132 bar to 10 bar. Calculate minimum power required to drive the compressor having two stages and compared it the power required for single stage compression. Assume value of index for compression process to be 1.3 for both cases also assume the condition for maximum efficiency


Answer:

-----------------------------------------------------------------------------------------------------
Q 5 b )

8

Question:

What do you mean by ‘Perfect Intercolling’ ? Explain with the help of P.V. diagram.


Answer:

Intercooling : In perfect intercooling the temperature of air after passing out of intercooler is same as that of the temperature of air before compression of LP cylinder. 

-----------------------------------------------------------------------------------------------------
Q 6 b )

4

Question:

Why majority of air compressors available in the market are multi staged ? Explain


Answer:

Multi-stage air compressors feature many benefits and so, they are mostly used in the market. Some of those features are given below 1. Higher air pressures are achievable by multi-staging (about 175 PSI against 120 PSI in single stage) 2. It requires less power for running 3. Light weight cylinders can are used 4. Leakages are less 5. Overall discharge temperatures are lower 6. Intercooler increases the efficiency of unit 7. It has a greater durability 8. Many multi-stage air compressors have the crankcase cast separate from the pump cylinders, which makes it easier to repair. 9. Multi-stage compressors Air compressors can perform (are suitable) many different functions in industrial applications

-----------------------------------------------------------------------------------------------------
Examination: 2016 WINTER
Que.No Marks
Q )

4

Question:

Explain with neat sketch working of lobe type air compressor.


Answer:

Lobe type air compressor: it is a rotary type of compressor consisting of two rotors which are driven externally. One rotor is connected to drive and second is connected to gear. These two rotors have two or three lobes having epicycloids, hypocycloid or involutes profiles. In the figure two lobes compressor is shown with a inlet arrangement and receiver. A very small clearance is maintained between surfaces so that wear is prevented. Air leakage through this clearance decreases efficiency of this compressor. During rotation a volume of air V at atmospheric pressure is trapped between left hand rotor and casing . this air is positively displaced with change in volume until space is opened to high pressure region. At this instant some high pressure air rushes back from the receiver and mixed with the blower air until both pressure are equalized .

 

-----------------------------------------------------------------------------------------------------
Q 1a)(b)

4

Question:

A two stage air compressor with perfect intercooling takes in air at 1 bar pressure and 27 °C. The law of compression in both the stages is Pv1.3 = constant. The compressed air is delivered at 9 bar from the H.P. Cylinder to an air receiver. Calculate per kg. of air i) The minimum work done. ii) The heat rejected to the intercooler.


Answer:

-----------------------------------------------------------------------------------------------------
Q 2 b )

8

Question:

A single stage reciprocating air compressor has a swept volume of 2000 cm3 and runs at 800 rpm. It operates on a pressure ratio of 8 with a clearance 5% of the swept volume. Assume NTP room conditions at inlet (p = 101.3 kPa t = 15°C) and polytropic compression and expansion with n = 1.25 calculate i) Indicated power, ii) Volumetric efficiency, iii) Mass flow rate iv) Isothermal efficiency


Answer:

-----------------------------------------------------------------------------------------------------
Q 5 b )

8

Question:

A single cylinder reciprocating compressor has a bore of 120 mm and a stroke of 150 mm. and is driven at a speed of 1200 rpm. It is compressing CO2 gas from a pressure of 120 Kpa and temp. of 20°C to a temp. of 215°C. Assuming polytropic compression with n = 1.3, no clearance and volumetric efficiency of 100% calculate (i) pressure ratio, (ii) Indicated power, (iii) shaft power with mech. efficiency 80%, (iv) mass flow rate


Answer:

-----------------------------------------------------------------------------------------------------
Examination: 2015 SUMMER
Que.No Marks
Q 1a)(b)

4

Question:

 Define : i) Compression ratio (Rc) ii) Swept volume (vs) iii) Cut off ratio iv) Clearance volume (vc)


Answer:

 

-----------------------------------------------------------------------------------------------------
Q 1a)(c)

4

Question:

Write uses of compressed air.


Answer:

 Uses of compressed air:- 1. Cleaning automobiles 2. Pneumatic tools 3. Supercharging in I.C. engines 4. Cooling of large building 5. Construction of bridges, roads etc. 6. Spraying points 7. Spraying fuel in high speed diesel engine 8. Starting of I.C. engines

 

-----------------------------------------------------------------------------------------------------
Q 1a)(d)

4

Question:

Draw a neat sketch of vane compressor and label the different parts.


Answer:

-----------------------------------------------------------------------------------------------------
Q 2 b )

8

Question:

What is the necessity of multistage compression ? Explain the working of two stage reciprocating air compressor with intercooler, with the help of p-v diagram.


Answer:

Necessity of multistage compression i) As index of compression ‘n’ increases it increases compression work. ii) Increase in pressure ratio (P2/P1) it increases work as well as size of cylinder. iii) Increment in pressure ratio (P2/P1) beyond certain limit, volumetric efficiency decreases while it increases leakage loss on either sides the piston and valves. Due to above pointes and for higher pressure ratio compressor needs multistaging.

Fig. shows arrangement of two stage reciprocating air compressor with inter cooler and its working shown on P.V. diagram plane. Processes occurred in the cycle  - 1 – 2 – suction process by LP cylinder to draw atmospheric pressure  - 2 – 3 – compression process by LP cylinder up to pressure P2  - 3 – 4 – delivery of compressed air into the air cooler  - 4 – 5 – during this process air rejects the heat to the cold water and at the same time suction process by HP cylinder to draw air from air cooler.  - 5 – 6 – compression pressure by HP cylinder up to required pressure P3  - 6 – 7 – delivery of compressed air at required pressure to the point of use. This completes the process and system has shown saving in work shown by shaded portion.

-----------------------------------------------------------------------------------------------------
Q 5 a )

8

Question:

Differentiate between reciprocating and rotary compressors


Answer:

-----------------------------------------------------------------------------------------------------
Examination: 2015 WINTER
Que.No Marks
Q 1a)(b)

4

Question:

State any four industrial uses of compressed air.


Answer:

1) To drive air motors in coal mines. 2) To inject fuel in air injection diesel engines. 3) To operate pneumatic drills, hammers, hoists, sand blasters. 4) For cleaning purposes. 5) To cool large buildings. 6) In the processing of food and farm maintenance. 7) For spray painting in paint industry. 8) In automobile & railway braking systems. 9) To operate air tools like air guns. 10) To hold & index cutting tools on machines like milling / cnc machines.

-----------------------------------------------------------------------------------------------------
Q 1a)(c)

4

Question:

Define the following terms related to compressor. i) Compressor capacity ii) Free air delivered iii) Volumetric efficiency iv) Mechanical efficiency


Answer:

i] Compressor capacity:-  It is the volume of air delivered by the compressor in m3 per minute  It is express in m3 /min ii) FAD:-  It is the volume of air delivered by compressor under the intake conditions of temperature and pressure.  Capacity of compressor is generally given in terms of free air delivery.  Unit = m3 /cycle iii) Volumetric Efficiency: It is the ratio of actual volume of air delivered at standard atmospheric condition discharge in one delivery stroke to the swept volume by the piston during the stroke.iv) Mechanical Efficiency: It is the ratio of Indicated power to shaft (brake) power.

-----------------------------------------------------------------------------------------------------
Q 2 a )

8

Question:

Compare Reciprocating air compressor and Rotary air compressor mentioning the basis of comparison (any 8 points)


Answer:

-----------------------------------------------------------------------------------------------------
Q 5 b )

8

Question:

A pneumatic rock drill requires 10 kg/min of air at 6 bar pressure. Find the power required to drive the single acting single stage reciprocating compressor receiving air at 1 bar and 27°C. Assume mechanical efficiency as 80% and value of index, n as 1.25. Take Cp = 1.005 kJ/kgk and Cv = 0.718 kJ/kgk for air. Also estimate isothermal efficiency of compression.


Answer:

-----------------------------------------------------------------------------------------------------
Q 6 b )

4

Question:

Define perfect and imperfect inter-cooling in air compressor and show it by graph also


Answer:

-----------------------------------------------------------------------------------------------------
Examination: 2014 WINTER
Que.No Marks
Q 1a)(ii)

4

Question:

Define following efficiencies related to compressors: 1) mechanical efficiency 2) polytropic efficiency 3) compressor efficiency 4) overall volumetric efficiency


Answer:

-----------------------------------------------------------------------------------------------------
Q 1a)(iii)

4

Question:

Show the effect of increase of compression ratio in a single stage reciprocating compresor on PV diagram and give its physical significance.


Answer:

Effect of Compression ratio in a single stage reciprocating compressor on PV diagram

 

Physical Significance:-

If compression in increased (usually it varies from 5 to 8) the final temperature increases and volumetric efficiency decreases flow and it compression ratio increases beyond usual value, compression ratio P2/P1 becomes zero as it can be observed from the figure. Increment in compression ratio will increase leakage past the piston and will need robust cylinder. If will also affect the operation of delivery valve and if will reduce lubricating properties of oil. It may increase the risk of ignition in piping and receiver.

-----------------------------------------------------------------------------------------------------
Q 2a)(i)

8

Question:

The criterion of the thermodynamic efficiency of a reciprocating compressor is isothermal compression while for rotory compressor it is isentropic compression. Discuss the reason for this.


Answer:

The compression process in reciprocating compressor may approach to low speed of compression and cylinder cooling. Therefore isothermal efficiency is used in reciprocating compressor. But in rotary compressor there is high friction and eddies formation due to high velocity air through the compressor. This causes heating of air during compression process. Therefore temperature of air leaving the impeller is higher than the isentropic compression. The compressor may be as high as 1.7 (n>t). Therefore isentropic efficiency is used in rotary compressor.

-----------------------------------------------------------------------------------------------------
Q 2a)(ii)

8

Question:

Compare reciprocating compressors and centrifugal compressors on the basis of the following parameters: 1) adaptability 2) suitability 3) mechanical efficiency 4) capacity of delivering volume.


Answer:

-----------------------------------------------------------------------------------------------------
Q 5 b )

8

Question:

A single stage single acting air compressor delivers 0.6 kg of air per minute at 6.1 bar. The temperature and presure at the end of suction stroke are 28°C and 1.1 bar. The bore and stroke of the compressor are 100 mm and 150 mm respectively. The clearance is 3 % of the swept volume. Assuming index of compression and expansion as 1.25, find: (i) volumetric efficiency of compressor (ii) power required if mechanical efficiency is 85% (iii) speed of compressor in rpm


Answer:

 

-----------------------------------------------------------------------------------------------------
Q 6 b )

4

Question:

Define displacement of compressor for two stage compressor. Why is free air delivered less than displacement of compressor?


Answer:

Displacement is the product of piston displacement and working stroke per minute is bared on low pressure only and the amount air passing through the other cylinder for two stage compressor. When free air wave from low pressure cylinder to high pressure cylinder through intercooler there is reduction of volume of air because of perfect cooling so free air delivered is less than displacement of compressor. ( Pl check)

-----------------------------------------------------------------------------------------------------