UNIT 4

1. (a) Explain the working of a single stage single acting reciprocating

compressor with a neat sketch and p-V diagram. Also explain the need and the advantages

of multi stage compression, with a p—V diagram. (16)

Or

(b) Estimate the volumetric efficiency and power consumption of a single stage single

acting reciprocating compressor, given the following data : Cylinder diameter : 30 cm

Stroke :22mm

Clearance ratio : 0.03

Delivery pressure : 8 bar

Suction pressure : 1 bar

Speed : 400 rpm

Compression and expansion follows pv1.3= constant.

2. (a) The following data relate to a performance test of a .single acting

14 cm x 10 cm reciprocating compressor:

Suction pressure = 1 bar,

Suction temperature = 20° C,

Discharge pressure = 6 bar,

Discharge temperature = 180° C,

Speed of the compressor = 1200 rpm,

Shaft power = 6.25 kW,

Mass of air delivered = 1.7 kg/min. Calculate,

(i) The actual volumetric efficiency.

(ii) The indicated power,

(iii) The isothermal efficiency.

(iv) The mechanical efficiency.

(v) The overall isothermal efficiency.

Or

(b) (i) How the volumetric efficiency of the reciprocating air compressor is affected

by the following parameters.

(1) The speed of the compressor.

(2) The delivery pressure.

(3) Throttling across the valves.

(ii) Discuss positive and non-positive displacement rotary compressors.

3. (a) A single stage, single acting reciprocating air compressor has a bore of 0.2 m

and a stroke of 0.3 m. It receives air at 1 bar and 293 K and delivers it at 5.5 bar. If the

compression follows the law pV13 = constant and clearance volume is 5% of the stroke

volume, Determine the mean effective pressure and the power required to drive the

compressor, if it runs at 500 rpm.

OR

(b) (i) Derive the expression for minimum work required for a two stage

reciprocating air compressor. . (12)

(ii) Estimate the minimum work required to compress 1 kg of air from 1 bar 300 K to 16

bar in two stages if the law of compression is pV125 = constant and the inter cooling is

perfect. Take R = 287 J/kg K. (4)

4. (a) A single acting single stage compressor is belt driven from an electric

motor at 400 rpm. The cylinder diameter is 15 cm and the stroke is 17.5 cm the air is

compressed from 1bar to 7bar and the law .of compression PV1.35 = const. Find the power

of the motor, ,if the transmission efficiency is 97% and the mechanical efficiency of the

compressor is 90%. Neglect clearance effects.

Or

(b) A two stage double acting air compressor operating at 200 rpm takes in air at 1.013bar

and 27°C. The size of the L.P cylinder is 355 x 375 mm, the stroke of HP cylinder is the

same as the LP cylinder and the clearance of both the cylinders is 4%. The LP cylinder

discharges the air at a pressure of 0.452bar. The air passes through the inter cooler so that

it enters the HP cylinder at 27°C and 3.850bar, finally it is discharged from the compressor

at 15.4bar. The values of n for both cylinders are 1.25. Cp= 1.0035 kJ/kgK and R=0.28

kJ/kgK.Calculate:

(i) The heat rejected in the inter- cooler.

(ii) The diameter of HP cylinder and

(iii) The power required to drive HP cylinder.

5. (a) (i) With a neat sketch, explain the working of single stage single

acting reciprocating air compressor. (6)

(ii) A single stage single acting reciprocating air compressor delivers 15 m3/min of free air

from 1 bar to 8 bar at 300 rpm. The clearance volume is 5% of the stroke volume and

compression and expansion follow the law pv1.3 = constant. Calculate the diameter and the

stroke of the compressor. Take L/D = 1.5. The temperature and pressure of air at suction

are the same as atmospheric air. (10)

Or

(b) (i) With a help of PV diagram explain the need of multi-stage

compression. (6)

(ii) A three stage reciprocating air compressor compresses air from 1 bar and 17°C to 35

bar. The law of compression is pV125 = constant and is the same for all stages of

compression. Find the minimum power required to compress 15 m3/min of free air. Also

find the intermediate pressures. Assume perfect inter cooling and neglect the

clearance.(10)

6. (a) (i) Explain with suitable sketches the working of two stage air

compressor with actual p-v diagram. (6)

(ii) A single acting single stage compressor is belt driven from an electric

motor at 400 rpm. The cylinder diameter is 15 em and the stroke is 17.5 em. The air is

compressed from 1 bar to 7 bar and the law of compression PV1.3 = constant. Find the

power of the motor, if transmission efficiency is 97% and the mechanical efficiency of the

compressor is 90%. Neglect clearance effects. (10)

Or

(b) A three-stage air-compressor delivers 5.2 m3 of free air per minute. The

suction pressure and temperature are 1 bar and 30°C. The ambient pressure and

temperature are 1.03 bar and 20°C. The air is cooled to 30°C after each stage of

compression. The delivery pressure of the compressor is 150 bar. The RPM of the

compressor is 300. The clearances of LP, I.P and H.P. cylinders are 5% of the

respective strokes. The index of compression and re-expansion in all stages is 1.35.

Neglecting pressure losses, find the B.P of the motor required to run the compressor

if the mechanical efficiency is 80%.

7. (a) (i) Derive an expression for the minimum work required for a two-stage

reciprocating air compressor with perfect intercooling and neglecting clearance. (8)

(ii) A two stage air compressor compresses air from 1 bar and 20°C to 42 bar. If the law of

compression is pV1.35= constant and the intercooling is perfect. Find per kg of air

(1) the work done in compression

(2) the mass of cooling water necessary for abstracting the heat in the intercooler, if the

temperature rise of the cooling water is 25°C. (8)

Or

(b) (i) With a neat sketch describe any one type of rotary compressor. (8)

(ii) A single stage single acting reciprocating air compressor delivers 14 m3 of free air per minute

from 1 bar to 7 bar. The speed of compressor is 310 rpm. Assuming that compression and

expansion follow the law pV1.35= constant and clearance is 5% of the swept volume, find the

diameter and stroke of the compressor. Take stroke length is 1.5 times the bore diameter. (8)

8. (a) (i) Define the volumetric efficiency of a reciprocating compressor and explain why it is

less than unity.

(ii) Determine the size of the cylinder of a double acting air-compressor of 32 KW LP. in which

air is drawn in at 1 bar and compressed to 16 bar according to the law pv1.25 =constant. R.P.M. =

300, Piston speed = 180 m/min, Volumetric efficiency = 0.8.

Or

(b) A two-stage double acting air compressor, operating at 200 r.p.m, takes in air at 1.013 bar and

27° C. The size of the L.P. cylinder is 350 x 380 mm, the stroke of H.P. cylinder is the same as

that of the L.P. cylinder and the clearance of both the cylinders is 4%. The L.P. cylinder

discharges the air at a pressure of 4.052 bar. The air passes through the inter-cooler so that it enters

the H.P. cylinder at 27° C and 3.850 bar, finally it is discharged from the compressor at 15.4 bar.

The value of n is both cylinders is 1.3. Cp = 1.0035 kJ/kg-K and R = 0.287 kJ/kg-K.

Calculate :

(i) The heat rejected in the inter-cooler.

(ii) The diameter of H.P. cylinder and

(iii) The power required to drive H.P. cylinder.

9. (i) Explain with suitable sketches the working of two stage air compressor with actual

p - v diagram. (6)

ii) A single acting single stage compressor is belt driven from an electric motor at 400rpm. The

cylinder diameter is 15 cm and the stroke 17.5 cm. The air is compressed from 1 bar to 7 bar and

the law of compression PV 1. 3 = constant. Find the power of the motor, if transmission efficiency

is 97% and the mechanical efficiency of the compressor 90%. Neglect clearance effects.

(10)

Unit 5

1. (a) With the help of a suitable sketch explain the working of lithium bromide water

based vapour absorption system. Also list the advantages and disadvantages of

vapour absorption systems. (12 + 4)

Or

(b) In an air conditioning system, re-circulated air is mixed with fresh air and the mixture

is sent through the conditioner coil. After the conditioner coil, the conditioned air enters

the room. Draw the block diagram of this system and show the related psychrometrjc

processes on a psychrometric chart. Also explain RSHF, GSHF and ESHF in connection

with this AC system. (16)

2.(a) (i) Explain the working principle of vapour absorption refrigeration system and

compare it with vapour compression refrigeration system.

(ii) A Freon vapour compression system operating at a condenser temperature of 40° C and

an evaporator temperature of -5° 0 develops 15 tons of refrigeration.- Using the p-h

diagram for Freon 12, determine,

(1) The mass flow rate of the refrigerant circulated.

(2) The theoretical piston displacement of the compressor and piston displacement

per ton of refrigeration.

(3) The theoretical horse power of the compressor and horse power per ton of

refrigeration.

(4) The heat rejected in the condenser.

Or

(b) (i) Explain the working principle of year round air conditioning system.

(ii) An air conditioned space is maintained at 27° C DBT and 50 percent RH. The

ambient conditions are 40" C DBT and 27° C WBT. The space has a sensible heat

gain of 14 kW. Air is supplied to the space at 7° C saturated. Calculate,

(1) Mass of moist air supplied to the space in kg/h.

(2) Latent heat gain of space in kW.

(3) Cooling load of the air washer in kW if 30 percent of the air supplied to the

space is fresh, the remainder being recirculated.

3. (a) A vapour compression refrigerator uses R-12 as refrigerant and the liquid

evaporates in the evaporator at 258 K. The temperature of this refrigerant at the delivery

from the compressor is 288 K when the vapour is condensed at 283 K. Find the coefficient

of performance if (i) there is no under cooling (ii) the liquid is cooled by 278 K before

expansion by throttling. Take specific heat at constant pressure for the super heated vapour

as 0.64 kJ/kg K and that for liquid as 0.94 kJ/kg K. The other properties of R12 as follows:

Temperature (K) Enthalpy (kJ/kg) Entropy(kJ/kg K)

Liquid Vapour Liquid Vapour

258 22.3 180.88 0.0904 0.7051

283 45.4 191.76 0.1750 0.6921

Or

(b) Explain the working of Lithium Bromide-water system with a schematic layout.

4. (a) Explain with a neat sketch practical Ammonia-water vapour absorption

refrigeration system. Also bring out any four important differences between vapour

compression and vapour absorption refrigeration systems. (12 + 4)

Or

(b) A restaurant with a capacity of 100 persons is to be air-conditioned with the

following conditions :

Outside conditions = 30°C, 70% RH

Desired inside conditions = 23°C, 55% RH

Quantity of air supplied = 0.5 m3 /min/person

The desired conditions are achieved by cooling, dehumidifying and then heating.

Determine :

(i) Capacity of cooling coils in tones of refrigeration

(ii) Capacity of heating coil and

(iii) Amount of water removed by dehumidifier.

5. (a) (i) With a simple block diagram, explain the working of a vapour

compression refrigeration system. (6)

(ii) The pressure in the evaporator of an ammonia refrigerator is 1.902 bar and the pressure

in the condenser is 12.37 bar. The refrigerant is in dry saturated condition at the entry of

the condenser. Calculate the refrigerating effect per unit mass of refrigerant and the COP.

(10)

Or

(b) (i) Define Relative Humidity, Dew point Temperature and Wet Bulb

Temperature. (6)

(ii) 100 m3 of air per minute at 15°C DBT and 80% R.H. is heated until its temperature is

22°C. Calculate heat added to air per minute, R.H. of the heated air and Wet bulb

temperature of the heated air.(10)

6. (a) (i) Draw a neat sketch of a simple vapour compression refrigeration system and

explain its principle of operation. (5)

(ii) An ammonia refrigerator produces 30 tonnes of ice from and at O°C in a day of

24 hours. The temperature range in the compressor is from 25°C to 15°C. The

vapour is dry saturated at the end of compression and an expansion valve is used.

Calculate the coefficient of performance. The properties of the refrigerant are given

in the following table : (11)

Temperature °C Enthalpy KJ/kg Entropy of Liquid Entropy of Vapour KJ/kg-K KJ/kg-K Liquid Vapour 25 100.04 1319.22 0.3473 4.4852 -15 -54.56 1304.99 -2.1338 5.0585

Or

(b) An office is to be air-conditioned for 50 staff when the outdoor conditions are 30°C

DBT and 75 RH if

the quantity of air supplied is 0.4 m3/min/person, find the following :

(i) Capacity of the cooling coil in tonnes of refrigeration

(ii) Capacity of the heating coil in kW

(iii) Amount of water vapour removed per hour

Assume that required air inlet conditions are 20°C DBT and 60% RH, Air is

conditioned first by cooling and dehumidifying and then by heating.

(iv) If the heating coil surface temperature is 25°C, find the by-pass factor of the

heating coil?

7. (a) A refrigeration system of 10.5 tonnes capacity at an evaporator temperature of -

12°C and a condenser temperature of 27°C is needed in a food storage locker. The refrigerant ammonia is

sub cooled by 6°C before entering the expansion valve. The vapour is 0.95 dry as it leaves the evaporator

coil. The compression in the compressor is of adiabatic type. Find

(i) Condition of vapour at the outlet of the compressor

(ii) Condition of vapour at the entrance of the evaporator

(iii) COP and

(iv) The power required.

Neglect valve throttling and clearance effect.

Or

(b) A sling psychrometer in a laboratory test recorded the following readings.

Dry bulb temperature = 35°C

Wet bulb temperature = 25°C

Calculate the following :

(i) specific humidity

(ii) relative humidity

(iii) vapour density in air

(iv) dew point temperature and

(v) enthalpy of mixture per kg of dry air

Take atmospheric pressure = 1.0132 bar.

8. (a) Freon 12 is compressed from 200 kPa to 1.0 MPa in an 80 percent efficient compressor. The

condenser exiting temperature is 40° C. Calculate the COP and the refrigerant mass flux for 100

tons (352 kW) of refrigeration.

Or

(b) 100 m3 of air per minute at 15° C DBT and 80% R.H. is heated until its temperature

becomes 22° C. Find the following

(i) Heat added to the air per min.

(ii) R.H. of the heated air. Assume air pressure is at 1.033 bar.

9. (a) A simple R-12 plant is to develop 5 tonnes of refrigeration. The condenser A and

evaporator temperatures are to be 400C and –l0°C respectively. Determine

(i) the refrigerant flow rate in kg/s

(ii) the volume flow rate handled by the compressor in m3/s

(iii) the compressor discharge temperature

(iv) the heat rejected to the condenser in kW

(v) the COP and

(vi) the power required to drive the compressor.

How does this COP compare with that of a Carnot refrigerator operating between 40°C and –l0°C.

(16)

Or

(b) An office is to be air-conditioned for 50 staff when the outdoor conditions are 30°C

DBT and 75 RH if the quantity of air supplied is 0.4 m3/min/person, find the following :

(i) Capacity of the cooling coil in tonnes of refrigeration

(ii) Capacity of the heating coil in kW

(iii) Amount of water vapour removed per hour.

Assume that required air inlet conditions are 25°C DBT and 65% RH. Air is conditioned first by

cooling and dehumidifying and then by heating. If the heating coil surface temperature is 28°C,

find the by-pass factor of the heating coil. (16)