· q.10 in a vapour compression refrigeration system using r-12, the evaporator pressure is 1.4...

MECHANICAL DEPARTMENT, OITM

1

SEM: 6TH

REFRIGERATION & AIR-CONDITIONING

PAPER: ME-302-E

UNIT-I

Q.1 (a) Derive an expression for the COP of Bell-Coleman refrigeration cycle in terms of

pressure. (10) [May-09]

(b) The atmosphere air at a pressure of 1bar and temperature -5°C is drawn in the cylinder of the

compressor of a Bell-Coleman refrigerating machine. It is compressed isentropically to a pressure

of 5 bar in the cooler. The compressed air is cooled to 15°C, pressure remains the same. It is then

expanded to a pressure of 1 bar in an expansion cylinder from where it is passed to the cold

chamber. Find:

(i) Work done per kg of the air.

(ii) COP of the plant for air. (10)

Assume the law of expansion pv1.2

=C and law of compression pv1.4

=C and Cp for air=1 kJ/kg-K.

Q.2 (a) Carnot refrigerator works more efficiently in winter than in summer. Explain (10) [May-

10]

(b) Air refrigerator working on Bell-Coleman cycle takes air into compressor at 1 bar and 268K is

compressed in the compressor to 5 bar and cooled to 298 K at the same pressure .it is further

expanded to 1 bar and discharged to take the cooling load. The isentropic efficiencies of

compressor and turbine are 0.85 and 0.90 res.Determine (I) Refrigeration capacity of the

air circulated is 40 kg /min (ii) power (iii) COP of the system.(10)

Q.3 In airplane simple air refrigeration is used. The main compressor delivers the air at 5 bar and

200°C. The bled air taken from compressor is passed through a heat exchanger, cooled with the

help of ram air so that the temperature of air leaving the heat exchanger is 45°C and pressure is

4.5 bar. The cooling turbine drives the exhaust fan which is used to force the ram air through heat

exchanger. The air leaving the heat exchanger passes through cooling turbine and then

supplied to cabin at 1 bar. The pressure loss between the cooling turbine and cabin is 0.2 bar.


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Mass flow through turbine is 20kg/min.Calculate (i) temperature of air leaving the turbines (ii)

power delivered to ram (iii) refrigeration load in tons, when the temperature of air leaving the

cabin is limited to 25°C. assume The isentropic efficiencies of turbine are 0.75. (20) [May-10]

Q.4 (a) A cold storage is to be maintained at – 4°C while the surroundings are at 34°C. The heat

leakage from the surroundings into the cold storage is estimated to be 30 kW. The actual C.O.P.

of the refrigeration plant used is one third that of an ideal plant working between the same

temperatures. Find the power required to drive the plant. (10) [May-12]

(b) A Bell-Coleman refrigerator operates between pressure limits of 1 bar and 8 bar. Air is drawn

from the cold chamber at 9°C, compressed and then it is cooled to 29°C before entering the

expansion cylinder. Expansion and compression follow the law pv1.35

= constant. Calculate the

theoretical C.O.P. of the system. For air take γ = 1.4, cp = 1.003 kJ/kg K. (10)

Q.5 (a) A simple air-cooled system is used for an aeroplane having a load of 9 tons. the

atmosphere pressure and temperature are 0.9 bar and 10°C res. During ramming pressure

increases to 1.013 bar, in the heat exchanger the temp of air is reduced by 55°C.The pressure in

the cabin is 1.01 bqr and the temperature of air leaving the cabin is 25°C.Find out (i) Power

required to take the load of cooling in the cabin (ii) COP of the system. Assume that all

expansions and compressions are isentropic. The pressure of the compressed air is 4 bar. (10)

(b) Write short notes on:

(i) Adiabatic demagnetization

(ii) Evaporative aircraft refrigeration. (10) [May-12]

Q.6 (a) a refrigerator working on Bell-Coleman cycle operates between pressure limits of 1.03

bar and 9.0 bar. Air is drawn from the cold chamber at 10°C.Air coming out of compressor is

cooled to 30°C before entering the exp. Cylinder. Expansion and compressor follow the law

PV1.35

=C. Determine the theoretical COP of the system and work done by compressor. (10)

(b) What is the necessity of cooling the aircraft? (10) [May-10]


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Q 7 The following data to refer to a reduced ambient refrigeration system:

Ambient Pr.=0.8bar, Pr. Of ram air=1.1 bar, Temp. of ram air=20°C , Pr. At the end of main

compressor=3.3 bar , Efficiency of main compressor=80%, Heat exchanger effectiveness=80%,

Pr. At the exit of auxiliary turbine=0.8 bar, Efficiency of auxiliary turbine=85%, Temp. of air

leaving the cabin=25°C, Pr. In the cabin=1.013 bar , Flow rate of air through cabin=60kg/min

Find:

1. Capacity of cooling system required;

2. Power needed to operate the system; and

3. C.O.P. of the system.( 20) [May-10]

Q.8 An air refrigeration used for food storage provides 25 Tr.The temperature of air entering the

compressor is 7°C and temperature at exit of the cooler is 27°C.

Find:

I. C.O.P.

II. Power per tone of the refrigeration required by the compressor.

The quantity of air circulated in the system is 3000kg/h.The compression and expansion both

follows the law PV1.3

=C and take ƴ=1.4 and Cp =1 kJ/kg k for air. (20) [Dec-11]

UNIT-II

Q.9 (a) With the help of neat sketch explain the working of Lithium bromide –water absorption

refrigeration system. (10) [May-09]

(b) Derive an expression for the maximum COP of vapour absorption refrigeration system.

(10)

Q.10 In a vapour compression refrigeration system using R-12, the evaporator pressure is 1.4 bar

and the condenser pressure is 8 bar. The refrigerant leaving the condenser is sub cooled to 30° C.

the vapour leaving the evaporator is dry and saturated. The compression process is isentropic; the

amount of heat rejection in the condenser is 13.42 MJ/min. determine:

(i) Refrigerating effect in kg/min


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(ii) Refrigeration load in TR

(iii) Compression input and

(iv) COP of the cycle (20) [May-10]

Q.11 (a) With the help of neat sketch and P-H diagram, explain the working of a simple vapour

compression refrigeration cycle. Derive the expression for the COP and power required. (10)

(b) A vapour compression refrigerator uses R-12 as refrigerant and the liquid evaporates in the

evaporator at -15°C, the temp of this refrigerant at the delivery from the compressor is 15°C,

when the vapour is condensed at 10°C. Find the COP if

1. There is no cooling.

2. The liquid is cooled by 5°C before expansion by throttling. (10) [May-09]

Q.12 In an absorption type refrigerator, heat is supplied to NH3 generator by condensing steam at

2 bar and 90% dry. The temp in the refrigerator is to be maintained at -5°C. Find the maximum

COP possible. If the refrigeration load is 20 tonnes and actual COP is 70% of the maximum

COP. Find the mass of steam required per hour. Take temp of the atmosphere as 30°C. (20)

[Dec-11]

Q.13 (a) Derive an expression for the COP of an vapour absorption system. (8)

(b) With the help of layout explains the working of modified H2O, NH3 absorption system using

the all modifications. (8)

(c) Why Hydrocarbons such as propane and butane are being tried as substitutes for Freon

refrigerants. Explain. (4) [May-10]

Q. 14 (a) With the help of neat sketch explains the working of Electrolux. What are its

advantages over absorption type refrigeration type refrigeration system? (10)

(b) What do you mean by cascading? Where is it used? (10) [May-10]

Q.15 Explain in detail simple vapour absorption system with block diagram. (20) [Dec-11]

Q.16 A refrigerating machine using NH3 operates between the temperature limits of 15°C and

30°C .find the COP of the system. The properties of NH3 are given below:


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Pressure Temp Liquid Vapour

bar °C hf Sf hg Sg

2.41 -5 351 3.95 1667.5 9.05

11.895 30 562 4.69 1711 8.48

(20) [Dec-08]

UNIT-III

Q.17 (a) Prove the following results (04)

Pw =0.622 , h=Cpm.t+w[(htg)dp=2.3td)

b) At a certain locality, the DBT of air is 30oC and RH is 40%.determine the specific humidity,

dew point and WBT of air. If this air is cooled in an air washer using recirculated spray water

and having a humidifying efficiency of 0.9, what are DBT and dew point temperature of the air

leaving the air washer? (8) [May-09]

(c) A quantity of air having a volume of 300 m3 at 30

o C DBT and 25

o C WBT is heated to

40o C DBT. Estimate the amount of heat added, final R.H, WBT of the air. (8)

Q.18 (a) Derive an expression for the adiabatic mixing of two air steams. (10)

(b) 30 m3 /min of moist air at 15

o C DBT and 13

o C WBT is mixed with 12 m

3/min at 25

o C

DBT and 50% RH. Determine the condition of the mixture also calculate the total volume

of the mixture. (10) [May-10]


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Q.19 Air at 40o C DBT and 15% RH is passed throughout an adiabatic humidified at the rate of

200 m3/min. The outlet conditions of air are 25

o C DBT & 20

o C WBT, Find-

i) Dew point temperature.

ii) Relative humidity of exit air

iii) Amount of water vapour added to the air per minute. (20) [May-10]

Q.20 (a) With the help of neat sketch, explain the working of an air-washer. Draw the all

processes on the psychometric diagram.(10)

(b) Explain the following:

Heat pumps, transport air-conditioning. (10) [May-09]

Q.21 Atmosphere air at 15 o

C and 75%RH is to be conditioned to a temperature of 25 o

C and

70%RH .the amount of air supplied to 200cu.M/min. The required condition is achieved

first by heating and then by adiabatic humidifying. Find (a) amount of steam required in

kg per hour through the heating coil at pr. 2 bar and 0.95dry. (b) The heating of water

required per hour in humidifier. (10) [May-10]

Q.22 Atmosphere air at 12oC and 75%RH is to be conditioned to a temperature of 22

oC and

60%RH .the amount of air supplied to 200cu.M/min. The required condition is achieved

first by heating and then by adiabatic humidifying. Find (a) amount of steam required in

kg per hour through the heating coil at pr. 2 bar and 0.96dry. (b) The heating of water

required per hour in humidifier. (10) [Dec-08]


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Q.23 (a) Room temperature at 20 o

C DBT and 50% RH is mixed with outdoor air at 40 o

C DBT

and 30%RH in the ratio of 4:1. The mixture is passed through a cooling coil whose

temperature is maintained constant at 10 o

C whose by-pass factor is 0.2.determine (i)

condition of air before entering the cooling coil. (ii) Condition of air leaving the coil (iii)

refrigeration load on the cooling when 250m3/min of air is supplied to the room. (12)

(b) Define ‘RSHF’ and ‘coil by-pass factor’. (8) [May-12]

Q.24 (a) 150 m3/min of Air at 35

o C DBT and 20% RH is passed throughout an adiabatic

humidified. The outlet conditions of air are 20o C DBT & 15

o C WBT, Find-

i) Dew point temperature.

ii) Relative humidity of exit air

iii) Amount of water vapour added to the air per minute. (15)

(b) Write a short note on ‘Psychrometric chart’. (5) [May-12]

Unit – IV

Q.25 (a) what is alignment circle? Using psychometric chart find the value of specific humidity,

WBT, DPT, specific volume, RH and enthalpy corresponding to the alignment circle.

(10)

(b) Draw a neat diagram for air-conditioning system required in winter season. Explain the

working of different components in the circuit. Is it possible to use the steam for such air-

conditioning system? (10) [May-10]

Q.26 The following data is given for the space to be air conditioned.

Out door air-conditions 43o C DBT & 27

o C WBT

Inside design conditions 23o C DBT & 50% RH


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Room sensible heat load=10kw

By pass factor of the cooling coil=0.2

The return air from the room is mixed with the outside air before entering the cooling coil

in the ratio of 3:1 by mass. Determine-

a) Supply air flow rate

b) Outdoor air sensible heat

c) Outdoor air latent heat

d) Grand total heat

e) Effective room sensible heat factor (20) [May-10]

Q.27 (a) with the help of neat sketch and psychometric diagram explain the hot and dry

summer air-conditioning. (10)

b) An air conditioning plant is designed for a small office for winter air condition outdoor

conditions are 10 o

C DBT and 8 o

C WBT.required indoor conditions are 20 o

C DBT

and 60 % RH. Amount of air circulated 0.3 m3/min/person. Seating capacity of office is

50 persons. Required condition is achieved firat by heating and then by adiabatic

humidifying. Find (i) heating capacity of heating coil in kW and its surface temperature if

the bypass factor of the coil is 0.32 (ii) capacity of humidifier. (10) [May-09]

Q.28 writes short notes on:

(i) Comfort air condition

(ii) Adiabatic mixing of two streams

(iii) Estimation of cooling and heating load. (20) [May-09]

Q. 29 The following data refer to summer air conditioning of a auditorium:-


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Indoor conditions = 27o C DBT, 55%RH

Ambient conditions = 25o C DBT, 28

o C WBT

Room sensible heat gain = 120000 KJ/hr.

Room latent heat gain = 45000 KJ/hr.

The condition of air leaving the coil is 17 o C.

The return air from the room is mixed with 40% of makeup air after the cooling coil. Find

(i) Apparatus dew point (ii) amount of make-up air (iii) by pass factor. (20) [May-12]

Q.30 The following data refer to room:

Indoor conditions = 21o C DBT, 50%RH by air supplied at a temperature of 16

oC.

outdoor conditions = 32o C DBT, 40%RH

Room sensible heat gain = 20000 KJ/hr.

Room latent heat gain = 4000 KJ/hr.

The ratio of recirculated air to fresh air is fixed at 3:1 by mass the plant consist of a direct

expansion cooling coil and after heater and a constant speed fan. Calculate (i) The

quantity of air supplied per minute in cubic meters.(ii) the load on refrigerating plant in

tons of refrigerating assuming the by pass facot is 0.1 (iii) The load on after heater in kW.

(20) [May-10] & [Dec-11]

Q.31 Air at 10oC DBT & 90% RH is to be heated and humidified to 30

o C DBT 22.5

o C WBT.

The air is pre-heated sensibly before passing to the air-washer in which water is recirculated. The

relative humidity of air coming out of the air washer is 90%. The air is again reheated sensibly to

obtain the final design condition. Find-(I) the temperature to which the air should be preheated.

(20) [Dec-08]


10

Q.32 The reading from a sling psycho meter is as follows-

DBT= 30o C

WBT 20o C

Barometer reading = 740 mm of Hg

Using steam tables, determine:

i) DPT (ii) RH (iii) Specific humidity (iv) Degree of saturation (v) Vapour density

(vi) Enthalpy of mixture per kg of dry air. (20) [Dec-08]

· q.10 in a vapour compression refrigeration system using r-12, the evaporator pressure is 1.4...

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