R& AC

QUESTION BANK

UNIT – I Introduction to Refrigeration: Necessity and applications – Unit of refrigeration and C.O.P. –

Mechanical Refrigeration – Types of Ideal cycles of refrigeration.

Air Refrigeration: Bell Coleman cycle and Brayton Cycle, Open and Dense air systems – Actual air refrigeration system problems –

Refrigeration needs of Air crafts.

1. Explain the boot strap air cycle refrigeration system with a schematic and cycle diagrams.

2. Derive an expression for the COP of Bell- Coleman cycle.

3. What is the difference between a refrigerator and a heat pump?

4. The capacity of a refrigerator is 200 TR when working between - 8oC and26oC. Determine the mass of ice produced per day

from water at 260C. Also find the power required to drive the unit. Assume that the cycle operates on reversed carnot cycle

and latent heat of ice is 335 kJ/kg

5. Explain the importance of Industrial Refrigeration citing specific cases.

6. Distinguish between vapour absorption and vapour compression system and what is the distinct advantage of vapour

absorption over vapour compression system.

7. Explain the working principle of vapour compression refrigeration system with a neat diagram and also plot the same on a P-

h and T-s diagram

8. Give a brief description of an ideal cycle of air refrigeration.

9. A Carnot refrigerator and a heat pump are supplied with equal amount of work. The refrigerator operates between -270C and

+270C and the heat pump operates between + 450 C and + 270C. The refrigerator absorbs 4000 kJ/ min at -270C. The

heat pump absorbs all the heat rejected by the refrigerator and supplies at 450C. Compute

(i) COP of refrigerator

(ii) COP of heat pump

(iii) Heat available at + 450C and

(iv)Work input to each unit.

10. Prove that the COP of a Heat Pump is greater than 1

11. Determine the temperature ratio for a Carnot refrigerator whose C.O.P is 4. What is the refrigeration capacity of the machine

in tons of refrigeration if the power consumption is 7.5 kW. If the cycle is used as a heat pump, find its COP.

12. A Bell-Coleman refrigeration cycle works between 1 bar and 6 bar. The adiabatic efficiency of compression is 90% and

expansion is 95%. Find the COP of the system and its tonnage when the air flow rate is 2 kg/sec. The ambient temperature is

25oC and refrigerator temperature is -5oC.

13. How is ideal reversed Carnot cycle modified to result in Bell-Coleman cycle? Explain with sketches.

14. Explain the working of simple air evaporative cooling system used for air crafts?

15. The Carnot cycle machine operates between the temperature limits of 470C and -300C. Determine the C.O.P. when it

operates as

1. A refrigerating machine

2. A heat pump and

3. A heat engine

16. What are the advantages of dense air refrigeration system over an open air refrigerating system?

17. A refrigerator works on the carnet cycle in temperature between -70 C and 270 C. It makes 500kg of ice per hour at -50 C

from water at 140 C. Find H.P required to drive the compressor and C.O.P. of the cycle. Take specific heat of ice as 2.1

kj/kg-k and latent heat as 336kj/kg?

UNIT – II

Vapour compression refrigeration – working principle and essential components of the plant – simple Vapour compression

refrigeration cycle – COP – Representation of cycle on T-S and p-h charts – effect of sub cooling and super heating – cycle

analysis – Actual cycle Influence of various parameters on system performance – Use of p-h charts – numerical Problems

1. A refrigerator using Ammonia works between the temperatures -10 deg. C and 25 deg. C. The gas is dry at the end of

compression and there is no under cooling of liquid. Using the tables, Calculate the theoretical COP of the cycle

2. Draw the refrigeration cycle on T-S diagram. When the refrigerant is dry and saturated at the end of the compression and

find an expression for the C.O.P in terms of

(i) Temperature and entropies

(ii) Enthalpy.

3. Derive an expression for COP of vapour compression cycle from t-s chart when the refrigerant is dry saturated before

compression.

4. Describe the use of liquid vapour regenerative heat exchanger in a vapour Compression system

5. A simple saturation ammonia compression system has a high pressure of 1.35 MN/m2 and low pressure of 0.19 MN/m2.

Find per 400,000kJ/h of refrigerating capacity the power consumption of the compressor and COP of the cycle.

6. A vapour compression machine is used to maintain a temperature of -230 C in a refrigerated space. The ambient

temperature is 37o C. The compressor takes in dry saturated vapour of F- 12. A minimum 100c temperature

difference is required at the evaporator as well as at condenser. There is no sub cooling of the liquid If the refrigerant

flow rate is 1 kg/min, F i n d

(i) Tonnage of refrigeration

(ii) Power requirement

(iii)Ratio of COP of this cycle to COP of carnot cycle.

7. Sketch the T-S and p-h diagrams for the vapour compression cycles when the vapour after compression is:

i. dry saturated, and ii. Wet.

8. A vapour compression refrigerator works between the pressure limits of 60 bar and 25 bar. The working fluid is just

dry at the end of the compression and there is no under cooling of the liquid before the expansion valve. Determine:

i. C O P of the cycle and

ii. Capacity of the refrigerator if the fluid flow is at the rate of 5 kg/min.

Pressure (bar) Saturation Temp (K) Enthalpy (kJ/kg) Entropy (kJ/jg.K)

Liqui

d

Vapour Liqui

d

Vapour

60

25

295

261

151.9

6

56.32

293.29

322.58

0.554

0.226

1.0332

1.2464

8. With a neat sketch, explain the working principle of vapour compression re- frigeration system.

9. A15TR freon22 Vapour compression system operates between a condenser temperature of 400 C and an evaporator

temperature of 50 C.

i) Determine the compressor discharge temperature.

ii) Calculate the theoretical piston displacement and power consumption f the compressor per ton if refrigeration.

Using the p-h diagram for Freon22. Assuming the specific heat of its vapour as 0.8 kj/kg.k

10. Explain the construction of T-S and P- h diagrams and state why P- h diagram is more suitable for calculations over T-S

diagram.

11. A refrigerating machine using NH3 operates between the temperature limits of -150 C and 300 C. Find the C O P of the

system. Also find the corresponding value for a reversed Carnot cycle operating between the same temperatures. The

properties of NH3 are given below:

Pressure(bar) Temperature0 C Liquid Vapour

hf(Kj /Kg) Sf(Kj /Kg-K) hg(Kj /Kg) Sg(Kj /Kg-K)

2.41

11.895

-5

30

351

562

3.95

4.69

1667.5

1711

9.05

8.48

UNIT III System Components: Compressors – General classification – comparison – Advantages and

Disadvantages.

Condensers – classification – Working Principles

Evaporators – classification – Working Principles

Expansion devices – Types – Working Principles

Refrigerants – Desirable properties – classification refrigerants used – Nomenclature – Ozone Depletion– Global Warming.

1. Explain the working of a rotary screw compressor.

2. How the capacity control is achieved in refrigerant compressor?

3. Give the normal boiling points and designations of floro chloro derivatives of methane.

4. Describe the chemical and physical requirements of a good refrigerant.

5. Describe the important components of a centrifugal compressor with the help of a neat sketch.

6. Name the two types of rotary compressors. Explain the working of anyone type of rotary compressor.

7. Derive an expression for the shaft work of a reciprocating commpressor as- suming zero clearance volume.

8. Single stage reciprocating commpressor is required to commpress 1.5 m3 /min if vapour refrigerant from 1 bar to 8

bar. Find the power required to drive the commpressor, if the commpression of refrirant is (i) Isothermal; (ii)

polytropic with polytropic index as 1.12; and (iii) isentropic with inentropic index as 1.31.

9. Explain the dry expansion evaporator with the help of a neat sketch.

10. Write short note on:

i. Ozone depletion

ii. Global warming.

11. What is the difference between dry expansion type and flooded type evaporator?

12. What are the different types of commpressors? Mention the fields for the use of each in refrigeration systems giving

reasons.

13. Explain why reciprocating compressor cannot be used as a vaccum pump for producing high vacuumm.

UNIT IV Vapor Absorption System – Calculation of max COP – description and working of NH3 – water system and Li Br –water ( Two shell

& Four shell) System. Principle of operation Three Fluid absorption system, salient features.

1. Explain the working of an evaporative condenser

2. Give the comparison of flooded evaporators and dry evaporators

3. What are the different types of evaporators used in a vapour compression refrigeration system? Explain the working of

any one of them

4. What are the advantages and disadvantages of capillary tube over other types of expansion devices?

5. Explain the working of a automatic expansion valve with the help of a neat sketch

6. Describe the working of shell and tube type and shell and coil type evaporators.

7. Explain the working of thermostatic expansion valve with the help of a neat

sketch.

8. Derive an expression for the C.O.P. of an ideal vapour absorption system in terms of temperature Tg at which heat is

supplied to the generator, the tem- perature Te at which heat is absorbed in the evaporator and the temperature Tc at

which heat is discharged from condenser and absorber.

9. In a vapour absorption refrigertion system, heating, cooling and refrigeration takes place at the temperatures of 1000 C,

200 C and -50 C respectively. Find the maximum C.O.P of the system.

10. Draw a neat line diagram of Electrro - Lux refrigerator and explain its working principle. What is the important

role of hydrogen in this refrigeration system?

11. Draw a neat sketch of a practical vapour absorption refrigeration cycle. Indi- cate there on the phases of various flieds and

the name of the equipments. Also indicate the direction of the external energy flow to or from the equipments.

12. Mention the function of each fluid in a three-fluid vapour absorption system

13. Draw a neat diagram of lithiem bromide water absorption system and explain its working.

14. What is the function of the following components in an absorption system:

i. Absorber

ii. Rectifier

iii. Analyser

iv. Heat exchangers

UNIT V Steam Jet Refrigeration System – Working Principle and Basic Components. Principle and operation of (i) Thermoelectric

refrigerator (ii) Vortex tube or Hilsch tube.

1. What are the characteristics of ideal absorbent and refrigerant mixture in vapour absorption refrigeration.

2. Explain with a simple sketch the construction of Electrolux refrigeration system

3. What is the effect of inert gas in three fluid refrigeration system.

4. What is Carnot COP of absorption refrigeration system. How it is differ from actual COP.

5. What are the disadvantages with water in absorption system?

6. Why the boiling point difference of absorbent-refrigerant should be high

7. What are the desirable requirements of a Refrigent - Absorption pair

8. Determine the HCOP of a vapour absorption refrigeration system when the temperature of generator is 120o C, the

temperature of the condenser is 30o C and the temperature of the evaporator is -20o C. What would be its COP if it were a

Carnot.

9. Explain, with the help of a neat sketch, the working of a steam jet refrigeration system.

10. What are the advantages of barometric condenser over surface condenser in steam jet refrigeration system?

11. List out the merits and demerits of thermo-electric refrigeration system over other refrigeration systems

12. What are the various applications of thermo-electric refrigerator

13. What is the situation unnder which the Steam Jet Refrigeration system is recommended? What are its limitations? Can

it be used for obtaining sub- zero temperatures?

14. Explain the various components of Steam Jet Refrigeration system and clearly discuss the function of each component.

15. What is the effect of latent heat of absorbate on performance of the absorption

systems.

16. What is absorption refrigeration system and how it differs from convention

vapour compression system and explain its working.

UNIT – VI Introduction to Air Conditioning: Psychometric Properties & Processes – Characterization of

Sensible and latent heat loads –– Need for Ventilation, Consideration of Infiltration – Load concepts of RSHF, GSHF- Problems,

Concept of ESHF and ADP.

1. What are the limitations of steam jet refrigeration system?

2. What are the merits of steam jet refrigeration system over other system?

3. How can you produce the cold with the vortex tube?

4. What is the principle in steam jet refrigeration. Explain with neat sketch.

5. With the help of a sketch of the processes on Mollier chart explain how the refrigeration is produced in steam jet

refrigeration system.

6. What difference does it make when the steam jet refrigeration system operates with a barometric condenser instead of

a surface condenser?

7. Explain thermo electric refrigeration system

8. Explain the term seebeck effect, Thomson effect

9. Where is steam jet refrigeration system widely used. Explain how the refrig- eration is produced with this system

10. What are the materials for the thermoelectric refrigeration?

11. Define relative humidity, specific humidity, and dew-point temperature and describe a theoretical method for

determining their values.

12. When do the DBT, WBT and DPT become equal?

13. Air at 370 C, 44 percent relative humidity, is cooled to 230 C by spraying water at 130 C into it. The mixture

pressure remains constant at 101.3 kPa. Assum- ing that all of the water evaporates and that the mixing occurs in

an insulated duct, calculate the mass of water added per kilogram of air.

14. Why does the enthalpy of an air-vapour mixture remain constant during an adiabatic saturation process?

15. 1100 m3 of air per minute at 250 C DBT and 45% R.H. is supplied to an auditorium.

The condition of the atmospheric air is 350 C DBT and 65% R.H. The required condition is achieved first by

cooling and dehumidifying and then by heating. Find the cooling capacity required in tons of refrigeration and

heating capacity required in kW.Take air pressure as 1.03 bar.

16. An air-water vapour mixture enters a heater-humidifier unit at 80 C, 110 kPa,

45% RH. The flow rate of dry air is 0.15 kg/s. Liquid water at 150 C is sprayed into the mixture at the rate of

0.003 kg/s. The mixture leaves the unit at 400 C, 110 kPa. Calculate

i. The relative humidity at the outlet, and

ii. The rate of heat transfer to the unit.

17. Explain the following terms:

i. Specific volume of humid air

ii. Density of humid air

iii. Enthalpy of humid air

UNIT VII Requirements of human comfort and concept of effective temperature- Comfort chart –Comfort Air conditioning – Requirements of

Industrial air conditioning, Air conditioning Load Calculations.

1. Define the term “effective temperature” and explain its importance in air- conditioning systems. Describe the factors

which affect the effective temper- ature.

2. Define the “human comfort” and describe the factors which affect the human comfort

3. A quantity of air having a volume of 300 m3 at 30o C (DBT) and 25o C (WBT) is heated to 40

o C (DBT). Estimate

the amount of heat added, final relative humidity and WBT. The air pressure is 1.01325 bar.

4. 800m3 /min of re-circulated air at 22o C (DBT) and 10o dew point temperature is to mixed with 300 m3 /min of

fresh air at 30o C (DBT) and 50% RH. Determine the enthalpy, specific volume, humidity ratio and dew point temp

of the resultant mixture

5. An auditorium of 100 seating capacity of conditioned for the given specifications

Out door conditions - 35 and 65% RH

Required air inlet conditions - 15o C

and 40 RH.

The required condition is achieved first by cooling and dehumidifying and then by heating. Find the following

(a) The capacity of the cooling coil in tons of refrigeration

(b) Capacity of the heating coil in kW

(c) By-pass factor of the heating coil if the surface temp of the coil is 22o C

6. Define “thermal shock” used to air-conditioning systems. Describe the meth- ods of reducing the thermal shock.

7. Why ventilation is required? Explain why different ventilation standards for different purposes are recommended

8. The amount of air supplied to an air conditioned hall is 300 m3 / min. The atmospheric conditions are 35o c DBT

and 55% RH. The required conditions are 20o c DBT and 60% RH. Find the Sensible Heat and Latent Heat removed

from air per minute.

9. Define human comfort and explain the factors which affect the human comfort

10. Explain h o w does the body attempt to compensate for a cool environment which tends to lower the internal

temperature?

11. Why ventilation is required? Explain why different ventilation standards for different purposes are recommended?

12. Ten grams of moisture per kg of dry air is removed from atmospheric air when it is passed through an air

conditioning system and its temperature becomes 20o

C. The atmospheric conditions are 40o

C DBT and 60% RH.

Find the following for the conditioned air

i. Relative humidity

ii. Wet-bulb temperature

iii. Dew point temperature

iv. Enthalpy change for the air

Assume standard atmospheric pressure

13. Represent the following process in a skeleton psychrometric chart.

i. Sensible cooling

ii. Cooling and humidification

iii. Adiabatic mixing of air streams

14. 100 m3

of air per minute at 15o

c DBT and 80% RH is heated until its tem- perature become 22o

c. Find the

following

i. Heat added to the air per minute

ii. RH of heated air

iii. WBT of heated air

Assume air pressure is 1.033 bar

15. Describe the following psychrometric processes

i. cooling with dehumidification

ii. cooling with adiabatic humidification

16. In a laboratory test a psychrometer recorded 36o

C DBT and 30o

C WBT cal- culate

(a) vapor pressure

(b) relative humidity

(c) specific humidity

(d) degree of saturation

(e) dew point temperature

(f ) enthalpy of the mixture

17. An air water vapor mixture at 1 bar and 26.7o C has a specific humidity of

0.0085. Determine the percentage saturation.

UNIT – VIII

Air Conditioning systems - Classification of equipment, cooling, heating humidification and dehumidification, filters, grills and

registers, fans and blowers. Heat Pump – Heat sources – different heat pump circuits

1. Explain the working principle of a heat pump which makes use of Rankine power cycle with a neat diagram

2. Explain solar energy assisted heat pumps by drawing the circuit

3. With a simple sketch explain the principle of centrifugal dust collector.

4. List the different methods of odour removal from air and write short notes on any three methods.

5. What are the different constructional features used in heat pump to improve the overall EPR?

6. Define the following in relation to the conditioned supply air to room ‘Throw’, ‘Drop’ and ‘Spread’

7. Differentiate between central and unitary air conditioning systems

8. Write notes on Grills and Registers.

9. Differentiate between central, district and Unitary a i r -conditioning systems.

10. Discuss relative merits and demerits of central system with District system.

11. Describe t h e different methods of air c on d i t i on i n g duct design. Why a r e dampers required in some systems?

12. What are the different methods of heat recovery and where they are used?

13. What are the different methods of controlling the temperature? Discuss their relative merits.

14. What are the advantages of all air-conditioned system over all water air- conditioned system?

15. Discuss the common considerations for humidification of air in air conditioning.

16. What are the different methods of humidifying the air? Explain the working of any one of the atomizing the

water type humidifier.

17. Describe the following types of air filters for cleaning air with simple sketches.

i. Viscous filter

ii. Wet filter.

18. Sketch a self contained air conditioning unit with air as the sink for the con-

denser and describe its working.

19. (a) State the different types of supply air out lots

(b) Describe the factors governing the selection and location of supply air outlets

20. A heat pump uses water at 10C as the source of heat and the air supplied to the room is to be at 350

C. If the

actual EPR attained is 70% of the reversed carnot cycle operating between the same temperatures, determine

i. The actual EPR of the heat pump system

ii. The power required to run the compressor if required heat load is 60,000KJ/hr.

The p ow er required to run the compressor if required heat load is 60,000KJ/hr.