VAPOR COMPRESSION REFRIGERATION QUESTION ANSWERS

VAPOR COMPRESSION REFRIGERATION

QUESTION ANSWERS

Question answers on basics of

refrigeration increases the level

of understanding very deeply.

Compressor, condenser, receiver,

evaporator are essential components of

refrigeration. Condensing unit is

motor, compressor, condenser

and receiver. Different prime

movers run refrigeration units.

  1. What is the meaning of condensing unit?

Condensing unit is the assembly of motor, compressor, condenser and receiver mounted on a single frame.

2. List the different names of high pressure in refrigeration.

There are three different names of the high pressure. These are

(a)  Head pressure

(b) Discharge pressure

(c) Condensing pressure

3. What are the different names of low pressure in refrigeration?

There are three different names of the low pressure. These are

(a) Back pressure

(b) Suction pressure

(c) Evaporator pressure

4. What are the different parts which belong to the low pressure side of a refrigeration unit?

There are six parts which belong to low pressure side. These are

(b) Liquid vapor pipe from the outlet of expansion valve to the inlet of evaporator

(c) Evaporator

(d) Suction pipe

(e) Suction service valve

(f) First half portion of the piston stroke and the crank case

5. What are the different parts which belong to the high pressure side of a refrigeration unit?

There are eight parts which belong to high pressure side. These are

(a) Later half portion of the piston stroke

(b) Discharge service valve

(c) Discharge pipe

(d) Oil separator

(e) Condenser

(f) Receiver

(g) Liquid pipe

(h) Inlet of expansion valve

6. What are the different names of the prime mover in refrigeration?

There are four different names of the prime mover. These are

(a) Diesel engine

(b) Petrol engine

(c) A.C. motor

(d) D.C. motor

7. What are the different processes and their sequence in refrigeration?

There are four different processes in the following sequence. These are

(a) Compression process

(b) Condensation process

(c) Expansion process

(d) Evaporation process

8. Give the definition of a compound gauge.

It  measures vacuum (less than atmospheric pressure) as well as above atmospheric pressure.

Fit this gauge on suction side of the compressor to know the back pressure or low pressure.

Its range is 32″ of Hg to 150 p s i g.

9. Define a pressure Gage.

Measures pressures above atmospheric pressure. Fit this gauge on discharge side

of the compressor. Its range is 0 to 300 p s i g.

10. What is crank case of a compressor?

Portion (space) in which the crankshaft is located. It contains lubricating oil in it.

11. Write the other common name for the piston ring. 

(a) Gudgeon pin

(b) Wrist pin

12. Why to compress the refrigerant?

Refrigerant gains heat at low temperature (say -15oC) in the evaporator from the items to be cooled. This heat is to be rejected to atmosphere which is at a much higher temperature (say + 30oC). Rejection of heat is possible only  if the temperature of the refrigerant is at much higher than the atmospheric temperature. Pressure & temperature of the refrigerant increase due to compression.

13. What is a receiver?

Receiver is a storage tank for the liquid refrigerant. Place this unit just after the condenser. Store liquid refrigerant in it.  It supplies refrigerant in the refrigerant circuit as per cooling requirements. Use it only for high cooling requirements. During winters, store the entire refrigerant in it. Refrigerant is more corrosive in the stationary condition than in the running condition. Therefore, provide a special inside coating to overcome corrosive effect. It keeps refrigerant safe for long periods.

14. DIFFERENCE BETWEEN CRANK SHAFT AND ECCENTRIC SHAFT
Sr. No.
Crankshaft
Eccentric shaft
1.
Make crankshaft a single piece by casting or by Forging
Uses a cast iron eccentric on a forged steel shaft
2.
 Multi-cylinder compressor use a crankshaft.
 One or two cylinder compressor use this shaft.
3.
Decreases vibrations by using counter weights on it
Its operation is smooth and balanced.
4.
This is  costly.
It is cheap.
 15. Define refrigeration
Refrigeration is a process of creating and maintaining lower temperatures as compared to the surrounding temperature.
For example
(i)   To cool a room in summer with a window air conditioner
(ii) To produce lower temp in a fridge.

16. What is air conditioning?

Air conditioning  controls simultaneously temperature, humidity, purity and velocity of air
i.e. (i) Temperature between 22 to 27C
(ii) Relative humidity 35 to 60 %
(iii) Dust particles of size > 100  microns are not permitted
(iv) Velocity 8 to 10 m/s
Examples of air conditioning are:
Window air conditioner for a room
Central air conditioning plant for a big auditorium  
 17. APPLICATIONS OF REFRIGERATION
(i) Fridge
(ii) Air conditioner
(iii) Deep Freezer
(iv) Water cooler
(v) Display cabinets used in confectionery shops
(vi) Incubators
(vii) Cold storage
(viii) Ice plants
(ix) Distilleries
(x) Milk plants
(xi) Dairy industry
(xii) Food processing, preservation and distribution
(xiii) Chemical and process industries
(xiv) Cold treatment of metals
(xv) Medical
(xvi) Construction,
(xvii) Ice skating
(xviii) Comfort air-conditioning

18.  Applications of Comfort air conditioning 

(i) Window air conditioner in homes, offices and shops
(ii) Air conditioning of cinema halls,
(iii) Air conditioning of libraries,
(iv) Air conditioning of computer centers,
(v) Air conditioning of hotels,
(vi) Air conditioning of hospitals,
(vii) Air conditioning of cars, buses, trains and aircrafts
(viii) Air conditioning of air-ports.
(ix) Air conditioning of saloons
(x) Air conditioning of malls
(xi) Air conditioning of restaurants

19. Applications of Industrial  air conditioning

(i)  Textile industry
Printing industry
(iii) Photography industry
(iv) Power plants industry

20. Which are the main parts of a mechanical refrigeration system?

(a) Prime mover (Diesel engine/electric motor)

(b) Compressor

(c) Oil separator

(d) Condenser

(e) Receiver

(f) Refrigerant control device (expansion valve)

(g) Drier cum filter

(h) Evaporator (Cooling coil)

(i)Accumulator

(j) Mufflers

(k) Refrigerant piping

21. What is basis for dividing the refrigeration system into high and low pressure sides?

Evaporator operates at low pressure. Condenser operates at high pressure and thus is the classification.

22. Why is pressure difference necessary in a refrigeration system?

There is heat transfer from low temperature to high temperature. Low temperature side will have low operating pressure while high temperature side will require a high pressure. Heat  transferred is latent heat. At low pressure, the boiling point is lower. It is high at high pressure. Thus a pressure difference is necessary in a refrigeration cycle.

23. Which is the best system to produce refrigeration?

Vapor compression refrigeration is the best and oldest method. Because  advanced technology is available for it.

24. What are advantages of mechanical refrigeration system over vapor absorption system?

  1. Mechanical refrigeration is more efficient.

  2.  Used in domestic, commercial and industrial units.

  3. It is cheap with respect to the initial investment for cooling capacity up to 50 tons of refrigeration.

  4. Its running cost is also less because of high Coefficient of performance.

  5. Space requirement is less for small capacity plants

Q25. Differentiate between a heat pump and an electric heater.

COP heat pump=1 + COP refrigerator =1 + N/W

In a heat pump, the working temperatures are 50C evaporator  and 200C room temperature. The working temperatures in a refrigerator are -150C evaporator and + 300C condenser.  Three times heating effect of the work (W) input in a heat pump. In electric heating, heating effect is only electric power input. Therefore, in winter, it is advisable to use heat pump in place of an electric heater for room heating.

Q26. Write the two limitations of single stage vapor compression refrigeration system.

The two limitations are

( i) Evaporator temperature is normally -150C. Further decrease of temperature beyond a particular limit is not possible because of the following reasons:

(a)  Lowering of evaporator temperature increases compression ratio and work input and thus decreases COP.

(b)  Lowering of evaporator temperature increases compression ratio. A compression ratio more than six is not achievable in any single compressor.

(ii) Condenser temperature is not increased beyond a particular limit. It is dependent of air or water temperature. This is beyond human control.

Q27. What is DRY compression?

Dry compression

Compression start from Dry  saturated vapor condition. It ends in the super heated region. This is shown as 1′-2′. 

Figure: 1-2  Wet Compression on p-h & T-s Charts

1′-2′ Dry Compression on p-h & t-s Charts

Q28.  What is wet compression?

When vapor is dry saturated at the end of compression.

Compression start from the wet region. It ends at the dry saturated vapor condition. This is shown in Figure as 1-2.

Q29. What is a Solenoid Valve? 

It is operated with electrically. This is fitted into the liquid line after the condenser. It has only two positions.

(i)Fully open

(ii)Fully closed

There is a coil and a plunger. Flow of electric current produces magnetic field. The plunger lifts up and  the valve is in open position. When compressor is working, electricity is there, valve is in open position. Electric power from the mains is to compressor through the solenoid valve.

When power is off, magnetic effect is lost. Plunger falls down and the valve comes to close position. Then it does not allow the flow of liquid refrigerant to  the evaporator. Because of this pressures in the evaporator and condenser equalize. Starting torque reduces. Life of the Motor increases.

Q30. What is an Accumulator in Refrigeration?

It is a cylindrical unit. This is placed just after the evaporator. It has baffles in it. More of vapor and less of liquid refrigerant enters the bottom of the accumulator. It allows the flow of vapor to the compressor. Liquid refrigerant is sent back to the inlet of the evaporator. It does not allow the flow of liquid refrigerant into the compressor. This is used with a flooded evaporator. A flooded evaporator has liquid refrigerant from the inlet to the outlet of the evaporator. The flooded evaporator gives more cooling effect as compared to dry evaporator system.

Q31. Where to Use Centrifugal Compressor in Refrigeration?

When Volume to be handled by the compressor is very large and pressure difference is small (< 1.5 bars for a single stage) between condenser pressure and evaporator pressure. It will be applicable for refrigerants having normal boiling point above 0 C. This refrigerant will operate under vacuum at the evaporator temperature of -15 C. Hence specific volume will be large under vacuum condition. Such refrigerants are R-11 (23.90 C) and R-113 (28.0 C).

Q32Give the brief description of Hermetically sealed compressor.

Hermetically sealed unit

  1. Both compressor and motor are  in one housing. Its main body is in two parts. Upper and lower domes are of black color.   Lower dome contains lubricating oil and springs. Compressor-motor assembly is mounted on these springs to have a quiet operation. These are welded after fitting motor compressor assembly in it.

    Upper dome has three pipes. First is the suction pipe which ends on the dome itself. Second is the charging pipe which also ends on the dome. The charging tube is normally pinched off. Charging of refrigerant is done through it. Third is the discharge pipe. It goes from the compressor to the condenser. There is no suction and discharge valves in it.

2. Sealed unit has suction reed and discharge reeds in place of suction and discharge valves to make it cheaper. The RPM of compressor and motor are same (2800 RPM). There are no vibrations as the compressor and the motor is mounted on three springs. This unit is almost sound free. These are very cheap and work satisfactorily. These have a long life. Hermetically sealed compressors are very common on low refrigerating capacity units. These are used in 

(i) refrigerators

(ii) water coolers

(iii) window air conditioners 

(iv) display cabinets.

Q33. What is super heat OR degree of super heat?

  1. Difference of actual temperature and boiling temperature of a vapor at a certain pressure. Normal Boiling Point (NBP) of R-22 is -410C at 1 atmosphere.

  2.  Vapor  is at a temperature of -300C at 1 atm. Degree of super-heat is 110C = -30-(-41) = 110C.

  3. Degree of SUPER-HEAT IS applicable only IN VAPOR FORM.

  4. Super-heating takes place at constant pressure.

 

Q34. Sub-cooling or degree of sub-cooling

a. Difference of temperature between the actual temperature and saturation temperature of a liquid at a certain pressure. It is applicable only in the liquid form.

b.

(i) Normal boiling point of R-22 is -410C at 1 atm. Suppose, it is in liquid form at a temperature of -500C at 1 atm. Then, degree of sub-cooling is 90C

= -41-(-50) =90C.

(ii) Water boiling point at 1 bar is 100 0C. water available in a tap is say at 200C. The degree of sub-cooling in this case is  800C=100-20=800C.

c. Degree of sub-cooling is applicable only in liquid form.

d. Sub-cooling takes place at constant pressure.

Q35. What is effect of super-heat on COP? 

(i)  If the degree of super-heat is greater than 50C, COP decreases. Due to super-heat, there is increase in cooling effect and work input to compressor. When degree of super-heat is greater than 50C, increase in N is much less than increase in W, so COP decreases.

(ii)  When degree of super-heat is less than 50C, increase in N is much more than increase in W, so COP increases.

Q36. What is effect of sub-cooling on COP?

  Increase of degree of sub-cooling increases the COP.

It increases cooling effect without increasing the work input.

So sub-cooling in refrigeration cycle is highly desirable.

Install Liquid suction heat exchanger between the suction and liquid line. This causes sub-cooling of the refrigerant.

Q37. Discuss various compressors in refrigeration.

(a)    Reciprocating Compressor

 (I) Where the normal boiling point (N B P) of refrigerant is lower than the application temperature.

(II) Where evaporator and condenser pressures are positive pressures.

(III) When the difference in condenser pressure and evaporator pressure is greater than 3.5 atm.

(IV) Where the volume of refrigerant vapor at entry to compressor is less.

For example: such conditions are satisfied in case the refrigerant is R-22, NH 3, Co 2, R-134 a.

(b) Rotary compressor

 (i)  Where the N B P is near to the application temperature.

(ii)  Where evaporator pressure is higher than atmospheric pressure but close to it.

(iii)  Volume at the inlet of compressor is moderate.

(iv) Where pressure difference between condenser and evaporator pressures is > 1.5 atm. but < 3.5 atm.

ISO-BUTANE satisfy such conditions as refrigerant.

 ©   Centrifugal compressor

(i) Where the NBP is much higher than the application temperature.

(ii) Where the pressure in evaporator is vacuum.

(iii) The volume handled at the inlet of the compressor is extremely large.

(iv) Where the pressure difference between evaporator pressure and condenser pressure is < 1.5 bars.

EXAMPLE: R-11 and R-114

 Q38.  List the various types of cooling medium in condensers and the practical applications of each type.

Various types of condensers with respect to cooling medium are

Air Cooled,

Water Cooled

Evaporative Cooled

Air Cooled Condensers:

(i) Natural air cooled as in small size domestic refrigerators

(ii) Forced air cooled Condensers for

Water coolers,

Window air conditioners,

Big size refrigerators,

Display cabinets,

Deep freezers,

Air conditioning of vehicles.

Water Cooled Condensers:

Used in packed units,

Used in air conditioning plants of libraries, cinema halls, hotels, computer centers, hospitals.

Evaporative Cooled Condensers:

Used in industrial units like

Ice plants,

Cold storage,

Breweries,

Milk plants,

Refineries

Q39. What is refrigerating effect OR cooling effect?

 Heat removed per kg of refrigerant from the products cooled. Its symbol is ‘N ‘. The unit of N is kJ/kg.  Point 4 is entry to evaporator. Point 1 is exit of evaporator, then

N=h1-h4   kJ/kg

Q40. What is specific work?

Specific work input to compressor for compressing 1 kg of refrigerant from evaporator pressure to condenser pressure. Its symbol is ‘W’.  Its units are kJ/kg.

W = h2–h1 kJ/kg

Q40.     What is COP (Coefficient of Performance)?

COP= N/W.  It has no units.

Q41.  DISCUSS COMPRESSOR CAPACITY.

Compressor capacity is in tons of Refrigeration: 5 TR compressor, 10 TR compressor.

Q42.  What is volumetric efficiency of a compressor?

  It is a ratio of sucked volume to stroke volume.

ηvol = volume actually sucked/stroke volume

ηvol = 1 + c – c (pd/ps)1/n

C % clearance

if 5% then c=0.05

pd is discharge (high) pressure

ps is suction (low) pressure

Q43. What is a heat pump?

Heat pump is a device to heat the room in WINTER. It uses a refrigeration cycle. The condenser in inside the room. The evaporator is outside the room.

Evaporator picks up heat from outside at low temperature. Condenser delivers heat at higher temperature inside the room.   Heating of the room takes place.

COP heat pump = (N+W)/W=1+N/W= 1+COPref

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