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DAIRY ENGINEERING

 

Refrigeration and Air Conditioning

 

Compiled By: - Mr. Girish Rajpurohit

 

Q. What is refrigeration?

A. Refrigeration is a process of removing heat from a space to reduce the temperature below the immediate surrounding temperature. Thus, refrigeration refers the pumping of heat from lower to higher temperature.

 

 

Q. Define one ton of refrigerant?

A. Capacity of refrigeration system is expressed as ton of refrigeration (TR). A ton of refrigeration is defined as the quantity of heat to be removed in order to form one ton (2000lbs.) of ice at 0 ºC in 24 hrs, from liquid water at 0 ºC.

 

 

Q. What is one ton of refrigerant equals to?

A. 12600kj/h or 12000BTU or 3.5KW.

 

Q. The component of vapor compression refrigeration system where the actual cooling takes place is known as?

A. Evaporator

 

Q. The purpose of compressor in vapour compression refrigeration system is ?

A. 1. To draw the cold vapour from the evaporator to maintain a pressure in

evaporator sufficiently low to achieve the desired evaporating temperature.

     2. To compress the vapour refrigerant and to deliver the compressed gas into the condenser where it can be liquefied at ordinary temperature by means of condenser.

 

 

Q. Explain primary refrigerant with examples?

 

A. Primary refrigerants are those fluids which are used directly as working fluids like in vapour compression and vapour absorption systems. Example-  R-22 etc.

 

Q. What does C.O.P means?

A. Coefficient of performance is the ratio of refrigerating effect produced to the work of compression.

 

Q. The unit of enthalpy in SI system is?

A. kJ/kg.

 

Q. As the suction pressure decreases, the specific volume of the refrigerant?

A. Increases

 

Q. What does 2nd law of thermodynamics states?

A. It sates that heat does not flow from a low temperature body to a high temperature body without the help of an external work.

 

 

Q. Enlist some properties desirable for VCRS?

A. Sub-cooling of refrigerant, Higher suction pressure and Lower condensing pressure.

 

Q. Block diagram of VCRS?

A.

 

 

 

 

 

 

  Q. Part of VCRS where the actual work of refrigeration is performed?

A. Evaporator.

 

Q. What is the purpose of a condenser?

A.  The purpose of the condensation is to provide a means by which the vapour rejects the heat absorbed at the evaporator as well as the heat of compression.

 

 

Q. What is the function of throttling device?

 

A. The throttling device is often referred to as an expansion valve but its real purpose is to achieve pressure reduction so as to lower the boiling point of the liquid refrigerant.

 

 

Q. What is the effect of evaporating temperature on the performance of refrigeration plant?

 

A. Lower evaporating temperature reduces the refrigerating effect per kg of refrigerant circulated in the system and increases the work of compression leading to reduction in COP of the plant. Therefore, it is desirable to operate a refrigeration plant with highest possible evaporating temperature and undue lower evaporating temperature should be avoided.

 

 

Q. What is the effect of condensing temperature on the performance of refrigeration plant?

 

A. Lower condensing temperature is desirable to get higher COP of the system. The efficiency of cooling tower is very important to get lower temperature o coolant medium.

 

 

Q. What is the effect of sub-cooling of liquid refrigerant on the performance of refrigeration plant?

 

A. Sub-cooling of liquid refrigerant is desirable as it increases the refrigerating effect without any change in work of compression.

 

 

Q. Excessive super heating of suction vapour is not desirable because?

 

A. it increases temperature of gas leaving the compressor.

 

 

 

Q. What is relative C.O.P?

 

A. It is defined as the ratio of actual COP to the theoretical COP.

 

 

Q. What happens to entropy during compression?

 

A. It remains constant.

 

 

Q. Carnot COP relation with actual COP?

 

A. Carnot COP is always lower than the actual COP.

 

 

 

Q. What is the work of compression of 35 TR refrigeration plant which operates with COP of 3.5?

 

A. 2100kJ/minute.

 

 

Q. What is the mass flow rate of refrigerant required for 100 TR plant, if the refrigerating effect at the operating evaporating temperature is 1000 kJ/kg?

 

A. 0.35kg/s

 

 

Q. Numbering system for Saturated Hydro-carbon refrigerants is?

 

A. R(carbon atoms-1)(hydrogen atoms+1)fluorine atoms.

 

 

 

Q. What are azeotrope refrigerants?

 

A. Mixture of refrigerants which do not separate in to their components.

 

 

 

Q. What is the purpose of shaft seal?

 

A. To prevent leakage of refrigerant and oil.

 

 

Q. Compression ratio is equal to?

 

A. Absolute discharge pressure divided by absolute suction pressure.

 

 

 

Q. What happens to enthalpy of air when it passes through the evaporative condenser?

 

A.  It increases.

 

 

 

Q. The lowest temperature which can be attained in cooling of water in cooling tower is?

 

A. Equal to wet bulb temperature of the air.

 

 

 

Q. Specific heat of water is?

 

A.  4.186 kJ/kg K.

 

 

Q. The material of pipeline commonly used in small capacity R-22 plant is?

 

A. Copper

 

 

Q. What is the name of expansion valve in which the valve opening varies depending on the super heat at the end of evaporator?

 

A. Thermostatic expansion valve.

 

 

Q. What is the use of low pressure switch?

 

A. It checks for the blockage of expansion valve.

 

 

Q. The function of permanent magnet provided in thermostat is?

 

A. To prevent spark problem.

 

 

 

Q. What does Removal of flash vapour by flash chamber do?

 

A. Decreases mass flow rate of refrigerant in the evaporator.

 

 

 

Q. Mass of vapour produced after expansion can be calculated by knowing?

 

A. Dryness fraction of refrigerant.

 

 

 

Q. Flash chamber helps in?

 

A. Reducing mass flow rate of refrigerant entering in evaporator.

 

 

Q. Multi-compression system is employed when compression ratio is?

 

A. >7

 

 

 

Q. Intermediate pressure to be maintained in multi-compressor system having suction pressure of 1 bar and condensing pressure of 16 bar is about?

 

A. 4 bar.

 

 

 

Q. The dryness fraction of refrigerant after perfect inter cooling will be?

 

A. 1

 

 

Q. The difference between DBT and WBT of air is called?

 

A. Wet bulb depression.

 

 

Q. Partial pressure of water vapor of saturated (PVS) air can be obtained from?

A. Steam table corresponding to DBT of the air.

 

 

Q. In air conditioning system, in which sensible heat removed is 10 kJ/kg dry air and latent heat removed is 10 kJ/kg dry air, what is SHF (Sensible heat factor) of the process?

 

A. 0.5

 

 

Q. Generally material used for insulation of cold storages?

 

A. PUF

 

 

Q. What do you mean by efficiency of cooling tower?

 

A. The efficiency of cooling tower is defined as the ratio of actual drop in temperature of water to the ideal drop in the temperature of water.

 

 

Q. What is Overload protection?

 

A. Motor Overload Protection as that device which is intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure of the motor to start.

 

 

Q. What is the need of defrosting?

 

A. To increase the heat transfer. The accumulation of ice over the heat transfer surface reduces the heat transfer rate as the ice is poor conductor of heat. Therefore, it is necessary to remove the ice deposited over the evaporator at periodic time interval.

 

 

 

 

 

References:

 

1.      R.S. Khurmi Refrigeartion and Air conditioning.

                   

2.      ICAR E-course

 

3.      NDRI class notes

                   

 

 

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