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. 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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