Q. What do you mean by refrigeration? What is the principle of refrigeration?
Ans: Refrigeration is a process of moving heat from one location to another in controlled conditions. The work of heat transport is traditionally driven by mechanical work, but can also be driven by heat, magnetism, electricity, laser, or other means.
The basic principle of refrigeration is simple. You simply pass a colder liquid continuously around the object that is to be cooled. This will take heat from the object due to the temperature difference.
Q. What is a thermostatic expansion valve?
Ans: A TEV is a regulator through which the refrigerant is metered from the high pressure side to the low pressure side of the system. The pressure drop causes the evaporating temperature (saturation temp.) of the refrigerant to fall below that of the evaporator.
a. Avoids liquid refrigerant return to the compressor suction.
b. Automatic expansion control and maintain 6~7 degree superheat.
Q. What is the function of an L.P controller?
Ans: It stops the compressor at low suction pressure caused by the closure of all compartmental solenoids. When the pressure in the compressors suction rises due solenoid opening, the LP controller restarts the compressor.
Q. What is the function of an L.P cutout?
Ans: To protect the compressor against the low suction pressure due to loss of refrigerant or blockage. If the a/c compressor suction pressure is allowed to fall below atmospheric pressure then there exists a risk of moisture and air ingress into the system.
Q. What is the function of a solenoid valve in the refrigeration system?
Ans: This is an electrical signal actuated v/v in the liquid line of the refrigeration system just before the TEV. This is being actuated by the thermostat in the refer compartment. The solenoid shuts off the refrigerant supply if the compartment is sufficiently cooled to the lower set point of the thermostat. Also it connects the supply when the temperature of the compartment goes high i.e. above the thermostat higher set point.
Q. What is the function of back pressure valve in the refrigeration system?
Ans: Back pressure valve is fitted just at the exit of the refrigerant from the evaporator coil in a multi temperature room system. This being fitted at the exit of the compartments whose temperature is set higher (usually at about4~5 degrees centigrade). The function of the v/v is to maintain equilibrium of the system as the pressure of the gas at the exit of each compartment differs. Moreover, the back pressure v/v creates a back pressure on the evaporator coil and ensures that most of the liquid refrigerant is made available to the lesser temperature requirement compartments as their demand for the refrigerant is higher than the compartments being maintained at a higher temperature. It is spring loaded non return valve.
Q. What is the function of a H.P cut out in a refrigeration system?
Ans: This is a safety device (trip) provided in the discharge of the compressor. It functions to trip the compressor if the pressure in the HP side goes high above the working level.
Q. What is the function of the driers in the refrigeration system?
Ans: The filter/drier is installed in the main liquid line of the system to absorb any moisture present in the refrigerant. It consists of activated alumina or silica gel in a renewable cartridge. It also accommodates the charging connection.
Q. Why is fridge compressor belt driven?
Ans:
a. A slight misalignment is a problem or vibration of the electric motor that may lead to shaft seal leakage, leading to loss of the refrigerant. So, to minimize this, fridge compressor is belt driven
b. In case of the liquid entry into the refrigeration compressor, the belt drive gives a limitation of the damage owing to the slip in the belt and flexibility of the belt material.
Q. What is the quality of the vapor coming back to the compressor suction in the refer system? In addition, how will you ensure that the vapor has adequate degree of superheat?
Ans:
a. The condition of the vapor coming to the compressor suction line should have adequate degree of superheat.
b. The condition is checked by reading the pressure of the returning vapor and its corresponding temperature from the P-T chart provided for the specific refrigerant. This temperature is compared with that of the evaporator outlet and the degree of super heat is expressed as the difference in the temperature.
Q. What are the safeties in the fridge system?
Ans: Safeties in fridge system are:
a. HP cut out
b. Differential lube oil pressure cutout
c. LP cutout
d. Relief valve in the condenser
e. Belt driven
f. Cylinder head relief valve
g. Cooling water low flow/high temperature alarm
h. Motor overload
i. Oil separator
j. Drier
k. Mech. Seal
l. Unloaders /capacity controllers
m. Non return shut off valves
n. Sensors – temperature, pressure
Q. How will you know the presence of air in the refrigeration system? Explain the procedure for purging air.
Ans:
a. High condenser pressure
b. Small bubbles in the sight glass
c. Compressor discharge pressure high/running hot
d. More superheat
e. Pressure fluctuations
f. Inefficient working
PROCEDURE FOR PURGING AIR
a. In the normal operation of the system, measure the liquid refrigerant pressure, temperature at the outlet of the condenser/reservoir.
b. Check the corresponding saturation temperature for the recorded pressure of the liquid refrigerant from the P-T chart for the same refrigerant.
c. Compare the measured temperature with the determined saturation temperature for sub-cooling and adjust the flow of the cooling water through the condenser to achieve near saturation condition inside the condenser,
Then,
d. With the condenser liquid refrigerant outlet valve closed, circulate cooling water, start the compressor and pump down the liquid to the condenser/reservoir, checking the pressure in the suction line. If this pressure is allowed to drop down below the atmospheric pressure then there could be chances of air ingress into the system..
e. Circulate the cooling water till the cooling water outlet and the inlet temperatures equal, a check to ensure complete pump down operation.
f. Check the condenser sea water out let temperature, check the refrigerant pressure corresponding to its temperature from the P-T chart of same refrigerant.