Answer EKG Question 48

Q48.With regard to refrigeration system explains;
A. What are the possible reasons for HP cut out in the refrigeration system?
B. What are the indication if the air is present in the refrigerant system and how to remove air from the refrigeration system?
C. How unloader works in the refrigeration system?
D. What are the possible reasons for Icing on evaporation coil?
E. Why compound gauge fitted in the refrigeration system?

Answer: A. What are the possible reasons for HP cut out in the refrigeration system?
Check that HP cutout is not malfunctioning and the pressure at the HP side is actually higher than the normal working pressure. If the pressure is high the following are the possible reasons:
(1) Air in the system, check for large bubbles in the condenser and check for fluctuations in the pressure gauges, see part B of this question to identify and purge.
(2) Condenser cooling water temperature may be high or flow is short, check for the line filters and inlet-outlet valves. Check the condenser tubes are clean and the partition in the end cover is corroded.
(3) Check if the stop valve in the delivery line after the compressor is completely open.
(4) Excess refrigerant may also cause high pressure. The cooling tubes in the condenser are if submerged in liquid refrigerant then the heat exchange area will reduce. Purge the excess refrigerant.


B. What are the indication if the air is present in the refrigerant system and how to remove air from the refrigeration system?
If delivery pressure is abnormally high and shall not be restored to normal pressure, even if condenser water flow was increased. It is an indication of non-condensable refrigerant gas or air is intermixed in the refrigeration circuit.
1) Stop the compressor and close the liquid outlet valve for the condenser. Increase the condenser cooling water flow until almost equivalent temperature between inlet and outlet water.
2) If in case of actual condensing pressure higher than refrigerant saturated pressure corresponding to condenser cooling water temperature, it is a confirmation of the presence of air.
3) For extract air, the following procedure to be taken for;
(a) Pump down the refrigerant from piping and compressor etc.
(b) Stop the compressor.
(c) Close the liquid inlet valve for the condenser.
(d) Store refrigerant into condenser for approximately five minutes.
(e) Result of above, refrigerant vapour shall be condensed gradually.
(4) Then connect an empty cylinder at the purging plug, keep that cylinder in a bucket full of ice, slightly open the air purge plug on the top of the condenser and purge the air.
(5) keep the condenser cooling water with the maximum flow during the above purging. When the air is purged completely, high pressure shall be lowered compared with air in the circuit.
(6) When this pressure shall not be lowered more, purging work is finished.


C. How unloader works in the refrigeration system?
This mechanism can be subdivided into hydraulic and mechanical components. The hydraulic assembly consists of a housing in which is fitted a piston operated by oil pressure.

The mechanical assembly consists of a pressure ring to which are attached two activating rods on the underside. Six valve lifting pins are located around the upper face of the operating ring and the whole assembly is free to move in a vertical direction. The actuating rods pass through a supporting ring which is rigidly held in a groove around the outside of the cylinder liner. Pressure Springs are fitted around each activating rod, tending to force the operating ring in an upward direction.

To the lower end of the actuating rods is attached a semicircular Lever, the fulcrum of which is fixed to the supporting ring. The movement of this Lever is controlled by the piston stem of the hydraulic assembly bearing against the upper position thus lifting the suction valve ring. When the hydraulic piston is moved towards the cylinder liner by the high-pressure oil it bears against the lever causing the actuating rod to be drawn down against the spring-load. This causes the valve lifting pin to retract. This enables the suction valve ring to descend on its seat thereby putting the cylinder on load.
Suction Port block type unloader(Loading state)

Suction Port Block type unloader (Unloading state)

Another type of unloader is the Suction valve open type. High-pressure gas is used to keep the suction valve in the open condition so that the compression in the unit will not take place. The gas is controlled with a solenoid valve.

D. What are the possible reasons for Icing on evaporation coil? 

 Icing or frosting on the evaporator coil takes place when the moisture present in the cold room gets accumulated on the evaporator coils and the temperature of the coil falls below the freezing point of the moisture. In a plant using R-22, the temperature of evaporator coils falls up to -35℃. any moisture particle that comes into contact will immediately get condensed and if it is retained over the coils it will turn into ice. Provision is made that as the droplet forms it should fall into the collecting tray, and in the event of frosting has already taken place the defrosting arrangement is there. Further, the fan is provided behind the coils to circulate the air in the cold room and that keeps the evaporator coils in contact with the average room temperature. Now it is understood that the reason for frosting is the presence of moisture in the cold room. 

The ingress of moisture takes place at the time of person entering the room. The sealing of the doors is to be checked and maintained. The circulating fan must be checked if it is functioning properly. De-Frosting arrangement checked for proper functioning, note that during defrosting the fan should not be running that will cause the water droplets to spread in the room and the purpose of defrosting which is to remove the moisture content from the cold room will not be met. 

 E. Why compound gauge fitted in the refrigeration system?
Compound gauge is a gauge, whose range starts in negative gauge pressure and ends in positive gauge pressure. That means it can be used to measure both pressure and vacuum on these gauges,. there are two more scales. One scale indicates pressure. Another scale on the gauge lists the saturation temperature of the refrigerant corresponding to the pressure. Using a compound gauge we are able to read the system pressure and corrosponding satureation temperature of the refrigerant used.