We know that there are different types of indicating and safety devices and also the expansion/control valves in a refrigeration plant. These devices sense the various properties like pressure, temperature, flow, liquid level etc. of refrigerant flowing indicates the value of these properties so that the operator and maintenance staff may see these values and have an idea of properly functioning of the plant. The safety devices sense the value of these properties and control accordingly the starting and stopping of the plant to keep these properties in a set range so that to ensure the safe and efficient working of the plant. The expansion/control valves also automatically control the flow and expansion of refrigerant through the system depending on the cooling load and evaporator temperature. In this way we see that the safe and efficient working of the whole plant depend on proper functioning of these control and safety devices just like the functioning of our body depend on functioning of our mind. Thus the ‘timely checking, setting or replacement’ of all the indicating/control devices is very much important for the safe and efficient operation of the plant. Many times a minor fault in the setting/adjustment of a control device greatly increase the wear and tear and hence the operation and maintenance cost. Controls should be checked at least four times a year, or often, if there is a fine adjustment to make. Making adjustments before hot weather operation is particularly important. The care and maintenance required in each type of these devices is discussed one by one as follows:
i. Thermostatic Expansion Valve
It is the main control valve in a refrigeration plant, which controls the flow of refrigerant depending on amount of superheat in the evaporator. It should be checked frequently at least once a month. Under normal operation, these valves are set for10 o C superheat. But through normal wear in the needle valve and valve seat or also sometimes because of a large particle of dirt or any other foreign material, the valve may open wider even at less superheating. It results the excessive flooding of refrigerant in the evaporator, which will cause liquid traces of refrigerant to pass on to the suction of compressor causing damage to the compressor and loss in efficiency. On the other hand if sometimes the gas leaks out of the feeler bulb, the valve will be closed due to spring action and will not allow liquid refrigerant in the evaporator, thus stopping the cooling. The thermostatic valve should be serviced or replaced as per the manufacturer’s recommendations.
ii. Float Control Valve
It keeps the liquid ammonia at proper level in the evaporator. If this valve is sticking closed, the evaporator is starved of liquid, which cuts its cooling capacity. It increases the running time for compressor. On the other hand if due to normal wear and tear or due to any dirt at the valve seat, the valve remains open, even at proper liquid level, it allow too much liquid in the evaporator. It may cause liquid traces of refrigerant to pass on to the suction of compressor. It not only causes damage to the compressor, but also circulates more ammonia than is necessary, which means a loss in efficiency. Thus float control valves should be checked very often, and float and valve kit should be replaced at least once every 6 months or more frequently if they do not respond to adjustment.
iii. Solenoid Valve
It is an electrically operated valve in which the force of a magnetic coil operates valve-stem. Normally, it remains in closed position, but when electric current is supplied to the magnetic coil, magnetic force generates, which open the valve.When current is stopped, it again comes to the closed position. These are used tostop the flow of either liquid or gas at different points in the system to isolate that particular cooling unit or room from the main suction and liquid lines. These valves are usually trouble-free, except for the burning of the magnetic coil, which lifts up the valve stem. On burning of magnetic coil it needs replacement.
iv. Back Pressure-Regulating Valve
In some refrigeration systems ‘back pressure regulating valve’ is fitted in the suction line of compressor. The function of this valve is to maintain a constant suction pressure and also so the temperature in the evaporator. Any faulty working of this valve may cause the temperature in evaporator too low allowing frost or ice to accumulate between the fins of evaporator and clog the entire unit. Hence regular check-up and setting for accuracy is needed for the back pressure-regulating valve.
v. Relief Valve
At some top point above the condenser, relief valve is given for purging of air and to release excessive pressure in the system. After the excessive pressure has been brought down to normal, it should reseat automatically. However, a particle of dust can hold the valve slightly open and allow ammonia to leak out. If it remains unchecked, a large amount of ammonia can escape through it. So, the valve must be checked and reseated at regular intervals.
vi. Water Flow-regulating Valve
In the shell-and-tube type condenser, a water flow-regulating valve is fitted, which is usually actuated by the condenser pressure. This valve automatically regulates the flow of cooling water so as to keep a constant pressure in the condenser regardless of the variation in flow rate and temperature of refrigerant vapours. If this valve is not checked occasionally, many thousands of liters of water can go waste.
vii. Filter/Strainer
We know that the working of a control/expansion valve is very much sensitive to the presence of any dirt, scale or rust particle in the flowing refrigerant liquid, which may come through fresh charging or rusting in pipes. Any such particle can be filtered from the refrigerant liquid before flowing to the control valve. Thus to install a filter/ strainer before the control valve in the refrigerant line is a usual practice.On running of the plant, this filter/ strainer may get clogged after some period and then needs to be cleaned/ washed or replaced. It should be checked at least once a year.
viii. Pressure Gauge
These are the main indicating devices, which give us the idea that how the refrigeration system is operating. After being in use for some time, the pointing needles of these gauges may either stick or show too high pressure. Any false readings may lead to wrong adjustments, which will affect proper functioning of the refrigeration system. There is an adjusting screw by which the pointing needles can be set to zero. If gauges become rusty and hard to read, replace them, as the cost is very small when compared with the loss due to false readings.
ix. Thermometers
These are installed in the various suction lines, compressor discharge lines, cooling water inlet and outlets, storage rooms, and at other convenient points in a refrigeration plant to indicate the temperature of flowing fluids. These should be frequently checked to see that they are not broken, easily visible and installed properly at the site.
x. Rotameter and Watermeter
These are some other types of indicating devices, which indicate the flow rate of refrigerant and cooling water through the condenser. These should be in working condition and should be replaced if not repairable.
xi. Temperature Controller
It must be kept properly adjusted and set to the required value, as wrong adjustments either will keep higher temperatures than desired in the refrigerated space or lower temperature which will tend to keep the compressors operating longer. The higher temperature than designed may effect the preservation of food products in a cold storage. And lower temperature than that designed will increase the power consumption of the plant. So any wrong adjustment or improper functioning of temperature controller effects the working of refrigeration plant to a large extent and its timely checking and setting must be taken care of.
xii. L.P and H.P. Cut-outs
These should be checked occasionally for their correct operation. Any wrong adjustment will either switch off the compressor unnecessarily or will not work even at dangerous limit of pressure in the suction and discharge line and leave the system unsafe. Thus from safety point of view correct setting of these pressure switches must be ensured.
i. Thermostatic Expansion Valve
It is the main control valve in a refrigeration plant, which controls the flow of refrigerant depending on amount of superheat in the evaporator. It should be checked frequently at least once a month. Under normal operation, these valves are set for10 o C superheat. But through normal wear in the needle valve and valve seat or also sometimes because of a large particle of dirt or any other foreign material, the valve may open wider even at less superheating. It results the excessive flooding of refrigerant in the evaporator, which will cause liquid traces of refrigerant to pass on to the suction of compressor causing damage to the compressor and loss in efficiency. On the other hand if sometimes the gas leaks out of the feeler bulb, the valve will be closed due to spring action and will not allow liquid refrigerant in the evaporator, thus stopping the cooling. The thermostatic valve should be serviced or replaced as per the manufacturer’s recommendations.
ii. Float Control Valve
It keeps the liquid ammonia at proper level in the evaporator. If this valve is sticking closed, the evaporator is starved of liquid, which cuts its cooling capacity. It increases the running time for compressor. On the other hand if due to normal wear and tear or due to any dirt at the valve seat, the valve remains open, even at proper liquid level, it allow too much liquid in the evaporator. It may cause liquid traces of refrigerant to pass on to the suction of compressor. It not only causes damage to the compressor, but also circulates more ammonia than is necessary, which means a loss in efficiency. Thus float control valves should be checked very often, and float and valve kit should be replaced at least once every 6 months or more frequently if they do not respond to adjustment.
iii. Solenoid Valve
It is an electrically operated valve in which the force of a magnetic coil operates valve-stem. Normally, it remains in closed position, but when electric current is supplied to the magnetic coil, magnetic force generates, which open the valve.When current is stopped, it again comes to the closed position. These are used tostop the flow of either liquid or gas at different points in the system to isolate that particular cooling unit or room from the main suction and liquid lines. These valves are usually trouble-free, except for the burning of the magnetic coil, which lifts up the valve stem. On burning of magnetic coil it needs replacement.
iv. Back Pressure-Regulating Valve
In some refrigeration systems ‘back pressure regulating valve’ is fitted in the suction line of compressor. The function of this valve is to maintain a constant suction pressure and also so the temperature in the evaporator. Any faulty working of this valve may cause the temperature in evaporator too low allowing frost or ice to accumulate between the fins of evaporator and clog the entire unit. Hence regular check-up and setting for accuracy is needed for the back pressure-regulating valve.
v. Relief Valve
At some top point above the condenser, relief valve is given for purging of air and to release excessive pressure in the system. After the excessive pressure has been brought down to normal, it should reseat automatically. However, a particle of dust can hold the valve slightly open and allow ammonia to leak out. If it remains unchecked, a large amount of ammonia can escape through it. So, the valve must be checked and reseated at regular intervals.
vi. Water Flow-regulating Valve
In the shell-and-tube type condenser, a water flow-regulating valve is fitted, which is usually actuated by the condenser pressure. This valve automatically regulates the flow of cooling water so as to keep a constant pressure in the condenser regardless of the variation in flow rate and temperature of refrigerant vapours. If this valve is not checked occasionally, many thousands of liters of water can go waste.
vii. Filter/Strainer
We know that the working of a control/expansion valve is very much sensitive to the presence of any dirt, scale or rust particle in the flowing refrigerant liquid, which may come through fresh charging or rusting in pipes. Any such particle can be filtered from the refrigerant liquid before flowing to the control valve. Thus to install a filter/ strainer before the control valve in the refrigerant line is a usual practice.On running of the plant, this filter/ strainer may get clogged after some period and then needs to be cleaned/ washed or replaced. It should be checked at least once a year.
viii. Pressure Gauge
These are the main indicating devices, which give us the idea that how the refrigeration system is operating. After being in use for some time, the pointing needles of these gauges may either stick or show too high pressure. Any false readings may lead to wrong adjustments, which will affect proper functioning of the refrigeration system. There is an adjusting screw by which the pointing needles can be set to zero. If gauges become rusty and hard to read, replace them, as the cost is very small when compared with the loss due to false readings.
ix. Thermometers
These are installed in the various suction lines, compressor discharge lines, cooling water inlet and outlets, storage rooms, and at other convenient points in a refrigeration plant to indicate the temperature of flowing fluids. These should be frequently checked to see that they are not broken, easily visible and installed properly at the site.
x. Rotameter and Watermeter
These are some other types of indicating devices, which indicate the flow rate of refrigerant and cooling water through the condenser. These should be in working condition and should be replaced if not repairable.
xi. Temperature Controller
It must be kept properly adjusted and set to the required value, as wrong adjustments either will keep higher temperatures than desired in the refrigerated space or lower temperature which will tend to keep the compressors operating longer. The higher temperature than designed may effect the preservation of food products in a cold storage. And lower temperature than that designed will increase the power consumption of the plant. So any wrong adjustment or improper functioning of temperature controller effects the working of refrigeration plant to a large extent and its timely checking and setting must be taken care of.
xii. L.P and H.P. Cut-outs
These should be checked occasionally for their correct operation. Any wrong adjustment will either switch off the compressor unnecessarily or will not work even at dangerous limit of pressure in the suction and discharge line and leave the system unsafe. Thus from safety point of view correct setting of these pressure switches must be ensured.
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