The main function of condensers and evaporators is to exchange heat between the refrigerant and outside environment. Thus these are maintained so as to exchange the maximum possible heat or to keep the heat transfer resistance at a minimum level. Various types of routine problems that come in operation of condenser and evaporator and their remedies are discussed as below:
i. Cleaning and De-scaling of Condenser
A condenser is either air-cooled or water-cooled or both i.e. evaporative type.Whatever be the type of condenser, its function is to transfer the latent heat of refrigerant vapours flowing inside the bundle of tubes to the surrounding air or water. This heat transfer can be badly affected if the outside surface of tubes is dirty or any scale or sludge is deposited on it. This problem is more serious in case of water-cooled condenser where the naturally available water is used for cooling purpose in condenser. This water may contain many types of salts and minerals,which deposit in the form of scale or sludge on the tube surface and cut the heat transfer. Deposit of dirt also cannot be avoided because the condenser is situated in open. So what we can do is only to follow the proper cleaning and de-scaling schedule to maintain the effectiveness of condenser. A compound named as potassium permanganate is generally added to the water to kill the fungi and keep away the slime deposit from accumulating upon the surfaces.
ii. Purging of Non-condensable Gases and Air
Theoretically, a refrigeration system is closed system. That means only the pure refrigerant flows through the system from one component to the other. But practically it is not completely closed. During the suction stroke in compressor, the pressure of refrigerant vapours above the piston becomes lower than that of atmospheric pressure. Due to low pressure in the compressor, outside air can leak through stuffing box and piston rings into the low-pressure refrigerant vapours. Air can also leak through the repair of compressor and other leaks in the suction line. Some air may also be initially present in ammonia while charging in the system.
Now, you must be thinking that what is the bad effect of presence of this air in ammonia vapours. Actually what happens, when the system runs, the air and other non-condensable gases do not condense in the condenser because their condensing temperature is considerably higher than the condensing temperature of ammonia.And due to their lightweight they collect at the top of the condenser or other high points in the system. The presence of these causes an excess condensing pressure, which increase power consumption. These also put an extra heat transfer resistance and make the condenser ineffective. Thus the air and any other non-condensable gas must be extracted or thrown out of the system for its efficient working. For this, the system is first shut down before allowing the purging of these gases through relief valves.
iii. Oil Traps Draining
In the compressor of a refrigeration plant, the lubricating oil layer between cylinder and moving piston come in contact with refrigerant vapours. Now if the refrigerant is miscible with lubricating oil, it will dissolve in the lubricating oil but the traces of lubricating oil will not mix in refrigerant vapours. It is the case with freon refrigerants and the system is safe against the problem of oil traces going with vapours.
However, the choice of lubricant oils in freon compressor need very much care because if lubricant oil is not of right type, its lubricating property can be damaged by dissolved freon refrigerant vapours. On the other hand if refrigerant is immiscible in lubricating oil, it will not dissolve in lubricant oil but the oil traces will mix in refrigerant vapours. It is the case with ammonia, which is most frequently used in large size refrigeration plant of a dairy due to its other favourable properties. These oil traces will go to condenser and evaporator along with the ammonia vapour and form a layer on the inside surface of evaporator tubes and decrease the heat transfer. If some method is not used to prevent this oil to accumulate in evaporator,it will make the evaporator ineffective. For that oil traps are used after compressor which separates out the oil traces from ammonia vapours and prevent these to pass on to the condenser and evaporator tubes. In this way this trapped oil accumulates in the oil trap and must be drained at regular intervals. And whatever amount ofoil is added to a crank-case of compressor the same must be drained out from the
oil trap.
iv. Purging of Oil out of Evaporators
In spite of that oil trap is installed before condenser in an ammonia refrigeration plant. Some oil traces may pass through it and enter the condenser and evaporator coil along with refrigerant. As the temperature is low in the evaporator, the oil traces accumulate at the inside surface of evaporator coil and cut the heat transfer.In this way lubricating oil layer is more harmful in the evaporator and makes it ineffective. This oil must be purged out at regular intervals by warming up the evaporator coil and then draining the oil after disconnecting the evaporator from rest of the system.
v. Defrosting of Evaporator Coil
This is the another problem with evaporator coil, which cuts the heat transfer, and makes the evaporator ineffective. As the temperature at the surface of evaporator coil is very less, the moisture present in the surrounding air of cold room condenses on the coil surface and then freeze there in the form of frost. This frost puts an extra resistance to exchange of heat from air to refrigerant inside the tubes. This problem is common in case of natural convection evaporator. But in most of the refrigeration plants, a blower type (forced convection) cooling coil is used in which a blower pass the air over the cooling coil with high speed. The speedily passing air does not allow the frost to accumulate on the coil. But sometimes due to very low temperature or failure of blower, frost may come and need to be removed.
There are many types of defrosting methods used in the plant:
i. Cleaning and De-scaling of Condenser
A condenser is either air-cooled or water-cooled or both i.e. evaporative type.Whatever be the type of condenser, its function is to transfer the latent heat of refrigerant vapours flowing inside the bundle of tubes to the surrounding air or water. This heat transfer can be badly affected if the outside surface of tubes is dirty or any scale or sludge is deposited on it. This problem is more serious in case of water-cooled condenser where the naturally available water is used for cooling purpose in condenser. This water may contain many types of salts and minerals,which deposit in the form of scale or sludge on the tube surface and cut the heat transfer. Deposit of dirt also cannot be avoided because the condenser is situated in open. So what we can do is only to follow the proper cleaning and de-scaling schedule to maintain the effectiveness of condenser. A compound named as potassium permanganate is generally added to the water to kill the fungi and keep away the slime deposit from accumulating upon the surfaces.
ii. Purging of Non-condensable Gases and Air
Theoretically, a refrigeration system is closed system. That means only the pure refrigerant flows through the system from one component to the other. But practically it is not completely closed. During the suction stroke in compressor, the pressure of refrigerant vapours above the piston becomes lower than that of atmospheric pressure. Due to low pressure in the compressor, outside air can leak through stuffing box and piston rings into the low-pressure refrigerant vapours. Air can also leak through the repair of compressor and other leaks in the suction line. Some air may also be initially present in ammonia while charging in the system.
Now, you must be thinking that what is the bad effect of presence of this air in ammonia vapours. Actually what happens, when the system runs, the air and other non-condensable gases do not condense in the condenser because their condensing temperature is considerably higher than the condensing temperature of ammonia.And due to their lightweight they collect at the top of the condenser or other high points in the system. The presence of these causes an excess condensing pressure, which increase power consumption. These also put an extra heat transfer resistance and make the condenser ineffective. Thus the air and any other non-condensable gas must be extracted or thrown out of the system for its efficient working. For this, the system is first shut down before allowing the purging of these gases through relief valves.
iii. Oil Traps Draining
In the compressor of a refrigeration plant, the lubricating oil layer between cylinder and moving piston come in contact with refrigerant vapours. Now if the refrigerant is miscible with lubricating oil, it will dissolve in the lubricating oil but the traces of lubricating oil will not mix in refrigerant vapours. It is the case with freon refrigerants and the system is safe against the problem of oil traces going with vapours.
However, the choice of lubricant oils in freon compressor need very much care because if lubricant oil is not of right type, its lubricating property can be damaged by dissolved freon refrigerant vapours. On the other hand if refrigerant is immiscible in lubricating oil, it will not dissolve in lubricant oil but the oil traces will mix in refrigerant vapours. It is the case with ammonia, which is most frequently used in large size refrigeration plant of a dairy due to its other favourable properties. These oil traces will go to condenser and evaporator along with the ammonia vapour and form a layer on the inside surface of evaporator tubes and decrease the heat transfer. If some method is not used to prevent this oil to accumulate in evaporator,it will make the evaporator ineffective. For that oil traps are used after compressor which separates out the oil traces from ammonia vapours and prevent these to pass on to the condenser and evaporator tubes. In this way this trapped oil accumulates in the oil trap and must be drained at regular intervals. And whatever amount ofoil is added to a crank-case of compressor the same must be drained out from the
oil trap.
iv. Purging of Oil out of Evaporators
In spite of that oil trap is installed before condenser in an ammonia refrigeration plant. Some oil traces may pass through it and enter the condenser and evaporator coil along with refrigerant. As the temperature is low in the evaporator, the oil traces accumulate at the inside surface of evaporator coil and cut the heat transfer.In this way lubricating oil layer is more harmful in the evaporator and makes it ineffective. This oil must be purged out at regular intervals by warming up the evaporator coil and then draining the oil after disconnecting the evaporator from rest of the system.
v. Defrosting of Evaporator Coil
This is the another problem with evaporator coil, which cuts the heat transfer, and makes the evaporator ineffective. As the temperature at the surface of evaporator coil is very less, the moisture present in the surrounding air of cold room condenses on the coil surface and then freeze there in the form of frost. This frost puts an extra resistance to exchange of heat from air to refrigerant inside the tubes. This problem is common in case of natural convection evaporator. But in most of the refrigeration plants, a blower type (forced convection) cooling coil is used in which a blower pass the air over the cooling coil with high speed. The speedily passing air does not allow the frost to accumulate on the coil. But sometimes due to very low temperature or failure of blower, frost may come and need to be removed.
There are many types of defrosting methods used in the plant:
- By blowing hot air over the evaporating coil.
- By using electric heater for heating the evaporator and melting the frost.
- By blowing hot discharge gas in the evaporator coil, etc.
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