Expansion Valve is a very important component in the refrigeration system. Its function is just opposite/reverse to the compressor. Compressor increases the pressure of refrigerant vapour by decreasing its volume while the expansion device decreases the pressure of refrigerant liquid by increasing its volume i.e. allowing it to expand.Actually compressor maintains high pressure of refrigerant liquid in the condenser and receiver. But the expansion valve prevents this high-pressure liquid refrigerant to go to the evaporator as it is. It allows the high pressure refrigerant liquid to flow through a very narrow section which offers resistance to flow and decrease its pressure. As pressure reduces after passing through narrow opening the saturation temperature also decrease to a lower temperature than that required in evaporator.Thus some of the low-pressure liquid evaporates at once cooling the rest of liquid up to that saturation temperature. Then this partially vaporized low-pressure liquid enters the evaporator. In some expansion device that narrow section is automatically adjustable according to the amount of cooling required around the evaporator in refrigerated space. In this way, the expansion device also acts as a control device.
The commonly used types of expansion/control devices are as follows:-
Capillary Tube
The commonly used types of expansion/control devices are as follows:-
Capillary Tube
Thermostatic Expansion Valve
Low Pressure Side Float Valve
i. Capillary Tube
It is only an expansion device and not a control device. So, its function is only to reduce the high-pressure liquid refrigerant from condenser to low pressure liquid refrigerant before being fed to the evaporator. It is generally used in domestic refrigerator, drinking water cooler, room AC, etc. because of its simplicity and low cost. As shown in fig.5.8. It is simply a copper tube of very small internal diameter and of varying length depending on its application. The inside diameter is generally about 0.6mm to 2mm and length varies from 0.5m to 5m. Due to frictional resistance offered by small diameter tube, the pressure of refrigerant drops when it flows through it. Pressure drop is greater when length of tube is larger and diameter is smaller. Conversely if length is reduced or diameter is increased, pressure drop will be less. The diameter and length of the capillary tube are decided as per the system requirement
ii. Thermostatic Expansion Valve
It is an expansion device and control device both. It maintains constant temperature of refrigerant vapour going out of evaporator automatically. It consists of a needle valve and a seat, a metallic diaphragm or bellow, spring and an adjusting screw Between the valve and valve seat a small passage/ orifice forms through which refrigerant flows. When valve rests on seat, the passage is closed and the flow is stopped. When valve moves away from seat, passage opens and refrigerant starts flowing. When valve further moves away, it create more wide passage allowing more refrigerant to flow through it. In addition to that, there is a feeler bulb, attached to the outgoing tube of evaporator. The feeler bulb is connected to a capillary tube, which is further connected to the valve chamber over the diaphragm. In the feeler bulb the same liquid refrigerant is filled partially. Since feeler bulb is installed on the outgoing tube of evaporator, therefore,it will be at the same temperature as the temperature of refrigerant vapour going out of evaporator. When the cooling requirement increases in the refrigeration space or putting more items to cool in the refrigerator, temperature of outgoing refrigerant vapour will increase by absorbing more heat from more items thus increasing the temperature in feeler bulb. Due to increase in the temperature in feeler bulb, pressure will also increase which will push the diaphragm through capillary tube. The diaphragm will move the valve away from valve seat and allow more refrigerant to flow in evaporator to meet the increased cooling requirement.
On the other hand, when cooling requirement is less, less will be the heat absorbed by refrigerant in evaporator and less will be temperature of outgoing refrigerant vapour. Due to that temperature of refrigerant in the feeler bulb will also decrease thus decreasing the pressure on diaphragm. On decrease in pressure, diaphragm will move by spring action and tend to set the value near to valve seat. In this way less refrigerant liquid will enter the evaporator according to less cooling requirement.The thermostatic expansion valve keeps the superheating of refrigerant vapour,coming out of evaporator, at a constant level and controls the quantity of refrigerant liquid going to evaporator coil according to the cooling requirement in refrigerated space.
iii. Low side Float Valve
It is also both an expansion and control device. As its name suggests, it is a type of float valve and as the float is situated on the low-pressure side, it is called low- side float valve. The working of this float valve is exactly same as that of a ball valve fitted in water tank at our homes for maintaining a fix level of water in tank. Like floating ball, there is a float, which floats on the liquid refrigerant level in the valve body. The high pressure liquid refrigerant from the receiver and high pressure pipe enters the float valve body through a needle valve and on passing through this valve becomes low pressure liquid. This low pressure liquid refrigerant fill the space in valve body and then evaporating coils connected to valve body. The float is connected to needle through an arm. When the low pressure liquid refrigerant level falls down due to more evaporation in evaporating coil, the float also comes down and moves the needle away from valve thus allowing more refrigerant to meet the requirement. On the other hand when cooling requirement lowers, the evaporation also lowers and so liquid refrigerant level increases in the evaporator.
Due to that the liquid level also increases in the connected valve body, thus lifting the float. The float in turn pushes the needle in the valve seat and reduces the inflow of refrigerant. The major advantage of the low-side float valve is that it maintains a constant liquid level in the evaporator under all loading conditions regardless of the evaporator pressure and temperature.
i. Capillary Tube
It is only an expansion device and not a control device. So, its function is only to reduce the high-pressure liquid refrigerant from condenser to low pressure liquid refrigerant before being fed to the evaporator. It is generally used in domestic refrigerator, drinking water cooler, room AC, etc. because of its simplicity and low cost. As shown in fig.5.8. It is simply a copper tube of very small internal diameter and of varying length depending on its application. The inside diameter is generally about 0.6mm to 2mm and length varies from 0.5m to 5m. Due to frictional resistance offered by small diameter tube, the pressure of refrigerant drops when it flows through it. Pressure drop is greater when length of tube is larger and diameter is smaller. Conversely if length is reduced or diameter is increased, pressure drop will be less. The diameter and length of the capillary tube are decided as per the system requirement
ii. Thermostatic Expansion Valve
It is an expansion device and control device both. It maintains constant temperature of refrigerant vapour going out of evaporator automatically. It consists of a needle valve and a seat, a metallic diaphragm or bellow, spring and an adjusting screw Between the valve and valve seat a small passage/ orifice forms through which refrigerant flows. When valve rests on seat, the passage is closed and the flow is stopped. When valve moves away from seat, passage opens and refrigerant starts flowing. When valve further moves away, it create more wide passage allowing more refrigerant to flow through it. In addition to that, there is a feeler bulb, attached to the outgoing tube of evaporator. The feeler bulb is connected to a capillary tube, which is further connected to the valve chamber over the diaphragm. In the feeler bulb the same liquid refrigerant is filled partially. Since feeler bulb is installed on the outgoing tube of evaporator, therefore,it will be at the same temperature as the temperature of refrigerant vapour going out of evaporator. When the cooling requirement increases in the refrigeration space or putting more items to cool in the refrigerator, temperature of outgoing refrigerant vapour will increase by absorbing more heat from more items thus increasing the temperature in feeler bulb. Due to increase in the temperature in feeler bulb, pressure will also increase which will push the diaphragm through capillary tube. The diaphragm will move the valve away from valve seat and allow more refrigerant to flow in evaporator to meet the increased cooling requirement.
On the other hand, when cooling requirement is less, less will be the heat absorbed by refrigerant in evaporator and less will be temperature of outgoing refrigerant vapour. Due to that temperature of refrigerant in the feeler bulb will also decrease thus decreasing the pressure on diaphragm. On decrease in pressure, diaphragm will move by spring action and tend to set the value near to valve seat. In this way less refrigerant liquid will enter the evaporator according to less cooling requirement.The thermostatic expansion valve keeps the superheating of refrigerant vapour,coming out of evaporator, at a constant level and controls the quantity of refrigerant liquid going to evaporator coil according to the cooling requirement in refrigerated space.
iii. Low side Float Valve
It is also both an expansion and control device. As its name suggests, it is a type of float valve and as the float is situated on the low-pressure side, it is called low- side float valve. The working of this float valve is exactly same as that of a ball valve fitted in water tank at our homes for maintaining a fix level of water in tank. Like floating ball, there is a float, which floats on the liquid refrigerant level in the valve body. The high pressure liquid refrigerant from the receiver and high pressure pipe enters the float valve body through a needle valve and on passing through this valve becomes low pressure liquid. This low pressure liquid refrigerant fill the space in valve body and then evaporating coils connected to valve body. The float is connected to needle through an arm. When the low pressure liquid refrigerant level falls down due to more evaporation in evaporating coil, the float also comes down and moves the needle away from valve thus allowing more refrigerant to meet the requirement. On the other hand when cooling requirement lowers, the evaporation also lowers and so liquid refrigerant level increases in the evaporator.
Due to that the liquid level also increases in the connected valve body, thus lifting the float. The float in turn pushes the needle in the valve seat and reduces the inflow of refrigerant. The major advantage of the low-side float valve is that it maintains a constant liquid level in the evaporator under all loading conditions regardless of the evaporator pressure and temperature.
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