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16 Cards in this Set
- Front
- Back
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the flow of liquid refrigerant from the high pressure side to the low pressure side of the system myst be controlled in order to |
maintain the desired temperature in the refrigerated space |
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a metering device |
measures and adjusts the flow of liquid refrigerant to the evaporator |
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without a metering device |
- the evaporator may receive and insufficient amount of refrigerant resulting in an insufficient cooling effect - the evaporator may receive too much refrigerant and not all of the liquid will evaporate |
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why is unvapourized liquid coming from the evaporator bad? |
it may be drawn into the compressor with the vapour and cause serious damage |
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in older systems the flow of liquid refrigerant to the evaporator is controlled |
manually |
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in modern systems the metering or flow control of refrigerant is controlled |
automatically *with possible exception of very large systems where qualified operators are in attendance at all times. |
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six types of metering or flow control devices: (which is manually operated?) |
- hand-operated expansion valve (lol) - automatic expansion valve - thermostatic expansion valve - low pressure float valve - high pressure float valve - capillary tube |
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Hand operated expansion valve |
- a manual expansion valve - used in small capacity refrigeration systems - made of brass - connected to the system by flared compression fittings *seldom used in modern systems. however, occasionally used in bypass lines around the auto control valve |
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in order to stand up to severe requirements of throttling service the hand operated expansion valve is equipped with |
- a valve stem tapered at the end and a matching tapered valve seat. - the spindle has a fine thread which makes precise adjustment possible - the valve is adjusted by removing the cap and turning the spindle by means of a wrench or key |
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Automatic expansion valve (constant pressure) |
- pressure controlled valve - maintains a constant pressure in the evaporator while the compressor is operating (regardless of load) - automatically shuts off the liquid flow when the compressor stops |
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a thermostatic switch |
stops the compressor when the temperature of a refrigerated space or substance drops below the desired level |
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automatic expansion valves are used extensively in |
small refrigerating units - refrigerators - freezers *use has now been generally discontinued in favour of more efficient types |
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an automatic expansion valve is suitable when |
- compressors are controlled by thermostats that are activated by the temperature in the controlled zone - cannot be used when more than one cooling coil is connected to the same compressor |
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Thermostatic Expansion valve |
- most widely used refrigerant control
- installed at the inlet of an evaporator - similar to auto expansion valve except uses thermal power element (bellows or diaphragm with a sensing bulb and capillary tube) - liquid in bulb is usually charged with same refrigerant used in the system - a change in temp will change the pressure of the refrigerant in the thermal power element |
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the thermostatic expansion valve adjusts the amount of liquid admitted to the evaporator so that
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- under all load conditions, nearly the entire surface is utilized for heat absorption by evaporating liquid - ensures that no liquid leaves the evaporator with the vapour |
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vapour leaving the evaporator must be slightly __________. |
superheated approx 5° to 6°C |
