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28 Cards in this Set
- Front
- Back
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What is a metering device |
Provides a small opening or orfice, through which only a certain amount of refrigerant can flow at one time. Separates low side from the high side. |
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Most common types of metering devices? |
- capillary tube (cap & tube) - metering orfice - thermostatic expansion valve (txv) - automatic expansion valve (axv) - electronic expansion valve (eev) - low side float (lsf) - high side float (hsf) |
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2 main fixed metering devices are? |
1- capillary tube 2- metering orfice or fixed orfice |
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Capillary tube is ... |
A metering device consisting of a length of seamless tubing with a small and precisely formed inside diameter. |
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Capillary tube performance depends on? |
Tube length Inside diameter # of turns |
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Capillary tubes do not have ? And are what kind of charge? What is the main advantage |
Capillary tubes do not have check valves or direction control valves. They are critically charged systems Main advantage = no moving parts |
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Metering orfice is? |
A fixed orfice metering device that consists of a fitting with a small hole. Is installed between the condenser and the evaporator and provides pressure drop between the high and low side. |
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Thermostatic expansion valve (TXV) is ... |
Is a type of expansion valve metering device that adjust the refrigerant flow rate based on a superheat pressure signal from a sensing bulb at the evaporators outlet. |
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TXV is operated by?? |
Operated by a pressure difference between the sensing bulb and the low side pressure. -Valve opens when the Sensing bulb pressure higher that the combined spring pressure and evaporator pressure. - Valve closes when the sensing bulb pressure is lower than the combined spring pressure and evaporator pressure. |
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Evaporators is starving when? |
The evaporator only has liquid refrigerant in part of it. |
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Minimum stable signal (MSS) is? |
Is the best super heat setting for an evaporator is the best point that causes the least sensing bulb temp change while the system is running |
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TXV adjustment are? |
Turning the adjuster clockwise increases spring tension. = larger difference in pressure required to open the valve. More sensible heat. Turning the adjuster counter clockwise, tension decreases. Smaller pressure difference required Only turn adjuster one 1/4 turn at a time while the system is running. Wait 10-15mins to stabilize before adjusting again. |
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Hunting refers to? |
Refers to a responsive mechanism going to far in one direction and than over correcting and returning too far the other direction. |
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Liquid charged sensing bulb is? |
Charged with the same refrigerant as the system. |
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Liquid cross charge sensing bulb is? |
Uses a refrigerant that is different than the one in the system. |
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Gas charged sensing bulb is? |
Uses the same refrigerant as the system. Requires that the diaphragm and the sensing bulb capillary tube be kept at a temperature warmer than the bulb during the operating cycle. |
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Gas crosses charged sensing bulb is ? |
Is charged with a refrigerant different from the system refrigerant. |
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Adsorption gas cross sensing bulb is? |
Contains two substances, one is a non condensing gas such as carbon dioxide. It provides pressure in the bulb. The other is a solid, such as carbon, silica gel or charcoal, that has the ability to absorb the gas. |
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Distributors are? And purpose is? |
Are connected to the outlet of the metering device. - it splits the flow of refrigerant into several paths. - reduces pressure drop across the evaporator. |
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Pressure limiters are the following.. |
1- gas charged sensing bulb 2- gas cross sensing bulb 3- mechanical pressure limiter. |
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Automatic expansion valve (AXV) is? |
Is a metering device that operates based on low side pressure. - no sensing bulb. - thermal element is clamped to the oulet of the evaporator. |
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Electric expansion valve (EEV) is? |
Is an electric operator instead of the diaphragm power assembly of a txv or a spring of a axv. |
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Stepper motor EEV is? |
Used to drive a piston or pin that operates the valve. - restricts super heat. - pressure transducer is used instead of a bulb. |
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Low side float (LSF) is ? |
Its job is to maintain a level of liquid refrigerant in the evaporator. - requires a large amount of refrigerant charge. - used it a flooded system. |
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High side float (HSF) |
Regulates the refrigerant flow into the evaporator based on the volume of liquid refrigerant in the high side receiver. |
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Why must a low side float unit be level? |
It the unit is not level, the oil will not return to the compressor. When this occurs the oil forms a layer on the surface of the liquid refrigerant. This is know as oil binding |
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What is a weight valve? |
Is a simple valve consisting of a weighted needle, an adjustment spring, a valve seat, an inlet port and an outlet port. - the refrigerant pressure in the liquid line must over come the weight of the needle and spring pressure in order to pass in to the evaporator. |
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What happens to oil with an high side float? |
At higher pressures, the oil dissolves and circulates more readily in the liquid refrigerant. |
