period four
Expansion Devices
notes
TXV Operation
external equalizer
TXV
remote | B | A |
bulb |
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C | Figure 47 |
For example, assume that an increasing system load causes the refrigerant within the coil to vaporize at a faster rate than desired. This moves the point at which the refrigerant becomes completely vaporized from A toward B. This increase in coil surface used for superheating results in the refrigerant vapor leaving the evaporator at a higher temperature. Sensing the rising superheat, the remote bulb transmits a higher pressure to the top side of the TXV diaphragm, causing the valve to open further and allow more refrigerant to flow into the evaporator.
This increased flow of refrigerant moves the point of complete vaporization back toward A, until the desired superheat condition is reestablished and the opening and closing forces within the valve equalize at a refrigerant flow rate that balances the new system load.
Conversely, a decreasing system load slows the rate at which the refrigerant vaporizes, moving the point of complete vaporization toward C. The resulting reduction in superheat creates a lower pressure inside the remote bulb and, therefore, on the top side of the diaphragm. This causes the valve to close more, reducing the flow of liquid refrigerant into the evaporator. This reduction in refrigerant flow moves the point of complete vaporization back toward A, reestablishing the desired superheat condition.
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