Metering Devices Overview:
- Primary Function: Feed refrigerant to the evaporator, reducing pressure and temperature, causing the refrigerant to boil and evaporate, lowering the temperature in the evaporator.
Types of Metering Devices:
Thermostatic Expansion Valve (TXV):
- Commonly referred to as TX or TXV.
- European-manufactured versions may have different configurations, such as liquid entering from the bottom and superheat adjustment on the side.
Automatic Expansion Valve (AXV):
- Not detailed in this section but mentioned as another common type.
TXV Components:
- Head and Diaphragm: Controls the valve operation by moving the needle up or down.
- Needle and Orifice: Regulates the refrigerant flow.
- Inlet Screen: Filters particulate matter and debris.
- Spring: Allows for field adjustment of superheat.
- External Equalizer Port: Balances pressure differences across the evaporator.
Operation of TXV:
- Primary Function: Maintain superheat in the evaporator by adjusting refrigerant flow.
- Forces Acting on TXV:
- Bulb Pressure: Applies an opening force on the diaphragm.
- Evaporator Pressure: Acts as a closing force under the diaphragm.
- Spring Pressure: Provides additional closing force, adjustable in some models.
Example System:
- Refrigerant: R-22.
- Suction Line Temperature: 50°F, correlating to 84 psig bulb pressure.
- Liquid Line Temperature: 125°F entering the TXV.
- Evaporator Boiling Temperature: 40°F (based on 69 psig suction pressure).
- Superheat: 10°F (measured at the evaporator outlet).
TXV in Action:
- Process:
- Liquid refrigerant enters the TXV and pressure drops, causing the refrigerant to evaporate at a lower temperature.
- The sensing bulb, filled with R-22, applies pressure to the top of the diaphragm to open the valve.
- The evaporator pressure and spring pressure act as closing forces to regulate the flow and maintain desired superheat.
- In this example, the closing force is calculated as 150 psig (spring pressure) + 69 psig (evaporator pressure).
No comments:
Post a Comment