CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A technician installs a TXV on an evaporator coil with a long refrigerant distributor and multiple circuits, resulting in significant pressure drop across the coil. Why should an externally equalized TXV be used instead of an internally equalized valve in this application?
- AIt allows the valve to operate without a diaphragm
- BIt increases the total refrigerant charge required by the system
- CIt senses evaporator outlet pressure directly, compensating for coil pressure drop to maintain accurate superheat
- DIt eliminates the need for a remote sensing bulb on the suction line
Show answer & explanationAnswer & explanation
Correct answer: C. It senses evaporator outlet pressure directly, compensating for coil pressure drop to maintain accurate superheat
An externally equalized TXV uses a separate equalizer line connected to the suction line near the evaporator outlet, allowing it to sense actual outlet pressure rather than inlet pressure. This compensates for pressure drop through long or multi-circuit coils, preventing superheat from being read inaccurately high.
Why the other options are wrong
- A. All TXVs use a diaphragm to modulate the valve pin.
- B. Equalizer type does not directly change total system charge.
- D. Both internal and external TXVs still require a sensing bulb.
Externally Equalized TXV
A TXV design using a separate equalizer line connected near the evaporator outlet to sense actual outlet pressure, compensating for pressure drop across long or multi-circuit coils.
- Used when coil pressure drop exceeds about 2-3 psi
- Internal equalizers only sense valve outlet pressure, not coil exit
- Improper equalizer connection causes erratic superheat control
Memory trick: External equalizer 'checks the exit' to keep superheat honest.