Control basis of a thermostatic expansion valve (TEV) A thermostatic expansion valve modulates refrigerant flow primarily in response to changes in which variable measured at the evaporator outlet?

Introduction / Context:
Thermostatic expansion valves (TEVs or TXVs) are widely used to meter refrigerant into the evaporator while preventing liquid carryover to the compressor. Understanding their control feedback is essential for proper selection and troubleshooting.

Given Data / Assumptions:

• A sensing bulb is strapped to the evaporator outlet (suction line) and filled with a charge.
• The valve experiences three forces: bulb pressure (opening), evaporator pressure (closing), and spring force (closing).
• Stable operation targets a set superheat, not just a fixed temperature or pressure.

Concept / Approach:
The TXV adjusts to maintain a set “superheat,” defined as the temperature rise above the saturation temperature corresponding to evaporator pressure at the outlet. The bulb temperature rises with superheat, increasing bulb pressure and opening the valve; low superheat reduces bulb pressure, closing the valve. Thus, the controlled variable is degree of superheat.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer literature uniformly describes TEVs as “superheat controllers,” with bulb placement critical to accurate SH sensing.

Why Other Options Are Wrong:

Temperature only or pressure only: neither alone defines superheat; both are required to infer Tsat and compute SH.Condenser subcooling and discharge temperature are not the TXV control variables.

Common Pitfalls:
Relocating the bulb to a poor thermal contact point or after a suction-line heat exchanger can distort sensed SH and cause hunting.

Final Answer:

Degree of superheat at exit from the evaporator