In refrigeration system controls, which expansion device maintains a nearly constant degree of superheat at the evaporator outlet?

Introduction / Context:
Expansion devices meter refrigerant into the evaporator and influence stability, capacity control, and compressor protection. Maintaining superheat at the evaporator outlet prevents liquid refrigerant from reaching the compressor, avoiding damage and ensuring efficient heat transfer.

Given Data / Assumptions:

• Goal: maintain roughly constant superheat at the evaporator exit.
• System uses a sensing bulb at the evaporator outlet.
• Evaporating pressure acts on the valve diaphragm via an equalizing line.

Concept / Approach:
The thermostatic expansion valve (TXV/TEV) compares the bulb pressure (proportional to outlet temperature) with evaporator pressure and spring force. As load increases, outlet temperature and superheat rise; the bulb pressure increases, opening the valve to admit more refrigerant. Conversely, reduced load lowers superheat, prompting the valve to throttle, thus maintaining near-constant superheat.

Step-by-Step Solution:

Verification / Alternative check:
Superheat trend can be confirmed by measuring suction line temperature and saturation temperature (from suction pressure). A stable difference indicates proper TXV control.

Why Other Options Are Wrong:

• Automatic expansion valve (a) regulates pressure, not superheat; poor at load variations.
• Float valves (b, d) control liquid level, not outlet superheat.
• Capillary tube (e) is a fixed orifice; superheat varies with load and ambient conditions.

Common Pitfalls:
Misadjusting TXV superheat setting too low, risking floodback; too high wastes evaporator area and reduces capacity.

Final Answer:
Thermostatic expansion valve