Question 62 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is C **Explanation for Option C (Correct Answer):** The thermostatic expansion valve (TXV) controls the flow of refrigerant into the evaporator. To protect the fine orifices and moving parts within the valve from debris (such as pipe scale, solder fragments, or dirt) circulating in the refrigerant system, an inlet screen (or strainer) is often fitted directly before the valve. Over time, this screen can become partially blocked, leading to a restricted flow of refrigerant and poor system performance (starving the evaporator). Routine or corrective maintenance often involves dismantling the inlet connection area of the TXV to remove and clean or replace this inlet screen, restoring proper refrigerant flow. **Explanation of Incorrect Options:** **A) The rate action may be increased.** * **Incorrect:** The terms "rate action" (or derivative control) and "proportional action" (Option B) are standard control engineering terminology typically applied to complex process control loops (like PID controllers) used for regulating variables in large industrial settings. While the TXV is a proportional controller in function (it modulates based on superheat error), it does not have user-adjustable parameters labeled "rate action." Its operating characteristics are primarily set by the spring tension, thermal charge, and diaphragm size. **B) The proportional action may be varied.** * **Incorrect:** While the operating superheat (which relates to the proportional band of the valve) can sometimes be adjusted by turning the superheat adjustment stem (changing the spring tension), the valve does not have a distinct, labeled control input called "proportional action" that is independently varied during maintenance in the way this option suggests. Furthermore, adjusting the superheat setting is often done only if the operating conditions require a different superheat target, not as routine cleaning or repair maintenance. **D) The thermal bulb may be recharged.** * **Incorrect:** The thermal bulb (and the capillary tube connecting it to the valve diaphragm) contains a specific, sealed refrigerant charge that drives the valve operation. If this charge is lost or degraded, the TXV has failed and cannot operate correctly. In modern refrigeration practice, these components are sealed units; if the charge is lost, the entire valve or the power element assembly must be replaced. The bulb itself is not typically designed or intended to be "recharged" in the field as a maintenance procedure.