Question 20

The Correct Answer is A. **Explanation for Option A (Correct):** 1. **Determine Saturation Pressure:** The centrifugal chiller is charged with R-134a, and the ambient temperature is $60^\circ\text{F}$. Looking at the R-134a Pressure-Temperature chart (or standard thermodynamic tables), the saturation pressure (the pressure at which the refrigerant boils or condenses) for R-134a at $60^\circ\text{F}$ is approximately **55.5 psig**. 2. **Compare Actual Pressure to Saturation Pressure:** The machine's actual pressure is currently $10\text{ psig}$. 3. **Apply the Decision Tree:** Since the machine is idle, the internal pressure should equalize to the saturation pressure corresponding to the ambient temperature ($55.5\text{ psig}$). The actual measured pressure ($10\text{ psig}$) is significantly lower than the expected saturation pressure ($55.5\text{ psig}$). According to standard leak test procedures (or the implied decision tree for low-pressure/high-pressure systems), if the system pressure is less than the ambient saturation pressure, it indicates that there is insufficient refrigerant charge, likely due to a leak. 4. **Determine Necessary Adjustment:** For leak detection in a high-pressure system using a halide or electronic leak detector, the pressure must be high enough to allow the detector to function effectively. The target pressure required for effective leak detection in a high-pressure R-134a system is typically around $35\text{ psig}$ or higher (well above atmospheric pressure). Since the system is undercharged but already contains R-134a, the recommended procedure is to add enough R-134a refrigerant to raise the system pressure to an acceptable level (in this case, $35\text{ psig}$) before conducting the leak test. **Why Other Options are Incorrect:** * **B) The machine definitely does not have a leak; therefore no attempt at leak detection is necessary.** This is incorrect. The actual pressure ($10\text{ psig}$) is much lower than the expected $60^\circ\text{F}$ saturation pressure ($55.5\text{ psig}$). This pressure discrepancy strongly suggests the machine is significantly undercharged due to a substantial leak. * **C) The machine has a suspected leak; therefore, nitrogen should be added to bring the pressure to 70 psig prior to checking for leaks.** This is incorrect as the primary/initial step. While nitrogen can be used for pressure testing, the system currently contains R-134a. EPA guidelines often require charging the system first with a small amount of the operating refrigerant (or the operating refrigerant mixed with nitrogen) to a pressure sufficient for detection (like $35\text{ psig}$) using R-134a-specific detection equipment. Raising the pressure to $70\text{ psig}$ solely with nitrogen is usually reserved for a pressure-only test or when the system has been fully evacuated and is being tested for leaks prior to charging, not when the system already contains R-134a. Furthermore, the preferred method is to raise the pressure using the system refrigerant to enable electronic leak detection. * **D) The machine may or may not have a leak; therefore the machine should be checked for leaks without any adjustments in pressure.** This is incorrect. At $10\text{ psig}$, the pressure is only slightly above atmospheric pressure. Many leak detectors, especially electronic ones, operate best when the system pressure is significantly higher (e.g., $35\text{ psig}$ or more) to ensure a high concentration of refrigerant escapes from the leak site. Additionally, the extremely low pressure compared to the saturation pressure clearly indicates a severe leak has already occurred, necessitating the adjustment before testing.