The coil temperature measured at the expansion valve sensing bulb of an operating system is 10°F. The low side pressure with the compressor running as shown on the gauge illustrated indicates 15 psig. What adjustments or changes, if any, should be made to the system? Illustration RA-0016
• Use the gauge to find the saturation temperature that corresponds to 15 psig for this hypothetical refrigerant. • Compute superheat as: coil (bulb) temperature minus saturation temperature at the same pressure. • Know the normal superheat range for a thermostatic expansion valve (TXV) on a properly operating system.
• At 15 psig on this particular gauge, what temperature on the outer scale lines up with that pressure? • If the sensing bulb temperature is 10°F, how many degrees above the saturation temperature is it, and is that amount of superheat considered normal or abnormal? • If superheat were too high or too low, which of the listed problems (plugged filter-drier, dirty evaporator, fouled liquid line strainer, or TXV misadjustment) would most likely be indicated?
• Be sure you read pressure and its matching temperature from the same point on the gauge (15 psig line). • Carefully calculate the exact number of degrees of superheat before considering any component problems. • Before choosing a mechanical fault (filter-drier, coils, strainer), ask whether the data actually show an abnormal operating condition or a normal one.
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