A newly installed commercial refrigeration unit using R-448A has been evacuated to 500 microns. The superheat is measured at 10 K (ºC) with a saturated suction temperature of -10 ºC, and the subcooling is 5 K (ºC) with a saturated liquid temperature of 30 ºC. The manufacturer's specifications call for 8 K superheat and 6 K subcooling. How should the technician adjust the charge?

Question: A newly installed commercial refrigeration unit using R-448A has been evacuated to 500 microns. The superheat is measured at 10 K (ºC) with a saturated suction temperature of -10 ºC, and the subcooling is 5 K (ºC) with a saturated liquid temperature of 30 ºC. The manufacturer's specifications call for 8 K superheat and 6 K subcooling. How should the technician adjust the charge?

Answer options: ✅ Add a small amount of refrigerant to increase subcooling and decrease superheat.

• Remove a small amount of refrigerant to decrease subcooling and increase superheat.
• The charge is optimal; no adjustment is needed.
• Adjust the expansion valve before adjusting the charge.

Correct answer: Add a small amount of refrigerant to increase subcooling and decrease superheat.

Explanation: The system has slightly high superheat (10 K vs 8 K required) and low subcooling (5 K vs 6 K required), both indicating a slight undercharge. Adding a small amount of refrigerant will increase the liquid available to the condenser, thus increasing subcooling, and reduce the superheat at the evaporator outlet, bringing both values closer to the manufacturer's specifications.