If expansion valve is opened too wide, which of the following system conditions may result?

• A. Discharge temperature decreases and suction pressure increases
• B. Discharge temperature increases and suction pressure decreases
• C. No change in pressures
• D. Both pressures rise

Answer: (A)

Opening the expansion valve wider lets more liquid refrigerant into the evaporator. That increases the amount of refrigerant being vaporized and raises the low-side (suction) pressure because the evaporator operates with more refrigerant being turned into vapor.

The high-side pressure (discharge pressure) is mainly set by the condenser and compressor capacity, so it doesn’t rise in step with the suction side. With the suction pressure higher and the discharge pressure relatively unchanged, the pressure ratio across the compressor becomes smaller. A smaller pressure ratio means the compressor does less work per pound of refrigerant, so the discharge gas temperature tends to fall.

So, over-opening the valve tends to raise suction pressure while the discharge temperature decreases.