walk in cooler is kept at aound 38 degrees F. ....

clean evap. coil every year, and every year after running for 6 or 8 months i go back and check it and the ball of ice is slowly getting bigger and bigger.

its been doing this for few years now.

its running on mp-39 or (r-401a) refrigerant.

checked psi on suction line and its running at about 15psi. witch i belive is to cold, way below freezing.

so evap. coil is clean, it must be low on
mp-39 refrigerant ?????????????? any guesses?