walk in cooler evaperator keeps freezing up

[Gary]

First it is a walk in chiller using R22, when it ice up it starts from the bottom until it cover the entire coil and the unit is a chiller it does not carry a defrost cycle. one other thing i notice, the low side pressure would go down to 25psi at times when the room temp. is at 39 degree F

A chiller is a machine which cools water. This is a walk-in cooler, not a chiller.

Why is the low side pressure dropping? Are the evaporator fans stopping? Is the liquid flow restricting? Is the system undercharged? Does the thermostat control a liquid line solenoid valve? Is that valve installed backwards?

The low side pressure should not drop. You need to find out why it is doing so.