Coil freezing with 5 degree c suction?

Hi all,

I'm working on a small air drier that's dehydrates air for an air compressor. When there is no air flow through the drier it doesn't turn off , but has a hot gas bypass valve that comes into play if the suction drops below 3 degree c. This would prevent the coil from icing up. The only place I can measure the evap pressure is at the compressor. The hot gas bypass valve tees of the discharge line straight into the suction ( bypassing the evap ) on my first call there I found the suction running at 5c , quite superheated.but I could see frost exiting the foamed in place evap? The evap had iced and blocked the air flow generating alarms on the site. I have realised there is a leak and since recharging all is good. My question is : How can my evap ice up if my suction is running above 0c?
I understand the that the hot gas valve would be active raising the suction and superheat but shouldn't that pressure be constant down the suction? How can I have a lower suction in the evap than at the compressor?

Curly one.

Maybe the hot gas needs to come on a bit earlier?

Most times I've worked on these it's a gas leak or dead compressor, patch 'em up & away they go again.

A differential pressure over the evaporator. Where is the hot gas injected: inlet or outlet evaporator?
What refrigerant? R407c (glide) ?

The bypass tees off discharge and then tees into suction , bypassing the evap. It tees into the suction maybe 200mm from compressor . It would have been better if the bypass was fitted just after the capillary expansion device, but the way it is configured is ok if the gas is good.
The way it is with the 2 parallel lines , one the bypas and the other the evap should maintain a constant suction pressure regardless of the fact that the system was short? Clearly the evap had iced to the degree of icing the evap closed, ice was evident exciting the evap , after an hour defrost ( left off) the system cleared itself and ran ok for a period with a 5 c suction ( compressor) until I could see ice starting to form again leaving the evap. I then recharged and all is good. I have always considered the compressor to have the lowest suction anywhere on a system, but I'm seeing the comp at 5 c and the evap must be lower than 0c?

Running on r 22.

Drew,

1/ Maybe ice forms for a period below 0 deg C for awhile before hot gas comes on.
2/. Your gauges are wrong
3/. Mixed or wrong refrigerant charge changing saturated temp of refrigerant.
4/. Tasmania defies the laws of physics
5/ Make sure water drain solenoid energizes & purges water, usually on a timer.

I thinks it's number 4 . Ha ha. Thanks for your input.