Duke sandwich unit, Duke Model# SUB-CP-TC60

[ectofix]

Read this.  It’s from an old post that I copied & pasted.  I hope it helps:

 

That cooler uses a coil-sensing thermostat.  By that, I mean that the thermostat sensing bulb is actually mounted within the evaporator (cooling) coil instead of being suspended out in the air stream (such as in a walk-in) .

 

A coil-sensing thermostat is designed for cut-out temperatures (to turn the compressor OFF) well below freezing range – from 21°F to 18°F (depending on your dial setting).  The designed cut-in temperature (to turn the compressor ON) is around 40°F± 3°.

This is a common thermostat configuration for counter-top and prep coolers.  Bear in mind that when temperature within the evaporator coil reaches…say…20°F, the actual air temperature to chill the box would be a much higher – probably around 35°F.

 

Obviously yours isn’t working properly.  Here are several possibilities:

The thermostat is faulty, or… The system’s refrigerant flow is inadequate, either due to a RESTRICTION or a LOW system charge.

 

That latter possibility may not make sense to you, so consider this:

 

If the system is low on charge or if the refrigerant isn’t feeding the evaporator coil as it should, then the coil isn’t getting fed enough refrigerant to supply the coil in its entirety.  As such, it’s likely that the coil section which envelopes the thermostat bulb is no longer performing in the cooling process.

Yet, the section of evaporator coil that IS getting adequate refrigerant – IS cooling.  Consequently, THAT section is doing all the work, chilling the air to sub-freezing temperatures, freezing your product…with that same frigid air passing back through the relatively dormant section of the evaporator coil…to THEN cool the thermostat bulb.  So at this point, the coil-sensing thermostat has been relegated to only sense the 18° AIR temperature.

 

The low refrigerant flow conditions that I’ve described will result in a thermostat satisfying ONLY at a box temperature WELL below what your desired range is, or in some instances, may not satisfy AT ALL.  So the compressor will just keep running…and running…and your lettuce freezes.

At some point when refrigerant flow becomes inadequate to even do THAT, then cooling compartment temperature will become too high.  Again, the compressor would keep running and running, but temperatures will remain higher than the desired range.

 

Obviously if this is the problem you’re encountering, you’ll need to connect the gauges, read the pressures and determine whether there’s a restriction (cap tube) or a leak.  The two can look very similar in pressures.

[davejohnsonnola]

Is sensing bulb in good contact with the evaporator coil?

[ryanreid1]

What temperature is it cycling at?(preferably coil temp but air temp is ok.)

Is it getting too cold or not much cooling at all? We need a little more info. 

[dixitpatel]

Thanks for every thing but i loss customer.

[ectofix]

Well, learn from your mistakes to serve the next customer better.  That’s the way it goes.