Pizza make table controls

[Capz]

Working on a Stanley Knight 10' table today, old R12 converted to Hot Shot although not 100% sure, not labeled anywhere. Newer Copeland condensing skid seems to be working good.

I am asked to lower temps slightly. But the previous service company's tech installed this skid and set it to operate on/off using a low pressure cut in/out switch. Using my low & high pocket gauges ( standard gauge screwed into a hose fitting) I am able to tweak the switch so the unit will operate slightly longer each cycle and cool the table box to 36F.
BUT, what am I missing? Seems to be working but with the compressor cycling on/off every 5 mins, not sure how long it will last? Is this a standard practice? No thermostat, rather use a Ranco low psi cut in/out switch to control the cycle? No liquid solenoid either.
Low side comes on at 28psi, falls to 15psi and opens the switch, slowly rises to 30 psi and closes the switch. I can't even get an accurate SH reading in such short period of time. BUT it's working!

[trippintl0]

Since you don't know what type of gas it use, put a electronic thermometer in the evaporator coil. Set the low-pressure control to cut-In at 38*f evaporator and cut out for whatever you need to obtain the temp you want (15 - 20*f coil temp is typical for this type of cooler usually, to obtain 33-35*f box)

Or just tell the customer, they need a new temperature control to make it get colder. Then install a Ranco A12-700 in the coil. Problem solved.

[Tjune54]

If the unit uses an expansion valve the pressure control is ok to use. If it is cap-tube ditch the pressure control and use the Ranco thermostat already mentioned.
Also the differintial you have on the control now is too close for proper operation of the condensing unit. Cut-in should be around 32 to 35 psig (37 to 40f).

[hvac wiz 79]

I'm not a fan of hot shot when using pressure to control box temp. That glide is far to great & while I've been told how to best achieve a target, it still involves to much fine tuning. I agree with coil mounting a A12-700

[Capz]

I'm not a fan of hot shot when using pressure to control box temp. That glide is far to great & while I've been told how to best achieve a target, it still involves to much fine tuning. I agree with coil mounting a A12-700

I like the idea of the A12-700 too. I knew I wanted a temperature control, wasn't sure which one to use. Briefly though of using a A419 and a relay or small contactor but that would be overkill. I have never seen pressure control used as temperature control, wanted to know if this was accepted practice. From what I am hearing, it has its place? Thanks Wiz and Tj.

[hvac wiz 79]

Its def accepted & i like it with the "right" refrigerant. Certain applications a regular temp control doesn't keep things tight within a temp range & causes evaps to freeze .. So many different methods of temp control it all depends on the application

[icemeister]

Low pressure controls, when used for temperature control, are really quite simple yet very effective. In addition, for medium temperature applications you can get the added bonus of automatic defrost.

In commercial refrigeration, I believe they actually preceded direct temperature control using thermostats...and they are still used today.

Here's a past thread which has a very good explanation:

http://hvac-talk.com/vbb/showthread.php?t=62989

One downside of LPCs is if the system loses some of its charge they will tend to cause the system to short cycle for very long periods...at least until the system finally goes flat. A manual reset LP control as a refrigerant loss safety control wold solve that problem, but is seldom used except for larger systems.

Another shortcoming is they can't be used on remote outdoor condensing units in climates where the ambient temperature drop below the pressure corresponding to the control's cut-in setting.

[david chamberla]

used 409a ranco a12 700 worked out good