compressor cycles per hour question

[kengineering]

Hi all, I'm working on a self-contained custom refrigerated box. Temperatures are ok but it cycles 9-10 times per hour. Cycles on for 3-4 minutes then off 3-4 minutes. I have heard it said that a run time of 18hours per day is about right. There is no history available. R134A. cut in 32°f- cut out 38°. SH-10. ANy suggestions

[Rob White]

.

Most compressor manufacturers do not like their comps
starting and stopping more than 6 times an hour.

If it is well insulated a fridge should be off for as long as it is on,
when used as a storage fridge.

12 hours a day is good, 16 run hours a day means the on to off
ratio is in the not very economic side but some systems with
a lot of duty and a lot of through put can run longer and cope ok.

What you don't want is the compressor chopping in and out.
It works best when it is running or when it is off, it will fail quicker
if it is cycling too often.

Run it longer with a wider control diff and it will stay off longer.

Rob

.

[chemi-cool]

If it takes 3-4 min. to cool it down then the unit is too big for the box.
But you can increase the differential and add a timer or use off delay parameter on the controller to keep it still for 0 min. before restarting.

Another thing is to check where the temp sensor is located, it might feels cold air from the evaporator outlet while it should be on the evaporator inlet air temp.

[al]

is it actually cycling on thermostat or is the compressor overheat kicking in and then resetting when the body cools down?

[monkey spanners]

Think smaller compressor can start 10 times an hour, bigger ones its 6 times an hour, maybe you could fit an anti cycle timer on if it has an electronic controller delayed start may be one of the parameters.

[Magoo]

Perhaps cycle evap fans with temperature controller.

[kengineering]

I should give you guys the whole scenario. I am trying to build a more energy efficient storage refrigerator. I have modified our standard storage box and as follows.
1) More insulation with thermal breaks
2) High efficiency fan motors
3) Replaced condensate evaporator with compressor hot gas assist condensate evaporator.
4) Reduced compressor size from our original (designed 10 years ago)

I have reduced our energy consumption by 24% through these measures, but it is not enough. Due to the case temps being good and the cycle times what they are reducing the compressor size seems to be the next step.

Yes thermostat sensor is in discharge air.
I sampled the avg. product temp on all shelves front to back and left to right. High-39.5°f, low-35.9°f = avg 37.5°f this fine.

which brings up additional questions;

1. How does reducing the coil size affect running kW if I keep the same condensing unit?
2. If I change the condensing unit will I need to change the coil too? Is there a way to know?
3. Any suggestions greatly appreciated.

I'm am sorry for not being more forthcoming in the first post.