Tip & How-To about Oasis Structural Concepts CO4778R-E3 Refrigerated Open Air Screen Case

Oasis co4778r if compressor will not start

If the compressor on this case does not start install a set of gauges on unit and make sure pressures for 404a are correct for your ambient temp. at 70F you should have around 155psi on suction and discharge. if pressures are good and no power from relay on emerson board then remove bottom shelves from unit exposing evap fan/txv/coil. you will find a high pressure switch and a low pressure switch. cut wiring to both switchs and ohm them out to make sure they are closed. if one or more is open and your pressures are okay replace the switch. There are dilcores on switches so it can be done with out reclaiming if you are quick. if high pressure switch is faulty and does not open during hot pull downs the unit may trip on internal relief in compressor. 404a will run very high temp and pressure and may shut down up to 5/6 times on hps before unit suction and discharge come down. make sure condenser coil is very clean. wash with hot water and degresser if needed.
General maintence should be carried out on these units often such as cleaning the condenser. cleaning condenser pan and float check proper operation **** all dirt from pan and float with shop vac and wipe up all sluge from heater elements. remove bottom shelves to the evap area and vacuum any loose particals from inside and make sure drain is clear.
if you have product in case that is not keeping cool you may have to remove all shelves and remove back wall and vacuum dust from grates in back to get good air flow.

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1 Answer

What is the normal operational pressure in lo/hi side at the compressor of commercial freezer like dispaly case, reach in cabinet or other any low temp refrigeration uses 502 or408A/404a. is there any rule of thumb to make it easy to remember those important parameters or formula in refrigeration.


why yes there is. Generally speaking, the head pressure should be around the "ambient +30" rule.
That is, measure the entering air temp to the condenser, say 75 degrees. Now add 30 to that and get 105 degrees. Look at a P/T chart and see what the pressure is for that temp which is about 253 for R-404-A.
The evap on a freezer leads the load so, as a general rule, the evap temp is going to be around minus 10 degrees to get a 0 degree box. Coolers are different. So, the pressure for a minus 10 evap is around 25 or so. But the unit has to be close to operating temp. If the box is warm, naturally the pressure is higher. And what influences the pressure is whether or not it's a TXV or a cap tube system.
If a TXV and no receiver, charge by subcooling. If a receiver, fill by sight glass. If Cap tube, charge by superheat.
Hope that answers your question.

Oct 03, 2010 | True 49 cu. ft. / 1388 liter Commercial...

3 Answers

Normal Operating pressure for true 2 door Freezer R404A Room Under 80 degrees F Box at -10 degrees F


The pressures depend, somewhat, on the expansion device,i.e. a TXV or Cap Tube. Take a pressure reading, then convert it to a temperature. The evap temp should be right at 10 colder than the box temp. Remember, heat flows to cold so the evap has to "Lead the Load" in order to cool. In many cases, the condensing pressure is usually ambient temp, plus 30, then convert to a pressure. These are general accepted practices that have been used for many years.

Hope this helps

Jul 11, 2009 | True 49 cu. ft. / 1388 liter Commercial...

1 Answer

compressor will not turn off but cannot find the location of the low pressure switch to change it. blows cold but will not cycle off...


there is a thermistor switch located in evaporator case(thin black single or 2 wires) that makes the compressor cut in and out (cycle) it monitors evaporator temperature.
check head pressures with air cond charging station or gauge set and monitor pressure on high and low side (150-200kpa on low, 1500-1800kpa on high good average depending on ambient temp) if it is low on gas due to leak this can also keep compressor running longer due to evaporator temp not reaching lowest threshhold to cut compressor out

Jun 13, 2009 | 1998 Chevrolet S-10 Pickup

1 Answer

evaporator unit keeps on building ice. question what is the nornal operating pressure on low side and on the high side for truee freezer using 404 refrigerant


I'll try to give you the "Readers Digest" version.
First, make sure there is no ice build up on the evap. Light frost is O.K. as long as it does not effect air flow. (Very Important). All fans need to be running. Again, an air flow thing. There should be some product in the box but not too close to the evap. Again, an air flow thing.
The expansion device can effect your pressures to some degree i.e. Cap tube vs. TXV.
As a general rule of thumb, which I use almost everyday when it comes to pressures:
Low side is based on the temp difference between evap temp and condition space in the box. In freezers, it is almost always 10 degrees. If the box is 0 degrees then the evap needs to be at a minus 10 degrees. That is a pressure for R-404A of 24.5 psi. This only holds true when the temp in the box is approching set temp. I would say, about +10 degrees. 0 Degrees for the evap for R-404A is 33.5 psi.
High side should be ambient temp +30 degrees and then convert to a pressure for the refrigerant.
Example: 75 degrees in the room, +30 degrees equals 105 degrees. Now 105 degrees converted to a R-404A pressure is 253 psi. This will get you very close to the desired high side pressure.
I would look at low side first to see if it is within reason. Don't let the high side get too high. Increases the compression ratio and overworks the compressor.
Your icing problem could be a defrost issue and not a refrigerant charge issue.

Good Luck and hope this gets you started.

Apr 07, 2009 | True 49 cu. ft. / 1388 liter Commercial...

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