A grocery store owner in Georgia does a daily inspection of all refrigerated and frozen case temperatures using a laptop computer with specialized commercial software. The store owner notices that the temperature in a frozen food reach-in case has been above normal and will not pull down below 15°F. The case temperature is 20° at the present inspection, but ideally it should be between minus 10° and 0°. Customers have been complaining of warm products in this same glass door reach-in freezer for about a week.
The store owner then proceeds downstairs where the condensing units are located. The computer software also shows that the compressor for the glass-door freezer seems to have a constant run time. The store owner decides that a service technician should be called in to remedy the problem.
The service technician arrives, and the store owner explains that there has been a gradual increase in case temperatures from 0° to 20° in about a month’s time and that customers are complaining of a warm product. The ice cream has been softer than usual and has a tendency to melt before customers arrive at their final destinations. Also, the frozen vegetables are a bit softer than usual. The store owner also lets the technician know that the condensing unit has a 100 percent run time and is only keeping the product at 20°.
The service technician enters the basement, where the temperature is 70°, and notices that the R-404A condensing unit is a 3-hp, semi-hermetic, reciprocating compressor with a forced-air condenser. The system also utilizes a thermostatic expansion valve (TXV) and a receiver. The technician installs gauges on the low and high side of the compressor and takes an amperage reading. The pressure on the low side reads 33.5 psig (0°), and the high side reads 174 psig (80°). The amperage reading is 5 A under the nameplate’s rated load amps (RLA).
The technician puts an insulated temperature probe on the condenser’s outlet and then travels back upstairs to install an insulated temperature probe on the evaporator outlet. The installation of these insulated temperature probes on the condenser and evaporator outlets, along with both a high- and low-side pressure reading, will allow the technician to calculate evaporator superheat and condenser subcooling amounts.
The evaporator outlet temperature reads 14°, and the condenser outlet temperature reads 71°. This gives the system 14° of evaporator superheat and 9° of condenser subcooling (see equations 1 and 2, respectively).
(Evaporator outlet temperature)
- (Evaporator temperature)
= Evaporator superheat
|14° - 0° = 14°|
- (Condenser outlet temperature)
= Condenser subcooling
|80° - 71° = 9°|
EQUATIONS 1 AND 2: Use these measurements to calculate evaporator superheat and condenser subcooling.
The technician realizes the unit cannot be low on refrigerant (R-404A) because the 9° of condenser subcooling indicates there is enough liquid refrigerant in the condenser. Also, evaporator superheat is usually much higher on systems that are low on charge.
After thinking for a moment, the technician wonders why the condensing temperature is only 10° hotter than the ambient temperature in the basement. This is an indication that the condenser is not rejecting very much heat from the system to the basement air. The difference between the condenser temperature and the ambient air temperature is known as the condensing split or condenser temperature difference. The R-404A standard efficiency condensing unit in the basement is about 20 years old and should operate with a 25° to 30° condenser split under normal heat loads on the evaporator.
The technician analyzes the system check again. A dirty condenser would give a high condensing temperature and pressure; however, this system has low condensing temperatures and pressures. A dirty evaporator or low airflow across the evaporator would give a low evaporating temperature and pressure. This system has higher than normal evaporator temperatures and pressures. Also, if the liquid line or metering device were restricted, the evaporating temperature and pressure would be low.
|New system check:|
|Evaporating temperature||minus 16° (20 psig)|
|Condensing temperature||96° (222 psig)|
|Condenser split temperature||26°|
|Basement ambient temperature||70°|
|Refrigerated box temperature||minus 5°|
MEASUREMENTS: By analyzing the original measurements, the technician was able to properly troubleshoot and resolve the issue.
The technician wonders what would cause a high evaporating temperature (pressure) and low condensing temperature (pressure) with low amperage draw. The technician then realizes that the compressor’s valves or piston rings could be worn and leaking. This would cause leakage of pressure between the high and low sides of the system as the pistons reciprocated, causing a lower condensing pressure with a higher evaporating pressure. The amp draw also would be low because of this pressure leakage within the cylinders and/or valves, which would cause a low compression ratio.
The technician then pumps down the compressor by front-seating the suction service valve and examines the valves and valve plate. Sure enough, the valves are not seating properly and are warped and discolored. A new valve plate with new valves, head gasket, and valve plate gasket for the valve plate are installed, and the oil in the compressor’s crankcase is also changed. The compressor is evacuated to a 500 micron vacuum and put back into commission. After about one hour of running time, the system pulls down to 0°. A new system check is taken, and everything seems to be running fine.
Both the condensing pressure and evaporating pressure are normal. The compressor was 20 years old, so the technician blames the valve problem on old age and wear and tear since no obvious system or mechanical problems existed. The compressor’s oil was a bit dark, but an acid test kit showed no signs of acidity in the oil.
Having solved the problem, the technician explained how the problem was fixed to the store owner, who is happy to have the freezer operating normally once again.
Publication date: 2/4/2019
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