Cooling - But Not Cold Enough

December 4, 2006

Let’s say one of your customers has been complaining for three weeks of warm products in a low temperature, glass door, reach-in freezer. The store owner inspects the thermometers in the case and notices that the temperature will not pull down below 15°F. The temperature has been 15° for about a week, but ideally it should be 0°. The owner then proceeds downstairs where the condensing unit is located. The compressor seems to have a constant run time. After about 10 hours of constant run time with a warming product load, the owner decides that a service technician should be called in to remedy the problem.

The technician arrives and talks to the owner about the problem. The owner explains to the service technician that there has been a gradual increase in case temperatures from 0° to 15° in about three weeks time. The owner also explains to the technician that customers are complaining of not-to-frozen products in the reach-in. The owner also lets the technician know that the condensing unit is running 100 percent of the time trying to keep the product cold.

The technician enters the basement and notices that the condensing unit is a three-horsepower, semi-hermetic, reciprocating compressor with a forced-air condenser. The system also has a TXV and a receiver. The temperature in the basement was 70°.

Equation 1.

The technician installs gauges on the low and high side of the compressor and takes an amp reading. The pressure on the low side read 33.5 psig (0°), and the high side read 174 psig (80°). The amp reading was 5 amps under rated load amps (RLA) on the nameplate. The service technician then puts a temperature probe on the condenser and evaporator outlets in order to get a condenser sub-cooling and an evaporator superheat reading, respectively.

The condenser outlet temperature read 71° and the evaporator outlet temperature read 14°. Therefore, the system has 9° of condenser sub-cooling and 14° of evaporator superheat, as shown in the equations worked out by the technician. (See Equations 1 and 2.)

The technician then realizes the unit cannot be low on refrigerant because of the 9° of condenser sub-cooling telling him that there is liquid in the condenser. (See Table 1.) Also, evaporator superheat is usually high on systems, low on charge. After thinking a moment, the technician wonders why the condensing temperature is only 10° hotter than the ambient or surrounding 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 in temperature between the condenser temperature and the ambient air is often called the ‘condensing split’ and it should run from 25° to 30° on standard efficiency condensers under normal heat loads.

Equation 2.

 The technician then sits down and analyzes the system check again. A dirty condenser would give a high condensing temperature and pressure. A dirty evaporator would give a low evaporating temperature and pressure. If the liquid line or metering device was restricted, the evaporating temperature and pressure would be low.

The technician wonders what would cause a high evaporating temperature (pressure); low condensing temperature (pressure) with low amp draw. The technician then realizes that the compressor’s valves or piston rings could be worn and leaking. This would cause pressure leakage between the high and low side 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.

So, the technician pumps the compressor down and examines the valves and valve plate. Sure enough, the valves are not seating properly and are warped. A new set of valves with gaskets for the valve plate and head are installed. The compressor is evacuated and put into commission.

Table 1.

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 right. Both the condensing pressure and evaporating pressure are normal. Since the compressor was 17 years old, the technician blames the valve problem on old age and wear and tear since no obvious system or mechanical problems existed.

The technician then explains his actions to the owner. The new system check shows an evaporating temperature of -16° (20 psig), a condensing temperature of 96° (222 psig), condenser sub-cooling of 9°, evaporator superheat of 7°, condenser split of 26°, and basement ambient temperature of 70°.

Publication date: 12/04/2006