Seven simple steps for service diagnosis
In order to diagnose any refrigeration fault quickly and accurately, a set procedure must be followed. The procedure described here takes the form of seven simple steps. If fully understood, these will prevent expensive call-backs and dissatisfied customers. No attempt to correct a condition should be made until the fault has been found, and therefore a thorough diagnosis is essential.
It is necessary to have the correct tools and instruments with which to carry out the procedure. These include a resistance thermometer, a leak detector, gauges, valve keys, a multitester or avometer and a compressor test cord, together with a complete set of engineering tools.
The seven steps are as follows:
1 Check the actual temperature of the product and compare with that recommended for the product.
2 Check the suction pressure, control switch settings and product classifica- tion to establish the temperature difference (TD) between the evaporator and the product.
3 Check the superheat setting of the expansion valve.
4 Check the condensing medium temperature.
5 Check the operating and idle head pressures of the compressor.
6 Check the refrigerant type and charge.
7 Check the drive pulley size on an open type system. If the compressor is hermetic or semi-hermetic, check the operating range; they may be for high, medium or low back pressure operation.
An incorrect size of pulley may be fitted to a drive motor. The refrigerating effect may be acceptable in cooler ambient temperatures, but when ambient temperatures rise the equipment will not have the capacity because of the compressor speed.
The same principle applies to the compressor operating range; older models were selected for specific operating conditions.
The following practical sequence is suggested for covering the first six steps:
(a) Ensure that the product has been stored for sufficient time to have become chilled or frozen (has not recently been deposited) before checking the product temperature. Take the actual temperature of the product and not the air circulating around it. Take care that the temperature has not been affected by the opening of the fixture door.
(b) Fit gauges and calculate the average suction pressure to establish the TD (see Chapter 5). Consult the classification in Chapter 5 to establish the product group, and note the TD for the type of evaporator employed in the system. If the TD is incorrect, an adjustment of the temperature control may be all that is necessary to rectify the fault.
(c) Ensure that the system has been operating for a sufficient period to be at an average suction pressure, and check the refrigerant charge. Check the operating head pressure and the idle head pressure. Check the superheat setting of the expansion valve. When checking the refrigerant charge, observe the condition of the evaporator (should be fully frosted) and the liquid sight glass. Do not adjust the expansion valve unless the refrigerant charge is complete and the system is operating at an average suction pressure.