A STARVED EVAPORATOR WITH AN UNEVEN FROST PATTERN resulting in poor cabinet refrigeration may be due to the following:
1. Loss of refrigerant from the thermal bulb (if undependable, erratic refrigeration is evidenced).
2. Valve needle stuck shut. (This very seldom happens. The evaporator no longer cools.)
3. Clogged expansion valve screen. (Evaporator loses its cooling ability.)
4. Moisture in the system. (The evaporator cools sometimes and sometimes does not).
5. Under-capacity valve installed. (Evaporator temperature never drops to the desired point.)
6. Inside of valve covered with wax. (Poor or no refrigeration). This occurs when the wrong type of oil isused. (Different types of lubricant are used for different temperatures.)
A FLOODED EVAPORATOR AS THE RESULT OF TOO MUCH REFRIGERANT FLOW IS EVIDENCED BY A FROSTED ORSWEATING SUCTION LINE The six most usual causes of this condition are the following:
1. Pressure drop too great in the evaporator coil. (Replace valve with one using an equalizer.)
2. Thermal bulb with wrong charge. (Replace valve with one having the correct charge.)
3. TEV orifice adjusted too large. (Turn valve stem clockwise).
4. Thermal bulb installed too far from evaporator or loose from suction line. (Correct as necessary.)
5. TEV needle stuck open. (Replace valve.)
6. Undersize evaporator. (This rarely occurs. Replace evaporator.)
As a general rule of thumb, externally equalized TEVs are used where pressure drop in the evaporator is more than:
a. 3 psi through an evaporator in high temperature application,
b. 2 psi through an evaporator in a medium-temperature application,
c. 1 psi through an evaporator in a low temperature application
d. Always use an externally equalized TEV when a distributor is used (See fig. 200). Depending on the make, size and the number of outlets, the pressure drop across the distributor alone can range anywhere from 5 to 30 psi.
e. An externally equalized valve must be used on any system in excess of three tons (regardless of application).