The art and science of troubleshooting finned coil evaporator performance varies as much as the applications in which this refrigeration component is placed. In this part 2 of the series, I will look at the refrigerant feed arrangements for evaporators.

There are three refrigerant feed arrangements for an evaporator: liquid recirculated, flooded and direct expansion.

Refrigerant for liquid recirculated coils can be delivered via a mechanically pumped system or by the use of refrigerant pressure (also known as control pressure receiver CPR). Cross-fed coils, designed for liquid recirculated mechanically fed coils, utilize orifices in the circuit inlets to meter the refrigerant flow and often use refrigerant distributors in refrigerant pressure fed systems, where in a vertically fed coil the circuits are unrestricted. Conversion of a cross-fed design liquid recirculated coil to another feed application is not possible; the orifices used to meter refrigerant flow are not conducive to a flooded or direct expansion feed type.

Flooded-fed coils are designed with open circuits for both coil designs and are convertible to an overfeed application in a vertical-designed coil only.

Direct expansion refrigerant-fed coils are designed with a distributor. This device is used to provide proper feeding of refrigerant to each circuit and is engineered to handle the refrigerant liquid and vapor mixture as the refrigerant is fed from a high pressure/temperature to the coil pressure/temperature. A direct expansion-fed coil can be converted to a liquid recirculated overfeed design but not to a flooded design. The overfeed rate for a converted direct expansion coil will be much higher than for a coil originally designed for liquid overfeed. This is due to the diameter of the capillary tubes from the distributor to the coil. For instance, for ammonia, they will range from 0.1875 to 0.25" OD.

Air volume over a coil surface is an important element to the performance of an evaporator. Air volume will vary by application and temperature. The specific heat of air is 0.24 BTU/lbm of air, and the air volume can be increased or decreased to affect the performance of an evaporator. Air volume for evaporators operating below freezing can be designed with as low or high face velocity as is economically feasible. Face velocity of a coil is calculated by the following equation:

Coils operating below freezing typically operate with a face velocity of 700 to 1,000 ft/min. A higher velocity usually has an energy cost associated with it, and the additional horsepower could have a negative effect on overall unit performance. A motor will produce 2,547 BTU/hp, and there is a point of diminishing returns for designing a coil with a higher face velocity. Coupled with the heat penalty associated with the horsepower, the velocity of air passing through the coil can become too great to be effective.

For evaporators operating above freezing, the common threshold for moisture carryover is 620 ft/min. Air temperature and moisture content will affect this level. Evaporators operating in high temperature and humidity applications may require a face velocity as low as 400 ft/min to prevent moisture from carrying over into the discharge air stream. Intermediate drain pans are beneficial with coils stacked in high humidity applications.

Troubleshooting

When an evaporator does not appear to be producing cold air, you must systematically evaluate the unit operation. It is important to determine if the performance problem is isolated to a single evaporator or is common among multiple evaporators. Depending on the answer to this question, you will look at either the evaporator and its control valve group, or expand your investigation to the system.

It must also be ascertained if the lack of performance is a new or a recurring event. If it is new, you must look into any changes in the system. Has there been a new addition to the system, like an additional load or changes in any of the components feeding the coil? Examining the refrigerant delivery system and control valve group becomes necessary to see if they have the capacity to handle the existing and new load requirements in the event of additional load to the system.

I will look more at troubleshooting in part 3.

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