The principles presented so far in this chapter have provided the foundation for making useful judgments about actual plant operation. The straight-line law and a few heat-transfer principles equip the refrigeration professional to predict qualitative trends and to know how to correct certain evaporator problems.
The predominant criterion for evaluating evaporator coil performance is the condition of air leaving the coil. The assignment of the evaporator is to hold a certain temperature in the space, which it does by removing heat at the proper rate from the air passing through the coil. The outlet air temperature from the coil is often sensed and used as the basis of controlling the evaporator. The outlet dry-bulb temperature is not the only concern because the amount of moisture removed from the air is often crucial too. In some situations, such as a high-humidity produce storage room, as little moisture as possible should be removed. But the coils at a loading dock for a frozen-food storage room should remove as much moisture as possible to reduce the amount of water vapor carried into the low- temperature space with the infiltration air.
Predicting the condition of outlet air can be approached by analyzing the state of the air as it passes through the coil. Figure 6.28 imagines the coil to be a flat surface with refrigerant boiling at a temperature of tr on one side of the metal. Air enters at a temperature of t1 and a moisture content of W1.
A key realization is that the wetted surface temperatures, ts1, ts2, etc., progressively decrease in the direction of air flow through the coil. The reason for this progression is that ts adjusts itself, so that the same rate of heat flows from the air to the wetted surface as from the wetted surface through the metal to the refrigerant. With the drop in temperature and moisture content as the air flows through the coil, ts also drops in response to the lower potential for heat transfer and moisture removal.
The straight-line law is now combined with the awareness of this progressive drop in wetted surface temperature to yield the curve showing the air conditions in the coil. Fig. 6.29.
Suppose that the flat surface of Fig. 6.28 was an idealization of a coil having 8 rows of tubes in the direction of air-flow, and each of the sections in Fig. 6.28 represents two rows of tubes. In Fig. 6.29, the entering air, in conformity to the straight-line law, drives toward ts1 on the saturation line. As the air moves through the coil, it drives toward progressively lower temperatures on the saturation line. The state of the air, called the coil condition curve, in Fig. 6.29 is the result.
A few observations from Fig. 6.29 are confirmed by catalog data from coil manufacturers. Each succeeding row of tubes does less work toward lowering the temperature or-removing moisture from the air. The greatest rate of heat transfer is where the air enters the coil, because the air temperature and moisture content are the highest here. The curve becomes steeper as air passes through the coil, which indicates that the ratio of moisture removal to the temperature drop is greatest at the coil’s air outlet.