When the surfaces of evaporator coils operate at temperatures below 0°C (32°F) and also below the dewpoint temperature of the air, frost will form on the coil. In certain special situations, icing can occur where moisture from the air first condenses to liquid water, then freezes to ice. The far more common situation is where the water vapor transforms directly into a solid state, frost. No redeeming merit has been discovered for frost, and no general means of preventing its formation are known.
After accepting that we must live with frost when cooling air to subfreezing temperatures, the approach is to lessen its penalty and to remove it periodically. The two detrimental effects of frost cited mosf often are: (1) resistance to heat transfer; and (2) restriction of air- flow.
Of these two penalties, restricting the air-flow rate is the most serious. Some laboratory tests12 were conducted where the rate of air-flow was maintained constant by progressively increasing the fan capacity while frost accumulated (Fig. 6.42). These tests indicated that the U-value was not severely reduced as long as the flow rate and velocity remained constant. The drop in air pressure, on the other hand, increases markedly with additional frost, as Fig. 6.43 shows’.
Fan-coil units operating in the field are unable to maintain constant airflow rates as frost accumulates, in contrast to the way the tests in Figs. 6.42 and 6.43 were conducted. Instead, because of its pressure-flow characteristics, the fan will deliver a lower air-flow rate as the pressure drop increases (Fig. 6.37). This reduction in air-flow rate and associated drop in air velocity reduces the U-value, as Fig. 6.42 shows, and is the major reason for the reduction in heat transfer rate. This combined behavior of the fan and coil suggests criteria to sense the need for defrost: air-pressure drop across the coil or air-flow rate.
When selecting coils to operate under frosting conditions, the designer should lean toward coils with wide fin spacing and large heat-transfer area. Figure 6.44 shows some experimental data demonstrating that coils with wide fin spacing are not subject to the rapid rise in air-pressure drop experienced by coils with closely spaced fins. Another goal is to select large coils with a low temperature difference between air and refrigerant. The benefit of this choice (see Section 6.17) is a lower rate of moisture removal and thus less rapid frost formation.