Some of the most important factors over which the system designer and operator have some control are the:
– face area of the coil, the cross-sectional area of the air stream entering the coil
– number of rows of tubes deep
– fin spacing
– air-flow rate (and air velocity)
– refrigerant temperature.
Table 6.5 shows the influence of an individual increase in these parameters.
Face area. An increase provides more heat transfer surface, and the surface also assumes a lower temperature (Fig. 6.30a).
Number of rows of tubes deep. Each additional row of tubes further reduces the air temperature and moisture content, limited by the saturation conditions at the refrigerant temperature (Fig. 6.30b).
Closer fin spacing. Closer spacing provides more heat-transfer area, resulting in lower outlet temperature and moisture content (Fig. 6.30c). Also see Section 6.20 on frosting of coils.
Higher air-flow rate With a higher flow rate, the outlet temperature and moisture content increase, but in the equation for the rate of heat transfer,
the flow rate increases by a greater percentage than the percent decrease in enthalpy difference. The refrigeration capacity q thus increases (Fig. 6.30d).
Higher refrigerant temperature. A higher temperature raises the coil surface temperature all along the coil. Thus the temperature and moisture content are not reduced by as much. Also, as seen from Fig. 6.30e, the ratio of moisture removal to temperature drop is not as great—a characteristic desired in high-humidity rooms.
The preceding discussion concentrated on changing only one condition at a time. System designers who select coils may adjust two or more conditions to obtain desired outlet air properties. For example, suppose that the amount of available space does not permit the coil face area desired. Instead, a deeper coil could be selected and combined with a slightly higher air-flow rate to closely match the original outlet air conditions.