Industrial Air conditioning fan

FAN AND MOTOR PERFORMANCE AND SELECTION

The selection of coils from a manufacturer’s catalogue,and that operation constituted the major decision with respect to the coil. There are several auxiliary choices often available to the designer, namely, whether to select—

1. draw-through or blow-through arrangement
2. a propeller or centrifugal fan
3. a single- or two-speed motor

One characteristic of a fan-coil combination is the throw, for which there should be a definition. Unfortunately, there doesn’t seem to be one. In air conditioning practice, throw is defined as the distance from an air outlet to the point where the velocity has dropped to 0.5 m/s (100 fpm). 

In industrial refrigeration practice, the common usage of the term is the distance from the outlet of the coil to a surface, such as a wall, or to the position where another fan-coil exerts an influence. When designers refer to throwing air 30, 60, or 90 m (100, 200, or 300 ft), they imply that there is enough air circulation at that distance to avoid objectionable pockets of high temperature.

One option available to the designer is to select either a draw-through or a

blow-through arrangement of the fan and coil (Fig. 6.36).

The blow-through arrangement has the thermal advantage that the heat introduced by the fan motor is absorbed by the air before entering the coil, while in the draw-through configuration, the motor heat warms the air upon leaving the coil. Thus, the coil operates somewhat more effectively in the blow-through arrangement because of the higher mean air temperatures. The advantage of draw-through is that the throw is greater than with the blow-through. Outlet air velocities from the fan in a draw- through arrangement might be as high as 20 m/s (4000 fpm) to achieve 60 m (200 ft) throw.

Several options are available to reduce the refrigeration capacity of the coil

at light refrigeration load:

1. interrupting the supply of refrigerant to the coil
2.  elevating the evaporating temperature
3. shutting off the fan or shutting off one fan in a multiple-fan unit
4. reducing the air flow over the coil by shifting a two-speed motor to low speed.