Section 6.16 investigated the selection of coils from a manufacturer’s catalog, 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—
– draw-through or blow-through arrangement
– a propeller or centrifugal fan
– a single- or two-speed motor
The manufacturer is responsible for offering coil, fan, and motor combinations that provide the rate of air-flow and the velocities that develop adequate heat transfer coefficients to transfer the specified rate of heat transfer. The
manufacturer is also responsible for determining the motor capacity that will drive the fan under the specific application conditions, for example, conveying high density air through a frosted coil in a low-temperature space.
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 objectional pockets of high temperature. Despite the vague use of the term, throw is an important concept and is affected by several designer choices.
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.
Coils are usually available equipped with either a propeller fan or a centrifugal fan. These two types of fans have somewhat different pressure-flow and power-flow characteristics, as shown in Figs. 6.37a and 6.37b. If the resistance to air-flow remains constant under all operating conditions, so will the air-flow rate and power. In this case, knowledge of the static pressure and fan power is required only for the design conditions. Resistance does change when frost forms on a low-temperature coil. In Figs. 6.37a and 6.37b, two pressure-flow curves are shown—one for a clear coil and the other for a frosted coil. One difference in the response of the fans is that the power required by the motor driving the propeller fan increases, while that of the centrifugal fan drops as frost builds. The propeller fan is used much more frequently than the centrifugal fan in conjunction with refrigeration coils. This is because the shape of the propeller fan curve generally permits a more compact package, and because it is more efficient than the centrifugal fan at the low static pressure that is typical of the coil application. A disadvantage of the centrifugal fan in low-temperature applications is that belt life may be short. A centrifugal fan might be chosen when a very long throw is needed, because the centrifugal fan in a draw-through arrangement can direct a jet for a long distance. An installation requiring long duct work attached to the coil is another situation where the centrifugal can be considered.
Several options are available to reduce the refrigeration capacity of the coil at light refrigeration load:
– interrupting the supply of refrigerant to the coil
– elevating the evaporating temperature
– shutting off the fan or shutting off one fan in a multiple-fan unit
– reducing the air flow over the coil by shifting a two-speed motor to low speed
The last two techniques in the above list are associated with the motor and its control. An attractive feature of two-speed operation is that when the flow rate drops to half its full value, the static pressure drops to 1/4 and the fan power to 1/8 their rated values. The electric power demanded by the motor may not drop fully by 1/8 because the motor efficiency at the low speed drops off abruptly at low loads, as shown in Fig. 6.38. Nevertheless, the power saving is usually significant, which provides a bonus by also reducing the heat load introduced to the space by the fan motor.
In a multiple-fan unit one or two fans can be cycled off to reduce the capacity and internal heat load from motors. When restarting an idle motor, the operating fan(s) should be stopped first. The reason for this procedure is that when one fan is operating, the idle one is likely to be spinning in reverse, and when power is applied to it for startup, the motor may overload. Another version of two speed fan operation that is adaptable to a centrifugal fan installation having several scrolls is to apply pony motors. Here a motor is connected to each end of the shaft, with the speed of one motor twice that of the other. The fan is then driven by one motor while the other idles.
One of the fan laws states that the power required by a fan operating at constant speed varies as the density of air that it delivers. Thus if the motor is sized for a fan operating with air at room temperature and a density of 1.14 kg/m3 (0.0714 lb/ft3) and actually operates in a space at -21°C (-6°F) where the air density is 1.39 kg/m3 (0.0870 lb/ft3), the actual load on the motor will be about 20% higher than rated. This potential misapplication is inherently compensated by the fact that the temperature of the motor windings is a principal criterion influencing the rating. Since the motor is operating in a low-temperature space, it usually can be loaded higher than its rating.