Condenser Capacity Control – Air Flow Rate

An equation8 for the air-flow rate comparable to Equation 7.7 for the rate of spray-water flow is:

This equation matches closely some other data14 that is shown in Fig. 7.16. If the air-flow rate is reduced by 50%, for example, the heat-rejection capacity of the condenser with a given combination of condensing and wet-bulb temperatures is 72% of the base capacity. One manufacturer15, on the other hand, suggests that a reduction in air-flow rate will result in 58% of rated capacity. Some of the ways in which the air-flow rate can be regulated are:
– Variable-frequency drive of fan motor
– Two speed fan motors
– Pony motors
– Fan dampers
– Fan cycling on a single-fan unit
– Shutting down one fan in a multiple-fan condenser

Variable-frequency inverters driving the fan motor give the most precise regulation, but currently the first cost of the assembly makes this method the most expensive in overall first cost. Two-speed fan motors are available that operate with 1800/1200 rpm combination using a two-winding construction or 1800/900 rpm using a single winding. The 1800/1200 rpm combination requires an expensive motor but a low-cost starter, while the 1800/900 rpm combination offers a low-cost motor but an expensive starter. The pony motor arrangement mounts a different-speed motor on each end of the shaft and only one is powered while the other idles. Fan dampers are sometimes used, but in the hostile environment of the condenser the parts sometimes fail to move easily. Cycling the fan of a single-fan unit is a direct approach, but the condensing pressure oscillates and may cause control problems elsewhere in the system. One of the widely used methods of controlling the air flow is to cycle one or more fans in a multiple-fan unit. Such condensers must be equipped with baffles between the cells or much of the air delivered by one fan simply flows backward through an adjacent fan.

Effect of air-flow rate on the heat-rejection capacity of an evaporative condenser with given condensing and wet-bulb temperatures.

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