Power for Fans and Pumps in Heat Exchangers of Refrigerating Plants

Div. of Applied Thermodynamics and Refrigeration, Royal Institute of Technology, KTH, Stockholm, Sweden. The energy consumption to operate auxiliaries such as fans and pumps in heat exchangers have a significant influence on the total energy demand for operating a refrigerating system. With a starting point in a simple entropy analysis a more practical approach is adopted for common cases of air coil fans in evaporators or condensers. Examples are given to illustrate how the power for evaporator or condenser fans will affect the capacity and total energy demand of refrigerating systems. It is shown that the two different criteria: -maximum of capacity or -minimum of energy demand (equivalent to maximum system COP) will give different optima for the power to be used in fans or pumps. Simple relations are derived for optimum power in fans or pumps for the two different criteria applicable for many general cases in refrigerating systems. Introduction The energy needed to operate fans or pumps is important when considering the total energy demand to operate a refrigerating plant or a heat pump. It is not unusual that the electric power of such auxiliaries is in the order of25% or more of the power to operate the compressor in a system. The purpose of this paper is to illustrate and exemplify this issue. Simple relations will be derived for optimum power in fans or pumps to reach criteria like maximum capacity or maximum system COP. Let us exemplify with the application of an evaporator. As a starting point let us assume that we have a given plant where we can adjust the fan speed in practice perhaps by means of an inverter control. It is obvious that by using a high fan speed the evaporator will operate with smaller temperature differences between the inlet air and the refrigerant evaporating temperature than if low speed is used. This will decrease the temperature lift of the cycle and thus decrease the compressor work. However we will have to pay for the fan power and what is of interest is the sum of the power for the compressor and the fan. It is obvious that there must exist a certain fan power that we can call optimal from the point of view of energy consumption. Two different approaches will be used: First a treatment minimizing the entropy generation and, secondly, a more practically oriented way of treatment will be demonstrated. This treatment will concentrate on refrigerating applications. Slightly different relations will be obtained for heat pump operation. Space limitations prevents a treatment for that case but the practical result for minimum energy demand are quite similar. Relations between pumping power and temperature difference The pumping power will influence the temperature difference for a case with given geometry. A reasonable assumption is that the overall heat transfer coefficient is proportional to V"u where Vis the fluid flow and nu is an exponent, which in most cases has a value in the range of 0,3 to 0,6. The pressure drop can be set proportional to V"P where for turbulent flow np = 1,8. The pumping power, Ep, will hence (assuming constant pump efficiency) be proportional to V(np+lJ. Based on a reasoning indicated we can write the overall temperature difference, 8, as where 8=:: C·(EPFnE: nE:: _!!!!____ which hence for most cases will be in the order ofO,l to 0,2 np+] C is a constant.