Capacity Control of Air Coils for Heating and Cooling: Transfer Functions, Drive Power and System Design

Liquid-to-air coils used as air heating system coils and air system cooling coils for air-conditioning, refrigeration etc. rarely use their design capacity. The capacity must therefore be reduced accordingly, traditionally by means of on-off operation or by means of control valves. Draw-backs of traditional control are excessive pressure drop and drive power to pumps due to high flows as well as the need for balancing valves and control valves with authority. There are, however, possibilities to substantially reduce the drive energy of pumps and fans for air coils, e. g. by replacing valve and damper control by direct control of decentralized pumps and fans. This may achieve better control at a lower cost while using substantially less drive energy. This paper includes basic analysis of heat transfer and control methods to study how coil design affects the transfer function of an air coil on capacity turn-down. The analysis indicates that direct flow control, using variable-speed pumps, may require only a fraction of the drive power needed by traditional valve control. Furthermore, system designs for low flow rate and pressure drop also provide opportunities for new types of laminar-flow coil designs. Results show that the transfer functions of alternative control methods for the capacity and outlet temperatures of air coils can be written as simple functions of the controlling variable (supply temperature, inlet temperature or coil liquid flow rate). The transfer functions may be tailored to specific needs by changing design parameters such as the design values of air and liquid flow rate and the heat transfer characteristics and heat transfer areas of the respective air and liquid sides. Also, the paper provides an example of alternative system design and control strategy.