300px, A run-around coil installation, serving air handling units on the roof of an office building A run-around coil is a type of

energy recovery Energy recovery includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but mos ...

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

most often positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, to recover the heat energy. Generally, it refers to any intermediate stream used to transfer heat between two streams that are not directly connected for reasons of safety or practicality. It may also be referred to as a run-around loop, a pump-around coil or a liquid coupled heat exchanger.D. A. REAY (1980), A Review of Gas–Gas Heat Recovery Systems, Heat Recovery Systems, Volume 1, No. 1, Pergamon Press Ltd., pgs 18 – 21

Description

A typical run-around coil system comprises two or more multi-row finned tube coils connected to each other by a pumped pipework circuit. The pipework is charged with a heat exchange fluid, normally water, which picks up heat from the exhaust air coil and gives up heat to the supply air coil before returning again. Thus heat from the exhaust air stream is transferred through the pipework coil to the circulating fluid, and then from the fluid through the pipework coil to the supply air stream. The use of this system is generally limited to situations where the air streams are separated and no other type of device can be utilised since the heat recovery efficiency is lower than other forms of air-to-air heat recovery. Gross efficiencies are usually in the range of 40 to 50%, but more significantly seasonal efficiencies of this system can be very low, due to the extra electrical energy used by the pumped fluid circuit. The fluid circuit containing the circulating pump also contains an expansion vessel, to accommodate changes in fluid pressure. In addition, there is a fill device to ensure the system remains charged. There are also controls to bypass and shut down the system when not required, and other safety devices. Pipework runs should be as short as possible, and should be sized for low velocities to minimize frictional losses, hence reducing pump energy consumption. It is possible to recover some of this energy in the form of heat given off by the motor if a glandless pump is used, where a water jacket surrounds the motor stator, thus picking up some of its heat. The pumped fluid will have to be protected from freezing, and is normally treated with a

glycol A diol is a chemical compound containing two hydroxyl groups ( groups). An Aliphatic compound, aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common ...

based anti-freeze. This also reduces the specific heat capacity of the fluid and increases the viscosity, increasing pump power consumption, further reducing the seasonal efficiency of the device. For example, a 20% glycol mixture will provide protection down to , but will increase system resistance by 15%. For the finned tube coil design, there is a performance maximum corresponding to an eight- or ten-row coil, above this the fan and pump motor energy consumption increases substantially and seasonal efficiency starts to decrease. The main cause of increased energy consumption lies with the fan, for the same

face velocity The face is the front of an animal's head that features the eyes, nose and mouth, and through which animals express many of their emotions. The face is crucial for human identity, and damage such as scarring or developmental deformities may af ...

, fewer coil rows will decrease air pressure drop and increase water pressure drop. The total energy consumption will usually be less than that for a greater number of coil rows with higher air pressure drops and lower water pressure drops.

Energy transfer process

Normally the heat transfer between airstreams provided by the device is termed as ' sensible', which is the exchange of energy, or

enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...

, resulting in a change in temperature of the medium (air in this case), but with no change in moisture content.

Other types of air-to-air heat exchangers

Thermal wheel A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy wheel, energy recovery wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of air-h ...

, or rotary heat exchanger (including enthalpy wheel and desiccant wheel) *

Recuperator A recuperator is a special purpose counter-flow energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat. ...

, or cross plate heat exchanger *

Heat pipe A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces. At the hot interface of a heat pipe, a volatile liquid in contact with a thermally conductive solid surface turns into a vapor ...

References

{{HVAC Heating, ventilation, and air conditioning Mechanical engineering Low-energy building Energy recovery Heating Sustainable building Energy conservation Industrial equipment Thermodynamics Heat transfer

Description

Energy transfer process

Other types of air-to-air heat exchangers

See also

References