Run around coil
Encyclopedia
A run-around coil is a type of energy recovery
heat exchanger
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.
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, as well as containing the circulating pump, will also contain an expansion vessel, to accommodate changes in fluid pressure; a fill device, to ensure the system remains charged; controls to bypass and shut down the system when not required, and various other safety devices and ancillaries. Pipework runs should be kept as short as possible and should be sized for low velocities to keep frictional losses to a minimum, and hence reduce pump energy consumption. It is possible however 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 water passing through the pump will pick up some of its heat.
The pumped fluid will have to be protected from freezing in certain climates, and as such is normally treated with a glycol 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 -10 C, 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 drop off. The main cause of increased energy consumption lies with the fan, for the same face velocity, fewer coil rows will decrease air pressure drop and increase water pressure drop but 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.
', which is the exchange of energy, or enthalpy
, resulting in a change in temperature of the medium (air in this case), but with no change in moisture content.
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...
heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
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.
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, as well as containing the circulating pump, will also contain an expansion vessel, to accommodate changes in fluid pressure; a fill device, to ensure the system remains charged; controls to bypass and shut down the system when not required, and various other safety devices and ancillaries. Pipework runs should be kept as short as possible and should be sized for low velocities to keep frictional losses to a minimum, and hence reduce pump energy consumption. It is possible however 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 water passing through the pump will pick up some of its heat.
The pumped fluid will have to be protected from freezing in certain climates, and as such is normally treated with a glycol 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 -10 C, 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 drop off. The main cause of increased energy consumption lies with the fan, for the same face velocity, fewer coil rows will decrease air pressure drop and increase water pressure drop but 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 'sensibleSensible heat
Sensible heat is the energy exchanged by a thermodynamic system that has as its sole effect a change of temperature.The term is used in contrast to a latent heat, which is the amount of energy exchanged that is hidden, meaning it cannot be observed as a change of temperature...
', which is the exchange of energy, or enthalpy
Enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
, 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 wheelThermal wheelA Thermal Wheel, also known as a Rotary heat exchanger, or Rotary air-to-air enthalpy wheel, or Heat recovery wheel, is a type of 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...
, or rotary heat exchanger (including enthalpy wheel and desiccant wheel) - RecuperatorRecuperatorA 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 pipeHeat pipeA heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces....
See also
- HVACHVACHVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
- Energy recovery ventilation
- Heat recovery ventilationHeat recovery ventilationHeat recovery ventilation, also known as HRV, mechanical ventilation heat recovery, or MVHR, is an energy recovery ventilation system using equipment known as a heat recovery ventilator, heat exchanger, air exchanger, or air-to-air heat exchanger which employs a counter-flow heat exchanger between...
- Regenerative heat exchangerRegenerative heat exchangerA regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where the flow through the heat exchanger is cyclical and periodically changes direction. It is similar to a countercurrent heat exchanger. However, a regenerator mixes the two fluid flows while a...
- Air handlerAir handlerAn air handler, or air handling unit , is a device used to condition and circulate air as part of a heating, ventilating, and air-conditioning system. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and...
- Thermal comfortThermal comfortThermal comfort is a term used by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, an international body. It is defined as the state of mind in humans that expresses satisfaction with the surrounding environment...
- Indoor air qualityIndoor air qualityIndoor air quality is a term referring to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants....
- CCSICCSICCSI specialises in developing software for Air conditioning and engineering applications as well as virtual reality and 3D visualisation tools....