Types of Coils in Heat Recovery from Flue/Exhaust Gases

Often called 'run-around coils', this type of system is characterised by the use of two separate heat exchanger coils, one placed in the donor airflow the other in the recipient airflow, with the heat from the donor being transferred to the recipient indirectly, using a liquid medium, most commonly water. This configuration is typically used where airflows need to be entirely separate either to avoid transfer of humidity and/or contaminants, or due the distance/location of the airflow systems. Below are some examples of scenarios were this is done and how it works:

• Industrial Dryer exhaust gas heat is transferred to the dryer air pre-heater  
  • Heat exchanger coil 1 located in dryer exhaust flue -hot gas circulating around tubes and fins of coil exterior transferring heat to water inside the tubes- water pumped & piped to coil 2 which is located in dryer’s air feed. 

• Kiln flue exhaust gas is transferred to a separate drying chamber 
  • Heat exchanger coil 1 located in kiln exhaust flue - hot gas circulating around tubes and fins of coil exterior transferring heat to water inside tubes water pumped & piped to coil 2 located at drying chambers fan forced air feed. 

Note: Run-around coils have pump-sets to regulate water flow which comprise a pump, motor, an expansion tank, a pressure relief valve, a pressure gauge, pipework, pipe fittings. 

Heat from flue/exhaust gas is used to heat another gas, typically air, without the use of an intermediate fluid such as water or thermal oil. These are commonly used to pre-heat combustion air and known as an Air-Preheater. Where the donor gas (i.e. exhaust gas) and the recipient gas (i.e. combustion air) is in the same system, the heat recovery unit is often known as a ‘recuperator’.  

Although plate type heat exchangers are often used for such applications, where heat is to be recovered from gas or air laden with particles, a finned tube or bare tube coil heat exchanger is often better suited as plate type exchangers can be prone to fouling. This is due to the airflow pattern in finned/bare tube designs which results in less build-up of contaminants around fins than in dimpled plate surface. 

• Steam boiler smoke box heat transferred to a combustion air pre-heater 
  • Cool air is pumped through the tubes of a heat exchanger located inside the steam boiler smoke box while hot exhaust gas in the smoke box circulates around outside the tubes. Heat is transferred through the tube walls to warm the circulating combustion air.