How To Recover Waste Heat from Flue Gas | Dry Coolers Inc.

Dry Coolers Inc.

Industrial Process Cooling Systems

Dry Coolers, Inc.
575 South Glaspie Street
Oxford, MI 48371 USA


How To Recover Waste Heat from Flue Gas

Save Energy – Save Money 

The widespread use of recuperative burners to preheatcombustion air has boosted efficiency throughout mostof the heat processing industry.  The savings in gasconsumption pays for the equipment and the “carbonfootprint” is reduced. However, for most industrial users, opportunities still exist for recuperating and putting to usemuch of the waste heat currently being exhausted fromtheir facilities. 

Tax incentives and various energy programs can help fund a project and potential upcoming legislation may putpressure on industry to reduce greenhouse gas emis­sions. Here are some practical approaches to this timelytopic. 

Heat your Wash or Rinse Tank 

Figure 1. Gas-to-Liquid Heat Recovery Unit

Measure the flow and temperature of your exhauststreams. A source of heat that is adequate to fully sup­ply or at least substantially augment an existing demandshould be identified. 

The vented exhaust from combustion can be ducted to a heat recovery unit with a gas-to-liquid heat exchanger to pre-heat boiler feed water or another nearby process(see figure 1).

Many heat related processes are precededor followed by a heated wash or rinse tank which are alsogood candidates for recovered heat. 

Rinse tanks, boiler feed water and other applications maybe heated directly in the heat recovery unit. Other fluids may have to be indirectly heated (using an intermediate heat exchanger) due to chemistry or lack of cleanliness. 

Process Control Methods 

One challenge is to match the demand for recuperatedheat with the available supply of heat. One way to ac­complish this is to pull only as much heat as neededfrom the exhaust stream by utilizing a simple modulatingdamper to bypass excess heat around the heat transfercoil, controlling the amount of heat reclaimed.  An alter­nate method is the utilization of variable speed exhaustblowers. 

A temperature probe in the tank can be used to signal the heat recovery unit to operate and a temperature probein the outlet of the heated fluid stream can be used to proportionately control the bypass damper or variablespeed blower. This will prevent any over-heating of the heat transfer fluid. 

If there is not enough heat to fully satisfy the demand,consider pre-heating or assisting the current system. For large exhaust streams consider a modular approach us­ing several units for different users (see figure 2).

Figure 2. Modular application for washer and boiler pre-heat

Heat Recovery Tips 

  • Match the materials of construction to the temperature of the exhaust stream 
  • Consider Variable Frequency Drives for blow­ers as a means of temperature control 
  • A backup heating system may be advisable 
  • Allow for thermal expansion in equipment, ducts and piping. 
  • Look for close proximity heat users to mini­mize installation costs 
  • Insulate ductwork and fluid piping for maxi­mum efficiency 
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