Capturing the heat blasted out the kettle flue can yield tremendous benefits. Yet very few galvanizers have attempted to capture this source of free heat.  Fear of interfering with flue exhaust design parameters and lack of competent engineering have hindered the use of this heat source.

Typical Kettle Arrangement

A typical kettle arrangement includes a process to mix natural gas with combustion air. Simple thermodynamics and common sense tell us that it takes energy to raise the temperature of air or liquid. That energy is proportional to the difference in temperature between the starting and ending points. So, if you are required to raise the temperature of the gas and combustion air from ambient (70ºF) to over 1,000ºF, the amount of energy required would be calculated by figuring the volume of air necessary to be heated times the increase in temperature. This figure is given in British Thermal Units (Btu's). It is easy to take the next step in logic and ask, "With all the high temperature exhaust gas going up the flue, what can we do to capture some of the energy?"  We will discuss three methods and show a few variations.

1. Simple Heat Recuperator

An extremely common method of obtaining additional value out of the flue is through the use of a "recuperator". This device is a simple baffled pipe placed around the outside of the flue and connected to the suction of the combustion air blower. This pre-heats the combustion air and provides a significant reduction in gas usage. Plants using this device have recorded reductions in natural gas usage between 8 and 30 percent. The most common numbers were in the 8 to 13 range, but it is easy to see that larger savings are possible in the most severe climates. One variable that is not usually mentioned along with the addition of a recuperator, is the analysis and tuning of the burners. This combination of the tuning and the heat recovery can substantially increase efficiency.

2. Flue Heat Recovery Using Exchangers

An excellent method of grabbing energy from exhaust combustion gas is through the use of a heat exchanger in the flue. This process has to be carefully engineered to avoid "starving" the exhaust duct and causing back pressure to the burners. This can be accomplished by making sure that there is a flue opening that fits this simple formula:

Total Btu Capacity of the Burners (in Millions) divided by 15,000=Square inches

Several of the installers have used good sense and placed a damper and "Y" shunted side stream on their flue. This allows easy access and maintenance of the heat exchanger as well as the ability to control exhaust direction. The diverted gas enters a widened box where simple steel or stainless heat exchangers are located to extract the energy. A heat transfer fluid (either a glycol loaded water or a special oil) is pumped through the exchangers and then through another set of exchangers that transfer the heat to the process tanks.

A variation on this method was used by a galvanizer who had been using the direct method for several years. After saving about $2,500 per month on gas bills by heating his caustic and one acid tank, he decided he wanted to expand the operation to all of his tanks. He placed a high temperature water heater in the loop between the flue exchanger and the "in tank" exchangers, thereby realizing value from his flue while controlling temperatures throughout the operation. The high temperature water heater maintains the exchange fluid between 185 and 205ºF with very simple controls and minimal use of gas. His two 8,500 gallon acid tanks are maintained at 140ºF, the caustic tank at 160ºF, and the pre-flux averages about 125ºF. His capital cost for the installation was about $85,000.  The flue heat exchanger was manufactured in-house out of mild steel. The immersion heat exchangers already existed and were not changed.

Both immersion (in tank) heat exchangers and "tube in shell" style have been used as transfer methods into the caustic and acid tanks. Both methods, with their advantages and disadvantages, have proven effective. The real focus of design concern should not be just the style of heat exchanger, however. The proper pump and expansion tanks arrangement, materials of construction, final flue exhaust temperatures, and exchanger design also must be addressed. When all of these parameters are met, a galvanizer can save several thousand dollars per month on gas bills and equipment repair. Many of the modifications to the plant can be done internally.  If the project is correctly planned, installation can occur within one week. The flue exchanger work can be done during the weekend or during maintenance downtime. A relatively simple set of controls will operate the system, and a built in expansion tank and system shut down should be worked into the process.

3. Direct Flue Heat Usage

It is fairly common practice to use the flue gas for product drying before going to the kettle. Some kettle design engineers suggest that it is risky to do this since the exhaust gas has a high liquid vapor level. Another concern is that the parts may reach temperatures greater than 160ºF, and the flux will be destroyed before entering the kettle. A more direct and interesting use of the heat is to direct the flue gas into a double boiler-like operation where the pre-flux tank is enclosed with an outer tank. The gas is directed into the outer tank to keep the preflux at about 150ºF. An in-line exhaust fan then drafts the gasses up the stack into the atmosphere. An interesting side effect of this installation is the deterioration of the flue stack. After less than two years of operation a steel stack has to be replaced due to corrosion. The reduction of stack temperatures to below 300ºF caused condensation of sulfonic acid that rusted through the stack. This may be a mixed blessing for the galvanizer. With the advent of the clean air laws, the reduced temperatures are much easier to scrub, and the capital cost of equipment is substantially reduced. The galvanizer may even be able to simply baffle scrub his off gasses to remove the majority of the offending sulfonate.

If you would like to receive a no-obligation proposal for a Heating and Heat Recovery System, please contact us  or one of our representatives.
 

last updated April, 2003