I’m doing a detailed rundown of the steps you should take to wring the best oven and furnace efficiencies out of your high-priced energy supply. The first major step is getting the best performance you can from your existing equipment, before considering rebuilding or replacing anything. In my last column, we examined Part A of that step, refining your scheduling practices so your heating equipment operates at the lowest energy cost per unit of production.
Part B is to maintain the equipment -- specifically, bringing it up to the operating specifications it had when it was first commissioned. There are several areas of attention.
Exhaust or Flue Gas Losses.Generally, these are the single biggest heat loss in furnaces and ovens, so they should get priority. Your objective is to keep the volume and temperature of those gases to a minimum consistent with the needs of your process.
Controlling the volume (actually, the mass flow) of exhaust gases involves different things, depending on the process. For high temperature furnaces, it requires setting the combustion ratio control system for no more excess air than necessary. It also means limiting the amount of ambient air drawn into the furnace by stack draft, because this air behaves the same as excess air going through the burners, absorbing heat from the furnace and then passing out the stack without doing any useful work.
On processes with exhaust fans, the flow through the exhaust system has to be balanced against the incoming flow of combustion products and makeup air. If the incoming flow doesn’t satisfy the exhaust system, the oven chamber will operate at a negative pressure, pulling ambient air in through any available opening, wasting fuel and creating cold spots at the entry points. The flip side of this coin is an exhaust system unable to remove all the gases entering the oven. Here, the problem is excessively positive chamber pressures forcing hot gases out to heat the surrounding area, instead of the product.
To keeping exhaust gas temperatures under control, watch two things -- product loading and heat transfer inside the oven or furnace. In my last column, I pointed out that running equipment at less than 100 percent capacity increases energy consumption per unit of production. Operating beyond rated capacity is even worse. When hot combustion products enter the heating chamber, they immediately begin transferring heat to the product. The more time allowed for this, the more complete and efficient the transfer will be. In an overloaded furnace or oven, the product can be heated properly only be overfiring the combustion system, either by raising the burners’ firing rate (higher gas flow, shorter residence time, less efficient heat transfer) or by raising the setpoint temperature, forcing the exhaust gases to leave at higher temperatures.
Despite careful attention to production rates, heat transfer efficiency can suffer if the workload isn’t properly exposed to the incoming heat. Where radiant heating is the dominant mode of transfer, as in high temperature furnaces and infrared heated ovens, arrange the work so it presents as much surface area as feasible to the heat source. In batch heating operations, this may mean decreasing load sizes to avoid burying pieces deep in a pile. In continuous processes, it may help to space workpieces farther apart or arrange them so more of their surface “sees” the heat source.
In convection heating applications, like most lower temperature ovens, the most efficient heat transfer results from intimate contact between the heating gases and the work. Avoid deep, dense piles of work -- the gases can’t circulate easily around and through them. Wherever possible, locate the workpieces as close as reasonable to the incoming hot airflow. Heat transfer rates are strongly affected by the velocity and turbulence of the hot gases at the point of contact. Excessive distances between the hot air outlets and the product allow the velocity to decrease quickly, and heat transfer efficiency will suffer.
Wall Losses. At this stage of the game, I’m not talking about adding insulation to existing walls -- that comes later -- but simply ensuring the existing insulation or refractory lining is doing its job. Look for discolored paint, blistered sheet metal or other signs the lining is breaking down. Invest in a hand-held infrared pyrometer to do a systematic survey of exterior skin temperatures. Catching hot spots before they become obvious will save you a lot of energy and minimize repair costs.
Radiation Losses. In high temperature processes, radiation losses can rob huge amounts of energy from the oven or furnace. Close all unnecessary openings. Don’t leave furnace doors open any longer than absolutely necessary. If certain openings must be left as is, for example, for access by material-handling devices, consider fitting them with radiation shields of flexible ceramic fiber or textile.
Cooling Media. If cooling air or water is required to protect parts of your furnace, monitor their flow and temperatures to be sure you’re not overdoing a good thing.
Storage Losses. Primarily a problem with batch-type furnaces and ovens, this is the heat required to bring the unit up to operating temperature. Whenever the equipment is shut down and restarted, some or all of it is lost and must be replaced. To minimize it, avoid unnecessary production shutdowns or idle periods. If idling is unavoidable -- no third shift, for example -- run some tests to determine the most efficient idling strategy. Meter the fuel flow to compare the effect of a complete shutdown vs. reduced temperature idling.
Next time, I’ll begin to look at improvements that can be made to existing equipment.