Saving Fuel in Batch Furnaces

If combustible volatiles are evaporating from the load, NFPA standards require that the atmosphere in the oven never exceed one-fourth or one-half (depending on the control system) of the lower explosive limit of the volatile gas. For noncombustable volatiles, the required volume for circulation is less severe, but based upon the ability of the circulating stream to absorb the vapor. If the vapor is water, humidity sensors should be used to automatically adjust burner input, circulated volume, and/or exhaust damper. If humidity is not a sensitive factor, simple temperature controls will suffice. 

The fuel economy of furnaces is commonly expressed in units of fuel or electrical energy expended to heat a unit weight of load. A generalized way to compare furnaces is furnace efficiency, or %thermal efficiency = 100% x (heat absorbed in the load)/(heat in fuel consumed for the load). 

From the preceding study of heat losses, one can conclude that the heat efficiency of a furnace depends not only on its design but also, to a large extent, on its operation and on the requirements for uniformity of heating. For example, if a few small pieces are heated in a large furnace, the fuel consumed per unit of material heated will be extremely high—whether the furnace was heated up especially for those pieces, or whether it had been kept hot all the time. 

The general method for calculating the energy consumption of a furnace heating a given amount of material is  

Step 2. Predict the %available heat (which is 100% - %flue losses) by reading it from an available heat chart (figs. 5.1 or 5.2). Section 5.1 explains how to determine fiue gas exit temperature. 

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