Furnaces stack loss

Furnace stack loss is calculated based on actual heater efficiency. For this example, 55% furnace efficiency is assumed for the charge heater and the diesel stripper heater, which have a radiant section only. A furnace efficiency of 85 % is used for the product fractionator heater and the debutanizer reboiler heater, which have both radiant and convection sections. 

Qloss stack loss from furnace, boiler, or gas turbine (kJ s )... 

The profile shown in Figure 15.21 represents the furnace efficiency, if the casing heat losses are neglected. Making this assumption, the process duty plus the stack loss represents the heat released by the fuel. 

As excess air is reduced, theoretical flame temperature increases. This has the effect of reducing the stack loss and increasing the thermal efficiency of the furnace for a given process heating duty. Alternatively, if the combustion air is preheated (e.g. by heat recovery), then again the theoretical flame temperature increases, reducing the stack loss. 

Example 16.4 The process in Figure 16.2 is to have its hot utility supplied by a furnace. The theoretical flame temperature for combustion is 1800°C and the acid dew point for the flue gas is 160°C. Ambient temperature is 10°C. Assume A7 = 10°C for process-to-process heat transfer but A7 = 30°C for flue gas to process heat transfer. A high value for A 7 mln for flue gas to process heat transfer has been chosen because of poor heat-transfer coefficients in the convection bank of the furnace. Calculate the fuel required, stack loss and furnace efficiency. 

A2. Because fuel costs are much higher in high-temperature furnaces than in lower temperature furnaces as a result of the higher flue gas exit temperature causing higher stack loss. 

In a process, energy losses consist of both thermal and mechanical losses. Thermal losses are typically originated from column overhead condensers, product mn down coolers, furnace stack, steam leaks, poor insulation of heat exchangers/piping and vassals and so on. Mechanical losses could also be significant, which usually occurs in rotating equipment, pressure letdown valves, control valves, pump spill back, heat exchangers, pipelines, and so on. Some of the thermal and mechanical losses are recoverable with a decent payback of investment but many others do not. 

Heat balance terms such as furnace stack heat loss as a fiiaction of fired fuel heating value and furnace efficiency are not calculated, mainly since these indices of performanee are not required in an optimization system that has the plant-wide operating profit as an objective function. These indices are remnants of local equipment or design optimization approaches. T5q)ically a plant s operation should not be constrained nor its performanee judged by these indices. Models can of course be easily used to ealculate these indices, and plant built to the furnace models to perform these calculations on-line. 

The heat balance shows that the heat loss from the furnace walls is only ca 11% of the energy suppHed by the fuel and just slightly more than the sensible heat loss with the slag. The principal heat loss is in the stack gases and is equivalent to ca 30% of the energy suppHed by the fuel. 

As everyone who has used a fireplace knows, when a fire bums in a furnace, a draft, or slight vacuum, is induced that causes the hot combustion gases and entrained particulate matter to flow up and out of the stack. The reason is that the hot gas in the stack is less dense than air at ambient temperature, leading to a lower hydrostatic head inside the stack than at the furnace inlet. The theoretical draft D(N/m ) is the difference in these hydrostatic heads the actual draft takes into account pressure losses undergone by the gases flowing in the stack. 

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