Stack heat loss

When lowering stack temperature, be aware of the dew point as discussed in the next topic. 

Woodward, A. M., Reduce Process Heater Fuel, Hydrocarbon Processing, July 1974, p. 106. 

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Stack (Heat) Loss - Sensible and latent heat contained in combustion gases and vapor emitted to the atmosphere. 

Lower stack heat losses due to internal air recycling... 

Lower stack heat losses due to internal air recycling Improved feed dispersion at the entrance to the system, thus providing the possibility for operation with sticky materials Increased residence time for heavier (wetter) particles... 

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. 

A more obvious energy loss is the heat to the stack flue gases. The sensible heat losses can be minimized by reduced total air flow, ie, low excess air operation. Flue gas losses are also minimized by lowering the discharge temperature via increased heat recovery in economizers, air preheaters, etc. When fuels containing sulfur are burned, the final exit flue gas temperature is usually not permitted to go below about 100°C because of severe problems relating to sulfuric acid corrosion. Special economizers having Teflon-coated tubes permit lower temperatures but are not commonly used. 

In all warm-air design applications, consideration must be given to the effects of stratification in tall buildings. Stratification increases the roof and high-wall fabric losses and the air change rate by the stack effect, and hence the ventilation loss. These effects may increase the heat loss by 25% over that of a radiant heating system. 

Exhaust gases from the gas turbine are used to raise steam in the lower cycle without the burning of additional fuel (Fig. 7.3) the temperatures of the gas and water/steam flows are as indicated. A limitation on this application lies in the heat recovery system steam generator choice of the evaporation pres.sure (p ) is related to the temperature difference (Tft — T ) at the pinch point as shown in the figure, and a compromise has to be reached between that pressure and the stack temperature of the gases leaving the exchanger, (and the consequent heat loss ). ... 

All planned heat balance measurements have been established and heat loss from both reactor and from vapour stack determined, the former more and the latter rather less satisfactorily. 

The stack of platelets is encompassed by two end caps bearing the external fluidic connections. If desired, a third housing part can be introduced in between the end caps to shield the stack. As a further design modification, ceramic Macor insulating plates can be inserted between the end caps and platelet stack to prevent heat losses from the stack to the housing. 

The welded Rh stack was welded to a cover plate and inserted into a ceramic holder, made of a special shrinkage-free material that can be machined as green compact [3]. The ceramic holder prevents heat losses from the metallic stack. 

Because of heat losses, the temperature at the top of the stack will be around 80°C below the inlet temperature. 

The split between the radiant and convection section heat varies according to the design. Casing losses are usually between 1 and 3% of the heat release from combustion. The heat loss from the stack is constrained by the desire to avoid any condensation of water vapor in the convection section. If there is any sulfur present in the fuel, then the condensate will be corrosive. The temperature at which the flue gas starts to condense is the acid dew point. For sulfurbearing fuels, the temperature of the flue gas is normally... 

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. 

Qloss = heat losses from the stack, casing and boiler blowdown... 

Many of the conservation measures require detailed process analysis plus optimization. For example, the efficient firing of fuel (category 1) is extremely important in all applications. For any rate of fuel combustion, a theoretical quantity of air (for complete combustion to carbon dioxide and water vapor) exists under which the most efficient combustion occurs. Reduction of the amount of air available leads to incomplete combustion and a rapid decrease in efficiency. In addition, carbon particles may be formed that can lead to accelerated fouling of heater tube surfaces. To allow for small variations in fuel composition and flow rate and in the air flow rates that inevitably occur in industrial practice, it is usually desirable to aim for operation with a small amount of excess air, say 5 to 10 percent, above the theoretical amount for complete combustion. Too much excess air, however, leads to increased sensible heat losses through the stack gas. 

Heat Loss - Less heat loss from the stack for similar levels of insulation. In particular radiation losses can be significantly less since these are a function of T". This can have a significant effect on the self-sustainability of the stack. 

The heat of combustion is a product of the amount of fuel consumed and the net heating value of the fuel. The heater s efficiency is a function of the flue-gas stack temperature, the excess air or oxygen, and the ambient-heat losses from the firebox and the convective-section structures. 

As shown in Figure 2.2, the radiation and wall losses of a boiler are relatively constant, and most of the heat losses occur through the stack. Under air-deficient operations, unburned fuel leaves, and under air-excess conditions, heat is lost as the unused Oz and its accompanying nitrogen are heated up and then discharged into the atmosphere. The goal of optimization is to keep the total losses to a minimum. This is accomplished by minimizing both excess air and the stack temperatures. 

The analyzers available for the detection of C02, CO, and excess 02 are discussed in Chapter 3. Excess air can be correlated to 02, CO, C02, or combustibles present in the flue gas. The most sensitive indicator of flue gas composition is CO. As shown in Figure 2.2, optimum boiler efficiency can be obtained when the losses due to incomplete combustion equal the effects of heat loss through the stack. These conditions prevail at the "knee" of each curve. Whereas the excess 02 corresponding to these knee points varies with the fuel, the corresponding CO concentration is relatively constant. 

Uarm uater loss 15447.4 Stack gas loss 451. 3 Heat losses 2321. 3... 

The available energy flou through five major sections of sulphuric acid plant is given in figure 2. The major inputs to this system are sulphur and pouer, with demineralised (DM) water uet air, process water and cooling water from environment. The useful outputs from the system are sulphuric acid and steam. Losses to environment include heat losses from various equipments blowdown water steam from deaerator vent warm water and stack gas. 

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