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#BoilerManual #AirAndGasFlow #Section3 #Page27

For clarity, recirculated gas introduced in the immediate vicinity of the initial burning zone of the furnace and used for steam temperature control will be referred to as gas recirculation. Recirculated gas introduced near the furnace outlet and used for control of gas temperature will be referred to as gas tempering. Figure 20 shows the application of recirculated gas and its two components.

An important feature of recirculated gas is that its use changes only the pattern of heat absorption through a boiler. Gas recirculation has a

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Alt = Labeled Fig. 20 Gas recirculation. Image is a simplified drawing of an entire boiler furnace focused on gas/air flow from just above the cyclone furnace firebox on the lower left, but a pair of arrows pointing in opposite directions at the point where the Gas recirculation fan ductwork enters, pointing up as well as down. The windbox ductwork around the cyclones runs to the right, to the Tubular air heater marked at the lower right. Just above that ductwork is located the Gas recirculation fan, appearing to be suspended in the middle between the air heater on the right and the windbox on the left. Following the upward pointing arrow up the furnace wall, then curving through the penthouse archway, then down the right side through the area marked Economizer, the flow continues on to both the Tubular air heater and the input to the Gas recirculation fan.

#BoilerManual #AirAndGasFlow #Section3 #Page27

For clarity, recirculated gas introduced in the immediate vicinity of the initial burning zone of the furnace and used for steam temperature control will be referred to as gas recirculation. Recirculated gas introduced near the furnace outlet and used for control of gas temperature will be referred to as gas tempering. Figure 20 shows the application of recirculated gas and its two components.

An important feature of recirculated gas is that its use changes only the pattern of heat absorption through a boiler. Gas recirculation has a

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Alt = Labeled Fig. 20 Gas recirculation. Image is a simplified drawing of an entire boiler furnace focused on gas/air flow from just above the cyclone furnace firebox on the lower left, but a pair of arrows pointing in opposite directions at the point where the Gas recirculation fan ductwork enters, pointing up as well as down. The windbox ductwork around the cyclones runs to the right, to the Tubular air heater marked at the lower right. Just above that ductwork is located the Gas recirculation fan, appearing to be suspended in the middle between the air heater on the right and the windbox on the left. Following the upward pointing arrow up the furnace wall, then curving through the penthouse archway, then down the right side through the area marked Economizer, the flow continues on to both the Tubular air heater and the input to the Gas recirculation fan.

#BoilerManual #FluidCirculation #Section2 #Page27

In the penthouse (Figure 21), the front, rear (screen tube) and side wall outlet headers are joined by connecting tubes to the roof inlet header. Roof tubes which travel from the roof inlet header form the furnace and convection pass (CP) roof enclosure. These tubes discharge into the roof outlet header located in the penthouse at the rear of the unit.

FURNACE MIX SYSTEM

The furnace mix system has been incorporated to achieve more uniform fluid temperature throughout the three furnace passes. Uniform temperatures, or as close as possible to uniform, are desirable in the furnace circuitry of a Universal Pressure boiler. This, however, cannot be achieved all the time due to some physical and operational characteristics of the boiler. For example, some furnace tubes do not absorb the same amount of heat as other furnace tubes. This can be attributed to the tubes which are located closer to the cyclones and receive more heat than those located in the corners. Also, some tubes have bends in them to allow room for sootblowers, cyclones, and other equipment. By allowing for these bends, there is more length in the tube, therefore, more heat is absorbed.

Slagging is still another factor which affects even heat absorption by furnace tubes. A build-up of slag will effectively insulate a tube from absorbing its maximum amount of heat. Therefore, the more slag on a tube, the less heat can be absorbed by the tube.

Different heat absorption rates do exist in various furnace tubes and yield different temperatures. But why is this so important? Elimination of fluid temperature variations in the furnace can reduce the thermals stress between tubes. If thermal stresses are not reduced, it is quite possible that (over a period of time) a tube failure may occur.

If a furnace tube should absorb more heat than what it was designed

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#BoilerManual #FluidCirculation #Section2 #Page27

In the penthouse (Figure 21), the front, rear (screen tube) and side wall outlet headers are joined by connecting tubes to the roof inlet header. Roof tubes which travel from the roof inlet header form the furnace and convection pass (CP) roof enclosure. These tubes discharge into the roof outlet header located in the penthouse at the rear of the unit.

FURNACE MIX SYSTEM

The furnace mix system has been incorporated to achieve more uniform fluid temperature throughout the three furnace passes. Uniform temperatures, or as close as possible to uniform, are desirable in the furnace circuitry of a Universal Pressure boiler. This, however, cannot be achieved all the time due to some physical and operational characteristics of the boiler. For example, some furnace tubes do not absorb the same amount of heat as other furnace tubes. This can be attributed to the tubes which are located closer to the cyclones and receive more heat than those located in the corners. Also, some tubes have bends in them to allow room for sootblowers, cyclones, and other equipment. By allowing for these bends, there is more length in the tube, therefore, more heat is absorbed.

Slagging is still another factor which affects even heat absorption by furnace tubes. A build-up of slag will effectively insulate a tube from absorbing its maximum amount of heat. Therefore, the more slag on a tube, the less heat can be absorbed by the tube.

Different heat absorption rates do exist in various furnace tubes and yield different temperatures. But why is this so important? Elimination of fluid temperature variations in the furnace can reduce the thermals stress between tubes. If thermal stresses are not reduced, it is quite possible that (over a period of time) a tube failure may occur.

If a furnace tube should absorb more heat than what it was designed

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