Natural-draft heater

Suppose we have a natural-draft heater operating very efficiently on the good side of the point of absolute combustion. The oxygen content of the firebox gases (just below the shock tubes) is 2.5 percent oxygen as shown in Fig. 20.5. What do you think the oxygen content of the flue gases in the stack will be ... 

Figure 20.5 Natural-draft heater showing tramp air leaks.

In Fig. 20.6, we see a simple natural-draft heater with no convective-section tubes. The laws of hydraulics tell us that fluids flow from regions of high pressure to regions of lower pressure, and yet the draft readings in Fig. 20.6 seem to contradict this principle. 

Figure 20.6 A simple natural-draft heater with no convective section tubes.

The air supply for a gas-fired, natural-draft heater consists of two portions primary air and secondary air. (In some newer burners, tertiary air is used to control nitrogen oxide emissions.) The primary air is educted or sucked into the burner through a venturi by the rapidly flowing fuel gas. The air is well mixed with the gas prior to combustion. Hence the name premix burner. A Bunsen burner is an example of a premix burner. 

Vlanv oil biirnor.s arc do.SLgnod a.s cornbination ga.s/oil biirnor.s, An example of a modern low-N(), oil/ga.s forced-draft burner Ls. shown in Fig, 27-30, This is an air-staged design, with the air divided into pri-rnaiv, secondarv, and tertiarv streams. An air-staged natural draft process heater oil/gas burner is illustrated in Fig, 27-3L... 

Fuel-Staged Burners Use of fuel-staged burners is the preferred combustion approach for NO control because gaseous fuels typically contain little or no fixed nitrogen. Figure 27-36 illustrates a fuel-staged natural draft refineiy process heater burner. The fuel is spht into primaiy (30 to 40 percent) and secondary (60 to 70 percent) streams. Furnace gas may be internally recirciJated by the primaiy... 

The technology of kerosene burners is quite mature. The most popular kerosene heater is the perforated sleeve vaporizing burner or range burner (Figure 1). It consists of a pressed steel base with concentric, interconnected grooves and perforated metal sleeves, between which combustion takes place. Kerosene is maintained at a depth of about 1/4 inch in the grooves. As the base heats up, oil vaporizes from the surface, and the flame lights from asbestos wicks. Combustion air is induced by natural draft. The flame is blue, and the burner is essentially silent, odorless, and smokeless. 

Gas-Fired water heaters are also made more efficient by a variety of designs that increase the recov-ei y efficiency. These can be better flue baffles multiple, smaller-diameter flues submerged combustion chambers and improved combustion chamber geometry. All of these methods increase the heat transfer from the flame and flue gases to the water in the tank. Because natural draft systems rely on the buoyancy of combustion products, there is a limit to the recovery efficiency. If too much heat is removed from the flue gases, the water heater won t vent properly. Another problem, if the flue gases are too cool, is that the water vapor in the combustion products will condense in the venting system. This will lead to corrosion in the chimney and possible safety problems. 

Most fired heaters operate with natural draft, and the stack height must be sufficient to achieve the flow of combustion air required and to remove the combustion products. 

FIG. 24-33 Low-NO fuel-staged burner for a natural draft refinery process heater. Callidus Technolo es, Inc.)... 

A typical natural-draft gas-fired process heater is shown in Fig. 20.1. Suppose we gradually close either the stack damper or the air register the flow of air into the firebox will then be reduced. If both the process-side flow and the fuel-gas rate are held constant, the following sequence of events occurs ... 

Figure 20.1 Typical natural draft gas-fired process heater.

Boilers normally are forced draft (FD) versus most fired heaters that are natural draft. It is cheaper to combine FGR with an FD system because the FD fan can be used to induce the flue gas and the plenums already exist to distribute the air. 

An FD system is required, which is one reason this technology has been more popular with boilers than with fired heaters. Boilers are typically FD while fired heaters are typically natural draft. 

Figure 2.11 shows a natural draft burner without an air inlet muffler. These burners are often quiet enough that no muffler is needed. Figure 2.12 shows a common type of muffler used on a natural draft burner used in the floor of refinery heaters where the air inlet has a baffle lined with sound-deadening insulation. Figure 2.13 shows a comparable standard muffler for a radiant wall burner... 

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