Natural draft fired heater

A furnace or fired heater can be classified as natural, induced, forced, or balanced draft. The pressure inside a warm furnace is typically lower because of buoyancy differences in the cooler outside air. A natural-draft furnace can operate using this approach however, when fans are used to push or pull the air through the furnace, greater heat transfer rates can be achieved. A natural-draft fired heater is severely limited in contrast to these systems. 

How can a positive pressure develop in a natural draft-fired heater ... 

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. 

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. 

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. 

In the summertime the heat load with humidity control is removed by cooling systems based on mechanical refrigeration or naturally cold water (see Perry s Chemical Engineers Handbook, 3d ed., pp. 758-797). Make-up heat and humidity control in the winter is incorporated in the air-conditioning system, if available. Separate heating systems use steam from the power plant, or. small gas- or oil-fired heat. Enclosed finned heat exchangers are popular in both forced draft and natural circulation systems with small hot-blast or unit heaters provided in many... 

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