Fired heaters draft

Air recirculation. Prevailing winds and the locations and elevations of buildings, equipment, fired heaters, etc., require consideration. All air-cooled heat exchangers in a bank are of one type, i.e., all forced-draft or all induced-draft. Banks of air-cooled exchangers must be placed far enough apart to minimize air recirculation. 

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. 

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. 

Fired heaters have either forced draft fans or induced draft fans to control air to the burners. This allows control of oxygen amount by direct measurement of air and fuel... 

I feel the need to provide additional comments on excess air as many plants have an O2 reduction program. O2 reduction (or minimum excess air) must be built upon the basis of proper draft control. Minimum excess air for the fired heater can be obtained when it is reduced to the point where combustibles begin to appear in the stack. For modern fired heaters, this occurs at 8% excess air equivalent to 1.8% of oxygen level in the flue gas. However, practical constraints prevent achieving this minimum excess air in operation, and these constraints include variations in fuel quality, feed rates, and other process variables. Thus, operation without flame impingement sets the limit for practical minimum excess air. The optimal flue... 

Draft should be monitored and maintained as required for the specific fired heater design. Fired heaters without draft control should be periodically checked. 

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. 

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. 

I d better phone Professor Peterson to apologize. I just now remembered that we did learn about this concept that density difference between two columns of fluid causes flow. Professor Peterson taught us the idea in the context of draft in a fired heater. Cold combustion air flows through the burners and is heated by the burning fuel. The hot flue gas flows up the stack. The difference in density between the less dense hot flue gas and the more dense cold air creates a pressure imbalance called draft. Just like the fish tank story. 

However, I can t call Professor Peterson. He s dead. I wouldn t call him anyway. I know what he would say "Lieberman, the analogy between the air lift pump and draft in a fired heater is obvious to the perceptive mind, which apparently excludes you."... 

A typical fired heater combustion air preheater is shown in Fig. 46.4. Combustion air at a pressure of 5 inches of water is shown on the tube side. The flue gas is on the shell side, flowing at a draft of 2 inches of water. This air preheater is clearly leaking cold air into the flue gas. How do I know this ... 

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. 

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.

When the combustion air was cut back this time, fire started to come out of the stack. Now Operator A was forced to admit that they would have to increase the combustion air again. The problem they then faced was that the amount of draft in the heater seemed to be less than before and they were not quite able to reestablish the same airflow. The reason for the restricted airflow was that they had caused afterburn in the... 

Combustion calculations show that an oil-fired watertube boiler requires 200,000 lb/h (25.2 kg/s) for air of combustion at maximum load. Select forced- and induced-draft fans for this boiler if the average temperature of the inlet air is 75°F (297 K) and the average temperature of the combustion gas leaving the air heater is 350°F (450 K) with an ambient barometric pressure of 29.9 inHg. Pressure losses on the air-inlet side are, in inFLO air heater, 1.5 air supply ducts, 0.75 boiler windbox, 1.75 burners, 1.25. Draft losses in the boiler and related equipment are, in inH20 furnace pressure, 0.20 boiler, 3.0 superheater, 1.0 economizer, 1.50 air heater, 2.00 uptake ducts and dampers, 1.25. Determine the fan discharge pressure and horsepower input. The boiler burns 18,000 lb/h (2.27 kg/s) of oil at full load. 

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... 

Fig. 4.25. Forced draft heater for petro-chem processing—may be cylindrical with one burner as shown, or a circie of vertically up-fired, high-velocity type H burners (fig. 6.2) or rectangular (a cabin heater) with rows of up-fired burners, or rows of side-fired type E flat-flame burners, shown in fig. 4.26 and 6.2.

Efficient heater operation requires that excess air entering the convection section be minimized, whieh is indicated by a very small negative pressure at the convection section inlet. To achieve this, it should have a well balanced draft pressure profile between the firebox and stack. The hot gas pushes so that the pressure is always greatest at the firewall while the stack draft pulls. When this draft is eorreetly balanced, the pressure at the bridge waU should be around 0.1-0.2 WG (water gauge). Too mueh draft allows cold air leakage into the fired box resulting in wasted fuel. 

Loss of draft will make it impossible to provide sufficient air to combust all of the heater fuel entirely. The oxygen concentration in the flue gas will approach zero carbon monoxide in the flue gas will increase significantly. If the furnace is oil-fired, the stack will emit black smoke. Some of the more common causes of reduced draft are listed below. 

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