a furnace—that is, a fired heater—is a

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A furnacethat is, a fired heateris

a device used to heat up chemicals

orchemical mixtures.

Fuel Heat Value

Different fuels release different amounts of heat energy asthey are

burned. The heat energy released, referred to as the heatvalue, is measured

in British thermal units per cubic foot.


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Fuel System

Located under or on the side of the furnace is acomplex network of lines

that provides fuel gas and air to the burners. Furnaces can be classified by several features:

type of draft, number of

fireboxes, number of passes, volume occupied bycombustion gases, and

shape


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Number of Fireboxes

A furnace can have one or two fireboxes. A

double-firebox furnace has a

center wall that divides two combustion

chambers. Hot gases leaving the

two chambers meet in a common convectionsection.


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Number of Passes

The chargethat is, flowentering a furnace is often split into two ormore

flows calledpasses. These passes usually are referred to as the east,

west, north, or south pass. As the names suggest, each goes to a specific

section of the furnace before they all enter a common discharge header. Furnace operators balance the flow rate of these passes equally before

starting the furnace. Balanced fluid flow is critical during furnaceoperation.

Another critical factor to be considered is the composition of the charge.

The components that make up the charge must remain consistentthroughout

the duration of the run or variations in operating conditions will occur.

This could involve pressure, temperature, flow, and analytical variations to

both the charge and furnace operation


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Direct Fired and Indirect Fired

Furnaces are classified as direct fired or

indirect fired. The class is based

on the volume occupied by combustion gases.

In direct-fired furnaces,

the combustion gases typically fill the interior.Direct-fired furnaces heat


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Cabin Furnace

The cabin furnace is a very popular direct-fired heater used in thechemical-

processing industry for large commercial operations. Most cabin furnaces

(Figure 10.10) are located above the ground, making it possible to

drain the tubes and provide easy access to the burners, which can belocated

on the bottom, sides, or ends. Radiant tubes may be configured in

a helical or serpentine layout. The radiant section in a cabin furnace is

designed to contain the flames while avoiding direct contact with thetubes. Cabin furnaces have several advantages. They can accommodate

radiantwall and end-firing burner designs. Their tubes can be drained, and their

two-phase flows are less severe than a single-phase flow would be.


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Compared with other designs, cylindrical

furnaces cost 10 to 15% less to construct, requireless space and money to

operate, have a higher firebox, have more paralleltube passes, and have

higher flue gas velocity. A major disadvantage ofcylindrical furnaces is that

they have a lower efficiency than other designsbecause stack temperatures

are higher.


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A space of 1.5 to 2 feet is considered to be a

safe distance between the

open flames and the radiant tubes. The

burners flame pattern should be

less than 60% the height of the firebox.


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Feed is introduced into a box furnace through

the convection

section and moves toward the firebox.


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Excess airflow will decrease furnace temperatures around the burners

and force the automatic controls to increase natural gas flow rates to

the burner, wasting money. As hot combustion gases rise, cooler air is

e ntrained causing the temperature to decrease. Excess air enhances this

process. When excess air is increased to the burner through the primary

and secondary air registers, a temperature shift occurs as heat is moved

away from the burners. Higher temperatures are found in the upper section

of the firebox due to the reduced heat transfer in the lower section

of the firebox. Temperatures in the convection section and stack will also

rise significantly. This will reduce the amount of heat available for heating

the hot oil and more fuel will be burned in order to maintain process

specifications. To be on the safe side, more air than is theoretically required

for combustion is used. When this occurs it is referred to as util izing

excess air. The percentage of excess oxygen by volume in the flue

gas can be measured using a graph. Each fuel has its own plotted curve

graph. Suppose for example that the oxygen analyzer digitally indicates

an O2 reading of 3% by volume in the stack. The curve in Figure 10.3

shows this to be equal to 10% excess air for natural gas. Air can enter the

furnace through:

Open peepholes

Leaks in furnace casing or joints

Damaged header box gaskets

Burners that have gone out

It is important to recognize the position of the measurement, either near

the burner or in the stack. Large leaks in the furnace can indicate high levels

of oxygen in the system


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a furnace—that is, a fired heater—is a

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