GAS FIRED HEATERS
Direct Fired Heaters
Direct-fired heaters are the usual equipment when large amount of heat input is required. Modern units consist of both radiation section and convection section. Since the temperatures in both are very high, an intermediate heat transfer fluid may be used. A
rather stable heat transfer oil, Dowtherm or some such relatively stable liquid, may be heated and then circulated to various fluid – fluid exchangers. There is a wide variety of models and heating configurations available. The choice depends on fuel cost, thermal efficiency, temperature desired, size of the heat load and
the fluid being heated.
Various types of Direct fired heaters:
Horizontal Fluid Tube and Vertical Fluid Tube.
Horizontal
Cabin Type
The fluid tubes are horizontal. The radiant section normally coils around the walls with burners in the floor level. Convection coils are horizontal as shown in the above picture. An economical and high efficiency unit that currently is the most popular of horizontal tube units. Normal duty range:- 30 – 30 MW (10 – 100 MM Btu/hr).
Cabin with bridge wall:
This divider at the centre of the cylindrical section provides two compartments, which can be fired individually. Can be fired horizontally or vertically. Convection coils are horizontal as shown in the below picture. Normal duty range: 6- 30 MW ( 20- 100
MM Btu/h).
Cabin with Dividing or bridge wall
Two- cell box bridge wall:
Only two boxes are shown but three or four can be used. Vertically fired from floor to give an economical, high efficiency design.
Normal duty rating: 30- 75 MW (100- 250 MM Btu/h).
End Fired Box type
Horizontally fired as name implies. Normal duty range: – 1- 15 MW 5- 50 MM Btu/h).
Vertical-Cylindrical all radiant type
All radiant vertical coils only. It has got no convection coils. Low cost low efficiency design that is compact Normal duty range 0.1 – 6 MW (0.5- 20 MM Btu/h).
Cylindrical Helical coil
Fluid coils are arranged in helical shape, in side the cylindrical part. No convection coils. A basic low cost, low efficiency vertical heater.
Normal range: 0.1- 6 MW (0.5- 20 MM Btu/h).
Cylindrical with cross flow convection coils
This is the most popular of new vertical units. The radiant section is having vertical coils. Convection section is having horizontal coils to have cross flow. Fluid to be heated will normally enter at the top and flow through the convection coils and then flow through the vertical coils and comes out as hot fluid for circulation. It is an
economical high efficiency compact unit. Normal duty range: 3 to 60 MW (10 – 200 MMBtu/h)
Direct-fired heaters have evolved over the years to reflect the need for greater efficiency and more reliable performance. With high flame temperatures and low
convection film coefficients, the development of “hot spots” and tube failure always
has been a problem. Tube metallurgy selection normally is a compromise between initial cost and service life. The choice of material, method of welding, configuration
used, etc. must be based on experience.
Indirect Fired Heaters
These are different configurations, two of which are shown in the below pictures. The hot combustion gas and the flame, heat an intermediate liquid, which, in turn, heats a fluid flowing through a coil or a series of tubes. The intermediate liquid must be
stable at atmospheric pressure and the maximum temperature involved. It is water, heat transfer oil or some other liquid depending on temperature level. Indirect heaters have proven safe, reliable and convenient. Both radiation and convection heat transfer are involved. The intermediate liquid transfers heat between the fire tube and the fluid being heated by natural convection. This limits the rate of heat flux per unit area. Indirect heaters are seldom used to produce outlet fluid temperatures above 260 degree C. The primary use is to heat oil and gas in production operations where the heat loads are not large.
Combustion of Fuels
Liquid fuels
For combustion, liquid fuels are vaporized or atomized in the combustion air. It is completely vaporized and homogeneously dispersed in the air before burning. Blue flame combustion and yellow flame combustion are acceptable.
Gaseous fuels
Because gaseous fuels are easily dispersed in air no fuel preparation is necessary. Combustion time is short once ignition temperature is reached and proper turbulence is provided. The combustion of gas take place in two ways, depending upon when gas and air is mixed. When gas and air are mixed before ignition, as in a Bunsen burner,
burning proceeds by hydroxylation. The hydrocarbon and oxygen form hydroxylated compounds, which become aldehydes; the addition of heat and additional oxygen breaks down the aldehydes to H2, CO, CO2 and H2O. As carbon is converted to aldehydes in the initial stage of mixing, no soot can be developed even if the flame is quenched. Soot and carbon black are formed if insufficient oxygen is present or if the combustion process is arrested before completion.
Burners
Premix Burners are used for many natural-draft applications and for forced-draft applications when accurate heater conditions must be maintained. Figure below indicates a common natural-draft industrial type burner with air being aspirated at the spud and burner throat.
The high velocity burner shown above can be adapted for use with various gaseous fuels. Although not strictly a premix burner, its temperatures and mixing produce results similar to premix burners.
Controls of Heater
Heater control is very important for better performance and safety. Firing must be coordinated with the flow of the fluid being heated due to following reasons.
To control process parameters
To minimize the fire hazard
To prevent hot spots
To prevent undue thermal stress
To avoid coke formation (Coke = a residue left after the incomplete combustion of
Hydrocarbons)
To reduce scale formation.
Sufficient purge must precede the lighting of the pilot (if it is not operated continuously). Automatic pilot ignition and use of flame scanner to confirm ignition follow this. Flow of fluid to be heated must commence before the main burner(s) light. Said burners also must shut off if flow reaches some minimum level or stops
completely (on purpose or accidentally). Control of maximum firing rate limits the temperature fluctuations. Heater should trip on very high temperature of the fluid by closing the fuel valve. When the burner turns off, the tubes and cabin are still hot.
The heat removal cycle flows fluid for sufficient time after shut off to cool down the unit. This minimize rapid thermal stresses.
Shut down controls are critical. They must shut of process flow entering, back flow of the process fluid leaving and, of course, shut off burner fuel. High temperature shutdown in the stack is also preferred. In some cases external fire detection shut down is also incorporated in the control system
