Fire-Side Heaters

So far, we have been discussing fired heaters, with the fire outside the tubes that is, the fire is outside the heat-exchange surface. Many fired heaters have the fire, or at least the hot flue gas, inside the heat-exchange surface. 

One common example of firing on the tube side of a heater is the glycol-regeneration boiler, shown in Fig. 21.7. This type of heater will typically have a high excess 02, to prevent high flame temperatures, which could overheat the fire tube. Also, the fire tube is kept submerged in liquid, to prevent tube overheating. 

In multiple-tube boilers (usually horizontal) the fire may be on the tube side. As long as the fire tubes are kept submerged in water, the tubes do not overheat. Boilers of this type are widely used in the regen- 

The great difficulty with such fire-side, multitube boilers is overheating the hot-side tubesheet. This tubesheet is exposed to the full temperature of the combustion gases, but is difficult to keep cool with the boiler s feedwater. Also, the ends of the tubes, where they are sealed in the tubesheet, are hard to keep cool. To protect these ends from the direct radiant heat in the combustion chamber, ferrule inserts, about lV2 in long, are cemented into the front end of each tube. 

If these ferrules fail, or if the tubesheet overheats, the ends of the tubes will pull away from the tubesheet. The result is called a roll leak. Boiler feedwater will blow out of these roll leaks. Rerolling and seal welding the ends of the tubes, while difficult, is then the only way to stop such leaks. 

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In the North American market, water heaters are almost always made with the cold water inlet and hot water outlet lines coming out of the top of the tank. The hot water outlet opens right into the top of the tank and so draws off the hottest water. The hot water has risen to the top of the tank because of its lower density. The cold water on the inlet side is directed to the bottom of the tank by a plastic dip-tube. In some models the dip-tube is curved or bent at the end to increase the turbulence at the bottom of the tank. This is to keep any sediment from settling on the bottom of the tank. As sediment— usually calcium carbonate or lime—precipitated out of the water by the increased temperature builds up, it will increase the thermal stress on the bottom of a gas-fired water heater and increase the likelihood of tank failure. On electric water heaters the sediment builds up on the surface of the elements, especially if the elements are high-density elements. Low-density elements spread the same amount of power over a larger surface of the element so the temperatures are not as high and lime doesn t build up as quickly. If the lower elements get completely buried in the sediment, the element will likely overheat and burn out. 

Joints in copper components may be a source of trouble. Copper/zinc brazing alloys may dezincify and consequently give rise to leaks . In some waters, soft solders are preferentially attacked unless in a proper capillary joint. Copper/phosphorus, copper/silver/phosphorus, and silver brazing alloys are normally satisfactory jointing materials. Excessive corrosion of copper is sometimes produced by condensates containing dissolved oxygen and carbon dioxide. Rather severe corrosion sometimes occurs on the fire side of fire-back boilers and on electric heater element sheaths under scales deposited from hard waters . 

At least one potassium bicarbonate dry chemical extinguisher (120-B C) should be provided at each fired process heater handling liquid fuel or a liquid process stream. They should be installed on opposite sides, or ends, and adjacent to fire aisles. 

These objectives are equally important in the operation of fired boilers whose principles of operation on the fire-side and flue-gas side are essentially the same as those of process-plant-fired heaters. 

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 ... 

At the ends of the heaters, tubesheets are provided through which the tubes pass outside the combustion chamber to a header compartment. Tubesheets are generally cast iron with plastic refractory facing on the fire side in sections holding five or six tubes and are bolted to the... 

Column was reboiled by a fired heater. Heater fuel was controlled by a tray temperature, and there was a hi -temperature trip at the process side heater outlet, l en the circulation pump briefly failed the column cooled and the controller increased heating rate. The trip failed to function. When circulation was reestablished an extremely hi v xnization rate resulted and produced a pressure surge that dislodged trays. 

Preparation for Lay-Up. When a boiler is being cleaned in preparation for lay-up, the water side of the unit should be cleaned and then the unit should be fired to drive off gases. The fire side should then be cleaned. An oil coating on the fire-side metal surfaces is beneficial when the boiler is not used for extended periods of time. Another helpful treatment would consist of completely filling the boiler with an inert gas and seating it tightly to prevent any leakage of the inert gas. This will help prevent oxidation of the metal. Fuel-oil lines should be drained and flushed of residual oil and refilled with distillate fuel. H all boilers are to be laid up, care of oil tanks, lines, pumps, and heaters is similarly required. 

Boiler and fired heater tubes develop scale on the fired side of the tube. Past procedures required furnace shutdown for cleaning on a regular basis. On-line cleaning with combustible abrasives allows treatment without shutdown. With on-line cleaning, one refinery experienced a CO2 emissions reduction of 1,800 toimes per year, 60,000 fuel per year savings, 300,000 per year yield improvement, 800,000 per year throughput increase, and an overall 1.5% improvement in efficiency for the unit. 

Larger-si2ed heaters are usually hori2ontal box heaters. The radiant coils can be located either on the side walls so that the units are fired from underneath, or in a center row of tubes in which the heater is fired from both sides to provide a higher heat flux for reducing the radiant surface. An access area at one end of the box is required in order to remove the tubes. Sometimes multiple coils are included in the same box, which may require access to both ends of the box. 

The simplest unit employing vacuum fractionation is that designed by Canadian Badger for Dominion Tar and Chemical Company (now Rttgers VFT Inc.) at Hamilton, Ontario (13). In this plant, the tar is dehydrated in the usual manner by heat exchange and injection into a dehydrator. The dry tar is then heated under pressure in an oil-fired hehcal-tube heater and injected directly into the vacuum fractionating column from which a benzole fraction, overhead fraction, various oil fractions as side streams, and a pitch base product are taken. Some alterations were made to the plant in 1991, which allows some pitch properties to be controlled because pitch is the only product the distillate oils are used as fuel. 

In the vei tical-tube single-row double-fired heater, a single row of vertical tubes is arrayed along the center plane of the radiant section that is fired from both sides. Usually this type of heater has an overhead horizontal convec tion bank. Although it is the most expensive of the fired heater designs, it provides the most uniform heat transfer to the tubes. Duties are 21 to 132 GJ/h (20 to 125 10 Btu/h) per cell (twin-cell designs are not unusual). 

Honzontal-tube cabin heaters position the tubes of the radiant-section-coil horizontally along the walls and the slanting roof for the length of the cabin-shaped enclosure. The convection tube bank is placed horizontally above the combustion chamber. It may be fired From the floor, the side walls, or the end walls. As in the case of its vertical cylindrical counterpart, its economical design and high efficiency make it the most popular horizontal-tube heater. Duties are 11 to 105 GJ/h (10 to 100 10 Btu). 

In the horizontal-tube box heater with side-mounted convection tube bank, the radiant-section tubes run horizontally along the walls and the flat roof of the box-shaped heater, but the convection section is placed in a box of its own beside the radiant sec tion. Firing is horizontal from the end walls. The design of this heater results in a relatively expensive unit justified mainly by its abihty to burn low-grade high-ash fuel oil. Duties are 53 to 210 GJ/h (50 to 200 10 Btu/h). 

The radiant section tube coils of horizontal fired heaters are arranged horizontally so as to line the sidewalls and the roof of the combustion chamber. In addition, tliere is a convection section of tube coils, winch are positioned as a horizontal bank of tubes above the combustion cham her. Nonnally the tubes are fired vertically from the floor, but they can also be fired horizontally by side wall mounted burners located below the tube coil. Tins economical, high dficiency design currently represents the majority of new horizontal-tube-t1icd heater installations. Duties run from 5 to 250 MMBtu/hr. Six types o) horizontal-tube-fired heaters arc-shown in Figure 3-21. 

Figure 3-21. Six bask designs used in horizontal-lube fired heaters. Radian section coil is horLmtai. (a) Cabin, (b) Two-cell box. (c) Cabin with dividing bridgewall, (d) End-fired box. e) End-fired box, with side-mounted convection section, (f) Horizontal-tube, single-row, double-h red. [From Chem. Eng., 102-103 (June 19, 1978).]...

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