Fire-tube heaters

Floor-fired vertical-circular heater Horizontal heater, fired from one end Horizontal heater, fired from both ends Horizontal heater, floor-fired Fire tube heater... 

For process liquid and gas stream heating, most designs heat the process stream as it flows through tubes that pass through fireboxes, convection sections, or combustion gas stacks, although a few fire-tube heaters exist. 

Fire-tube heaters—furnaces consisting of a battery of tubes that pass through a firebox. Fired heaters or furnaces are commercially used to heat large volumes of crude oil or hydrocarbons. Basic designs include cylindrical, cabin, and box. 

Fire-tube heaters contain the combustion gases in tubes that occupy a small percentage of the overall volume of the heater. The basic components of a fire-tube boiler include a large shell that surrounds a horizontal series of tubes. A large, lower combustion tube is attached to a burner that admits heat into the tubes. The upper tubes transfer hot combustion gases through the system and out the stack. Airflow is closely controlled with the inlet air louvers and the stack damper. Water level in the shell is maintained slightly above the tubes. 

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. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

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). 

Bath-type heat exchangers can be either direct or indirect. In a direct bath exchanger, the heating medium exchanges heat directly with the fluid to be heated. The heat source for bath heaters can be a coil of a hot heat medium or steam, waste heat exhaust from an engine or turbine, or heat from electric immersion heaters. An example of a bath heater is an emulsion heater-treater of the type discussed in Volume 1. In this case, a fire tube immersed in the oil transfers heat directly to the oil bath. The calculation of heat duties and sizing of fire tubes for this type of heat exchanger can be calculated fom Chapter 2. 

In an indirect bath heat exchanger, the heating medium provides Iil u to an intermediary fluid, which then transfers the heat to the fluid h)cuig heated. An example of this is the common line heater used on many gas well streams to keep the temperature above the hydrate formal ion lem perature. A fire tube heats a water bath, which provides heat to tlie v.all siieam flowing through a coil immersed in the bath. Details pertaining to dcsi jit of indirect bath heaters are presented in Chapter 5. 

In fire tube type heaters, the coils are immersed in a bath of water. The water is heated by a fire tube that is in the bath below the coils. That is, the fire tube provides a heat flux that heats the water bath. The water bath... 

In order to adequately describe the size of a heater, the heat duty, the size of the fire tubes, the coil diameters and wall thicknesses, and the cor lengths must be specified. To determine the heat duty required, the maximum amounts of gas, water, and oil or condensate expected in the heater and the pressures and temperatures of the heater inlet and outlet must be known. Since the purpose of the heater is to prevent hydrates from forming downstream of the heater, the outlet temperature will depend on the hydrate formation temperature of the gas. The coil size of a heater depeiuLs on the volume of fluid flowing through the coil and the required heat duty. 

Fire tubes, especially in heater treaters, where they can be immersed in crude oil, can become a source of ignition if the tube develops a leak, allowing crude oil to come in direct contact with the flame. Fire tubes can also be a source of ignition if the burner controls fail and the tube overheats or if the pilot is out and the burner turns on when there is a combustible mixture in the tubes. 

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. 

Both types of boiler systems may incorporate finned copper heating coils, which are located above the furnace and gas-pass tubes (smoke tubes or fire tubes) and provide for indirect heating of domestic HW. Where coils are fitted and the boilers are only fired during winter months, domestic HW heating usually is provided via gas heaters for the summer. 

The most common type of commercial pyrolysis equipment is the direct fired tubular heater in which the reacting material flows through several tubes connected in series. The tubes receive thermal energy by being immersed in an oil or gas furnace. The pyrolysis products are cooled rapidly after leaving the furnace and enter the separation train. Constraints on materials of construction limit the maximum temperature of the tubes to 1500 °F. Thus the effluent from the tubes should be restricted to temperatures of 1475 °F or less. You may presume that all reactor tubes and return bends are exposed to a thermal flux of 10,000 BTU/... 

Oil and emulsion rise user heater fire tubes to a coalescing section where sullicicnt retention tune is provided to allow small water particles in the oil continuous phase to coalesce and settle to the bottom Treated oil Hows out the oil outlet Gas. flashed from the heated oil. flows through an equalizing line to the gas spare above Oil level is maintained by pneumatic or lever-operated dump valves Oil-water inter-fare height is regulated hy an interface controller or by an adjustable external water leg... 

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. 

Investigators determined that during a hectic day of operations, the chemical process operator erred. On this afternoon, he inadvertently tried to startup the heater with the burner firing and the heater tubes isolated from the circulating pump by closed blocked valves. Shortly after firing the heater, the lead operator checked the flame pattern but observed nothing out of the ordinary. 

In general, the less heat is applied the greater the cost savings. As heat is applied at production facilities by fuel-gas-fired heaters, any increase in heat is reflected in fuel-gas consumption. The addition of heat also boils lighter hydrocarbon fractions from the crude oil less product at a lower API gravity (defined in the Glossary) results. Addition of heat also accelerates rates of corrosion and increases the likelihood of scale formation on vessel internals, particularly the fire tubes. 

F). They provide energy savings of 18 to 20% compared to quartz-tube heaters. They heat only the sheet, not the ambient air. The complete machine that includes the molds remains cool enough to touch. Risk of fire is lower because of no ambient heat. Sheet heats faster and cools faster. Finer temperature control exists than with most quartz heaters. See halogen. 

Industrial furnaces that do not show color, that is, in which the temperature is below 1200 F (650 C), are commonly called ovens in North America. However, the dividing line between ovens and furnaces is not sharp, for example, coke ovens operate at temperatures above 2200 F (1478 C). In Europe, many furnaces are termed ovens. In the ceramic industry, furnaces are called kilns. In the petrochem and CPI (chemical process industries), furnaces may be termed heaters, kilns, afterburners, incinerators, or destructors. The furnace of a boiler is its firebox or combustion chamber, or a fire-tube boiler s Morrison tube. ... 

Aluminum heat treating (aging, homogenizing), uses indirect-fired air heaters, with a bank of radiant tubes positioned across an air duct. Circulation rates are typically at 8 to 10 air changes per minute. The process temperature levels are well below 1000 F (538 C). 

Most of the preceding discussions related to liquid flow heaters in which the liquid was inside tubes and the furnace gases outside the tubes. Figure 4.27 shows some fire-tube boilers wherein the opposite is the case that is, furnace gases inside tubes that are surrounded by liquid water. These are mostly used in smaller boiler installations. 

---