1. Indirect Heaters or Line heaters
    1. General: Indirect Water Bath Heaters have a useful application for both natural gas and oil and water emulsion streams. They may be used at wellheads, in flow lines, tank batteries, and refineries. Indirect heaters may be readily sized from standard models for specific applications by your authorized Sivalls representative.
    2. Gas Streams: A major problem which occurs in the handling and transporting of natural gas is the formation of hydrates which will plug lines and equipment. For specific conditions of temperature and pressure, hydrates will form in natural gas streams (see hydrate formation curves on Figures 1 and LA). These hydrates consist of a loose association of water and liquid hydrocarbons present in the gas streams which collect to form solid particles. These particles usually collect in any convenient restriction or obstruction in a gas pipeline such as valves, drips, return bends, elbows, et cetera, and will eventually form a large enough mass to block the gas flow. A collection of these hydrates often resembles ice in appearance and has caused the term “freezing up” to be used. However, hydrates will form in gas streams for most flowing pressures encountered at temperatures well above 32 F. High-pressure natural gas is produced from the wellhead at usually some pressure in excess of the sales or transmission line operating pressure. It is, therefore, necessary to reduce this pressure before the gas enters the line. This pressure
      reduction may be easily accomplished by expanding the gas through a small orifice or choke. The expansion of natural gas in a choke follows the Joule-Thompson or throttling effect which is an irreversible adiabatic process. Briefly, this is a process where the heat content of the gas remains the same across the choke (constant enthalpy), but the pressure and temperature of the gas are reduced. This reduction in temperature is the cause of the hydrate problem. If the reduced temperature after choking is below the hydrate formation temperature for the line
      pressure, then hydrates will tend to form downstream from the choke. However, if the temperature of the gas is raised by heating in an indirect heater and then the pressure reduced by expansion through a choke, the final reduced temperature will be above the hydrate formation temperature and no problem will be encountered therefore line. This pressure Indirect heaters are also used on gas transmission lines where the ground temperature may drop below the hydrate formation temperature of the gas at the line pressure. Gas flowing in buried lines will rapidly approach the ground temperature, it may then be necessary to place indirect heaters along the pipeline to periodically reheat the gas so that it will remain above the hydrate temperature at all times.
    3. Oil and Water Streams: Indirect heaters are often used in conjunction with Gunbarrel settling tanks to heat pressure oil and/or water emulsion streams to aid in separation of the oil and water. They are also sometimes placed in flow lines ahead of free water knockouts to raise the temperature of the fluid streams to assist in the settling of the free water and oil within the vessel.
    4. Heater Construction: The standard horizontal oilfield indirect water bath type heater consists of a cylindrical atmospheric pressure shell with a directly fired firetube in the lower portion of the shell and a pipe coil in the top (See Figure 7). Although the basic design is the same, many configurations of pipe coils are available for indirect heaters. The size, weight, and the number of tubes in a coil will vary with the specific requirements of the heater. Table 5 and 6 in the Appendix list the specifications of standard size indirect heaters as well as the various tube coils available with each basic BTU/hr rating. Table 3 and 4 list the properties of the steel pipe and a cast iron pipe used in indirect heater coils Table 7 lists the specifications for indirect heaters designed for use with 10,000 psi steel coils for extremely high-pressure gas streams.
      The standard heater specifications do not indicate the placement of the choke on the heater as would be required for wellhead use. A single pass coil with the pressure reducing choke on the outlet is generally recommended. Sometimes it is desirable to heat the gas, expand it through a choke, and then reheat the gas to a higher temperature to further prevent hydrate formation downstream in the line or equipment. This can easily be done in a single heater by using a split pass coil, i.e., putting the choke between two coils. Generally, a long nose heater choke is used with the seat of the choke immersed in the water bath portion of the heater to further assist in hydrate prevention directly at the choke Another type of heater configuration is to use one heater to heat two or more separate well streams by placing separate coils in the same water bath. This type of heater is also referred to as a split pass heater. It may be easily sized by considering each pass as a separate heater and then combining the requirements for firebox and coil area to arrive at the indirect heater size required. Another method is to put the choke on the coil inlet or near the inlet after only a few tubes preheat. This is not recommended because in actual cases freezing of the choke will occur. Not enough heat can be put into the gas to keep the temperature from dropping far below the hydrate temperature immediately
      downstream from the choke. This can happen even if long nose chokes are used.