Over the last couple of weeks I was involved in a Linkedin conversation (Chemical Process Engineers group) for the appropriate use of indirect bath heaters. The topic sparked my interest and I started exploring the uses of such systems in more detail. A number of experienced process engineers also commented on the topic and I have tried to incorporate as much of this as possible into the following summary.
Indirect bath heaters are mainly used in the oil and gas industry to heat process fluids. The mechanism takes place indirectly through a bath (utility bath) as opposed to heating directly by flame or furnace (direct fire-tube heaters). The use of a bath promotes uniform heating and reduces the chance of hotspots. Hotspots often occur in direct fire-tube heaters as a result of carbon build up on the fire side of the tubes. If left unchecked this build up can be a source of process fluid degradation and may lead to ruptures, explosions and leaks (Green and Perry 2007). Due to the indirect nature of bath heaters the safety risks are much lower (combustible streams isolated) compared to direct fire-tube heaters, they can therefore be used in hazardous areas where safety is a concern.
The “indirect” name comes from the fact that a fire-tube is submerged in a bath that provides heating through the bath medium to a submerged coil containing the process stream. Usually the bath fluid is water but depending on the climate and heating requirements it can also be oil (capable of heating process fluids to higher temperatures) or a mixture of water and glycol (used in colder climates to lower the freezing point).
Typically indirect bath heaters should achieve around 50-55% heating efficiency. This relatively low efficiency compared to direct fire-tube heaters is a result of the number of heat transfer steps involved. In catalytic infrared fire-tube heaters for example, there are only two steps: 1 – infrared from flame to HX, 2 – HX to process fluid. While indirect (fire-tube) bath heaters have four: 1 – heat from flame to fire-tube, 2 – fire-tube to bath fluid, 3 – bath fluid to process stream coil, 4 – process stream coil to process fluid. It’s also worth mentioning that a bath heater is slow to react to changes in the inlet stream temperature, mostly due to the large thermal capacity of the bath medium.
Typically indirect bath heaters are used where the flow rate of the process stream is low and operations may be unattended, they could also be used to prevent coke formation on the tubes when low heat fluxes are required.
Preheating of crude oil, heating of gas/crude at the well head (prevents cooling effect of gasses expanding), heating of high viscosity fluids to reduce pumping pressures, heating fuel gas at power generation sites, heating at compressor stations, fuel gas dew point control, heating high pressure hydrocarbon gas streams, vaporisation of process fluids and reboiler heating.
Found some more information? Feel free to share in the comments section below.
Green, Don, and Robert Perry. Perry’s Chemical Engineers’ Handbook. McGraw-Hill Companies, 2007.
G.L.Hayett. “Engineering Handbook: Technical Information.” New York: Industrial Press Inc., 2004.
Manning, Francis S, and Richard E Thompson. Oilfield Processing of Petroleum: Crude oil. Oklahoma: Pen Well, 1995.
Pietro Fiorentini. “Indirect Fired Heaters.” Pietro Fiorentini. 2010. http://www.fiorentini.com/media/files/503_systems_-_indirect_fired_heaters_-_eng_-_apr2010.pdf (accessed 08 2012).