This kind of equipment is used where a direct fired heater (FH) would have an excessive peak heat flux. It is typically applied to processes requiring very low heat fluxes, such as crude oil with very high viscosity, or crude/water emulsions.
The heat transfer to the process fluid occurs through an indirect medium (typically water) heated by the flue gas produced by the fuel combustion. This application is commonly used near wells where the first heating of the extracted crude is required to separate it from water or to reduce the viscosity of the oil to be pumped away.
This equipment is characterized by a water solution as the heating medium, maintained below the boiling point, to heat the process fluid in the coils (in the form of an immersed tube bundle). The water is heated by a single-fire tube burner that can burn either the available gas or liquid fuels, by forced or natural draft.
Due to the nature of the equipment that contains water in a shell vessel, the design pressure is near the atmosphere so that the maximum temperature level of the water is around 95°C. If freezing may occur, ethylene glycol can be added to the water bath for protection.
The duty of these units is generally very low – up to max 3 MW – therefore, the full heating requirements are mostly met by adding units in parallel, typically designed according to API standards.