This kind of equipment are used where a direct Fired Heater would result in an excessive peak heat flux and are 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 is through an indirect medium (typically water) that is 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, maintained below the boiling point as the heating medium for the purpose of heating the process fluid in the coils (in the form of a immersed tube bundle). Water is heated by a single fire tube burner that can burn either gas or liquid fuels as available, forced or natural draught. Due to the nature of the equipment which contains water in a shell vessel, the design pressure is near the atmosphere so that the maximum water temperature level is around 95°C. When freezing is possible, ethylene glycol may be added in the water bath for protection. Duty of these units is generally very low (up to max 3 MW) and the whole heating requirements are mostly covered by adding units in parallel tipically designed accordng to API.