Process heaters and heat exchangers preheat feedstocks in distillation towers and in refinery processes to reaction temperatures. The major portion of heat provided to process units comes from fired heaters found on crude and reformer preheater units, coker heaters and large-column reboilers, which are fueled by refinery or natural gas, distillate and residual oils. Heaters are usually designed for specific process operations, and most are either cylindrical vertical or box-type designs. Heat exchangers use either steam or hot hydrocarbon, transferred from some other section of the process, for heat input.
Heat is also removed from some processes by air and water exchangers, fin fans, gas and liquid coolers and overhead condensers, or by transferring the heat to other systems. The basic mechanical vapour compression refrigeration system is designed to serve one or more process units, and includes an evaporator, compressor, condenser, controls and piping. Common coolants are water, alcohol/water mixture or various glycol solutions.
A means of providing adequate draft or steam purging is required to reduce the chance of explosions when lighting fires in heater furnaces. Specific start-up and emergency procedures are required for each type of unit. If fire impinges on fin fans, failure could occur due to overheating. If flammable product escapes from a heat exchanger or cooler due to a leak, a fire could occur.
Care must be taken to assure that all pressure is removed from heater tubes before removing any header or fitting plugs. Consideration should be given to providing for pressure relief in heat exchanger piping systems in the event they are blocked off while full of liquid. If controls fail, variations of temperature and pressure could occur on either side of the heat exchanger. If heat exchanger tubes fail and process pressure is greater than heater pressure, product could enter the heater with downstream consequences. If the pressure is less, the heater stream could enter into the process fluid stream. If loss of circulation occurs in liquid or gas coolers, increased product temperature could affect downstream operations, requiring pressure relief.
Depending on the fuel, process operation and unit design, there is a potential for exposure to hydrogen sulphide, carbon monoxide, hydrocarbons, steam boiler feedwater sludge and water treatment chemicals. Skin contact with boiler blowdown which may contain phenolic compounds should be avoided. Exposure to radiant heat, superheated steam and hot hydrocarbons is possible.
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