Process heat

Process heat is supplied by a variety of equipment types including direct heating units, boilers, kilns, furnaces and electro-technologies. The processes that use heat range across a number of different industry sectors, vary in size from small to very large systems, use a variety of energy sources and technologies, and operate at a wide range of temperatures.

Industrial process heat systems typically fall into the following categories:

  • combustion-based process heating systems
  • electric process heating systems
  • heat recovery and heat exchange systems.
Figure 1. A fuel-based process heating system and opportunities for improvement. A schematic of a typical fuel-based process heating system, as well as potential opportunities to improve the performance and the efficiency of the system. Most of the opportunities are not independent, for example, in the case for heat recovery and heat generation. Transferring heat from the exhaust gases to the incoming combustion air reduces the amount of energy lost from the system, but also allows the more efficient combustion of a given amount of fuel, thereby delivering more thermal energy to the material to be heated.

Figure 1. A fuel-based process heating system and opportunities for improvement.

Source: US Department of Energy (2007) Improving Process Heating System Performance: A Sourcebook for Industry (Opens in a new window) PDF 2.4 MB

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Boilers and steam systems

Steam is used by many industries as a useful medium to transfer and deliver heat to industrial and chemical processes. Boilers usually form part of a larger steam system.

Steam is also used for heating applications in industrial processes to control temperatures and pressures, to remove contaminants and drive mechanical processes. The technology is also used widely in the commercial sector.

Figure 2. Steam System Schematic: a general schematic illustration of the four principal areas of a steam system. The following sections discuss the components in these areas in greater detail. Consider opportunities to cascade heat recovery from exhaust gases to lower temperature process heating equipment. Develop procedures for regular operations, calibration, and maintenance of process sensors (i.e., pressure, temperature, and flow) and controllers.

Figure 2. Steam System Schematic

Source: US Department of Energy (2012) Improving Steam System Performance: A Sourcebook for Industry (Opens in a new window) PDF 2 MB

There are four key components of steam systems:

  • Generation – where steam is generated in a boiler or heat recovery generator
  • Distribution – where steam is carried from the boiler to the points of end use
  • End use – includes process heating, mechanical drive and moderation of chemical reactions using equipment such as heat exchangers, turbines, and chemical reaction vessels
  • Recovery – where condensate is returned to a collection tank

Boilers and water systems generate hot water or steam by using fuel to raise the energy content of water. The fuel input can include natural gas, coal and oil, Bagasse, wood waste, black liquor or electricity.

There are two main types of boilers, based on the type of heat transfer system they use:

  • Firetube boilers – store water in the main body of the boiler. They heat the water via conduction where a tube carrying the combustion gases is immersed in water. Firetube boilers are simple and low cost and best suited to producing hot water or low pressure steam.
  • Watertube boilers – Combust fuel in a central chamber. Gases flow around tubes that contain the water. Heat radiation from the flame and conduction and convection from the gases heat the watertubes. They can be more expensive and complex, but can produce steam at very high temperatures.

Industry sectors vary in the boilers they use, for example:

  • Food processing – use some large boilers, but generally a large number of small boilers
  • Pulp and paper – generally use large steam boilers
  • Chemicals – use both small and large boilers
  • Primary metals – use large boilers.

Taking a systems approach, United States analysis estimated that the overall system efficiency of steam systems was only 55%, with around 30% of losses occuring in the boiler and 15% occurring in the distribution system.

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