Industrial heaters are used to covert energy from a fuel or energy source to thermal energy in a system, process stream or closed environment.  The process by which thermal energy is transformed from an energy source to a system can be described as heat transfer.

Methods of Heat Transfer

There are three fundamental methods of heat transfer: conduction, convection and radiation. Industrial heaters use one or a combination of these three methods to transfer heat.

Conduction

Thermal conduction is the transfer of thermal energy across a solid object. When an energy source is directly applied to a solid object the molecules in that substance start to vibrate at a higher frequency and transfer that energy to the adjoined molecules.

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Convection

Convection is a method of heat transfer where thermal energy is carried through a fluid or medium. The fluid or medium creates a convection current that is either forced or free flowing. In forced flow convection the medium is circulated by either a fan or pump. In free flowing systems the convection cycle is solely dependent on temperature changes and density gradients.

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Radiation

Thermal radiation is an indirect method of heat transfer. In this method thermal energy is transferred through electro-magnetic waves. Since these waves have negligible mass the transfer of heat requires no physical contact between substances and the waves are free to pass through empty space until they make contact with a physical substance. When the waves are absorbed we experience an excited state and the affected molecules begin to vibrate at a higher frequency. The resultant effect of absorption and vibration frequency is heat or more fundamentally, an increase in temperature.

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The temperature of the heat source can be determined by the peak wavelength emitted as described by Wein's displacement law:

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Almost all industrial heaters that use thermal radiation as a mode of heat transfer emit radiation in the infrared spectrum.

Classification of Industrial Heaters

There are many different types of industrial heaters and they are available in many different shapes, sizes, and configurations. The specific type of industrial heater can be described by its application, medium, physical properties, fuel source or other specific type of heater.

Application

Common types of heaters described by their application include: circulation heaters, duct heaters, immersion heaters, and space heaters and room heaters.

Heaters by Application
Circulation Heaters
Circulation heaters are used to heat moving, flowing, or circulating fluid streams. Fluid flows through the heater, which transfers heat to the stream.		Image Credit: Process Heating Company, Inc.
Duct Heaters
Duct heaters are used to heat moving gas streams. They are installed in the middle of a moving air / gas stream in order to heat the air as it moves through the heater.		Image Credit: WATLOW
Immersion Heaters
Immersion heaters are used in applications that require immersing the heater in the substance to be heated.		Image Credit: EXHEAT
Space Heaters and Room Heaters
Space heaters and room heaters are designed for use in confined areas or spaces. They do not emit hazardous pollutants or noxious fumes, and are generally portable.		Image Credit: Chromalox

Medium

Common types of heaters described by the medium that they are used to heat include: air heaters and water heaters.

Heaters by Medium
Air Heaters
Air heaters are devices used to heat air.  They include forced air products as well as all types of radiant and space heaters.		Image Credit: BERNER International Corp.
Water Heaters
Water heaters are gas, oil or electric-fired devices used to heat water for residential and commercial use.		Image Credit: Johnson Pump

Physical Properties

Common types of industrial heaters that are described by their physical properties include: flexible heaters and explosion proof heaters.

Heaters by Physical Properties
Explosion Proof Heaters
Explosion proof heaters are constructed with housings that can withstand explosions, protecting the materials being heated. This classification may also include housings, which can withstand sparking and flames.		Image Credit: STEGO, Inc.
Flexible Heaters
Flexible heaters are devices that can be molded to the shape of a heated object.		Image Credit: Tyco Thermal Controls

Fuel Source

Common types of heaters described by their fuel source include; electric heaters, gas/propane heaters, and oil/kerosene heaters.

Heaters by Fuel Source
Electric Heaters
Electric heaters generate heat by passing an electric current through a high-resistance material.		Image Credit: Acrolab Ltd.
Gas Heaters and Propane Heaters
Gas heaters and propane heaters burn natural gas, liquefied natural gas (LNG), propane, or other gaseous fuels to generate heat.		Image Credit: CCI Thermal Technologies Inc.
Oil Heaters and Kerosene Heaters
Oil heaters and kerosene heaters burn liquid petroleum products to generate heat. They consist of metal columns with cavities and a heating element.		Image Credit: Grainger

Selection Criteria

When selecting industrial heaters, industrial buyers should consider the distinguishing features of different types of heaters. Once the appropriate type of heater is determined, performance specifications, fuel or energy source requirements, specific applications and  RoHS / WEEE compliance should be considered in order to make the best selection.

Performance Specification 

Performance specifications for industrial heaters include maximum operating temperature, heating capacity and for air heaters, maximum air flow.

Maximum Operating or Sheath Temperature 

Maximum operating temperature is the maximum temperature that the sheath covering the heater may reach. Maximum operating temperature may be the single most important specification for a wide range of industrial heaters, as materials of construction and power requirements are design factors affecting the operating temperature.

Heating Capacity

Heating capacity is a measure of how much thermal energy the device delivers over a period of time, usually measured in kilowatts.

Image Credit: Spirax Sarco

                                                          
=
Mean heat transfer rate (kW (kJ/s)

                                                         m 
=
Mass of the fluid (kg)

                                                         c_p 
=
 Specific heat capacity of the fluid (kJ/kg °C)

                                                         ΔT
=
Increase in fluid temperature (°C)

                                                         t
 =
Time for the heating process (seconds)

Maximum Air Flow

The maximum flow of air through the heater is an important specification related to all air heaters. The heater's air flow will define the volume of space that the heater is capable of heating. Airflow will also dictate the heating capacity required to maintain a specific temperature.

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Energy Sources

Industrial heaters are primarily powered by electricity, combustible fluids, thermal fluids, solar energy or combustible solids (e.g. wood, wood pellets). 

Electrically powered  

Electrically powered industrial heaters may use either single-phase or three-phase power. Typically, these devices use alternating current (AC) instead of direct current (DC) and differ in terms of watt density. When listed as a range or maximum amount, watt density provides a good measure of how quickly the industrial heater can transfer heat. 

Image Credit: Allen & York

Combustible Fluids

Combustible fluids used to power industrial heaters are composed of simple or complex hydrocarbons. They can be further classified into two subgroups related to the stable physical state of the substance at standard temperature and pressure. The two physical states of matter observed are either a liquid or gaseous state. Combustible fluids that exist in a liquid state at standard temperature and pressure are diesel, fuel oil, gasoline and kerosene. Combustible fluids that exist in a gaseous state at standard temperature and pressure are natural gas and propane.

Image Credit: Eberspächer UK Ltd

Thermal fluids

Thermal fluids are used to carry thermal energy in process heating applications. In these applications the thermal fluid is used to convect thermal energy from a heat source to a process, system or closed environment. Steam and hot water are the two most common types of thermal fluid used in industrial heating applications.

Image Credit: Alfa Laval

Solar Energy

Solar energy is a renewable energy source that can be used to operate or supply thermal energy to a system by the use of photovoltaic cells or a solar collector and a thermal mass transfer system.

Image Credit: Renewable Solar Energy | DOYOURBIT.net

Combustible Solids

The primary power source burns wood, wood pellets, or other combustible solid products.

Image Credit: Science Learning Hub

Applications

Annealing / Heat Treating
Process annealing is used to treat work-hardened parts made from low-carbon steels (< 0.25% carbon). Annealing can be used to stress-relieve or fully soften a part. A full anneal allows the parts to be soft enough to undergo further cold working without fracturing.

Curing / Tempering
Heaters are designed for curing or tempering applications. They can heat adhesive bonds to curing or setting specifications.

Drying
Heaters are used to dry products or materials with in-process or batch operation.

Melting
Heaters are used to transform materials from a solid to a liquid state for further processing (e.g., casting).

OEM / Custom

Heaters are designed as OEM modules and are designed to be embedded or installed in another piece of equipment.
  

RoHS / WEEE Compliance

RoHS Compliant

Image Credit: Industrial Safety Solutions

Restriction of Hazardous Substances (RoHS) is a European Union (EU) directive that requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of the following hazardous substances: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether. RoHS became effective on July 1, 2006.

WEEE Compliant

Image Credit: EPM Global Services 

Waste Electrical and Electronics Equipment (WEEE) is a European Union (EU) directive designed to encourage the reuse, recycling and recovery of electrical and electronic equipment. WEEE is also designed to improve the environmental impact and performance of this equipment. The WEEE directive establishes requirements and criteria for the collection, treatment, recycling and recovery of electrical and electronic equipment. It also makes producers responsible for financing these activities. Retailers and distributors must provide a way for consumers to return used or obsolete equipment without charge.

Resources

The Engineering ToolBox - Air Heating Systems

Integrating Engineering into G9 - 12 Education - Heat Transfer

CHERESOURCES.COM - Heat Transfer Basics