Air Heaters Information
Air heaters are devices used to heat air. They include forced air products as well as all types of radiant and space heaters. Forced air products moderate and control air temperature by circulating air past a heat source with a fan or blower. As the air current flows through the heater it absorbs thermal energy and then exits the heater at an elevated temperature. Radiant heaters operate by radiating heat from a point source, heating the objects near that point source. The temperature differential causes a free flowing convection cycle to form, which in turn heats the surrounding air mass. Space heaters are self-contained devices that can operate by either of the aforementioned methods.
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.
Forced Flow Convection Cell
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In free flowing systems the convection cycle is solely dependent on temperature changes and density gradients.
Free Flowing Convection Cell
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The method of heat transfer may be used to describe the heater, but it is independent of the application. Air heaters may be used for residential, commercial or industrial applications. Residential and commercial air heaters are primarily used for HVAC systems and room or space heating. Industrial applications include calcining, drying, melting, preheating and other chemical processes. Air heaters are used in a variety of packaging, automotive, plastics, rubber, textile, and electronics applications. Air heaters are used to cure adhesives, de-flash molded parts, weld plastic or vinyl fabrics, and dry ink. In the semiconductor industry, air heaters are used in preheating process gases, soldering lead frames, wafer and PC board drying, and heat shrinking wire insulation.
Calcining is a high temperature heat treatment process performed on metals, ores and minerals. The process is used to removed hydrates and other impurities in order to alter the physical and chemical properties of the solid material.
Mill System with Flash Calcining. Image Credit: ALSTOM
Preheating is a general term that may describe a process used to prepare unfinished products and materials for further processing. It is important to regulate temperature or slowly warm products and materials in order to prevent undesired temperature gradients. Rapid temperature fluctuations may cause variances in deposition thickness, problematic grain boundaries, stress fractures and other undesired outcomes. Preheating may also describe an economical means of elevating air or gas temperature without the need of high capacity heating systems.
Stationary-plate Regenerative Air Preheater. Image Credit: Citizendium
Drying is a process used to remove water or other solvents through evaporation. Air heaters may be used to dry inks, coatings, raw materials as well as other products and materials.
Types of Air Heaters
Air heater is a broad term that may be used to describe several specific types of heaters. More concisely defined types of air heaters include duct heaters, enclosure heaters, forced air heaters, heat torches and flame heaters, radiators, space heaters and room heaters, and tubular heaters.
|Types of Air Heaters|
|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: OMEGA Engineering, Inc.
|Enclosure Heaters||Enclosure heaters are designed for use within enclosures. They prevent freezing and provide protection and humidity control.||
Image Credit: STEGO, Inc.
|Forced Air Heaters||Forced air heaters use forced convection to move air past a heat source with a fan or blower.||
Image Credit: Global Equipment Company
|Heat Torches and Flame Heaters||Heating torches and flame heaters are devices that emit a concentrated flame or stream of unusually hot air or gas.||
Image Credit: Hotwatt, Inc.
Radiators have coils that contain heated, circulated media such as oil, hot water, or steam.
Image Credit: CCI Thermal Technologies Inc.
|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: CCI Thermal Technologies Inc.
|Tubular Heaters||Tubular heaters are generally used in custom heating applications. Base (tubular) heating elements can be bent into standard or custom shapes.||
Image Credit: Chromalox
After defining the type or application of the heater, it is important to identify the required air temperature, heater capacity, flow rate, and energy source.
Formulas used to calculate wattage requirements, expected temperature rise and required air flow:
Image Credit: BTU Electric Heaters
Maximum Operating or Sheath Temperature
Maximum operating temperature is the maximum temperature that the sheath covering the heater may reach.
Heating capacity is a measure of how much thermal energy the device delivers over a period of time, usually measured in kilowatts.
Calculating Wattage Requirements
Image Credit: ASB Heating Elements Ltd.
Maximum Air Flow
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.
Image Credit: The Engineering ToolBox
Air heaters are primarily powered by electricity, combustible fluids, thermal fluids, solar energy, or combustible solids (e.g. wood, wood pellets).
Electrically powered air 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.
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Combustible fluids used to power air 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 include diesel, fuel oil, gasoline and kerosene. Combustible fluids that exist in a gaseous state at standard temperature and pressure include natural gas and propane.
Image Credit: TLG GROUP
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 is a renewable energy source that can be used to heat air by the use of a solar collector and a thermal mass transfer system.
Image Credit: REUK.co.uk
The primary power source burns wood, wood pellets, or other combustible solid products.
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