Whether it's an open coil duct heater, a tubular or a finned tubular duct heater, each duct heater serves the main purpose which remains the same – to heat up a large area in the most efficient and economical method. However, the type may vary based on their construction, advantages, disadvantages and functionality differences.
Tubular duct heaters are electric heat transferring products whose construction is composed of various components that include a terminal housing, individual elements and controls. At the tubular center, it has a heating resistive wire made of chrome and nickel alloy. Magnetic contactors can be present to ensure full power cut-off when needed. The power wiring is also set to standard values to meet your industrial requirements.
An airflow switch is present to provide proper air flow near the heating elements. An automatic switch is present called primary thermal switch which is used to stop the whole unit when the temperature limit is reached to maximum value. As soon as the value of temperature comes below the maximum limit, the switch is reset on its own. Similar switch is present which is operated manually. It is called secondary thermal cut-off.
A disconnect switch is present to interrupt the electric power supply at the time of heater's service and maintenance. There is another relay whose purpose is to make sure the fan is ready and energized before the heater. This relay is called fan interlock relay. The duct heater has bushings to bear the mechanical stress on device. These bushings are made of steatite ceramic due to its high value of dielectric strength and resistivity to mechanical and thermal shocks.
The terminals of open coils are composed of steatite as well, while those of tubular duct heaters are not. This way these insulators give secure and safe electrical power transfer between elements and the control panel. A transformer is also present with a primary voltage rating of 120-600 volts. To protect the aluminum and copper wires, there is a protective covering in the form of a terminal block made of stainless steel. A ground connection is also present for extra safety.
There are various types of duct heaters. Some of the commonly used duct heaters are named below.
The main difference are the fins that are present around tubular elements. Each fin wraps around the tubular element in order to maximize the surface area of a tubular element. The heat that is generated is then dissipated more quickly and, increases the life of the tubular element. Steel fins are normally used in cases where there is low moisture areas. They are also a good heat transfer source as oppose to stainless steel fins. Stainless steel fins have other advantages such as their rigidity in processes that have more vibrations.
The following dimensions are same for all tubular heaters
Diameter of Tube (inches)- 0.260, 0.315, 0.375, 0.430, 0.475
The most commonly used are 0.315 inches and 0.430 inches.
However, the extra dimension added in this type of tubular heater is that of fins.
Typical Thickness of Fin (inches)- 0.020
Typical Number of Fins/ inch- 5
Typical Width of Fin (inches)- 0.375
The following dimensions may be changed as per individual requirements.
Total value of Square Inch/linear inch-8.30, 9.20, 10.10, 10.83, 11.60
Maximum length of Sheath (inches) - 240
Minimum length of Sheath (inches)- 11
Outer diameter of Fins (inches)- 1.01, 1.07, 1.13, 1.18, 1.23
Shown below are the maximum values of voltage for duct heaters.
Maximum value of Voltage (volts)-120, 240, 300, 480, 600
Maximum value of wattages can be:
Maximum value of Wattage (watts)-3600W, up to 2 MW
The tubular duct heaters provide a great mechanical strength, preferable for harsh environments.
There are two main factors which greatly control the reliability of these tubular duct heaters. They are:
-High watt densities offer quick heat by calculating the amount of wattage over a surface area. For applications that require quick heating time, a high watt density is desired. In those cases however, the life of the heater might be shortened as per the laws of thermodynamics.
-Material of sheath
-This factor is of critical significance for two major reasons. The first one is the temperature of operation. The second one is related to material corrosiveness under which the heater will work. Given below are certain temperature values at which the heater is safe to operate.
Material of the Sheath- Steel, Copper, Incoloy, Stainless Steel
Operating Temperatures (F) - 750, 350, 1500, 1200
Those duct heaters whose power rating is 50 KW or less are suitable for installation in places where there is no distance between the combustible materials and heater. While, those who have ratings that exceed 50 KW must be placed with minimum spacing.
Tubular Duct Heaters may be used in many industrial applications to ensure constant heat provision. A few of these applications include: painting, load banks, process heating:
Finned Tubular duct heaters are used in demanding applications such as dirty locations or when airflow is not uniform.
Rugged construction eliminates hazard of electric shock
Explosion proof environments for hazardous locations.
Process air duct heaters designed for ovens, autoclaves and other forced air industrial systems for drying, baking or heat treating.
These heaters are used in commercial refrigeration to prevent freezing.
The duct heaters are readily used in industries where forced air system is needed. It can be used there for heating, drying and baking purposes.
Other applications include: