Open coil heating elements have open circuits consisting of coiled resistance wire (usually Ni- Chrome or FeCrAl) fixed onto a supporting element that heat the medium directly. Termed as the most efficient and versatile while also the most economically feasible solution for heating, these elements have fast heat up times that improve efficiency and have been designed for low maintenance and inexpensive replacement parts. When an electrical current is applied to the wire, it gives off heat. The wire is connected to the control panel which regulates the amount of heat provided by the electric heater and fills the tunnel of the air handling unit. Because of the low mass and fast response time, SSR or SCR switching devices are advisable. They serve as an indirect solution to decrease watt density requirements and prevent heat sensitive materials from breaking down. The heater can be formed into a compact, coiled nozzle heater supplying a full 360 degrees of heat with optional distributed wattage.
The face velocity of the air passing over the open coil elements must not be less than a minimum specified value when the heater is energized. There are three factors that are considered when an appropriate face velocity is calculated, i.e. kW, frame size and heater element type. Sufficient airflow for the required kW in a given frame prevents an overheating condition. Heat must be dissipated away from the heating elements. The following calculation is used for determining face velocity :
Face velocity = CFM/Face Area
Another consideration is the amount of current draw the electric heater will place on the incoming power source. Electric heaters should be divided into individual circuits drawing 48 amps or less. The amp draw can be calculated using the kW and voltage of the heater.
Amps = (kW x 1000)/ (Vac x 1.732)
Attention must also be paid to the geographical area in which the open coil heater will be located.
Different Configurations :
One, two or four resistance wires
Parallel coil or straight wire
Drawn or swaged sheaths
With or without thermocouples
Round, rectangular or square cable cross sectionals