Open coil heating elements

Open coil heating elements have proven design that exposes the maximum heating element surface area directly to the airflow. We specialize in custom engineered heating solutions to fit exacting requirements.

Build the future today!

With growing demands on energy efficiency and environmentally friendly white goods, the pressure for heating element manufacturers to perform with quality is on. Withstanding the higher loads, temperatures and environments in new models are among tomorrow's element challenges, while system design and materials knowledge will be key factors. Further design possibilities can be right within your reach. It's just a matter of exploring them together. Through real-case simulations and experiments, together with our materials experts, we can pioneer solutions for you.

Type of alloyDescription
Kanthal and Alkrothal FeCrAl alloys* High-resistivity iron-chromium-aluminium alloys for use up to 1400ºC (2550ºF).
Nikrothal NiCr alloys High-resistivity nickel-chromium alloys for use up to 1250ºC (2280ºF).
Nifethal NiFe alloys Low-resistivity nickel-iron alloys with high temperature coefficient of resistance for use up to 600ºC (1110ºF).
Cuprothal CuNi alloys Medium- and low-resistivity copper-nickel alloys for use up to 600ºC (1110ºF).

* Kanthal FeCrAl alloys are available as both conventional resistance heating alloys and powder metallurgical alloys (Kanthal APM).

Design variables – basic specifications

Wattage

In the design of a custom open coil heating element several factors need to be considered when selecting the optimum coil(s) for an application. First, the watts, volts, and resistance must be determined for each coil in the heater. Typically, the line voltage is specified by the customer and/or end user. The total wattage is calculated using the amount of airflow (SCFM) and the temperature rise (∆T) desired. (W=SCFM∙∆T /3 ) The wattage of each coil is the total wattage divided by the number of coils in the circuit. Most heaters use identical coils for throughout the circuit to ensure that the load is balanced.

Voltage

The voltage across each coil is determined by the circuit used. A single phase heater will 'see' full line voltage unless some of the coils are in series. A three phase heater is the same if it is wired as a delta circuit. A wye (star) circuit will 'see' only 57.7% of the full line voltage on each leg. If there are two coils in series on the legs, the voltage each 'sees' will be one half of the voltage on the leg.

Resistance

The resistance is calculated using Ohm's Law (R = V2/W). The result is Rhot. But Rhot is the resistance of the wire when it's hot. Resistance increases with temperature, so compensation must be made to get Rcold. Rcold is the resistance when at room temperature. The various alloys have different temperature factors (T.F.) from hot to cold. We generally use a T.F. of either -3% or -5% to figure Rcold from Rhot depending on the alloy. For example, a 1000W 120V heater would have an Rhot of 14.4Ω. (120V2/1000W=14.4Ω) Using -5% T.F. will result in Rcold of 13.68Ω. (14.4Ω x .95 = 13.68Ω)

Detailed Specifications and Coil Geometry

Alloy

The next thing to be determined is which alloy to use for the resistance wire. Each alloy has different properties that need to be considered for each application. Some alloys are stronger at high temperatures, some resist oxidation better at high temperatures, and some have non-magnetic properties.

Dimensions

The coil designer must now fit the coil to the heater frame he will use. This allows him to calculate how long each coil must be, and this is called the Stretch Length (S.L.). A coil usually exits the coiling machine with the coil completely 'closed', with each wrap of the coiled wire against one another. When the coil is stretched it will open up so that the wraps are not touching. The amount it opens is referred to as the Stretch Ratio (S.R. = Stretch Length/Closed Length). Stretch Ratio is the geometric pitch from one coil wrap to the adjacent one, expressed in units of the wire diameter. A closed coil (not stretched) has an S.R. of 1.00. An S.R. of 2.00 has a Space Between Turns (S.B.T.) equal to one diameter of the wire. A properly designed coil will have its stretch ratio between the minimum and maximum values necessary for that type of heater, usually between 2.5 and 4.0. This allows the air to circulate around the wire for better heat transfer.

The I.D. of the 'donut' bushings used to support the coil, in a duct heater for instance, determines the maximum O.D. for the coil. The O.D. is controlled by specifying a suitable arbor on which to wind the coil.

Wire gauge

The wire gauge selected has even more properties to consider, such as "the finer the wire, the higher the resistance", measured in Ω/ft. Using a finer wire increases the watt density (W/In2) for a given O.D. The arbor to wire ratio (A/W) will get larger with a finer wire. If the A/W ratio is too large, the coil will be too flimsy and will sag. If it's too small the coil will be too stiff to stretch, and will also be more difficult to wind onto the heater frame.

So why Kanthal?

  • Increased efficiency
  • Deliver on reliability
  • Reduce size
  • Environmentally friendly
  • Longer life