What is Watt Density and How Does It Relate to Heaters

Watt density is a measure of the rate of heat being transferred through the surface of the heater. That is, if you were to draw a 1”x 1” square on the surface of the heater, how much heat would pass through that 1 square inch area. This is call watt density and it is measured as watts per square inch. Other things being equal, the higher the watt density, the higher the temperature inside the heater. As the temperature inside the heater increases, the materials inside the heater are operating closer to their breaking point resulting in shorter heater life. Generally, for satisfactory life in a mica band heater, the watt density should be less than 50 watts per square inch for heater diameters less than 3 inches, less that 40 watts per square inch for band heaters with diameters between 3 and 6 inches and less than 35 watts per square inch for mica bands with diameters between 6 and 10 inches.

 

Determining Size

The mica band heaters inside diameter should be equal to the outside diameter of the plastic machine’s barrel. The diameter must be exact since a difference would be compounded by a factor of pi (3.1415926) on the circumference of the band. This would mean that a 1/4” (0.250”) error on the diameter results in a shortage or overage of approximately 3/4” (0.785”) on the circumference. In those rare cases when the barrel diameter cannot be measured with accuracy, it is advisable to specify an adjustable heater band with the approximate range. When all else fails, it is better to estimate heater diameter slightly smaller as opposed to too large. This would ensure adequate tightening of the heater, provided the difference is no greater than 1/4” on the diameter dimension. If a heater with too large a diameter is used, the ends will come together and prevent the heater from being drawn to the cylinder. However, the exact dimension is necessary for optimum heater performance.

 

Mica band heaters are designed for “contact” heating as opposed to radiant heating, and therefore must be secured against the barrel of the plastic molding machine. If the band heater is not tightly clamped against the barrel it is possible for the heater to expand away from the barrel. This would cause air gaps between the barrel and the band heater, resulting in poor heat transfer, shorter life, excessive heat loss, and wasted electrical energy. Operating sheath temperatures should not exceed 850° F° (454°C). (Note that this rating is for the heater’s sheath-not the process.)

 

A rough spot or foreign material on the barrel can greatly reduce the life of your band heater.  A small particle can leave an air gap on either side of itself which can cause a “hot spot.”  Air will not conduct the heat away from your heater as fast as the metal of the barrel.  This results in a spot on the heater where the temperature will be significantly hotter than the rest of the heater.

 

When you are ready to call BRIGHT HEATER and speak to one of our sales engineers to order a heater, please specify the following information:

 

Quantity and Part Number (if known)

 

Inside Diameter (I.D.)

 

Width (W)

 

Voltage (2-pc construction; specify total voltage)

 

Wattage (2-pc construction; specify total wattage)

 

Basic Construction Type

 

Lead Lengths Desired (stock length is 12”) or Post Terminals (stock is 10-24 thread size)