Introduction
In the heating industry, we are often asked what is the difference between a distributed wattage heater and a zoned heater. These various construction techniques can be applied to many different types of conduction heaters such as cartridge heaters, ceramic and mica strip heaters, and band heaters. Almost any heater that uses a wound coil can use one of these construction methods of controlling watt density. Before beginning, some definitions should be discussed.
Definitions
Coil: A resistance wire (usually Nichrome) that is wound around a round mandrel or ceramic core to hold the shape. When a coil is wound, it is usually done in a uniform manner.
Close Wound: If all the turns of a wound coil touch, this is called a close wound coil.
Open Coil: If the close wound coil is held by its ends and stretched, it becomes an open wound coil. Coils can also be initially wound as open coils if there is space between each turn of the coil.
Pitch: The distance between two turns of an open coil is the pitch.
Watt Density: The wattage produced by a heater divided by the surface area of the heater. Common units of measure are W/in² and W/cm².
Distributed Wattage Heater
What makes a distributed wattage heater? Distributed wattage is when a coil winding’s pitch is not consistent throughout its entire length. Some parts of the winding are more tightly packed with turns than other sections, to “force” the heat profile to be more concentrated in some areas of the heater than another. The non-uniform pitched coil will have sections of closely wound coils and sections of loosely wound coils. These methods of non-uniform stretching or non-uniform winding of the coil pitch allow the heater manufacturer to increase or decrease the watt density of the heater in certain sections. Because the coil winding in some of the sections will be closer together than other sections, the watt density will be greater in those sections. These higher watt density heater sections will be hotter than the other sections. Certain sections of the heater can be now be made to run hotter than other sections. Only one set of power leads is used in this configuration, as the entire heater turns on as one unit.
Application of Distributed Wattage Heaters
A distributed wattage heater allows the end-user to have higher heat where needed. This method of distributing heat is especially beneficial if only a small section of the heater is providing the majority of the heating or in applications where heat is needed over a long length and there tends to be temperature loss or drop-off near the heater ends. Distributed wattage heaters are often used in platens and sealing bar applications where heat loss compensation near the ends of the heater is important.
Zoned Heater
Zoned heating is where a specific section of the heater has its own “dedicated” wound coil with uniform pitch and its own set of power leads for each particular section of a heater. The internal coil construction within this particular type of heater has sections or zones of dedicated uniformly wound coils. The key word in this construction style is “dedicated”. The customer can literally control each section of the heater individually. The heated area is very specific and controllable. For example, a 2-zone heater will have 4 power leads coming out of it; a pair of leads for each zone. Sometimes a common wire may be used; in this 2-zone example, the number of leads can be reduced because each zoned coil shares a common lead. A heater like this would have 3 power leads.
Application of Zoned Wattage Heater
Zoned heaters can be used in any application distributed wattage heaters are used but in most cases, this would probably be overkill and not justify the added expense of the zoned heater. Zoned heaters are best used in applications requiring a significant level of heat control over the length of the heater. Zoned cartridge heaters can have various sections literally turned off entirely while only a fraction of the entire heater is energized. These heaters are commonly used in heater bar applications where heat control at precise locations is an important factor.
