What Is a Heating Coil?
A heating coil is a coil utilized in induction heating.
Heating is achieved by placing the object to be heated inside the coil. The shape, number of windings, and diameter of the heating coil vary depending on the heating requirements, such as the characteristics and shape of the object to be heated and the area to be heated. It is important to optimize the heat distribution by designing a coil that meets the requirements.
A coil design that meets requirements is also necessary to maximize work efficiency by making it easy to insert and remove the object into and from the coil. Furthermore, during heating, the heating coil itself is indirectly heated by the heat emitted from the object. Therefore, it is common to have a structure that can be constantly cooled.
Uses of Heating Coils
Heating coils are used in induction heating, and they are used in situations where precise and controllable heat treatment is required without direct contact with the object. Specific examples of industrial applications are as follows:
- Preheating for welding
- Quenching
- Tempering
- Annealing
- Brazing
- Hardfacing
- Soldering
- Metal melting and forging
- Getter heating
- Floating Melting
- Material Testing
- Cap sealing
- Material hardening
- Metal-to-glass bonding
- Crimping
- Susceptor Heating
Heating coils are flameless, which not only reduces carbon dioxide and environmental impact but also ensures clean and waste-free heat treatment. A well-known induction cooktop is a cooking appliance that applies this characteristic to ordinary households.
Principle of Heating Coils
The principle of a heating coil is that a coil is connected to an AC power source, which generates magnetic field lines around the coil to heat the object. This method of heating by electromagnetic induction is called “induction heating.” There are two types of induction heating methods:
1. Direct Heating Method
In induction heating, a conductive object is placed inside a heating coil and an electric current is applied to the object, which itself generates heat. This is the “direct heating method.”
This heating method uses a mechanism in which eddy currents flow in the object in a direction that prevents the magnetic flux from changing, and Joule heat is generated due to electrical resistance. The eddy currents are larger near the surface and smaller toward the interior. This is called the “proximity effect,” and the direct heating method is suitable for surface heating.
2. Indirect Heating Method
When heating insulators such as ceramics with a heating coil, the object is placed in a conductive container, and the container is heated directly, causing heat transfer and heating the object. This is the “indirect heating method.”
In this heating method, the heating element and the object are in contact with each other, or the heating element and the object are separated. The latter is called far-infrared heating because infrared rays heat the object.
Structure of Heating Coils
The shape of the heating coil is not simple, as the optimum one is selected according to the dimensions and shape of the object. A wide variety of coils exist, such as an outer surface coil in which the product is inserted inside the coil when heating the outer surface of a shaft, or an inner surface coil in which the coil is inserted when heating the inside of a steel pipe or other object.
For example, the high-frequency quenching method using induction heating includes the “stationary one-shot quenching method,” in which the object is heated without moving, and the “moving quenching method,” in which the object is sequentially quenched while moving. The heating coils used are different for each method.
1. Heating Coil Structure of the High-Frequency Quenching Method
In the “in-situ one-shot quenching method,” heating coils are manufactured according to the shape of the object, so they must be prepared according to the type of product. The shape of the heating coil is important, and experience and know-how are required for design and fabrication.
However, the advantage of this method is that even complex shapes can be heated uniformly, and the time required for heating can be shortened compared to the moving quenching method. The moving quenching method, on the other hand, uses single or multiple coils. The shape is simpler than that of the stationary one-shot quenching method and is selected in consideration of the shaft diameter and length of the object.
2. Heating Coil Structure with Coolant Injection
When used for quenching, the structure is provided with a quenching water jet. This is because the heated object needs to be cooled rapidly.
There are two types of water jets:
- A type that injects hardening water from inside the heating coil toward the object.
- A separate cooling jacket is installed near the heating coil.
The type that injects cooling water from inside the heating coil has a structure that allows both the cooling of the heating coil and the object with the quenching water. The object can be cooled efficiently since it can be cooled directly from the heating point, but in most cases, the shape is complicated.