Friday, November 21, 2014

Basics of OEM and Industrial Electric Heating Elements - Part 1

This blog entry, reproduced from an electric heating element basics white-paper from Hotwatt, a leading US manufacturer of OEM and industrial heating elements. To download the PDF version, click this link.

Electric heating elements for OEM and Industry
Electric heating elements
for OEM and Industry
(courtesy of Hotwatt)
The simplest definition of an electric heater is any device that changes electrical energy into heat energy. But from that simple explanation, electric heaters explode into a myriad of types, sizes, applications, and designs depending upon what’s being heated, the degree of heating needed, and the method by which the heat is applied.

The measure of electrical energy is called the Joule after its discoverer, James Prescott Joule. Through numerous experiments, Joule determined that the quantity

(Q) of heat transferred from electrical energy is proportional to the square of the current (I2 ) multiplied by the resistance (R) for the period of time (t) through which it passes:

Q ∝ I2 × R × t

However, one seldom sees a reference to Joules used in modern electric circuits. Instead, the controlling factor becomes that of power (P):

P = I2 × R

You’ll note the only difference between the formula for determining power and that of determining Joules is the time component. The time factor in heating becomes readily apparent in any device that gets hot when an electric current flows through it: its temperature rises as time passes.

Types of Heaters

Heaters come in many different styles and configurations, each design optimized for a specific application. The selection of a heater depends on the material being heated, the heater style, the sheath material that surrounds the heating element and protects it from the material being heated, and the operating voltage of the heater.

Styles of heaters include strip, ring, rope and cable, cartridge, tubular, band, immersion, circulation, process air and duct, radiant, comfort, flexible, tote, and drum.

The style of heater chosen depends on the application. The type chosen depends on the heated object’s shape, size, and mass, as well as performance requirements.

For example, band heaters work best on cylindrical object such as tanks and pipes. Cartridge heaters are typically inserted in close fitting holes in large blocks of metal such as platens and molds. And immersion heaters may, as their name implies, be immersed directly into the material being heated. As added protection, an immersion heater may use a heat well, a protective device that isolates the heater from the material being heated but still allows heat transfer to the material.

Heat wells also allow immersion heaters to be replaced without the need to drain the tank.

Sheath materials can include steel, Incoloy, iron, silicone, Kapton, copper, ceramic, and glass overbraid among others to handle specific materials. The choice of sheath material depends upon the material being heated.

Transfer of Heat

When a body starts to generate heat, meaning its temperature rises above that of other nearby objects, it is called a heat source. As its temperature rises, it starts to raise the temperature of the materials in its vicinity using any combination of three different methods. Those three methods are known as conduction, convection, and radiation.

Conduction transfers heat energy from one material to another via direct contact. It is the most direct method of transferring heat energy, and is usually considered the most efficient: the highest percentage of heat energy created transfers to the colder object from the heat source.

Convection also uses physical contact to transfer heat energy, but the contact entails the use of an intermediary gas, typically air. In convection heating, the heat source warms the gas. The warmed gas now weighs less per volume measure, producing a buoyancy effect. This buoyancy is easily seen in the flight of hot air balloons. The hot gas rises, creating convection currents that move the heated gas into contact with the colder objects, warming them. The now cooled gas flows back to the heat source where it is warmed again, and the process repeats.

The third method of transferring heat energy, radiation, does not rely on any physical contact between the heat source and the object to be heated. Instead, heat energy is transmitted through space from the heat source to the object in the form of electromagnetic radiation: typically as infrared wavelengths, although that’s not the only frequency used. For example, microwave ovens use wavelengths many times longer than infrared to heat food while medical diathermy machines use even lower radio frequencies to warm parts of the human body.

Electric heaters use all three methods to warm specific objects. Which of the three methods becomes the best choice depends upon the application.

Heat Losses

As stated previously, an object subjected to electric heating continues to become hotter as long as current passes through it until it reaches its burn out temperature and the heat source is destroyed. Obviously, when used properly modern electric heaters do not experience this burn out condition.

The answer lies in heat losses, or the transfer of heat away from the heat source that keeps its temperature down.

The term heat loss is typically reserved for heat energy transferred to undesired areas, but heat energy is also transferred to the desired point. All three methods of heat transfer can lead to heat losses. For example, a contact heater placed on the bottom of a container heats the contents of the container. The primary form of heat transfer is via conduction from the heater to the container and then its contents, but conduction losses can occur between the container and any supporting structure. As the container and heater are both in air, a convection heat loss occurs as air circulates around the container. In addition, a hand held one foot away from the side of the container can feel the radiated heat loss.

Needless to say, the size of these objects also plays a considerable role. For a container the amount of radiated heat energy lost is determined by the outer surface area. Assume the container is a cylinder 6-in. diameter and 5-in. tall. The surface area of the container (AS) equals the circumference of the container (calculated as either π times the diameter D, or 2π times the radius R) multiplied by its height (h):

AS = πD × h or 6π × 5 = 94-1/4 in

Just enlarging the diameter of the container by one inch means the surface area of the container increases by 15.7 in2, an almost 17% increase in surface area and thus heat loss. When determining the size of the heating element needed to reach a specific temperature, all of these heat losses must be taken into effect.

Note: All information, data and dimension tables in this article have been carefully compiled and thoroughly checked. However, no responsibility for possible errors or omissions can be assumed.

It is the express responsibility of the customer to determine the suitability of the product for the intended application and Hotwatt Inc. nor Belilove Company makes no claims or provides no guarantee in this respect, either written or applied.