Saturday, July 18, 2009

Electric Heating Elements

electric tubular elements by Watlow
Electric Heating Elements make the industrial world Hot.

Anderson-Bolds has been selling electric Heating Elements since 1934.
Electric Heating elements are a very simple yet extremely effective way to heat in the industrial work place as well as commercially and in the home.

Electric heating elements can be applied in just about every facet of industrial heating and commercial heating, from surface heating, to tank heating, air heating, drying, heat treating, freeze protection, cooking, air conditioning and frost prevention.

The heating element itself is very simple, made of three main parts, a resistance wire, insulator and metallic sheath. The element then functions under Ohms Law with the relationships of Resistance (ohms), current (amperes), watts and volts. Which one of these can be the constant?
To make an element the three main parts mentioned above are brought together, the sheath material is usually in a tube form (except for plastic cable and strip heaters) and hung in a vertical fashion up to 30 or 40 feet and the resistance wire is then also hung through the center of the tube with spacers at each end (a plug on the bottom end) and stretched taught to keep it centered in the sheath tube. The insulator material is then entered into the tube gently, the material is usually magnesium oxide or MGO. [MGO has the two properties necessary to be very good at its job, to insulate the metal sheath from the electrically charged resistance wire AND be a very good heat transfer medium. MGO does this job very well, unless it gets wet.] The MGO is in a fine sand like consistency when poured and shaken into the tube. [The manufacturing plant has to be fairly tall to make the long elements.] The resistance wire is not straight but in a curl formation running up through the center of the sheath tube. The light shaking of the operation allows the MGO to get in between the curls of the wire to fill all the gaps. No air gaps can be present with the element.

After sufficient time on the vibrators or shakers the filled tubes are capped off and removed and taken to the rollers or presses. The elements are now pressed and squeezed to the desired diameter. There are two other results to this operation, the element is lengthened as it is squeezed and the MGO gets compressed into a solid and fills all remaining spaces within the element and resistance wire spaces.

The Heating element is now ready to be formed into the correct shape, be it straight, hairpin, "M" shaped or any other shape desired for the application. After the element is shaped it is repressed to consolidate any of the MGO that has separated and then terminations are added to the ends of the element.

Heating elements can have sheath materials of Iron, copper, stainless steel, incoloy, inconel, monel or any material needed for temperature ratings or corrosion resistance. The common material are copper for water heating, iron for oil heating, and stainless or incoloy for higher temperatures or corrosion.

Lets return to Ohm's law which is W=I2R or V=IR or I=W/V
are the common formula's used with heating with resistance heating elements. With W = watts, I = amps, v= volts and R=ohms.

When making a heating element, the resistance is the constant in the actual element. The wire diameter and length is chosen to meet the wattage requirement when a specific voltage is present on the element. However different wattages and amperes can be easily achieved with different applied voltages; this can be good and bad. In short, if the voltage is doubled the wattage is quadrupled and conversely if the voltage is halved, the wattage drops to 1/4 of what is was. Example; an element rated at 1000 watts on 240 volts will be 250 watts when 120 volts is applied or will be 4000 watts if 480 volts is applied. The resistance wire is the fixed constant in a heating element and is the key to manufacturing consistent product.

The ease of which electric heating elements can be used to heat product makes them an excellent choice in all manners of heat, from manufacturing, to water heating, floor heating, pipe heating, coffee makers, heat treating and plastic injection molding.

Of course in a heating system, we need to control the temperature, sense the temperature and apply the voltage required, this items make up a heating system. Another blog will describe this system.

Some of our manufactures of heating elements and heaters include, Watlow, Danfoss, Q'Mark, Chromalox, Brisk Heat and Process Heating.

Cleveland, Ohio USA

Friday, July 17, 2009

Photo Electric Switches

How do garage doors know when something is in the way?
How do elevators know where to stop?
How does the store keeper know when someone enters a store?
How does a car wash know where the car is?
How does a manufacture count the cans on an assemble line?


Photoelectric switches use a light source to detect an object. The switch then changes it's logic or switch to make an action, like an alarm or movement of a device.

The photoelectric switch uses either visible light or infrared light as it's source. The object being sensed should be reflective for a diffuse switch or the use of a reflector is required and the object breaks "the beam" as in an elevator or garage door switch.

A thru beam system utilizes a transmitter and receiver. the Transmitter sends out the light beam and is directed at a receiver unit. Both units need to be powered and interconnected together for the system to work. Advantages are the longest field of view is achieved and the most secure sensing is realized via polarization. The main disadvantage is that each unit must be wired and interconnected together. A "thru beam" system can also be utilized using a reflector. this is called retro-reflective. The retro-reflective system has the advantage over a thru beam of all the logic and power supply being located in one box and one area. It is also more secure like the thru beam system by using polarizing filters that turn the light and only allow the correct "polarization" to be sensed by the receiver. The retro-reflective unit as the receiver and transmitter housed in a single unit. Thru Beam and Retro-reflective systems are used for counting, sorting and sensing the presence of an object or person for safety or security reasons.

The Diffuse system relies on the object being reflective, these are usually used in an industrial setting sensing a product or color code or marking on a product or box. Like the retro-reflective unit the transmitter and receiver are in the same unit. The distance is usually quite short so that the reflected light is able to be sensed. The target does not usually have the reflective qualities of a reflector. A diffuse system is used when a thru beam or retro-reflective cannot be used or when sensing color or a bar code.

All Photo Electric systems need an output to communicate with the PLC or system logic. Most use NPN or PNP logic but some have built in relays (1 amp usually) to give the output signal. The signal can go to a computer, PLC or directly to the system like a motor or contactor or alarm.