Frequently Asked Questions
Mica Band Heaters, Mica Insulated Heaters, Metric Band Heaters
Welcome to National Plastic Heater, Sensor and Control Inc’s frequently asked heater questions page. These, address Mica, Ceramic Band and Cartridge Heaters. Other helpful measuring and installation tips, tools, engineering links and formulas are included. If your question is not addressed here you may send a question to our engineers for review.
Question: What is the best way to install a band heater?
Answer: These suggestions are for conductive heaters only. Ceramics, or “knuckle bands” have different considerations when installing.
After removing the defective band heater from the barrel or nozzle, inspect the area to be covered by the new heater and make sure it is clean and free from obstructions. Assure that there are no hydraulic fluids or materials that will become trapped between the heater and the barrel. If need be, use a wire brush and solvent to prepare the barrel, then wipe it down and make sure it is totally dry and free of debris before installing your new heater.
REMEMBER: If there are any holes, nicks or air spaces between the heater and the area to be heated, the life of the heater will be significantly shortened. These imperfections should be addressed before installing the heater or when ordering a new heater, assure that there will be no heater coverage over the area in question. The better and more complete the contact between the heater and the area being heated, the better the heat transfer. The better the heat transfer, the longer the heater will live under normal operating conditions.
If using post terminals, make sure you are using high temperature connectors. Typical industrial connectors will fail rapidly and nickel ring terminals should be used. If the environment is exposed to material dust to the point of heaters failing due to carbonization of particulate dust, try ceramic post terminal covers and fiberglass tape (with silicone or non-organic adhesive) to insulate exposed lead wire and ring terminal from the atmosphere. Be sparing with the tape and make sure you give as much space as you can between the connector and the heater. When installing, make sure the strap(s) or clamping mechanism are tight.
After you are satisfied that the heater is installed and wired correctly, check to make sure your temperature sensors are seated firmly and power it up. Give a quick check to assure it is drawing the right current and bring it up to 75% of operating temperature, and cut the power to it. Make sure there is no power going to the heater and retighten all appropriate clamping mechanisms before the heater contracts.
You are now ready to power your heater back up and the job is done. For maximum heater life, check the tightness of the clamping mechanisms at least once a week. Remember, most heaters that fail prematurely in the field, fail due to contamination or improper installation. Both these reasons are avoidable and with a little extra care and common sense, you can extend the life of your heaters and save your machines from that dreaded unscheduled downtime.
Question: What’s the best way to test a heater before installing it?
Answer: A disconnected heater can be expected to work properly when installed if its resistance measures within ten percent of an easily calculated optimum level.
To calculate the heater’s resistance value, first solve for R using the formula R=VxV/(Wx1.065), where R equals ohms, V equals voltage and W equals wattage.
Volts and watts should be stamped on the heater. The 1.065 factor accounts for the fact that the resistance of the nichrome element wire increases resistance by about 6.5 percent from room temperature to hot temperature.
Next, test the heater using an ohmmeter. Make sure at least one of the two lead wires is not connected to anything. Measure the resistance from one lead wire to the other lead wire.
If the resistance you measure is within 10 percent of the calculated value of R, the heater should be good, though it is possible to measure some resistance if part of the heater is good and part is bad.
Question: How can I test a heater without stopping production?
Answer: During operation, a heater that’s working properly will draw current in amperes within ten percent of an easily calculated optimum level.
To check a heater on a running machine, you first need to figure the optimum level of current your heater should draw. Use the formula I=W/E, where I is the current in amperes, W is the wattage and E is the voltage.
You should find the watts and volts stamped on the heater. To find I, the current in amperes, use a clamp-on ammeter. For the meter to read correctly, you must measure the current in only one of the lead wires going to the heater. You must also make the measurement while the controller is calling for heat, because the temperature controller turns the heater off some of the time.
If the current you measure is within ten percent of the optimum performance value of I that you calculated, the heater is most likely good.
A bad heater may still draw current. For example, the inside of the heater may be made up of two parallel heaters. If one of the parallel heaters is bad, the current will be half the calculated amount. Depending on how the inside of the heater is constructed, it is possible to measure several different current values, indicating that part of the heater is bad and part is still working.
Question: Ceramic Knuckle Bands or Mica Band Heaters?
Answer: It seems that at least once a week, we have a customer changing from mica heaters to ceramic heaters or from ceramics to mica. If you’re thinking of a change or if you’re simply curious, we’d like to offer a little information here to help you make an informed decision. The two greatest things about using ceramic heaters are the potential energy savings and the fact that less heat is released to the atmosphere than with mica heaters. While we wouldn’t recommend touching the sheath of a ceramic heater, we can say that the consequences of an accidental encounter are less severe than an experience with a mica band. From our field studies, we have determined that our ceramic band heaters typically use 10% less energy than a mica band heater when manufactured with extra heat saver insulation.
National Plastic Heater’s ceramic heaters have two standard insulation options as shown below. There are many other options available as to clamping options, terminations, leads and lead protection. Please see our Ceramic Heater pages http://ceramic-band-heaters-knuckle-band-heaters.com/products.html for listings and sketches of available options.
The ‘business end’ of our ceramic heater is the construction of high quality ceramic “knuckles” which support coiled nickel-chromium element wire. This assembly is sheathed in stainless steel. These heaters are extremely flexible allowing for easy installation, can operate at higher temperatures than mica heaters and are longer lived as well! This can mean quicker start up times translating to extra production time.
Another item of consideration is the fact that extremely tight fits and irregularities in the mounting surface are not as critical with ceramic versus mica due to the fact that much of the heat is transferred via radiant energy.
Downsides to ceramic heaters would be that they are thicker than mica heaters, and widths are only available in ½” increments. When selecting a knuckle band, you need to consider that they do not lend themselves to repeated reinstallations. While price is higher than mica heaters, the savings in energy and longer lived heaters more than makes up for any price difference.
Mica band heaters are great low cost heaters that are easily customized and quickly manufactured. They can last for years or months depending upon many different variables.
Positives for mica band heaters in addition to the low cost would have to include their versatility, low prices and ability to be manufactured in less time than ceramic heaters. When installed properly, they are an efficient and durable answer to process heating in an industrial environment.
Whichever route you take in your industrial process heating needs, you can depend on the National Plastic Heater team to do our best to provide you with the best solution available today.
Question: What is watt density and how is it related to heater life?
Answer: 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”x1” square on the surface of the heater, how much heat would need to pass through that 1 square inch area. This is called watt density and it is measured as watts/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 mica band heaters, for satisfactory heater life, the watt density should be less than 50 watts/square inch for heater diameters less than 3 inches, less that 40 watts / square inch for diameters between 3 and 6 inches and less than 35 watts / square inch for heaters with diameters between 6 and 10 inches.
For cartridge heaters, the watt density should be less than 200 watts / square inch. For ceramic knuckle band heaters, the watt density should be less than 35 watts / square inch. Also, higher operating temperatures require lower watt density for equivalent temperatures inside the heater and equivalent heater life.