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What is Ohm’s Law?

Electric heater characteristics can be described fairly well with four simple variables. They are wattage (power), voltage, current (in amps), and resistance (in ohms). There is an interesting relationship between these four variables in that if you know any two of them you can calculate the other two. This is called Ohm’s Law and it is important when determining how to set up to use an electric heater or heaters; what kind of dielectric protection is required, how much current will be pulled by a heater when energized, and how to check a heater to make sure it’s good to use are all things you can use Ohms Law to help determine.

Those units above are often abbreviated with letters that appear to make only partial sense. They are P for watts (makes sense, P = Power), E for Voltage (this comes from E meaning Electromotive Force, which is measured in Volts), I for Current (from the French, ‘Intensité de Courant’), and Ω or R for Resistance, where Ω is the greek uppercase letter omega.

First, determine what variable you need to calculate (select one of the two at a time) and that will direct your attention to the correct quadrant of the Ohm’s Law Calculator tool graphic.

Each quadrant contains three wedges, where you will find one of the wedges to contain the two variables that you already know values for.

Perform the math as shown in the wedge to determine the missing value that you need for that quadrant.

First Example.

Let’s say you want to maximize the power output using the voltage source and current available to you. Let’s say you have a 240V circuit available that can handle 15 amps. What is the maximum wattage of heaters that this circuit will allow you to power?

Start in the quadrant in the upper left because you want to know the wattage. You already know the voltage and current, so you’ll use the lower wedge in the quadrant for this example. EI means Voltage (E) times Current (I) so 240 x 15 = 3600 watts. There’s your upper limit. With a 240V supply and drawing no more than 15 amps, you can generate 3600 watts.

Second Example.

You have a heater marked with 1000W and 240V, but you don’t know if it’s any good anymore. A good way to check it is with an ohmmeter or multimeter to measure the resistance. We start in the lower right quadrant of the ohm’s law circle because we want to know the resistance. We know the E and W values so we’ll use the center wedge (of the three) to calculate the resistance using E2/W. That gives a resistance of 2402/1000 = 57.6Ω. If your heater measures a resistance of 57.6 Ω or something close to that you can be confident that your heater has the correct resistance.

You can find a tool on the TUTCO website that will perform these calculations for you. Go to tutco.com / Engineering Resources / Calculators / and click on OHM’S LAW CALCULATOR on the right side.

 
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How to Correctly Install a Band Heater

When installing a band heater, it is important that it be well seated on a clean smooth surface and be tightened so that good heat transfer occurs between the heater and the body being heated. The more circular the object to be heated is, so much the bet- ter. Conductive heat transfer is the best type of heat transfer for a band heater, which relies on physical contact between the heater and body being heated. Any air gaps between the two will cause hot spots and are detrimental to the life of the heater. A small piece of built-up contamination can cause a lengthy air gap between the heater and the object to be heated. The higher the operating temperature, the more detrimental a poor fit can be.

If the surface you’re installing the band heater around has pits or divots, it’s a good idea to apply a layer of high-temperature thermal transfer compound between the heater and what you’re heating, in order to aid with heat transfer over those uneven areas. The thermal transfer compound should be used sparingly and kept away from any termination areas and edges of the heater, due to the possible conductive nature of some of those transfer compounds. Heater efficiency is also negatively affected by a poorly fitting band heater.
 

General Procedure:

  • The surface to be heated should be clean and smooth. Projections of baked-on contaminants or surface cavities may reduce heater life and decrease heater efficiency. Eliminate them as much as possible, especially for high operating temperatures.
  • Seat the heater in its intended position.
  • Securely and evenly tighten the heater clamping screws.
  • Electrically connecting the heater.
  • Energize the heater, allowing it to reach operating temperature.
  • TURN OFF THE POWER TO THE HEATER and…Immediately retighten the clamping screws of the heater to take up elongation caused by any thermal expansion. Try to do this quickly while the heater is as hot as possible. Take care not to burn yourself. For safety reasons, don’t retighten the heater when it is energized. Just don’t do it.
  • Energize the heater again and allow it to operate normally.
  • There are applications where, because of the heater size, the clamping screw re-tightening operation should be repeated several times. Don’t forget to turn off the power to the heater for each retightening.

Notes:

If you have a torque wrench and the space in your heater installation location,

  • 6-32 screws should be tightened to 15 in·lbs.
  • 10-24 screws should be tightened to 75 in·lbs.
  • 1/4-20 screws should be tightened to 100 in·lbs.
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Applied Voltage vs. Rated Voltage?

The ratio between applied and rated voltage is a square relationship. That means that applying twice the rated voltage to a heater does not produce twice the power (wattage). It actually produces 4 times the power. Applying half the rated voltage to a heater does not produce half the power,but only a quarter of the power.

Here’s the general formula:
Vr = Rated Voltage
Va = Applied Voltage

A few examples:

For a heater rated at 1000W and 240V, applying 208 volts will generate the following power. Output Power = 1000 x 2082/2402 = 751.1 Watts
For a heater rated at 1000W and 208V, applying 240 volts will generate the following power. Output Power = 1000 x 2402/2082 = 1331.4 Watts

And to show what was discussed earlier:
For a heater rated at 1000W and 240V, applying 480 volts will generate the following power. Output Power = 1000 x 4802/2402 = 4000 Watts !
Double the voltage and you quadruple the power !

Sometimes it’s perfectly fine to apply a voltage to a heater that’s different from its rated voltage. Sometimes it’s not. Be aware and always think of what you’re doing, making sure that it’s safe.

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Cartridge Heaters: Heater and Hole Fit

Cartridge Heaters can be a great choice for many applications that require heat. If used and controlled properly, cartridge heaters can provide a long-term solution for many conductive process heating applications.

When using a cartridge heater for your application, there are two key factors to ensure optimal results:

  • Make sure the heater is sized correctly for the hole it’s intended for
  • Make sure the surfaces of the heater and hole are clean

If you’re heating a solid mass, usually metal, you’ll need a hole in the material for a cartridge heater to be inserted into. Usually, it’s easy to find a heater for a hole size you have. If a hole measures 0.500” across its inside diameter (ID) you’ll want to use a 1/2” diameter cartridge heater. Cartridge heaters are slightly undersized, so they’ll fit into a standard hole size. In the case of a 1/2” diameter heater, it’s actually sized to 0.495” so it will fit into a hole with a small amount of clearance. It’s the same general rule for all TUTCO Cartridge heaters. It’s a lot easier to drill a 0.500” diameter hole using a standard drill bit or gun drill (due to availability) than a 0.505” diameter hole. Heaters are typically sized 0.005” smaller than the hole size for a good fit. The same rule applies to our metric heaters too.

Make sure the heater and inside of the hole are clean. This will enable the heater to be inserted into a hole without too much difficulty or excess force. Contamination will interfere with the heat transfer from the cartridge heater to the hole’s surface.

There are three types of heat transfer; conduction, convection and radiation. These will be discussed in another article, but for the time being please know that conduction is the best method of heat transfer for instances where you have a cartridge heater inserted into a hole. Conduction can be described as heat transfer between objects in physical contact.

If you have a heater that doesn’t fit into its hole well, as in, it’s a sloppy fit, then you’re asking for trouble. With a loose fit there will be a lot of convective heat transfer instead of conductive. That convection occurs because of the air surrounding the heater.

An inexpensive way to fill that air gap is to coat the heater with a few layers of Milk of Magnesia (yes, the stuff you can buy at any pharmacy). By applying thin layers of Milk of Magnesia and allowing it to dry between applications you

can build up the outside diameter of the heater so that it completely fills the hole when inserted. The coverage doesn’t have to be perfect since some of the material will shave off the heater when it’s inserted. That extra layer of material takes up the excess space and promotes good conduction.

Another material is Boron Nitride spray, available from a popular industrial distributor. Though more expensive, boron nitride is easy to apply (as a spray) and lubricates the heater very well for easy insertion.

There is also a large variety of thermal transfer pastes that can be used to fill gaps, but with caution. Some of these pastes are conductive so try to avoid them. If that’s not possible, keep the paste away from the termination area of the heater, and make sure the temperature rating of the paste is appropriate for your application.

heatingliquids

Cartridge Heaters: Heating Liquids

In this article, we discuss heating liquids and the wattage requirements for heating various materials. If used and controlled properly, cartridge heaters can provide a long-term solution for many conductive process heating applications.

Make sure that the heater will always be surrounded by the liquid you are heating. The liquid acts as the heat sink and it is required to keep the heater at a safe operating temperature. Be sure to consider a worst-case scenario where the liquid could drop as low as it can go, even in the case of a leak in your tank. Without the liquid around your heater to help pull the heat away, the heater will be exposed to air and run hotter. That excess temperature may decrease the life of your heater depending on the operation temperature.

Have safety precautions in place. TUTCO recommends a high-limit thermostat to turn off the heater in case the liquid level gets too low, for example. Using a closed loop control system with the high limit thermostat will help monitor and control the voltage to the heater.

Keep liquid away from the leads or screws of the heater. Lead connections are electrically live and must not be submerged to prevent electrical hazards. Please contact the factory for proper terminal connections for liquid applications.

If using a temperature controller with a thermocouple (or other temperature sensing device), make sure that the sensor’s location in the tank is carefully considered. If the sensor is too far from the heater, it may never turn off. If the sensor is too close, the heater may cycle too frequently. If the liquid level drops allowing the sensor to be exposed to air, that would be bad too.

Correct Heater Wattage

Heater wattage can vary depending on the material being heated. Here are a few quick rules of thumb for heating different mate- rials (these are rough numbers):

  • For Water in a tank – Wattage Required = Gallons x Temperature Increase (in °F) x 3.08 / Hours to get to temperature
  • For Flowing Water – Wattage Required = Gallons Per Minute x Temperature Increase (in °F) x 0.16
  • For Oil in a Tank – Wattage Required = Gallons x Temperature Increase (in °F) x 1.25 / Hours to get to temperature
  • For Flowing Oil – Wattage Required = Gallons Per Minute x Temperature Increase (in °F) x 0.40

After determining the wattage requirement, try to hold yourself to that limit. Excess wattage may not be pulled away by the application quickly, resulting in an increase in heater temperature. That increase in temperature stresses the internal components of the heater causing them to oxide faster (than they normally would) and reduces the dielectric strength (electrical insulative properties) inside the heater.

Cartridge heaters have a watt density limit, meaning there is a maximum wattage you can get out of a heater based on its surface area. The smaller the heater and the larger the heat sink, the more you can bend this rule. For large heaters, over 1/2” in diameter and longer than half a foot in length, that rule must be more strictly adhered to.

Larger surface areas can put out more wattage and the wattage must go somewhere. Unless the heat sink is sufficiently large enough for the application, the heat won’t have anywhere to go, and the heater temperature will increase. Reminder, a hotter heater the shorter the life span due to excessive internal oxidation and dielectric issues.

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Cartridge Heaters: Minimizing Failures

There are several bad things you can do to a cartridge heater (any heater actually) that may damage it or shorten its life. In this installment of Ask Ian, we look at heater abuse and things to avoid.

Don’t let contamination get onto a heater, especially near the lead-wires or screw terminals. Even a dry contaminant could liquefy when heated and get to where it’s not supposed to. I have no idea if chocolate is conductive or not, and I don’t particularly want to find out by having powdered cocoa on the armored section of the lead-wires melt and onto the lead-wires within. It’s probably a nice smell for a few minutes, yes, but probably not good for the heater.

Try to hold and transport heaters by handling the heater body. It’s very tempting to handle a heater by its lead-wires, and often there’s no problem in doing so, but it’s good to get into a habit of handling heaters by their bodies to avoid possible damage to the lead wire connections. If the heater was powered, obviously, wait for the heater to cool down before handling!

A heater that is moving while operating will put stress on the lead-wires. This could be as simple as repetitive movement or abrasion against a static object. Even slight abrasion can result in lead wire damage, not due to the force of the abrasion but the repetition of it. Even the strongest wire will fail after enough repetitive abrasion. A good solution is to specify the heater with some lead-wire protection. There is simple fiberglass sleeving (which will eventually wear away too), and stainless-steel braid or stainless-steel armor along with a few other options. The last two choices (the stainless-steel ones) will last longer. If the wire is not abrading against anything, the simple movement can lead to breakage in the actual wire from the repetitive movement. Specify a lead-wire length that is long enough for the wire to drape and hang as low as it can, preferably to a flat surface. Try to have the wire loop from the heater at least twice on that surface before traveling off to the power source.

Aside from contamination and damage caused by movement, one of the worst things for a heater is heat. That sounds counterintuitive, but if heat can be pulled away from a heater via conduction (the best method of heat transfer for most applications) then the heater will operate at a cooler temperature and last longer. Proper fitting holes for the cartridge heater, voltage control, and a safe ramp rate will keep the heater within safe operating temperatures.

We are here and happy to help with any technical questions related to our heaters and your application. We look forward to working with you.

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Increasing Conductive Heater Element Life

Does continuous operation shorten heater element life? It’s a common question, and frankly, the answer is yes. All heating wire will eventually fail over time, but it is possible to improve the life of the heater by following these recommendations. All heater types require a balance of cooling and voltage control to avoid overshoot failures. If a heater’s watt density is too high, the heater will quickly get too hot. It’s this accumulation of heat and the resulting oxidation of the internal components that ultimately lead to the death of a heater. The higher the temperature the higher the oxidation. Over time, the heater will fail. How much time depends on how well the temperature is controlled.

In the case of a conductive product, like a cartridge heater, if the heater is inserted into a tightly fitting hole in a large steel block and operated with a good voltage control and a moderately safe temperature of 300º or 400°F then the heater could last for many years.  In that instance, the heater has a low setpoint temperature and the low level of heat that is generated is sucked away quickly by the surrounding mass. Less temperature means less oxidation and longer heater life.

A wide range of variations is possible based on the application the heater operates in. Some heaters come back from the field operating for over 30 years while others can fail in weeks if not properly installed and controlled. In some cases, applications requiring higher temperatures may be detrimental to heater life but if high temperatures are required, the element life expectations should be shorter and a back up heater element kept in stock for quick field replacement.

Here are some quick pointers to help develop a conductive heating system for sustained performance:

Cooling – Conductive products use heatsinking to another mass to cool the element wire. It can be liquid, a tightly fitted metal mass, and air.

Voltage Control – Closed Loop control measuring process temperatures keep the heater running based on safe operating set points.

Ramp Rates – Managing the temperature increases and decreases through time avoids temperature overshoot. In combination with a closed-loop system, it’s an effective solution to increase heater life.

Process Temperatures – It’s important to understand process temperatures are not actually element temperatures. To get to a process temperature quickly, the element temperature must be very high to heat the surrounding parts, mass, air, or medium. A safe balancing act occurs when using a fast-acting temperature controller in a closed-loop system. Proper setup, thermocouple type, and thermocouple placement are essential.

We are here and happy to help with any technical questions related to our heaters and your application. We look forward to working with you. 

Ian Renwick
Director of Engineering
TUTCO, LLC

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