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watt density on heater

What is Watt Density and How is it Related to Heater Life?

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Home > Blog Archive > Category: Industrial Heating Technology > What Is Watt Density and How Is It Related to Heater Life?

Watt Density: Basic Definition

Watt density is a measure of the rate of heat being transferred through the surface of the heater.

For example, if you were to draw a 1″ x 1″ square on the surface of the heater, how much power (or heat) would pass through that area? The answer is the heat per unit area, so basically the watts per square inch.

So, How Is it Related to Heater Life?

Basically, the higher the watt density, the higher the temperature is inside the heater (assuming all other variables are constant). As the temperature inside the heater increases, the materials inside the heater are operating closer to their breaking point. This results in shorter heater life.

Mica Band Heaters

For satisfactory heater life of mica band heaters, the following density should be maintained:

Heater DiameterWatt Density
Less than 3"Less than 50 watts/square inch
Between 3" and 6"Less than 40 watt/square inch
Between 6" and 10"Less than 35 watts/square inch

Cartridge Heaters

For satisfactory heater life of cartridge heaters, the density should be less than 200 watts/square inch.

Ceramic Band Heaters

For satisfactory heater life of ceramic band heaters, the density should be less than 35 watts/square inch.

Higher operating temperatures require a lower density for equivalent temperatures inside the heater and equivalent heater life.

watt density of a Thermal Corporation ceramic band heater is less than 35 watts per square inch

Written by Jennifer Taylor
Edited by Shelby Reece
Date Published: 09.25.2018
Last Updated: 09.06.2019

photo of the 3 phase option on a Thermal Corporation mica band heater

Tips for Extending the Life of Band Heaters

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Home > Blog Archive > Category: Industrial Heating Technology > Tips for Extending the Life of Band Heaters

image of a mica band heater

#1 Avoiding Contamination

Both mica band heaters and ceramic band heaters are vulnerable to contamination.

  • Moisture is one of the most immediately noticeable contaminants. If sufficient moisture is present at start-up, the heater fails as soon as power is applied. Sometimes, when the heater has been cool for a prolonged period and has been subject to a high humidity environment, there will be enough moisture absorbed to cause failure at start-up. When insulated lead wire has gotten wet near the heater, the moisture can be wicked into the heater and cause failure. Anytime heaters fail as soon as the power is supplied, moisture is a strong suspect as the culprit.
  • Oil and other organics can cause heater failure if they get inside the metal sheath, but they behave differently than water. Most oils and organics do not conduct electricity very well as lower temperatures, but carbonize and become conductors at elevated temperatures. Thus, if such contaminants are present, the failure occurs only after the heater has gotten hot enough to cause carbonization. Like water, oil that gets on the lead wire insulation can be wicked into the heater.

#2 Avoiding Over-Temperature

  • A loose fit of the band heater on its cylinder will reduce the heater’s lifetime because the heat generated is not transferred efficiently to the cylinder being heated, causing the heater to run at a higher temperature to transfer its energy. The higher the operating temperature, the shorter the lifetime. It is good practice to retighten the clamping strap or clamping tabs after the first time that the heater has been used, because the heater will be better conformed to the cylinder during its first heat-up. Anything that prevents a smooth fit of the heater on the cylinder, such as a cylinder surface irregularity, will shorten the life of the heater because the net effect of the irregularity is to make a “loose fit” in spots.
  • Choose the lowest wattage heater than will maintain the desired operating temperature of the cylinder being heated and still provide a short enough start-up time. Choosing a heater with higher wattage than required will result in the controller turning the heater on and off to maintain the desired temperature and a higher operating temperature during on-time. These conditions will shorten the heater life.

#3 Excessive Cycling

  • The way that Thermal Corporation evaluates the lifetime of their own band heaters and those from competitors is to cycle the units from 150°F to 900°F and count the cycles to failure. Cycling reduces lifetime because the surface of the element wire oxidizes rapidly at higher temperatures; if the higher temperature is maintained, the oxide coating actually protects the wire from further rapid oxidation, but if the wire temperature is reduced substantially, the oxide coating breaks off due to contraction and exposes fresh metal to more oxidation. With continuous cycling, the wire diameter is eventually reduced, and the resistance of the element is increased to the point that it becomes too hot. At that point, the element wire either melts and breaks open the circuit, or it causes the insulation over the wire to break down, causing a short to the sheath.

#4 Physical Abuse

  • Never carry a band heater by the lead wires. If the heater has post terminals, use two wrenches; one to hold the bottom nut and one to do the tightening on the top nut.

Edited by Shelby Reece
Date Published: 07.05.2018
Last Updated: 09.06.2019

photo of Thermal Corporation cartridge heater

Tips for Extending the Life of Cartridge Heaters

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Home > Blog Archive > Category: Industrial Heating Technology > Tips for Extending the Life of Cartridge Heaters

Listed below are some tips for extending the life of cartridge heaters.

#1 Avoiding Contamination

photo of Thermal Corporation ease aid

  • When using a release agent, such as Thermal Corporation’s Ease-Aid, to ease heater removal from its hold, be sure to wait until agent is bone dry before inserting the heater into the hole. If the heater is inserted before the agent is dry, some of the liquid will usually be pushed towards the lead end and then will soak into the heater through the lead insulation or through the lava or ceramic plug at that end. When that happens, the heater can be expected to fail as soon as the power is applied. (Choosing the lead option of Teflon wire insulation and a Teflon plug can reduce the likelihood of water entering the heater, but this does not provide a fully hermetic seal, and the lead end of the heater must be restricted to 400°F).
  • Moisture, oil, and other liquids on the lead wires can be wicked into the heater and cause early failure.
  • Oil or other organic material on the lead end cap of the heater will carbonize at elevated temperature, causing a short from the leads to the sheath.

#2 Avoiding Over-Temperature

  • A loose fit of the cartridge heater in its hole will reduce the heater’s life time because the heat generated is not transferred efficiently to the object or material being heated, causing the heater to run at a higher temperature to transfer its energy. The higher the operating temperature, the shorter the lifetime. A rule-of-thumb for the fit is to make the hole diameter no more than .005 inches greater than the diameter of the heater.

    • Thermal Corporation cartridge heater

  • Choose the lowest wattage heater that will maintain the desired operating temperature of the part being heated and still provide a short enough start-up time. Choosing a heater with a higher wattage than required will result in the controller turning the heater on and off to maintain the desired temperature and a higher operating temperature during on-time. These conditions will shorten the heater life.
  • If used as an immersion heater, the type of fluid and its velocity passing over the heater are important factors. Ask for assistance from a Thermal Applications Engineer.

#3 Excessive Cycling

  • The way that Thermal Corporation evaluates the lifetime of their own cartridge heaters and those from competitors is to cycle the units from 150°F to 1,400°F and count the cycles to failure. Cycling reduces lifetime because the surface of the element wire oxidizes rapidly at higher temperatures; if the higher temperature is maintained, the oxide coating actually protects the wire from further oxidation, but if the wire temperature is reduced substantially, the oxide coating breaks off due to contraction and exposes fresh metal to more oxidation. With continuous cycling, the wire diameter is eventually reduced, and the resistance of the element is increased to the point that it becomes too hot. At that point, the element wire either melts and breaks open the circuit, or it causes the insulation over the wire to break down, causing a short to the sheath.

Edited by Shelby Reece
Date Published: 07.03.2018
Last Updated: 09.06.2019

photo showing a close up of the 3 phase option on a Thermal Corporation mica band heater

What is Wrong with this Band Heater Request?

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Home > Blog Archive > Category: Industrial Heating Technology > What Is Wrong with This Band Heater Request?

close-up image of three phase option on mica band heater

The Band Heater Request:

4.5″ Diameter x 1.5″ Wide, 480V/3 Phase, 700 Watts

Introduction

Virtually all industrial power is generated as 3 Phase power. At the generating plant, it is generated as three phase power, because it is MUCH more economical to generate power as three phase. The power is then transmitted as three phase power. Just look at power transmission lines. You always see the lines in sets of three. Sometimes the heaters use all 3 phase voltages and sometimes they use one phase of the power. A single phase heater can run off of one of the phases of the 3 Phase power, connected either between two of the phases or between one of the phases and a neutral return path (0 Volts).

Why would you want to build the band heater as a 3 Phase heater?

You want to build a band heater as a 3 Phase heater to reduce the current in the wires going to the heater. When you carry the power through three wires rather than the same power through two wires, the current is less. Why do you want the current to be less? Higher current requires larger wire which costs more. Also, failures frequently occur where connections are made and the higher the current in a connection, the greater the likelihood of a failure. Some of these connections are inside the heater.

Generally, you can start considering making the band heater a three phase heater when the total wattage gets in the range of 2000 watts. You probably should make the band heater a three phase heater when the wattage reaches 5000 watts and almost definitely be a three phase when the heater reaches 10,000 watts.

So, what happened?

What happens sometimes is that someone is told that the heater will be running off of three phase power. That does not necessarily mean the heater has to be a three phase heater. All single phase power comes from three phase power. The question is how much is the current or wattage? In the heater we are talking about, the current is 700 W/480 v = 0.69 amps. Thus, even connecting the heater as a single phase heater, the current is 0.69 Amperes. This is just a little more than a 60 watt light bulb. Making this band heater a three phase heater would make the current even less. A three phase heater is more complex to build than a single phase heater. Thus, there are more things that can go wrong and it cost more. Why choose a heater that is more expensive and less reliable?

In Conclusion…

All power starts out as three phase power. Just because the power source is three phase does not mean the heater has to be a three phase heater. To answer the question, does the heater need to be three phase, look at the current or wattage. Don’t order a heater as a three phase unless it really needs to be one.

Written by Jim Dixon
Edited by Shelby Reece
Date Published: 10.30.2017
Last Updated: 09.06.2019

Why Higher Voltage / Lower Wattage Parameters Do NOT Make Good Industrial Heaters

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Home > Blog Archive > Category: Industrial Heating Technology > Why Higher Voltage/Lower Wattage Parameters Do NOT Make Good Industrial Heaters

The Problem

Occasionally, customers specify industrial heaters that have a higher voltage such as 480 volts in combination with a wattage of only several hundred watts. This is sometimes done because 480 volts is readily available or sometimes because it may seem that this combination would produce good heater life. In fact, this frequently produces a heater that has rather poor life.

But, Why?

The wattage of industrial heaters is the product of the voltage (E) multiplied by the current (I). Thus: W = E x I.

If for a particular heater, the voltage is on the high side, to have medium or low wattage requires that the current (I) be rather low. Now, to get a low current for a particular voltage, we need typically a higher resistance. Higher resistance is obtained either by using longer (more) element wire or by using wire that has a high amount of resistance (ohms) per foot. Usually, there is a limit on the amount of wire you can put in a heater unless it is a large heater, so in most cases we must use a wire with high ohms per foot. The higher the ohms/foot, the smaller the diameter of the element wire. Thus, most of the time we end up using wire that is very fine (small diameter). Perhaps, a wire that is finer than the hair on someone’s head.

Sometimes the wire size required is smaller than any wire we have in stock.

One of the Main Ways Industrial Heaters Fail

One of the main ways industrial heaters fail in manufacturing processes, like injection molding, is for the element (heater) wire to oxidize. As the wire oxidizes, the outside of the wire becomes a nonconductive oxide and the current is carried by the smaller diameter of unoxidized metal on the inside of the wire. All nichrome heater element wire goes through this aging process. But, if we start with a very small diameter wire, it does not take as long for the element wire to oxidize and burn into.

Thus, in most cases, the combination of higher voltage and lower wattage results in a heater that has a rather short life. Sometimes, we may try to improve industrial heater life by lowering the watt density or lowering the wattage, where the real problem is that the element wire is just too small. In general, if the wattage is lower than the voltage you are probably getting into an area where the heater life is being compromised.

So, What is the Alternative?

Usually, the best alternative is to locate a lower voltage such as 240 volts or 120 volts. Sometimes, this can be achieved by putting two heaters in series, such as two 240 heaters of equal wattage across 480 volts. Sometime with 480 volts, 277 volts is also available or with 240 volts, 138 volts may be available. (Line-to-neutral voltage from a 3 phase line-to-line supply voltage).

Written by Jennifer Taylor
Edited by Shelby Reece
Date Published: 06.09.2014
Last Updated: 09.06.2019