A customer phoned in one day about an issue with two heaters. The two heaters were wired in series on a barrel. One of the heaters, however, was getting red hot and the other was not. He did not know why. Our first thoughts were that maybe the heaters were accidentally wired in parallel rather than in series. After checking with the customer, we found that the heaters were wired correctly. But, why was one of them getting so hot?
One heater was 610W at 230V. The other was 1260W at 230V. The customer applied 460V across these two heaters when they were wired in series.
Hint: to successfully wire two heaters in series, they must be equal wattage AND equal voltage, i.e. they must have equal resistance.
Now to see what happens when we calculate the resistance of each heater:
Now when we apply 460V across these two heaters that are wired in series the circuit looks like this:
Written by Jim Dixon and Shelby Reece Edited by Kyle Otte Date Published: 07.02.2019 Last Updated: 09.03.2019
A customer ordered a mica band configuration 528 that was 9.55 inches in diameter and 1.5 inches wide. The heater was rated at 2,000W, 480V. The customer asked that the post terminals be moved so an incorporated strap could be used.
The application required that the heater have 1/2″ nickel return ribbon and an extra layer of mica inside the heater. The recommended wattage was 1,200W, yet the customer ordered a 2,000W heater. The customer called in and mentioned that the heater glowed orange hot and he wasn’t sure why. Immediately, I knew this was because the heater was getting well over 1,000 degrees F. But, why? See the diagram below.
The first thing I suspected was watt density. Watt density is calculated as follows: Heated Width = 1-1/2″ – 1/4″ – 1/4″ = 1″ Heated Length = 29-1/4″ – 1-7/8″ = 27-3/8″ Heated Area = 1″ x 27-3/8″ = 27-3/8″ in2 Watt Density = 2,000W / 27-3/8 in2 = 73 Watts/in2
This is about twice what it should be! This is why it was orange hot, but what can we do to fix this issue? There are two options: A. Increase Heated Area B. Reduce Wattage This is how I handled it:
I asked the customer about possibly increasing width. The customer said we could go to 1-3/4″ wide. We also talked about possibly converting to a configuration 500 heater which would remove the 1-7/8″ circumference length used for the post terminals. This would also remove the 1/2″ nickel return ribbon and a layer of sandwich mica.
I, then, asked the customer how they came up with 2,000W for their quoted heater. The customer said that this was the wattage on a ceramic band heater they had used in that place beforehand. This did not line up, however, because ceramic band heaters start to fail around 40 Watts/in2.
My next question surrounded whether the customer was using PID control and he said yes. I asked him to measure the on-time and off-time and calculate the percentage of time the heater was on, using this formula: on-time / (on-time + off-time) He calculated 75%. I asked him if that was the exact calculated number or if he had read it off of the control panel. He, then, told me that he was able to set it to any percent he desired. That was my AHA!-moment. This was not PID Control, it was a percentage timer. He thought he was able to operate on even less! 75% of 2,000W is 1,500W.
After all of our discussion, the customer decided to try a similar-style heater with a 1-3/4″ width and 1,500W. The heated area would now be: Heated Width = 1-3/4″ – 1/4″ – 1/4″ = 1-1/4″ Heated Area = 1-1/4″ x 29-1/4″ = 35.56 in2 Watt Density = 1,500W / 36.56 in2 = 41 Watts/in2 This heater is much better than the first iteration of this heater. We provided the updated heater and the customer had no more problems.
Written by Jim Dixon Edited by Shelby Reece and Kyle Otte Date Published: 10.31.2019 Last Updated: 11.01.2019
A customer was having an issue with a stainless steel hose getting too hot when the heater was powered on. This happened when the heater, a new standard item, was being powered up for the first time. The customer stated that after 8 to 10 minutes, the hose temperature was getting up to 138°F.
The Details
This heater was one of four that Thermal Corporation built with stranded nickel lead wire. Now, the reason the leads on this heater were getting hotter was either because more heat was generated or less heat was removed. It was difficult for us to visualize how less heat would be removed from the leads. Therefore, it was more likely that more heat was being generated. The first question we asked was how much heat is normally supposed to be generated by the leads on this heater.
The Math Behind the Mystery
The heater was a 1,000W, 240V heater. Its lead length was about 100 inches. We measured the resistance of one lead using a bench model volt ohm meter. It measured 0.35Ω. Because there are two wires generating the heat, the total resistance would be twice that measurement.
2 x 0.35Ω = 0.7Ω I = 1000W / 120V = 8.33 Amp Heat(W) = I2R = (8.33)2 x 0.7Ω Heat = 48.6 Watts
The hose was approximately 3/8″ in diameter. For a 3/8″ diameter surface:
Surface Area = 0.375 x π x 100 = 118 in2 Watt Density = 48.6 / 118 = 0.41 Watts/in2
If we look at a combined radiation convection losses curve for 0.41 Watts/in2 under oxidized steel, the temperature should be at least 150°F. The hose, most likely, had more surface area than a 3/8″ diameter smooth surface. So, the actual temperature would be less than 150°F. So, the 138°F does not seem wrong.
A Solution
A possible solution is to lower the temperature by using a nickel clad copper wire. Nickel clad copper wire has less resistance than stranded nickel, so less heat would be generated.
The Mystery
But, the question still remains: why were the leads on the other heaters not hot? Perhaps the hose was touching something and conducting heat to another object? This seems like a likely explanation. All lead wires generate some heat, but often the lead wires are in a hot environment and the heat is not noticed. Or, sometimes the lead wires are short, thus they have a low resistance, and not enough heat is generated to be noticeable. But every once in a while a long lead wire length in a cooler environment will lead to this type of question.
Have a Problem with a Heating Application?
Let us troubleshoot the issue by contacting our engineering department!
A customer was having some issues with a set of cartridge heaters in a mold. The mold had two sides, each side contained four cartridge heaters. The technician would start the process and the heaters would all come on and begin heating the mold. Then, after about 45 minutes, the heaters would begin blowing the fuses. Once the fuse blew, then power would stop flowing and the heaters and mold would cool down. What was going on?
Leads and Fuses
I went to visit the customer and looked at their actual machine as well as some old heaters that had been taken out of the mold. The first thing I noticed was that the leads were black where they exited the heater. The second thing I noticed was that the heaters still had the correct resistance. So, the heaters were not burned out, the power was somehow returning to ground and blowing the fuses (which is the equivalent of tripping the breaker).
The Explanation
This is what was happening: the mold was getting hot and because the heaters were not long enough to stick completely out of the mold, the fiberglass leads were getting hot and the insulation was breaking down. The particular leads we use at Thermal Corporation are rated at 600V, but as the fiberglass insulation breaks down, it is not a stretch to think that 480V could jump to ground through the broken down fiberglass.
Silicone BootEx-Flex
Another issue is that the heaters had a silicone boot on each lead end as well as ex-flex. In the first picture, the silicone boot is the shiny black material over the leads. Silicone boot is used as a protector for the leads and is rated at 500°F. Ex-flex is a high-temperature fiberglass product that is shown in the second picture as the white sleeving. It is rated for a similar temperature as the silicone boot. While these additions can certainly protect the leads from damage, especially from mechanical or cyclic wear, in this particular situation they were really acting as a thin layer of insulation inside the mold and causing the leads to hold more heat. This was also contributing to the breaking down of the fiberglass and causing the leads to arc.
Solution to the Problem
The solution was to add a 1-1/2″ long cold section to onto the lead end of the cartridge heaters. This got the leads completely out of the mold and out into the cooler air. This stopped the leads from breaking down due to heat and kept the fuses from blowing.
Have a question about a heating application?
Contact our engineers for advice! Our engineers excel at problem-solving heating application issues. Don’t hesitate to shoot us an email (engineering@thermalcorp.com) or give us a call ((800) 633-2962) and we will be happy to answer any questions you are having about any industrial heating application.
Written by Kyle Otte Date Published: 10.10.2019 Last Updated: 10.10.2019
We had a customer once that had been using a strip heater, similar to our configuration 022, in a foil stamping process. The heater measured 6” long, 1-1/8” wide, 150W at 120V, and had two holes in it— each 1/4” in diameter spaced 4” apart on the centerline of the heater. The heater fit in a 10” x 2-3/4” space. The customer asked for a quote from us and when we quoted it, our configurator alerted us with a note warning high watt density. The customer ordered 4 pieces of this heater. He thought these heaters would work for his process that required it to reach 400°F; however, the heaters only allowed his process to reach 300°F.
What Happened?
Both the original heaters and the newer heaters were rated for 150W at 120V. What was the difference that caused this result? The Thermal Corporation tech sheet had recommended a wattage of 115W.
The industry standard for strip heaters specifies the wattage tolerance as +5, -10%. The tolerance does not specify as ± x% because from a safety standpoint it is safer to have a lower limit on how hot the heater can get. +10% of a specified wattage will often lead to premature heater failure and can cause damage to the customer’s process.
Wattage is determined using the following formula:
Considering the bi-lateral tolerance on wattage, it is possible to get a 15% variance in wattage between the two heaters and both could be in spec.
This is usually not a problem because, under normal circumstances, the heaters would be within a control loop. The temperature controller adjusts the amount of heat provided to produce the desired temperature.
To measure wattage it is easier to measure the resistance, BUT you must remember a couple of things:
(1) When first heating up the resistance of the heater will increase about 4% or so. (Heating anneals the wire/ribbon removing the work hardening which was added to the wire in the drawing and forming process.)
(2) When the heater cools down, the resistance will be about 2% higher than before it was initially heated up.
This is how the 4% breaks down: 2% is due to removing the work hardening.
The other 2% is due to the resistance of the NiCr wire increasing with temperature.
Look at it this way…
Resistance happens when the current (electrons) run into other electrons. If the temperature rises, then non-current electrons are moving around more than before and are sort of “getting in the way” of the current electrons to a slight degree. The higher the temperature is, the more the non-current electrons get in the way.
We, at Thermal Corporation, design our heaters to produce the target wattage (and resistance) when the heater is hot. NOT ALL MANUFACTURERS DO THIS!
What about the high-watt density warning? This warning means the heater could result in decreased heater life. Could the heater be made larger? Could we also raise the wattage, if desired?
One of our engineers, Kyle Otte, has an answer to this: “Yes, making the heater larger will allow for more total wattage while also lowering the watt density thus removing the high-watt density alert. This will yield more even heating over the entire space as well as more total energy into the process with the added benefit of greater heater life.”
Written by Jim Dixon and Shelby Reece Edited by Kyle Otte Date Published: 06.07.2019 Last Updated: 09.03.2019
