space heaters visibly glowing with normal current - is it normal / acceptable? 5

[electricpete]

We have an 8000hp vertical motor with six 480 volt 500w heaters (3000w total) in the lower bearing bracket area.

The current measures 3.5A in each leg as expected.

However the heaters are visibly glowing dark orange. I tried to take a picture and they came out more purple looking (both images are normal camera, NOT infrared camera).

Have you seen this before? Would you consider it normal / expected? Is it safe considering there is a nearby oil reservoir. (Google tells me things shouldn't visibly glow until they get to 900 degrees F).

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[TugboatEng]

It certainly is normal and safe for heating elements to glow. We've all use space heaters that glow in our homes. With that out there, I have never seen one glow in an enclosure. It kind of makes sense to do this in a large enclosure because radiant heating will help better control condensation. Otherwise, what is the current reading like to this heater? Maybe you should mechanically attach it to the enclosure better so that it conducts more heat to the frame. This might keep it from glowing, but the current consumption will also increase.

 

[itsmoked]

For starters those are NOT space heaters. Space heaters are for heating an air space like a floor heater in a bathroom. You know, a "house burner".

Those are Bar Heaters because, well, they're bar shaped.

As for glowing, think about it, think 500W incandescent light bulb, blinding bright white. However, that's because the 500W are spread across a tiny filament, here it's spread over a foot long bar. Glowing dully in a dark space can be expected. In fact they will get even brighter if they can't get rid of their heat somehow.

Last week a steel 35lb die holder suspended in the air 2" thick by 12" by 12" with four 500W cartridge heaters in it had two shorted to ground so they couldn't be controlled. It had that huge die holder bolted to steel on one side glowing in a not bright production facility.

As for your glowing bar heaters. I'll say they can do that. I'll also say they don't last years if they're doing that. I would not want them doing that because I wouldn't want to replace them frequently. You can stop them from doing that if you get them properly thermally connected to the wall. I believe they're not well connected. The wall has rusted? or corroded or deformed enough so the heater isn't fully coupling to it. That often means it's not actually heating the surface like someone wanted it to.

They often have simple end tabs that they bolt down with. I've seen many cases where further clamping was added by the user. This is especially important for ones longer than about 6 inches as they will deform away from the surface rather quickly.

This also seems to show that a bunch of the output of those heaters is going radiantly off the back into the air, close to 50% of the heat. You could probably use 250W bars properly connected to the surface for the same effect.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Never seen glowing space heaters inside the motors since they are a fire sparking hazard for contaminants like grease and oil. 3 KW seems a bit high for this motor. EASA recommended thumb rule is watts=2dl, d= stator OD and l=stator length in inches.

From your calculations, it looks like they are connected in 3 phase, which is odd since they are normally connected in single phase. I provide space heaters rated usually at 240 V with 2 of X KW rated heaters connected in series so that final output is X/2 KW. This ensures the surface heat density is well below the glow point.

Muthu

http://www.edison.co.in

 

[electricpete]

One piece of info I forgot to mention, we have 8 identical motors, all have the same physical size heaters with the same electrical power and this is the only one that visibly glows.

At first glance it sounds counterintuitive, how could they be different temperature if they are the same physical size and same power?!? (same watts per square inch).

I think Tugboat and itsmoked hit the key. These are not heaters mounted on standoffs which are designed to radiate all their heat into the air. They are mounted to a structural steel plate and if that mounting is solid then they conduct most of their heat to the steel. But if the mounting is not solid then there may be an airgap which interrupts that conduction path and the heater will be hotter (for the same power).

This also seems to show that a bunch of the output of those heaters is going radiantly off the back into the air, close to 50% of the heat.

What is it that you see in the photo which makes you say that?


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[itsmoked]

The entire heater is glowing. Half the radiation is going to be coming off of each side. If instead the heater was properly conducting into the metal it's bolted to far more of the heat would be exiting the heater thru the much, much more efficient conduction mechanism.

e.g. Because it's glowing I can say most the heat is leaving as radiation and around half is coming off the side away from the surface.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Are you sure the others have the elements connected in parallel to the supply? The others might be connected in a delta wiring configuration but with two 480V elements in series for each leg. That would drop the output of each element to 125W or the total output to 750W which would stop the glowing.

 

[Gr8blu]

electricpete: If you have a 3-ph 480V system powering six heaters (each rated 500 W - 240 V), connected with two heaters in series in each phase leg, the resulting current will be 3.6 A, and the measured line-to-line resistance after connection will be 153.6 ohm. In effect, you are applying full rated (240) volts to each heater element. That means the surface temperature of the heater will reach its normal maximum design temperature ... which is in the 650-750 C range for the "standard" element length. This is why hazardous locations often have what seem to be an abnormally high number of heaters connected in a fairly complex circuit - they are trying to reduce the watts-per-element to a reasonable range, thereby limiting the observed surface temperature.

If these same elements were misconnected to form a 3-PH wye with two elements in parallel for each phase leg, we'd be seeing about 4.8 A and a line-to-line resistance of roughly 115 ohms. But we'd be putting out 4 KW of heat (since we're applying 277 V effective to each heater), which will definitely drive the surface temperature up even further.

edison123 The applied circuit voltage is up to the end user. Some want to run off single phase supplies, others want to draw from a 3-ph connection. The motor manufacturer just builds the heater circuit to conform to the supplied data - like what is the terminal voltage, whether the surface temperature is an issue (which it is, for hazardous/classified locations), and how much physical space is available to mount the elements themselves. Elements come in many sizes and shapes, ranging from the standard "bar" version to finned to tubular, to plate, to silicon wrap/blanket. All have their pros and cons.

Converting energy to motion for more than half a century

 

[jraef]

I’m thinking the same thing as Lionel. Misconnected in delta after a PM service by someone who wasn’t paying attention. Check the connections against the others that are not glowing.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[electricpete]

e.g. Because it's glowing I can say most the heat is leaving as radiation and around half is coming off the side away from the surface.

Ok I understand what you were saying now.

If you have a 3-ph 480V system powering six heaters (each rated 500 W - 240 V), connected [in delta] with two heaters in series in each phase leg, the resulting current will be 3.6 A,

That is exactly what we have. Two 500-w 240V heaters in series per delta leg and we measure the expected current 3.5 or 3.6A. Photo of heater nameplate

And we measure in the ballpark of 3.6A on any phase for space heaters on all 8 motors, so I don't think there is any misconnection anywhere.

In effect, you are applying full rated (240) volts to each heater element. That means the surface temperature of the heater will reach its normal maximum design temperature ... which is in the 650-750 C range for the "standard" element length

I agree this configuration does not reduce the heater voltage in attempt to reduce heater temperature like many other applications do. This configuration is also mounted directly to structural steel which would certainly reduce heater temperature compared to a heater on standoffs. At any rate the one motor in question is the only one where the heaters glow (all 6 heaters on that motor) in spite of same current measured on heaters for all 8 motors and same heater type (same dimensions and nameplate) on heaters for all 8 motors. It suggests to me that there are no differences in electrical resistive heat generation but rather there are probably differences in the mounting and associated heat conduction characteristics like tugboat and itsmoked suggested.

Thanks for all the input so far. And sorry that I posted the info piecemeal (I hate when other people do that!).

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[zlatkodo]

Maybe some thermal paste is missing for better heat transfer.

 

[electricpete]

This was noticed immediately after the motor had been refurbished. We reburbish each of these motors about once every 10-14 years, and each time the space heaters are (proactively) replaced.

Maybe some thermal paste is missing for better heat transfer.

Yes thermal paste is a logical thought that I had been thinking about before. But...

But now I do have some new information. I was finally able to talk to someone at the shop which just refurbished this motor who was very familiar with the space heater replacement practices. He said no thermal compound is used, but the normal practice is to bend the heater into a gentle U shape (preferably bend against a large radius surface for smooth bend); then place the u-bent heater onto the steel so that only the center is touching; then tightening the mounting fasterners on each end of the heater will unbend that U (which helps assure good contact along the entire length). He suspects this step may not have been done on this motor. His recommendation is to deenergize, remove, bend, and reinstall the heater. Also inspect for smooth clean surface of the structural steel underneath during the process.

They often have simple end tabs that they bolt down with.

Yup, thats' what we have.

I've seen many cases where further clamping was added by the user. This is especially important for ones longer than about 6 inches as they will deform away from the surface rather quickly.

That fits very well with what the repair shop guy told me about prebending the heaters. Prebending or additional clamping seem like alternative ways to address the same basic concern.

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[3DDave]

A comment on the photo - many cameras have an IR cut filter for near IR light. Typical sensors are sensitive to near-IR, without that filter the light will be picked up in error and, in this case, produce that purplish glow. This sort of filter is put in place to remove near-IR from daylight exposure as many fabrics are somewhat transparent to near IR. I'm interested to know what camera that is.

 

[electricpete]

Interesting.

The photo was taken with a Nikon Coolpix either model L610 or model L620 (camera is currently at work, had to find the model from my amazon purchase history, I have previously purchased both models).

It's an old camera so I can believe it might be missing some things we have now.

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[che12345]

Dear Mr. electricpete (Electrical)(OP)27 Oct 22 19:35
"...The current measures 3.5A in each leg as expected....However the heaters are visibly glowing dark orange. ....Have you seen this before? Would you consider it normal / expected? Is it safe considering there is a nearby oil reservoir...."
There had been numerous learned advice but don't seems to resolved the observation/question " why only one is glowing. Would you consider it normal / expected? Is it safe considering .....?
1. In general, strip (space heaters) are usually designed with [surface temperature at about 70 [sup]o[/sup] C.] Note: the (heating element) is able to operate at a [much higher temperature; up to red hot (glowing) without burnout]. Attention: space heater operating with surface temperature above 70[sup]o[/sup]C shall be protected with enclosure; preventing it from a) accidently touch by personal and b) any combustible material that may come into contact with it ; e.g. loose grease or oil spill etc.
2. a) Cleaning the rust ensure good heat conduction/contact surface between the heater strip and the heating body is essential. b) At 70 [sup]o[/sup] C, thermal conduction grease is NOT essential. c) for long strips, additional clamping to ensure good heat conduction area is essential. d) refrain from bending the original straight heater trip into U-shape. You may? break the heating element or the mica insulation.
3. With the same (voltage, current, wattage and physical dimension); it is possible to have different [hearting element temperature]. The heat/wattage is governed by i[sup]2[/sup]R. Therefore, with V,A and W known, R is fixed. But the heating element R is governed by R=rho. l/A. where l is the length and A the cross-sectional area of the heating element. With fixed R, it is possible to design the heater element with different length or cross-sectional area. Note: the heating element temperature will differ, but retaining the same wattage. Note: the heater that "glows" is at a higher heating element temperature than the others which [remained at ambient temperature colour], but operating at the same wattage.
4. Check the heaters, they may be designed with same (V,A, W and physical dimension) but with different [heating element temperature]?.
Che Kuan Yau (Singapore)

 

[Gr8blu]

electricpete: Interesting to see the close-up on that heater element. It may be that the shop used a carbon steel sheath in place of a stainless sheath design. This ultimately means that the surface temperatures will be noticeably different (up to 150 C in some cases, depending on other factors). In general, the more exotic the sheath material, the higher the surface temperature is going to be (because the sheath can withstand it, more than anything else). I mentioned 650-750 C range in my earlier post: this was incorrect. What I should have said was the following: (Sheath Material // Max Surface Temp in F) Iron // 750. Monel // 900. Chrome Steel // 1200. Incoloy // 1500.

What this means is that a circuit designed to use a more exotic alloy (e.g., Incoloy) will be severely stressing a lesser alloy (e.g., iron or Monel).

Converting energy to motion for more than half a century

 

[edison123]

Presently, we are doing a refurbishment of a 11 KV, 15 MW, 1500 RPM flood damaged generator, where the stator weighs 18 tons and rotor weighs 9 tons. The space heater is only 2x1000 W, 240 V, single phase for the entire mass of 27 tons. The helical length of the heater ensures low surface heat density thus preventing any glows.

Muthu

http://www.edison.co.in

 

[macmckim]

ElectricPete

Have you measure the skin temperature of purple picture strip heaters - using a thermal gun and compared it to one of the other 7 units?
Depending on length of heater (500W) - skin temp can be up to around 200C

Mac

 

[stevenal]

Pete,

Have you checked the voltage balance across the series connection? A poor connection might shift the midpoint voltage so that one heater sees more than half the 480 and the other less.