Unusual Transformation/Transformer Needed 1

[Laplacian]

We have an application with an 81KW heater bank controlled by a 480V 3-phase SCR controller. Each heater probe is 240V, 750W, 1-phase. The probes are connected in series-delta; 2 probes in series per delta leg, 18 parallel legs for 108 total probes.

We are experiencing several failure modes with these heaters. It is not feasible to change probe type or voltage at this point; 81KW is absolutely required.

Fuses blow frequently; each leg of each delta is fused. One fuse removes 50% of the KW from the respective delta leg. I am seeking to fuse each probe independently for both ease in troubleshooting and process reliability.

My idea was to install a step down transformer at the heater location to 240V l-l versus the existing 480V l-l so all probes can be paralleled and fused independently. I wanted to go with a delta-wye transformation as there are no delta-delta transformers in our plant (the mere thought would bring grievances from the electricians).

I began searching for a multi-tapped 3-phase transformer to no avail. I then searched for single phase multi-tapped transformers to connect in a bank. I need 139V l-g on the wye side to get my nominal 240V l-l heater probe voltage. The best I could find was a Sola transformer that would deliver 133V @ 480V input.

Is it possible to get a custom transformer to yield 480V delta - 139V wye? I'm used to dealing with oil filled transformer manufacturers, not small dry types. Any custom dry type manufacturers out there?

 

[itsmoked]

Laplacian; How can you sustain any blown fuses in your process or afford regular fuse replacement? Rather than re-fuse why not figure out why you are getting shorts/blown fuses? Do yo know why? I mess with heaters all the time and I just have the feeling you are going in the wrong direction(quickly).

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Before I make any transformer suggestions let me say that I agree with itsmoked.
The first thing I would check is the fuses. Are you using renewable fuse links? I found out decades ago that renewable fuse cartriges have to be renewable because they are so undependable. They blow so often that you are convinced that you can't afford one time fuses. You can afford one time fuses because the one time fuses only blow when they should. If good quality one time fuses are blowing, check your fuseholders and wire connections for any sign of heating. Cheap fuse holders or fuse holders bent out of shape will quickly overheat the ends of the fuses and cause fuse failure.
I suggest new, good quality fuse holders with dual element fuses. That should solve your fuse problems.
Better yet replace the fuses with a good quality circuit breaker.
Transformers and individual fusing.
When I think about an enclosure to contain 108 fuses, or 216 if you fuse both legs and the engineering involved to ensure adequate ventilation, my mouth waters. It sounds like a lucrative contract for someone or a long time in the shop if your own people do it.

Fuses. Forget fuses. Use a 480 volt/200 amp 42 cct panel and put 3 or 4 heaters on each breaker.

Transformers
Option #1 ask the heater manufacturer about the posibility of running the heaters at 277 volts and use a standard 480 delta to 480/277 wye transformer. Some heaters have a low enough watt-per-unit-area density that they take overvoltage easily. Your input current will be more unless you drop off a few elements to keep the load at 81 KW.

Option #2 Use 3 x 37.5 KVA dry types rated 480:240 volts.
Connect them 480 volt delta primary, 240/416 volt wye secondary.
Connect the heaters line to neutral on 240 volts.

Option #3 Use a 480 to 120/208 volt transformer. Add three 120:24 volt boost transformers in autotransformer configuration to boost the voltage to 244 Volts. Don't worry about the extra 5 volts. It's only 3.6% and the voltage drops of the transformers will use that up. You may still have to go up a tap on the main transformer to maintain 139 volts at full load.

I like option 2 the best. I would ask break the good news to the plant electricians that the delta connections were being changed to grounded wye and ask for their suggestions for an appropriate flourescent orange warning decal to identify the 416 volt panel.
yours

 

[dpc]

http://www.hammondpowersolutions.com/specialty.html

http://www.uptegraff.com/

You can get a special transformer built - dry type should not be a problem. A couple of options might be Hammond and Uptegraff.

 

[Laplacian]

Thank you gentlemen for your suggestions and advice. If only it were so simple as just making the equipment work as designed. In this application, process is the top priority. The material being heated (catalyst) is worth up to 10^6 dollars per batch. The process must be repeatable and all parameters must be the same for each batch. This process has evolved over 30 years and is perfected to make top tier quality product.

Unfortunately, not much thought really went into heater design during the evolution process. They are in an application that pushes their metallurgy to the limits given the type elements they are (nichrome with incoloy 800 sheaths). Probe temperatures see over 2000F; process is 1600F. The process temperature must reach certain temperature points within precise time constraints and be held at various points along the way.

The heaters suffer moisture ingress from the humid environment. An adequate purge can't be placed on the heater termination junction box without flowing massive volumes of instrument air due to the clearances required between the probe entry nozzles into the vessel. There is a lot of movement during heating. This moisture causes many blown fuses initially, but they can be replaced and usually hold prior to the next batch cycle.

The other issue is failure from overheating on random probes. My suspicion is that it is partially from resistance drift at very high temperatures causing an unequal voltage distribution across the series pairs.

There has been a history of proposed solutions, but the risks of implementing any corrections outweighs the benefit of increased heater reliability. This is just now getting major attention as temperatures are being pushed up for experimental runs. This is bottlenecking plant production and costing big $$$. The vessels and all associated equipment are constructed of incoloy 800.

Another issue is that the probes are inserted horizontally (each probe is 48" long) and are unsupported on one end. The heaters sag down at extended 100% output periods which cracks the magnisium oxide insultation resulting in resistance wire to sheath short circuits. This is unfortunately something that operations has chosen to live with because heaters are cheap compared to the product they activate.

I can justify almost $100K to make heater connection/configuration improvements. The constraints are that the controller must stay the same, the heater probes must stay the same, and KW must stay the same. There are 5 of these installations on site with common spares. I am only making modifications to one unit for evaluation.

The existing fuse type is a one-time dual element fuse for each set of probes. When 1 fuse blows, 2.78% of total heater output is lost. 5 blown fuses results in pushing the remaining heater probes to the point of melt down. Sometimes, the probe fuses race with the SCR fuses in the heater controller resulting in total heater shutdown. This would be another added benefit of a transformer (more line impedance to reduce the fault current seen by the SCR fuses).

The probe fuses are installed in the junction box for the heaters; during the process online fuse checks and replacements can't be made for safety reasons. I'm adding an auxiliary junction box to install all fuses or circuit breakers away from the vessel so online replacements or resets can be made. I want to protect each probe seperately to reduce the effect of one blown fuse to 0.9% of total heater output.

waross:

The option 2 transformer you mention; that would be a wye connected heater load right? I wanted to stay delta for reducing line-ground potential to reduce the stress on the MgO insulation.

 

[waross]

Hello laplacian.
Yes the heater load would be connected wye.
The advantage is that the scheme uses standard, readily available transformers.
Lets go option #4
Use three 240:480 volt transformers in auto transformer wye configuration.
Advantages; Standard transformers,
Wye connection,
240 volts delta available.
Dis-advantages;
The transformers will be over-sized/under-utilized. The 240 volt windings will only see 277/2 = 138.5 volts. Size the transformer on rated current rather than on KVA. The supply must be a grounded system or the transformer (and the SCR controller) will act as a grounding transformer for the whole system.
Two pole breakers or two fuses should be used.
In regards to probes overheating,

Another issue is that the probes are inserted horizontally (each probe is 48" long) and are unsupported on one end. The heaters sag down at extended 100% output periods which cracks the magnisium oxide insultation resulting in resistance wire to sheath short circuits. This is unfortunately something that operations has chosen to live with because heaters are cheap compared to the product they activate.

This is probably causing this

The other issue is failure from overheating on random probes.

A resistance to sheath short circuit near the probe to probe connection will result in as much as 277 volts across the probes.
It's been a long time since I have worked with Magnesium oxide, but my impressions were that moisture related failure was not particularly voltage dependant. A damp cable would fail. It may take hours or days, depending on the amount of moisture. The voltage may have accelerated the time to failure but if it was damp, it would fail.
I agree with your assesment of the moisture problem. I would suggest another low voltage transformer to provide a warming current to dry the probes. Your choice will be a low level current to keep the probes dry, or a higher level current for a number of hours to dry the probes before use.
I would not use the SCR control for drying because we want to avoid the voltage spikes on a possibly damp probe.
I suspect that voltage stress with a wye connection will not be a problem. The good news is you can size the transformers for the 139/240 scheme but connect them as a 240/416 system. When we get away from theory to practice we may find that the closest standard transformer size is the same for both options.
You may want to consider din rail mounted IEC breakers with trip alarm contacts.

Don't forget dpc's links to transformer manufacturers. You may be able to get the transformer you want at a better price than an off the shelf transformer which may have to be oversized. eg; Standard 25KVA transformers will provide only 75 KVA. The next standard size is 37.5 for a capacity of 112.5 KVA. That's over 30 KVA excess capacity. The difference may give a custom transformer a price advantage.

Thank you for your detailled description of your process.
respectfully

 

[Laplacian]

I just received a quote for an Olsun encapsulated 112.5KVA unit for $5500. 480V delta - 240Y/139.

Thanks for the considerable detail in your last post waross. The heater manufacturer has had limited involvement in this issue. They basically throw their arms up when they fully understand the application and stress placed on the probes. Plus, I don't think they mind selling us $200K worth of heater probes per year.

We've discussed with operations the option to bake out the heater probes before running each batch. The heater controller has the option built in. Operations response: "We don't have time for that"

I have a request for quote out to Uptegraff for the transformer also. I have had good experience with them on substation transformers.

Our 480V system is solidly grounded at the X0 bushing of the transformer wye secondaries that feed the MCC's where the heater controllers are connected.

It's tough to engineer a way around operator and operations negligence, but this plant has that philosophy.

 

[BJC]

How well matched are the heaters? Are they pretty close in resistance over their entire operating range?
Are the "pairs" of heaters located where they are in the same ambient heat? Do you have the heaters logged and a record of which ones fail and how often? Have you recorded current on any of the heaters? Does the current creep up or do they just blow? Ditto with recording the temperature in the fuse boxes? Log the temperature and humidity as well. If the process is shut down during rainy damp weather do you get more blown fuses on the next start up?

"The heaters suffer moisture ingress from the humid environment. An adequate purge can't be placed on the heater termination junction box without flowing massive volumes of instrument air due to the clearances required between the probe entry nozzles into the vessel. There is a lot of movement during heating. This moisture causes many blown fuses initially, but they can be replaced and usually hold prior to the next batch cycle."

Why does it need to be insturment air? Fuses installed in a hot location have different T-C charesterics.
In addition to cooling fuses during the process you may need to heat them during off times. If the heaters are not sealed they may absorb moisture during cooling. I would suspect this is the case as your getting fuses to blow on start-up and when the fuse is replaced the heater functions?
FWIW I don't think much of wiring things in series that were designed to run at a constant voltage. Try wiring a 60 and a 150 watt light bulb up in series and running them at 240 volts.
Who makes the heater and have you asked them for help? Long ago I use to et really good help from Chromalox. they probably make everything in China and just salemen working in this country?
I would avoid a custom transformer, when goes south on you you better have a spare or have a standard transformer that is quickly replaceable.
In other words make sure you know enough to solve your problem not a symptom.

 

[waross]

I suggest a "check'" price on 3 x 50 KVA 240/480:120/240 volt transformers. They can be wired as wye:wye secondary and after derating will have a combined rating of 87 KVA.
If you can use an autotransformer configuration you can use 3 x 25 KVA transformers.
If the operators won't pre-dry the heaters, consider an automatic scheme to heep a low voltage on the heaters at all times. The current to keep the heaters dry will be a lot less than the current required to dry them.
BJC, "Why does it need to be insturment air?" Instrument air is typically dried to a dew point of -30 or -50. F or C take your pick at those temperatures.
Plant air in an old plant is often quite wet, and can sometimes even "Burp" raw water. Some plants have lubricant added to plant air.
respectfully

 

[BJC]

I wouldn't use plant air. I would consider a seperate cooling system of some kind.

 

[Laplacian]

BJC,

All the suggestions you've made have been done in the 1980's and again in the mid 1990's. There is not much instrumentation to measure online operating characteristics of the individual heater probes. As I said earlier, there is more than one failure mode taking out the heaters. One is transient and the rest are permanent.

At one time, temperature in the fuse/probe termination boxes was quite high due to poor insulation between the vessel and the box. Presently, temperature gets up to 110F in the termination box.

The dew point of our plant instrument air is around -135F nominally.

In addition to two probes being in series; each probe is configured internally as 4 nichrome resistance elements in series. That makes 8 resistance elements in series with 2 probes.

The resistance varies up to 20 ohms between probes while cold (new they have a variance of a few ohms). I mentioned in an earlier post about the lack of mfg support once they learn the application (It's not chromalox btw). Can't make online checks presently for safety reasons. Once I relocate the fuses to an aux junction box away from the vessel, then any online measurement can be performed.

I will go with the custom transformer and buy a spare. This will more than likely be duplicated on the other 4 activators.

 

[rbulsara]

I haven't read evey responses in every detail above..but itsmoke and waross appear on right track.

First up, if you need 81kW minimum, you need to install more than (108) 750W heaters. 750W*108 equals 81kW exactly. Yoy need to install something like 25% more capacity heaters and not run them at full rating. What good is your SCR control when they have to run full blast all the time.

Is heat itself causing fuses to overheat and causing them to blow prematurely? After all fuses are thermal devices.

Are fuses removed from the heat??

 

[itsmoked]

Yep! Too high a watt density for your heaters... Which ultimately is thrashing them.

Punch holes and add 20 more if you can.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Laplacian]

We know the heaters are undersized for the present application. As the years have progressed, temperatures have increased from 1200F up to 1600F. The heaters didn't increase in size. In addition to the heater probes in the vessel, there is a pre-heater of 65KW that heats instrument air into the vessel (up to 1100F). This heated air fluidizes the bed. The velocity of the air can't increase as this would fracture the bed particles. We are looking to add an intermediate preheater to further increase temperature of air going in, but are not sure if the dry air will get enough heat to the bed. Main vessel modifications are not feasible at this point due to the downtime required to make them. A new vessel would be in the order of $15M which is really what we need to avoid production issues.

This is the whole reason for individually fusing each probe. With all probes in service, performance is marginal. One fuse takes out too much power and is not caught soon enough. Like I said, sometimes the fuse holds and sometimes it doesn't. If the fuse does hold and can be replaced online this would reduce stress (a little) on the remaining heater probes.

I do appreciate all of your input though. Many of your suggestions have come up in the past. There is just too much bureaucracy to do anything about it.

 

[rbulsara]

What are the fuses sized at? for what load?

 

[itsmoked]

How about monitoring the fuses electrically so you essentially have a display covered with LEDs that show at a glance which blew?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Is your product electrically conductive?
If so, you may be able to use electrodes to heat the product. I used to service a glass bottle making plant. The glass furnaces were heated with gas, but there were electrodes that used the resistance of the molten glass to generate heat and increase the capacity of the furnaces. Later, I worked in a plant where molten glass was spun into fiber-glass for building insulation. The sole source of heat was electric resistance heating. They brought in a crew of specialists with truckloads of portable gas heaters to melt the initial charge, and then kept the process going with electric resistance heat only.
Todays trivia;
One of the most interesting valves that I have ever seen is the one used to control the flow of molten glass into the fiber spinning machines. A resistance element was cast into a disk of platinum. The disk was about 3/4" thick and about 7" in diameter. It had a hole about 1 1/2" diameter through which the molten glass flowed. The temperature of the disk could be controlled, and as it was allowed to cool, the glass would "Freeze" around the edges of the hole and slow down the flow. Control response was slow but adequate. With the aid of an air hose to help the cooling they could cool them enough to stop the flow completely if a spinning machine had to be removed for service.
And yes, molten glass does conduct electricity quite well.
respectfully