Ceramic heaters

[frecossais]

Okay, this was a thread that was posted in the main forum. The guy has 8 heaters which are cabled to a control panel where they are switched on and off by contactors.

The control panel is supplied with a 3-phase supply from a local DB and each heater is protected individually by a 16A MCB.

His problem is that the heaters have been cracking because of the rapid temperature change during start up. He was thinking about installing dimmers in order to manually heat them up slower over several minutes by gradually turning the dimmer higher. Problem is that 16A dimmers will cost an arm and a leg so I suggested to him that it would be possible to install some extra contactors and a timer. The heaters would then be grouped in two's and each group would start wired in series so each heater only gets 115v and after several minutes the timer causes contactors to change the wiring so each heater is supplied individually again as they are now.

I though it might be a nice exercise for trainees to come up with a circuit for a control panel that could do this.





.....Anyway now I have a question of my own to pose concerning the heaters in my local village hall. They are ceramic heaters and and because they are not often used moisture builds up in them and sure and certain after a while when you go to switch them on at full load, because they are full of moisture then crack! either the breaker goes or in the worst case the ceramic cracks and a new heater is required.
Now I know there is such a thing as a soft start switch which would prevent the full load being whacked on at once and would gradually allow it to build up but I find them rather expensive so what I would like to know is can I fit a dimmer switch instead as long as it is able to carry the full load? .......

 

[telectrix]

why not fit timer/s so that they come on for an hour or so each day?

 

[frecossais]

I suppose I could do that but I would have to see with the community council to see if they would be happy with that, you know council cutbacks,etc. Thanks all the same.

 

[Marvo]

Running them for some time every day might be a hard sell in this day and age of saving the rain forests and expensive electricity.

How many kW are the heaters. You could use a dimmer but you're possibly getting into the realms of very large dimmers which will be expensive. Depending on the layout maybe you could could use something similar to a star/delta starter to first switch them on in groups of two in series connection so they only get half voltage, then after a few minutes it changes over so they're back to standard supply voltage. Just a thought, how easy this would be would depend on the existing switching arrangement, if they're not switched from a central location t might not be viable.

 

[guest119]

Or just replace them with something more suitable for the environment?

 

[Knobhead]

How many heating elements of xxKW are there?

There’s a way of building up the heat that we used on analytical ovens (1600°C). By connecting the elements in various series parallel networks the power is regulated during start up. But as Marvo said, if the feeds to the various elements aren’t taken to a central point it wouldn’t be viable.

The switching won’t be cheap but the system is reliable. It comes down to what the failures are costing.

 

[frecossais]

Hi guys, sorry I didn't get back to you last night as I was tired and went off to bed.So today I went and had a look at this job and it is a bit more involved than the guy who asked me thinks it is. Anyway I couldn't see the wattage of the heaters as they are all high up on the walls but there are 8 of them all individually protected by a 16A mcb. The control circuit has its own mcb and the supply side is fed from 3 3-phase contactors. I think that I would have to fit a switch for each of them and it would cost too much money as the village isn't a very big one and doesn't have a lot of money and I would be doing this as a favour anyway. I think that I'll wait until the heaters are at the end of their natural and suggest either another way of supplying them or tell them to stop buying ceramic heaters as they are liable to this kind of thing.(Funnily enough I've just had to change one at work a couple of months ago which had the same problem). Thanks again guys for all your replies, I really appreciate it.

 

[Marvo]

If all the heaters are individually supplied from one control panel and as long as all the neutrals are there as well then with a couple of contactors and a timer you could feasibly reconfigure them to start up in pairs supplied in series so they'd be at half voltage for a few minutes then change over to them being individually supplied after a few minutes. I'm sure this would be a lot cheaper than sourcing 16Amp dimmers and less complicated for the user as well.

 

[Marvo]

Okay trainees. Any thoughts to start with??????

 

[happysteve]

This reminds me of a cooker/hob switch I came across in my course notes:



Maybe the middle diagram isn't so relevant (unless you only wanted to put half the heaters on), but the top and bottom ones sound like what you're describing.

I'll just leave that dangling, I've been doing assignments all weekend and now I'm tired...

 

[Marvo]

Absolutely spot on Steve, I like the way you're thinking. The cooker switch is a very good illustration of what I was suggesting the OP does with contactors and as you say the middle diagram wouldn't apply in this case, only the first and last one.