Resurrection of a CFEI 100 KVA Induction Furnace

[David Jupp]

You need to work out the overall heat transfer co-efficient for the heat exchanger - I did some basic Chemical Engineering on a course years back.  All the equations are probably available on-line...  but not sure you'll have all the data.

It's a bit of a 'chicken and egg' situation though as the OHTC is influenced by flow rates in both circuits, any fouling, boundary layers, etc.  The exchanger manufacturer should be able to provide some guidance.

First thing is do you know the heat load to be dealt with? 

You're going to end up with quite a few estimated values - and of course the water flows will change when you connect things together.

[awemawson]

David,

I need to remove an estimated 40 kW of heat from the primary circuit. The primary circuit is water (with a bit of glycol) that will be pump circulated at approx  25 litres / minute. I don't want the primary to go above 30 Centigrade. The secondary will be fed with water at (say) 10-15 degrees C at 14 litres / minute.

But the unknown of course is the heat exchanger and it's characteristics as I 'ain't got one yet !

I have a feeling that to err on the large size is sensible, and throttle the secondary cooling water if need be - if indeed 14 litre / min at 15 Degrees is sufficient 

I suppose if one assumed 100% H.E. efficiency and the cooling water rising from 15 to 30 degrees at the 14 litre / min rate how much energy does that take?

Now I found this 'handy rule' on the web:

Handy rule: multiply the hot water flow rate in litres/minute by the temperature rise in celsius. Divide by 100 and multiply by 7 to get the boiler DHW output in kW. Example: 12 litre/min at 40C rise: 12 x 40 = 480.
 

Applying that says that it takes 14.7 kW to raise a flow of 14 litre / min by 15 degrees C so I am out by quite a margin from my requirement 

I need three times that amount of cooling so presumably three times the water flow rate which is a problem 

[David Jupp]

40kW = 40 kJ/sec

14 ltr / minute = (roughly) 0.25 ltr/sec.  or 0.25 kg/sec assuming water s.g. = 1

A 15 C temperature rise for that = 0.25kg x 15 C x 4.2 kJ/kg/C = 15.75kJ

So yes, quite a bit out.  You can assume more or less 100% efficiency of heat transfer - the heat transfer calculations relate to how big the exchanger needs to be and how fast the water has to flow in the internals.

[russ57]

So, how much water can you get out of the bore... And how cold is it..

This is a bit like the penultimate sub-plot of 'Trustee from the Tool Room' where Keith discovers a discrepancy in the hydraulic cooling calculations for a saw mill and earns himself a tidy little commission...


-Russ

[awemawson]

It's certainly a bit of a roller coaster isn't it !

Cheap bore hole pumps on eBay claim a figure of 2000 - 3000 litres / hour plus so are in the ball park, but whether the bore hole can sustain that sort of draw, and whether the pumps can deliver it from the depth of the hole are more variables!

. . . argh . . why is life never simple . . . 


 

[awemawson]

Well according to those records I found, this bore hole was able to produce 150,000 gallons a day  in 1947 

But IS it the same bore hole - I strongly suspect so, as if I put the grid reference on the bore hole survey  (TQ 7782 1741 ) into a 'grid reference locator' on the web that uses Google Maps it shows me a point about 15 yards away from where it actually is, which I suspect is within the margin you'd expect a reference to be accurate to in the circumstances.

So: Still need to gain safe access to the bore - possibilities:

A/ Smash the cover and replace - probably not too easy it's pretty solid - sledge hammers bounce off !

B/ Chip away flaunching cement round manhole surround - pull the lot off - leave in citric acid for a few months and re-cement with modifications

C/  Lift off entire upper works as previously partially done and re-build with new incorporating control chamber above ground

B/ Is probably favourite, and C/ is potentially dangerous so when the rain stops I'll tentatively attack the flaunching and see how tough it is.


(Typically as these things go, as I was typing this I got a phone call response to my plea for a water chiller from a fellow who has an 8.5 kW unit - I assume that is 8.5 kW electrical consumption - 30 kW chilling - so right order of magnitude but sadly the price isn't the right order of magnitude!)

[awemawson]

The rain has stopped so I ventured out to have a poke at the cement flaunching round the man hole.

The good news is that it is not impossibly hard - actually it looks suspiciously as though there's quite a bit of soft sand in the mix (should be sharp sand) so perhaps with the Kango and a newly sharpened point I may make a bit of progress.

Need to clear all the turf and earth back first - I may well get started tomorrow, as I have half a foundry floor to paint and when that's done I can't work in there anyway until it's dried for the next coat !

[philf]

But IS it the same bore hole - I strongly suspect so, as if I put the grid reference on the bore hole survey  (TQ 7782 1741 ) into a 'grid reference locator' on the web that uses Google Maps it shows me a point about 15 yards away from where it actually is, which I suspect is within the margin you'd expect a reference to be accurate to in the circumstances.

Andrew,

A 4 figure grid reference specifies the position to 10 metres. (10km, 1km, 100m, 10m).

15 yards I would have thought is close enough.

Phil.

[Pete.]

I've been thinking about this and comparing it to the kit we use at work, a lot of which is water cooled.

Our most powerful concrete saw uses a 27kw motor, and the drive electronics and motor are both cooled by the (same) water feed, which is usually just fed from a 1/2" hose up to 25metres long off a domestic water supply. The motor is just a cooling jacket. Very often the supply is quite marginal and you can get away with surprisingly little flow without tripping the thermal protection.

Last year I ran three of these at full load for hour after hour some days. The coolant was by a single 110v submersible dropped in the Thames feeding a 3-branch manifold. I think that if your bore hole is deep enough you could simply drop a decent sized submersible in the top and set up a return hose with a weighted end to return the water back into the bore after doing a lap of the heat exchanger. The thermal store of the bore hole will be immense.

[awemawson]

Yes Pete, I've been thinking along similar lines

I've just worked out that a 1000 litre IBC starting at 15 deg C will take an estimated 27 minutes to get to my (self imposed) upper limit of 30 degrees C - so for a 90 minute 'session' I could even get away with 3 IBC's that have filled up gradually over the preceding days. And that time can be extended a bit if I had a tap filling them at the same time.

This is ASSUMING the heat put into the water IS about 40 kW - I have no ideas if that figure is correct - 39/40 kW was the cooling ability of the dead chiller which I thought at time was marginal, but looking at some of my notes from back in 2005 I was working on 17 kW - I can only suppose that lower figure came from the firm I bought the furnace from !

I've been trying to find other examples of similar sized induction furnaces to compare their water cooling, as I'm sure that they must all be much of a muchness, but so far I've drawn a blank.

Today I've cut all the turf and earth back from the manhole, as the builder who rents one of my containers has volunteered to chip the man hole cover surround out of the flaunching this coming Monday - I'd intended to do it myself but he's far better equipped to do it than I.

[awemawson]

At last, a useful reference to required cooling from:

https://www.tct-tesic.com/wp-content/themes/tct/assets/documents/induction-furnace-handbook.pdf

"Cooling water volume
is determined mainly by the electrical output. One can say roughly that approx.
27% of the furnace output and the heat losses from the crucible wall will have to
be dissipated. In rough terms, one can reckon on approx. 35% of the furnace output as the total loss performance that must be dissipated."

So for my 100 KW furnace  35 KW cooling, which is right on the money for my original 39 WK chiller 

[awemawson]

I've been thinking about this and comparing it to the kit we use at work, a lot of which is water cooled.

Our most powerful concrete saw uses a 27kw motor, and the drive electronics and motor are both cooled by the (same) water feed, which is usually just fed from a 1/2" hose up to 25metres long off a domestic water supply. The motor is just a cooling jacket. Very often the supply is quite marginal and you can get away with surprisingly little flow without tripping the thermal protection.

Presumably the motor being sizable will be fairly efficient? The cooling I assume only has to lose the energy lost by inefficiency so if it is 75% efficient it's only having to carry away 6.75 kW

[RussellT]

Hi Andrew

You do have interesting problems don't you.

I've been thinking about this at odd moments.  Presumably the main argument against a total loss system is that it would scale up quickly and even a thin layer of scale would reduce heat transfer significantly.  Can the pipework be descaled with acid? I don't know if citric acid would dissolve scale - I expect it would.

I'm intrigued by the idea of using the stream.  That would give the possibility of a counter current heat exchanger  which would potentially be more efficient.  My usual interest in trying to do jobs for little or no expenditure is imagining a series of old domestic radiators lying in the stream.

I'm sure there are design guides for heat exchangers, but without having consulted any I recall from physics lessons that heat transfer is proportional to wall thickness, surface area, temperature difference, etc and I wondered whether this might help in some ball park calculations.

I had in mind if you know the area extracting heat from the furnace and the temperature difference and you know the temperature difference you are aiming for in the cooling section then you can calculate the area needed - and similarly for different  materials.

I'm not sure whether to mention this, but even though the borehole is there already, do you need some sort of abstraction license?

That enough rambling thoughts for today. I look forward to seeing your progress.

Russell

[awemawson]

Russell,

From what I've recently discovered about 'welded plate' heat exchangers they seem pretty efficient in terms of heat transfer so I don't think there is any need to use direct cooling, in that any cold water source can be isolated from the furnace plumbing by the heat exchanger. This is really needed anyway as the water needs to be low conductivity and with added glycol.

Although the stream sounds an attractive proposition, it's a long way from the foundry both for laying pipe and pumping the water, whereas the bore hole is only 15 yards away.

The bore hole water will undoubtedly be heavily laden with iron, as that's why it ceased being used for public supply, so my heat exchanger will presumably eventually clog up with it, but as Seadog said, it can be acid flushed, and frankly they are fairly cheap for what they are. (A 50 plate welded one rated at 90-100 kW is less than £100 from Poland or Czech including postage). I would err on the large size for the H.E. as I can easily throttle the cooling side if it proves excessive.

As far as I can tell an abstraction licence is only required if drawing more than 20 cu M per day which I wont be.

[awemawson]

Today I managed to open up the Bore Hole 

My friend Andy came with his big Kango clone and in a matter of 20 minutes freed the man hole cover frame from it's flaunching. Whoever did it originally used soft sand (should be sharp sand) so it came away remarkably easily having been given a seeing to with the breaker.

I slid the forks of the fork lift truck under the assembly and carried it away for dealing with at a later time - it's a blooming heavy lump of iron !

So this exposed a 24" pool of water - again I slid my fibreglass duct pulling rod down it and it went down to 174 foot 

I rigged up my petrol 2" water pump, with a foot valve and rigid pipe into the bore hole, and flat pipe out to a reasonably convenient 'rodding eye' on my roof and surface drain network that leads about 60 metres to the stream. Starting the pump water fairly gushed out down at stream level and the bore hole level started falling.

As the level went down, man steps were revealed set into the side of the bore hole - it's a brave man that goes down those 

It was my intention to carry on pumping until I got to the limit of the suction hose to see what was revealed, but at this point the engine died on the pump - would briefly start on choke but immediately dies. So pumping operations had to cease until I find the pump problem.

A quick tidy up and a temporary cover put over the hole was enough excitement for one day 

(Pete. will recognise the cover  )

[hermetic]

The only living boy in New Cross eh? Is this a sign (see what I did there) of a misspent youth robbing sub stations and knocking off policemans helmets? I used to have one from Reading Rock festival (1978) that said "No glass in the arena please" It was a good fit on the floor of the transit van, and covered the ribs to give us a much more confortable, and warm area to sit on the way back! I think I eventually sliced it up for under bench shelving!
Phil.

[hermetic]

There is more to this than meets the eye Andrew (at present anyway) 174 ft of draw wire, did you come to a stop, or was that the extent of the wire on the reel? 174 ft of man irons would seem to be totally impractical, and very dangerous, more pumping may reveal surprises! Would you return the water to the borehole, or dump it in the stream? I don't know what it is like in Suffolk, but in East Yorks the demands of agriculture have meant the increased use of boreholes, and susequent fall of the water table, so your borehole would not neccasarily be as productive as when last tested. OTOH if it has high iron, and is unusable as a potable supply, it could be very good indeed! Watching with great interest, and gagging to get back to my workshop!
Phil

[awemawson]

Sussex Phil not Suffolk - miles apart ! (The rigid fibreglass duct rod bottomed out at 53 metres)

The sign was obtained during a legitimate demolition (I am assured!)

The really GOOD thing about it being still 174 deep is that that is well below where the records say perforated pipe was installed so hopefully water will still seep in at a perfectly adequate rate for my needs.

OK so man hole & cover now transportable by fork lift (I'm certainly not carrying them!!!!) so putting them in the rear farm yard balanced on two baulks of wood, and slogging the cover via a 14 lb sledge hammer and a third block of wood produced . . .

 . . . zilch - a splintered block of wood 

Resorting to 'metal on metal' I had a very stout go at the cover, working round it's periphery until eventually the sound changed and ever so slight movement was detected. A few more really meaningful thumps and the cover dropped out of the frame. No wonder it was jammed - some serious rust had accumulated. And note, there is some pre-existing damage on the vertical rim of the cover - obviously old from the rust on it and lack of the missing bit!

Frame and cover need a good sand blasting, but I have too many other things on at the moment, and anyway too many other peoples vehicles in the line of fire.

[AdeV]

Nicely done!

I must admit, with the water that high in the borehole, I'm surprised you've not been suffering from soggy ground around it - The water must be no more than 6 inches under the surface?

Anyway - now you've got the lid off, it's obvious where you can drill holes for outlet and return pipes 

[awemawson]

Well Ade, we are always fairly damp here !

My plan at the moment is to build a low brick plinth on the concrete rings of the hole to give me head room to bring a pipe up from the pump and through 90 degrees, and to introduce the pumps cable. Also to give me something to tether a stainless wire to support the pump. Then the original man hole surround and cover can go back on.

[hermetic]

A good go around with the needle descaler would make some headway before the blasting! Sussex!! sorry about that, now do you understand why they won't let me cross the Humber! Can't wait to see the results of more pumping, you may have some sludge to pump out of the bottom after all the years unused, but what a result!
Phil

[awemawson]

Good idea using a needle descaler Phil 

[PekkaNF]

Hmm... Now when everybody is hot and poised to water/water cooling what about water to air cooling? You probably have few truck radiators and industrial fans floating around....100Kw to dissipate doses not sound much. Unless you need close to ambient temperature cooling output to start with. I have seen few hundred kW hydraulic unit oil to water cooling radiators and they were not that big (but fan was big and LOUD).

[awemawson]

I think the cooling is getting sorted Pekka because . . . . .

I've been looking at heat exchangers that will isolate the furnace cooling water from the cold source water in the borehole. Many available on eBay, most seem to be Chinese, but many coming from Poland and Czech that also could be originally Chinese. Lots of amazing specifications, remarkable low prices and quality promises, and often with free delivery - but what spec do I need.

Well, in this application, erring on the large size is no disadvantage - I can't think over cooling the furnace water is an issue, and if it is I can easily throttle the cooling side feed water accordingly, so when a HUMONGOUS heat exchanger popped up on eBay, UK made, and brand new never commissioned it seemed too good an opportunity to pass up. Manufacturer was still in business, the H.E. was still a current 'off the shelf' model and the full specification was available and emailed to me - now could I win the auction  Half the issue was that it was 'collect only' and the seller wasn't responding to my requests for contact to involve couriers.

OK auction was ending today at about 5 pm, so I put the item on Shiply  last night to get courier quotes, and then put a 'best offer' to the seller of the original starting price LESS my delivery quotes 

Surprise phone call this morning from the seller accepting my offer. Shiply courier happened to be close to the sellers location (Esher), promptly collected it and has just delivered it - well actually THEM as it turns out that there were two 

So what is it?  - it's a UKE BR205-50C 50 plate copper brazed heat exchanger intended for an air source heat pump. 54 mm soldered copper inlet ports and 2" BSP outlet ports. It weighs 34 kg ! and is 528 mm x 246 mm x  134 mm so I don't think there is any danger of it being under sized, and I have a spare 

So I still need to source a pump for the borehole, a tank for the furnace water reservoir (might come from the original chiller) and work out quite a bit of plumbing lay out with big pipes.

[David Jupp]

And you can always use both together if you find the cooling isn't quite good enough !