tool heater oil coking

[RobWard]

I wonder if anyone could please help me...

We are using some small tool heaters running at 125 to 150 degrees C on some low pressure casting machines and the oil is coking up. Is there a detergent that can be added to thermal oil prior to an oil change which will help remove the deposits? The heat is no longer transfering to the machines and it's really causing problems now.

Many thanks,

Rob

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[Pud]

hi Rob,

Bad news I'm afraid. We have oil tool heaters used on plastic mould tools which we run at 150 deg C and suffered the same problem. The reason for it (as i am informed) is that as the oil ages it suffers from reduced viscosity and molecular breakdown. This gives a lower flash point and a tendency for coking as the maximum allowable film temperature is also then lowered (film temp = actual temp on the heater elements). Answer is to change oil at regular intervals (after decoking!) Once coking starts it gives rise to increased film temps which cause coking which etc., etc., ad nauseum.

Silicone oils last longer but cost an arm and a leg.

 

[RobWard]

Thanks Pud.
I managed to empty the system (with the help of a compressed air line, which isn't the best idea if it can be avoided), flush some clean oil through, switch the flow direction and get everything running again, but it's a recurring problem.
Once, I was supplied with an additive to be added prior to an oil change which acted as a detergent and helped clean everything out, but I've been unable to locate any more supplies.
Ideally I'd like to fit a filter on the return line and with the detergent, and some very regular oil changes, try to get the system clean.
I phoned the manufacturer but they seemed unaware that such a thing could even happen. (Funny, when I speak to other users they all say they've encountered the same thing...)


So any ideas on how best to decoke it?

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[Pud]

Hi Rob,

As it is literally coke, we found the only way to remove it was to physically scrape/chip it off with hardwood and/or brass scrapers.

In retrospect it might be worth finding out how much new ones (heater elements) are as it took ages!!

I dont think a filter would help much to prevent the problem as the coke is formed directly on the elements. If you have coke loose in the system I would take the heaters out and have a look at them. Ours was a solid chunk - you could not see the elements - just a black mess!!

As regarding manufacturers, I have a book on fluid heating published by a Swiss manufacturer, dated 1983, which specifically mentions oil degradation and coking.

Rgds


Harry

 

[RobWard]

Actually the tool heater itself seems ok. (I've not stripped it down but it works just fine)

The problem for me lies in the heat exchange coil of the low pressure casting machines: its a double, conical, helix, expensive, difficult to replace, and has a bore of about 6mm which makes physical cleaning nigh on impossible.

That's why I need to find a cleaning fluid or oil additive that I can pump around it.

Rob

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[Pud]

Hi Rob,

Oh dear...

The only suggestion I have is to try some flushing oil as used for car engines - that is a thin, detergent loaded oil. Certainly gets the crud out of engines!! Not too expensive either. Worth a try for a few quid?

btw, whats the coil in question exchanging heat from/to?

Oxalic acid is also ok for dissolving metal oxides without attacking the base metal (if it 'is' oxides rather than 'coke' - toxic via skin absorbtion though. Use rubber gloves. (I use it to clean plastic flow meter tubes of water (rust) deposits.)


Rgds


Harry

 

[RobWard]

Well here's a thing: I went to the local car spares place and he tells me you can't buy flushing oil anymore!
(Hopefully my days of engine rebuilding are behind me, because I used to use it a lot then.)

The coil melts out methylene-bis-chloro-aniline granules which then fall into a holding tank. The problem is if the coil doesn't get hot enough because of coking up, the granules don't melt, the tank runs dry and the machine will, well, pretty much destroy itself.

I'll ask our lubricant supplier if he can get hold of any.
I wonder what temperature it would be best to run it at to make best use of the detergents in it?

Thanks for your help Pud!


Rob.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[Pud]

hi Rob,

Your spares man is wrong - flushing oil is available (several apparently) - our local mechanic tried it to free hydraulic tappets on wife's car last year.

Have spoken to him and it's made by Forte - Called Motor Flushing Oil (surprise!). Apparently (so I'm told) it's one of the best you can get - detergent based rather than solvent based.

I would guess it would be good for 100 - 110 Deg C as engines run around that.

Rgds

Harry

 

[shoobaka1]

What type of heat transfer fluid are you using. Do you purge it out of the tool with air? Air can actually damage some heat transfer fluids and cause a build up. Paratherm makes a cleaner for PM on oil circulating units. They also make several grade of transfer fluid.

 

[RobWard]

Right...
I used some diesel engine flushing additive.
Ran it for half a day at 90 deg C.
Changed the oil twice and it's now running OK.
I shall have to do more frequent oil changes in future, I think.

To answer your questions shoobaka; I had to blow the oil out of the line simply because the pump wouldn't do it. I didn't want to, and usually wouldn't, but when you've no alternative...
The oil is a generic type: 150 deg C thermal transfer oil. I asked our lubricant supplier for some information on any different types they could supply, but they haven't responded. After asking four times I'm sick of phoning them, and in future may well look for a different supplier.

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams