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Hospital "dish"washer


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#1 Keinokuorma

Keinokuorma

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Posted 17 July 2006 - 02:26 PM

I don't know if this appliance was from hell, but I'm sure it was raised from the dead.

I remember a case some 10 years ago, I was in my training, at a hospital. They handed me over an old large industrial grade "dish"washer to troubleshoot. Well you know what their "dishes" were: pee bottles and such, nothing that I'd take into the dining room. The tech crew had done some servicing on it, but failed to shotgun the real problem. There was already a new one installed to replace the old 1970-some machine... so they handed it over to me, the fault report said it goes some 3 minutes into the cycle and stops. Well, what was there for me but troubleshoot a little more.

So I checked that timer wheels didn't stick, checked that everything worked like the railway loo... right up to 3 minutes, after which nothing seemed to happen. What the heck? After 15 minutes I got bored to wait, and I started knobbing around... turned the thermostat down, and whallah! The cycle advanced right to the end from there.

So I tried again, turned the T-Stat up to the nominal 85 Celcius that was to be used, and tried... Again, seemed to jam at 3 minutes, after 10 mins I turn the tstat down again, and see that the cycle proceeded around 40 C... so I opened tha door, felt into the tub... just slightly warmer than my hand, so it was calibrated right...  what was left, I thought I'd try and see if the heater was bad.

OK, it was a "star" or "Y" wired triple element, a.k.a. 3x240v-to-neutral, 6kW spec. Measured resistance for each element, each gave different results. Now, it should have been around 27 ohms per element at 10 C, provided it should still give 2kW per element arund 70 C... but the readings I got were the better side of 100 ohms around that 40C. That would give about 3x550W or 1650W... clearly not enough to heat the large amount of water used, not before millennium. :(

I knew the facts, the machine was not to see much use anymore. I took my freedom to wire the heaters into triangle and try it out... would be around 5kW. And what happened, I got it working neatly up to 70C and a little more. The tech crew settled to put their greasy coveralls into the racks once in a while and running it through... They even got some jolly good results with the hospital grade detergent. And it was faster than the laundromats, that were there for the shiny white linen anyway, and off-limits for filthy worker's coveralls...

A couple years ago I went back to see the crew, they still had the machine working fine the way I left it... we may count this as a miracle. As to what had caused the original problem, somehow the timer must have jumped right over to heating back then, before the tub was full. The elements were almost busted, but not enough to be completely out, just that the special alloy changed it's properties, and their resistance became too sensitive to heat. 

What's the lesson here? Usually, heating elements are made of a precise alloy like constantane, that has high resistivity and low res/temp factor. So Grasshoppers, my brothers in evolution, remember this when you find out that your stove seems to take forever to boil your coffee water, your clothes dryer seems to take forever to dry your clothes, or your dish/clothes washer stops dead in it's tracks at a specific point. You may have run into the trouble of burnt-out heating elements. So don't boil your coffee water out. You may need to enjoy your coffee luke-warm in the future. Yes, this is one of the most usual cases of burning out a heating element: Boiling your coffee water out.

Umm, these disfigured pearls of partially off-topic information were thrown overboard by a dumb fumbler of questionable honor. :?
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