Expansion and contraction

Tony Shelton

Tony Shelton

BS Detector, Esquire
Alright, I've got a question for the mechanics and scientists.

I have a couple of instant water heaters in my house. One of them is 5 years old. Every time I turn on the water the burner fires up instantly. When I close the tap the burner shuts off automatically.

When I first got this thing I called the mfg (Bosch) and asked if we needed to turn it to cold before shutting it off and they said no and explained it like this:

As soon as the heat source is stopped the water begins cooling instantly. Therefore there is no need to run cool water through it.

Try it with a pan of boiling water. As soon as you take it off the stove it stops boiling.

With that in mind let's change over to the pressure washing skid. Why is is necessary to ever turn your burner off? Why is it necessary to run cool water through it. It looks to me like that would be more damaging than just letting it cool on it's own with all the expansion and contraction temperature extremes cause.

I'm looking for scientific answers. Who's got em?
 
I only cool for safety reasons, I have had guys shut down the skid without cooling it down and leaving the ball valve off trapping steam in the line, thats a no no!!
 
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Quick someone go wake up Nigel :jester:
 
I wish I had the scientific answer. I like the idea of cooling the burner down with water, think it might be a good idea.

One of the jobs where I wash I have to apply hot soap then shut off the machine but cool the burner first so I can take the downstream injector out (it does restrict the flow on my surface cleaner) then re-connect the hose and fire up the burner again to clean with very hot water.

I am working on building a manifold so I don't have to do this, just switch with ball valves and "T"'s between using the injector and not using it. Just have not got around to doing it yet.

The reason for cooling the burner before taking out the downstream injector is so I don't burn the skin off my hand AGAIN, it really sucks the first time! And that is slowing down the machine and de-pressuring the system before disconnecting the injector, it shot me with boiling hot water/steam. Now I cool it down first so no burn possibility.
 
Scientific answer would be, heating and cooling over time will eventually break down the molecular structure of the steel and cause the coil to weaken and fail.
 
Here's the science. Water contains minerals. As the water is heated, the minerals become soluble. If the coil cools on it's own, the minerals that are in solution solidify and build up on the interior of the pipe...scale. Cooling the coil rapidly drives the solvent minerals out and leaves behind water with minerals in suspension, thusly preventing or slowing scale buildup.
 
Maybe that is why my machine had a lot of rust build up in the coil causing a loss of about 1000psi.

The guys at Hotsy were suprised to find rust in the coil instead of Scale, they say that it is usually scale in the coils causing problems, not rust.

I have always killed the burner and ran water through it until it comes out either warm or cool then shut down the engine.

Do I really need to keep doing this Russ? or Does it really matter?
 
I'm going with the mineral build up, I've heard that many times 1 , safety, so you dont blast water or steam when you disconnect the hose, and it sure sounds like the right thing to do. It really doesn't take any extra time, just let it run for 2 min while your rolling up hose or packing up or Tony, hook your cool remote to the burner and turn it off while your spraying.
 
Russ Johnson said:
Here's the science. Water contains minerals. As the water is heated, the minerals become soluble. If the coil cools on it's own, the minerals that are in solution solidify and build up on the interior of the pipe...scale. Cooling the coil rapidly drives the solvent minerals out and leaves behind water with minerals in suspension, thusly preventing or slowing scale buildup.
Click to expand...

Thanks Russ. That makes sense.

I wonder why that doesn't affect the instant water heater. When we had a broken part on it we opened it up and there was a lot of scale/corrosion at the connector, but virtually none in the coil. That was after about 4-5 years of daily operation. Maybe the coil is made out of some other metal?
 
David Potter said:
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Quick someone go wake up Nigel :jester:
Click to expand...

Sorry, missed this one, but I did not know the answer.

Russ Johnson said:
Here's the science. Water contains minerals. As the water is heated, the minerals become soluble. If the coil cools on it's own, the minerals that are in solution solidify and build up on the interior of the pipe...scale. Cooling the coil rapidly drives the solvent minerals out and leaves behind water with minerals in suspension, thusly preventing or slowing scale buildup.
Click to expand...


Thanks Russ.
I know that alkota has some Stainless Steel coiled boilers.
 
Sounds expensive. I heard there was something problematic with the stainless, expansion characteristics or something.
 
I don't think schedule 80 has a 'real' rating by anybody, but has an accepted burst strength above 20,000 psi.
 
David Potter said:
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Quick someone go wake up Nigel :jester:
Click to expand...
:haha::rotflmao1:
 
tomtucson said:
I don't think schedule 80 has a 'real' rating by anybody, but has an accepted burst strength above 20,000 psi.
Click to expand...

Manufacturers have to specify a maximum working pressure for their coils, its a liability issue.

The spec strength may even be different for the same coil exact coil, if the methods of forming are different, cold rolled vs heat formed.
 
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