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Author Topic: Adhesives and sealants  (Read 6921 times)

Simtech

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Adhesives and sealants
« on: November 30, 2011, 06:33:49 PM »
I've been designing the cogeneration domestic hot water system and I've realized I need a flexible sealant for the many tank penetrations thats capable of handling water at or near boiling.

The coolant coming from the engine thermostat will be @ ~195 degrees not to mention the heat derived from the exhaust.

Silicone isnt a sealant, its a gasket material:  There's no strength to it at all, however it has excellent chemical and temp resistance.  I dislike silicone immensely - I'd never use it in a permanent installation.

The only two common sealants that I know of are 3M polysulfide and 3M 5200 (polyurethane), both of which I've used on my boats.  (BTW, the boat-life polysulfide is crap - dont ever use it)  Both take up to a week to cure.  Both have excellent adhesion, 5200 is incredibly strong.

3M Polysulfide has a service temp of 180F
3M 5200 has a service temp of 190F

There are some exotic epoxies that I've used professionally like Duralco, but they're fairly expensive.  Duralco has a service temp of 450F and is still very strong yet flexible enough to tie it into a knot.

So, what other commonly available sealants are available that can withstand boiling water?

Thanks

dieselgman

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Re: Adhesives and sealants
« Reply #1 on: November 30, 2011, 06:47:03 PM »
For flanged penetrations and handling water/glycol coolants we tend to use a high-temp variety of silicone... that is a correct application of the product because it forms a gasket for the flanges. I presume the penetrations you are referring to do not have mechanical supports?

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LowGear

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Re: Adhesives and sealants
« Reply #2 on: November 30, 2011, 08:40:58 PM »
J B Weld!

It cures gasket problems once and for all.

Casey
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Chris R

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Re: Adhesives and sealants
« Reply #3 on: November 30, 2011, 11:01:00 PM »
I have found that Marine Tex to be a better product than JB Weld. Not as easy to find and cost about twice as much. Marine Tex has worked for me where JB Weld failed.

Regards
Chris R

Simtech

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Re: Adhesives and sealants
« Reply #4 on: December 01, 2011, 12:36:45 AM »
I'm envisioning a compression fit:  Two threaded flanged connections on a threaded piece of pipe that meet in the middle and clamp down to the tank wall.  However I see potential gaps where the flat flanges meet a rounded tank wall.

I'm sure there will be vibration conducted by the radiator hoses, thus the need for a flexible but extremely durable sealant.  From what I understand JB weld and/or other epoxies will eventually crack from the vibration.

So far I've got several loops in/out of the tank:

Engine cooling loop
Exhaust gas heat recovery loop
Excess heat removal loop
Domestic heating loop
Domestic hot water loop

Because they all depend on thermosyphon flows (except for the domestic heating loop) placement of the inside tank pipe ends is critical, thus I cant "re-use" a penetration and have two loops share one hole.

I'm also using a phase change material inside the tank for thermal storage, which complicates the problem even more.  When I get the drawings done I'll post them for comment in a separate thread.

Dave

dieselgman

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Re: Adhesives and sealants
« Reply #5 on: December 01, 2011, 12:43:57 AM »
A Polyurethane sealant might be the best material for a combination of strength and flexibility, I do not know about the temperature issue though.

What phase-change material/s are you referring to?

dieselgman
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fabricator

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Re: Adhesives and sealants
« Reply #6 on: December 01, 2011, 01:03:49 AM »
High temp silicone is perfect for applications where it is between two flanges, the oil pan gasket and the transmission pan gasket on your car are silicone.
BioDiesel Brewer

Simtech

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Re: Adhesives and sealants
« Reply #7 on: December 01, 2011, 01:56:16 AM »
@Dieselman:
The phase change material is paraffin wax with a ~130F melting point, it'll be in 2-5/8" copper DWV pipe tubes in a horizontal crosshatch grid starting 2/3rds of the way up the tank.  A lot of copper and a lot of wax.  At least it'll displace some of the 75/25 water/proplene glycol mix I'll be using as my working fluid.

I've used wax as a storage medium before in my solar hot water experiments, its amazing how much heat it stores - and it moderates temperature swings, especially during the thermal discharge cycle where a steady ~130F is maintained until its re-solidified.  Its also extremely stable and can undergo thousands of cycles without breaking down.
Gotta be careful filling the tubes though, because wax expands so much when melted it the tubes have to be filled hot with melted wax (leaving 1.5" unfilled at the top) and the cap soldered on immediately.  Its also real easy to screw up a solder joint if wax gets on it.  Its REAL easy to overfill (tube not hot enough and solidifiies the wax during the fillling process) and have a split pipe during its next thermal charge cycle.

@Fabricator
A tranny gasket is only at most a few mm's thick, with supporting metal nearly encapsulating it.  I'm expecting gaps of up to a 1/8".  I just dont think silicon is strong enough, or adheres well enough.  The tank is going to be tall but fairly narrow, its intended to stagnate the water so the incoming heat doesnt mix with the cooler strata.

I think I might just go ahead and spend the money on the flexible epoxy.

Simtech

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Re: Adhesives and sealants
« Reply #8 on: December 01, 2011, 09:13:45 PM »
Found the perfect sealant:  MAS Flexbond 5000 sold at West Marine or other marine stores.  Its a polyurethane based hybrid, unfortunately it's pretty spendy at $25.00 for a 9.5oz bottle

Service temp is -40F to over 212F with short term surges to 350F.

From all accounts its incredibly strong and will stick anything to anything permanently and adheres to metal particularly well.  One guy described it as "welding out of a plastic tube".

mobile_bob

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Re: Adhesives and sealants
« Reply #9 on: December 02, 2011, 02:21:49 AM »
i would use the rtv gray high temp silicone, it will handle all the heat you want to throw at it with your cooling system.

it is used for flanges on transmissions, rear differentials and all over the cooling systems of heavy trucks with excellent results

i will stay just a bit pliable, meaning it will never crack, it bounds to any clean surface like iron, and is impervious to oil, fuel, water, antifreeze etc. and can handle several hundreds of degree's F. for years and years.

i have never seen it fail in my experience.

at 5-10 bucks a tube it is the sealant of choice in my books

the only thing i would not use it for is pipe fittings and for air fittings, it seems to blow out of the threads unless you wait for it to cure properly.

if you are concerned just use an o-ring between the flanges of the bulkhead fittings, problem permanently solved!

bob g
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fabricator

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Re: Adhesives and sealants
« Reply #10 on: December 02, 2011, 10:48:52 PM »
Most store bought bulkhead fittings come with square o rings, I've used em on round cone bottom tanks lots of times with excellent results, you can get bulkhead fittings at tractor supply, or McMaster Carr.
BioDiesel Brewer

Horsepoor

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Re: Adhesives and sealants
« Reply #11 on: December 04, 2011, 09:02:10 AM »
Instead of parfin wax, have you researched or considered Glauber's salt? We know that thermal storage is a fundamental problem in any heat storage system and phase change materials can make tremendous logical sense when you go through the numbers. It takes many times the energy to achieve a phase change as a normal degree change. For example, think a stone or brick filled tube - this would have a heat capacity of about 0.2 cal./gram/deg C. Now, look a some of the different mixtures of Glauber's salts which have about 50 to 80 cal./gram during the phase change. Water is 1 cal./gram/deg C. So if you cycle brick, through about 5 degree Celsius change, you only have 1 cal./gram. If you used Glauber's salt in this case, you have an advantage of 50 to 80 times the thermal storage using Glauber's salts as you do with brick.

Suppose you had a concrete walled box (4 feet by 8 feet by 2 feet deep) in the ground well insulated. Then you run a closed loop heat exchanger (tubes of another workin fluid - engine coolant) through your concrete box filled with Glauber's salt. Ohyou could run another closed loop through the box from the exhaust to transfer more heat. After awhile the salt melts (90 to 100 F depending upon make up), this is giant low heat sink, that could be used for low temperature applications.   

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bschwartz

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Re: Adhesives and sealants
« Reply #12 on: December 04, 2011, 03:31:05 PM »
Not easy to locate, but I found it for sale here

http://www.dharmatrading.com/html/eng/1716-AA.shtml
-Brett

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listerboy

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Re: Adhesives and sealants
« Reply #13 on: December 04, 2011, 08:58:13 PM »
Quote from the site for the Glauber's Salt: 

" Note - this is the anhydrous form of Sodium Sulfate, not the deca-hydrated form ".

Does that make much of a difference?

Horsepoor

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Re: Adhesives and sealants
« Reply #14 on: December 05, 2011, 06:06:40 AM »
Yes, you want the stuff that melts or phase changes at 34C instead of 884C. Here is a brief comment stolen from another web site:

Thermal storageThe high heat storage capacity in the phase change from solid to liquid, and the advantageous phase change temperature of 32.384 °C (90 °F) makes this material especially appropriate for storing low grade solar heat for later release in space heating applications. In some applications the material is incorporated into thermal tiles that are placed in an attic space while in other applications the salt is incorporated into cells surrounded by solar–heated water. The phase change allows a substantial reduction in the mass of the material required for effective heat storage (the heat of fusion of sodium sulfate decahydrate is 25.53 kJ/mol or about 19 cal/gm), with the further advantage of a consistency of temperature as long as sufficient material in the appropriate phase is available.

GTC 20/2 down rated to 850 rpm - ST 15
Metro 6/1 800 rpm on cart - ST 7.5