stainless steal versus aluminum heat exchanger

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Old 10-31-11, 03:32 PM
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stainless steal versus aluminum heat exchanger

does any one have an opinion
 
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Old 10-31-11, 04:20 PM
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This would be for a hot water boiler? I would not prefer either over cast iron or steel. Check this, but I believe that the thermal conductivity of stainless is lower than iron or carbon steel. Also, stainless is susceptible to cracking if there are chlorides present in the water - you'd need to check the chloride concentration of your city water supply.

Aluminum - great thermal conductivity, but can develop problems with stresses due to uneven heating, has lower strength, and it, too, can have problems with chlorides.
 
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Old 10-31-11, 04:24 PM
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I believe the stainless steel is supposed to resist corrosion from the condensate in a mod/con better than the other choices. It is true that stainless doesn't conduct heat as well as the other choices.
 
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Old 10-31-11, 05:11 PM
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The cast aluminum boilers have about 8 times the thermal transfer over stainless steel. A cast iron boiler is 5 - 8 times the thermal transfer dependent on water temperature. So listing in greater transfer from lowest to highest.
Stainless Steel
Steel
Cast Aluminum
Cast Iron

Cast Aluminum
Ph 6.5 - 8.5
< 2500 ppm dissolved solids
Hardness <7 grains

Stainless Steel
Ph 8.2 - 9.5
< 2500 ppm dissolved solids
Hardness 3 - 9 grains
All the above may change slightly from manufacturer to manufacturer.

Stainless and Aluminum are also affected by chlorides. Chlorides affects the welds on stainless more which is called crevice corrosion. Cast iron also is affected by chlorides but more by oxygen causing interior corrosion.
All the above is a mute point if the system is tight and not adding a lot of make-up water.
 
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Old 10-31-11, 06:22 PM
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If aluminum or stainless steel are preferred for mod-con boilers, then that would be another reason, at least for me, to avoid mod-cons.

Another reason is cost. We spend about $1,000 per year for natural gas, including space heating, cooking, water heating, clothes drying, and outdoor grilling - it's difficult to determine how the costs should be apportioned among those uses. But whatever, how can a mod-con be justified, economically? Particulary, if stainless steel or aluminum is called for?
 

Last edited by gilmorrie; 10-31-11 at 06:57 PM.
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Old 10-31-11, 07:06 PM
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Couple things i don't understand.
In a prior life i had a great involvement in the restaurant industry.
We had a Groen convection steamer that, for at least the last 15 years and to this day is one of the finest most reliable mechanical devices as could be desired.
This machine has cooked countless thousands and thousands of Lobsters, among innumerable and massive quantities of other foods.
Now then, lobsters contain a hugh amount of salt. After cooking a batch - that essentially meant craming in as many as could physically fit - (the) inside of the compartment - would be covered with white lobster/salt residue. Multiply this times days weeks and years after years.
This cooking takes place at over 212* at a few lbs of steam pressure, with the steam condensing over the food and out the back of the compartment. Add to this a squirrel fan that blows the steam all around for faster heat transfer.
In addition to all this the water used for steam production is softened with a salt softener.
Granted the steam generator is made of steel, and is blown down daily, but the boiler component is not the object of this discussion.
However the point is, good old American high quality Stainless Steel exposed to massive amount of salt(chlorides,) condensing steam, oxygen and all types of various food acids can seemingly last forever.
Granted these are not combustion by products, but it's hard to understand that a stainless steel condesing heat exchanger can not be made to last for a good long time. Given that, it is not made of Ch@#$se crap stainless.

Maybe the heat transfer is lower, but wouldn't increasing the size of the exchange area make up for the difference?

Respectfuly Submitted,
Peter
 
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Old 10-31-11, 07:48 PM
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You can get a rough idea how much gas you use for everything but heat by look at your bills for the summer months.

Your probably aren't going to see much payback by using a mod/con for the amount of gas that you use. They are more expensive to buy and more expensive to maintain.
 
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Old 10-31-11, 07:55 PM
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They obviously get some good heat transfer out the stainless steel heat exchangers with 97% AFUE numbers.

Maintenance is key for a mod/con no matter what the exchanger is made of. You need to have it cleaned every year. I've heard of stainless steel exchangers going quite fast when things aren't working right.
 
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Old 11-01-11, 05:44 AM
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There are different quality stainless'. Cookware is always a higher grade due to regulations which is food quality. Secondly the boiler flame temperatures are up to 2800f. When heating appliances are designed/engineered all parameters applied during design are assumed proper application. This includes but not limited to proper sizing, venting, flow rate, gas pressure, clocked meter, delta-T, near boiler piping, no system leaks limiting amount of addition of fresh water, etc. Unfortunately in many more installations than not, some or all of the above are not correct or tested. I find most gas valves (non-mod/con) are being sent out at about 3.65 in. wc. when manufacturers state 3.5 in. wc. max. I realize that 0.15 over means very little but the code states that a heating appliance cannot be over fired at all. Of course 3.65 does not mean the boiler is over fired. Over firing is determined by proper gas pressure and clocking the meter. Gas pressure alone does not mean the input is correct.
As many of the above are not correct it will affect the product.
Think about you lobster pot. Let's wrap the entire pot with temperatures around 2300f to 2500f, smash in enough lobsters that there is no steam circulation, don't clean it for the next 10 years, how much will the surface areas of the pot be affected and what quality is the water and lobsters.
 
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Old 11-01-11, 08:28 AM
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Originally Posted by rbeck View Post
hink about you lobster pot. Let's wrap the entire pot with temperatures around 2300f to 2500f, smash in enough lobsters that there is no steam circulation, don't clean it for the next 10 years, how much will the surface areas of the pot be affected and what quality is the water and lobsters.
OK, I see it better. Thanks.

I can add i'd still pick the Stainless over Aluminum any day!
Over my 30+ year career, we must have gone thru at least a dozen aluminum sauce pans with pinhole leaks vs none/zero for the mega buck$$$ All Clad Stainless sauce pans. Expensive- yes, worth it? - every penny, imho.

Peter
 
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Old 11-01-11, 07:49 PM
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In Europe they sell cast aluminum for quality and stainless steel for lower cost and less quality.
 
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Old 11-01-11, 09:46 PM
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That's intersting.
I guess i'm just biased, based on m past.
I need to keep open mind and clear view.
Yea, we had some thick aluminum stock pots that were old when i got them 40 years ago. They are still good and must be at least 60 yrears old.
Tehy have spent the majority of their existance sitting on top of a blue propane flame with varying temeratures of liquids on the inside.
I think the 60 quart monster is close to a half an inch thick 3/8's at least.

Peter
 
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