Copper -Fin Tube Heaters & Minimum Flow 3

[pesy]

Why do copper fin-tube water heaters require minimum flows? What would happen if it was provided 40F inlet water, and set to 200F outlet water?

a) Is it an issue of scale build-up?
b) Is it an issue of melting fins?

What is the reason for (a) or (b)--Does it occur only because there is unequal flow distribution among the various tubes, or is the distribution relatively equal and it occurs anyway?

Are there fin-tube products that don't have these limitations?

I ask because I'd like to design a new fin-tube heat exchanger that doesn't require minimum flows and I'd like to know what to look out for...

Thanks.

 

[lilliput1]

Because of poor heat transfer performance in laminar flow condition. What is the heating medium?

 

[pesy]

Water is the medium.

Is poor heat transfer the only problem, or can damage occur to the unit? Typically the warranty of such a product is voided with insufficient flow, so it must be something worse than merely reduced heat transfer.

 

[IRstuff]

If the flow is too low, the boiling may occur

TTFN

Eng-Tips Policies FAQ731-376

 

[pesy]

In a perfect system, the temperature sensor / switch would detect how much heat is going into the water and either modulate the input or shut the unit off before the water boiled.

Scaling and/or fin-melting would not be prevented if those are the failure modes (or maybe there's another I haven't thought of). But boiling could be prevented with sufficiently advanced controls.

Is the problem solely boiling, or are there other factors?

 

[ChrisConley]

The low inlet temperature on that system would cause flue gas condensation, which would be a major concern, unless managed.

Minimum flow would also be to prevent hot spots and potential boiler failure.

 

[ChrisConley]

I'm of course assuming that we're talking about a gas or oil-fired copper-fin tube boiler. Ignore my comments if I've misunderstood the OP.

 

[pesy]

Good point about the condensing. Yes, it would be for gas/propane/oil. I'm actually considering the new design to be a condensing product by using alternate materials so the condensate will not corrode anything. I'm trying to find out the mechanism of failure on the copper-fin products so that I can [hopefully] prevent those issues on the new product.

You mention hot spots--do these products typically have a significantly imbalanced flow distribution?

 

[willard3]

snip
You mention hot spots--do these products typically have a significantly imbalanced flow distribution?
snip

If you have low reynolds numbers, you have hot spots.......

If you have hot spots, you will have thermal failure from expansion even in homogeneous and isotropic materials.