Copper Fin-Tubes Heater & Minimum Water Flow

[pesy]

Why do copper-fin 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.

 

[vpl]

How about an "issue of heat transfer"?

Have you read through a good heat transfer textbook lately?



Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[pesy]

I am extremely well versed in heat transfer, thank you.

"heat transfer" is not an issue. Heat transfer occurs to whatever degree the thermal resistance, heat transfer coefficient, and LMTD allow.

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.

Scaling and/or fin-melting would not be prevented if those are the failure modes. But boiling could be prevented with sufficiently advanced controls.

My question still remains--what is the true problem?

 

[vpl]

Pesy

If you're extremely well versed in heat transfer, and given that it's your setup, I'm surprised that you don't know the answer. I'm also surprised you don't normally post in this forum, as your expertise would be welcome.

You, of course, have looked up the melting point of copper. And you have investigated whether you have scaling occurring (because that depends on your system and water chemistry).

Perhaps you would like to describe your situation a bit further as to what you mean by a no-flow situation.


Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[kenvlach]

pesy,
If you know the heat source -- open flame, electric resistance, steam, Dowtherm @ some temperature, etc. -- kindly provide that information & some details.

 

[pesy]

I have run some calculations and I don't think the fin tip temperatures in this product get much above 1500F (melting at 2000), having calculated a gas-side-convection coefficient of about 21 BTU/hr-ft-F (using estaimtes since I haven't found any textbook equations for heat transfer in low-#-of-row fin-tube banks). However, there my be local high-convection-coefficient areas in the HX. If, for example the local gas-side h is greater than 30, I calculate the fin tip temperature to be over 2000.

Without running some seriously heavy-duty CFD, it's very difficult to get an accurate localized h.

However-and I go back to my original question-I'm not sure how these products behave in real-world systems. What goes wrong first?

My experience is not in fin-tube heat exchangers, it is in more conventional plain-tube, or enhanced-tube heat exchangers.

Scaling is another factor which is a bit nebulous. Scaling occurs differenlt around the country, so my experience in NJ is not necessarily the same as someone else's in WI. High heat flux is a contributor to increased scaling, but I don't know if it becomes the primary damaging factor to a low-flow heat exchanger.

Typical fin-tube heat exchangers require recirculation. If recirculation were not provided, the flow rate would be at teh mercy of how many people turn on faucets. If all faucets are turned off at once, the flow drops to 0 (with no recirc) but the heater is still on. It will turn off at some point, but not before one or many of the following things occur:
a) fins melt
b) water boils
c) significant scale is deposited
d) something else I haven't thought of

Which one is it?

 

[pesy]

Open flame

 

[vpl]

Pesy

I must be totally missing something. I don't understand how your fins are getting to the temperatures you are citing. This may be because the applications where I've seen heat exchangers are different than the application you're discussing.

Based on your posts, I'm trying to understand the configuration. I read that you have gas and water, but then the words about faucets make me think this is a commercial hot water heater where the water is on the shell side and the gas is on the tube side. But if that's the case then I'm not understanding why fins are needed. I also would think that any commercial hot water heater would have a temperature feedback loop to prevent scalding.

Nevertheless, I don't want to increase your frustration (any more than I have already !) So you might find the following link helpful:

http://www.wlv.com/products/databook/databook.pdf.

This is to the Wolverine databook. They make commercial air/water finned heat exchanger and they discuss how to calculate heat transfer in low fins.



Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[btrueblood]

Pesy,

Yes, any of the problems you noted can occur. Stagnant flow conditions will allow water in the tube to boil locally, which can cause the material temperature to rise high enough to melt solder joints and/or cause accelerated corrosion, and deposit scale. More importantly, if the flow stops, the implication is that all downstream valves are closed, and the plumbing is now a closed volume (assuming there are anti-siphon check valves on the incoming line). If the fire stays "on" in this state, the thermal expansion and/or steam expansion in the hx can cause the pressure in the system to rise to extreme values, possibly causing a steam explosion.

 

[TXiceman]

Often the minimum water flow is based on laminar flow and the Reynolds number. Once you drop to laminar flow, the boundary layer is basically insulating the tube wall and not alowing heat transfer. Keep the flow up out of the laminar zone, and you get some turbulance which aids in heat transfer.

Ken

Ken
KE5DFR

 

[eaboujaoudeh]

laminar flow is an enemy to heat transfer, the minimum flow rating is that u stay in the turbulent flow region which gives u good heat transfer. The limitation of this ,on the upper side, is friction. while increasing flow speed u'll add turbulence which technically is good, but u reach a pt where the added turbulence generates more pressure drop than heat transfer and is not worth it.

Fins are placed because they help make the flow turbulent,and because they increase the heat transfering area...

As for the 200F, i thought u said u needed water and not water vapour.

Elie Abou Jaoudeh