Output of indirect water heaters

Greg_41

The only time I encountered laminar flow was in an old house piped for gravity water, 2 1/2" mains, with an indirect, and one zone of baseboard in an addition. Supply manifold piped with ifc circulators and multi zone relay. When the hot water or baseboard zones ran, the return on the main radiator zone would get smokin' hot. We added a flow check on the return and problem solved, but the tech guys at WM called it laminar flow.

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John_205

Indirect water heaters

Most manufacturers of indirect w/htrs have a table that shows the capacity of the heater and the amount of hot water delivered at certain btu inputs.Many show the same size tank giving various outputs at different btu inputs.I had questioned one manufacturer how this could be possible because the flow rate of the coil would only let a certain amount of btu's flow thru.After his explaination I sort of had a better hand on things. I just got of the phone with another manufacturer because they omitted the output of one of their indirects. When he gave me the informattion I asked at what btu input that was. Then I asked what will the output be if I put in more btu's.He siad it would be the same because thats all the btu's that can flow thru the coil.I'm sure I'm missing something. Any body Help?

Doug_7

Heat Input Rate

Heat input to an Indirect hot water tank (or any other heat exchanger) is calculated by the equation H = UAdT.

This means that H (the Btu / hour heat rate) is a function of: U (the heat transfer coefficient), multiplied by A (the surface area of the coil), multiplied by dT (the average temperature difference between the water in the coil and the water in the tank).

Sounds a bit technical - but here is the deal:

For a given size of tank, the coil area (A) is constant and the heat transfer coefficient (U) is almost constant - So the amount of heat transferred is strictly a function of the delta T or temperature difference.

For a given size tank a higher delta T will will transfer more heat and a lower delta T will will transfer less heat.

A bigger tank with more coil surface area will transfer more heat at the same dT.

Does that explain it ?

Doug

ALH_4

IDWH HX Data

The only IDWH manufacturer I know of that publishes the appropriate data for their heat exchangers is Viessmann.

It's a function of heat exchanger area, flow rate, supply temperature, tank temperature, material, hx thickness, etc.

I am sure the engineers at every manufacturer have this data available. However, I wouldn't bet that anyone outside the company could get a copy.

hot_rod

and a manufacturer

will sometimes install the tank on a test bench with data logging, and btu meters to confirm output calculations.

My question is how an indirect tank will perform with the low 1gpm or so flow rates when used with solar panels? Is it possible for the flow to go laminar under these low flow conditions?

hr

Wayco Wayne_2

What do you mean

by laminar? Also different btu input rates by my reckoning means a bigger boiler can raise it's temp quicker to a high delta tee thus getting a jump on heat transfer. Also more gpm can help given a higher input rate.

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Steamhead (in transit)

Here's a discussion

of laminar flow. I think those W-M guys need to read it.

http://en.wikipedia.org/wiki/Laminar

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Wayco Wayne_2

Oh, I think I understand

Thanks for the link Frank. I think the issue HR was refering to is there might not be enough turbulence in the HX at such low flow to get good heat transfer in the HX. Please correct me if I'm wrong. This might explain my intuitive reasoning to not use a TT Smart tank for a solar project. I thought at such low flow the solar water would just rise through the outer tank and not envelope the inner tank for good heat transfer. WW

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