Why does hot water rise to the top of a water heater?

[Tom.G]

The designers may prefer hot coming down a coiled heat exchanger pipe, working with rather than against the (separate) convection within the heating fluid

That configuration is the well-known Counterflow Heat Exchanger. Its advantage is that the hottest part of the hot side heats the hottest part of the cold side fluid.

If both fluids are flowing in the same direction thru the heat exchanger, then the cold fluid can typically get only as hot as the average of the hot and cold sources. (unless the hot side is grossly over-provisioned)

https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node131.html

Cheers,
Tom

 

[Ken Fabian]

That configuration is the well-known Counterflow Heat Exchanger. Its advantage is that the hottest part of the hot side heats the hottest part of the cold side fluid.

If both fluids are flowing in the same direction thru the heat exchanger, then the cold fluid can typically get only as hot as the average of the hot and cold sources. (unless the hot side is grossly over-provisioned)

https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node131.html

Cheers,
Tom

A hot water system exchanges heat but a dedicated heat exchanger is designed differently.

Typically in a hot water system it is a tank - and the OP appears to be about hot water rising to the top in HW systems like that - and typically the hot side IS over-provisioned (and very quickly an overturning circulation is generated). Whilst that mixing is short of absolutely homogeneous there is very little vertical temperature differentiation left.

For most of the heating process the temperature difference between top and bottom within the tank is insignificant; the heating pipe will still have a temperature gradient between inlet and outlet but the water being heated will not be much hotter at the top than the bottom.

 

[snorkack]

That configuration is the well-known Counterflow Heat Exchanger. Its advantage is that the hottest part of the hot side heats the hottest part of the cold side fluid.

If both fluids are flowing in the same direction thru the heat exchanger, then the cold fluid can typically get only as hot as the average of the hot and cold sources. (unless the hot side is grossly over-provisioned)

https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node131.html

Cheers,
Tom

The thing is, there is no flow in the cold side of the tank, just convection if any.
If you put an electric heating coil at the top of a cold water tank then the thin layer at the top of the tank will be heated to boiling while the bottom of the tank is heated by conduction only - very inefficiently.
The same will happen if you put a heat exchanger pipe on top only.
If you put a heat exchange pipe in a tank with inlet on top and outlet in bottom then the contents of the tank will be heated layer by layer from top down. There will be top layer of the tank where the heat exchange is inefficient because the tank contents are heated to almost the temperature of pipe inlet, middle layer where the active heat exchange occurs and bottom layer where the heat exchange is again inefficient because the pipe outlet has cooled to almost tank bottom temperature.

 

[Tom.G]

Well, I seem to have been un-clear in my post about counterflow heat exchangers. (post #61, above. https://www.physicsforums.com/posts/7250242)

I was referring to post #60 by @Ken Fabian where he describes that exact configuration, hot heat exchanger inlet towards the top of the water tank and cold exchanger return closer to the bottom of the tank.

Our previous hot water system was a rooftop solar one, with hot water tank above the solar collectors. Heating fluid was glycol that flowed through a sleeve around the bottom third of the water tank and the flow of that heating fluid was entirely convection driven and that requires the inlet (hot) to be at the highest point of that sleeve. (Not a big height difference with the outlet - more horizontal flow than top to bottom.)

I was pointing out that the configuration he describes is the most efficient/effective for heating with a heat exchanger.

If you care to go thru the math, the link I provided in post #61 has the details.
(https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node131.html)

I hope this helps clear up any ambiguity.

Cheers,
Tom

 

[Ken Fabian]

@Tom.G - I still maintain it makes no significant difference to the efficiency of heat exchange; it is done like that to enable passive (no pump) convection driven circulation within the heat source side of the system.

Without a pump you would impede that circulation if the hottest point of the exchanger piping is at the bottom instead of the top. With a pumped system (solar HWS' with water tank below solar heat collectors use pumps) it could run in reverse - that convection is weak compared to a suitable pump.

The reason I think it doesn't matter much to the efficiency of heat exchange is convection driven overturning circulation in the water tank mixes the water and eliminates any vertical temperature gradient. The hottest part of the exchange pipe (where it enters the tank) is in contact with water of the same temperature as the coldest part (where it leaves the tank).

One more time - overturning convection mixes the water in the tank and eliminates a temperature gradient.

We can quibble about initial conditions - before that overturning circulation mixes the tank contents - and end state conditions where the contents approach the same temperature as the heating pipe. Or quibble that the temperature is not absolutely homogeneous even when that overturning circulation is dominant and quibble that the rising hotter water from a lower coil will flow past higher coils. I just don't think that will make significant difference to the heat exchange.

I have tried to be clear in my language that I am talking about most of time that heating is taking place and the difference between strong heating that will drive mixing and weak heating that will not.

 

[sophiecentaur]

The thing is, there is no flow in the cold side of the tank, just convection if any.

Draw a sketch of the convectional flow in the tank. For every gram of water that flows upwards there's a gram flowing downwards. Up past past the coil and down the outside skin; there's your flow. (In the case of the coil being outside , the flow is still upwards for the heated water by the input water).

You could ask why the tanks are always connected that way. Could it just be general ignorance?

Remember the old 'gravity systems? Hot water flowed up from the back boiler and dropped down through the hot tank; there was no way you could persuade it to ignore gravity and flow the other way. If you connected it so that hot water flowed into the bottom of the coil you could end up with a plug of hot water with nowhere to go. (Stratification?)

 

[sophiecentaur]

Why would heating the water be too vigorous to allow stratification? Please explain this using simple language.

You can only get stratification when there's no horizontal gradient of temperature. There will always be some gradient from the heating coil and the skin of the tank so the outer (virtual) column will drop