head loss caused by wrong inlet /outlet connection of heat exchanger ?

[YuriB]

What mey be typical head loss of an AHU heating coil being fed at bottom in comparison to one fed at the top ? The medium being water.

 

[Drazen]

water direction is related to heat transfer, uniformity of heat distribution, eventually influences heat capacity, not pressure loss.

wrong direction of control valve is another story, but you are not mentioning control valve.

 

[willard3]

Pressure drop will be the same.

 

[YuriB]

I thought water flow should decrease as hot water going up in the coil get cooled, thus denser, thus affected by gravitation drawn downwards thwarting the upwards flow of the hot water. The coil pressure drop should in effect decrease, while pump static pressure drop increase. Is not vertically positioned heat coil bottom connection wrong in principle ?
Mixing valve being connected correctly.

 

[IRstuff]

Conservation of mass; what goes in at a rate must come out at the same rate

TTFN
faq731-376

 

[Drazen]

What you described is called stack effect within hydraulics, and such physical process indeed exists, but for water you would need at least several meters height difference for such effect to be measurable at all. 20 meters height difference at 20 degrees temperature difference gives about 2,5 kPa of stack effect, so you can get a picture.

Such an effect has influence on distribution heating pipework calculation in high-rise buildings.

 

[YuriB]

Drazen, then, swapping pipes on a heating coil fed at bottom and giving problems (insufficient flow on an unknown reason) would scarcely improve the flow ? Such as now connection of the pipes cannot be the reason for the inadequate flow ?

 

[Drazen]

yes.

going back to practical probabilities - if you change direction of waterflow it is possible that your control valve will receive flow in wrong direction, and whil i cannot be certain, i can suspect that could be cause of problem - if your problem only exists when you swap connections.

you mentioned in your previous post that "mixing valve is connected correctly", but correctness of mixing valve connection can directly depend on water flow, i.e what is correct in one flow direction can be incorrect in opposite flow direction.

mixing valves normally have symbols on their bodies which, first show whether they do hydraulic mixing or hydraulic diverting, second they show what are inlet and what are outlet port(s).

 

[YuriB]

The valve is ESBE, series MG, it can pass in any direction - what I deduce seeing the descriptions I found. Most important : the supply at bottom of a heating exchanger is not so critical as I asumed previously. Thank you !

 

[YuriB]

Just now I have found a picture of the same exchanger I believe - and fed at bottom as it is shown it should direct water against the air flow, so, all is connected correctly.

 

[ChasBean1]

YuriB, your concern is sound but you're missing the reasoning behind typical heating coil installation with supply at the bottom (on the air outlet side) and return at the top (on the upstream air side). Water velocity increases and flow directions change upon entering the coil. This tends to strip gases out of solution. Gases rise in an aqueous solution. Running water from top to bottom goes against this natural occurrence and could cause head loss and vapor binding. The difference in pressure drop due to density changes in the water from a hot (top) inlet to a warm (bottom) outlet is negligible. The addition of waterborne air issues and head loss due to flow against vapor bubble release/buoyancy is more substantial. Mathematically, the density of a 140°F return is 61.38 lb/ft3. A 180°F supply is 60.58°F. A typical coil might have 12" between inlet and outlet heights. The density difference would cause a pressure difference of 0.006 psid, which really is negligible relative to coil pressure drop and other factors.

 

[YuriB]

Water velocity increases and flow directions change upon entering the coil.

What do you mean ?
I am sorry that I cannot post the picture I drawn. The installation have performed some absolute morones, that is the problem.

 

[ChasBean1]

What do I mean, what do YOU mean? "The installation have performed some absolute morones, that is the problem." You lost me there.

What I meant was that a typical coil might have 3/4 inch piping to and fro, but reduce to 3/8 inch at the inlet and outlet fittings. This is by design, as a velocity increase lessens the laminar boundary layer and provides better heat transfer. For a 3 gpm coil (as an example), our 2.2 feet per second velocity would increase to about 9 fps through the coil tubing. The water then turns 180° a few times before exiting.

So with this hot water, the big rise in velocity, and the bends through a coil, gases can tend to leave solution.

 

[YuriB]

We have received a cite to service recently and a more idiotic istallation I did never seen.

The heat exchangers in AHU's have two vertical collectors (possibly, one such HE may have 3/4 pipe collectors) feeding between them a dozen or so coils (maybe, one HE having them of 3/8 diameter). But as there are many of them, water velocity in them should rather decrease.

 

[YuriB]

Why there is not allowed post editing ?

To clear, I described in my previous post not the idiotic installation I mentioned, but AHUs in general.