coil flow direction - why?

[kevindgas]

have been reading another forum and the question being asked is why do we put hot flow in the top of the coil and the return out the bottom and not the other way round?
anybody know a reason why?

 

[kevplumb]

afik it's cos we used to use gravity

i did read somwhere its more efficient the other way round

 

[namsag]

As kev says legacy from the gravity days, but every fully pumped servowarm system i have seen the flow is at the bottom .

 

[kevindgas]

yes it probably is, but why as heat rises ?

 

[kevplumb]

ahh methinks the theory was as the water cools in the coil it falls

old habits and myths

 

[Biggles..]

Its by location,we we're taught flow in the bottom and return out the top,full heat is then delivered throughtout the entire cylinder(not that you loose much heat throught he length of the coil)

 

[bengasman]

Afaik, typical case of: we have always done it that way, left over from gravity era.
No regs or technical reason that I'm aware of.

 

[doitall]

Technical reason No 1

Think cold feed and vent and where they used to be prior to fully pumped systems.

My cylinder is flow in the bottom

 

[kevplumb]

made it then

 

[doitall]

made it then

And one

 

[Onetap]

Close, but no coconut.

It used to be done that way on gravity systems because that was the only way they would work, as others have said.

The other reason is that any heat exchanger is most efficient if piped contra-flow and not parallel-flow. You'd have to read a thermodynamics book to grasp the maths behind that, but the entering (hottest in this case) primary fluid should be in contact with the leaving secondary fluid. The flow in the cylinder is bottom to top; it also stratifies with the hottest water at the top. The primary fluid should go top to bottom.

It is quite possible that you could have 70/50 degC primary Flow/Return and DHWS stored at 60 degC. If piped bottom to top, the stored hot water at 60 degC would be heating the LTHW primary which should be leaving the cylinder at 50 degC. It wouldn't happen, the return temperature would be too high and the boiler efficiency would be reduced.

The only reason I know for piping bottom to top would be to shift air bubbles.

The gravity explanation seems to be 'common knowledge' among pipe fitters who install air handling units, who usually install the pipework incorrectly rather than read the labels on the pipe connections. I've never yet seen one that wasn't meant to be piped contra-flow.

 

[bengasman]

Technical reason No 1

Think cold feed and vent and where they used to be prior to fully pumped systems.

I actually said: "there is no technical reason."

I never claimed there never has been one, like before the mid 20th century.

The romans used stone and clay to pipe the water; I think that became outdated as well with the arrival of commercial availability of metal pipe.

 

[Onetap]

The romans used stone and clay to pipe the water; I think that became outdated as well with the arrival of commercial availability of metal pipe.

And you're wrong about that as well.
They used lead pipes. The word for plumber comes from the latin word for lead, plumbum. The lead lined baths uncovered at Bath were still watertight. They used stone and clay in their aqueducts. Smart guys, them Romans.

 

[kevplumb]

The romans used stone and clay to pipe the water; I think that became outdated as well with the arrival of commercial availability of metal pipe.

And you're wrong about that as well.
They used lead pipes. The word for plumber comes from the latin word for lead, plumbum. The lead lined baths uncovered at Bath were still watertight. They used stone and clay in their aqueducts. Smart guys, them Romans.

go get it floyd

 

[dextrous]

. Smart guys, them Romans.

Yeah, but what did they do for us