Low loss header

[3yearsapprentice]

Hi there,would like to know why low loss headers are designed with a max velocity of .5 M/s,the other question would be why is the diameter 3 times the diameter of the secondary pipework & for what reason?Is there a calculation for this or just a rule of thumb,if a calculation does exist could someone please explain in detail what the figures are what they represent.
The company i work for don't really tell me much about the techi side of things,one job we are doing at the moment comprises of a header with flow/return from boiler feeding into each end,off of this we have 3 heating zones with flow & return for each circuit piped very close to each other,surely the last zone would be fed with water cooler than flow temperature from boiler.Not much info on the so thought i would ask you peeps.

Sorry for long post.

Adrian.

 

[Onetap]

Maybe they're not telling you because they don't know? Are they working to a rule-of-thumb they've picked up somewhere?

There should be negligible pressure loss across the LL header at the primary design flow rate; I think CIBSE's design guide recommends 40Pa maximum. There will be a pressure loss due to friction along the LL header, at the primary (boiler) flow rate. Any significant pressure difference between a secondary circuit's flow and return branches will induce flow in that circuit, without the pump being on.

You should have the 3 secondary flow branches at the same end as the primary flow inlet and 3 secondary return branches and boiler return connection at the other end. You will otherwise get mixing in the LL header and a reduced secondary flow temperature.

If the secondary flow is greater than the primary flow, you will get recirculation and mixing in the LL header. Just sketch out the header and mark on the flow rates at each point. It is surprisingly simple, the flow rates, pressure losses and temperatures at every point can be calculated.

 

[monoxide62]

Seems you have whats known as 'closely spaced' tees , what this achieves is to eliminate any interaction between primary/secondary circulators , much like a one pipe system wherby flow & return pipework is taken off the ring main with very little distance between the two , anyone familiar with one pipe systems will know water won't flow through the emitter , the further appart the tees are placed the more the water will flow through it.

Water velocity is kept low in order to release any air that is carried back from sytem water , the higher the velocity the more the air is pulled around the system & not released , with low velocitys air can be eliminated via air vents situated on top of LLH which in part is an 'air catcher' , this is the reason the LLH should be installed vertically , this also helps collect any debri from system water as it passes through LLH , valves at low point of LLH are useful for cleaning any debris collected.

LLH can be designed in a way whereby the first set of closely spaced tees don't cause any issues with regards to other zone temperatures down stream of header.

The '3 times the diameter of secondary pipework' is a given when pipe sizing for a velocity of .5 m/s.

 

[ajrobb]

I would have thought that a LLH should also maintain nearly 20°C between boiler flow and return in order to maximise condensing efficiency while protecting the boilers. Ideally the LLH pump(s) should modulate with boiler output to keep to 20°C rise. However, it the LLH tries to maintain 20°C but the heater circuits have a minimum drop of 11°C then somethin' got to give...

 

[DeltaT2]

Seems you have whats known as 'closely spaced' tees , what this achieves is to eliminate any interaction between primary/secondary circulators , much like a one pipe system wherby flow & return pipework is taken off the ring main with very little distance between the two , anyone familiar with one pipe systems will know water won't flow through the emitter , the further appart the tees are placed the more the water will flow through it.

Water velocity is kept low in order to release any air that is carried back from sytem water , the higher the velocity the more the air is pulled around the system & not released , with low velocitys air can be eliminated via air vents situated on top of LLH which in part is an 'air catcher' , this is the reason the LLH should be installed vertically , this also helps collect any debri from system water as it passes through LLH , valves at low point of LLH are useful for cleaning any debris collected.

LLH can be designed in a way whereby the first set of closely spaced tees don't cause any issues with regards to other zone temperatures down stream of header.

The '3 times the diameter of secondary pipework' is a given when pipe sizing for a velocity of .5 m/s.

Close spaced tees.
This is called Primary/Secondary circulation in the States, though commonly used here for years in small semi-commercial jobs. It's a great way of controlling multi-zone installs. If I recall correctly; the tees should be no more than 150mm apart.

Thanks to everyone I love threads like this.

 

[DeltaT2]

I would have thought that a LLH should also maintain nearly 20°C between boiler flow and return in order to maximise condensing efficiency while protecting the boilers. Ideally the LLH pump(s) should modulate with boiler output to keep to 20°C rise. However, it the LLH tries to maintain 20°C but the heater circuits have a minimum drop of 11°C then somethin' got to give...

I think the boiler/boilers manufacturer will state the flow rate & head loss through their boiler to give the optimum 20°C Delta T on the Primary F&R. Of course the 'Low Loss' on the heater would also need to be considered.

 

[monoxide62]

Close spaced tees.
This is called Primary/Secondary circulation in the States, though commonly used here for years in small semi-commercial jobs. It's a great way of controlling multi-zone installs. If I recall correctly; the tees should be no more than 150mm apart.

.

Correct P/S systems.

The '150mm' referred to the 'close coupled tee arrangement' not to be confused with 'closely spaced tees' , these referred to the cold feed expansion/open vent , same sort of thing with regard to low friction loss , the 'closely spaced tee' arrangement is based on a max distance of 4 pipe diameters appart which would put the tees for a 42mm secondary circuit at a max distance of 168mm , although the tighter the better.