Max flow velocity in 15mm pipe?

[sharpener]

Is 1.6m/s reasonable for intermittent use in a domestic setting?

 

My existing S-plan installation has bizarrely only got a 15mm feed and return to the coil in the OSO 210l tank. The performance is fine on my Kidd oil boiler. Perhaps the original installer in 1995 thought that using 22mm pipe would steal all the flow from the radiator circuit?

 

Am now planning to replace the boiler with a Vaillant 12kW heat pump. The existing cylinder is in an inacessible void above the airing cupboard and V have eventually agreed I do not need to replace it*. I intend to heat up a 260l thermal store in parallel with the cylinder, so plan to divide the flow 60%/40% in proportion to the typical usage using a flow setter for the TS.

 

This will result in a flow through the coil of 2065/60 * 0.4 = 13.8 l/min so from this calculator the velocity would be 1.6m/s.

 

And the pressure drop over a 6 m run using 20% glycol of 190mbar, which is well within the HP's capacity of 550mbar.

 

* Provided I fit a secondary circulation pump to improve the heat transfer from coil to tank. Not germane to this question.

[SimonD]

Maybe I've missed something but this calculation doesn't seem to include how much heat you're moving through the system, which is what you need to determine the flow rate in your heating system. What Delta T are you assuming? At what temperature are you heating your cylinders to?

 

Your system currently works because I'd assume that your oil boiler was commissioned to work at a 70C flow with 50C return - so as an approximate difference, at Delta T 20 and a flow rate of 0.9m/s you'll move about 11kW in water, if you're running a heat pump at 7C Delta T this will move about 4kW. With glycol this will be less as you'll only move about 3,600J/kg K with 20% glycol content.

 

Max velocity is usually considered to be 1.5m/s but more ideally around 1.

[sharpener]

10 minutes ago, SimonD said:

Maybe I've missed something but this calculation doesn't seem to include how much heat you're moving through the system, which is what you need to determine the flow rate in your heating system. What Delta T are you assuming?

 

Yes it does, it assumes the nominal 12kW will be divided as 7.2 kW to the TS and 4.8kW to the cylinder in proportion to the flow rates.

 

5 deg delta T multiplied by the max flow rate of 2065l/hr from the Vaillant manual equates exactly to the 12kW output.

 

I am not concerned about the heat transfer, that has been dealt with in other calcs (and a long discussion involving a simulation thanks to @JamesPa), only whether I can get by with the existing 15mm pipework (even if it is less than ideal).

 

The pressure drop across the TS will be negligible in comparison so I will need to restrict its flow with a flow setter to get the right apportionment otherwise the coil will be starved.

 

14 minutes ago, SimonD said:

Max velocity is usually considered to be 1.5m/s but more ideally around 1.

 

Can you provide a source or link for this please? If I tweak the proportions to 7.5 and 4.5 kW it will come out at 1.5 exactly which sounds OK then.

 

 

[JohnMo]

1 hour ago, sharpener said:

1.6m/s

The normal max velocity for hot water below 60 degs through copper is 1.5m/s, above 60 the flow velocity is less than half that. Cold water can be up to 2.4m/s, but likely to be pretty noisy.

 

High velocity will strip the corrosion layer away inside the pipe and never lets it reform and you get high erosion rates.

 

As a side note Hep2O is only happy with 1.3m/s for hot water.

 

Sounds like the pipe is bit small.

[SimonD]

3 minutes ago, sharpener said:

Can you provide a source or link for this please? If I tweak the proportions to 7.5 and 4.5 kW it will come out at 1.5 exactly which sounds OK then.

 

Domestic Heating Design Guide

 

27 minutes ago, sharpener said:

5 deg delta T multiplied by the max flow rate of 2065l/hr from the Vaillant manual equates exactly to the 12kW output.

 

I think you're slightly low on your figures. My calculation at Delta T 5 and 12kW equates to 0.67 kg/s mass flow rate which is about 40.20 l/m  and 16l/m if your assumption of 40% distribution to the cylinder works out. So you're looking at a higher flow rate that you expected, I think.

 

Formula taken from Domestic Heating Design Guide.

[sharpener]

52 minutes ago, SimonD said:

I think you're slightly low on your figures. My calculation at Delta T 5 and 12kW equates to 0.67 kg/s mass flow rate which is about 40.20 l/m  and 16l/m if your assumption of 40% distribution to the cylinder works out. So you're looking at a higher flow rate that you expected, I think.

 

That would equate to 1.8 m/s. Maybe the difference is a correction for the density of glycol solution? My formula is 1kWh = 861 deg - litres, which is 3600 seconds in an hour / 4.184 specific heat of water. Agrees exactly with Vaillant data sheet.

 

1 hour ago, JohnMo said:

The normal max velocity for hot water below 60 degs through copper is 1.5m/s, above 60 the flow velocity is less than half that. Cold water can be up to 2.4m/s, but likely to be pretty noisy.

 

High velocity will strip the corrosion layer away inside the pipe and never lets it reform and you get high erosion rates.

 

Yes, I see the 1.5 m/s is widely quoted and comes from the Copper Development Assn (it agrees with the erosion energy method here on the assumption it is running 10h 40 mins a day).

 

My worst case scenario is in summer where 6kW (which is the min output of the HP) is reheating the cylinder on its own from stone cold, which will take 90 mins. According to the same method 90 mins/day at 2m/s equates to 360 (mins x velocity^2) which is only 25% of the permissible 6 hours (1440 mins x velocity^2).

 

Because of the square law 1.6 m/s is much much better and 90 mins of that corresponds to 16% so it is in fact allowable for 9h 22.5 mins in every 24.

 

John Hearfield states that anything up to 2.0 m/s is OK from the POV of noise. I will see if I can find the performance curves of the Grundfos pump in the existing boiler and try it on max, that might give me a feeling for it. Currently it is on min and inaudible everywhere.

 

1 hour ago, JohnMo said:

 

Sounds like the pipe is bit small.

 

 

Wouldn't be my first choice but access to change it is massively difficult, it would need precision drilling up through a multilayer acoustic ceiling with metal laths in it, then the chipboard flooring 8in above that and finally up a duct where the boiler flue is. I might just tackle it with my long drill extensions but I would not want the installers to try as the acoustic tiles are easily damaged and can't be painted over.

 

Thanks for the help guys, looks like it's OK from the erosion perspective, and we can assess the noise before putting the expensive gylcol in.

 

[JohnMo]

10 minutes ago, sharpener said:

My worst case scenario is in summer where 6kW

Maybe, you have to checked if your heat pump will modulate the circulation pump when doing DHW, mine just goes to full speed and stays there until DHW cycle is complete.

 

Also have you checked the pump curve, to verify the actual flow rate you achieve with your expected pressure drop.  The bigger the pressure drop (head required) the lower the flow rate you get.

[SimonD]

46 minutes ago, sharpener said:

4.184 specific heat of water

 

This is where your difference lies as a glycol solution has less heat capacity than water (at 100% it's about 2500 J/kg K). Also, don't forget you'll need to add extra pumping resistance as a consequence of this.

[sharpener]

49 minutes ago, JohnMo said:

Maybe, you have to checked if your heat pump will modulate the circulation pump when doing DHW, mine just goes to full speed and stays there until DHW cycle is complete.

 

AFAIK the Vaillant Arotherm Plus does modulate the flow along with the output in order to maintain the 5C delta T. They have approved the calcs which show the heat transfer reducing as the tank heats up which imply this. I can also use the noise reduction mode to turn the whole thing right down in summer, I am positive that turns down the pump too.

 

49 minutes ago, JohnMo said:

Also have you checked the pump curve, to verify the actual flow rate you achieve with your expected pressure drop.  The bigger the pressure drop (head required) the lower the flow rate you get.

 

The data sheet describes the 550 mbar as "Remaining Feed Pressure" so I am assuming this is what there is left to drive the external building loop at the full flow rate of 2065l/hr. Even if it means just the static head at zero flow, it is in any event so large in comparison with the projected 195mbar drop across the coil circuit that I am not worried assuming the usual sort of pump curve that is convex towards the top rh corner.

 

14 minutes ago, SimonD said:

This is where your difference lies as a glycol solution has less heat capacity than water (at 100% it's about 2500 J/kg K). Also, don't forget you'll need to add extra pumping resistance as a consequence of this.

 

The 195 mbar result from this simulation was for the 20% glycol case, so was on the pessimistic side; we don't need as much as that because we are in the SW, the installers have yet to tell me exactly what they think is appropriate. 10% would give 4016 J/kg K or a 4% drop which is not exactly a disaster in the context.

 

 

Edited February 15 by sharpener

[Beelbeebub]

On 15/02/2024 at 13:26, JohnMo said:

The normal max velocity for hot water below 60 degs through copper is 1.5m/s, above 60 the flow velocity is less than half that. Cold water can be up to 2.4m/s, but likely to be pretty noisy.

 

High velocity will strip the corrosion layer away inside the pipe and never lets it reform and you get high erosion rates.

 

As a side note Hep2O is only happy with 1.3m/s for hot water.

 

Sounds like the pipe is bit small.

I found massive turbulence corrosion in a 15mm cold water copper pipe running around 8lpm albeit after about 9 years 24/7 and just downstream of a restrictor that would have generated significant turbulence.

 

The pipe was paper thin and full of pin holes for about 40mm downstream of the restriction. 

[sharpener]

15 hours ago, Beelbeebub said:

I found massive turbulence corrosion in a 15mm cold water copper pipe running around 8lpm albeit after about 9 years 24/7

 

Sounds worrying! As @JohnMo says upthread, the issue is a balance between eroding the film in service and leaving time for it to re-form in between, 24/7 operation doesn't allow that.

 

I am hoping there is plenty of time in my case, the model shows a safety factor of > 4x, I don't know how authoritative it is but that is a big margin.

 

I will be sure to have them fill the circuit from the mains. Despite treatment my harvested rainwater is a bit acidic but I know the mains is slightly alkaline.

[JohnMo]

1 hour ago, sharpener said:

will be sure to have them fill the circuit from the mains. Despite treatment my harvested rainwater is a bit acidic

I am doing an offshore job at the moment they have a pH of 5 from the water they make (reverse osmosis), and have been using copper unvented cylinders, which have to be replaced every two years, as they start to leak. Think the slight acidic nature stops the corrosion skin forming and copper disappears quite quickly. So good call on the use of tap water instead of rain water.

 

1 hour ago, sharpener said:

eroding the film in service and leaving time for it to re-form in between

You still end up eating the base material away, as the corrosion skin is just base material corroded. So each cycle takes the odd micron of material away.

 

I would be tempted to add a gate valve and simple flow meter downstream or a flow setter, you can tune the flow rate, to get the best compromise between reheat time and lowest erosion risk. Add it to section of 22 or 28mm prior to reducing down to 15mm.

[sharpener]

On 15/02/2024 at 11:44, sharpener said:

I intend to heat up a 260l thermal store in parallel with the cylinder, so plan to divide the flow 60%/40% in proportion to the typical usage using a flow setter for the TS.

 

As upthread the flow setter is planned for the TS branch as that will have almost zero resistance otherwise and steal all the flow. At £51 a time not sure I need a second one on the coil cct as the Arotherm display will show the total anyway.

 

3 hours ago, JohnMo said:

I am doing an offshore job at the moment they have a pH of 5 from the water they make (reverse osmosis), and have been using copper unvented cylinders, which have to be replaced every two years, as they start to leak.

 

Why on earth do they not use s/s tanks then (like the OSO tank I have got)? Or treat the water.

[JohnMo]

2 hours ago, sharpener said:

 

Why on earth do they not use s/s tanks then (like the OSO tank I have got)? Or treat the water

No idea, but duplex ones will be going in when I'm finished. Whole design is pretty poor overall.