Increasing Pump Head

[JohnD]

... allowing the water to enter the rad quicker than it is leaving. ..

presumably only until they burst?

 

[sanj.varah]

Thanks for replying everyone. I do appreciate it when I get constructive replies. I do understand the London eye principle but I think I might have read a little too much into this.

There have been some replies stating the height of the radiators is irrelevant. I disagree with this because the central heating pump (technically an accelerator) pumps against a restriction (radiator and valve). The losses on a vertical section of pipe are more than a horizontal section of pipe of the same length.

The whole block of flats idea is that the suction and discharge pressures are equal but in reality the gain you get from the suction side is less than the pain on the discharge side. This is due to pipe friction / throttling of valves but also orientation (which nobody ever considers). You get more resistance moving fluid in a vertical direction than you do moving fluid in a horizontal direction. There is more “internal” fluid loss when pumping vertically than horizontally. Additionally there is more loss when moving fluid upwards than downwards.

In a nutshell what I’m saying is if you had a 100m length of pipe, the frictional losses are the same irrespective of orientation. But if the pipe is mounted vertically the Total losses (frictional + internal fluid) will be greater vertically than horizontally. This is completely different from head but still related to “head loss”.

I took the liberty of phoning up grundfos today and spoke to a guy in the tech dept. He said if the total resistance of the system was greater than approximately 150kPa, the pump would self circulate. He went on to say that this corresponds to a height of about 12m, beyond that the pump won’t pump as the resistance would be greater than 150kPa just in internal fluid and friction losses.

If I gave the impression that the head loss was purely as a height change then I apologize. What I meant was the internal fluid losses (caused by the change in height) and associated valves, not just 3.2m of height difference, when I guesstimated the loss it just so happened to come out at the same height as my rad. I actually worked it in KPa and then converted it to equivalent head but upon getting a colleague to re-read (English is not my first language) what I had written I see why it may have given the impression that I was talking about pure elevation change.

And to answer a few more questions.

Softus, Yes I did increase the size of my rads by 50-100% (on average), if you have a greater surface area at lower temperature, the flow temp is lower. My boiler manual tells me that the boiler becomes more efficient. If you think that’s Nonsense Softus, then that’s up to you. And if you don’t think there is a greater pressure loss through a larger radiator (effectively heat exchanger) then please could you justify it.

D Hailsham, I designed my heating system for a transient state not a steady state, So when I said I doubled my rad size, this was a bit of a simplification. The actual process involves the shape of the rooms, which way the room faces convection currents, position of furniture, temperature against time, predicting when the TRV’s would click off etc. Thus its not possible to actually say that my rads are emitting X kW of heat. This might have sounded OTT but I got fed up of having a heating system that would work in the steady state scenario but in order to get to the steady state would take 3 hours. I’ve only got half my house done but my aim was to get the house up from 10C to 22C in 15 minutes with a temperature outside of 1C. This is largely helped because of the parameters on my weather compensating boiler which increase the flow temperature for a predetermined period before backing off.

 

[Onetap]

You get more resistance moving fluid in a vertical direction than you do moving fluid in a horizontal direction. There is more “internal” fluid loss when pumping vertically than horizontally. Additionally there is more loss when moving fluid upwards than downwards.

In a nutshell what I’m saying is if you had a 100m length of pipe, the frictional losses are the same irrespective of orientation. But if the pipe is mounted vertically the Total losses (frictional + internal fluid) will be greater vertically than horizontally. This is completely different from head but still related to “head loss”. .

Bolleaux, IMHO.

Any references to this orientation losses theory, e.g. the CIBSE guide?
ISTR that the last time you quoted a text book to support your opinion, I had the book on my shelf (Eastop and McConkey) and couldn't find anything in it like you had clamied.

The truth is the opposite; you will get (apparent) reduced head losses if you're pumping the flow upwards because of the gravity circulation effect; the hotter flow column is less dense than the descending return column.

I took the liberty of phoning up grundfos today and spoke to a guy in the tech dept. He said if the total resistance of the system was greater than approximately 150kPa, the pump would self circulate. He went on to say that this corresponds to a height of about 12m, beyond that the pump won’t pump as the resistance would be greater than 150kPa just in internal fluid and friction losses.

Utter cobblers, IMHO.
Either you've made that up or they misunderstood what you meant; I think the former.

The system head-loss is proportional to the square of the flow-rate, so a system with too large a resistance would have a low flow rate, if the pump were undersized for it's duty. You'd still get some flow.

The actual process involves the shape of the rooms, which way the room faces convection currents, position of furniture, temperature against time, predicting when the TRV’s would click off etc. Thus its not possible to actually say that my rads are emitting X kW of heat. This might have sounded OTT but I got fed up of having a heating system that would work in the steady state scenario but in order to get to the steady state would take 3 hours. I’ve only got half my house done but my aim was to get the house up from 10C to 22C in 15 minutes with a temperature outside of 1C. This is largely helped because of the parameters on my weather compensating boiler which increase the flow temperature for a predetermined period before backing off.

Deary me, you do post some twaddle; do you think all Plumbers are stupid?

If you want to get the air temperature up that quick, put in a recirculating AHU; you'd still feel cold, due to the radiant losses to the cold surfaces in the room.

 

[Balenza]

sanj wrote

When I did my heating calcs I opted to increase the size of my rads by 50-100%

Don't you think that was OTT considering you have WC ?.
Why do you consider the pressure losses through your oversized radiators to be significant ?.
And just how much pressure drop difference is their between say a small ( K1 type) 438mm and a 2165mm (Length) assuming heights are the same ,lets say 530mm. (Myson).

 

[Agile]

This "head loss" is only a convenient way of converting frictional losses into an equivalent head but thats at FIXED FLOW RATE.

I am inclined to test Sanji's suggestions on a mouse in a tread wheel! If the wheel is over 12 m high then he says that the mouse will run out of steam and will be unable to climb up. We all know that mice can climb ups several floors.

I have to admit that he should know a lot more than I do about fluid mechanics but with an incompressible liquid I dont see that the direction of flow should make much difference. Just how does the liquid in the pipe know which direction its travelling in a closed loop?

I accept that a compressible and a vaporising liquid would be different. I would do my test with mercury at a low temperature perhaps.

Tony

 

[sanj.varah]

Sometimes I don't know why I bother, onetap you are clearly on a higher plain than me and I'm going to let you feel all high and proud. I would suggest you do a bit more research before you say something is "Ballcocks". Eastop and Mcconkey is thermodynamics - it isn't going to be in there?

Not sure its in this one but Fluid Mechanics by FM White.

Definitely in this one Advanced Engineering Fluid Mechanics by K. Muralidhar and G. Biswas P.436


Closed loop circulation with no heat transfer. If you start transferring heat then other things happen but I would have thought the restriction would have more effect than the thermosyphon.

I would just also point out that a number of people doubted I was telling the truth when I said the pressure loss on a reducer was greater in one direction than another. Nobody believed me that time either.

Tony, the wheel theory is fairly straight forward. Basically the pump in central heating is not positive displacement, so to actually get it going you need to overcome both frictional losses and internal fluid losses. If you increase the height of the column enough, the pump will not circulate irrespective of it being in a closed loop or not. Once the fluid is actually going, you enter a transient state and dependent on the elbows and orientation of the pipework you could "choke" the system.

Bit of a bad analogy but if you are diving, there will come a certain depth where you can no longer breathe without a pressurized supply. The same thing happens with the pump. If the column of water is too big - it won't pump.

OneTap, As regards to my heating calcs. Its part of some research I do so once the paper is published you can decide if you think its utter twaddle or not.

 

[sanj.varah]

sanj wrote

When I did my heating calcs I opted to increase the size of my rads by 50-100%

Don't you think that was OTT considering you have WC ?.
Why do you consider the pressure losses through your oversized radiators to be significant ?.
And just how much pressure drop difference is their between say a small ( K1 type) 438mm and a 2165mm (Length) assuming heights are the same ,lets say 530mm. (Myson).

This is dependent on the flow rate, at greater flow rate you get more loss. the loss is proportional to the flow rate squared.

The loss is usually given by

Loss Factor x 1/2 x Density (998) x Velocity (m/s)^2

This has the units of Pascals, Rearranging that and you get

a head loss

h = Loss Factor x 1/2 x Velocity^2/2gravity (9.81)

I don't think it was over the top. I just had my first gas bill on this system and it was £45 for a quarter (sep - dec). Its not quite the whole system i've got 4 more rads to install but my target was less than £150 for a whole year. I'm planning on increasing the loft insualation to 400mm so this should save me more money.

 

[Balenza]

Sanj wrote

I would just also point out that a number of people doubted I was telling the truth when I said the pressure loss on a reducer was greater in one direction than another. Nobody believed me that time either.

I think I may be one of those people.
You never answered my final point on that thread about velocity and the wide variations between the two sets of tables.

 

[ChrisR]

Sanj you seem to be saying that dear old Darcy Wiesbach (sp?) is wrong, or has forgotten about something.
head loss due to friction, head loss due to height gain. What else?
I don't believe there's a lot that new in the world of fluid mechanics, so perhaps you could refer to an equation which accounts for the additional "upwards friction loss" you're referring to?

I would just also point out that a number of people doubted I was telling the truth when I said the pressure loss on a reducer was greater in one direction than another. Nobody believed me that time either.

Can't imagine why they would, it's perfectly standard stuff.
That fact that you weren't believed on one thing does nothing to make you credible on another, I'm afraid!

Bit of a bad analogy but if you are diving, there will come a certain depth where you can no longer breathe without a pressurized supply. The same thing happens with the pump. If the column of water is too big - it won't pump.

Completely INVALID analogy! When you're diving, you don't have water INSIDE your lungs to help you push your chest out!

 

[D_Hailsham]

D Hailsham, I designed my heating system for a transient state not a steady state, So when I said I doubled my rad size, this was a bit of a simplification. The actual process involves the shape of the rooms, which way the room faces convection currents, position of furniture, temperature against time, predicting when the TRV’s would click off etc. Thus its not possible to actually say that my rads are emitting X kW of heat.

What I wrote has nothing to do with transient or static states and I did not say that you rads are emitting X kW of heat.

You said the following:

I did point out that I had circa 40kW (@80C flow, 60C return) of rads hooked up to my boiler

You also said that you were using 50°C flow and 30°C return. This means that the mean water temperature is 40°C and, assuming a room temp of 20°C, the differential temperature between radiator and room will be 20°C.

If you look in any radiator catalogue, you will find a table which shows the factor by which the radiator output must be multiplied if the differential is not 50°C. In your case the differential is 20°C, so the factor is 0.304. A radiator which delivers 1kW with a differential of 50°C will only produce 304 watts if the differential is 20°C. Radiators which produce 40kW will only produce 12.16kW.

Are you actually saying that, while the house is warming up, the boiler will be running at 80°C/60°C - i.e the rads will be delivering their advertised out put; but, when the room is up to temperature, the boiler will automatically throttle back to 50°C/30°C to maintain the room temperature?

The flow rate you quote of 1100 litres/hour will deliver 25.67kW for a temperature differential of 20°C.

Most designers allow a 10-15% over-sizing to allow for warming up. You seem to have allowed about 100%. I'm not sure whether this is over-sizing or overkill

So you have 40kW of rads being fed by a boiler which can only deliver 25.67kW?

Somehow I think you are trying to be too clever. You mention some factors you have taken into account, but what about factors such as doors opening and closing, people entering and leaving a room etc etc?

 

[sanj.varah]

D Hailsham, I designed my heating system for a transient state not a steady state, So when I said I doubled my rad size, this was a bit of a simplification. The actual process involves the shape of the rooms, which way the room faces convection currents, position of furniture, temperature against time, predicting when the TRV’s would click off etc. Thus its not possible to actually say that my rads are emitting X kW of heat.

What I wrote has nothing to do with transient or static states and I did not say that you rads are emitting X kW of heat.

You said the following:

I did point out that I had circa 40kW (@80C flow, 60C return) of rads hooked up to my boiler

You also said that you were using 50°C flow and 30°C return. This means that the mean water temperature is 40°C and, assuming a room temp of 20°C, the differential temperature between radiator and room will be 20°C.

If you look in any radiator catalogue, you will find a table which shows the factor by which the radiator output must be multiplied if the differential is not 50°C. In your case the differential is 20°C, so the factor is 0.304. A radiator which delivers 1kW with a differential of 50°C will only produce 304 watts if the differential is 20°C. Radiators which produce 40kW will only produce 12.16kW.

Are you actually saying that, while the house is warming up, the boiler will be running at 80°C/60°C - i.e the rads will be delivering their advertised out put; but, when the room is up to temperature, the boiler will automatically throttle back to 50°C/30°C to maintain the room temperature?

Unless I have misunderstood it looks like you have assumed a steady state, because you have assumed my room temperature is constant. I assume it starts at 10C and rises, In this case I get added benefit from a greater delta T both in reduced room temperature and 80C flow from the boiler (its actually 74C limited). the boiler output at 50C/30C is 35kW.

Your right, I haven't modelled doors opening and closing - which is something I should look at as it will be significant. Once i've finished my paper i'll put it on here.

ChrisR, There has been quite a lot on fluid flow in closed circuits in the past few years, the main reason being the growth of the use of heat pumps and also solar panels (which are commonly found on blocks of flats on the continent). I don't know if you have this book but Pump Theory by Igor Karassik also mentions the upward fluid friction phenomenon and has a graph showing the losses dependent on flow rate as a function of elevation change. This is an empirical graph and not a strict formula, you could plot a regression on it but i think it would be at least a quartic.

The problem is most formula are reliant on a number of conditions, usually inviscid/adiabatic flow. In reality this never occurs and the theorems are useless - an example would be bernoulli's equation in compressible flow, has no meaning in the real world because it assumes an isobaric fluid.

If you delve deeper then there are ways to model things but usually involves rather complex maths.

 

[sanj.varah]

Sanj wrote

I would just also point out that a number of people doubted I was telling the truth when I said the pressure loss on a reducer was greater in one direction than another. Nobody believed me that time either.

You never answered my final point on that thread about velocity and the wide variations between the two sets of tables.

what was the question again or do you have a link?

 

[Onetap]

Sometimes I don't know why I bother, onetap you are clearly on a higher plain than me and I'm going to let you feel all high and proud..

Nothing to do with me, you've posted some very dubious stuff, I've challenged it.

Just explain in small words why it won't circulate any water.

The static pressure at inlet and the outlet is the same. When the pump is started, it generates a small differential pressure and will start a small circulation. The water will accelerate until the losses are equal to the applied differential pressure.

How do vertical pipes stop all flow?

OneTap, As regards to my heating calcs. Its part of some research I do so once the paper is published you can decide if you think its utter twaddle or not.

You keep claiming academic superiority to support your claims, without actually explaining your theories.
I call your bluff; what you've posted is nonsense.
Your claims as to your qualifications are fictitious.

 

[Balenza]

Sanj wrote

I would just also point out that a number of people doubted I was telling the truth when I said the pressure loss on a reducer was greater in one direction than another. Nobody believed me that time either.

You never answered my final point on that thread about velocity and the wide variations between the two sets of tables.

what was the question again or do you have a link?

//www.diynot.com/forums/viewtopic.php?t=101309&postdays=0&postorder=asc&start=60

 

[sanj.varah]

Sanj wrote

I would just also point out that a number of people doubted I was telling the truth when I said the pressure loss on a reducer was greater in one direction than another. Nobody believed me that time either.

You never answered my final point on that thread about velocity and the wide variations between the two sets of tables.

what was the question again or do you have a link?

//www.diynot.com/forums/viewtopic.php?t=101309&postdays=0&postorder=asc&start=60[/QUOTE]

I can get to the iklimet table but the table referenced on photobucket is coming up blank for me.