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how much heat 42mm,35mm,28mm,22mm copper pipe carry


 
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corgigazza

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PostPosted: Thu Nov 15, 2007 4:26 pm Reply with quote

Need to do system design for 55kw of radiators in office the floor area is 28m x 13m approx with 2.5m ceilings worked all the raditor sizes but am unsure of what the pipesizing will be, also the pipe work will be in the ceiling void is it worth insulating the main runs, looking to install buderus 800 60 kw bioler28mm flow & return tappings, this system only does rads.
Also need to fit ex vessel what is the guide lines for this was thinking of 24 litre
thanks gazza
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chrishutt

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PostPosted: Thu Nov 15, 2007 6:12 pm Reply with quote

The heat carrying capacity of a tube is limited by the maximum velocity that can be accepted for noise reasons. This is generally taken as 1.5 m/s. The other key factor is the temperature drop of the system which is generally taken as 20K for a condensing boiler, although greater or lesser temperature drops could be specified.

For 1.5 m/s and 20K temp drop, the carrying capacity of each pipe size is as follows -

8mm = 4.5 kW, 10mm = 7.3 kW, 15mm = 18.1 kW, 22mm = 40.2 kW and 28mm = 67.5 kW.

However these figures are maxima and may need to be reduced to take account of frictional resistance and available pump head. That can only be calculated for the individual installation, based on the index circuit.

With a temperature drop of 20K or more, care has to be taken with radiator sizing because the average temp of the rad will reduce with temp drop. For example with a max flow temp of 80C and a 20K drop the average temp will be 70C. With a room temp of 20C the differential temp (delta t) will be 50K. Many radiator outputs are quoted based on a delta t of 60K so such output figures would have to be reduced by 17% for delta t of 50K.
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D_Hailsham

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PostPosted: Thu Nov 15, 2007 6:46 pm Reply with quote

chrishutt wrote:
Many radiator outputs are quoted based on a delta t of 60K so such output figures would have to be reduced by 17% for delta t of 50K.


I thought all manufacturers now quoted outputs to BSEN442 i.e Mean Water Temp = 70degC and Room Temp = 20degC icon_question.gif

The only thing which is different from normal installation practice is the the Standard specifies radiator connections as Top and Bottom, same end.
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chrishutt

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PostPosted: Thu Nov 15, 2007 6:52 pm Reply with quote

You could be right. I haven't actually checked that, but I'm sure I've seen some delta 60K figures recently - ah yes, here.
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corgigazza

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PostPosted: Thu Nov 15, 2007 7:39 pm Reply with quote

thanks for that
seems surprising & 67kw for 28mm so thats why the buderus 800 has only 28mm f & r i had visions of using 35 & 42mm pipework, how can i work out the ex vessel size is it worth insulating pipes in ceiling void.
cheers gazza
found out how to work out in the net if anyone is interested
http://www.rwc.co.uk/content.aspx?page=ABOUT_EXPVESS#-1_g-ABOUT_EXPVESS_idMainTop(2)
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Terrywookfit

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PostPosted: Thu Nov 15, 2007 7:57 pm Reply with quote

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gas4you

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PostPosted: Thu Nov 15, 2007 9:58 pm Reply with quote

All rads I fit, stelrad and myson have outputs quoted at 50C which was brought in to align us with eec about 2 years ago.
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bengasman

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PostPosted: Thu Nov 15, 2007 11:52 pm Reply with quote

If you use 2 radial lines in stead of one loop, you will only have to deal with 30 kw, that solves half the problem.
Secondly, the heating capacity seems enormous. Yes it is a large area, but heatloss will be predominantly from the walls, not the floor space, as I am sure you know. Any chance you made a mistake in the heatloss calculation? I used to live in house with house with about 30 metres of external wall, 3 metres high, times 2 floors is 180 square metres. 20 kw keeps it nice and warn in midst of winter, even with sash windows. Your office is about 200 square metres wall surface, seems a bit odd that it would need 3 times more heat.
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D_Hailsham

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PostPosted: Fri Nov 16, 2007 10:08 am Reply with quote

chrishutt wrote:
You could be right. I haven't actually checked that, but I'm sure I've seen some delta 60K figures recently


And they even mix their units by giving output in BTU (BThU?).

I was thinking of manufacturers catalogues, e.g Stelrad, not suppliers.

Why the K? I thought system design allowed for a lowest temp of -1C or -3C not -273C. And with global warming this may be too low icon_wink.gif

The OP might like to read:

Copper in small bore heating systems
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chrishutt

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PostPosted: Fri Nov 16, 2007 11:53 am Reply with quote

D_Hailsham wrote:
Why the K?

Just showing off really, but I didn't get it quite right!

The kelvin (symbol: K) is one of the 7 SI base units, which ideally should be used in preference to non standard units. As it happens a temperature difference of 1K is exactly the same as 1C, so when referring to temperature intervals it could be considered better (from an academic or anal point of view) to use K than C, plus it saves having to do that Alt 0176 thing on the keypad to get the symbol.

When referring to relative temperatures something like 20C is obviously better understood than 293K. So that is what I attempted to do above, except that I thought it was K when the correct form is just K.
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corgigazza

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PostPosted: Fri Nov 16, 2007 6:08 pm Reply with quote

worked out rads sizes then compared this with total floor space which gave very similar readings

used this web site for working total input which comes out at 163254 btu(2 outside walls) then divided this by 3412 to get kw. 47kw times this by 20% for pipeloss then add 10kw for existing rads in system
http://www.radcalcs.com/
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ChrisR

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PostPosted: Fri Nov 16, 2007 6:15 pm Reply with quote

Quote:
add 10kw for existing rads in system

EH?

Size fits with 50 watts per cubic metre, which is nearly always on the high side.
28 x 13 x 2.5 /20 = 45.5kW. Saves hours for a quick guesstimate!
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chrishutt

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PostPosted: Fri Nov 16, 2007 6:28 pm Reply with quote

corgigazza wrote:
used this web site

That web site looks like a joke. I really don't think you should base a 50 kW system on such an amateurish calculator. There are much better calculators available.

Besides which offices aren't the same as homes. There's all the heat given out by equipment and lighting to take account of, plus the heat from the workers themselves. It would be sensible to get a professional assessment of the heating needs.
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corgigazza

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PostPosted: Fri Nov 16, 2007 6:30 pm Reply with quote

thanks need to write all this use full info down

the office has some rads and small system bioler, the rest of the office uses warm air in ducts supplied from central commercial gas heater this keeps breaking down even the manufacures cant seem to keep it going longer than two weeks. hence rads need to be fitted

a quick ball park figure was needed asap
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