90o water supply at 6lpm.

Discussion in 'Electricians' Talk' started by Deleted member 33931, Aug 4, 2016.

  1. Anyone know of an instant electric heater that can supply water at 90oC at around 6 lpm?
     
    btiw likes this.
  2. Phil the Paver

    Phil the Paver Screwfix Select

    Why so hot. ???
     
  3. Mr. Handyandy

    Mr. Handyandy Screwfix Select


    2 large kettles?

    Mr. HandyAndy - Really
     
    Deleted member 33931 likes this.
  4. Rulland

    Rulland Screwfix Select

    That would have to be a hefty heater to basically boil 6 litres in 60 seconds!:eek:
     
  5. Yup.

    It's for a product test. Not mine...
     
  6. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    What do you get out of these new 'Quooker' taps?

    Mr. HandyAndy - Really
     
    Deleted member 33931 likes this.
  7. Bazza

    Bazza Screwfix Select

    Deleted member 33931 likes this.
  8. btiw

    btiw Well-Known Member

    A quick and dirty calculation (1 sig fig) to get an idea of scale.

    Heat capacity of water say 4 J g^-1 K^-1 (a little low, but this is back of envelope stuff)
    6 L = 6000g
    Temp rise say 80 K (10C in 90C out)

    Requires 4*6000*80 = 1920 KJ

    Power = 1920KJ / 60 sec= 32KW

    So that's roughly a 130Amp fuse (umm...) or (say) a dozen big kettles with half a litre in each.
     
    Deleted member 33931 likes this.
  9. Thanks Mr Ha and Bazza.

    That's the sort of thing they were looking to use, but having checked out the details, I can't see it working.

    I can't see any way it can continue to supply water at near boiling point after the 7 litres are expelled (and I think btiw has put the kabosh on that for certain!)

    The other issue is that the water needs to be at 90oC and not boiling.
     
  10. Blimey! Many thanks for the figs, btiw.

    Blimey - that's high. But I guess should be no surprise.

    A wee bitty more info: Water needs to be delivered at 90oC at 6lpm for 90 seconds. Then there's a 90 seconds pause before it repeats. And repeats. And repeats. etc. It needs to be automated.

    So, there is a 90 second 'recycling' time for the system to recover.

    Since btiw's calcs seem to suggest that 'instant' heating is gonna required some truly hefty power, the other solution is a larger store of heated water.

    Using a domestic cylinder's capacity as a starting point - around 200 litres - kept at 90oC by at least one 3kW but possibly two if needed, what are the chances of it being able to maintain that 90o temp when 9 litres is drawn off over 90 seconds, replaced with the same amount of 'cold' water?

    Obviously the cold comes in at t'bottom and t'hot flows from t'top. And would a couple of 3kW elements - one mounted high and one mid or low - maintain this?

    Btiw - how's yer calcs going?! :D


    (If it's essential, it may be possible to catch and recycle that used hot water so's the cylinder doesn't have to heat 9 litres of cold each time. But this would add considerable extra complexity.)
     
    btiw likes this.
  11. KIAB

    KIAB Super Member

    You can get 230v 4kw, even 6kw elements.
    Looking at my book, I see Santon do a industrial single phase 12kw element.
     
    Deleted member 33931 likes this.
  12. retiredsparks

    retiredsparks Super Member

    Dead simple.............providing you use a large capacity 'pre heated' storage tank behind the instant heater.
    But guess thats not an option ?
    RS
     
  13. Bazza

    Bazza Screwfix Select

    The largest commercial water heater i have seen is 3 phase 27KW. But these (many of them) are designed to deliver water at 60degC, not as much as 90.

    You could try a bank of these:D

    [​IMG]
     
  14. KIAB

    KIAB Super Member

    Deleted member 33931 likes this.
  15. Jeepers, not sure it's looking good even using the 'stored' method...

    Say we had a 180 litre cylinder (typical domestic) heated to 90oC. From this we extract 10 litres of hot water and replace it with with 10 litres of 'cold' (would 10oC be typical in this climate? I'm trying to keep the figures simple...)

    What would the cylinder temp fall to? From this it should be 'simple' to work out how much energy is needed to get it back up to 90oC

    Gawd, this must be a simple calc, but darned if I...

    Ok, it must be ratios of hot to cold water?

    180l : 10l = 18:1

    So the cold water would only affect the temp by 1/18th?

    So if the hot is 90o and the cold is 10o, then the difference is 80oC. And this will affected by that volume ratio above?

    So 80oC x 1/18th = 4.4oC drop?

    So 90oC would drop down to almost 85oC when 10 litres is replaced.

    Is that right? I honestly haven't a clue.
     
  16. Even their Model 280-3 (27kW x 3!) will only heat 50oF water to 125oF at 2.5 galls per minute.

    The flow rate is ok (11lpm) but 125oF is nowhere near 90oC - it's only 52oC :-(

    So it's looking like either a large thermal store of water either heated up and used until empty, or else a large thermal store of water with constant heating. (Mind you, in theory the latter option shouldn't work either...?)
     
  17. So, if the cylinder temp would fall from 90oC to around 85oC when 10litres of hot is removed and replaced by cold, what energy is required to get that 5oC drop back?

    Found this site: https://elementsofheating.wordpress...-heat-a-volume-of-water-in-a-particular-time/

    They reckon that this formula gives the kW required to raise a given vol by a given temp in one hour:

    volume in litres x 4 x temperature rise in degrees centigrade / 3412

    So, 180(l) x 4 x 5(oC) / 3412 = 1.055kW will do this in 1 hour. So to do this in 1.5 minutes would require 42kW

    Oooookkkaaaayyyyyy.... :(

    The only realistic option I can see now is to slowly heat up a large cylinder to 90oC, maintain it at that temp, and use it all up. Before refilling and repeating.

    I'm not sure how many 9 litres cycles are needed - that will be the deciding factor.

    Thanks for all your thoughts - what a forum :D
     
  18. philthespark

    philthespark Active Member

    Cascadeing tanks are the only way I can see you doing it,basically 3 immersion units@ 90C,draw off the first one which fills from the second one which fills from the third one,the third fills off the cold tank.depending on tank sizes this should give you enough water at the required temp.
     
    Deleted member 33931 likes this.

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