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Heating water to 55c

  1. Jul 22, 2015 #1
    How long would it take to boil 1.5 litres of water to achieve 55c if the temperature heating up the water was 55c?
    The starting temperature of the hot water being heated would be 50c.
  2. jcsd
  3. Jul 22, 2015 #2
    This is not clear. At normal pressure water boils around 100 Celsius and not at 55.
    You mean just to heat up the water?

    If it is this last one, you cannot calculate the time from the data given. Can be anything from forever to fraction of a second, depending on how fast is the energy transfered to that water.
  4. Jul 22, 2015 #3


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    Staff: Mentor

    Hi lee121. :welcome:

    You probably mean how long to "heat" water until it reaches ambient surroundings temperature (which you choose to be 55C here)? I'll fix your thread title for you.

    The answer, if you are a mathematician, is it takes forever, because as time passes the water gets closer and closer to ambient, but never quite reaches it. The approach is an exponential, so never quite touches the target value, until at time = infinity.
  5. Jul 22, 2015 #4


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    Staff: Mentor

    Consider two different situations:
    1) The 55 degree heat source is a microscopic wire filament immersed in the water and held at a constant 55 degrees by a minute electrical current through it.
    2) The 55 degree heat source is a slab of cast iron, three meters by five and many centimeters thick, and we pour the water onto its surface so it forms a pool .1 millimeter deep.

    In case #1, we'd expect the water to warm up very slowly. In case #2, we'd expect the water to warm up almost immediately. So there's no single answer to your question - as nasu said, it depends on how quickly heat is transferred from the source to the water.

    And as Nascent said, to heat something to any given temperature you need a heat source that is at least slightly hotter than the desired temperature. A 55 degree heat source will never quite get the water to 55 degrees, although you can get as close as you want if you're willing to wait long enough.
  6. Jul 23, 2015 #5
    Apologies if my question is a little vague so will provide a little more detail.

    If you have a commercial gas boiler with a flow temperature of 55c feeding an external 1.5litre hot water heat exchanger, how long would it take to heat the water up to an average temperature of 54c.

    The heat exchanger flow is fitted with a temperature stat that ensures the temperature never falls below 50c within the heat exchanger, so I suppose my question is, how long would it take to heat up the hot water from 50c to 54c on average, assuming the flow rate to the heat exchanger is a constant 55c? If that makes sense?
  7. Jul 23, 2015 #6
    A flow rate will be measured in units like kg/s or m3/s or maybe l/s. What do you mean by flow rate of 55c?
  8. Jul 23, 2015 #7


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    Staff: Mentor

    How long? As long as the water spends in the heat exchanger - these things are often called "instantaneous" (unless there is a tank somewhere, which you didn't say). But I'm not sure if this is really what you want to know, because it still makes little sense. This could be a control responsiveness issue. Or you just need to change the setpoint from 50 to 54.

    Also, what do you mean by 1.5iter heat exchanger? Do you mean 1.5 L/min? Or is that the internal volume of the heat exchanger? And as said, 55c is temperature, not flow rate.

    Can you draw a diagram of this? I don't mean to sound rude, but you really need to try harder here if you want help.
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