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Excel spreadsheet calculating costs

  1. Jul 23, 2015 #1
    Can anyone tell me how to work out the Kw usage of a plate heat exchanger using Primary and return flow rates when no demand for hot water or heating is needed; assuming 0.04p per Kwh is charged.

    Basically a valve is passing when the heat/hot water is off and want to work out the heat loss and costs incurred due to the valve passing by using the flow rate, flow/Return temperature and 0.04p Kwh cost.

    assume the following:

    • Primary flow rate to the plate is 80c and the return temperature is 79.5c.
    • the flow meter fitted to the Primary return is registering 700 litres per hour passing from the flow back through the return
    • the cost per Kw is 0.04p per Kwh
    What else would I need to calculate temperature loss from the plate heat exchanger In Kw per hour?

    I am trying to form a spread sheet in Excel and enter formulas to work out the maths and ultimately provide the total costs of losses due to this valve passing.

    The spread sheet I have made up has all the data I think I need but for whatever reason it does not work and gives me some crazy figures.

    Many thanks for any help provided
  2. jcsd
  3. Jul 23, 2015 #2


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    Gold Member

    700 litres/hr is 700 Kg/hr mass flow .

    Temperature rise 0.5 degC

    Specific heat of water is 4.2 KJ/Kg/deg C

    Therefore energy lost/hr = 700 * 4.2 * 0.5 = 1470 KJ .

    1 KW = 1 KJ/sec

    Therefore 1 KW - hour = 3600 KJ

    1470 KJ = 1470/3600 KW-hr = 0.4 KW-hr .

    Cost - not very much @ 0.04p/KW-hr .

    You are getting very cheap electricity - can I run an extension cable across to you ?
  4. Jul 23, 2015 #3


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    Education Advisor
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    Nidum gave you the actual values of the energy used by your system. However, heat exchangers are not 100% efficient. If you can actually meter the electricity you would then be able to determine your exchangers efficiency too and maybe improve that as well.
  5. Jul 23, 2015 #4
    eMany thanks, I have entered your solution into a spread sheet and it works fantastic.

    To explain why I need the calculation is as follows.

    The heat exchanger holds 1.5 litres of static water at an average of 52c, this is like a small storage tank that feeds the hot water taps. In effect holds a bank of hot water to ensure hot taps reach the taps sooner. The same heat exchanger is heated by 22mm copper pipework, with a flow of hot water running through it at an average temp of 55c-60c to maintain the temperature within the heat exchanger.

    The 22mm copper pipework and water content, are heated from an external gas boiler.

    I thermostatic valve set at 52c is fitted to the ‘flow 22mm copper pipework’ turning off the flow when the heat exchanger reaches the correct temperature.
    On the Return pipework from the heat exchanger, back to the external gas boiler is a flow meter fitted that measures KW usage and charges per kw used.

    As mentioned, when the heat exchanger reaches temperature the Thermostatic valve closes, stopping the flow and in turn, stopping the kw charge per kw on the flow meter.

    However, the thermostatic valve has failed and the hot water flows around the circuit, through the flow meter and back to the return pipe to the external boiler, charging for hot water not being used.

    The calcs provided will work out how much money is lost due to this circulation through the flow meter that measures kw used.
    Last edited: Jul 23, 2015
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