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Sizing Heat Exchanger Flow

  1. Jun 23, 2016 #1
    I'm trying to figure a range of flows of coolant that will pull 200,000 Joules per minute from a heat exchanger cooling oil. The oil is passing through a head that adds 200,000 Joules per minute to 0.5 L / min. The 0.5 L is dumped into a sump with ~7 gallons of oil. The oil then passes through the heat exchanger at 2.7 Gallons / min and is cooled to 120 C. It enters the head from here... any excess is rerouted to the sump.

    So far I know 200,000 Joules / min will need to be removed by the heat exchanger (assuming perfectly insulated and incompressable fluid) or else a perpetual increase in temp will occur.

    So with P = (Mass Flow) * (Specific Heat of Oil) * (Delta T)... T_initial (temp of oil in sump) = ~133 C

    So the entering temp of oil is 133 C so the coolant cannot be hotter than 133 C or else it will heat the oil instead of cooling it.

    For the coolant Tf = 133 C and Ti = 16 C (another assumption.. entering coolant temp is a random cool temperature). Again using P = (Mass Flow) * (Specific Heat of Coolant) * (Delta T) this gives a min value for the flow at 1.74 L / min.

    The problem I'm having arises when trying to find a max flow..

    The heat exchanger is rated at a max pressure of 300 psi. Using bernoulli..

    P1 + 0.5 * density * V1^2 = P2 + 0.5 * density * V2^2

    with P1 = 300, V1 = 0, P2 = atmospheric (assumption since I cannot determine a pressure drop) and solving for V2.

    With V2 I calculate the flow and get ~262 L / min.

    This is obviously unrealistic and it is because I'm assuming such a huge pressure drop. Any idea how I can rectify this to get a more accurate idea of the max flow?

    In case you're wondering.. I am an intern at a company and this is a project given to me to get an idea of the pump sizing that will be needed. I'm hitting a wall on this and would prefer to not look like a fool. Appreciate any help that can be offered.
  2. jcsd
  3. Jun 26, 2016 #2
    I think what your work found was peak flow rate possible through the heat exchanger, not the flow rate you need..

    I think the first thing you need to do is figure out how much flow of your coolant is needed in order to carry away the heat (200kJ/min), with an acceptable rise in coolant temp (IE, from 16C to 60C) an then size the pump for that.. Bear in mind that depending on the coolant, it may boil long before it gets to 133C.

    So if you find you need 50L/min of coolant through your exchanger, find out how much pressure drop is required through it to get that flow, then size the pump to be able to deliver that flow at that pressure
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