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Homework Help: Multiple closed feedwater heaters in series

  1. Dec 8, 2012 #1
    1. The problem statement, all variables and given/known data

    Doing a T-s and P-v diagram from a real power plant Rankine cycle.

    The cycle, after the condenser, goes through one pump, and then in series:

    gland steam condenser
    4 closed feedwater heaters
    1 open feedwater heater
    another pump
    2 closed feedwater heater

    Temperature and enthalpy values are given on the cycle diagram. Pump pressures were not given, and can't be found in the documentation.

    2. Relevant equations

    I thought that the pressure of the feedwater remained constant across the feedwater heater.

    3. The attempt at a solution

    I'm doing this in EES.

    My understanding is that the feedwater line going through the GSC, first 4 closed FWH and into the open FWH should all have the same pressure from the first pump (with some head loss maybe), and then it is pumped to a higher pressure for the last two FWHs.

    But when I do a thermodynamic property call on the pressure at each state, it shows that the pressure is increasing.
    As an example
    after 1st closed feedwater heater
    Given values are 163.7 F, 132.7 btu/lb
    EES tells me the pressure is 5.178 psi (reasonable as the condenser exit is 1.25 psi)

    after 2nd closed feedwater heater
    Given 198.6 F, 167.7 btu/lb
    EES: 11.2 psi

    after 3rd closed feedwater heater
    Given 257 F, 226.5 btu/lb
    EES: 33.65 psi

    and so on. I do notice that it's closely related to the pressure of the steam extraction from the turbines.
    After passing through the 1st FWH, heated with steam extracted at 5.88 psi, it's 5.2 psi
    After passing through the 2nd FWH, steam extracted at 12.56 psi, it's 11.2 psi
    after 3rd FWH, with steam at 37.2 psi, it's 33.7 psi

    so the steam extraction pressure seems to have an effect on the closed FWH exit pressure. This makes sense to me for an OPEN FWH but not for a closed one? So what's going on here? How is the pressure being raised? I checked into my thermo textbooks and they say that a closed FWH stays at the same pressure for the feedwater.

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
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