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Pressure across multiple closed feedwater heaters

  1. Dec 8, 2012 #1
    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
    boiler

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

    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.

    Thanks,
    Owen
     
  2. jcsd
  3. Dec 9, 2012 #2

    Astronuc

    User Avatar
    Staff Emeritus
    Science Advisor

    Pumps raise pressure in a line, or a change in elevation. Pressure will drop a long a circuit due to shear forces (friction) with the piping.

    Sometimes one many need to determine pressure from the thermodynamic values, e.g., saturated condition (there is a relationship between temperature and pressure), subcooled or superheat - in the case of water/steam.

    In a closed fluid circuit, it helps to think of a pump like a battery (or voltage), which increases pressure (analogous to electric potential or emf) and piping and vessels as resistance and resistors.
     
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