Multiple closed feedwater heaters in series

[unknownowen]

Homework Statement

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.

Homework Equations

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

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.

Thanks,
Owen

 

[KataKoniK]

Hi Owen,

You are correct that the pressure should remain constant across the feedwater heaters in a closed system. However, in this case, the pressure is increasing due to the steam extraction. The steam extracted from the turbines is being used to heat the feedwater in the closed feedwater heaters, and as a result, the pressure of the feedwater is increasing. This is because the steam is at a higher pressure than the feedwater, so when it is used to heat the feedwater, it also increases the pressure of the feedwater.

In an open feedwater heater, the steam and feedwater are in direct contact, so the pressure remains constant. But in a closed system, the steam and feedwater are separated by a heat exchanger, so the pressure of the feedwater can increase when heated by the steam.

I hope this helps clarify any confusion. Good luck with your diagram!