Cycle Efficiency Problem

[HeterosexusOfRome]

I am looking at problems 27.4 and 27.5 in the MERM companion and they both ask for the thermodynamic efficiency given the pump and turbine efficiency. The answer shows to use the steam tables then find all the enthalpies at all the stages and then find the actual  enthalpies  due to the pump and turbine inefficiency. I understand this method is the method shown in the book. However i did it another way and was wondering if my method is just as valid as i get the same answer.

The method i use is i found the enthalpy of the fluid right after the turbine and then determined how much heat will need to be ejected at the condenser to bring the vapor back down to the temperature of the saturated liquid as it enters the boilerfeed pump. As i said i got the same answer but what concerns me is i did not need to use the pump efficiency at all in my calculation (had to use the turbine efficiency though). Can some one please confirm if my methodology is valid and if any conditions make it invalid?     

 

[P-E]

A fit bit will make it invalid.  

Hmmm Ill check on this one later.  

 

[HeterosexusOfRome]

A fit bit will make it invalid.  

Hmmm Ill check on this one later.  

Fit bit? I don't understand. Basically I find the needed Qout required on the condenser to get the fluid to the condition before the boiler feed pump then add that value to the turbine work out to get the total Qin. Then I divide the work out from the turbine by the Qin.

 

[P-E]

Go to the last results threads and look for fitbits

As far as your power cycle problem, it seems you found h'e (real turbine leaving enthalpy) and h'a (real pump enthalpy) a different way which includes the efficiencies.

nth = ((ha-h'e)-(h'a-hf))/(hd-h'a)

read the section of the merm again and see if that makes sense to you and try to understand h'a and h'e.    It's good to know this power cycle inside and out.  

 

[HeterosexusOfRome]

Go to the last results threads and look for fitbits

As far as your power cycle problem, it seems you found h'e (real turbine leaving enthalpy) and h'a (real pump enthalpy) a different way which includes the efficiencies.

nth = ((ha-h'e)-(h'a-hf))/(hd-h'a)

read the section of the merm again and see if that makes sense to you and try to understand h'a and h'e.    It's good to know this power cycle inside and out.  

I understand the method in the merm. I was wondering if the way I did it will always be valid as well.  Basically the merm method looks at net work out (w from the pump subtracted from work from the turbine )divided by the heat addition at the boiler represented by (hd-ha). The method I used didn't require the calculation of the pump work since it dealt exclusively with the actual work out of the turbine and the heat ejected at the condenser.  I am reasoning that the inefficiency of the pump translated into heat energy into the fluid and was then ejected in the condenser. Would this be correct?  

 

[P-E]

Did you use the theoretical he leaving the turbine or the actual h'e?

Edit: Yes, pump inefficiency will end up as heat in the fluid.   That is correct.  

 

[HeterosexusOfRome]

Did you use the theoretical he leaving the turbine or the actual h'e?

Edit: Yes, pump inefficiency will end up as heat in the fluid.   That is correct.  

i used the actual h'e leaving the turbine. however i didn't calculate the work at the pump at all.