9 replies to this topic

[saneaous]

Hi guys. I'm new on this forum so I apologies for my mistakes.

I found the text below in a book. But does not make sense for me, what the temperature of condensed water (on surface condenser) have to do with amount of work extracted from the motive steam while that temperature is on condenser not on exit from turbine. I mean some energy is extracted by turbine and some by condenser.

 

So here is the text:

The Second Law of Thermodynamics states: W = (ΔH) (T 2 − T 1 ) 

where W = amount of work that can be extracted from the motive steam

ΔH = enthalpy of the motive steam minus the enthalpy of the condensed water

T 2 = temperature at which the steam is generated in the boiler

T 1 = temperature at which the steam is condensed in the barometric condenser

The motive-steam supply to a condensing steam turbine is 360°F and 150 psig saturated steam. The turbine is exhausting to a surface condenser. The cooling water to the condenser is 92°F. The turbine is driving a centrifugal compressor. The calculated horsepower produced by the turbine is 10,000 hp.

The colder 62°F well water is to be substituted for the 92°F cooling-tower water.  And:

• The steam jets are oversized for the noncondensable flow, which consists of only a very few pounds of air in-leakage.

• The motive-steam flow to the turbine is not known, but won’t change.

• The pressure in the surface condenser is unknown and cannot be measured.

• The cooling-water flow rate is not known, but will not change either. The water will just be colder.

• The efficiency of the turbine and compressor is not known, but is presumed to remain constant.

Calculate the new compression horsepower output from the compressor. Using Eq. W = (ΔH) (T 2 − T 1 ), we note:

• Compression work W is proportional to horsepower.

• ΔH will go up from its prior value a little because the enthalpy of the condensed steam will be lower (because it’s colder).

• As the cooling-water supply is 30°F lower, we will assume that the condensation temperature T 1 in Eq. W = (ΔH) (T 2 − T 1 )  is reduced by 30°F.

• The enthalpy difference between 150-psig saturated steam at 360°F and steam condensed under a good vacuum is roughly 1000 Btu/lb.

• The enthalpy reduction of condensing the steam at a 30°F lower temperature will increase ΔH by 30 Btu/lb.

• ∆H in Eq. W = (ΔH) (T 2 − T 1 )   thus increases by 3 percent.

• T 2 , the temperature of the motive steam, is always 360°F. With 92°F cooling water, we will assume that T 1 is 120°F.

• (T 2 − T 1 ), with 92°F cooling water, is (360°F-120°F) = 240°F.

• T 1 , with 62°F cooling water, is 30°F cooler than T 1 , with 92°F cooling water; that is, the new T 1 is (120°F - 30°F) ? 90°F.

• (T 2 − T 1 ), with 62°F cooling water, is then (360°F ? 90°F) = 270°F.

• With the cooler well water substituted for the warmer cooling-tower water (T 2 - T 1 ) has increased by (270°F-240°F)/240°F=12.5%

Combining the ∆ H effect of 3 percent with the larger (T 2 - T 1 ) effect of 12.5 percent in Eq. (21.1) results in a compression horsepower increase of 16 percent or about 11,600 hp total.

 

Thanks in advance. 

 

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Edited by saneaous, 26 June 2015 - 07:29 AM.

[shan]

The lower condensate temperature is, the better the turbine efficiency achieves. 

[Zauberberg]

Condensate may be sub-cooled (as it usually is to a certain degree) and its temperature does not reflect the true enthalpy change. The true measurement of extracted power is vapor outlet temperature. See also the old guru's presentation at: http://asknormlieber...-condenser.html

[saneaous]

The lower condensate temperature is, the better the turbine efficiency achieves. 

Yes, that's right about the temperature at turbine exit nozzle, but further in the surface condenser I don't think that temperature affect the horsepower of turbine, only pressure does, which influence directly the turbine exit pressure. 

[saneaous]

Condensate may be sub-cooled (as it usually is to a certain degree) and its temperature does not reflect the true enthalpy change. The true measurement of extracted power is vapor outlet temperature. See also the old guru's presentation at: http://asknormlieber...-condenser.html

Thanks a lot for the link.

Actually that text is from guru's book "A Working Guide to Process Equipment,3rd ed" 

And there is something that I don't understand... 

[shan]

Lower temperature condenser is the necessary condition for lower turbine exhaust pressure.  Why do you want to sub-cool the condensate purposely, which may cost you more cooling duty at the condenser and more heating duty to heat up the BFW? 

[Pilesar]

saneaous, the pressure of the steam at the turbine exit depends on how much of that steam condenses downstream. As the condensing temperature gets lower, more steam is condensed and the vapor pressure is lower. This lower outlet pressure provides a greater driving force for the turbine.

[Bobby Strain]

I never considered Norm to be an expert on thermodynamics, or much else. You should use information from him (and Elizabeth) only if you know what you are doing. And, you should start with a Mollier diagram for steam. With this, you can understand what is happening.

 

Bobby

[Zauberberg]

I wouldn't say that someone who has so much experience in plant troubleshooting and revamps is poor in thermodynamics. I believe actually the opposite - Norm's ability to make things simplified gives the impression to me that he really understands the basic principles and the way things work in the industry. I consider Liberman, Art Montemayor, and Dave Simpson being the best teachers and professors I have ever had in my career. If one follows their advices, one can hardly make mistakes.

[Bobby Strain]

I know both Norm and Art. But who is Dave Simpson? Is he published? And I think we only confused Saneaous.

 

Bobby