Rankine cycle with low pressure condenser

[Manesh Karsan]

Hi guys, first post on the physics forum!

I am slightly confused by the information provided by the following resource, the website is as follows - http://www.learnthermo.com/T1-tutorial/ch09/lesson-B/pg09.php

The point that I do not understand is - "For starters, the quality of the turbine effluent drops and this can result in damage to the turbine".

How can the quality of the saturated steam drop entering the turbine when we are lowering the pressure at the condenser stage in order to lower the temperature more efficiently before it passes into the compressor pump? Do they mean that the saturated water entering the pump consists of too much steam and so the compressor will get damaged rather than the turbine at the other end of the cycle?

Also if this is the case then how can the compressor pump get damaged when we are trying to increase the amount of saturated water entering it by cooling it down as best as possible by lowering the pressure of the condenser below atmospheric pressure?

Thank you!

 

[SteamKing]

What the site is trying to explain, is that if the exhaust conditions on the steam coming out of the last turbine stage are not properly designed, the vapor in the turbine will be mixed with a large percentage of moisture droplets as part of the steam has condensed before actually leaving the turbine. The presence of these droplets will erode the blades of the turbine over time, possibly leading to failure of the machine if an eroded blade breaks off. To ensure an efficient cycle, the presence of some moisture is tolerated in the turbine exhaust; as a rule of thumb, the quality of the exhaust steam should not fall below about 90%.

The problems described have nothing to do with the circulating pumps and the condensate coming out of the condenser should be saturated liquid. In between the condenser and the main feed pumps supplying feed water to the boiler, there should be a de-aerating tank which removes any dissolved gases in the condensate.

As to the last comment on the site, "it is very unusual to find a condenser in a power plan that operates below atmospheric pressure", this doesn't appear to be true, at least in my experience. Typically, marine steam plants operate with the main condenser at a pressure of 1.5" Hg absolute (about 5 kPa), and large shore side generating plants can have their main condensers operate at pressures of about 10 kPa absolute. Any steam plant which operates with the pressure in the main condenser above atmospheric is wasting fuel.

 

[Manesh Karsan]

Ah, I think I get it now , thank you very much!

 

[gmax137]

...As to the last comment on the site, "it is very unusual to find a condenser in a power plan that operates below atmospheric pressure", this doesn't appear to be true, at least in my experience. Typically, marine steam plants operate with the main condenser at a pressure of 1.5" Hg absolute (about 5 kPa), and large shore side generating plants can have their main condensers operate at pressures of about 10 kPa absolute. Any steam plant which operates with the pressure in the main condenser above atmospheric is wasting fuel.

SteamKing is correct. Whoever wrote that site hasn't spent any time in real power plants. The condenser pressure is generally just above saturation at the cooling water supply temperature. For stationary plants this is close to ambient temperature. I've never seen a real plant with a condenser that doesn't run at vacuum. Even in hot and steamy south Florida I remember condenser pressure about 3 to 4 inches mercury, absolute. Lower in the "winter" and at night.

 

[Manesh Karsan]

I see that makes sense, does this also mean that it is common to see boilers operating under pressure to increase the boiling point of the working liquid in order to have a larger temperature difference leading to a greater efficiency? Id assume super heating and reheating is more common in this respect to achieve a greater inlet temperature at the turbine

Thank you!

 

[SteamKing]

I see that makes sense, does this also mean that it is common to see boilers operating under pressure to increase the boiling point of the working liquid in order to have a larger temperature difference leading to a greater efficiency? Id assume super heating and reheating is more common in this respect to achieve a greater inlet temperature at the turbine

Thank you!

That's the key to high efficiency in thermal cycles: large temperature differences. Turbines typically operate using superheated steam also because there is no moisture present, except possibly in the last few stages before the steam exhausts from the turbine to the condenser. Re-heating adds energy to partially expanded steam, allowing for higher overall efficiency, but at the cost of more complex machinery.

 

[Manesh Karsan]

Thank you guys :)