3 replies to this topic

[rsk]

As i understand presence of noncondensables degrades performance in two ways Q=UA(DT)lm
1. Decrease in U overall heat transfer coefficient, because of additional resistance of air film.
2. reduced LMTD,decrease in partial pressure of steam results in condensation at a lower temperature.

Both of above results in higher condenser back pressure, again resulting in the same condenser duty but at higher back pressure.
out of the above two factors which one has a major contribution, i would like forum members to have discussion on this.

[fallah]

As i understand presence of noncondensables degrades performance in two ways Q=UA(DT)lm
1. Decrease in U overall heat transfer coefficient, because of additional resistance of air film.
2. reduced LMTD,decrease in partial pressure of steam results in condensation at a lower temperature.

Both of above results in higher condenser back pressure, again resulting in the same condenser duty but at higher back pressure.
out of the above two factors which one has a major contribution, i would like forum members to have discussion on this.

I think major effect of noncondensables presence in powe plant condensers , is breaking the vacuum shall be maintained in prespecified value during operation.This vacuum in necessary for preventing early condensation.

Regards

[Zauberberg]

Hello RSK,

The question is general, but here is my personal opinion.

I think it depends on the amount of noncondensables present in the steam and type of application: if CO2 or air or noncondensable gas buildup is high in steam reboilers, reduction of overal heat transfer coefficient usually becomes dominant because of increased veloicity of steam-gas mixture. This increase in fluid velocity is also a consequence of higher steam demand due to reduced condensation temperature, so the effect is actually combined. Apart from additional air film resistance, increased velocities inside heat exchanger have detrimental effect on steam (or any other fluid) condensation ability (higher velocities are beneficial for sensible heat transfer - and for latent heat transfer it is the way opposite). If noncondensable is CO2, additional corrosion problems may occur.

You can calculate, based on type and quantities of noncondensables present in the system, what is the actual reduction of steam partial pressure.

As Fallah said, in vacuum condensers this can be a critical issue. Every piece of equipment which operates under vacuum is designed to handle certain amount of noncondensibles - usually air, originating from in-leakages. If noncondensables are present in ejector motive steam, the biggest concern is increased back-pressure on ejector system. Above certain value, if maximum allowable discharge pressure of ejector is exceeded, its performance will break and cause suction pressure to increase. This surging way of operation is unacceptable and is usually transferred across the whole plant.

[mishra.anand72@gmail.com]

Non -condensibles affects getting desired temperature. Check out said equipment specification.