Pressure in closed condensate tank

[Diulf]

Hi,

I was told that pressure in closed condensate tank depends on condensate temperature because pressure in the tank will be same as saturation pressure due to flashing... But I find this hard to understand.

For example,

A system which has a steam-to-water heat exchanger, closed condensate tank right below the heat exchanger and heating power of steam is controlled using condensate valve. Steam pressure is 3 bar(g) and pressure drop due the heat exchangers is 1,5 bar(g).

If heat exchanger sub cools condensate to 95°C, does this mean that pressure inside of the condensate tank is below 0 bar(g) because the saturation pressure will be under 0 bar(g) on that temperature. This assumption will lead to negative pressure in pump suction which doesn't make any sense.

What if temperature of condensate is 120°C. Does it flash in a tank or because condensate valve?

How I can determine the pressure inside of the condensate tank?

Please help me to understand this topic.

 

[Snickster]

Can you provide a sketch of the system?

 

[georgeverghese]

There is usually a hot vapor pressure balancing line connecting the vapor space of the HE with the vapor space of the condensate tank. In this case, vapor space pressure in the condensate tank will = HE shellside ( steam ) pressure, while vapor pressure of the subcooled condensate will correspond to sat vap pressure of condensate at subcooled temp.
See PID for this pressure balancing line

 

[EdStainless]

In steam surface condensers the pressures are way below atmospheric.
Values in the range of 1-4 in Hg absolute are typical.
They are usually dealing with outlet temps of 100-120F.
If you are below 100C you will be below atmospheric pressure.

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P.E. Metallurgy, consulting work welcomed

 

[goutam_freelance]

Please check the details of the heat exchanger. There could be a drain sub-cooling zone like in power plant feedwater heaters (example attached) which cools the condensate below the saturation temperature. This cooling is done when they lose contact with steam space so that steam pressure is not affected by this.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[EdStainless]

And this works because the thick baffle marked 'drains subcooling zone enclosure' uses very tight-fitting holes for the tubes to minimize bypass.

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P.E. Metallurgy, consulting work welcomed