Makeup tank outlet temperature

[Corben76]

Hello,

While I was investigating a current process flow diagram of an existing plant, I didnt understand a point. The return from process steam users is 90 degrees celcius and it is approximately 80 percent of process steam, 20 percent is supplied as makeup water which is 15 degrees. These two streams enter a demineralisation water system, then a condensate makeup tank. The outlet water temp. of this tank is 35 degress celcius. As far as I know, this tank like a mixing chamber, so the outlet temp. should be like 75 degrees celcius according to energy balance. So, why could there be a such low temperature? Do u think there is a cooling in the tank? Thanks for the posts
corben

 

[25362]

Some thoughts which I'm confident you have already considered yourself:

-There may be an error in either the estimation of water flow rates or in the readings by the temperature sensing instruments.

-Have you tried to check by a (careful) hand-touch whether the outlet temperature is in fact higher than 35 degrees Celsius ? Where are the thermometers located and what type are they ? Can you interchange them to verify readings ?

-Since the makeup water is at 15 deg C, it seems that the environment is a cold one, and heat losses may also play some role, subcooling the warm condensate return stream to temperatures lower than 90 deg C.

-If the boiler feed water collecting tank doesn't manage to do a thorough mix of both streams, the temperature reading instrument may be submerged/contacting/sensing a partially segregated "colder" tank outlet stream.

Kindly keep us posted when you grasp the nettle.

 

[Windybottom]

A need to look at your system would be required but you can get a "layering" of water temperature i.e. the mixing may not be as complete as you think. This all depends upon water inlet type of water i.e. purity of water, inlet and outlet does it go through a De-airator etc also as in previous reply accuracy of instruments.