9 replies to this topic

[steamreformer]

Dear learned members
greetings
I Need your help once again ,i wants to know how to calculate loss of water from a tank by evaportaion .Tank is closed not open to atmosphere and has one vent at top.
i want to calculate how much water is lost by evaporation.plz tell me how to proceeed .ur help as usual is highly appreceated.i search transport phenomenon book by Bird(BSL ) but i cant find the right subject.
thanks

[MrShorty]

You haven't given near enough information to give specific suggestions. At it's core, my first reaction is this is a basic VLE question. 1st step would be to determine the vapor composition (y) of water. Then determine the amount of vapor "lost"/removed. The amount of water is then y*total vapor removed.

Perhaps a very simple case to illustrate: A flask at 1 atm and ambient temperature containing pure water with a nitrogen blanket. I would first estimate the partial pressure (Py) of water: Py=P*(T) [P*(T) is the vapor pressure of pure water at temperature T]. This is essentially Raoult's law assuming the liquid is straight water (at 1 atm, the amount of N2 dissolved would be neglibible). Then y(water), from Dalton's law, is Py/Ptot. From there, you need a measure of the moles of vapor removed, and the total water loss becomes y(water)*moles of vapor removed.

It should be clear that this is a very simplified algorithm for a very simplified scenario. You will need to fill in gaps based on your specific system. (Such as, another of your posts asked about MDEA. If this were an MDEA solution, you'd need to use what you know about the the volatility of water over MDEA using activity coefficients or whatever model you prefer to get the partial pressure of water).

Hope that helps.

[steamreformer]

Dear shorty

thanks for your reply

well i am thinking the other way before your reply .i am trying to calculate heat input to tank because of solar heat.then by enthaly balances trying to figure out the solution.
well what more info u need more.pl tell.

[steamreformer]

Dear members:

Please help me to calculate the heat input to a tank. Do any of you have any formulae for this? The heat input to the tank is through its top and side walls.

I appreciate your nice help.

[ankur2061]

Steamreformer,

Please check out the following links for calculations related to evaporation losses:

http://www.engineeri...anks-d_286.html

http://www.slideshar...ks-presentation

Heat loss from tanks includes conduction (tank walls), radiation and convection due to air or wind currents. The calculations are complicated and not striaghtforward since there is an element of time also involved ie. transient conditions such as variance in ambient temperature, wind speed and solar radiation over the daytime duration.

Hope this helps.

Regards,
Ankur.

[sheiko]

FYI, attached is probably the best article on tank heat transfer ever written.

Attached Files

•  predict_storage_tank_heat_transfer_precisely.pdf   1007.67KB   163 downloads

[steamreformer]

Dear sheiko,

Thanks for a very nice paper on heat transfer in storage tanks. However, I found it difficult to calculate the overall heat transfer coefficient.

Mine is a simple case. I only have cooling water inside the tank, and I only want to calculate how much water is lost by evaporation. For that, I need the heat gain into the tank and after that, I need the diffrence in enthalpies between water inside the tank and the water vapor in the air. Then, I will get the amount of water lost. So, if there is any simple formulae to calculate the heat gain into the tank, it is highly appreciated.

[steamreformer]

Dear members (especially sheiko and ankur):

Thank you for your nice inputs but i calculated the evaporation losses by approach from transport phenomeneon.

I have attached the Excel spreadsheet that I created for this calculation. If you have some time, can you plesae comment on my work so that I can have more confidence on my method?

Please find the spreadsheet atatched to this post. Are the results reasonable?

[steamreformer]

Hi all,

Please find my attached Excel spreadsheet on evaporation losses. Feel free to add your valued comments if you should find any errors.

I await your valuable comments.

Attached Files

•  evaporation_losses.xls   11.58KB   93 downloads

[steamreformer]

Dear members,

Greetings.

I am requesting our learned members to please review the Excel spread sheet that I created for calculating evaporation losses through a closed loop cooling water tank.

I would be very grateful to have some comment from learned members so that I can release this spreadsheet for use. I await your valued comments.