from the Flow Calculation department...
Thermodynamic Calculation
 Posted by Mxromen on 14 June, 2011 - 3:36 pm
I have 260t/h steam flow at 24 bar and 471C. I spray 147C and 55barg water to that steam. Finally steam becomes 465C.

How can I calculate that spray flow and is there any good book to learn that kind of calculations?

Because I'm very enthusiastic to learn these kind of mechanical calculations as an electrical engineer.

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 Posted by Steve Myres on 14 June, 2011 - 6:24 pm
Calculate the energy content of each component above some arbitrary datum, say 100C liquid or vapor. Then total the energies and the mass flows (260t + x) and divide the energy by the mass flow to get the resulting temperature delta above your original datum. Solve for x (H2O mass flow) such that the resulting temp is 465C.

Remember that the 147C liquid will consume 970btu/lb or whatever when flashing, so your final mixed temperature will be less than if you were introducing the same mass flow of 147C steam.

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 Posted by Process Value on 14 June, 2011 - 10:03 pm
Attemperator/Desuperheater calculations

well, i am guessing that this data is taken from a attemperator / desuperheater. well the calculations is actually simple, you just have to equate energy balance in each sides.

Heat energy of incoming steam + heat energy of BFW sprayed = heat energy of outgoing steam

enthalpy of steam at 24 bar and 471 ded = 813 kcal/kg
enthalpy of water at 55 bar and 147 deg = 147 kcal/kg
enthalpy of steam at 24 bar and 465 deg = 810 kcal/kg

i hope you need to calculate the BFW spray for 260 tonnes of steam . (note - actually 471 deg for a 24 bar steam is a very very high degree of superheat, never seen it can you please tell me where exactly this steam is used, usually a 24 bar MP steam has a temp of 300-325 deg)

260 * 813 + X *147 = (260+x) 810

x = 1.17 t/hr

this is the quantity of BFW required. you can use the same principle in any heat exchanger.

I am a electrical engineer too, i learnt this from these text books and i had help from my manager.

Basic and applied thermodynamics - PK Nag
Power plant Engineering - PK Nag , Black and Vetach
BEE text books

but i guess any thermodynamic textbook will suffice.

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 Posted by Bruce Durdle on 14 June, 2011 - 10:17 pm
Get hold of some steam tables. They will list the enthalpy of superheated steam at various conditions. http://www.nist.gov/srd/upload/NISTIR5078.htm
is a good source if you don't have anything to hand.

Find the enthalpy of the steam you are putting into the desuperheater. From the mass flow (260,00 kg.hr) you can find the total energy in the steam supply. Also find the enthalpy of the product conditions.

The same tables give the enthalpy of the compressed water. You now have enough information to carry out an energy balance around the system:

Steam energy in + water energy in = total energy out.

Qsteam(in) * h steam(in) + Qwater(in) * hwater(in) = (Qsteam + Qwater)(out) * hsteam(out).

You should be able to find more info in a basic thermodynamics textbook. If not, I have some general notes I can make available.

Cheers,
Bruce.

 Posted by mxromen on 15 June, 2011 - 9:20 am
Thanks for everyone for the calculation suggestions.
It seems easy to calculate, sorry for asking an easy question like that. but really appreciated for your kind responses.

Dear PV,
you said: "note - actually 471 deg for a 24 bar steam is a very very high degree of superheat, never seen it can you please tell me where exactly this steam is used, usually a 24 bar MP steam has a temp of 300-325 deg"

this steam is our reheater steam. we reheat it before and after desuperheating. Finally we get 23 bar 570C steam. Then this steam goes to steam turbine.

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 Posted by Don Parker on 21 June, 2011 - 7:48 pm
You can build your own calculation spreadsheet if you wish using X-steam (free) at www.x-eng.com to construct the mass/energy balance equations. Note also that the RH spray pressure won't be as high as you indicate because it is reduced through the control valve to only slightly above the steam pressure, but it doesn't make much difference...

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