@Erma, you have now the classic case of heat exchanger design: you know only how much steam to condense, but do not know the area OR the U value (for heat transfer) and the friction factor (for pressure drop). You need one to find the other, which requires an iterative calculation. If you have a senior faculty member to show how to start this iterative process, it would be fine. Or if you have data on an existing design, you can, as a first approximation, prorate it for your design (i.e., adjust the flow area in proportion to your flow rate). If you have no such help, I can write down the procedure and mail it to you. The effort to do so is rather intensive and I may not find the time for several weeks. Meanwhile, you can get my mail ID from the corresponding author of my attached file.

Please tell me for which kind of geometry? cylindrical or other? 

If it is inviscid incompressible flow, the stream velocity can be obtained by dividing

@khalifa the geometry like a house, under the steam exhaust turbine pipe is trapezoidal 3D.

@ahmad how?

It looks to me as if your condenser is like what is called 'church window' design. From temperature and pressure, you can calculate the specific volume which will enable you to calculate the volume flow rate at start of desuperheating. The freestream velocity at inlet can now be got. But for pressure drop calculation, you will need the mean velocity which will exist in the bundle and is continuously variable. Therefore you might need to do a row-by-row iterative calculation that stops when all desuperheat is lost. Even then, you will need to treat the results with considerable caution. 

@vijay if I get volume flow rate, I can't calculate that velocity, because the dimension of each zone is unknown. I just have condenser's drawing, but no dimension for each zone. What should I do to determine area of each zone?

@Erma, you have now the classic case of heat exchanger design: you know only how much steam to condense, but do not know the area OR the U value (for heat transfer) and the friction factor (for pressure drop). You need one to find the other, which requires an iterative calculation. If you have a senior faculty member to show how to start this iterative process, it would be fine. Or if you have data on an existing design, you can, as a first approximation, prorate it for your design (i.e., adjust the flow area in proportion to your flow rate). If you have no such help, I can write down the procedure and mail it to you. The effort to do so is rather intensive and I may not find the time for several weeks. Meanwhile, you can get my mail ID from the corresponding author of my attached file.

use full navier stokes equations couppled with VOF echnique

• @vijay , my senior faculty member use iterative calculation method, but I don't understand to check that result is accurate or not. He write a formula : A= Q/ U . Tlmtd ; He make an assumption for U to find required area. But, in his case, he calculate Tlmtd without pressure drop. According to my opinion, I can't use my senior's method because I must calculate the pressure drop first. My steps is like this : first, I find V free stream, after that I fin Vmax based on pitch geometry to find Remax , After calculate Remax, I get friction factor and correction factor to find pressure drop in desuperheating zone (based Incropera's book). so the formula is : pressure drop = (Number of row . correction factor. friction factor. density . Vmax2/2). How?
• @M.j how I can get the film thickness?

I did'nt fully get your questions. However, we generally use energy equation in similar form to Bernouli equation in both internal and external flows and add some head loss (or pressure loss) as follows:

P1/gama+V1^2/(2*g)+Z1=P2/gama+V2^2/(2*g)+Z2+h_L

P1=pressure at point 1

P2=pressure at point 2

V1, V2= velocities at points 1 and 2

Z1 and Z2= height levels of points 1 and 2

gama= density * g

g=9.81 m/s^2 (gravity acceleration)

together with volume flow rate (Q1=A1*V1 and Q2=A2*V2) we usually find some useful relation to handle these problems.

the head loss corresponds to pressure loss due to friction. You may find more details in general Fluid Mechanics books.

@ahmad mmm...sorry,but pressure at point 1 is unknown