Question

An air-cooled steam condenser is operated with air in cross flow over a square, in-line array of 400 tubes (NL=NT= 20), with an outside tube diameter of 20 mm and longitudinal and transverse pitches of SL= 60 mm and ST= 42 mm, respectively. Saturated steam at a pressure of 2.455 bars enters the tubes, and a uniform tube outer surface temperature of Ts= 390 K may be assumed to be maintained as condensation occurs within the tubes.
(a) If the temperature and velocity of the air upstream of the array are  Ti=300 K and V=4 m/s, what is the temperature To of the air that leaves the array? As a first approximation, evaluate the properties of air at 300 K.
(b) If the tubes are 2 m long, what is the total heat transfer rate for the array? What is the rate at which steam is condensed in kg/s?

I just need help figuring out the bolded part. I looked up in the steam tables to find that at 2.455 bars, the specific volume of the steam is 0.731 m^3/kg. However, I am confused as to how we use this to find the mass flow rate. Vmax = 7.363 m/s, Avg Nu = 77.436, To = 343.075K, Log-mean temp diff = 66.141k, qtot = 338540.0148 W. Explanations are well appreciated!

Answer 1

To calculate the steam condensation rate, you don't have to look at the specific volume of steam from steam table.

You need to look at latest heat of vaporization from steam table.

See in first part of option (b) you calculate the heat transfer rate for the array. Now if you divide this by latent heat of vaporization of water at 2.455 bar, you will get the amount of steam condensed.

I hope I could explain your query. Let me know if you need any further explanation in this query.