Here's the calculation that I would do.
Hypotheses:
a) NH3 gas in the upper exchanger shell reaches the maximum temperature of C.W. (39°C)
I neglect the thermal resistance of the tubes
c) I consider a linear temperature gradient inside to the tubes (from the periphery to the center)
d) being the tube diameter of reduced entity (20 mm), I do not image large convective movements inside.
Based on the above assumptions, I can apply the equations of heat conduction:
heat flux density (W/m2) = -lambda * (dT/dx) and
Power absorbed (W) = flux density * exchange surface
lambda is the ammonia conductivity (W/[m*K])
S (m2) is obtained from the exchanger drawings
From the hypothesis of the linear gradient, I can write:
Power absorbed (W) = -lambda * S* (Ti-Te)/r
As told above Te is 39 °C
For Ti I have two choices. 1) overestimating, I consider the minimum tube side temperature (-33 °C), 2) I consider an average tube side temperature between Tin (-33 °C) and Tout (15 °C).
In case 1, the power absorbed (FI) is 760.4 kW and applying the equation (1) from API 521 (6° Ed., January 2014, page 33) I can calculate the volumetric flow rate to evacuate from the PRV: q=0.633 m3/h (W=431 kg/h)
Similarly in he case 2, I can calculate FI=480.4 kW, q=0.400 m3/h (W=273 kg/h)
What do you think of what is written above?
Are the assumptions acceptable (for me the weaker one is D))?
What kind of calculation would you have done?
Thank you very much in advance to those people who will find time to answer me
Edited by Aznadif, 06 February 2017 - 10:36 AM.