For this proposed design, 4500 kg/h of ammonia vapor at 6.7 bara pressure is to
ID: 1841854 • Letter: F
Question
For this proposed design, 4500 kg/h of ammonia vapor at 6.7 bara pressure is to be cooled from 120 degree C to 40 degree C, using cooling water. The maximum supply temperature of the cooling water available is 30 degree C, and the outlet temperature is to be restricted to 40 degree C. The pressure drops over the exchanger must not exceed 0.5 bar for the ammonia stream and 1.5 bar for the cooling water. A contractor has proposed using a shell and tube exchanger with the following specification for this duty. Shell: E-type, inside diameter 590 mm. Baffles: 25% cut, 300 mm spacing. Tubes: carbon steel, 15 mm inside diameter, 19 mm outside diameter, 2400 mm long, number 360. Tube arrangement: 8 passes, triangular tube pitch, pitch 23.75 mm. Nozzles: shell 150mm inside diameter, tube headers 75 mm inside diameter. It is proposed to put the cooling water though the tubes. Is the proposed design suitable for the duty? Physical properties of ammonia at the mean temperature of 80 degree C: specific heat 2.418 kJkg^-l degree C^-1, thermal conductivity 0.0317 Wm^-l degree C^-1, density 4.03 kg/m^3, viscosity 1.21 times 10^-5 N m^-2 s.Explanation / Answer
GIVEN:- Information as mentioned in the problem
TO FIND:- To discuss whether design specifications mentioned are suitable for the duty?
SOLUTION:- As per standards and experimental data (not mentioned), the mentioned proposal design is not suitable for the duty.
Proposed changes in the current design are as follows:-
> Baffle spacing to be increased from 300mm to 400mm to avoid eddy currents & turbulence and so ensure smooth flow of fluid (ammonia).
> Tube outside diameter to be increased to 25mm from 19mm with thickness of tube remaining constant.
> Number of tubes can be reduced from 360 to around 250 if outer diameter is increased to 25mm, thereby reducing the pressure drop in the tubeside liquid i.e. water.
> Pitch of the tube arrangement can be increased by decreasing number of passes and so keeping the shell diameter constant. Increasing pitch will help in reducing shell side pressure drop, better surface area and better shell side fluid flow.
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