Rectangular pressure vessel operating under vaccum

[pstile81]

I have to design rectangular pressure vessel.Inside vessel there is a vaccum of 6 mm of Hg.So plz tell me some guidlines to carry out this design .It's urgent

 

[heatexguy]

See Appendix 13, (13-14)

 

[MikeG7]

Appendix 13 and the vacuum is treated same as internal pressure but the bending tensile stresses will be on the inside surface and compressive on the outside at the mid-span positions.
Don't worry about the signs to much it will come out of the formula. Other forum members gave advice to me on similar subject - refer thread404-329472 (search).