I agree with PEinc here. A building is a complete system, not just a beam here and a column there. In your case, the beams could carry axial loads or even resisting moments given the column framing, especially when you factor in lateral loads. But as you have requested I have analyzed the following for you....use at your own risk and consider the services of a structural engineer before you build.
Beam Span: 22'
Tributary Area per Beam: 15'
Total Load:40psf
w=600 PLF
Results:
W14x26 is the lightest beam that will work in your 'gravity load only' example. This beam has an ubraced length of Lc=5.3 ft. It must be braced to meet the noted span. The deflection at mid span is 0.46", critical is 0.55" for l/480. Reactions on each end are 6886#. USE WITH CAUTION, there are other variables that can come into play here which cannot be accounted for over a casual thread across the internet.
This result came from Beam Check which is good for gravity load analysis and preliminary sizing, but again there is a lot more than meets the eye here.
Consider changing your spacing and use somehing smaller like LVLs. The outsided walls can support load also, therby giving you 12' bays (4 beams and two walls=5 bays). I don't have the room to discuss the unbraced length, you will need a copy of the AISC to aid you in that area.
FYI the 2x8s are really pushing it at the 15' span between beams. Not to mention the flat roof raises snow and rain load issues. The connections to the wall must be factored into also (way too much to cover here), this is where it gets back to my 'complete system' statement.
Best of luck and please consider the services of a structural engineer. If encountered with it, I wouldn't try industrial engineering, I would refer it to someone who specializes in it like yourself.
