Like all things in life it depends. It depends on:
How your beams are supported.
How your load is mounted.
How your load is distributed.
What material you're using.
How thick the flanges are on your beams.
What if any local codes apply.
Whether there is any dynamic loading.
Whether there are additional loads (wind, snow, etc.).
What safety factor is required.
What the desired maximum deflection is.
How long you want the thing to last.
and so on, and so on.
That being said, its pretty basic engineeering that most structural or mechanical engineers would be glad to handle for an appropriate fee.
