Shashank,
Your question infers too low a velocity is causing fouling. Are you sure about that? Obviously, I don't know what is going on, and you may be doing something that is not routine in my experience. Is the latex stable? Is the line kept hydraulically full, thus no drying? Is the line emptied and exposed to dry air or gas each time? What is this fouling? Skin formation (drying), coagulation, settling/sedimentation, incompatibility with previous latexes, maybe something else. Frankly, in my experience (45-65% solids) a maximum velocity is more of a concern so the wall shear does not destabilize the latex and cause scrap/coagulum.
Good luck,
Latexman
To a ChE, the glass is always full - 1/2 air and 1/2 water.