how to create a positively pressured room to shun all insects in food production floor?

Answer 1

Its a matter of balancing the air flow. You're going to have to total up all your exhaust fans and provide a make-up air unit that is slightly higher volumetric flow rate than the total of your exhaust.

You don't want the positive pressure to be much higher or you will be blowing doors open etc. Also I've never seen this to be very effective in preventing insects from entering.

The best way is vestibules with the insect zappers. And occasional fumigation when the plant is down.

Answer 2

Even if it is positively pressured, it will only keep insects out by not letting them through the door. If a person carrying a fly got through the door however, the insect will feel right at home, even in the higher pressure.

What you'd ideally want to do is not only have good entrance and exit seals, but have a constantly blowing wind inside to shuffle the insects back out somewhere.