Why do industries prefer pre-filter?

Pre-filters help in the long run of the main filter. It will inhibit debris to enter the main duct. So it will increase the lifetime of the main filter.

Which are the standard filters available in the market?

Strata Cap, Top Spin,DonaSpin, In-Line Separators etc

What removal efficiency of prefilters?

The removal efficiency varies according to the brand. Generally, the removal efficiency is 70 to 80%.