We have a cement mill,2.4m dia 7.9m length with 3 comp. The residue in 90 microns is always fluctuate from 3 to 7 % for same feed rate and same composition and same air vent quantity. What may be the reason.?

There could be a number of reasons: (i) changes in grindability of the clinker, or (ii) variation in the condition of the internal diaphrams between the chambers of the mill with regard to build-up. Have you inspected the mill internally?

we have inspected the mill but no build up, nor the diaphrams broken.The clinker composition is also same .only difference from earlier readings is air quantity of exhaust earlier 15000m3/hr,now 12000m3/hrs

Air flow rate will affect residue as fines will be stripped off the charge in the mill before they have time to be ground.