Query regarding cenvat credit

In case of exports, you can claim refund of service tax. But in case of sale of exempted goods or services, you cannot. You also need to reverse the cenvat credit taken for exempted goods or services. 

Agreed. with Mr.Prateek Ji

how can we seggregate the expenses related to export sale and exempted goods sale?

For that you hav to maintain separate books of accounts. If they are not maintained, then you have an option to reverse cenvat credit according to Rule 6(3) of the Cenvat credit Rules.