I create reusable worflow in SharePoint designer, Everytime I modify the workflow and attach it to the library a new column for the workflow is being created. They are not even shown in library settings. Can any one help me how do I remove those old workflow columns.
You can remove them from view by modifying the library view and unticking the oxes so they are not displayed.
Removing old instances of your workflows should help as well. Go to the library settings --> Workflow settings --> Remove a workflow