Fundamental Rules:

Only one fan can be on the Workbench at any time.

A fan can be moved back and forth between the workbench and the Cart, but will only be at one or the other, never both.

Time Savers: 

After a fan has been placed in the cart, the "Copy Blower Options From" drop-down box is available. Selecting a fan from the box will copy its options to the Workbench fan.

Along the same line, The "Place Copy Fan" button on the Workbench view will place the current fan in the cart and place a clone on the Workbench. This should be helpful for fans that differ in few details, such as Tag.