The CF Backpack (Construction Foam Backpack) was created so that you don't have to repeat the refilling process for the CF Sprayer over and over. Combine the CF Backpack (and a CF Sprayer, of course) with some Scaffolds, and you can quickly build walls for large buildings and towers.
The CF Backpack is able to hold up to the equivalent of 129 CF Pellets (two stacks plus one pellet), equivalent to 1677 blocks of Construction Foam, and is recharged in a Canning Machine. The CF Backpack goes in the lower slot of the Canning Machine and the CF Pellets go in the upper slot.
Unlike when using the BatPack, using the CF Backpack won't refill the CF Sprayer, simply maintaining its initial charge level.
To refill the CF Backpack, place it in the bottom slot of a Canning Machine and CF Pellets in the top slot.