The optional ProSelect® Cooling System features a non-toxic freezer pack insert and a plastic housing cartridge that snaps onto the front of either the ProSelect Standard or ProSelect Deluxe Crate Fan to add extra cooling power that lasts for up to two hours. We recommend placing the attachment to the outside of the crate only. Additional replacement freezer pack Refills are also available and sold separately (see item ZW11036). Our Crate Fan Cooling System Cartridge is attractively packaged for resale. Patent pending. Place core pack in freezer and chill for at least four hours. Store Insert core pack into the plastic cartridge housing. Attach cartridge to front of Crate Fan and twist counterclockwise to lock into place. To remove, twist clockwise. Additional core packs can be kept in freezer until needed.