If you place only the zImage file on the CF card
Obviously if you have anything other than a 5500/5000D (collie), you'll want to check the naming of zImage(.bin) and also place the updater.sh file on the CF card.
As long as you don't have an initrd.bin on the CF card, it won't flash the filesystem,
P.S. Do check that the kernel versions are the same (`uname -r`) otherwise, although the kernel will flash, it won't have any modules available (unless you install them before flashing).
P.P.S. Afaiu the kernel is contained in a seperate part of the flash.