I feel chill in my knees because I sit down for long hours.

Knee chill. The chill may be from sitting so long that the nerves to the knees are being compressed by the edge of the chair pushing on your legs. Try getting up frequently and stretching your legs and see if the chill goes away. If this gets worse then see your doctor for an examination.
See below. Stand up and stretch th pot and try to walk a few feet. You can so this every 30 minutes, good luck.