Frequent occurrence of acne on head.

Hello experts. I have very thick, smooth and generally healthy hair on my head. But the problem lies with my scalp. It gets dry very quickly and gets dandruff covered with dead skin. I have tried medicated dandruff control shampoos but they worsened the condition by making the scalp drier. I realised I did not have dandruff occuring from oily skin instead my dandruff was dried skin. It is a general occurence throughout my entire body. My skin is relatively dry, even as I drink about 5-6 litres of water a day on an average. Also on my scalp I get tiny, tender acne like bumps frequently. They are painful to touch when they first occur, and within 3 days they get dry and release solid crystal like substance (white/off-white in colour). After that the empty acne gets dry and becomes dandruff like. These acnes have the same size of the pimples you get on your skin. Please help me out on how to control this and make my scalp healthy.