Hi Erik, there are direct buses from Beijing Airport to Tianjin, you can take the bus at the following places:
T1/T2: In front of Gate 15 on the 1st Floor of T2,
T3:In front of Gate 3 on the 1st Floor
The bus takes around 2.5 hours to Tianjin and cost CNY70 per ticket. From 07:30 to 23:00, there is one bus leave for Tianjin around every half an hour.

Well, at the long-distance bus station of Beijing Airport, you could catch a bus to Tianjin directly. If you miss the bus, please go to take the shuttle bus Line 2 to its terminal at Xidan and then transfer to Metro Line 4, by which you could go to the Beijing South Railway Station directly and promptly.