I thought this info will be useful to those staying in the city. I have made use of this service twice while in CMai. It is convenient and on time.
There is a free shuttle service departing Central Airport Plaza. The free "songtheow" leave the Plaza at 10.30, 11.30, 13.00, 14.00 15.00 16.00 17.00 18.00 and last one at 19.00.
The Service passes by the following hotels :-
Amora Thapae, Imperial Mae Ping, Centara Duangtawan, D2, Suriwong Hotel, Royal Princess, Empress and Holiday Inn.
You can also hop on to the songtheow at any of the hotels (you can wait at the lobby) if you wish to go to Central Airport Plaza.
Get hold of the schedule at the info counter and you will know what time the songtheow will pass by each respective hotels.