What do air tanks do?  Air Tanks help the compressor cycle less which reduces related maintenance costs on the inlet of the compressor as well as reduce the amount of energy being consumed by the compressor motor.
Air Tanks Air Tanks
How large should I size my air tank? It is recommended to have a minimum of 2 gallons of storage per every CFM and we highly recommend having 4 gallons of storage per CFM. Here is a quick sheet showing the air compressor horsepower, average CFM or output and recommended tank size;
Compressor Size Capacity Gallons Per CFM Recommend
Horsepower CFM Air Tank
5 HP 17 CFM 4 68
7.5 HP 23 CFM 4 92
10 HP 35 CFM 4 140
15 HP 51 CFM 4 204
20 HP 80 CFM 4 320
25 HP 100 CFM 4 400
30 HP 125 CFM 4 500
40 HP 170 CFM 4 680
50 HP 214 CFM 4 856
60 HP 245 CFM 4 980
75 HP 345 CFM 4 1380
100 HP 450 CFM 4 1800
125 HP 600 CFM 4 2400
150 HP 700 CFM 4 2800
200 HP 911 CFM 4 3644
250 HP 1150 CFM 4 4600
300 HP 1350 CFM 4 5400