Common capacity control methods used in free cooling design schemes include variable speed drives, valve control, and bypass control. Variable speed drives allow for adjustments to fan and pump speeds to match changing cooling demands. Valve control involves regulating the flow of water or air through the system to modulate capacity. Bypass control diverts excess flow around the heat exchanger to reduce capacity. Each method has its advantages and disadvantages, and the selection of the most suitable method depends on the specific system design and operating conditions.