A combination unit is required to prepare the air that flows through a system before letting it into the main plant/machine. Due to the imperfect design and natural causes moisture, dirt, or dust might get accumulated inside the tubes or mechanisms the air flows through. This air must be cleaned before entering the main processing unit. Combination set-ups include air filters, pressure regulators, lubricators (if necessary), air distribution blocks and other parts specific to the situation. You can buy air combination units for the lowest prices one can find on the market.