Piston air compressors are the most common industrial air compressors available, following displacement principles to increase the pressure of the enclosed air. Using a valve system supported by two stainless steel discs, the piston draws air into the cylinder as the largest disc folds downward, allowing air to move down the line. As the piston retracts upward, the disc folds back, sealing against the valve seat. The compressed air is then forced through a hole in the seat and delivered to the end process. Benefits to purchasing piston compressors include low pricing, low maintenance, and high availability of spare and replacement parts. However, piston compressors can generate higher levels of noise and heat, and are more susceptible to oil contamination of the compressed air.