Generally, they are divided into four categories based on the pressure they generate:
(1) blowers, with pressures not exceeding 0.015 MPa (0.15 atm or 1500 mm H2O) gauge pressure;
(2) blowers, with pressures between 0.015 and 0.2 MPa (0.15 and 2 atm) gauge pressure;
(3) compressors, with pressures above 0.2 MPa (2 atm) gauge pressure; and
(4) vacuum pumps, with pressures below atmospheric pressure.