These Leybold Roots blower cooling fans are designed for the WS and WSU series pump models; WS-251, WSU-251, WS-501, & WSU-501. The WS and WSU are chemical series pumps which do not have a shaft seal and contain a canned motor (motor inside the vacuum). The canned motor is cooled by an external electric cooling fan, operate on 1-Phase 208 - 230 VAC, and have Leybold Part Number 38091123 (Old Part Number 200 13 527).