VTB or SIAV Series blowers from Hibon Inc. (a division of Ingersoll Rand) produce high vacuum levels and are not limited by the heat generated inside the blower. Air injection ports allow ambient air to enter the blower and cool the lobes. A properly designed air-injected blower will run in a blanked off condition (27 to 28 inches Hg or hose fully submerged) indefinitely without worry of the blower overheating. No vacuum relief valve is needed. 888/704-4266; www.hibon.com.