A medical vacuum pump system must meet NFPA99 design standards. The minimum size is a duplex system where one vacuum pump can carry the entire load of the hospital and the 2nd pump (lag pump) then provides 100% backup. Vacuum serves a multitude of medical surgical operations such as chest drainage, gastro intestinal and pharyngeal/tracheal treatments. NFPA99 regulates the vacuum level at all hospitals shall be 19”HgG and the lowest allowable vacuum level at the furthest outlet form the source to be 12”HgG. Capacity at all hospitals is given in SCFM and is generally defined as 70F air, 0% humidity and 14.7psia.
Vacuum pump manufacturers will use units of pump performance such as “HgG (inches mercury gauge) and Torr or mmHgA (millimeters of mercury absolute) same measurement two different terms. To convert 19”HgG to inches of mercury absolute (“HgA) 29.92”HgA – 19”HgG = 10.92”HgA. To convert from 10.92”HgA to say PSIA then 10.92”HgA/29.92”HgA x 14.7psia = 5.365 psia.
Because all vacuum pump curves are rated in ACFM from the manufacturer we need a way to convert from SCFM to ACFM so we can select a pump. In our example lets assume the hospital requires 25scfm @ 19”HgG, since air or gas under a partial vacuum is expanded our answer will always be a number larger than the 25scfm. Then 29.92”HgA/10.92”HgA x 25scfm = 68.5acfm @ 19”HgG. Since medical vacuum shall always operate at 19”HgG then you may memorize the expansion factor of 29.92/10.92 = 2.74, this will always remain the same in medical surgical vacuum applications.
Controls on most medical vacuum pump systems are simply load/no-load. Medical vacuum controls most found today include load/No-Load, On/Off or VFD, rarely does any vacuum pump system use a modulated inlet valve. Typical vacuum pump will be single stage, standard materials of construction, common Hp size for a single pump will be anywhere from 5Hp to 40Hp range.
Liquid ring medical vacuum pump systems offer the greatest reliability and durability of any other type of mechanical vacuum pump and feature lowest noise level, a 40 year pump life, can compress up to ¼” soft solids, cool discharge temperatures and minimal maintenance. Water sealed liquid ring vacuum pumps offer the greatest safety and protection when considering WAG compression. Be cautions when considering technologies such as Claw vacuum pumps, especially for higher elevations as these machines are limited in maximum achievable vacuum levels (24”HgG max) and may not meet the NFPA 99 partial vacuum level (19”HgG) requirement. Other issues that should be considered with the Claw technology include high discharge temperatures (300F>) 3600rpm operations speed creates loud noise levels, and the inability of labyrinth seals to provide a 100% seal between the gear box and pumping chamber. Since claw technology does not include any type of clean up equipment on the pumps discharge it is possible to exhaust hot oil vapor directly into the discharge pipe.