TM 5-3895-374-24-2 OPERATING CHARACTERISTICS Roots  UNIVERSAL  RAI^®  blowers,  as  covered  in this manual.  are designated as air blowers, and may be used for handling air in either pressure or vacuum service.      They   are   unsuitable   for   handling   gases because   shaft   seals   are   not   designed   to   prevent leakage to atmosphere. The Roots rotary lobe blower is a positive displacement   type   unit.        whose    pumping    capacity    is determined    by    size.        operating    speed    and    pressure conditions.  It employs two double-lobe impellers mounted on parallel shafts and rotating in opposite directions within a   cylinder   closed   at   the   ends   by   headplates.      As   the impellers rotate.  air is drawn into one side of the cylinder and   forced   out   the   opposite   side   against   the   existing pressures. The differential pressure developed. therefore,  depends  on  the  resistance  of  the  connected systems. Effective   sealing   of   the   blower   inlet   area   from   the discharge   area   is   accomplished   by   use   of   very   small operating    clearances.        Resulting    absence    of    moving contacts eliminates the need for any internal lubrication. Clearances between the impellers during rotation are maintained by a pair of accurately machined timing gears, mounted   on   the   two   shafts   extending   outside   the   air chamber. Operation  of  the  familiar  basic  rotary  lobe  blower  is illustrated in FIGURE 1.  where air flow is right to left from inlet    to    discharge    with    the    bottom    impeller    rotating clockwise.    In  Position  1  it  is  delivering  a  known  volume (A)  to  the  discharge,  while  space  (B)  between  the  upper impeller and cylinder wall is being filled. Counterclockwise rotation of this impeller then traps equal volume (B) in Position 2.  and further rotation delivers it to the  discharge  in  Position  3.    At  the  same  time.    another similar  volume  is  forming  under  the  lower  impeller,  and will be discharged when rotation reaches Position 1 again. One complete revolution of the driving shaft alternately traps four equal and known volumes of air (two by    each    impeller    and    pushes    them    through    to    the discharge,    the    pumping    capacity    of    a    lobe    blower operating at a constant speed therefore remains relatively independent   of   reasonable   inlet   or   discharge   pressure variations.    To  change  capacity,  it  is  necessary  either  to change speed of rotation or vent some of the air. No  attempt  should  ever  be  made  to  control  capacity by  means  of  a  throttle  valve  in  the  intake  or  discharge piping.  This increases the power load on the driver.  and may    seriously    damage    the    blower.        Likewise.        if    a possibility exists that flow to the blower inlet may be cut off during  normal  operation  of  a  process,  then  an  adequate vacuum relief valve must be installed near the blower.  A pressure  type  relief  valve  in  the  discharge  line  near  the blower    is    also    strongly    recommended    for    protection against cut-off or blocking in this line. When  a  belt  drive  is  employed.    blower  speed  can usually be adjusted to obtain desired capacity by changing the diameter of one or both sheaves.  See pages 18 and 20  for  minimum  sheave  diameter.    In  a  direct  coupled arrangement.    a  variable  speed  motor  or  transmission  is required,    or    air    may    be    vented    through    a    manually controlled unloading valve and silencer.  If discharge air is returned  to  the  blower  inlet,  it  must  be  cooled  to  100F (38C) through a cooling by-pass arrangement. Before   making   any   change   in   blower   capacity   or operating  conditions,  contact  the  nearest  Distributor  for specific information applying to your particular blower. In  all  cases.    operating  conditions  must  be  maintained within  the  approved  range  of  pressures.    temperatures and  speeds  as  stated  under  LIMITATIONS.    Also.    the blower must not be used to handle air containing liquids or solids, or serious damage to the rotating parts will result. OPERATING LIMITATIONS To  permit  continued  satisfactory  performance,  a Roots  UNIVERSAL  RAI®  blower   must   be   operated within   certain   approved   limiting   conditions.      The Manufacturer's warranty is, of course, also contingent on such operation. Maximum   limits   for   pressure,   temperature   and speed  are  specified  in  Table  1  for  various  sizes  of UNIVERSAL RAI® blowers.    These  limits  apply  to  all blowers   of   normal   construction,   having   operating clearances as listed in Table 5 when operated under standard atmospheric conditions.  Do not exceed any of these limits. Example: The listed maximum allowable temperature rise    increase    in    air    temperature    between    inlet    and discharge) for any particular blower may occur well before its maximum pressure or vacuum rating is reached.  This can easily occur at high altitude or at very low speed. Figure 1-Flow Through a Basic Type RAI Blower. page 3-801