The  air  thus  introduced  to  the  condenser  is  cumulative,  and  would

soon  set  up  a  large  back
pressure  unless  re-
moved. The  amount  of
air  by  volume  in  in-
jection water  is  said  to
average  5  per  cent,  of
the 'volume  of  the  water.
The  condensed  steam,
injection  water,  air,  and
vapour  are  pumped  into
the  hot  well  HW,  and
thence  flow  chiefly  to
waste.  The  feed-supply
for  the  boilers  is  taken
from  this  source.
The  suction  valve  of
the  air-pump  is  called
the  foot  valve,  and  the
delivery  valve  is  called
the  head  valve.
Another  form  of  jet
condenser  and  air-pump  used  for  horizontal  engines  as  made  by
Messrs.  Tangyes  of  Birmingham  is  shown  in  the  diagram,  Fig.  195.
The  air-pump  rod  is  an  extension  of  the  piston-rod  through  the
back  end  of  the  cylinder.  The  exhaust  steam  enters  the  condensing
chamber  C,  where  it  is  met  by  the  cold-water  spray  J  and  condensed.
The  condensed  steam  and  injection  water  are  removed  from  this
chamber  by  the  air-pump  AP,  which  draws  it  through  the  suction-
valves  FV,  and  forces  it  forward  through  the  delivery  valve  HV  into
the  hot  well  HW,  from  which  the  boiler-feed  may  be  taken.  The
remainder  overflows.
The  steam  should  enter  the  upper  portion  of  a  jet  condenser  so  that
there  is  no  danger  of  the  water  flowing  back  to  the  cylinder,  and  in
all  cases  the  injection  supply  should  be  carefully  regulated,  and  shut
off  before  or  at  the  same  time  as  the  steam-supply  is  closed  when  ths
engine  is  stopped.
The  condenser  should  be  so  shaped  that  the  water  may  flow  readily
by  a  fall  from  the  condenser  to  the  bottom  of  the  air-pump.
