The  length  of  the  stroke,  L,  in  incnes,  will  be  obtained
from  the  speed  and  the  number  of  strokes  such  that,

12  V
*  =  _  (.6)

All  direct  acting  steam  pumps  are  designated  by  diam-
eter of  steam  cylinder  X  diameter  of  water  cylinder  X  length
of  stroke;  as,

14"   X    12"   X    18"

Duplex  pumps  have  twice  the  capacity  of  single  pumps
having  the  same  sized  cylinders.
To  find  the  indicated  horse  power,  /.  H.  P.,  of  the  pumps,
reduce  the  pressure  head,  p,  in  pounds  per  square  inch,  to
pressure  head  in  feet,  h;  multiply  this  by  the  pounds  of
water,  W,  pumped  per  minute  and  divide  the  product  by  33000
times  the  mechanical  efficiency,  E.
W  Ji

I.  H.  P.  =  (96)

33000  E

To  reduce  from  pressure  head  in  pounds  to  pressure
head  in  feet,  divide  the  pressure  head  in  pounds  by  weight
of  a  column  of  water  one  square  inch  in  area  and  one  foot
high.  The  general  equation  for  this  is

144  p
h  =


w

where  w  =  the  weight  of  a  cubic  foot  of  water  at  the  given
temperature  and  p  —  differential  pressure  in  pounds  per
square  inch.
DISTRICT  HEATING  219

In  pump  service  of  this  kind  the  pressure  head,  p,  against
which  the  pump  is  acting,  is  not  the  result  of  the  static
head  of  water  in  the  system  but  is  due  to  the  inertia  of  the
water  and  to  the  resistance  to  the  flow  of  water  through
the  piping  system  and  the  heaters.  This  frictional  resist-
ance may  be  calculated  as  shown  in  Art.  144.  Read  this
part  of  the  work  over  carefully.
