64.  The  first  and  last
forms  shown  are  made
Fig.  62.  with  two,  three  or  four


PLENUM   WARM   AIR   HEATING


125


pipes  in  depth.  The  standard  number  of  pipes  in  any  one
section  is  four.  Sometimes  these  pipes  are  spaced  in  straight
lines  parallel  with  the  wind  and  sometimes  are  staggered.
Staggered  spacing  no  doubt  makes  each  pipe  slightly  more  ef-
ficient but  it  adds  friction  to  the  fan.  Efficiency  tests  of  both
spacings.  however,  show  little  difference  in  these  methods.
The  horizontal  sections  and  the  mitre  sections  present  this  ad-
vantage over  the  vertical  pipe  sections,  that  the  steam  and
condensation  is  always  flowing  in  the  same  direction  and


JHg.    63.
=)


drainage  is  very  simple.  With
the  vertical  pipe  section,
steam  in  one  half  of  the  pipes
must  pass  upward  against  the
di  ection  of  the  flow  of  con-
densation or  it  must  carry  the
condensation  with  it.  That
half  of  the  header  supplying
pipes  which  carry  steam  up-
ward is  usually  drained  for
condensation  by  a  small  hole
directly*  into  the  return  with
the  result  that  steam  often
blows  through  the  header


Fig.  64.
126


HEATING  AND  VENTILATION


without  traversing  the  pipe
circuits.  The  third,  or  mitre
section,  in  addition  to  per-
fect drainage,  has  perfect  ex-
pansion; the  vertical  header
serving  as  a  steam  supply,
and  the  horizontal  header  as
a  drain,  permit  every  pipe
to  assume  any  position  nec-
essary to  account  for  a  rea-
sonable change  of  length.
