CONVECTION  01'    HEAT   FRO  vl   AN   EXTERNAL   SURFACE

Air C=.2849<'-'«

Hydn^gen C=  .9827/'  -^^

Carbonic  acid C  =  .  2759i!'  -^■*

Olefiant  gas C=  .3817/'-33

C  =  quantity  of  heat  in  B.T.U.  conveyed  away  from  a  solid  body  by  a  gas
external  to  it,  per  square  foot  of  surface  per  hour  under  one  atmosphere
of  pressure.
i  =  excess  temperature  of  surface  in  degrees  F.
Summari/. — From  tiie  foregoing  experimental  data  and  from  other  experimental
sources  it  appears  that  the  transference  of  heat  from  steam  through  iron  pipes  to  air
increases  considerably  with  the  {)ressure  of  the  steam  which  ma}'  be  explained  by  the
greater  velocity  and  force  with  which  the  molecules  of  the  steam  impinge  on  the  walls
of  the  pipe  at  the  higher  pressure  and  the  correspondingly  higher  temperature  combined
therewith.
Accordingly  the  coefficient  n  or  the  transfer  of  heat  from  steam  to  air,  varies  from
1.8  to  3.5  B.T.U.  according  to  pressure  per  hour  for  every  square  foot  of  pipe  surface
and  for  every  Fahrenheit  degree  iliffercncc  on  each  side  of  pipe.  For  average  conditions
about  three  units  are  frequently  adopted.  The  transfer  of  heat  from  steam  to  water
is  variously  estimated  at  from  200  to  240  B.T.U.  for  iron  pipe  (280  B.T.U.  for  copper
pipe).
The  transference  of  heat  or  of  refrigeration  from  water  to  water  or  to  brine  and
water  or  the  reverse,  brine  to  water,  or  brine  through  iron  pipe,  is  about  the  same,  and
is  generally  assumed  to  amount  to  80  B.T.U.  per  hour  in  counter-current  arrange-
ments or  in  a  Buudelot  cooler.
