462 
JOURNAL  OF  HORTICULTURE  AND  COTTAGE  GARDENER. 
May  30,  1901 
Tl\e  Heating  and  Ventilating  of  Hothouses. 
*f  _______ _____ __ 
With  the  enormous  increase  in  the  prosperity  and  wealth  of  the 
country  during  the  last  fifty  years,  hothouses  for  the  cultivation  of 
fruits  and  flowers  have  increased  in  a  full  proportion.  The  maintaining 
of  an  equable  temperature  in  such  houses  when  the  outside  temperature 
varies  sometimes  as  much  as  20°  to  30°  in  twenty-four  hours  is  not 
without  difficulty,  The  means  used  to  accomplish  this  is  in  nearly 
every  case  now  hot  water  circulating  in  pipes;  the  days  of  the  old 
brick  flues  have  gone. 
The  theory  of  the  circulation  of  hot  water  in  pipes  is  very 
interesting,  for  we  are  presented  with  an  apparent  anomaly  by  the 
El 
_  _  n;  _  _ 
Fig.  1. — Boilers,  showing  Flow  and  Return. 
rapid  rise  of  the  water  in  the  flow  pipe,  apparently  against  the 
universal  law  that  water  flows  to  the  lowest  point — finds  its  level 
But  this  is  not  the  occasion  for  discussing  this  aspect  of  the  question 
to  any  great  extent.  In  passing,  however,  I  may  be  allowed  very 
briefly  to  draw  attention  to  the  cause  of  the  circulation  in  a  hot- water 
apparatus. 
Fig.  1  represents  an  ordinary  apparatus  with  a  saddle  boiler  to 
which  is  attached  in  the  ordinary  way  a  flow  and  return  pipe  ;  the 
flow  in  all  cases  and  in  all  classes  of  boilers  must  be  from  the 
highest  available  point  of  the  boiler,  and  the  return  should  rejoin 
the  boiler  as  near  the  bottom  as  practicable.  There  is  thus  an 
endless  pipe,  the  boiler  bemg  practically  a  part  of  the  pipe  enlarged 
and  shaped  for  the  application  of  heat.  When  such  an  apparatus  is 
filled  with  water  through  the  cistern  and  feed  pipe  it  is  ready  for 
use.  When  heat  is  applied  to  the  part  of  the  endless  tube  called 
the  boiler  what  happens  is  this,  the  water  expands — expands  equally 
in  all  directions,  downwards  as  well  as  upwards ;  but  inasmuch  as 
there  is  less  resistance  in  the  upward  direction  the  whole  expansion 
is  diverted  that  way,  the  longer,  or  rather  higher,  column  of  water  in 
the  return  pipe  resists  the  push  of  the  expansion,  and  the  hot  water 
is  forced  upwards,  thus  the  circulation  is  begun  and  in  the  same 
manner  continued.  It  is  clear  that  the  cause  of  the  upward  flow  of  the 
hot  water  in  the  flow  pipe  is  the  greater  density  and  weight  of  the 
higher  and  colder  column  of  water  in  the  return  pipe. 
It  can  be  proved  that  with  an  apparatus  having  a  height  of 
5  feet  from  the  lowest  to  the  highest  point,  and  with  an  average 
difference  of  10°  between  the  flow  and  return  pipe,  the  water  in 
the  return  is  continually  falling  with  a  theoretical  velocity  of 
fi8‘4  feet  per  minute.  With  an  average  height  of  10  feet  the  fall 
per  minute  is  96'6  feet,  and  in  an  apparatus  having  a  height  of  20  feet 
the  theoretical  fall  is  136-2  feet  per  minute;  in  short,  the  motive 
power  in  a  hot-water  apparatus  is  entirely  in  the  return  pipe,  the 
amount  depending  on  the  height  and  on  the  difference  of  temperature 
between  the  flow  and  return.  In  quoting  these  figures  no  account  is 
taken  of  the  friction,  which  may  be  very  small,  or  may  be  sufficient  to 
wholly  stop  the  circulation. 
In  the  heating  of  hothouses  it  is  of  the  utmost  importance  to 
remember  that  the  motive  power  is  in  proportion  to  the  difference 
in  height  between  the  lowest  and  the  highest  points  of  the  apparatus, 
which  practically  means  the  depth  of  the  stokehole  and  rise  of  the 
pipes.  Attempts  are  sometimes  made  to  avoid  sinking  a  stokehole, 
but  such  attempts  are  bound  to  be  failures,  and  are  only  attempted 
by  people  without  any  knowledge  of  the  underlying  principles  which 
govern  the  circulation  of  hot  water  in  pipes.  It  is  well  known  to 
all  experienced  heating  engineers  that  a  boiler  quite  powerful  enough 
to  heat  1000  feet  of  pipe  where  there  is  a  height  of  25  feet  or  30  feet 
O/rKat/re 
will  not  efficiently  work  more  than  750  feet  when  the  height  is  only 
5  feet  or  6  feet.  Along  with  this  must  be  considered  the  frictional 
resistance,  which  is  the  work  to  be  accomplished. 
For  hothouse  work  there  is  a  general  agreement  that  a  4-inch 
pipe  is  the  most  suitable  in  regard  to  the  quantity  of  water  and  the 
friction  on  the  walls  of  the  pipe;  3-inch  and  2-inch  pipes  may  be, 
and  often  are,  used,  but  probably  80  per  cent,  of  the  hothouses  erected 
are  heated  with  4-inch.  In  very  large  apparatus  larger  pipes  are  often 
used  for  mains,  but  the  radiating  pipes  are  almost  invariably  4-inch. 
The  relation  between  the  size  of  the  structure  to  be  heated  and  the 
amount  of  heating  surface  is  of  the  greatest  importance ;  and  although 
there  are  no  scientific  rules  for  this,  practice  has  been  much  on  the 
following  lines,  which,  I  think,  except  in  the  most  exposed  situations, 
are  safe  lines.  Of  course  provision  must  be  made 
against  the  coldest  weather,  which  may  be  taken  at  32° 
of  frost. 
For  conservatories  where  a  temperature  of  not  more 
than  45°  or  50°  is  wanted  there  should  be  1  foot  of  4-inch 
pipe,  or  its  equivalent,  for  every  35  cubic  feet  of  space. 
For  plant  houses,  where  a  higher  temperature  may  be 
required,  the  proportion  should  be  1  foot  of  pipe  to 
every  25  or  30  cubic  feet  of  space.  For  stoves  and 
Orchid  houses,  and  also  for  early  vineries,  the  proportion 
of  heating  surface  should  be  still  higher.  An  Orchid 
house  12  feet  wide  requires  four  rows  of  4-inch  pipes 
along  emh  side,  which  gives  1  foot  of  heating  surface  to 
every  12  or  33  cubic  feet  to  be  heated.  The  lean-to 
and  semi-span  type  of  early  vinery,  16  feet  wide,  should 
have  eight  rows  of  p  pes,  being  about  1  foot  of  pipe 
to  every  15  cubic  feet  to  be  heated.  An  intermediate 
vinery,  if  spau-roofed,  and  24  feet  wide,  should  have 
twelve  rows  of  4-inch  pipes,  giving  1  foot  to  about 
16  or  17  cubic  feet.  A  span-house  naturally  requires 
a  larger  proportion  of  heating  surface  than  a  lean-to. 
Peach  houses  14  feet  wide,  with  four  rows  of  pipes  have  a 
proportion  of  about  1  foot  of  heating  surface  to  every  28  cubic 
feet,  which  may  be  taken  as  a  fair  medium  where  early  forcing  is  not 
attempted.  Melon  and  general  forciog  houses  often  have  a  forcing 
bed  on  each  side,  with  four  rows  of  pipe  below  each  bed.  When  this 
arrangement  is  adopted  it  is  desirable  to  have  more  than  the  usual 
proportion,  as  those  pipes  in  the  chamber  below  the  bed  cannot  be 
counted  upon  but  to  about  one-half  their  heating  value.  It  is  usual  to 
put  a  row  of  pipe  along  the  side  above  the  bed,  close  to  the  front  ; 
hut,  in  addition  to  this,  it  is  desirable  to  have  some  pipes  in  the 
lootway  covered  with  an  iron  grating.  These  various  circulations 
should  be  controlled  by  valvis,  as  there  will  be  times  when  no  surface 
Fig.  2. 
Terminal  Saddle  Boiler. 
Fig.  8 
Terminal  Saddle  Boiler, 
another  Pattern. 
*  Paper  read  by  Mr.  a.  Donald  Mackenzie  oetore  the  Royal 
Horticultural  Society,  December  4th,  1900. 
heat  may  be  required,  whilst  a  good  strong  heat  i  i  needed  below  the 
forcing  beds.  There  should  be  ventilators  in  the  wall  of  the  forcing 
bed  for  the  admission  of  air,  and  other  ventilators  above,  close  to  the 
glass,  for  the  escape  of  the  heated  air.  In  this  way  the  temperature 
can  be  regulated  as  required. 
It  is  not  necessary  to  go  into  the  question  of  boilers.  There  has 
been  more  controversy  about  the  merits  of  boilers  than  any  other 
detail  in  connection  with  hothouses.  There  are  numerous  patent 
boilers  in  the  market,  each  one  put  forward  by  the  maker  or  patentee 
as  being  the  best.  I  have  had  very  considerable  experience  of  these 
during  the  last  forty  years,  and  my  opinion  is  that  a  good  deal  of  what 
is  said  in  their  favour  may  be  discarded.  The  old  saddle  boiler  still 
keeps  its  hold  as  one  of  the  simplest  and,  under  reasonable  conditions, 
one  of  the  most  economical  ;  but  1  could  not  advise  its  use  (except 
under  special  conditions)  for  quantities  over  750  feet  of  4-incH  pipe. 
For  quantities  from  500  to  2000  feet  the  terminal  saddle  boiler  is 
powerful  and  economical.  (Figs.  2,  3.)  It  takes  more  depth  of 
stokehole  than  the  plain  saddle.  For  larger  quantities  than  2000  feet  I 
very  strongly  recommend  a  steel  Cornish  or  annular  riveted  boiler,  and 
where  there  is  a  reasonably  good  draught,  water  bars  are  a  great  saving 
of  fuel.  (Fig.  4.) 
