PIH-102                                           HOUSING


                 Earth Tempering of Ventilation Air

Warren D. Goetsch, University of Illinois
Larry Jacobson, University of Minnesota
Randall Reeder, Ohio State University
Dennis Stombaugh, Ohio State University

Eldridge Collins, Jr., Virginia Polytechnic
  Institute and State University
Dexter Johnson, North Dakota State University
Richard Phillips, Pennsylvania State University
Harold and Dean Rogers, Petersburg, Illinois
David Shelton, University of Nebraska

     Earth tempering of ventilation air for  swine  buildings  is
being  considered by many producers because of the moderate fluc-
tuations in soil temperatures at shallow depths. Depending on the
season, incoming ventilation air is heated or cooled as it passes
through a buried tube. The soil serves as a heat sink in the sum-
mer  and as a heat source in the winter, thus giving almost year-
round temperature modification. It has the potential to  signifi-
cantly  reduce heating costs during winter and provide zone cool-
ing during summer.

Soil Temperature

     Soil temperature  is  one  of  the  most  important  factors
affecting  design  and  performance  of earth-tube heat exchanger
systems. Soil temperatures vary with soil type,  depth,  moisture
content, time of year, and geographic location.

     The mean annual ground temperatures for various locations in
the  United  States are given in Figure 1.*  In the central U.S.,
these mean annual ground temperatures range  from  49o F.  in  St.
Paul,  Minnesota, to 58o F. in Lexington, Kentucky, and from 52o F.
* The drawings in Figures 1, 2, and 3 first appeared in  ``Under-
ground  Building  Climate''  by Kenneth Labs, in the October 1979
issue of Solar Age, c 1979  SolarVision,  Inc.,  Harrisville,  NH
03450.  All  rights  reserved. Reprinted and published by permis-
in Ames, Iowa to 55o F.   in  Columbus,  Ohio.  The  variation  of
ground temperature from this yearly mean at any site is suggested
by Figure 2. The amount of  temperature  variation  decreases  as
depth  increases.   For  example, at a depth of 6 ft., the yearly
variation of a typical clay soil can be expected to range from 11
degrees  above to 11 below the mean annual ground temperature, or
a total yearly variation of approximately 22 degrees.  At a depth
of  10  ft., this variation is reduced to plus or minus 6 degrees
F.  or a total variation of 12 degrees.

     The time of year when  the  ground  temperature  is  at  the
extreme is also important in the design and performance of a sys-
tem. Soil temperature fluctuations lag behind surface temperature
changes  due  to  the heat storage capacity of the soil. The soil
surface reaches maximum temperature during the heat of  the  sum-
mer,  but  soil 10-12 ft. deep may not reach its peak temperature
until almost three months later. This thermal lag at the  10  ft.
depth  (Fig. 3) helps both the heating and cooling performance of
these systems. During the winter, soil temperatures at this depth
are  at  the  fall  season  level,  making the soil near the mean
annual ground temperature, thus adding to the  heating  capabili-
ties  of  a system. The reverse is true during the summer months,
when the soil temperatures at the 10-12 ft.  depth are springlike
and can cool the ventilation air.

     Soil types and moisture content also affect the ground  tem-
perature  variation.  Soils  with increasing sand content tend to
have larger temperature variations at  deeper  depths  than  clay
soils.  Soil moisture and ground water elevation also affect soil
temperature. Seasonal temperature variation  is  larger  in  very
moist  soils  as compared to very dry ones due to the increase in
heat transfer through soils whose voids are filled with water.

System Design

     The typical earth-tube tempering or  heat  exchanger  system
consists  of a heat exchanger field, a collection duct/fan house,
and a building air distribution system.  Each of  these  portions
must  be adequately sized to insure proper performance.  The fol-
lowing sections may help to explain the many tradeoffs in  system

     Airflow Capacity. In general, much more air is required  for
summer  ventilation than for winter. If zone cooling is used, the
difference between the two rates is  much  less  (Table  1).  For
example,  the  recommended summer zone cooling rate for a sow and
litter is 70 cu. ft. per minute (cfm) of uncooled air per farrow-
ing  crate,  50  cfm  for  evaporative cooled air, and 30 cfm for
air-conditioned air. Air tempered by an earth-tube system  should
be somewhat cooler and dryer than evaporative cooled air (depend-
ing on climate), but for planning purposes use  the  50  cfm  per
crate.  During  winter,  the recommended cold weather ventilation
rate is 20 cfm per crate. With the system designed for a capacity
of  50  cfm per crate, there is an additional 30 cfm which can be
used for mild weather room tempering as needed or it can be  used
to preheat the winter air of a compatible nearby nursery. Similar
design capacity figures are shown for gestation sows, boars,  and
growing and finishing pigs in Table 1.

     Comparison of the air volume requirements  for  a  farrowing
house  with  and  without the use of an earth-tube heat exchanger
system is shown in Table 2. A properly designed and managed  sys-
tem allows the producer to reduce whole building ventilation rate
by one-half during the summer (50 cfm/crate of earth zone  cooled
air  plus  200  cfm/crate  outside  air  versus  the  normal  500
cfm/crate outside air recommendation). If  zone  cooling  is  not
desired  or  possible because of interior room design, whole-room
cooling may be an option. For whole- room-cooling  planning  pur-
poses  use  an air volume of 100 cfm per farrowing crate or twice
the normal zone cooling rate. Adequate building insulation levels
and  proper room air distribution systems are extremely important
to ensure successful ventilation with this type  of  system  (See
PIH-65, Insulation for Swine Housing, and PIH 87, Cooling Swine).
Zone cooling is recommended over whole-room cooling because it is
more  cost  effective,  especially in the farrowing and gestation

     Heat Exchanger Field Design. Both soil  characteristics  and
tubing  factors  affect  the  design and performance of a system.
Soil characteristics include soil  type,  moisture  content,  and
water  table elevation. Temperature levels for various soil types
indicate the less favorable performance of sandy soils; so  avoid
these  if  possible.  If  sandy soils must be used, the number of
lines, line lengths, and/or depth should be increased  by  10  to
20%  to  offset this effect. Moisture content increases the heat-
transfer capability of the system. Therefore, a system  installed
in  an  area  with  a  shallow  water table should have the lines
buried below the average yearly elevation of the water table  for
maximum  performance.  Such a system must be well sealed to mini-
mize ground water seepage and additional pumping costs. Construc-
tion  should  take  place  during  periods  of low water table to
reduce the use of pumps and possibly unstable  trench  sides  and

     Air-tubing factors include diameter, length, depth of place-
ment,  and shape of the tube. Typically, nonperforated corrugated
plastic drainage tubing is used because of its  availability  and
cost.  The recommended airflow rates for various tubing diameters
are shown in Table 3. These airflows are based on an air velocity
in  the  tube  of  500-600 ft. per minute (fpm). Divide the total
airflow needed for the system by the recommended  flow  rate  per
tube  to  indicate the number of tubes needed for a given system.
Table 3 also shows the  recommended  tubing  length  for  various
diameters  of  tubing.  This  length  is  based on an air contact
(heat-exchange surface) of 1.3-2.0 sq. ft. of  tube  surface  per
cfm  of  airflow (figures are based on smooth pipe for simplicity
of calculation). Small diameter tubing, such as the  3-,  4-,  or
5-in. sizes, are impractical because of the large number of lines
needed to provide enough air capacity for a typical system;  thus
the 8-, 10-, and 12-in. diameters are the most practical.

     Layout. Several system layouts are possible,  including  the
wagon wheel (radial) or the lateral (see Figs. 4 and 5). Material
and trenching costs are normally less for the wagon wheel pattern
because  no  manifold  lines are used; however, excavation can be
difficult near the collection duct. Manifold lines must  be  much
larger than lateral lines, and tubing materials and trenching are
more expensive. However, a lateral system with a manifold may  be
the  only  option  when  surrounding  buildings, roads, or fields
limit the area available for installing the system.  The  spacing
between  lateral  lines  need  not  be  uniform, but each lateral
should be of equal length to keep the airflow equal.  Laterals do
not need to run straight, but abrupt turns should be avoided.

     Placement. The tubing should be buried to a  depth  of  7-12
ft. depending on installation costs and geographic location. Sys-
tem thermal performance will be better  with  maximum  depth.  If
installation costs are prohibitive, somewhat shallower depths may
provide a more beneficial economic return.

     Space lines at least 8-10 ft. apart to  maximize  soil  heat
storage  and  minimize the chance of tubing deflection and damage
during construction.  Trenches with  multiple  tubes  and  closer
tube  spacings may be used to reduce construction costs; however,
line length should be increased to maintain  adequate  soil  mass
for  heat  transfer.  For example, four 6-in. diameter tubes have
about the same airflow capacity as one 12-in. diameter  tube.  If
four  6-in.  lines are installed in a single trench, their length
should be the same as the 12-in. recommendation  of  200-250  ft.
instead  of  the  normal 6-in. tube recommendation of 100-130 ft.
Slope lines at a minimum of 2-3 in. per 100 ft.  to a U-trap  and
gravity  drainage  line at the outer tube ends or to a drain sump
at the collection duct. Constant slope is  critical  because  any
low  spots  in  the  lines could fill with water and restrict air

     Tubing should be installed  carefully,  in  accordance  with
ASTM Standard Designation: F 449-76.- Either trenchers  or  back-
hoes  can  be  used  for  excavation,  but hand blinding (careful
placement of select material over and on the sides of the tubing)
and narrow trenches with rounded bottoms should be used to ensure
constant slope and minimal tube  deflection  and  damage.  Modern
trenchers  are equipped with laser plane-grade guides that ensure
a constant slope.  However,  most  trenchers  are  restricted  to
depths of less than 7 ft.  unless a special adapter is available,
and 2-3 ft. of topsoil may need to be removed before trenching if
trenchers  are  to  be used (Fig. 6). Backhoes are more expensive
but are available for depths down to 12 ft. and can,  with  care,
maintain a constant slope (Fig. 7). They can be used when trench-
ers are not practical; however, due to  the  extreme  depths  and
possible  cave-in  problems,  trench  sides  should be sloped and
bulkheads may be needed to ensure a safe  working  area.  Minimum
trench  width  should be 6 in. wider than the outside diameter of
the tubing. If extremely  wide  trenches  are  used,  the  tubing
should  be  placed  in  the corner of the trench against a trench

     At the outer end of the system, the tubes  should  curve  up
and  extend  3  to  4  ft. above the soil surface to form the air
inlet. Either rigid PVC pipe or corrugated plastic tubing can  be
used  for  the inlet risers; however, the tops should be screened
to keep out debris and rodents and  should  be  very  visible  to
prevent damage from nearby machine traffic.

     Collection Duct/Fan House Design. Common materials for  col-
lection  ducts  below grade include reinforced concrete, concrete
blocks, and round steel. An example of one such  reinforced  con-
crete  collection  duct is shown in Figure 8.  Size is determined
by system airflow and wall area requirements to make  the  tubing
connections.  In  general, collection ducts should provide enough
wall area to connect the lines and enough cross-sectional area to
keep  airflow  velocities  below  500 fpm. Above grade, insulated
wood construction is acceptable to enclose the airstream. A prop-
erly  sized  fan  must be installed at the connection between the
underground system (collection duct) and the building air distri-
bution  ducts.  Determine the size of the above-grade duct by the
size of fan to  be  enclosed  and  the  type  of  service  access
entrance  to  be  used. Normally, the above-ground portion can be
constructed to the same dimensions as the below-grade portion and
still  provide  enough  area  for  fan  installation, access, and

     Insulate the entire collection duct/fan house  to  at  least
R-19  to a depth of 6 ft. below grade with moisture-proof insula-
tion. A closed cell polystyrene  or  polyurethane  insulation  is
recommended. A reverse tempering effect has been noted on instal-
lations in Illinois where no insulation was used below the 3  ft.
depth.  In  one  case,  air that had been cooled in the tubes was
reheated 5 degrees as it passed through the duct/fan  house  into
the building.

     A fan should be located between the underground tubing  sys-
tem  and  the  building air distribution system. Size this fan to
deliver the desired airflows against  to 1/2-in. static pressure.
Usually,  a  two-speed fan would be best, with the maximum volume
matched to the summer zone cooling rate and  the  smaller  volume
matched  to the winter continuous ventilation rate.  Tightly seal
the collection duct and all connections to prevent short circuit-
ing  of  air  from outside directly into the duct, thus bypassing
the tubing system.

     Building Air Distribution System.  The  distribution  system
for the earth-tempered air consists of a fan, main duct or ducts,
and downspouts (or drop ducts) located as needed for each  animal
(Figs.  9 and 10). In a farrowing house, locate a downspout above
each individual crate with the airstream directed  at  the  sow's
head.  The  downspout  should be located as close to the animal's
head as possible to make full use of the cooled  air.  If  spouts
are  within  the  animals'  reach, they should be made pig-proof.
Include dampers in the downspouts to close individual lines  when
crates are empty and to adjust airflow if needed.

     Main duct and downspout dimensions are  given  in  Table  4.
These are minimum duct dimensions and should be increased if duct
framing is located inside the airstream.  Insulate  ducts  to  at
least  R-6  to  prevent  heat gain and condensation during summer

     For winter  operation,  earth-tempered  air  can  be  routed
through  an  existing  room air distribution system, through room
make-up air heaters, or the summer downspouts can be removed  and
tempered  air  can  be introduced into rooms via the distribution
duct openings along the room ceiling.

Design Example

     Design an earth-tube heat exchanger for a  24-sow  farrowing
house. The summer zone-cooling ventilation rate equals 50 cfm per
sow and litter, and the continuous winter rate is 20 cfm per  sow
and litter (Table 1). Therefore, the maximum airflow for the sys-
tem (zone cooling) equals 1,200 cfm (50 cfm per sow x  24  sows),
and  minimum  airflow  equals 480 cfm (20 cfm per sow x 24 sows).
During the winter, the extra 720 cfm capacity of the system could
be used to heat and ventilate an adjoining nursery.

     From Table 3, find that 6-in. tubing can carry 110  cfm  per
tube.  Eleven  6-in. tubes are required (1,200 cfm divided by 110
cfm per tube). For 8-in.  lines, use six tubes (1,200 cfm divided
by  200  cfm  per tube). For 10-in.  tubing use four tubes (1,200
cfm divided by 300 cfm per tube). The suggested length  for  each
tubing  size  is  given  in  Table 3. A system using eleven 6-in.
tubes, each 100-130 ft. long (depending on soil type); six  8-in.
tubes,  each 130-170 ft. long; or four 10-in. tubes, each 160-210
ft. long, would be satisfactory. Check the cost of trenching  and
materials  in  the  area  to determine which system would be most
economical. The relative costs of different tubing sizes are also
shown  in  Table 3. As the size of the tubing increases, the cost
of the material goes up. The material cost  increases  are  espe-
cially large if tubing of 10-in. diameter or more is used.

     Manifold lines, when used, must carry the entire  flow  that
goes  through  them  at an appropriate velocity (refer to Table 3
for size). If six lateral lines of 8-in. tubing are installed, as
arranged  in  Figure 5, the manifold running in each direction to
the first lines needs to be 15 in. in diameter (200 cfm per 8-in.
line  x 3 lines = 600 cfm). The manifold can then be decreased to
a 12-in. size to the second lines (200 cfm per  8-in.  line  x  2
lines  =  400 cfm). After the second line is connected, the mani-
fold can be reduced to an 8-in. diameter tube  out  to  the  last
line.  The vertical tube coming out of the ground should be a 24-
in. tube or larger.

     Size the fan to supply 1,200 cfm at the high setting and 480
cfm  at  the  low  setting  while  working against  to 1/2 in. of
static pressure.

     From Table 4, an 18- by 18-in.  or  10-  by  30-in.  (inside
dimensions)  main duct will carry the 1,200 cfm airflow. If crate
layout is such that two ducts are needed, two 12-  by  12-in.  or
two  6- by 24-in. ducts could also be used.  Also from Table 4, a
4-in. diameter downspout or a 3- by 3-in. square downspout  would
carry the desired 50 cfm per crate airflow to each animal.

System Costs

     Major costs encountered when installing an  earth-tube  heat
exchanger  system include: excavation, tubing, fan, and the inte-
rior distribution system.  Cost will vary with the depth of  ins-
tallation,  excavation  method,  layout,  and  site  constraints.
Obtain cost estimates for the specific site, layout, and  desired
depth  before selecting a final design. Figure 11 shows a typical
breakdown between trenching and tubing costs for tubing  of  dif-
ferent  diameters  in  a  system  delivering  2,000  cfm  of  air
installed at an average depth of 9 ft.  For a 2,000  cfm  system,
the  8-, 10-, and 12-in. tubing sizes were the most economical in
this case. The figure also indicates average tubing and  excavat-
ing  costs  are approximately $2 per cfm of air capacity. Fan and
distribution system costs usually average approximately 50  cents
to  $1  per cfm of air capacity. Thus, total costs for an average
system should range from $2.50 to $3 per cfm of system air  capa-
city (1985 costs).

Performance Data

     Several functioning systems have been monitored in  Illinois
during  the  past few years, including systems designed according
to the guidelines presented here. Summer and  winter  performance
curves  for  a  30-crate  farrowing facility located near Spring-
field, Illinois, are shown in Figures 12 and 13.  The system con-
sists  of  five  12-in. lines, each 260 ft. long, buried about 10
ft. deep.

     The outside temperature for a  three-day  period  in  August
1981  (Fig.  12)  varied  from 60 to 92o F. The earth-tempered air
temperatures ranged from 64 to 69o F. The average sensible cooling
effect  during  the  three-day  period  was  equivalent to 20,773
Btu/hr. The temperature of the outside air during  the  three-day
period  in  January  1982  (Fig.  13)  varied from +20 to -19o F.,
whereas the earth-tube output temperature was  steady  at  46  to
48o F.,  a  maximum temperature increase of 67 degrees. The earth-
tube heat exchanger provided about 40% of  the  heating  required
during  the  winter  of 1981-82 by delivering tempered air at the
approximate rate of 920 cfm.

Economic Payback

     As with other alternative energy  systems  (solar  and  heat
exchangers),  tempering  of ventilation air by earth-tubes is not
free. Since the costs and returns vary  considerably  for  earth-
tube  systems,  a rigorous economic analysis would be both diffi-
cult and lengthy. However, to give some  indication  of  economic
payback  for  a  system, the following example is provided, using
the performance  data  and  costs  given  above,  plus  estimated
returns and expenses.

     Figure 13 gives the ``heating'' performance of a system over
three days in January from a 30-crate farrowing barn in Illinois.
A relatively constant exhaust air  temperature  from  the  earth-
tubes of 48o F. was recorded over this period. If one assumes this
same temperature over the entire heating season (will probably be
greater  in  the fall and less toward spring), then the amount of
energy recovered per heating month can be found using the follow-
ing relationship:

     Q = 1.1 x 920 cfm x (To -T) x 24 x (number of days in month)

     Q = Btu/month

     To = temperature exiting tubing (48o F. for our example)

     T  =  average  monthly  outside  temperature  Using  average
monthly temperatures for central Illinois during the heating sea-
son (November through March), a total of 61.5  million  Btu's  of
energy  would  be  recovered.  If this total is divided by 75,000
Btu's (amount of usable energy per gallon of L.PP. gas) then this
is  the  energy contained in 820 gal. of propane. At 75 cents per
gallon, a total of $615 would be saved per year.  Since a  larger
fan  (1/2  h.p.)  is needed in this system than with conventional
ventilation, a total of $100 (2,000 kwh x 5 cents/kwh) should  be
subtracted  from  $615 for a net return of approximately $500 per
year from heating.

     Estimating the cooling benefits during the  summer  is  much
more  difficult than calculating heat savings. It would be unfair
not to consider the returns from cooling, especially when compar-
ing  the  earth-tube  system with solar units and air-to-air heat
exchangers. Some animal scientists have estimated  that  1  extra
pig per litter occurs if a summer cooling system is used, because
of reduced sow heat stress, more efficient sow  milk  production,
and  faster  breeding. If that assumption is used in our example,
then 30 extra pigs per farrowing would result for a total  of  60
extra  pigs  (assume  2  farrowings  per summer). If an estimated
value of $15/extra pig is assumed, this results in  a  return  of
$900  due  to cooling. Adding this amount to the annual estimated
heat savings ($500), a total return of $1,400 per year results.

     The costs of  the  above  1,500  cfm  earth-tube  system  is
estimated  at $4,500, when using the $3/cfm figure discussed ear-
lier (1,500 cfm x  $3/cfm).  The  simple  payback  period,  which
excludes  fuel  price  increases  and  interest, would be between
three and four years. Consideration of L.PP. gas (propane)  price
increases  would  reduce  paybacks  while  the  inclusion of high
interest rates would extend them considerably.

     As is apparent from the above example, the  economic  feasi-
bility  of an earth-tube system should be thoroughly investigated
before beginning construction. While the heat savings can be cal-
culated  accurately,  one  should  also  give adequate weight (or
value) to the estimated cooling benefits. Solar systems and  heat
exchangers  provide no summer cooling while mechanical air condi-
tioning has proved to be too costly. Earth tempering of  ventila-
tion air may be the least-cost alternative for providing tempered
air during all times of the year.

NEW 5/85

Table 1.  Recommended ventilation rates for swine in  environ-
mentally regulated buildings.
                                                   Hot weather
                                              Zone cooling
    Swine        Cold     Mild     Uncooled   Evaporative   Air-condi-
     type       weather  weather     air      cooled air    tioned air   Normal
                                         cfm per head
Sow and litter    20       80         70          50            30         500
(12-0 lb)         2        10         -            -            -           25
(30-75 b)         3        15         -            -            -           35
(75-150lb)        7        24         -            -            -           75
(150-20 lb)      10        35         -            -            -          120
Gestation sow
(25 lb)          12        40         45          30            20        150*
(400 lb)          1        50         60          40            20         180

* 300 cfm for gestating sows or boars in a  breeding  facility
due to low animal density.

Table  2.   Ventilation   comparison  between  a  farrowing
house  with  and  without   an earth-tube heat exchanger 
                     Ventilation rate requirement
Ventilation       Normal building       Building with
rate type*     without earth system*     earth system
Cold weather   20 cfm/crate            20 cfm/crate
               (outside air)           (earth-tempered
                                       outside air)
Mild weather   80 cfm/crate            50 cfm/crate
               (outside air)           (earth-tempered
                                       outside air)
Hot weather    500 cfm/crate           250 cfm/crate
               (outside air)           (50 cfm/crate
                                       plus 200 cfm/
                                       crate outside
* Same as Table 1.

Table 3.Earth-tube heat exchanger line dimensions and capacities.
                                         Suggested    Suggested
    Tube     Nominal tube    Relative      line        airflow
  diameter       area        cost per     lengths     per tube
   (in.)       (sq. in.)       ft.*       (ft.)**      (cfm)***
     4            12.6        $0.25      65 -- 85          50
     5            19.6         0.35      80 -- 105         80
     6            28.3         0.55     100 -- 130        110
     8            50.3         0.90     130 -- 170        200
     10           78.5         1.85     160 -- 210        300
     12          113.1         2.30     200 -- 250        450
     15          176.7         3.80     250 -- 320        700
     18          254.5         6.30     300 -- 380       1000
     24          452.4        14.40     400 -- 500       1800
* Costs vary with  different  manufacturers  and  change  over
time.  These  costs are only offered to give a relative figure
for different sizes of tube.

** Line length ranges indicate the  effect  of  soil  type  and
moisture  content on line dimensions. The low end of the range
corresponds to a wet clay soil type and is based  on  1.3  sq.
ft.  of  tube surface area per cfm of airflow. The high end of
the range corresponds to a dry  sand  soil  condition  and  is
based  on 2.0 sq. ft. of tube surface area per cfm of airflow.
All surface area calculations were made assuming  smooth  pipe
for simplicity.

*** These airflow rates allow for a air velocity of 500  to  600

Table 4.Building  air  distribution  main  duct  and  downspout
dimensions. Main duct sizes  are based  on  duct  air velocity of
600  fpm.  Downspout  sizes  are based  on air velocities of 800-
1,000 fpm.*
                                  Inside duct dimensions if:
                  Air flow rate
                   within duct    Rectangular       Round
                  cu. ft./min.     in. x in.      in. diam.
Main duct sizes        250            6 x 10              9
                       500           10 x 12             12
                       750           10 x 18             15
                      1000           12 x 20             18
                      1250           15 x 20
                      1500           18 x 20
                      2000           18 x 27
                      2500           18 x 34
                      3000           18 x 40
                      3500           24 x 35
                      4000           24 x 40
                      5000           24 x 50
                      6000           30 x 48
                      7000           36 x 48
                      8000           36 x 54
Downspout sizes         20             2 x 2           21/2
                        30             2 x 3           21/2
                        40          21/2 x 3              3
                        50             3 x 3           31/2
                        75          3 x 41/2              4
                       100          4 x 41/2              5
                       125          4 x 51/2
                       150          4 x 61/2              6
                       175             4 x 8
                       200             6 x 6
                       250          6 x 71/2              8
*  It  is  the  minimum   cross-section  area,  not  the  actual
duct  dimensions  given  in  the table, that is important. Almost
any  duct  shape  of  comparable size  should  deliver  the  same
amount of air.

Figure 1.  Well-water isotherms indicating the mean annual ground temperatures
           for the 48 contiguous states.

Figure 2.  Yearly variation of soil temperature with relation to depth below
           surface for average soil.

Figure 3.  Annual ground temperature curves at the soil surface, at a depth of
           10 ft., and the annual mean for generalized conditions at
           Lexington, Kentucky, showing the degree of thermal lag at the 10-ft.

Figure 4.  System layout using the wagon wheel or radial pattern.

Figure 5.  System layout using the lateral tubing pattern.

Figure 6.  Chain-trencher excavating and installing 12-in. diameter tubing
           for a 1,600-cfm capacity system in Menard County, Illinois. Three 
           to 4 ft. of soil was removed, using a bulldozer before the trencher
           was used to install the tubing an additional 6 ft. into the soil.

Figure 7.  Backhoe excavating and installing 12-in. diameter tubing
           to a depth of 12 ft. for a 2,000-cfm capacity system in Sangamon
           County, Illinois. For safety reasons, the trench walls run up
           vertically only 6 ft. and the upper 6 ft. is set back to reduce
           cave-in problems. The evaporative cooler on the building roof is
           being replaced by the earth-tube heat exchanger system.

Figure 8.  Reinforced concrete collection sump on a 4,000-cfm capacity system
           in Peoria County, Illinois. At the sump, the tubing lines are 
           approximately 8 ft. below grade but slope away from the building 
           where they become 10-12 ft. deep. A portion of the sump was chipped 
           out after the producer decided to increase the number of lines in 
           the system.

Figure 9.  Tempered air to this 24-crate farrowing facility in Shelby County,
           Illinois, is carried down the center of the building via the insulated
           main duct shown. Downspouts are 4-in. diameter PVC pipe with flexible
           dryer hose used to allow opening and closing of crate doors.

Figure 10.  Downspouts can be used to bring the tempered air down into the
            crate a few inches away from the sow. Here PVC tubing has been used
            to direct the air through the crate door directly on the
            animal's snout.

Figure 11.  Tubing and trenching costs for a typical 2,000-cfm capacity system.


Cooperative Extension Work in  Agriculture  and  Home  Economics,
State  of Indiana, Purdue University and U.S. Department of Agri-
culture Cooperating. H.A. Wadsworth,  Director,  West  Lafayette,
IN. Issued in furtherance of the Acts of May 8 and June 30, 1914.
It is the policy of the Cooperative Extension Service  of  Purdue
University  that  all  persons  shall  have equal opportunity and
             access to our programs and facilities.