1. Heat stress is a major
problem during
the summer
months in many countries
where poultry are
produced. High levels of
temperature, humidity and
radiation lead to a
significant drop in
production, especially
when the temperature
exceeds 28°C.
As the temperature
begins to rise, birds
dissipate their metabolic
body heat by the sensible
transfer means -
convection, conduction
and radiation. But sensible
heat transfer depends on a
temperature difference.
When the ambient
temperature continues to
rise towards the birds’
body temperature, they
increase evaporative heat
loss but this costs them
energy and performance can fall dramatically.
To keep birds comfortable and productive, many
types of cooling systems have been tried. They rely on
methods to maximising sensible heat loss: evaporative
cooling and/or air movement but they only work well
as long as the ambient air temperature is less than
animal body temperature. Evaporative cooling is only
successful under low humidity climatic conditions and
moving more air through the poultry house when the
outside temperature gets above 29°C is not a good
solution to provide a comfortable environment for the
birds.
Evaporative cooling (cooling air by evaporating
water) can be effective during most days in most
climates. Hot, dry climates are the best suited to
evaporative cooling because the relative humidity (RH)
is low. RH is generally lowest during the hottest part of
the day so evaporative cooling can successfully reduce
the air temperature in poultry houses during this time.
When the humidity remains high all day, evaporative
cooling is less effective and is not generally used during
these weather conditions.
The most common methods of evaporative cooling
use pads (cellulose or wood shavings), spinning discs,
and either high- or low-pressure fogging (misting)
systems. They can be very satisfactory when the outside
temperature is moderate.
Problems really come when summer temperatures
climb towards 50°C. Assuming an RH of 20% and a 60%
efficiency of the evaporative cooling system, the dry
bulb temperature will still read 40°C. So, evaporative
cooling alone is not really effective in reducing the air
temperature even when the RH is low. At higher RH
and where the house has poor insulation, bad design
and/or siting and at high stocking densities, evaporative
cooling systems alone are certainly not adequate to
maintain production. In some countries, production
must cease during the hottest months, with inevitable
financial effects.
The solution lies at your feet!
The two main sources of heat are from the birds
themselves and from the sun shining on the building
walls and roof. Generally speaking, the soil below the
surface will be a more constant temperature as it heats
and cools with the seasons and there is a time lag as it
does so. These characteristics can be turned to
advantage by the design of a cooling system based on
a heat exchanger.
In terms of design of an earth tube heat exchanger
system, the most important criterion is the soil
temperature. The temperature variation decreases as
one goes deeper beneath the surface. Changes in soil
temperature lag behind those on the surface because of
the heat storage capacity of soil. The soil surface
temperature reaches its maximum during the summer
but at depth, it may not reach a peak until three months
later.
This thermal lag helps both the heating and cooling
performance of heat exchange systems. During the
Combine the simple idea of a pipe buried
underground with evaporative cooling to keep birds
comfortable and performing well. One system is “road-
tested” here. — Dr Dhia Alchalabi
Two-Stage Air
Cooling
forVery Hot
Environments

2. winter, the temperature of
the soil 3m below the
surface is at the autumn
seasonal level, thus
adding to the heating
capacity. The reverse is
true during summer
months when soil at a
depth of three metres is at
its spring level and can be
used to cool the
ventilation air.
Soil temperature varies
with soil type, depth,
moisture content, time of
the year and geographic
location. Sandy soils tend
to have temperature
variations than clays and
soil moisture also
increases the range.
Seasonal rains can affect
soil temperature in winter.
Two-stage air cooling:
the theory
Two-stage cooling is a
thermodynamic process.
The air is cooled in two
steps to reach the final
target economically and
practically. The first stage
uses sensible cooling to
reduce the temperature
with constant moisture
content. The second stage
is to reduce the
temperature by
evaporative cooling
(adding moisture to the
air) and increasing the RH
at the end.
There are several ways
of two-stage air-cooling
used in agriculture. The
most popular one is the
sensible cooling heat
exchanger (subsoil),
followed by an
evaporative cooling unit
(see figure). The air is
drawn through metal
Two-Stage Air Cooling
Figure 1 Design of a two-stage cooling system
Figure 2 Temperature changes over a 24-hour period (°C)

3. pipes buried in the soil and then through an
evaporative cooling unit. A study was conducted to
evaluate the efficiency and feasibility of such system.
A practical two-stage air cooling system
The system is shown in Figure 1. A metal tube, 30 m
long and 30cm in diameter was buried around three
metres deep in the soil. It was connected to a 4mm-
thick iron pipe in a box made of 20mm thick plywood.
The box measured 1.2m square and had three
openings, each 60cm square (to service the air cooler
unit). Inside the box was an evaporative cooling unit.
The connection between the pipe and the box was
insulated with glass wool 5cm thick. For these trials, the
air flow rate fixed at 1200m3/hr and the power of the
motor at 200 watt.
The results of the test are shown in Figure 2. The
outside temperature was coolest at 06:00 (23.5°C) and
highest at 15:00 (43.0°C), a range of 19.5°C. Within the
house, the range was just 3.8°C between the minimum
of 19.1°C at 02:00 and the maximum of 22.9°C between
17:00 and 18:00.
The study showed that two-stage air cooling in
poultry housing has the upper hand over conventional
cooling. Combining the two processes resulted in a
stable inside temperature. Relative humidity will not
have any negative effect as long as the temperature of
the air remains below 24°C. Running the system
continuously helps the soil to lose heat gained during
the day and it heats the air at night so the system will
be ready to work efficiently the next day. In winter, the
system can be used without the evaporative cooling
step. The relatively warm soil helps to heat the air,
reducing fuel costs to heat the incoming air during the
colder months. A study with this system showed a
saving of 0.57 L/bird/month on LP gas. — Dr Dhia
Alchalabi is a consultant in poultry environment and
house design in Auckland, New Zealand.
Two-Stage Air Cooling