Biomedical Engineering Reference
In-Depth Information
of about 30 ft. A 1-hp 48″ fan provides 21,000 cfm
with an effective throw distance of about 40 ft.
These fans should be mounted out of reach of the
cows and angled downwards slightly. Overhead
paddle fans can also be used to provide airflow, but
they offer limited air movement unless cows are
directly under a fan. The paddle fans do have much
higher efficiencies of operation, with ratings of
100 cfm/W of power input, as compared to 15-20
cfm/W for conventional propeller-type fans.
For a given airflow, a larger diameter fan is
more energy efficient than several small diameter
fans. When two fans have equal diameter and
rpm, the fan with lowest motor current rating is
usually more efficient. When two fans have equal
airflow, the fan with slower speed is usually qui-
eter and more efficient.
1991
). The body surface exposed to moving air is
reduced when cows huddle, as well as when cows
adopt a recumbent posture during resting or rumi-
nation. Mature Holstein cows in stall housing
systems spend about 15 h/day lying, and the dura-
tion and frequency of lying probably are indica-
tors of cow comfort (Haley et al.
2000
) . A smaller
body surface reduces both the convective and the
evaporative components of heat loss by skin. The
time spent lying was reduced in cows exposed to
heat stress (
F
razzi et al.
2000
) . Ambient tempera-
ture has been observed to negatively impact on
the percentage of cows lying (Shultz
1984
;
Overton et al.
2002
) . Animals standing were
observed to have higher respiration rate and body
temperatures (Frazzi et al.
2000
) . Spray cooling
of lying cows increased their lying time (Hillman
et al.
2005
), supporting the contention that a lying
cow is more sensitive to heat stress than a stand-
ing cow. Respiratory heat loss is recruited when
the heat loss by skin is insufficient to maintain
thermal stability. A high demand for heat loss by
respiration may, by itself, be a stressing factor
because it reduces the time spent lying.
Reasonable Fan Goals
3 6 ″ fan = 11,000 cfm
4 8 ″ fan = 20,000 cfm
3.3
Evaporative Cooling
Animal shades provide shield from solar radia-
tion, but there is no effect on air temperature or
relative humidity, and additional cooling may be
necessary for lactating cows in a hot and humid
climate. A number of cooling options exist for
lactating dairy cows based on combinations of
the principles of convection, conduction, radia-
tion and evaporation. Air movement (fans), wet-
ting the cow, evaporation to cool the air and shade
to minimise transfer of solar radiation are used to
enhance heat dissipation. Any cooling system
that is to be effective must take into consideration
the intense solar radiation, high ambient tempera-
ture and the typically high daytime relative
humidity, which increases to almost saturation at
night. These challenging conditions tax the abil-
ity of any cooling system to maintain a normal
body temperature of any animal. Evaporative
cooling utilises energy from the air to evaporate
water, that is, adiabatic cooling. This cooling sys-
tem lowers the temperature of the air and increases
the relative humidity, making evaporative cooling
most effective in dry environment (Bucklin et al.
3.3.1 Evaporative Cooling Pads and Fan
Systems (Desert Cooler)
Evaporative cooling through water evaporation in
the incoming airflow is an economical method
resulting in a temperature reduction of 8-10°C,
but it causes an increased relative humidity up to
90% (Chiappini and Christiaens
1992
) . This system
uses pads and pumps to pour water through the
pads (Kelly and Bond
1958
) . Cooled air must
have a short way through the building to avoid
high temperature and high humidity. It should be
installed in an adequate insulated building
(Chiappini and Christiaens
1992
) .
3.3.2 High-Pressure Foggers
Foggers disperse very fine droplets of water
which quickly evaporate and cool the surround-
ing air, but they also raise the relative humidity.
A ring of fog nozzles is attached to the exhaust
side of the fan, and then the cooled air is blown
down over the animal's body (Jones and Stallings
1999
). This system is expensive and requires a lot
of maintenance, and the water must be kept very
