Agriculture Reference
In-Depth Information
ventilation results in high interior humidity, whereas excessive minimum ventila-
tion simply uses excess energy as interior air is brought into the space and must be
heated. The simplest early implementations of intermittent operation of stage 1 fans
involved the use of a mechanical timer connected in parallel with a thermostat, so
that if the thermostat was not activated the timer would assume control. Problems
with these approaches include the difficulty of determining appropriate minimum
timer settings as external weather and internal occupant thermal loads changed. This
is particularly challenging for animal production systems with high stocking density
and rapidly growing animals whose heat and moisture production increases substan-
tially over time. In smaller facilities or in rooms within larger facilities, such as in
pig gestation or nursery housing, the use of a variable speed fan is common. This can
provide fairly uniform proportional control of ventilation rate, but the challenge of
properly setting the minimum ventilation rate remains.
Static pressure is commonly used as a control input to adjust inlet opening size.
As temperature inside the facility increases, additional fans are actuated to main-
tain temperature setpoint, necessarily increasing static pressure. Linear actuators or
powered cable winches are then used to adjust inlet opening distance to maintain a
constant static pressure (Gates et al., 1991a).
8.6.3 C
OOLING
AND
H
EATING
C
ONTROL
S
YSTEMS
As outside air temperature approaches the desired temperature setpoint in warm
weather, additional cooling is necessary to offset heat loads in the facility. Evaporative
cooling has long been an economical means to reduce air temperature in animal pro-
duction facilities (Hahn et al., 1981; Wiersma and Short, 1983; Gates and Timmons,
1988a, 1988b; Timmons and Gates, 1988a, 1988b, 1988c; Gates et al., 1991c; Bridges
et al., 1992; Turner et al., 1992; Bridges et al., 1998). Evaporative cooling systems
in animal facilities are of the direct type and are composed of either misting or pad
systems (Timmons and Gates, 1989; Singletary et al., 1996). Misting systems can be
staged to provide variable rate cooling by manipulating line pressure (which affects
droplet size and enhance evaporation efficiency) or by providing multiple stages of
cooling (Haeussermann et al., 2007; Bottcher et al., 1991; Singletary et al., 1996), or
variably intermittent operation (Geneve et al., 1999; Zolnier et al., 1999). However,
misting generally suffers from reduced evaporative efficiency when compared to pad
systems in all but the driest climates. Full ventilation capacity is typically reached
before initiation of evaporative pad usage in poultry housing. Many environmental
controllers offer the ability to restrict evaporative cooling usage to certain periods
of the day to maximize effectiveness, e.g., restricting use at night when outside air
is at near-saturation.
Radiant heating is commonly used in commercial and poultry housing to provide
supplemental heat for neonatal animals. Radiant heating systems provide efficient
heat transfer directly to the animals and heat the floor or bedding to provide zone
heating as well. Radiant heating systems heat surroundings, and the resulting air tem-
perature is used as the process variable, rather than the mean radiant temperature.
However, animals receive a portion of the radiant load, and typically at different rates
than the surroundings given the differences in absorbance and surface temperatures.
