Agriculture Reference
In-Depth Information
and deactivation, typically on the order of 0.1°C above and below the nominal stage
activation temperature.
During hot weather, the outside temperature exceeds desired inside temperature,
and ventilation is often provided at the maximum that the ventilation system can pro-
vide (Naas et al., 2006; Albright, 1990; Scott et al., 1983). This can have the advan-
tage of providing higher velocity of air over the animals although that depends very
much on the air inlet system design and operation (Tinôco et al., 2003; Pratt et al.,
1983; Scott et al., 1983; Hellickson et al., 1983). However, as temperatures continue
to rise, high ventilation rates simply bring in additional hot air and further exacerbate
an undesirable condition. Most commercial animal production systems do not use air
conditioning systems; rather, they either allow interior temperature to rise above out-
side temperature, or use some form of lower-cost cooling system typically based on
evaporation. Evaporative cooling systems can include evaporative pads, sprinklers, and
fogging systems. Their selection and operation is dependent on considerations of cost,
water supply capacity, water quality, and the climate (Palmer, 2002).
8.6.2 C
ONTROL
S
YSTEMS
In mechanically ventilated animal facilities, ventilation rate is controlled primarily
by means of thermostatic devices. Individual fans, or groups of fans in larger facili-
ties, are sized to provide specific ventilation rates that increase as inside temperature
rises above a setpoint. These incremental increases in building ventilation rate are
commonly referred to as ventilation stages. In the simplest approach, fans are con-
trolled by direct wired thermostats with different setpoints. However, over the past
three decades, microprocessor-based controllers have become increasingly prevalent
(Gates et al., 1992a, 1992b). The vast majority of automated HVC control systems
in agriculture use the so-called “staged ventilation,” which is depicted in schematic
form in Figure 8.14 (Gates et al., 2001). In its simplest form, building or building
zone temperature is the input process signal. It is compared to a nominally desired
value, labeled “Set Point” in Figure 8.14. If the actual temperature is warmer than
the heater activation temperature and cooler than the stage 1 activation tempera-
ture, only a prescribed minimum ventilation is provided that is adjusted to maintain
moisture and gas control for cold or cool conditions. This deadband is not defined
the same for all controllers, with some assigning the setpoint to the lower or the
upper activation point, but is a key feature in energy conservation gained from use of
such systems. As temperature rises sufficiently, stages of ventilation are sequentially
activated. These may involve single or large numbers of ventilation fans depending
on the size of the facility. Most controllers are programmed to provide a degree of
hysteresis for a stage's activation and deactivation, typically on the order of 0.1°C
above and below the nominal stage activation temperature.
For cold weather operation, minimum ventilation is provided either by intermit-
tent operation of the stage 1 ventilation system, a separate minimum ventilation stage
of ventilation, or in some cases by use of variable speed controllers that operate at a
fixed minimum speed and act as proportional controllers as temperatures rise above
setpoint. The primary purpose of minimum ventilation is to assist in maintaining a
moisture balance for the conditioned space (Gates et al., 1996). Insufficient minimum
