Agriculture Reference
In-Depth Information
pointed out that the design and management of the underlying HVC equipment to be
controlled substantially affects the interaction between different control strategies
and controlled systems.
Ventilation is critical in animal housing because fresh outside air is used to
replace the air within the facility (Hinkle and Stombaugh, 1983; Albright, 1990).
During mild or cool conditions with sufficiently high stocking densities, animal heat,
moisture, and gas production is removed by ventilation alone, and desired inside
temperature can be maintained without added heating. This requires a system that
is properly designed and configured with respect to the control of fans and inlets.
Typically in these situations, relative humidity of the interior air is uncontrolled.
When the outside conditions are substantially colder, or occupant heating load is
low, ventilation rate is reduced to a minimum intended to primarily control mois-
ture and gases within the facility, and some amount of supplemental heat must be
provided to maintain a minimum desired inside temperature. If supplemental heat is
not provided, then the building temperature will fall according to the sensible heat
balance of the space.
For facilities that utilize supplemental heat, many options are available and found
in commercial settings. A common option is for direct-fired gas (natural gas or LPG)
heaters that are activated once the temperature drops a specified amount below the
setpoint temperature. Less commonly found are various hydronic systems, which
are generally prone to fouling from the dust loads encountered in animal facilities.
When used, they may rely on simple thermostatic control, or may use proportional
controllers on valves controlling flow to unit heaters. Increasingly popular are radi-
ant tube heaters, which can also be either on/off or proportionally controlled.
The vast majority of automated HVC systems in agriculture use so-called “staged
ventilation,” which is depicted in schematic form in Figure 8.14. 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
6
Fan stages:
5
4
Minimum ventilation
3
2
1
Heat
Temperature
Set
point
Stage differentials
FIGURE 8.14
Heating and ventilation stage control system diagram. Each ventilation stage
of control involves switching additional fans and/or evaporative cooling; the heater control
circuit is activated when inside temperature drops.
