Figure 1. Ideal air flow for negative pressure ventilation in a broiler house.
The incoming cold air must be conditioned correctly by easily attaching to the ceiling, moving towards the peak of the house where it is heated, expand and its relative humidity reduced. The air jet must then start detaching from the ceiling near the centre of the house generating a return movement at floor level in the opposite direction. This return movement towards the side walls will ensure distribution of the warmer dry air thus bringing fresh air to the birds while also removing moisture and waste gases from the litter, the birds, and the heating system (Figure 1). The stale moist air is then extracted from the house through the fans. The system runs when the house is at or below set point temperature and will operate on a timer program – see the example of a simple timer fan program opposite (Table 3).
Hottest air in the house
RH-25 % 27 °C (80.6 °F)
RH-50 % 16 °C (60.8 °F)
RH-75 %, 10 °C (50.0 °F)
RH-100 % 4 °C (39.2 °F)
Table 3. Minimum ventilation timer program
Fresh air at bird level, dry litter and low heating costs.
Minimum ventilation timer settings (5 min (300 sec) timer)
attach to the ceiling to provide effective conditioning of the cold incoming air? These systems will require good quality environmental controllers, perimeter inlets and fans. They are not suitable for older (leakier) houses and will generally require a higher level of technical knowledge. Example calculation
Day
ON (seconds)
OFF (seconds)
1
45 to 60 (20 %)
240
3
45 to 60
240
5
75
225
8
90
210
House dimensions used in example:
11
105
195
150 m long, 14 m wide and 2.88 m average height.
14
120
180
18
135
165
Average height = 2.5 m + (0.5 x 0.75 m) = 2.88m
22
150
150
House floor area: 150 m × 14 m = 2,100 m²
25
165
135
Fans used for example:
30
180 (60 %)
120
Fans capacities used in the examples are rated at 25 Pa
The timer fan capacity should be a minimum of 0.3 m3/min per m² (1.0 cfm per ft²) in hot climates where outside temperatures rarely drop below 20 to 25 °C (68 to 77 °F). In cold climates a timer fan capacity of 0.61 to 0.77 m3/min per m2 (2 to 2.5 cfm/ft2) of floor area, will be needed. The
0.75 m (2.5 ft)
Exhaust or side wall fans: 900 mm, working capacity of 340 m³/ min. Air exchange range: 0.3 to 0.60 m³/min per m² of floor area
2.5 m (8 ft)
150 m (500 ft)
14 m (46 ft)
Note: These fans can be fixed volume or variable speed.
challenge will always be matching the requirement with fans available in the market. Do not use large tunnel fans for the minimum ventilation system, smaller capacity fans will always afford more stable environmental control and flexibility. Some ventilation systems do not use cycle timer programs for minimum ventilation, but instead run a continuous ventilation system using variable speed fans. These systems are designed to produce a more uniform house environment, but it is strongly dependant on the fans pressure stability at low rpm. In practical terms can the incoming “air jet”
The 0.61m3/min per m2 of floor area fan capacity is only needed in cold climates.
How many fans to install or allocate for minimum ventilation:
Number of fans required = (House floor area X Air exchange rate) ÷ Working capacity 2,100 m² × 0.3 to 0.61 m³/min per m² of floor area = 630 to 1260 m³/min
2
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