Hello, everyone! It's been a while since I last updated, and I haven't had much good material to share. Recently, however, a few people have asked me about some basic calculations for environmental control fans, such as how to set the airflow rate and operating time of ventilation fans. So, in this update, I'd like to explain how to calculate these values. The data provided is for reference only.
As you all know, during cold weather, many environmental control systems operate in minimum ventilation mode. The fans run in a cyclical manner, starting and stopping to ensure good air quality inside the livestock environment while minimizing heat loss, thus creating an optimal environment for the livestock. So, what exactly does "cyclical operation" mean? Simply put, the fans run for a set period, then stop for a set period, repeating this cycle. The question now is: how do we calculate the run time and stop time for these cycles? I'll explain this next.
To calculate the fan's operation cycle, the first parameter we need is the fan's airflow rate. Airflow rate refers to the volume of air that the fan exhausts or intakes within a given time period. Generally, the airflow rate can be calculated using the following formula:
Airflow (Q) = Cross-sectional area of the fan’s inlet/outlet (A) × Air velocity (v).
The air velocity can be calculated by measuring the pressure difference between the fan's inlet and outlet and factoring in resistance coefficients. It’s important to note that the calculated airflow rate might be affected by system resistance, fan efficiency, and other factors. Therefore, corrections may be necessary in practical applications.
This is why I mention "airflow" and not "exhaust airflow" specifically. The actual exhaust airflow is measured under certain operating conditions. For example, the airflow of a commonly mentioned 50-inch fan will vary depending on the negative pressure. The market offers fans for poultry farming with the same power rating, but prices range from over 1,000 to 5,000–6,000 yuan. As long as you haven’t been misled, I believe the price reflects the quality. This is also related to the manufacturer's quality standards. Some manufacturers cut costs by overstating the exhaust airflow or using aluminum wire motors, among other tactics.
Environmental control is crucial in the farming industry, and fan selection is vital. Choosing the right fan can make your work much more efficient, while selecting the wrong one can double your efforts and potentially cause unnecessary losses. Therefore, it's best to choose fans from reputable manufacturers, preferably ones with relevant certifications.
The most direct way to measure air velocity is by using an anemometer. Ordinary anemometers on the market cost only a few dozen yuan, and I personally think it's worth buying one. Before measuring, adjust the fan opening size and negative pressure to meet the required operating conditions. Negative pressure is tricky to control because it is directly related to how well-sealed the structure is. The same fan can have vastly different effects on environmental control depending on how well the structure is sealed. So, when constructing a poultry house, it’s essential to consider the sealing, insulation, and internal design.
Back to the main point. When measuring, please prioritize safety. Hold the anemometer close to the fan's intake safety net and use a "tic-tac-toe" pattern to take measurements at nine points. Then, average these values to obtain the air velocity. To calculate airflow, use the formula: air velocity * intake area * time. For example, if we measure an air velocity of 5 m/s (5 meters per second) at the fan inlet, let's calculate it using a common 50-inch fan with an inlet size of 1.38 m by 1.38 m. The calculation would be as follows:
5 * 1.38 * 1.38 * 3600 (one hour) ≈ 34,000 cubic meters per hour.
Now, let's calculate the ventilation volume using an example of 50,000 laying hens, each weighing an average of 1,000 grams. Use the formula:
number of chickens * minimum ventilation requirement / fan airflow.
I found a table online with minimum ventilation requirements that you can reference. I don't strictly endorse or reject it, as obtaining these figures requires long-term experience and experimentation. Thanks to the pioneers in the farming industry for sharing their knowledge.
For this example:
50,000 chickens * 1,000 grams of weight with a ventilation requirement of 0.7 / airflow of 34,000 ≈ 1.029 fans. Adding some margin for losses, we estimate about 1.1 fans are needed to achieve the minimum ventilation for these chickens over a long period.
We often use a 5-minute ventilation cycle, meaning the combined runtime and downtime of the fans should total 5 minutes per cycle. Now, how do we set up 1.1 fans? Should we run one fan continuously and have the other run for 30 seconds and stop for 270 seconds? While this approach might seem correct, it could cause instability in the negative pressure inside the poultry house. The size of the inlet openings remains the same, but if the number of running fans changes drastically, the airflow through the small windows will vary. This could result in cold air failing to reach the center of the poultry house, preventing it from mixing well with the warmer air near the roof, causing the cold air to fall directly. This could lead to issues like chickens catching colds.
Next, let me explain how to configure the operation of 1.1 fans. Use the formula:number of fans * runtime * 12 (the number of cycles in one hour) / 3600 seconds.Since this exceeds the runtime of one fan, we’ll need to run two fans.