Friends who are engaged in environmental protection projects must have encountered such problems: when the odor from sewage plants, garbage stations, chemical plants and other places wafts out, not only do nearby residents have objections, but environmental inspections cannot be passed. To solve the odor problem, the collection and treatment system is the core, and duct calculation and fan selection are the "skeleton" and "heart" of this system. Today, let's talk in plain language about how to calculate the odor collection duct and how to choose the fan, ensuring that you can see it clearly.
1、 First understand: Why do odor collection ducts need to be counted?
Some people may say, 'Isn't it just a pipe? Just find a size and install it?' But that's not really the case. If the size of the air duct is calculated incorrectly, it can be very troublesome:
-The pipe is too thin, the wind cannot run, the odor cannot be collected, and the place that should smell is still smelly;
-The pipe is too thick, although the wind runs smoothly, it wastes materials, and when choosing a fan later, larger ones have to be chosen, causing electricity bills to rise rapidly;
-If the wind speed in the duct is not appropriate, it may also blow up dust and impurities in the pipeline, which can clog the pipe over time and even corrode it. The maintenance cost in the later stage can make you headache and unable to sleep.
So, the calculation of air ducts is not just about fooling around, but about making the odor "obediently" enter the pipeline, while also saving money and peace of mind.
2、 Odor collection duct calculation: just look at these steps
The calculation of air ducts may sound complex, but in fact, the core consists of three things: air volume, wind speed, and pipe diameter. There is an "iron triangle" relationship between the three, knowing two can calculate the third. Let's take it step by step.
Step 1: Calculate how much air is needed - determine the air volume
Airflow refers to the amount of air (including odors) that needs to pass through a duct per unit time, typically measured in cubic meters per hour (m ³/h). There are several methods for calculating air volume, and it depends on the actual situation.
1. Calculated by "collection area size": suitable for enclosed or semi enclosed spaces
If the odor source is a closed pool (such as an anaerobic tank in a sewage treatment plant) or a closed workshop, it can be calculated based on spatial volume. The formula is simple:
Air volume=space volume x air exchange rate
-The spatial volume is easy to calculate, length x width x height is enough. For example, a closed pool that is 10 meters long, 5 meters wide, and 3 meters high has a volume of 150 cubic meters.
-The key is the number of air changes, which depends on the concentration of odor. The higher the concentration, the more air changes are required to extract the odor in a timely manner. Generally speaking:
-Mild odor (such as ordinary sewage tanks): 3-5 air changes per hour;
-Moderate odor (such as garbage transfer stations): 6-10 air changes per hour;
-Severe odor (such as chemical plant reactors): 10-20 air changes per hour, or even higher.
For example, in the enclosed pool with a capacity of 150 cubic meters above, if there is a moderate odor, the air exchange rate is calculated as 8 times, and the air volume is 150 × 8=1200 m ³/h.
2. Calculated by "pollution source area": suitable for open or local pollution sources
If the odor comes from an open liquid surface (such as an aeration tank in a sewage treatment plant) or a local leakage point (such as a valve interface), it must be calculated based on the area of the pollution source. The formula is:
Air volume=Pollution source area x Capture wind speed x 3600
-The area of the pollution source is the area where the odor is emitted, for example, an open liquid surface that is 8 meters long and 4 meters wide has an area of 32 square meters.
-The "capture wind speed" is an important parameter, and it is necessary to ensure that the wind speed is sufficient to "press" the newly emitted odor into the air duct and prevent it from drifting into the surrounding environment. Reference for capture wind speed in different scenarios:
-The liquid level is calm and the odor spreads slowly: 0.3-0.5 meters per second;
-The liquid level is stirred (such as aeration), and the odor spreads quickly: 0.6-1.0 meters per second;
-Local leakage points (such as pipeline interfaces): 1.0-2.0 meters per second.
For example, for a 32 square meter open liquid surface with aeration and agitation, the capture wind speed is calculated at 0.8 meters per second, and the air volume is 92160 m ³/h (note that multiplying by 3600 here converts seconds to hours).
3. Calculated by "device parameters": suitable for situations where there are ready-made devices
Some devices themselves will indicate the required ventilation volume, such as biological filters, activated carbon adsorption towers, and other processing equipment. The instructions will state "processing air volume XXXX m ³/h", and in this case, simply follow the required air volume of the equipment, without having to calculate it yourself.
