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Assume the flow velocity method----------------------------------

The commonly used design calculation method for ventilation systems is the assumed velocity method.

I. Design calculation methods for general ventilation 1. Calculation based on the number of air changes per hour (this method can be used in the absence of any specific requirements) The number of air changes per hour refers to how many times the air in a room is replaced in one hour; the unit is usually times/hour. This value is known and can be found in design manuals or standards, or it may be specified in the requirements put forward by the main responsible party. 　　It is necessary to calculate the volume of the room; the product of this volume and the number of air changes per hour gives the ventilation rate. For example, in the case of a substation with an area of 18X9, and a room height of 4.7 meters (generally, if the ceiling height is above 6 meters, it is assumed to be 6 meters): The volume of the room is V = 18×9×4.7 = 761.4 m3 ； 　　Air volume: L=n·V=12×761.4＝9136.8m3/h ； 　　After calculating the required ventilation volume, it is necessary to select a ventilator. Taking into account air leakage from the ventilator, adjustments need to be made to it; generally, the air leakage coefficient for ventilation is between 1.05 and 1.1. If we use a coefficient of 1.1, then the adjusted value is: L’ = 9136.8 × 1.10 = 10050.5 m3/h ； 　　At this point, we should calculate what the pressure head of the fan is in Pa. Generally, an estimate of 3 to 6 Pa per meter for duct connections is sufficient, as the calculation is rather complicated. If there are no duct connections, we can generally assume that the fan head pressure is very low. 　　After calculating the ventilation volume, we need to select a fan. Based on the calculated data and by referring to fan catalogs, we can choose a model that is suitable; this makes it easier for us to determine the fan’s power consumption, as well as its size and weight. It also helps in planning the electrical wiring, determining the necessary openings in the building, and taking into account the weight of the fan in terms of structural requirements. 　　The arrangement of fans can generally be done in a free and even manner based on the number selected. 　　What was mentioned above refers to the most common calculations for room ventilation, which generally aim to remove excess heat, moisture, and unpleasant odors, with no special stringent requirements. 2. Thermal balance calculation method: It is mainly based on the heat generation amount; the power of the relevant equipment is specified in specialized sources, and the heat generation amount is calculated from this power. The exhaust volume is then determined using the heat generation amount and the temperature difference between indoors and outdoors (formulas are provided in the manual). -------------------------------------------------------------------------------- 　　II. Several scenarios in ventilation design 1. Is make-up air consideration included? 　　Sometimes, a room has no windows or only fixed windows that are designed for exhaust only; in such cases, a negative pressure is created inside the enclosed room, which makes it even more difficult to remove harmful gases. In such situations, it is necessary to consider installing supply vents, and the location of these vents should be chosen so that there are no dead corners in the airflow. 　　Generally, exhaust air is at the top while intake air is at the bottom. 　　2. What is the height position of the exhaust fan (or air outlet)? 　　Under normal circumstances, top exhaust ventilation can be used to remove residual heat and unpleasant odors; specifically, top exhaust ventilation is feasible as long as the density of the gases to be removed is lower than that of air, with the fan placed in the upper part of the room. 　　If the gas being exhausted is denser than air, it will sink; in such cases, bottom exhaust is required. However, with bottom exhaust, the fan is not usually placed at the bottom of the room, but rather connected to the upper part via ducts, with the exhaust taking place through fans located there. Openings are provided in the ducts at the lower level, with the wind speed at these openings controlled at around 3 m/s, while the wind speed within the ducts is kept below 7 m/s. 　　3. Should accident ventilation be installed? 　　Emergency ventilation is required in the following situations: A. The accumulation of gases to a certain concentration can cause explosions or poisoning. 　　B. Electrical substation, inside the distribution room. (This is mainly to consider the exhaust of smoke after a fire). -------------------------------------------------------------------------------- III. Calculation of local exhaust ventilation 　　The concept of exhaust ventilation discussed earlier refers to overall air exchange, while local exhaust ventilation means ventilation at specific locations where harmful gases are generated, such as in laboratory cabinets and fume hoods. 1. Calculation method for local ventilation Local ventilation is generally not calculated based on the number of air changes, as it is a localized system and it is not possible to determine the volume of air that is exchanged. 　　At this point, we need to use the calculation method based on the wind speed at the operating surface; for example, in the case of local exhaust hoods, it is the wind speed passing through the opening of the hood, while for fume hoods, it is the wind speed at the opening of the fume hood – generally, a wind speed of 0.5 m/s is used. You can check the design manual for details. 　2. The design dimensions and height of the exhaust hood can be referred to in the design manual; no one remembers the formulas. -------------------------------------------------------------------------------- 　　IV. Selection of fan type: There are various styles of fans; under what circumstances should a particular model of fan be chosen? 　　1. Installed on the wall without ductwork; axial flow fans are generally used, with the T35 model being common, or low-noise wall-mounted axial flow fans can also be chosen. An axial flow fan can also be connected to a section of ducting as needed; the length to which it can be connected depends on whether the fan’s pressure head is sufficient to cover the pressure losses. 　　2. Generally, when the air ducts are long, mixed-flow fans, diagonal-flow fans, or centrifugal fans need to be used, as these types of fans have a relatively high pressure head. 　　3. Sometimes, a rooftop fan can be chosen; a foundation can be built directly on the roof and the fan installed there, which is convenient. 　　4. Bathrooms usually have ceilings, so ceiling-mounted room ventilators can be used for exhaust; the air can be vented directly outside, or the building can be equipped with an air duct (a concrete shaft) to which the pipes can be connected. If there is really no ceiling, choose a wall-mounted exhaust fan, which can be installed on the wall or on a fixed window. 　　Note: The wind speed (the speed at which air flows) within ducts and air channels is generally kept between 3 and 7 m/s, as this results in less noise. 　　Keep the wind speed at the louver opening below 4 m/s, and use an effective coefficient of 0.6 to 0.9 for the louvers

The commonly used design calculation method for ventilation systems is (the assumed flow velocity method)

The commonly used design calculation method for ventilation systems is (the assumed flow velocity method)

The commonly used design calculation method for ventilation systems is (the assumed flow velocity method)

Assume the flow velocity method! ! ! ! ! ! ! ! ! ! ! ! ! !