Shutters are widely used in HVAC systems and building façade systems and are also part of their foundation, but in practical applications, they are often overlooked or do not have enough time for design and proper consideration, and most projects are simply quoted. In view of this situation, the correct method of selecting the louver and the problems existing in the application of the louver are summarized.
According to the structure, fixed form, shape, material, and characteristics of the louver, the louver is divided into the following types of louvers according to its performance: ordinary ventilation louver, rainproof louver, rainproof louver ventilation louver, sound-absorbing ventilation louver, sand-proof louver. Ventilation louvers, etc. Among them, ordinary general ventilation louvers and rainproof ventilation louvers are the most widely used, and the selection method between the two is universal.
According to the problems encountered in actual work, we mainly discuss with you the basic selection of louvers, the detailed design of louvers, and some common misunderstandings about louvers.
Basic selection of shutters
In developed countries such as Europe and the United States, blinds are usually provided by professional suppliers, and each blind has detailed information for customers to choose from and use. In order to facilitate the reader's understanding, two basic concepts are introduced here.
1: Pressure drop refers to the pressure difference on both sides when the airflow passes through the louver;
2: The ventilation area (free zone area) refers to the minimum area where the airflow can pass through the louvers. For ordinary rectangular shutters, it is usually described as the sum of the minimum distance between all adjacent two shutters in the middle plus the minimum distance from the top shutter to the top frame of the shutter plus the sum of the minimum distance from the bottom shutter to the top frame of the shutter. The bottom frame is multiplied by the distance between the two frames. Product of minimum distances.
The following is a brief introduction to the steps of how to choose the shutter that suits your needs from the various shutter series
1. Determine airflow, airflow direction, and maximum pressure drop. These data usually have corresponding data provided by the bidding documents or mechanical and electrical engineering;
2. On the airflow resistance graph, draw a horizontal line at the desired static pressure drop coordinate until it intersects the intake or exhaust curve. At the intersection, draw a straight line down to the free zone velocity axis. Obtain the free zone flow rate corresponding to the pressure drop.
3. Check the water seepage map to ensure that the free zone velocity determined in the previous step is less than the starting point of the seepage velocity.
4. Calculate the ventilation area required for the louvers by dividing the airflow by the free zone flow determined in the steps above.
5. Go to the ventilation area table and select the louver with a ventilation area equal to or greater than the value calculated in step 4. There may be two or more louver sizes for the same ventilation area. Determine the width and height of the blinds according to your needs.
According to the requirements of louver performance, the method of choosing a louver is also different. This article selects three classic cases for your reference, see the attachment for details. It should be noted that this selection method does not consider the influence of factors such as anti-bird nets. Therefore, it is recommended to multiply the actually used louver area by a factor of 1.2~1.4 on this basis to ensure that the ventilation needs are met.
Detail design of shutters
1. The cross-sectional shape of the louver determines its performance. The overall shape of the river rainproof ventilation louver is designed to be streamlined to reduce wind resistance. There are sinks at both ends. The purpose is to collect the water entering the louver and discharge it from the two borders, so as to avoid the "waterfall" phenomenon of the louver and reduce the seepage water source. The louver frame has internal grooves and multiple grill-like "wings" to ensure airflow between the louver layers.
2 Bird net/insect net behind the blinds. Common insect nets/bird nets are made of stainless steel and aluminum, and the nets are in the form of woven nets and stretched nets. According to the requirements of AMCA Standard 500, the louver test is carried out without installing the bird/insect net, so the ventilation performance of the louver provided by the manufacturer does not consider the bird/insect net.
Common misunderstandings of louvers
1. The ventilation area percentage (opening ratio) of the louver and the ventilation coefficient of the louver
The ventilation area percentage of the louver is a physical index reflecting the density of the louver, and the ventilation coefficient is an index reflecting the ventilation performance of the louver. For the same type of louver, the louver with a larger percentage of ventilation area also has a higher ventilation coefficient. However, the percentage of ventilation area of different louvers cannot reflect the level of ventilation. Taking Jianghe rainproof ventilation louver as an example, its size is 1220mmX1220mm, its ventilation area percentage is about 50% and its ventilation coefficient is 0.33. In practical applications, the ventilation area percentage of the louver combined with the pressure drop is a common method to determine the louver. However, we must first be able to screen the requirements of the bidding documents for louvers and select suitable louvers according to the requirements.
2. Static rainproof performance
The data shown in the static rain performance table is based on AMCA standard testing. The experiment is to install a 1220mm×1220mm shutter on a wall and simulate a certain amount of rain falling down on the wall. It is tested that the rainwater can penetrate into the room under the wind speed. This is only under experimental conditions and does not consider the actual rainfall under natural conditions, the installation position of louvers, and the influence of factors such as wall wind pressure. The AMCA test is similar to a mild spring rain that does not affect a person's sleep. Unfortunately, many rainstorms were more violent than the test conditions. This means that even if the correct louver is selected according to the static rainproof performance, it will still penetrate into the rainwater.
3. Dynamic rainproof performance
Dynamic rainproof performance is an important indicator to measure the rainproof performance of louvers. Generally, the rain resistance rate or rainproof grade of the louver is required under a rainfall of 75mm/h and a wind speed of 13m/s. However, it cannot be said in general how much rain-blocking efficiency of the louver or the level of rain protection under this condition. Another important factor is the air intake (air output) of the louver or the wind speed passing through the core area. If we install a louver on a small and closed hole, and install another same louver on a hole with "passing" wind, under the same test conditions, the amount of water seepage of the louver is different. The dynamic rainproof performance levels corresponding to different wind speeds are also different.
Under normal circumstances, the wind speed in the core area is required to be 3.0m/s or 3.5m/s, and the requirements for the rain-blocking efficiency of the rain-proof louvers or the achieved rain-proof grade are put forward.
4. About the choice of rainproof grade
How to choose the rainproof shutters suitable for the project should comprehensively consider the needs, application, and cost of the project. In our reapplication, some consultants asked for 100% rainproof efficiency. In fact, this is very unreasonable and unnecessary, because rainproof ventilation louvers need to ensure two important indicators, one is ventilation and the other is rainproof performance, and The trade-off between the two requires a comprehensive balance.
In general, it is recommended that for applications with very high requirements for static rainwater protection (such as areas where electrical switches are located), louvers of performance class 4 (1 to 0.99 efficiency) are optional. This choice may sacrifice some ventilation performance. For high water seepage protection, a small amount of water seepage can be ignored, or occasional cleaning work is not a problem (such as the area where a water pump with a drip-proof motor or a fan is located), then it is recommended to use class 3 (0.989 to 0.985 efficiency) ) of the shutters. For most projects, the first concern is the ventilation performance and then the rainproof performance of the louver. performance) is relatively good, and it is also the most widely selected in practical engineering applications. Therefore, it is the key to choosing the louver correctly and reasonably according to the needs.