In the design of clean room purification air conditioning, air flow organization design is critical to the clean room’s success or failure. The airflow organization design is achieved by rationally arranging the air supply, return, and exhaust vents. If the air supply volume, return air volume, and exhaust air volume are calculated correctly but the arrangement of the air vents is unreasonable, the required cleaning effect will not be achieved or even cause design failure.
First, I would like to explain several common concepts about clean rooms, which are prerequisites for setting up air vents.
How many types of airflow patterns are in a cleanroom?
There are three types: Unidirectional flow, Non-unidirectional flow, and mixed flow.
Unidirectional flow: airflow with parallel streamlines in one direction and consistent wind speed on the cross-section;
There are two types: vertical one-way flow and horizontal one-way flow.
Non-unidirectional flow: Refers to airflow that does not meet the definition of unidirectional flow.
Mixed flow: Airflow that is a combination of unidirectional and non-unidirectional flow.
Generally, the cleanliness of unidirectional flow clean rooms can reach levels 1-100, non-unidirectional flow clean rooms can reach levels 1000-100,000, and mixed flow clean rooms can reach levels 1-100 in some areas.
About unidirectional flow and non-unidirectional flow in clean rooms
Since the filtration and purification technology controls the concentration of suspended particles in the clean room’s air, the air volume sent into the production area or clean room is much larger than the air volume required for ventilation and dust removal or general air-conditioned rooms. Hence, its airflow organization pattern is different. I suggest below aspects:
- Clean airflow should reduce or avoid eddy currents conducive to rapidly and effectively removing pollutants.
- Limit and reduce the spread of dust and bacteria emitted by indoor dust sources.
- You need to consider the maintenance of indoor temperature and humidity and the comfort requirements of staff.
Static pressure difference and number of air changes in the cleanroom
To prevent the cleanliness of the clean room from being contaminated by adjacent rooms or contaminating adjacent spaces, an appropriate static pressure difference must be maintained between clean rooms with different cleanliness levels or between clean rooms and general rooms. The static pressure difference shall not be less than 5Pa.
Since the air supply volume sent into the clean room is different, the indoor cleanliness is also different. Based on theoretical calculations and practical experience, I believe that the general empirical air exchange times are as follows, which can be used as an estimate of the preliminary clean room air supply volume:
- The number of air changes for a class 100,000 clean room is generally more than 15 times/hour.
- The number of air changes for a class 10,000 clean room is usually more than 25 times/hour.
- Air changes for a Class 1000 clean room are generally more than 50 times/hour.
- For a Class 100 clean room, the air supply volume is calculated based on the air supply cross-section wind speed of 0.2-0.45m/s.
How to set HEPA filter boxes and return air vents in a cleanroom?
After understanding the clean room knowledge, let me talk about how to set up HEPA filter boxes and return air vents.
Set an example
For example, a clean room requires a cleanliness level of 10,000. The calculated air supply volume is 900m3/h, and the return air volume is 850m3/h. Figure 1-1 below shows that one HEPA filter box A with a rated air volume of 1000 m3/h and one return air vent B are arranged on the short side. This airflow organization design will not achieve the required cleanliness (despite the correct air volume calculation).
With this airflow organization design, the vortex area is too large, see Figure 1-2 and the effect of controlling pollution in the clean room could be poor.
In this case, set 2 small HEPA filter boxes and two return air vents.
The aspect ratio of this kind of clean room is relatively large. A good cleaning effect can be achieved if the return air outlet is arranged, as shown in Figure 2-1 below.
Two air supply vents with an air volume of 500m3/h are selected, and the return air vents are arranged on the long sides. To reduce the vortex area, two return air vents are chosen. The airflow streamlines are shown in Figure 2-2 below.
The vortex area is significantly reduced, and the cleanliness can be improved. The purification effect will be better if double-sided lower air return is used.
How to determine the number of HEPA filter boxes?
As can be seen from the above example, calculating air volume is only an essential condition.
There are many factors to consider regarding the number and placement of HEPA filter boxes and air return vents.
Many HEPA filter boxes are currently used with rated air volumes of 500m3/h, 1000m3/h, and 1500m3/h. As long as the clean room’s air supply is accurately calculated, the number of HEPA filter boxes equals the air supply volume divided by the rated air volume of the selected model.
In the above example, you can choose either one HEPA filter box with an air volume of 1000 m3/h or two HEPA filter boxes with an air volume of 500 m3/h. Which model is better depends on the specific situation.
As the example shows, the aspect ratio of the clean room is relatively large, so it is necessary to choose two HEPA filter boxes with small air volume. Only such airflow organizations can meet the requirements. If the aspect ratio of the clean room is not large, you can choose a HEPA filter box with a large air volume.
This can make the airflow organization meet the cleanliness requirements and save the initial investment (because the cost of two HEPA filter boxes of 500m3/h is greater than the cost of one 1000m3/h air volume). Of course, it is also good to choose two HEPA filter boxes with small air volumes, which are also suitable for improving cleanliness.
How to determine the position of the HPEA filter box in the cleanroom?
First of all, consider the location of the production area, and the HEPA filter box should be arranged above the production area so that the mainstream area of the air supply outlet covers the protected production area as much as possible.
Secondly, consider the balance of the HEPA filter boxes. When there are many production areas and uniform cleanliness is required in the room, the HEPA filter box should be arranged as symmetrically as possible (for the sake of appearance, the same type of HEPA filter boxes should be arranged in the same clean room). The purpose of the symmetrical arrangement is to spread the airflow as evenly as possible and reduce the vortex area.
How to determine the number and position of the return air outlet in the cleanroom?
The number of return air outlets is equal to the calculated return air area divided by the return air outlet area of the selected model. The return air area can be determined by dividing the return air volume by the recommended return airspeed.
To ensure a good airflow organization in the clean room, the aspect ratio of the return air outlet should be larger. Similar to the HEPA filter box, for the same return air volume, there are options for large return air volume with fewer return air vents and small return air volume with more return air vents.
Regarding the air purification effect, it is better to have more return air outlets, but when the number of return air outlets increases, the number of return air branch pipes increases, the system becomes complex, and the cost increases. Therefore, various factors (number and location of HEPA filter boxes, clean room width and aspect ratio, cleanliness level, etc.) should be considered to avoid being one-sided.
My last suggestions
Based on my many years of practical experience in air purification engineering, I have summarized the following suggestions:
Tip for setting HEPA filter boxes
Non-unidirectional flow clean rooms with high cleanliness requirements or clean rooms with a large aspect ratio should use a small air volume and a large number of HEPA filter boxes as much as possible; the return air outlet should also use a solution with a small air volume and a large number of return air outlets.
Tip for set return air outlets
A double-sided lower air return is used for clean rooms with a cleanliness level of 1000.
For clean rooms with a cleanliness level lower than Class 1000, when the width of the clean room is not greater than 3m, use single-sided lower return air; when it is greater than 3m, use double-sided lower return air.
When the width of the clean room is large, and the return air on both sides cannot meet the airflow requirements, add a return air outlet (in the form of a return air column, etc.) at 1/2 of the width of the clean room to reduce the vortex area.
The specific design should be flexible according to the class of cleanliness, the location of production equipment, and other conditions. The general principle of arranging return air vents is:
- Adapt to the HEPA filter box and cooperate with the HEPA filter box to fully diffuse the clean airflow.
- Fully dilute the indoor airflow.
- Airflow flows out of the room evenly.
Can you use upper supply and upper return airflow?
The upper supply and upper return airflow organization should be used outside the clean room. They can be used in non-working areas like clean corridors and locker rooms.
However, in non-working areas, try to use the airflow form of the upper delivery and the lower return because the airflow form of the upper delivery and the upper return has the following defects: at a certain height, there are more large particles of 5 μm (Because of its hard to follow airflow), the cleanliness standard can be met when measured by a particle concentration of 0.5 μm, but it cannot meet the standard when measured by a particle concentration of 5 μm.
