Clean room is a specially designed room that will eliminate the dust particles, harmful air, bacteria and other pollutants in the air within a certain space range, and control the indoor temperature and humidity, cleanliness, indoor pressure, airflow speed and airflow distribution, noise, vibration and lighting, and static electricity within a certain demand range.

Clean room air volume calculation

In the design of a clean room project, the air supply volume is very critical, which can be said to be an important index related to whether the clean room meets the production needs. So how to determine the new air volume? First of all, meet the health standards of fresh air volume: to ensure that the supply of fresh air per person per hour in the clean room is not less than 40m3/h, while considering the occurrence of harmful gases.

The fresh air volume that meets the health standard is divided into two situations:

1) For the situation that there is no obvious harmful gas in the workshop, according to the "Clean Plant Design Code", ensure that the fresh air quantity supplied by the purification workshop to the clean room is not less than 40m3/ (h- person).

2) For the occurrence of harmful gases in the workshop, the new air volume of diluting indoor harmful gas should be calculated according to the allowable concentration of harmful gases contained in the workshop.

Clean room air volume calculation mainly refers to the calculation of air volume under the conditions of known clean level or allowable bacteria concentration.

A, positive pressure clean room air supply QⅠ calculation 1. Turbulent clean room air supply calculation Q1-1

2. Maintain indoor positive pressure required fresh air volume Q2

(1) Local exhaust air volume = Q2-1;

(2) The air volume through the residual pressure valve = Q2-2, which can be found in the manual of the residual pressure valve;

(3) Air leakage from the gap = Q2-3

Air volume in the clean room

Where: F1 -- gap area;

E1 -- Flow coefficient is usually 0.3 ~ 0.5

V1 -- Leakage wind speed

ΔP -- pressure difference between indoor and outdoor

ρ -- air gravity density is often 1.2kg/ m3.

The calculation of the above gap method formula is cumbersome, and another method can be used to calculate the number of air changes. The number of air changes is estimated according to the empirical value, that is, when the pressure difference of the clean room is 5Pa, the corresponding number of air changes of the differential pressure air volume is 1 ~ 2h-1, and when the pressure difference of the clean room is 10Pa, the corresponding number of air changes of the differential pressure air volume is 2 ~ 4h-1. Because the size of the differential pressure air volume in the clean room is related to the air tightness and the differential pressure maintained in the maintenance structure of the clean room, the upper limit can be taken for the room with poor air tightness, and the lower limit can be taken for the room with good air tightness.

To sum up, keep indoor positive pressure fresh air volume:

Q2 + Q2 = Q2-1-2 + Q2-3

When the requirements for the positive pressure chamber are particularly strict, the air leakage of the opening and closing door and the transfer window should also be added in Q2.

3. Meet a certain proportion of the new air volume Q3

When the number of personnel or leakage situation cannot be known, or as an estimate in the preliminary scheme, the method that the fresh air should account for a certain proportion of the total air volume can be used to determine the new air volume.

According to the "clean room design code", for turbulence clean room, the fresh air volume should not be less than 10% ~ 30% of the total air volume, for unidirectional clean room, the new air volume should not be less than 2% ~ 4% of the total air volume. The principle is: the lower the cleanliness, the greater the fresh air ratio. Of course, for all the occasions of circulating air, or when the process needs or allows, the fresh air ratio can not be taken according to the above proportion.

4. Supplement the fresh air volume Q4 needed for leakage of the air supply system

Q4= system air volume ×εΣ=Q Ne ×ε

5. The new air volume of the system QⅢ

Compare ΣQ1, AA Q2 and O Q3, take their maximum value, and "H" is the sum of the air volume in each room. Then add the system leakage volume Q4, that is, the final new air volume required by the system.

QⅢ= (int Q1, int Q2, int Q3) Max +Q4

Four, the system of the return air (circulation air) QⅣ calculation of the circulation air volume of the system should be the total air supply system minus the new air volume. Namely: Q Ⅳ Ⅲ Ⅱ - Q = Q

Usually, clean rooms are most affected by the wind speed of the HEPA filter. The high wind speed means the large resistance. If the service life of the filter is based on the final resistance, the service life of the filter is short if the wind speed is high. It is difficult for the average user to actually observe the effect of wind speed on filtration efficiency, but it is much easier to observe the effect of wind speed on drag.

For HEPA filters, the speed of air flow through the filter material is generally 0.01 ~ 0.04m/s. Within this range, the resistance of the filter is positively proportional to the filtration air volume. For example, a 484×484×220mm HEPA filter has an initial resistance of 250Pa at the rated air volume of 1000m3/h. If the actual air volume in use is 500m3/h, its initial resistance can be reduced to 125Pa. For the general ventilation filter in the air conditioning box, the air flow through the filter material speed in the range of 0.13 ~ 1.0m/s, the resistance and air volume is no longer a linear relationship, but an upward arc, air volume increase by 30%, the resistance may increase by 50%, so to calculate the clean room air volume, we should start from many aspects to think.