Air Change Rate
4.3.1 Influence of the Air Change Rate
To achieve a certain cleanliness class and degree of asepsis in a cleanroom, a certain supply air volume is necessarily required, together with a certain air change rate and self-purification time. The air change rate is the indicator that clean operating rooms and clean auxiliary rooms—except Class Ⅰ operating rooms and Class Ⅰ clean auxiliary rooms—must satisfy in order to ensure the required cleanliness and not exceed the specified self-purification time. These three technical indicators are all interrelated. However, in actual engineering practice, in order to achieve a higher (compliant) cleanliness level, the supply air volume is often increased, which invisibly increases the initial investment and raises the daily operating costs; therefore, this clause specifies the upper and lower limits of the air change rate. The total supply air volume (i.e., the air change rate) is related to the room volume, the total indoor load, the number of surgical participants in the room, the dust generation from garments, etc. The air change rates in the Code take into account a certain range of variation of these conditions; unless the conditions change too much, they can generally meet the requirements.
The air change rate is the most important indicator determining the cleanliness level in turbulent-flow cleanrooms, including Class Ⅱ, Ⅲ, and Ⅳ clean operating rooms and clean auxiliary rooms of all classes.
Within the practical range, cleanliness improves as the air change rate increases. Figure 4.4 shows the relationship between the air change rate and the indoor dust concentration as a straight line on log-log paper (assuming particles are uniformly distributed in the room) and as a broken line (in reality, particles are non-uniformly distributed in the room).
Figure 4.4 Comparison of the concentrations calculated by the two distribution models with the measured concentrations
In Figure 4.4, straight line A is the static calculation result based on the uniform distribution theory; broken line B is the static measured result. Broken line C is the static calculation result based on the non-uniform distribution theory. Broken line D is the static calculation result based on the non-uniform distribution for air change rates exceeding 100 changes/h.
Figure 4.5 shows the curves of the relationship between the air change rate and the cleanliness class in turbulent-flow cleanrooms under static (non-operating) and dynamic (operating) conditions published abroad. As can be seen from the figure, the larger the air change rate, the higher the cleanliness class.
The air change rate can be obtained through formulas based on the required air cleanliness class, the number of people in the room, the room area, other indoor dust generation, and the fresh air ratio; especially under dynamic conditions, calculations must be carried out for the specific conditions.
Now that the cleanliness class is required to be achieved under static conditions, how should the minimum air change rate be determined? As can be seen from line C in Figure 4-4, at an air change rate of 20 changes/h, the indoor dust concentration is 40 particles/L, i.e., Class 10 000 is reached; even with an air change rate below 10 changes/h, it can also Figure 4.5 Relationship between air change rate and class in turbulent-flow cleanrooms
to reach Class 100 000.
Figures 4.4 and 4.5 are basically consistent. From the static curve, it can be seen that at 20 air changes, the cleanliness is only slightly worse than Class 1 000.
It can thus be seen that under static conditions, the air change rates required to achieve the various classes can differ greatly, and the absolute values can be quite low. Moreover, the air change rates actually adopted are often raised considerably above the values specified in the code. Therefore, as a standard, a reasonable, slightly safety-oriented value should be given for the most important parameter ensuring cleanliness—the air change rate—such that following it will basically meet the requirements.
[Reference] Since the technical approach for compiling the Code has changed considerably—first, it adopted internationally advanced technical modes, and second, it cited a number of domestic research achievements, all of which have been proven successful through years of application in engineering—the air supply volume or air change rate differs from that of the past. However, the final results are very close to those of developed countries such as the UK, the US, and Germany, while differing somewhat from other relevant Chinese codes, as shown in Table 4.3.
Table 4.3 Comparison of cleanroom air change rates in different standards
| Class | Operating room name | Cleanliness class of operating area | Construction Standard and Architectural Technical Code for Hospital Clean Operating Departments | Architectural Technical Code for Clean Operating Departments of Military Hospitals | Code for Design of Cleanrooms |
|---|---|---|---|---|---|
| Ⅰ | Special clean operating room | 100 | 0.25～0.3m/s | 0.3～0.35m/s | 0.2～0.5m/s |
| Ⅱ | Standard clean operating room | 1 000 | 30～36h⁻¹ | ≥55h⁻¹ | 50～60h⁻¹ |
| Ⅲ | General clean operating room | 10 000 | 18～22h⁻¹ | ≥30h⁻¹ | 20～25h⁻¹ |
| Ⅲ | General clean operating room | 100 000 | ≥20h⁻¹ | 15h⁻¹ | |
| Ⅳ | Quasi-clean operating room | 300 000 | 12～15h⁻¹ |
Note: Class 100 cleanrooms are expressed in terms of cross-sectional air velocity, while cleanrooms of other classes are expressed in terms of air change rate.
4.3.2 Values of the Air Change Rate
For air change rates, all past standards only specified lower limit values, such as 25 changes/h for Class 10 000 and 15 changes/h for Class 100 000, etc. In actual engineering, in pursuit of compliance, these were increased several times over, causing enormous waste. In fact, even air change rates such as 25 and 15 were intended for dynamic conditions, whereas testing is now all done under empty or static conditions, so the result is that cleanliness is improved by one to two classes; in other words, values such as 25 and 15 are already sufficient. Taking into account the self-purging air change rate (detailed later), the Code slightly increases the values for operating rooms, while slightly reducing them for clean auxiliary rooms in consideration of their static conditions, and sets the upper limit at about 1.2 times.
For the same cleanliness requirement, there can be different air change rates. According to theoretical calculations with reference to actual measurements, under static conditions Class 100 000 can be as low as less than 10 changes, and Class 10 000 can be less than 15 changes. Although this standard is based on static or as-built (empty) conditions, the air change rate cannot be considered only in terms of static cleanliness. This is because the air change rate should serve two functions: one is to ensure cleanliness, and the other is to ensure the self-purging time, the latter of which is often overlooked. The self-purging time is of great significance for determining how much earlier the system must start up in the morning when there is no standby fan; however, if the self-purging time is long, the startup time must be advanced considerably, which is a waste. For operating rooms, there is another function: determining when the second operation can begin after the first one is finished. If a long self-purging time is required before starting, it will obviously both delay operations and reduce the turnover efficiency of the operating room, so the shorter the self-purging time, the better; however, if it is too short, the air change rate would inevitably have to be greatly increased, which is also unrealistic. After investigation, the code determined that for Class Ⅰ operating rooms with local Class 100 areas, since they are mainly used for operations that have a decisive impact on life, a self-purging time of no more than 15 min is preferable; for Class 10 000, no more than 25 min; for Class 100 000, no more than 30 min; and for Class 300 000 quasi-clean operating rooms, no more than 40 min is acceptable. From the perspective of morning startup, starting 40 min earlier is not too much. If it exceeds 1 h, then Too long.
Based on the above principles, the air change rates can be calculated as listed in the table of the provisions; this is the "self-purification air change rate" that takes the self-purification time into account, and this is also how it is adopted in China's military standard for clean operating departments. In practice, however, safety margins are often added again and again, with air change rates increased by one to several times in some cases, which is truly a waste; therefore, the "Code" specifies a range, keeping the gap between the upper and lower limits within 1.2 times.
【Reference】 The "Code" sets the design occupancy of operating rooms as: extra-large 12 persons, large 10 persons, medium 8 persons, and small 6 persons. Meanwhile, in order to curb unnecessary pursuit of larger sizes, upper and lower limits of floor area are given, and where enlargement is required for special needs, an increase of 1.25 times beyond the upper limit is allowed; this brings it close to the Japanese standard and above the military standard.