4.4.1 Cross-sectional Air Velocity

According to the principle of the lower-limit air velocity theory, the cross-sectional air velocity in the work area of vertical unidirectional flow should be 0.3m/s. However, for the local vertical unidirectional flow in Class I clean rooms where the air supply outlets are centrally arranged as specified by the code—commonly known as local Class 100—because the airflow diffuses to the area below the Class 100 zone, and this diffusion is affected by factors such as whether the air supply face has baffles and how far its four edges are from the walls, extensive field measurements show that 0.3m/s is a rather stringent value. In the earlier "Technical Measures for Air Cleanliness" and "Design Code for Clean Rooms", this value was 0.25m/s, but as for the measurement point height, the former specified two locations, 0.8m and 1.5m, with the results to be averaged; the latter's measurement point height was 1m. The "Code", like the "Code for Construction and Acceptance of Clean Rooms", sets the measurement point height at 0.8m. In view of the above-mentioned reason of the local centralized arrangement of air supply outlets, this value is specially relaxed to a range, i.e., 0.25–0.3: it is not desirable to be below 0.25, so as not to impair the anti-interference capability, while an excessively high value tends to cause the incision to lose moisture too quickly, which is also undesirable; during measurement, the average value will be taken.

In ophthalmic surgery, a high air velocity causes rapid evaporation from the conjunctiva and consequent loss of moisture, so for ophthalmic surgery the value is reduced by about 1/3 based on experience.

【Reference】 In China's "Code", Class I special clean operating rooms are specified to adopt the local unidirectional flow mode, requiring the average cross-sectional air velocity at the working-surface height of the operating area to meet 0.25–0.3m/s (see Table 4.1); this is regarded as the lower-limit air velocity that must be maintained during operation and is essential for ensuring the quality of the local unidirectional flow. Of course, there is also no need to use a large air volume to maintain the momentum required for typical full unidirectional flow. To achieve 0.25–0.3m/s at the working surface, the air velocity at the air supply face should be 0.40–0.45m/s, comparable to the American standard (see Table 4.4); the corresponding air supply volume is about 10 000m³/h, similar to the British standard.

Table 4.4 Comparison of operating room air supply volumes between China and other countries

Operating room class in China China's clean operating department standard German standard DIN1946/4 US ASHRAE Handbook—Applications British standard HTM2025 Japanese standard HEAS-02
Ⅰ 0.25～0.30m/s 0.15m/s 0.45m/s 0.2m/s 0.35m/s
Ⅱ 30～36$h^{-1}$
Ⅲ 20～24$h^{-1}$ 20～24$h^{-1}$ 25$h^{-1}$ (15$h^{-1}$ full fresh air) 22～33$h^{-1}$ 20$h^{-1}$

Note: Class I clean rooms are expressed in terms of cross-sectional air velocity, while clean rooms of other classes are expressed in terms of air change rate.

Considering that the protected area of an operating room is relatively small, China's "Code" explicitly requires clean operating rooms to adopt the local air supply mode and specifies the corresponding air supply volume range. If we take 6m×6m×3m as a standard-sized operating room for analysis (see Table 4.5), it can be seen that, because surgical incisions differ, healing requirements differ, protection levels vary, and the required characteristics of the supply airflow differ, the outlet velocity at the air supply face also differs. Class I special clean operating rooms require the unidirectional flow air pattern, which works on the extrusion-displacement principle. Class II and III clean operating rooms, because of their lower outlet velocity, do not have sufficient momentum to maintain unidirectional flow and adopt a low-turbulence displacement airflow, which works on the filling principle; however, the outlet velocity is not desired to be below 0.15m/s, so in design, for small operating rooms, one should take Upper limit air change rate: the Class Ⅳ quasi-clean operating room follows general turbulent flow parameters, which is based on the dilution principle; since the air supply outlet area is too small, the outlet air velocity is expected to be no less than 0.25m/s, so as to maintain sufficient momentum to deliver the air downward.

Table 4.5 Operating Room Air Supply Volume and Supply Air Area

Operating room class Class Ⅰ operating room Class Ⅱ operating room Class Ⅲ operating room Class Ⅳ operating room
Air supply volume 0.25～0.30m/s 30～36h$^{-1}$ 20～24h$^{-1}$ 12～15h$^{-1}$
Supply air area/m×m 2.4×2.6＝6.24 1.8×2.6＝4.68 1.4×2.6＝3.64 Not specified
Outlet air velocity/(m/s) 0.40～0.45 0.23 0.2 ≥0.25
Air supply volume/(m³/h) 9500～10 100 3875 2620

Note: Class Ⅰ cleanrooms are expressed in terms of cross-sectional air velocity, while cleanrooms of other classes are expressed in terms of air change rates.

4.4.2 Velocity Uniformity

In the Standard, the Class Ⅰ operating room is expressed in terms of air velocity, because achieving the Class 100 standard for an operating room can only be accomplished by adopting a unidirectional flow air supply method. To meet the requirements of unidirectional flow, three basic conditions must be satisfied: ① a certain air velocity should be achieved at the working surface; ② the airflow streamlines should be parallel; ③ the supply airflow should be sufficiently uniform, with its non-uniformity not greater than 0.25. The calculation formula is

$$beta_v = frac{sqrt{dfrac{sum(v_i - bar{v})}{n-1}}}{bar{v}} leqslant 0.25$$

where: $beta_v$ — air velocity non-uniformity;

$v_i$ — measured air velocity at a point;

$bar{v}$ — average air velocity;

$n$ — number of measurement points.