The first column of minimum static pressure in Table 4.1 refers to the degree of the minimum static pressure difference between clean rooms of each grade and adjacent lower-grade clean rooms during the operation of the air-conditioning and purification system. The positive pressure requirement is both the requirement of the clean operating department on the external non-clean area and the relative positive pressure requirement among the internal clean rooms.

The stipulated values of no less than 5 Pa between different clean rooms in industrial buildings and no less than 10 Pa to the outdoors are both too small, especially when the two rooms differ by more than one grade; the appropriate values are shown in Table 4.2.

Therefore, in the Code, for high-grade operating rooms that may differ from adjacent lower-grade rooms by one grade or by two grades, the pressure difference ...are all taken as 8Pa; for other lower-grade rooms, the difference from the adjacent lower-grade room is mostly only 1 grade, so 5Pa is still taken. Since the clean area is certainly more than 1 grade higher than the non-clean area, it is set at 10Pa, while 15Pa is taken for outdoors. This is where it differs from the current standards of other industries (such as the Code for Design of Clean Rooms, human pharmaceutical GMP, veterinary drug GMP, etc.).

Table 4.2　Recommended Pressure Differences

Purpose Turbulent-flow cleanroom with any communicating adjacent room differing by one grade/Pa Turbulent-flow cleanroom with any communicating adjacent room differing by more than one grade/Pa Unidirectional-flow cleanroom with any communicating adjacent room/Pa Cleanroom and outdoors (or rooms communicating with outdoors)/Pa
General (preventing penetration through gaps) 5 5～10 5～10 15
Strict (preventing contamination from entering through open doors) 5 40, or 5 for buffer room 10, or 5 for buffer room 10 for buffer room
Aseptic cleanroom 5 5 for buffer room 5 for buffer room 10 for buffer room

Regarding the degree of positive pressure, the following points shall be met:

(1) When two cleanrooms of different cleanliness grades communicate with each other, the higher-grade one should maintain a relative positive pressure; the maximum static pressure difference should be less than 30Pa, and the pressure difference should not generate noise.

(2) If two cleanrooms are of the same cleanliness grade and communicate with each other, the airflow direction should be maintained from inside to outside, with a pressure difference slightly greater than 0 kept between the two rooms.

(3) To prevent harmful gases from overflowing, auxiliary rooms such as the pre-anesthesia room should maintain a negative pressure relative to adjacent rooms.

(4) The clean area should maintain a positive pressure of more than 10Pa relative to the non-clean area communicating with it.

(5) The clean area should maintain a positive pressure of not less than 15Pa relative to outdoors or to areas directly communicating with outdoors.

[Reference]

① If the clean surgical department is regarded as an assurance system, positive pressure control should be an even more important means within this assurance system. Any attempt to maintain asepsis in the operating room by relying on spatial sealing is of no avail; only by keeping the pressure inside the aseptic area higher than that of the outside can the intrusion of contamination be prevented. Moreover, aseptic spaces of different grades within the aseptic area require different pressure-difference controls—this is the ordered gradient pressure, which ensures the directional flow of clean airflow. Airflow can only flow from a higher-grade aseptic space to a lower-grade aseptic space, and from the aseptic area to the non-aseptic area. Only by ensuring that under all circumstances (with particular emphasis on non-design conditions or non-normal operating conditions) the ordered gradient pressure distribution within the clean surgical department can be maintained unchanged can the risk of infection in the surgical area be truly and effectively reduced.

② Regarding positive pressure control methods. The positive pressure value is satisfied during the initial commissioning of the system, but during operation, due to the opening of doors, dust accumulation on the three-stage filters, or the influence of outdoor interfering airflow, the positive pressure value may also change. As long as one of the supply, return, or exhaust air volumes in the system changes, the positive pressure of the entire surgical department becomes unbalanced. The most common case is dust accumulation on the system filters, which increases system resistance and reduces the supply air volume, causing the positive pressure value to drift.

(i) Mechanical air valve (manual). The air valve adjusts the supply, return, and exhaust air volumes; this is the most primitive and simplest control method. When the positive pressure value changes, timely adjustment is difficult.

(ii) Pressure relief valve. The pressure relief valve is simple to install and has high control sensitivity; when it is fully closed and the indoor positive pressure value is still below the preset value, it cannot provide control. Pressure relief valves are fairly widely used in industrial cleanrooms, but are rarely used in biological cleanrooms. To prevent cross-contamination, biological cleanrooms do not adopt corridor return air, which can easily cause corridor pressure fluctuations. Biological cleanrooms often have odors, and organized exhaust is preferred over unorganized discharge. In addition, the pressure relief valve damages the flat, smooth interior surface of the room, and Chinese codes do not encourage its use. (iii) Electric control method. The pressure difference between indoor and outdoor is detected by a differential pressure transmitter, and then the supply, return, or exhaust air volume is regulated by motorized air dampers. The control is relatively complex and costly, and system instability often occurs.

(iv) Variable frequency regulation. The supply fan of the operating room clean air-conditioning system is regulated by variable frequency, where sensing elements such as differential pressure sensors or air velocity sensors are used to control the frequency converter and adjust the air volume of the supply fan. This approach features sensitive control, stable operation, and obvious energy-saving effects, and is widely used in newly built operating rooms.

When variable frequency regulation is adopted, the fan has the following characteristic: $dfrac{H_1}{H_2}=left(dfrac{n_1}{n_2}right)^2$. When the rotational speed decreases, the total pressure of the fan will also decrease according to the square of the speed ratio, while the air volume through the HEPA filter remains unchanged as required by the cleanliness requirements, or should vary only slightly; in this case, the resistance of the HEPA filter changes little. At this point, it should be checked against the pressure variation on the fan characteristic curve whether the operating condition requirements can be met.

(v) Mechanical constant air volume device. The mechanical constant air volume device is very simple and effective in design. If installed at the supply, return, and exhaust outlets of the operating room, it keeps the supply, return, and exhaust air volumes constant no matter how the system resistance changes, thereby ensuring the stability of the indoor positive pressure value. At present, only the German mechanical constant air volume devices have relatively good performance.

Viewed comprehensively, all the above measures are aimed at individual rooms; most of them can hardly control the orderly pressure distribution of the entire surgical department as a whole, and their stability is poor and not very reliable. In order to maintain a stable positive pressure indoors, the system schematic introduced in Chapter 7 may be adopted, whose function is very clear and simple. The practice of an independent positive pressure air supply system has been proven very effective in practice, and it also fully embodies the characteristics of the safeguard system.

③ Regarding the pressure difference of auxiliary rooms.

(i) The required pressure difference between the pre-anesthesia room and the adjacent lower-grade cleanroom is −8 Pa, which is the minimum requirement for preventing the leakage of anesthetic gas. The analysis of air flow under negative pressure is just the opposite of that in a positive pressure room. Single-leaf swing doors are often used for entrances and exits, which is different from the situation of air disturbance caused by the sliding doors of positive pressure operating rooms.

(ii) The pressure difference of the scrub room with respect to the adjacent lower cleanliness-grade room is >0. There are two arrangements for the scrub sinks: one is to arrange them on the inner corridor, in which case the problem of an adjacent lower grade does not exist; the other is to arrange the scrub sinks inside the scrub room. In such a scrub room, there is no partition wall between it and the inner corridor. In this arrangement mode, a supply air outlet is required to be installed on the inner side of the scrub room to reduce the moisture inside the scrub room. Since the supply air flows toward the inner corridor, it indicates that the pressure in the scrub room is greater than that of the inner corridor. The pressure difference requirements for other rooms such as clean corridors, changing rooms, and recovery rooms are all 0–5 Pa, which in fact all indicate a direction of air flow; any number other than 0 within this range is acceptable. A special case needs to be explained here: a Class IV operating room is set up in the clean surgical department, and the air cleanliness of its inner corridor is Class III, see Figure 4.3; in this case, the operating room is of a lower grade and should be under positive pressure, while the inner corridor is of a higher grade, but a large amount of leakage into the inner corridor is not allowed. A buffer room one grade higher may be set up outside the Class IV operating room, with the pressure gradient decreasing as shown by →, or a buffer room of the same grade may be set up, with the pressure gradient decreasing as shown by ⇢.

Figure 4.3　An example of the pressure difference of the scrub room