The technical indicator of self-purification time is of great significance to operating rooms. The self-purification time represents the time required for the dust concentration of the indoor air in an operating room to drop from a high initial value to a stable value after the air-conditioning and purification system is started up. For users, the shorter this time, the better. The shorter this time, the stronger the self-purification capability of the operating room, and the faster indoor dust particles can be removed. As can be seen from Table 4.1, the higher the cleanliness class of the operating room, the shorter the self-purification time and the stronger the self-purification capability; the lower the cleanliness class, the longer the self-purification time and the weaker its self-purification capability.

The self-purification time of an operating room is the maximum time that ensures the room can achieve self-purification between two operations under the specified air change rate; where there is a special need to substantially shorten the self-purification time, the air change rate should be re-determined based on calculation.

Self-purification time and air change rate are closely related to each other, yet designers often pay attention to the air change rate while neglecting the self-purification time. The air change rates in Table 4.1 are precisely the "self-purification air change rates" obtained by taking the self-purification time into account. Under normal circumstances, the table preparation work before surgery in the morning takes about 20～30min; when performing consecutive operations, including cleaning work and table preparation, it can also be completed in about 30～40min.

The self-purification time is solved using Figure 4.6. In the figure: $N_0$ is the original dust concentration of the cleanroom; $N$ is the design concentration value for the class to which the cleanroom belongs; $n$ is the existing air change rate of the cleanroom (or conversely, the value to be determined); $T$ is the required self-purification time (or conversely, the actual or design value). Figure 4.6　Self-purification time and pollution time

First, $frac{N_0}{N}$ must be determined. Assume that before startup $N_0$ is already close to the atmospheric dust concentration $M$. Since cleanrooms are generally located in areas with relatively light pollution, $1times10^5$ particles/L can be taken.

$N$ is generally taken as 1/3 to 1/2 of the upper-limit concentration of the corresponding class; for example, for Class 10 000, 100–200 particles/L is taken. Then $N_0/N$ can respectively be taken as:

  • Class 10 000　　500–1 000
  • Class 100 000　　50–100
  • Class 300 000　　17–34

Then, looking up Figure 4.6 gives the values of $n_t$:

  • Class 10 000　　650–680
  • Class 100 000　　520–560
  • Class 300 000　　450–480

Assume the self-purification time for each class:

  • Class 10 000　　25 min
  • Class 100 000　　30 min
  • Class 300 000　　40 min

Then the self-purification air change rate $n$ is obtained:

  • Class 10 000　　30 changes/h
  • Class 100 000　　18 changes/h
  • Class 300 000　　12 changes/h

These are the lower-limit values of the air change rate given in the Code.