Purification Air-Conditioning System of the Clean Operating Department
7.2.1 Special Requirements
The clean air-conditioning system is an important link in establishing the overall guarantee system of the operating department. The system is required to control the indoor parameters of rooms of corresponding grades and to maintain the airflow direction between rooms of different grades; of course, the most important of all is to effectively control the bacterial concentration in the controlled areas, especially in the clean operating rooms. However, ordinary air-conditioning systems, especially the heat and moisture exchange equipment within them, are often favorable places for bacterial growth and dust accumulation, and thus cannot meet the requirements of the guarantee system of the clean operating department. Compared with the air-conditioning requirements of general buildings, the system should have the following characteristics:
(1) The system should be clean, dry, and easy to clean, ensuring the cleanliness and sterility of the supply air.
(2) Effective comprehensive measures for bacteria removal, bacteria prevention, and antibacterial protection should be adopted to prevent secondary contamination within the system.
(3) Ensure reasonable airflow direction and pressure distribution between different areas.
(4) Meet the temperature and humidity required for medical purposes in different areas; humidity control in operating rooms should be given priority.
(5) Discharge waste gas and harmful gases to prevent pollution of the external environment. Heat recovery may be carried out where conditions permit.
【Reference】 The clean operating room requires the air-conditioning system to control the indoor temperature, humidity, dust, bacteria, harmful gas concentration, and airflow distribution, to ensure the fresh air volume required by indoor personnel, and to maintain a reasonable airflow direction between indoors and outdoors. Among these, the most important is to control the indoor bacterial concentration, so as to prevent infection of surgical wounds during operations and improve the success rate of surgery. According to the above design concept, to create an ideal aseptic environment for surgery, the following conditions must be met:
① Air filtration removes all microbial particles from the supply air.
② Airflow technology renders the room sterile and dust-free. ③ Positive pressure control achieves an orderly pressure gradient distribution throughout the entire operating department.
④ Appropriate temperature and humidity adjustment suppresses bacterial reproduction and reduces the amount of bacteria dispersed by the human body.
⑤ Undesirable indoor gases are removed to maintain good indoor air quality.
7.2.2 Selection of Air Conditioning Systems
For large general hospitals, operations may be scheduled throughout the entire day, so the entire operating department should preferably adopt a centralized air conditioning system. Considering that the cleanliness classes of the various rooms differ considerably, it is desirable to at least set up two separate systems—one for the clean operating rooms and one for the auxiliary rooms. This approach can achieve relatively satisfactory results in providing temperature, humidity, air purification, and odor removal; since the machine room and the rooms are separated, noise can also be handled relatively well, and problems of losing control over the aseptic environment generally do not occur. However, it is inferior in terms of the flexible use of each operating room: as long as one of them is still in operation, the entire system cannot be shut down, resulting in relatively high energy consumption.
A decentralized air conditioning system refers to one in which each operating room and the corridor are each equipped with an independently operable air conditioning system. Each operating room system can be started and stopped flexibly. To ensure the aseptic environment within the operating department, each air conditioning unit needs to be set with two levels—operating air volume and positive-pressure air volume—to meet the minimum positive pressure control requirement. When an operating room is closed, its air conditioning unit switches to the positive-pressure air volume to maintain operation. Even so, decentralized clean air conditioning systems are inferior in ensuring the overall controllability of the clean operating department.
A semi-centralized system involves installing a separate air conditioning unit (also called a terminal unit) in each control zone to achieve the temperature, humidity, and cleanliness within each zone, plus a centralized independent system for supplying outdoor air. For high-class clean operating rooms, this is a relatively reasonable choice, but for low-class clean operating rooms or auxiliary rooms, the terminal units are too small, the fan pressure head is difficult to increase, and high-efficiency filters cannot be fitted. It should be permitted for one air conditioning unit to serve several low-class clean operating rooms, or for the auxiliary rooms to adopt a separate air conditioning unit.
[Reference] The reliability of return air in clean operating rooms was affirmed as early as the 1970s and has been adopted by the standards of various countries. For high-class clean operating rooms, return air from the room itself is required; corridor return air should not be used for the entire operating department, so as to avoid cross-infection. Under normal circumstances, there is no need to adopt an all-outdoor-air clean air conditioning system. If an all-outdoor-air system is truly needed, a total heat recovery device should be equipped in the system. However, the actual application of total heat recovery devices in China is not very satisfactory.
7.2.3 The Clean Air Conditioning System Should Keep the Clean Operating Department in a Controlled State
This requires that the clean air conditioning system not only ensure overall control of the clean operating department but also allow flexible use of each clean operating room. Emphasizing that the clean operating department is in a controlled state means requiring the entire operating department to always maintain an orderly gradient pressure distribution, so as to maintain the directional flow of positive-pressure airflow between the operating rooms and effectively prevent cross-infection or contamination of the indoor environment. Therefore, it is emphasized that no matter what kind of clean air conditioning system the operating department adopts, whatever operating state it is in, and no matter which clean operating room is shut down, the orderly gradient pressure distribution of the entire operating department must not be affected; the entire operating department should always remain in a controlled state. Otherwise, the directional flow of positive-pressure airflow between the rooms will be disrupted, causing cross-infection or contamination of the indoor environment.
Clean operating rooms should have clean air conditioning systems set up separately from those of the auxiliary rooms; each clean operating room should preferably adopt an independently installed clean air conditioning unit; Class Ⅲ and Class Ⅳ clean operating rooms are allowed to share one system among 2～3 rooms; outdoor air may be supplied by a centralized air supply system. Operating rooms should be equipped with an exhaust system.
A system with independent outdoor air treatment commonly used in China is shown in Figure 7.1.
[Reference] The greatest technical feature of this system is the adoption of two-position controlled constant air volume devices, which simplifies control and makes it easier to separate the various control functions—the supply air and positive pressure of the clean operating room, as well as the indoor air conditioning and the outdoor-air cooling and dehumidification. In this way, the requirements set forth in the Architectural Technical Code for Hospital Clean Operating Department can be implemented and met very simply and effectively. The following features can be seen from the figure:
① The entire clean operating department is composed of an independent outdoor air system and the clean air conditioning units and exhaust units of each clean operating room. The outdoor air branch ducts of the outdoor air system are connected to (inserted into) the supply air ducts of the clean air conditioning units of the clean operating rooms, or connected directly to the supply air plenum. A two-position controlled constant air volume device is installed on each outdoor air branch duct. A check valve is installed on the supply air duct of the clean air conditioning unit, Figure 7.1 Dedicated Outdoor Air Treatment System (China)
to prevent backflow when only outdoor air is supplied. Each operating room is equipped with an exhaust fan unit with independent exhaust, and the exhaust air volume of the exhaust fan can be controlled by regulating dampers.
② The dedicated outdoor air system undertakes both the positive-pressure air volume of each clean operating room (see Table 7.11) and the normal outdoor air volume, so each operating room's independent air conditioning unit becomes a recirculation unit that controls only the indoor temperature and humidity. The exhaust fan removes the indoor polluted gases and odors.
③ When an operating room is in normal use, the dual-position controlled constant air volume device on the outdoor air branch is switched to the high setting, and the high-setting outdoor air volume enters; the air volume exhausted by the exhaust fan unit equals the difference between the outdoor air volume and the positive-pressure air volume, and the excess positive-pressure air infiltrates outdoors, establishing the positive pressure. When an operating room is not in use, the dual-position controlled constant air volume device on the outdoor air branch automatically switches to the low setting, the low-setting outdoor air (positive pressure) volume enters, and the exhaust fan unit is shut off; the positive-pressure air volume infiltrating outdoors can still establish the positive pressure. Since the entire surgical department is supplied by only one outdoor air system, the orderly gradient pressure distribution within the clean surgical department can be effectively kept unchanged, and the safeguard system is truly implemented.
Table 7.11 Air change rates required to achieve the positive pressure values
| Indoor positive pressure value /Pa | Cleanroom without exterior windows, single door /h⁻¹ | Cleanroom without exterior windows, double doors /h⁻¹ | Requirement of Code for Design of Clean Room /h⁻¹ |
|---|---|---|---|
| 5 | 2.6 | 4 | 2～4 |
| 10 | 3.3 | 5.2 | 3～6 |
④ According to the requirements of China's "Code", the outdoor air system shall be provided with three-stage filtration, and the final stage of filtration should be a sub-high-efficiency air filter. The "Code" stipulates this provision mainly in view of the relatively high dust concentration of outdoor air in China. In terms of positive pressure control, this measure can ensure that the terminal high-efficiency filters of the operating rooms basically do not accumulate dust, so that changes in the supply air volume of the operating rooms will not be caused by this.
[Reference] The main automatic control requirements of this system are described as follows:
① When an operating room is in operation, the turn-on signal in the operating room requires the air conditioning unit and the exhaust fan unit to start, and at the same time requires the dual-position constant air volume regulator to operate in the high-setting, large air volume state.
② When an operating room is not in operation, the turn-off signal in the operating room requires the air conditioning unit and the exhaust fan unit to shut down, and at the same time requires the dual-position constant air volume regulator to operate in the low-setting, small air volume state.
③ For the control of the fan of the dedicated outdoor air unit, when there are few operating rooms, a fan with a two-speed motor can be used to regulate the air volume. When there are many operating rooms and many states of outdoor air volume variation, methods such as the on/off status (digital signals) of the operating room air conditioning units can be adopted to control a variable-frequency fan and regulate the air volume, so as to achieve the purpose of energy saving.
④ The outdoor air unit is interlocked with the air conditioning units: only after the outdoor air unit is started can the air conditioning units of each operating room be started. As long as one operating room is in operation, the outdoor air system continues to run. Only when the entire surgical department is shut down does the outdoor air unit stop, and the outdoor air airtight damper closes automatically.
⑤ The indoor exhaust fan is interlocked with the automatic door of the operating room and is provided with a time-delay device. When the door is opened momentarily, the exhaust fan stops immediately; when the door is closed There is a time delay: the exhaust fan is started only after the positive pressure has been established. This not only ensures that the positive pressure is maintained when the door is opened, but also avoids frequent stopping and starting of the exhaust fan caused by the door being opened and closed.
In order to keep the entire clean operating department under control in a simple and effective manner, the following points are its main design starting points:
① Since mechanical constant air volume devices currently still have to rely on imports and are relatively expensive, it is desirable to use them only at critical locations and to adopt small-flow constant air volume devices as much as possible, thereby reducing the project construction cost.
② It is desirable to simplify the two fresh air systems into one; this system combines the functions of both the fresh air supply system and the positive pressure system, greatly simplifying the original system.
③ The fewer fresh air inlets the system has, the more effectively the interference of the high-temperature, high-humidity fresh air on the operating rooms can be controlled, so it is desirable to reduce the system fresh air inlets to one. As long as an airtight damper is installed at one fresh air inlet in accordance with the code requirements, outdoor air can be prevented from backing into the entire operating department when it is shut down.
④ The supply air units of each operating room are simplified into recirculation units, so that dust accumulation on the unit filters, system resistance, and air volume changes no longer have anything to do with the orderly pressure gradient of the entire operating department, and there is no need to adjust the supply and return air volumes of each unit.
⑤ It is desirable that the commissioning workload be small and the operation management be simpler.
【Reference】 German sources recommend a conceptual system with dual fresh air (Figure 7.2). An air conditioning system is installed in each control zone to achieve the temperature, humidity, and cleanliness within each zone; in addition, a centralized fresh air system is installed to maintain the positive pressure distribution within the entire operating department, without undertaking the task of regulating indoor temperature and humidity. Furthermore, the air conditioning systems installed in the individual operating rooms and auxiliary areas have their own fresh air and exhaust. The feature of this new concept is that it can both separate the two major functions—air conditioning and positive pressure—for each clean operating room, and link the entire operating department together, keeping the operating department under control at all times. In this way, the management of the operating department becomes more flexible, convenient, and effective. The operation schemes are as follows:
Figure 7.2 Dual fresh air system (Germany)
① During the normal working period of the operating department, the two systems run simultaneously, which simplifies commissioning and operation management work.
② When only some of the operating rooms in the operating department are in use, only the independent air conditioning units of those operating rooms and the positive pressure supply air system need to be run, which ensures both the normal operation of those operating rooms and the normal pressure distribution and directional flow of the entire operating department.
③ During the non-working period of the operating department, only the positive pressure supply air system is run; in this way, the indoor temperature and humidity requirements can be greatly reduced, and it is only necessary to maintain the positive pressure of the entire operating department so as to keep it clean and sterile.
This is merely a system concept; the concept is a combination of two systems, and if it is to be actually implemented in a specific project, its ductwork system is relatively complex.
【Reference】 Figure 7.3 shows a local supply air system for operating rooms with independent fresh air and recirculated air recommended by French sources. The features of this system are:
① The fresh air is treated independently and connected directly into the supply air duct to enter the supply air plenum. When the system is shut down, supplying fresh air alone can maintain the positive pressure.
② The air conditioning unit of the operating room bears the indoor load, so the air volume is not large, and a filter is installed at the return air inlet. ③ In addition, circulation fans are used to increase the air change rate to meet the control requirements for indoor bacterial concentration.
The keys to implementing this system: whether the outdoor air treatment can meet the indoor humidity control requirements under China's meteorological conditions, and the noise control issue when the three fans operate simultaneously.
Figure 7.3 Dedicated outdoor air, dedicated recirculated air system (France)
- Multi-layer low-speed return air grille with a 90% efficiency (ASHRAE 52-76) filter 2. Tertiary circulation fan (when an increased air change rate is needed, all three fans in the system operate simultaneously) 3. Electric valve (the valve closes when the circulation fan stops) 4. HEPA filter 5. Air handling unit 6. Supply fan 7. Heating coil 8. Heat recovery coil 9. 90% efficiency outdoor air filter (ASHRAE 52-76) 10. Humidifier 11. Electric valve
[Reference] A return air and outdoor air heat exchange system used in Japan is shown in Figure 7-4. Its features are: ① Variable outdoor air operation can be performed according to the outdoor meteorological state, with the return air volume and exhaust air volume determined accordingly. ② A total heat exchanger is installed in the system to fully recover the energy in the exhaust air. Whether the exhaust air undergoes total heat exchange with the outdoor air depends on the state of the outdoor air. ③ Local supply air and upper and lower exhaust outlets are provided. In implementing this system, the relationship between variable outdoor air operation and indoor humidity control as well as gradient positive pressure control must be properly handled.
Figure 7.4 Return air and outdoor air heat exchange system (Japan)
- Medium-efficiency filter with colorimetric efficiency above 60% 2. Cooler 3. Heater 4. High-medium-efficiency filter with colorimetric efficiency above 85% 5. HEPA filter with particle counting efficiency above 99.99% 6. Total heat exchanger
7.2.4 Class Ⅲ (inclusive) and above clean operating rooms shall adopt the local concentrated air supply method
There are two air supply methods in a cleanroom: uniform air supply and concentrated air supply. Industrial cleanrooms, in order to meet process requirements, usually need uniform Instead of dispersing the air supply outlets, since the key areas to be protected in an operating room are relatively few, it is recommended to arrange the air supply outlets in a concentrated manner above the operating table, and to specify the air supply requirements in detail based on theoretical calculations and with reference to domestic and international practice.
The concentrated arrangement of air supply outlets above the operating table is based on the theory of the main stream zone. It utilizes the clean, sterile airflow of the main stream zone to protect the critical area, i.e., the operating zone. Adopting the local air supply method will make it possible to obtain a cleanliness class one grade higher and a bacterial concentration at least half lower with a smaller air volume. In other words, a better cleanliness result can be achieved with less investment.
Other purification equipment should not be installed directly in Class I, Class II, and Class III clean operating rooms with concentrated air supply, because the airflow distribution of the concentrated air supply performs well, and the airflow from local purification equipment in the room would interfere with the unidirectional or quasi-unidirectional flow characteristics of the concentrated air supply.
7.2.5 Air Filtration of the Purification Air-Conditioning System
The entire system shall have at least three stages of air filtration. The first stage of air filtration shall be located at the fresh air intake, the second stage in the positive-pressure section of the system, and the third stage at the terminal of the system or as close to the terminal as possible.
Since air filtration is the most effective, safe, economical, and convenient means of removing bacteria, the use of appropriate filters can ensure that the required dust concentration and bacterial concentration are achieved at a reasonable operating cost. A good air filter should feature a large air volume, low resistance, high efficiency, and a large dust-holding capacity.
[Reference] In China, air filters are classified according to their efficiency. The national standard Methods for Testing the Performance of High-Efficiency Air Filters (GB6166-85), promulgated in 1985, adopted both the oil mist method and the sodium flame method as statutory performance test methods. The national standard Test Methods for Performance of Air Filters for General Ventilation (GB12218-89) (Table 7.12), promulgated in 1990, uses atmospheric dust as the test dust and adopts the atmospheric dust fractional counting method. Table 7.13 presents a comparison of several foreign air filter standards.
Table 7.12 Classification of air filters for general ventilation and atmospheric dust fractional efficiency (%)
| Air filter classification | ≥5μm | ≥1μm | ≥0.5μm | Initial resistance/Pa |
|---|---|---|---|---|
| Coarse filter | <40 | 50 | ||
| Coarse filter | 40~80 | 50 | ||
| Medium-efficiency filter | 20~70 | 80 | ||
| Medium-high-efficiency filter | 70~99 | 100 | ||
| Sub-high-efficiency filter | 95~99.9 | 120 |
Table 7.13 Comparison of several air filter standards of major countries at home and abroad
| Chinese standard | European manufacturers' standard EUROVENT4/9 | ASHRAE standard, gravimetric method, efficiency/% | ASHRAE standard, colorimetric method, efficiency/% | US DOP method (0.3μm) efficiency/% | European standard EN779 | German standard DIN24185 |
|---|---|---|---|---|---|---|
| Coarse filter | EU1 | <65 | G1 | A | ||
| Coarse filter | EU2 | 65~80 | G2 | B1 | ||
| Coarse filter | EU3 | 80~90 | G3 | B2 | ||
| Medium-efficiency filter | EU4 | ≥90 | G4 | B2 | ||
| Medium-efficiency filter | EU5 | 40~60 | F5 | C1 | ||
| Medium-high-efficiency filter | EU6 | 60~80 | 20~25 | F6 | C1/C2 | |
| Medium-high-efficiency filter | EU7 | 80~90 | 55~60 | F7 | C2 | |
| Medium-high-efficiency filter | EU8 | 90~95 | 65~70 | F8 | C3 | |
| Medium-high-efficiency filter | EU9 | ≥95 | 75~80 | F9 | — | |
| Sub-high-efficiency filter | EU10 | >85 | H10 | Q | ||
| Sub-high-efficiency filter | EU11 | >98 | H11 | R | ||
| High-efficiency filter A | EU12 | >99.9 | H12 | R/S | ||
| High-efficiency filter A | EU13 | >99.97 | H13 | S | ||
| High-efficiency filter B | EU14 | >99.997 | H14 | S/T | ||
| High-efficiency filter C | EU15 | >99.9997 | U15 | T | ||
| High-efficiency filter D | EU16 | >99.99997 | U16 | U | ||
| High-efficiency filter D | EU17 | >99.999997 | U17 | V |
[Reference] To prevent bacterial growth, a new generation of antibacterial filters has emerged in recent years with the development of filtration technology. However, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has repeatedly cautioned that antibacterial products should be adopted with great caution in HVAC systems, so as to avoid chemical contamination of the system, which would directly endanger the indoor environment. Therefore, the safety performance requirements for antibacterial filters are even higher than those for bactericidal efficiency. It is required that the antimicrobial additives of the filter not be sprayed onto the surface of the filter media, but that the media be treated during its production process, so that the entire filter layer possesses antimicrobial efficacy. It is also required that the filter have a long-term antibacterial effect against the bacteria that may come into contact with its surface (this lasting effect should be longer than the service life of the filter), and that it exert a lasting bactericidal action against a broad spectrum of microorganisms (it must not be a bacteriostatic agent; it must be able to directly kill both bacteria and their spores; otherwise, the bacteria cannot be completely killed and will instead develop drug resistance). Furthermore, it is required that the filter not release any gaseous chemical contaminants (including odors).
[Reference] Due to the long-term use of chemical drugs and antibiotics, nosocomial infections nowadays often involve M.R.S.A. infections. M.R.S.A. is a strain of Staphylococcus aureus that is resistant to multiple antibiotics; it is a very troublesome bacterium that is difficult to treat with drugs. The best approach is to eliminate it, rather than to treat it after it has caused an infection. Some antibacterial filters have a special killing effect on it, which is a very good technical measure.
Antibacterial filters can kill the microorganisms listed in Table 7.14 upon contact (including spores); this is a lasting antibacterial measure and plays a special role in the control of aseptic environments in hospitals.
Table 7.14 Types of Microorganisms Killed by Antibacterial Filters
| Bacteria | Bacteria | Yeasts | Fungi | Algae |
|---|---|---|---|---|
| Salmonella | Listeria species | Conjophgoraputeana | Streptomyces species | Green algae |
| Escherichia coli | Bacillus species | Candida albicans | Aspergillus species | Oscillatoria species |
| Pseudomonas species | Proteus vulgaris | Candida species | Cephalosporium species | Navicula species |
| Proteus species | Clostridium species | Rhodotorula species | Fusarium species | Scenedesmus species |
| Klebsiella species | Streptococcus mutans | Cryptococcus species | Paecilomyces species | Melsira algae |
| Flavobacterium species | M.R.S.A. cocci | Saccharomyces | Cladosporium species | |
| Alcaligenes species | "0157" Escherichia coli | Penicillium species | ||
| Streptococcus species | ||||
| Staphylococcus species | Viruses | |||
| Aeromonas species | MS2 Escherichia coli phage | |||
| Achromobacter species | Artificial test virus |
7.2.6 Air Supply Methods for Quasi-Clean Operating Rooms and Class Ⅲ and Ⅳ Clean Auxiliary Rooms
Purified fan coil units or indoor cabinet-type purified air conditioners equipped with sub-high-efficiency filters or high-efficiency filters may be used; ordinary fan coil units or air conditioners must not be used in any clean rooms.
Fan coil units have a relatively poor moisture removal capacity, resulting in excessively high indoor humidity, and their built-in air filters are of low performance and cannot meet the requirements for dust removal and bacteria removal. Coupled with insufficient fresh air, the stale indoor air cannot be exhausted, making the medical staff feel stuffy; in addition, the filters and fan coil units have to be inspected, cleaned, and serviced indoors, which not only affects the normal use of the operating room but may also leave hidden dangers of infection in the operating room.
[Reference] A large body of domestic and foreign literature has reported the problems of mildew growth and bacterial proliferation on the coils and condensate drain pans of ordinary air conditioners and fan coil units during summer operation, causing a tremendous increase in indoor bacterial concentration and odor; the risk of indoor air pollution is even greater in summer. Older versions of foreign standards explicitly prohibit the use of such equipment in operating rooms. The hospital design and management guidelines published in Japan in 1998 stipulate that low-class clean operating rooms are allowed to use air recirculation units equipped with air filters no lower than sub-high-efficiency; therefore, purified air conditioners and purified fan coil units are permitted in quasi-clean operating rooms, but their use is not advocated.
[Reference] Considering that the condensate water of ceiling-hung ordinary air conditioners and fan coil units may overflow and cause bacterial contamination, it is advisable to recommend cabinet-type purified air conditioners and column-type purified fan coil units.
7.2.7 Requirements of the Purification System for Outdoor Air Handling Units
When a centralized outdoor air handling system is separately provided for the entire clean operating department, the outdoor air handling unit shall, during the cooling season, treat the outdoor air at least to the enthalpy value of the indoor air state point [see Figure 7.5(a)].
[Reference] When the system is provided with an independent outdoor air handling unit, emphasis is placed on its final treatment state point, with the intention of minimizing, as far as possible, the influence of high-temperature and high-humidity outdoor air on system control; this applies even more to the air conditioning units of clean operating rooms. At present, in most regions of the country, the out
Figure 7.5 Various outdoor air treatment state points Air treatment still cannot reach the state point shown in Figure 7.5(c), which would allow the terminal units to operate under dry conditions. However, it is hoped that, when conditions permit, the treated outdoor air can undertake part of the indoor moisture load [see Figure 7.5(b)]. At the very least, none of the moisture load from the outdoor air should be borne by the terminal units.
[Reference] The new versions of foreign standards have all raised the requirements for indoor humidity control. Numerous facts show that although purification air-conditioning can effectively filter out bacteria in the supply air, emphasis still needs to be placed on humidity control of the entire system. Whether in the air-conditioning units and their ductwork or inside the clean operating room, as long as the humidity is out of control, indoor bacteria gain the conditions to breed and new hidden dangers emerge. If conditions become ripe, bacteria may multiply in large numbers and ultimately cause the entire control to fail. Therefore, great attention should be paid to the hazards of loss of humidity control. The final state point of outdoor air treatment has a significant impact on indoor humidity control. The outdoor air treatment states are shown in Figure 7.5.
7.2.8 Outdoor Air Volume Standards and Requirements for Operating Rooms
The outdoor air volume for a clean operating room shall be determined according to the following requirements, and the maximum value shall be taken:
(1) The outdoor air volume calculated based on the outdoor air change rate in Table 4.1 of Chapter 4.
(2) The outdoor air volume that compensates for the indoor exhaust and maintains the indoor positive pressure value.
(3) The outdoor air volume required for personnel respiration.
(4) For each of the above calculated values, the outdoor air volume should not be less than 1 000m³/h per Class Ⅰ clean operating room, not less than 800m³/h per Class Ⅱ or Ⅲ clean operating room, and not less than 600m³/h per Class Ⅳ clean operating room.
[Reference] After air conditioning was adopted in operating rooms, especially small ones, medical staff have consistently complained that the rooms are too stuffy. The Hospital Design and Management Guidelines published in Japan in 1998 stipulate a minimum outdoor air volume of 5 air changes/h; the Applications volume of the 1999 edition of the Handbook of the American Society of Heating, Refrigerating and Air-Conditioning Engineers also stipulates a minimum outdoor air volume of 5 air changes/h; Part 4 of the former Federal German standard DIN1946 gives 70m³ per person per hour for wards, and although no value is given for operating rooms—obviously it would be higher than that rate—a total outdoor air volume of 1 200m³/h per operating room is given; the Swiss standard adopts 80m³/h per person. Taking into account the setup of the exhaust system, the specified number of people (12 for extra-large operating rooms, 10 for large operating rooms, 8 for medium-sized operating rooms, and 6 for small operating rooms), and the stipulation of a minimum of 60m³/h of outdoor air per person, all of the above standards are relatively high, with the German outdoor air volume being the largest. Its rationale is that the halothane consumption in an operating room is 500ml/h; if the outdoor air reaches 1 200m³/h, the halothane concentration can be maintained at $frac{500text{cm}^3}{1,200text{m}^3} approx 0.4text{ppm}$, while the concentration near the anesthesiologist will be 10 times higher, i.e., 4ppm, which is just below the maximum allowable concentration of this gas, 5ppm. The considerations of the Code are: ① The German rationale can be used for reference, but for Class Ⅳ operating rooms performing minor ordinary surgery, the amount of anesthetic used may all be smaller, and the release of anesthetic gas should not be continuously at high concentration, whereas the Code stipulates that the exhaust is continuous; therefore, the outdoor air volume may be considered for reduction to half of it, approximately 600m³/h. ② The most important consideration is that, with the door open, the indoor airflow can flow outward at a certain velocity to resist the intrusion of outside air. Assuming that a Class Ⅰ operating room maintains an outward airflow velocity of 0.1m/s and the door area when open is 1.4×1.9＝2.66m², then 956m³/h of outdoor air is required; for Class Ⅱ and Ⅲ operating rooms maintaining a flow velocity of 0.08m/s, 766m³/h is required; in addition, it is quite commonly reported that operating rooms feel rather stuffy. Therefore, this clause appropriately increases the outdoor air; besides the outdoor air change rate and the outdoor air volume per person stipulated later, the minimum outdoor air volumes for Class Ⅰ, Class Ⅱ～Ⅲ, and Class Ⅳ are set at 1000m/h (for ophthalmology-dedicated operating rooms, where the surgical staff is generally very small and the room is also small, 800m³/h may be adopted), 800m³/h, and 600m³/h respectively, so as to avoid problems in small operating rooms.
7.2.9 Requirements for the Cleanroom Air-Conditioning System
The requirements for the cleanroom air-conditioning system are: first, to satisfy the special functions of the clean operating room; second, to prevent secondary contamination of the air-conditioning system from affecting indoor environmental control.
(1) Airtightness measures. When the clean operating department is not in operation, the reverse inflow of outdoor air shall be prevented to avoid destroying the indoor aseptic state; the tightness of the system is extremely important. The outdoor air intake of the operating room purification air-conditioning system, as well as the system's supply air duct, return air duct, and exhaust air duct, should be fitted with airtight dampers at locations near the operating room. Emphasis is placed on the tightness of the purification air-conditioning system and on its normal constant-air-volume operation state, so as to avoid the influence of dust accumulation in the air filters on the system air volume.
(2) Constant air volume operation. The indoor cleanliness is mainly ensured by the air change rate, but due to the three-stage air filtration in the system The filter continuously accumulates dust, causing the system resistance to rise and the supply air volume to decrease; or, when the three-stage air filters are replaced, the system resistance drops and the supply air volume increases. These are unfavorable for the control of the indoor state. Constant air volume operation of the system must be emphasized; to this end, electric control valves, frequency converters, mechanical constant air volume devices, etc., may be used for control.
(3) Humidity-priority control. Operating rooms do not require constant temperature and humidity control, but the requirements for controlling the upper and lower humidity limits are rather strict. The standard only specifies the indoor temperature and humidity control range for operating rooms, which is not a constant temperature and humidity requirement. Therefore, in terms of temperature and humidity control, humidity-priority control should be implemented, ensuring that temperature control is achieved on the premise that the indoor humidity is controlled at no higher than 60% and no lower than 40%. For dedicated units for Class I–III operating rooms, it is also required that the humidity before the air reaches the terminal air filter does not exceed 75%, which creates conditions for the energy-saving operation of operating rooms.
(4) Preventing secondary microbial contamination. Moisture and dust are essential nutrient sources for bacterial growth. In the past, the hazards of moisture and dust accumulation in air-conditioning units and duct systems (especially in unit fittings and plenum boxes), particularly on air filters, did not receive sufficient attention. The code emphasizes preventing secondary microbial contamination of the system. To reduce bacterial growth caused by dust accumulation and retained water, first, static control should be addressed, i.e., avoiding the possibility of bacterial growth in the configuration of the functional sections of the air-conditioning unit, eliminating hidden dangers of bacterial growth in the design and manufacture of components, and adopting antibacterial measures. Second, dynamic control should be pursued, i.e., eliminating dust accumulation and locally formed high-humidity hazards inside the air handling unit, implementing humidity-priority control indoors, and keeping the unit dry and dust-free when the air-conditioning unit stops running.
In addition, with reference to the relevant requirements of Part 4 of the German hospital standard DIN1946, it is emphasized that the air-conditioning ductwork of operating rooms should be short, straight, and smooth, that fittings should be minimized, that fittings and plenum boxes with good airflow performance and small vortex zones should be used, and that flexible hoses should not be used in the duct system.
[Reference] Why is it difficult for traditional air-conditioning units to meet the requirements of clean operating rooms? The main reason is that traditional air-conditioning units give priority to improving heat and moisture exchange efficiency while neglecting the secondary microbial contamination caused by dust accumulation and retained water.
The main difficulties include the following aspects:
① Dust accumulation and dampness in the air-conditioning unit mainly occur at the fresh air intake, especially on the fresh air filter, and are difficult to eliminate.
② The moisture-producing parts in the air-conditioning unit are mainly the cooling and dehumidifying coils and the humidifier, which cannot be eliminated.
③ To control bacterial growth on the coils, dust entering the coils should be minimized as much as possible, requiring that a high-efficiency medium-efficiency filter be installed upstream of the coils. However, to prevent the air filtered by the medium-efficiency filter from being contaminated again, purification measures require that the medium-efficiency filter be installed in the positive-pressure section of the system (i.e., downstream of the coils). This is a pair of contradictions that is difficult to resolve.
④ The places where water is retained in the air-conditioning unit are the condensate drain pan and the water seal; since the coils are located in the negative-pressure section of the unit, the problems of water accumulation and retention in the drain pan and the water seal cannot be changed.
⑤ There are two types of humidifiers in air-conditioning units, namely atomization and steam. The water tank of the humidifier is a place where bacteria breed, and atomization is the best way for bacteria to enter the air. Steam should in principle be sterile, but steam coming directly from the boiler contains chemical substances harmful to humans and cannot be used directly.
⑥ Air filters are an effective means of dust removal. Dust accumulated on the filter surface generally does not pose a bacterial growth problem, but if the filter remains damp for a long time, bacteria may multiply. It is really a difficult thing to keep the medium-efficiency filter located downstream of the cooling and dehumidifying coils and the humidifier always in a dry state.
⑦ To prevent the accumulation of dust particles inside the air-conditioning unit, uneven internal surfaces or transverse joints should be avoided, and the angle between the bottom and the side panels should be a rounded corner, etc.; conventional air-conditioning unit structures can hardly achieve this.
7.2.10 Fresh Air Intake of the Clean Air-Conditioning System
(1) The orientation and rainproof function of the fresh air intake are particularly important. Especially in the southeastern coastal areas, it is necessary to prevent heavy rain, typhoons, etc. from driving large amounts of rainwater back into the fresh air ducts. Therefore, the system should adopt fresh air intakes with good rainproof performance and effective rainproof or even drainage measures. The intake air velocity at the fresh air intake should not be greater than 3m/s. If conditions permit, the area of the fresh air intake can be enlarged to reduce the intake air velocity; the 3m/s air velocity is only an upper-limit control value.
(2) The fresh air intake of the system should not be located above the exhaust outlet; it should be set more than 5m above the ground and more than 3m horizontally away from the exhaust outlet ...above and in a clean area on the windward side of the exhaust outlet that is free from interference by pollution sources; if located on the roof, it should preferably be at least 0.9m above the roof surface. The fresh air intake should also not be located inside the machine room.
【Reference】 To improve the dilution effect of fresh air, the quality of the fresh air entering the room must first be improved. The concepts of fresh air age and pathway pollution can be used to quantify the quality of fresh air entering the room. The shorter the transport distance of fresh air from the intake to the room, the shorter the transport time, the smaller the fresh air age, and the better the quality. The smaller the pathway pollution, the better the fresh air retains its characteristic odor and performance. Therefore, for the high-quality fresh air collected, the shorter the transport ductwork and the smaller the pathway pollution—this is the key factor determining indoor air quality.
【Reference】 Pay attention to the siting of the fresh air intake. To avoid the influence of the ground or roof on fresh air quality, the height above the ground or roof is specifically stipulated, and there should be sufficient distance between the fresh air intake and the exhaust outlet; it is required to be far away from cooling tower discharge outlets, chimney smoke outlets, and all exhaust outlets, so as to avoid drawing in polluted air, foul odors, etc. In particular, the drifting water mist from cooling towers contains Legionella bacteria; if it enters a building through the fresh air intake used for air conditioning, it can cause a large-scale outbreak of infection, and there have been reports of such cases both in China and abroad.
7.2.11 Exhaust Outlets of the Purification System
(1) Since medical staff in the operating room often gather around the patient, human body odor is considerable, especially the foul odor emitted from the patient's body cavity during surgery, compounded by the smell of disinfectants, residual anesthetic gas, etc., which deteriorates indoor air quality. To address such a concentrated pollution source, the exhaust system of the operating room should preferably be set up independently and should be interlocked with the supply air system; it should preferably be located at the ceiling above the right side of the patient's head, exhausting directly, so that harmful gases in the room can be effectively removed.
(2) The exhaust air volume of each operating room should not be less than 200m³/h.
(3) Along the exhaust flow direction, the exhaust outlet should be equipped with a high-medium efficiency filter with a counting efficiency for atmospheric dust ≥1μm of not less than 80% and a check valve, or an electrically operated airtight damper interlocked with the exhaust fan, to prevent outdoor air from backflowing through the exhaust outlet.
【Reference】 To control positive pressure effectively and flexibly, and to directly exhaust the pollution emitted by concentrated pollution sources so as to ensure indoor air quality, it is not desirable to provide exhaust in the return air ductwork; this approach is commonly used in ordinary civil buildings where pollution sources are dispersed and uniform. If used in operating rooms, harmful substances indoors would circulate within the system, and the indoor pollution concentration could only be gradually reduced through continuous dilution, continuous circulation, and continuous exhaust by the system, with rather poor actual results.
【Reference】 The exhaust fan should operate continuously just like the supply fan; to maintain indoor positive pressure, the exhaust and supply air systems are required to be interlocked. Likewise, the indoor exhaust fan is required to be interlocked with the automatic door of the operating room, and a time-delay device should be installed. When the door opens momentarily, the exhaust fan stops immediately; when the door is closed, after a delay and once positive pressure is established, the exhaust fan is started again. This both ensures the maintenance of positive pressure while the door is open and prevents the exhaust fan from being frequently stopped and started due to the opening and closing of the door.
7.2.12 Restrictions on the Use of Radiators
Heating radiators shall not be installed in Class Ⅰ, Ⅱ, and Ⅲ clean operating rooms or in Class Ⅰ and Ⅱ clean auxiliary rooms, but radiant heating panels may be used for on-duty heating. If Class Ⅳ clean operating rooms and Class Ⅲ and Ⅳ clean auxiliary rooms need heating radiators, types that do not easily accumulate dust and are easy to clean, such as bare-tube radiators or radiant panel radiators, should be selected, and protective covers should be installed. The heat medium for the radiators should be hot water not higher than 95℃.
【Reference】 Considering that radiators easily accumulate dust and are difficult to clean, the selection of types such as bare-tube radiators or radiant panel radiators is emphasized, and the installation of protective covers is required. In addition, the thermal convective airflow generated during operation and the thermal deposition of dust particles on the cold wall surfaces are unfavorable to indoor purification; therefore, regulations should also be made on the situations in which radiators may be used.
7.2.13 Cold and Heat Sources for the Surgical Department
The possibility that the clean air-conditioning systems of the entire surgical department or of several operating rooms can be operated during transition seasons should be considered.
【Reference】 Due to the special requirements of indoor environmental control in operating rooms, the cooling (or heating) supply period is usually required to be advanced or extended. When designing the surgical department, the possibility of providing cold and heat sources so that the clean air-conditioning system can be used during transition seasons should be considered, so that it is not necessary to start up the cold and heat sources of the main system. It is highly unreasonable for the cold and heat sources of the main system to operate at low efficiency in order to meet the cooling (or heating) needs of a single surgical department. [Reference] The coefficient of simultaneous use in the surgical department is relatively small, and cooling (or heating) is often required immediately due to emergency operations. Therefore, when considering the cooling and heating sources for the clean air-conditioning systems of the entire surgical department or several operating rooms, the possibility of their independent use should be taken into account.