7.4.1 Overview

Air-conditioning equipment refers to air-conditioning units (with a refrigeration machine, cooling capacity above 16.3kW), air conditioners (with a refrigeration machine, cooling capacity below 16.3kW), and air-handling units (without a refrigeration machine). They are the most commonly used important components of the clean air-conditioning system. Their fabrication and material selection should meet the characteristics of daily maintenance; requirements for cleaning, disinfection, filter replacement, rust prevention, corrosion prevention, and smooth drainage should all differ from those for ordinary air-conditioning equipment, especially since they serve clean operating rooms where bacteria control is the primary objective. The main factor affecting bacterial reproduction is whether the humidity inside the equipment can be controlled, which is often the basic condition determining secondary microbial contamination inside. Therefore, sufficient attention should be paid to the material selection of various components.

7.4.2 Materials and Structure of Clean Air-Conditioning Units

(1) The inner surfaces and built-in components of clean air-conditioning units should be made of materials or surface layers resistant to corrosion by disinfectant chemicals, with smooth and clean surfaces; polished stainless steel or color steel plate is commonly used as the lining panels.

[Reference] The properties of commonly used disinfectant solutions are briefly described as follows:

① Peracetic acid. This is a fast-acting, highly effective, broad-spectrum disinfectant that can rapidly kill viruses, bacteria, fungi, and spores, and it retains its killing function at low temperatures. It is supplied as a 20% or 40% solution. For air disinfection, use 0.75g/m³ of the 20% solution, heated and vaporized with an electric stove in a sealed room for 1h, or 8mL/m³ of the 2% solution (equivalent to 0.16g/m³) atomized by electric spraying and then kept sealed for 30min.

The concentrated solution of this product is toxic, and high-concentration solutions are flammable and explosive (the 20% solution has no such danger), so open flames should be avoided. Solutions of higher concentration are strongly corrosive, and their vapor is irritating to the respiratory tract. This product can corrode metals such as iron and copper, but its effect on stainless steel and aluminum is not significant within ordinary disinfection periods.

② Glutaraldehyde. It is an excellent fast-acting, broad-spectrum disinfectant for articles, capable of killing various bacteria, fungi, viruses, and spores. It is colorless, odorless, and non-irritating, with low corrosivity; organic matter does not affect its sterilization effect, and it does not easily damage instruments. It is suitable for disinfecting various medical devices, including rubber, plastics, synthetic fibers, glass, leather, metals, and sharp instruments. Because of its high price, it is used only for disinfecting medical devices, instruments, and endoscopes that cannot withstand heat, are susceptible to corrosion, are difficult to sterilize, or have high sterility requirements.

③ Ethylene oxide. It is an excellent broad-spectrum gaseous disinfectant that can kill bacteria, fungi, spores, and viruses. It has strong penetrating power, does not damage articles, volatilizes rapidly after disinfection, and leaves no residual toxicity. It is especially suitable for disinfecting various precision instruments, such as electronic instruments, heart-lung machines, pacemakers, radios, cameras, films, books, and documents.

④ Chlorhexidine. It is a white powder, hardly soluble in water, with low toxicity and irritation, and relatively safe. It can kill bacteria (including Pseudomonas aeruginosa) and fungi, but is ineffective against tubercle bacilli and spores. For ward spray disinfection, a 0.1% solution is used, sprayed 2–3 times a day for about several minutes each time; patients in the ward experience no obvious discomfort.

⑤ Formaldehyde solution (formalin). Formaldehyde solution contains 36%–40% formaldehyde and is also known as formalin; it can kill bacteria, viruses, fungi, and spores. For surgical instrument disinfection, soak in 10% formalin for 1–2h and then rinse thoroughly with water. Because this disinfectant is carcinogenic, the Ministry of Health's "Hospital Disinfection Code" specifies that it is not suitable for air disinfection in cleanrooms.

There are many kinds of disinfectants. In selecting materials for clean systems, attention should be paid to their properties; for reference, see the "Medical and Nursing Technical Operations Code》.

(2) The internal structure of the air conditioning equipment should be easy to clean and able to smoothly discharge the cleaning wastewater, and should not easily accumulate dust or breed bacteria; this is a basic requirement for achieving daily maintenance and management. Given that in the past, the air conditioning units used in central air conditioning systems lacked the necessary management systems, and their structure also lacked the necessary provisions, the interior of the casing was often very dirty, and after long-term operation it was impossible to enter the interior for cleaning. For air conditioning units used in operating rooms, daily cleaning is extremely important, as it is a necessary means of preventing secondary contamination; however, because the components inside the casing are installed very compactly, it is very difficult to enter the casing interior to carry out cleaning. Therefore, the structure should allow as many as possible of the front and rear wall panels of the casing to be opened, so that a person outside the casing can wipe the interior directly. Since the casing panels can be removed, there is no need to make access doors; it is sufficient to provide sight holes on the panels, and the cleaning wastewater can also be completely drained from the bottom panel of the casing.

(3) Airtightness of the air conditioning unit casing. The sealing of the casing is an important issue and also an old one; it is caused by aging of the sealing materials or an unreasonable casing structure, and in some cases by reasons such as low elasticity of the gaskets, gaps at the lap joints, or loose door handles. Whatever the cause, in severe cases the air leakage rate can be as high as 20%～30%, a large amount of energy is wasted, and it may even result in insufficient indoor supply air, causing a decrease in indoor cleanliness. The main reason is still the poor quality of the air conditioning unit and the lack of attention to quality during installation; therefore, when placing an order, index requirements for the air leakage rate should be specified. According to the working-section combination with the air conditioning unit in the positive-pressure section, it should include the (fresh air) return air section, fan section, flow equalizing section, medium-efficiency filtration section, surface cooling section, heating section, humidification section, intermediate section, and air outlet section, with sealant applied at the joints; its total resistance is about 300Pa, the residual pressure outside the unit for the high-efficiency filter is 500Pa, and the resistance of the duct components is 200Pa. Therefore, when testing the air leakage rate of the air conditioner, the static pressure inside the casing should preferably be 1000Pa, while the resistance of the negative-pressure section of the casing is relatively small. For convenience of testing, a unified pressurization method is applied to the casing during the test, keeping the static pressure inside the unit at 1000Pa. If the manufacturing quality is good and the joints are sealed, and the air leakage rate test is carried out on the air conditioning unit, the prototype units all meet the air leakage rate standard for operating rooms with cleanliness higher than Class 1000, i.e., an air leakage rate of less than 1%; for operating rooms with cleanliness lower than Class 1000, the air leakage rate of the casing should not be greater than 2%.

7.4.3 Components of the Air Conditioning Unit

(1) Condensate drain outlet. The condensate drain outlet of the surface cooler should be equipped with a device that can automatically prevent back-siphonage and smoothly discharge the condensate under negative pressure; this is very important for preventing the inhalation of contaminated air from the drainage pipes.

【Reference】 Under summer operating conditions, the pipe of an ordinary water seal contains condensate; under winter operating conditions, it becomes a dry water seal, i.e., there is no stored water. If the air conditioning unit is operating under negative pressure, contaminated air at the drainage pipe opening can easily be drawn in through the condensate drain outlet and enter the air conditioning system directly with the supply air. The earliest 《Technical Measures for Air Cleanliness》 once stipulated that a valve be installed on the condensate drain pipe; under winter operating conditions the valve is kept in the closed position, but during operation in the transitional seasons the condensate appears and disappears intermittently, making this valve very difficult to operate. By the end of the 1980s, an air-seal auxiliary device appeared, which used the positive pressure at the fan outlet to run a small tube to the drain outlet of the condensate drain pan; when there was no water in the water seal, the drain outlet was in a positive-pressure air state. Later, in the early 1990s, another type called the pulse-type water seal (Figure 7.7) appeared, which used a float ball to seal the outlet of the water seal: when draining, the water level rose to a certain height and the float ball rose to allow drainage; when not draining, the float ball settled onto the valve seat to close the water seal; however, the height of the water seal should correspond to the static pressure value of the air conditioning unit. If the air conditioning unit is supplying air under positive pressure, it is only necessary to seal the drain outlet, but the buoyancy of the float ball should correspond to the positive pressure value inside the casing.

(2) Connections for differential pressure gauges should be provided before and after each stage of air filtration. The service life of the air filters at each stage is greatly related to the atmospheric dust concentration. And the fil-

Figure 7.7 Pulse-type water seal The dust holding capacity of a filter has a definite value. When the specified dust holding capacity is reached, the filter has reached its final resistance value, at which point the air passage capacity of the filter begins to decrease. Usually, when the final resistance reaches a certain multiple of the initial resistance, the filter should be replaced with a new one.

【Reference】 Differential pressure gauges are produced by many instrument manufacturers, and their product models vary. They can be broadly divided into two major categories: the indicating type and the indicating type with alarm. Figure 7.8 shows the style of a liquid column indicating type differential pressure gauge.

Without stand　　With stand

Figure 7.8　Liquid column differential pressure gauge (Mark-Ⅱ type)

Each filter section should be equipped with a resistance monitoring instrument, so that the final resistance of the filter is indicated by an instrument rather than relying on the operator's perception. A differential pressure switch may also be used to give a signal prompt when the final resistance is reached. The recommended value ranges for the final resistance of filters are shown in Table 7.20.

Table 7.20　Value ranges for filter final resistance

Filter efficiency class Recommended final resistance/Pa
Coarse (equivalent to foreign G3) 100～150
(equivalent to foreign G4) 100～160
Medium efficiency (equivalent to foreign F5～F6) 140～180
High-medium efficiency (equivalent to foreign F7～F8) 150～200
Sub-high efficiency (equivalent to foreign F9～F11) 150～200
High efficiency (excluding ultra-high efficiency) 300～400

(3) Cooler. Spray-water cooling should not be used in air conditioning units. When a surface cooler is used, the air velocity through the section where the coil is located should not exceed 2m/s; otherwise, water elimination devices should be installed.

With the spray-water cooling method, since the spray water is recirculated and in contact with the air for a long time, and the spray water also has a high dust removal efficiency, a large amount of dust and bacteria in the air dissolves into the water. The water quality of such water is very poor; in particular, the bacterial content in the water is very high and the bacterial species are very mixed. Therefore, this method should not be used. However, when a surface cooler is used, the air velocity through the coil should be kept below 1.8～2m/s. If the air velocity is too high, it will carry away the condensate droplets on the surface cooler; these droplets will be intercepted by components such as air ducts located in the positive pressure section, forming re-evaporation, which will directly affect the control of relative humidity in the operating room. If the components remain in a damp state for a long period, mildew is also likely to occur. Furthermore, when the unit stops running, it is not easy to blow dry within a short time, which also provides favorable conditions for bacterial reproduction. The disadvantage of this practice is that the cross-section of the air conditioning unit is slightly larger, with its side length increased by about 10%. If a higher face velocity is adopted, in order to prevent water carryover, a water eliminator should be added after the surface cooler, but this also increases the length of the air conditioning unit, and the water eliminator itself also brings many inconveniences to cleaning work.

(4) Humidifier. The humidifier in the air conditioning unit should preferably be a dry steam humidifier, and the water quality of the steam used for humidification should meet the standard of softened drinking water, so as to prevent calcium and magnesium ions in the water from precipitating out and forming white powder. There are many types and models of humidifiers, and careful selection should be made.

【Reference】 Dry steam humidifier. It consists of a drying chamber and a spray pipe. The steam supplied from the boiler room passes through a pressure reducing valve and a regulating ...throttle valve, steam solenoid valve, pressure gauge, etc., and enters the drying chamber. The steam is depressurized in the spray pipe cavity wall through repeated throttling and expansion with dehydration, becoming dry steam almost close to atmospheric pressure, and is then fed into the spray pipe. If the air handling unit does not have a sufficient empty section for humidification and the air does not have enough distance to absorb the moisture, the sprayed steam will not be fully absorbed. Therefore, there must be sufficient distance between the humidifier and the subsequent air conditioning equipment section; generally a spacing of more than 700 is required. For the purification air-conditioning systems of Class Ⅰ–Ⅲ clean rooms, the relative humidity of the air in the system before the high-efficiency filter should not be greater than 75%. The schematic of a dry steam humidifier installed in the air handling unit is shown in Figure 7.9, and its piping is shown in Figure 7.10. The solenoid valve should be interlocked with the humidity sensor and the fan.

Figure 7.9　Steam humidifier installed in the air handling unit

  1. Humidification spray pipe　2. Humidifier drying chamber　3. Air handling unit casing

Figure 7.10　Piping of the dry steam humidifier

  1. Steam solenoid valve (interlocked with fan)　2. Filter　3. High-pressure gauge　4. Pressure reducer　5. Low-pressure gauge　6. Regulating valve　7. Working regulating valve　8. Humidification spray pipe　9. Humidifier　10. Steam trap　11. Working pressure gauge [Reference]　Wet film humidifier. In a wet film humidifier, sterile water—obtained by first softening the water—is pressurized by a pump and delivered to the upper distribution pipe of the wet film humidifier. Under the action of gravity, the water flows through the wet film into the water pan and is drained; this water should not be recycled for reuse, because the surface of the wet film can be severely contaminated by dust and bacteria. Since the sterile softened water is continuously discharged, the routine operating cost is relatively high, but it is not restricted by the hospital's steam supply status. Its configuration is shown in Figure 7.11. When humidification is not in progress or the system has stopped, the air conditioner should extend its operation to blow-dry the humidifier, so as to prevent bacterial growth.

Figure 7.11　Configuration of the wet film humidifier

  1. Pressure gauge　2. Solenoid valve　3. Wet film humidifier　4. Surface cooler and heater

[Reference]　Electrode humidifier. The electrode humidifier is a commonly used type of humidifier. However, since it uses electricity to boil water to produce steam, its power consumption is relatively large, and the humidifier itself is also more expensive, requiring a larger investment. Nevertheless, it offers good flexibility and can well serve situations where a steam source is lacking, and the steam quality is relatively good. When the local water hardness is relatively high, the raw water should undergo water treatment. Its working principle is shown in Figure 7.12.

Working principle of the electrode humidifier: When water contains trace amounts of salts, the water becomes an electrical conductor. When water with a conductivity of 125–1 250 μs/cm enters the electrode humidifier, the computer can measure the conductivity of the water. When the water level submerges the electrodes inside the humidifier, the electrodes form a current circuit through the water, heating the water until it boils and produces steam, and the steam output is controlled by means of the water level and the magnitude of the conductivity.

After the electrode humidifier is started, the computer controller first opens the water inlet valve 8, and the water passes through the water inlet box 6 (at this point the computer can measure the conductivity of the incoming water) into the bottom of the humidifier. The water level gradually rises to the electrodes, and the electrodes form a current circuit through the water and heat the water until it boils. The higher the water level rises, the greater the current flowing through the electrodes; when it rises to the highest point, the computer controller detects the maximum water level signal through the high-water-level detection electrode 1 and closes the water inlet valve 8. As steam is continuously delivered, the water level in the humidifier 3 keeps dropping, and the computer controller 2 will open the inlet valve 8 again to replenish fresh water, so as to meet the humidification capacity requirement. When the salt content in the humidifier 3 keeps increasing and the conductivity of the water becomes too high, the computer controller will automatically open the drain valve 4 to discharge part of the water and impurities, thereby keeping the humidification in a stable working state. When the air conditioning unit is shut down, the interlocked vent solenoid valve 5 will open to completely drain the remaining water in the humidifier, so as to prevent bacteria from growing in the residual water. At the next startup, fresh water will enter again.

There are many types of humidifiers. For example, the high-pressure spray humidifier has a very large humidification capacity and is suitable for humidifying large spaces; the duct-mounted wet film humidifier is suitable for use where there is a relatively long straight duct section and a relatively small humidification capacity; the centrifugal humidifier is usually installed exposed with multiple units in parallel to provide humidification for large spaces, but it has no special requirements for water quality; the ultrasonic humidifier is usually installed with multiple units in parallel inside relatively large air conditioning units Figure 7.12 Working Principle of the Electrode Humidifier

  1. High water level detection electrode　2. Computer control board　3. Electrode humidifier
  2. Blowdown valve　5. Emptying solenoid valve (interlocked with the air-conditioning fan)
  3. Water inlet box with conductivity-measuring electrode　7. Overflow pipe　8. Water inlet valve

for humidification, they have relatively high requirements for water quality. These humidifiers are generally not used in places with very high purification requirements such as operating departments, so no further introduction will be given here.

When the humidification steam source is restricted by various conditions and other heat sources are available, facilities using secondary steam should be considered. Secondary steam uses other heat sources and, through a small heat exchanger, indirectly heats a medium that meets the service conditions, raising purified water to its boiling point so that it becomes steam for humidification; its heat source may be primary energy such as primary steam unsuitable for direct use, high-temperature hot water, or electricity.

(5) Silencers. In a clean environment, all kinds of noise must be effectively controlled, which includes two major aspects: airborne sound transmission and solid-borne sound transmission. Air-conditioning equipment is a sound source in both of the above aspects, while the Code stipulates that the upper limit of the noise level is 50～52dB(A). As for silencers, it is stipulated in terms of materials that they should be corrosion-resistant, non-dust-generating, not prone to dust adhesion, non-hygroscopic, and not made of wooden materials, and it is required that their filling material should not use glass fiber or its products.

[Reference] Silencers commonly used in clean operating rooms include impedance-composite silencers fabricated to the dimensions of the T701-6 standard drawing atlas, silencer elbows, folded-plate silencers, tubular silencers, and micro-perforated panel silencers, etc. When the micro-perforated panel silencer is compared with the impedance-composite silencer at the same amount of noise attenuation, the micro-perforated panel silencer is twice as long, its external cross-section is also slightly larger, its installation space position is relatively difficult, and it should be made of aluminum plate or stainless steel.

[Reference] The micro-perforated panel silencer belongs to the type without filling material; its sound absorption performance is good over a wide frequency band. In construction, it can be divided into two types: the narrow rectangular silencer and the sound-stream type silencer. For the narrow rectangular micro-perforated silencer, when the flow velocity is below 20m/s, the airflow has little influence on the silencer's attenuation performance and the pressure loss can be neglected; for the sound-stream type silencer, at flow velocities below 15m/s, both the attenuation performance and the aerodynamic performance are good. The noise attenuation of a silencer decreases as the pressure loss increases. Since the micro-perforated panel silencer has a very small friction coefficient, as long as attention is paid to the geometry of the passage and the local resistance is minimized, it is possible to achieve a micro-perforated panel silencer with low pressure loss, low regenerated noise, high flow velocity, and small external dimensions.

In terms of the sound-absorbing structure, it can be further divided into two types: single-layer micro-perforated panel and double-layer micro-perforated panel, with the latter being better.

[Reference] There are also many types of silencer elbows. The micro-perforated panel silencer elbow is a new type of silencer elbow; it is a filler-free sound attenuator that eliminates resistive sound-absorbing materials and uses micro-perforated panels as sound-absorbing elements. Because the density of the micro-perforations is high and the holes are very fine, it therefore has a very high acoustic resistance. Measurements show that it exhibits excellent performance in terms of sound absorption coefficient and sound absorption frequency band. In engineering practice, two layers of micro-perforated panels with different perforation ratios and two air layers of different thicknesses are commonly used to broaden the sound absorption frequency band.

Features: excellent silencing effect, wide frequency band, particularly effective at medium and low frequencies, low airflow resistance, and low regenerated noise.

Applications: suitable for use in air conditioning and ventilation projects in special environments such as low temperature, high temperature, high-speed airflow, and ultra-clean conditions.

The appearance is shown in Figure 7.13.

Figure 7.13　Outline drawing of the micro-perforated panel silencer elbow

There are two models: one is the single-cavity micro-perforated panel silencer elbow, whose silencing frequency band is dominated by medium frequencies; the other is the double-cavity micro-perforated panel silencer elbow, which has an even wider silencing frequency band and excellent medium-low frequency performance.

The silencing performance of the micro-perforated panel silencer elbow is shown in Table 7.21. Type Ⅰ: $B leqslant 500mathrm{mm}$; Type Ⅱ: $500mathrm{mm} < B leqslant 100mathrm{mm}$ (with guide vanes)

Table 7.21　Performance of the micro-perforated panel elbow

Model Air velocity/(m/s) Pressure loss/Pa Sound level difference 125 Sound level difference 250 Sound level difference 500 Sound level difference 1K Sound level difference 2K Sound level difference 4K Sound level difference 8K Sound level difference A
VKW-Ⅰ (single cavity) 5 24 3 9 13 15 12 7 4 10
VKW-Ⅰ (single cavity) 10 36 4 9 14 17 12 9 6 12
VKW-Ⅰ (single cavity) 15 72 4 10 13 14 13 9 7 13
VKW-Ⅰ (single cavity) 20 35 5 12 12 13 14 12 10 14
VKW-Ⅱ (double cavity) 5 25 8 12 13 14 10 8 5 10
VKW-Ⅱ (double cavity) 10 40 7 13 15 15 12 9 6 11
VKW-Ⅱ (double cavity) 15 82 7 10 15 16 13 10 6 11
VKW-Ⅱ (double cavity) 20 128 6 14 16 15 13 12 7 13

The overall dimensions are shown in Figure 7.13, where the dimensions $A times B$ are determined according to the duct flange. When two or more silencer elbows are used in succession, tests have proven that only when the connection spacing between the two elbows is $L geqslant 2.5x$ ($x$ being the diagonal length of the air outlet cross-section) can the silencing effect of two silencer elbows be achieved; when $L < 2.5x$, due to the interaction between the elbows themselves, the silencing effect is 1.2 to 1.5 times that of a single elbow. Therefore, when using continuous silencer elbows, the connection distance should be increased as much as possible to ensure the silencing effect.

(6) Vibration isolation pad

【Reference】　Vibration isolation of air conditioning units is an important means of preventing solid-borne sound transmission; it adopts measures to cut off and suppress the transmission from the vibration source to Preventing the transmission of vibration to the outside, with the aim of preventing the vibration of the air conditioning unit from affecting the environment, belongs to the category of active vibration isolation. For the fans inside the air conditioning unit, manufacturers have generally already installed damping vibration isolation supports on their fixed bases; small fans often use steel-wire damping vibration isolators, while slightly larger fans use JG-type rubber shear vibration isolators. Since transmissibility and vibration isolation rate are reciprocally related in percentage terms, even though vibration isolators are installed beneath the fans, 20% to 30% of the vibration force is still transmitted to the foundation or the floor slab; therefore, it is necessary to carry out an additional vibration damping treatment under the base of the air conditioning unit. Especially when the air conditioning unit has been running for a period of time and the bearings gradually become short of oil, this vibration is noticeably intensified, and such vibration isolation facilities become all the more necessary. SD-type vibration isolation pads are commonly selected; they are not only relatively inexpensive and lenient in their requirements for center-of-gravity deviation, but also have good damping performance and are convenient to install. When selecting them, selection calculations should be performed, and generally 2 to 3 layers are appropriate; a single-layer vibration isolation pad often fails to achieve the desired vibration damping effect. It is preferable to provide 6 to 8 support points for each air conditioning unit; the installation is shown in Figure 7.14.

Figure 7.14　Installation of vibration isolation pads for the air conditioning unit