Overview of Cleanroom Air Conditioning Systems
0. Introduction
With the advancement of science and technology and economic development, the application of cleanroom technology is becoming increasingly widespread. Nobel laureate and renowned scientist Yang Zhenning predicted at the 2001 China Association for Science and Technology Annual Conference that in the next two to three decades, ① the widespread application of chips, ② the rapid development of medicine and pharmaceuticals, and ③ bioengineering will become the driving forces of technological development, representing the three major strategic directions for future scientific and technological progress. To meet these three directions, besides improving the technology of the processes themselves, the most important thing is to challenge the cleanliness of the environment and create an indoor microenvironment with even higher cleanliness.
Today, modern industrial product manufacturing and modern scientific experiments demand miniaturization, precision, high purity, high quality, and high reliability. This has led to the healthy and rapid development of cleanroom technology, which, along with the advancement of science and technology and the rapid changes in industrial products, is also showing better performance in industries beyond the three major strategic directions, such as food, cosmetics, and military. To create a good cleanroom microenvironment, an efficient, safe, reliable, practical, and simple air purification system is the core of all its complementary systems. Air handling units are important components that perform functions such as heat exchange, filtration, humidification, and air supply. Unlike ordinary comfort air handling units, this article discusses the application of air handling units in cleanrooms from the perspective of cleanrooms.
1. Characteristics of cleanroom air conditioning systems
1.1 Large air volume
Cleanrooms primarily filter airborne dust and bacteria through air circulation, controlling both non-biological and biological particles to achieve cleanliness standards. Therefore, sufficient airflow is necessary to maintain indoor cleanliness. Cleanroom airflow is generally calculated based on the number of air changes per second, typically 10 times, or even tens of times, the volume of the room. This is especially true for unidirectional flow cleanrooms, where the air change rate can reach hundreds of times the room volume.
High airflow puts a strain on the strength of air handling units. Currently, most air handling units on the market use aluminum alloy frame structures or square steel structures. If the panel thickness and frame strength are insufficient, the air handling unit can easily deform. This is particularly true for medical air handling units, which generally employ positive pressure designs. If the panel and frame are not properly secured, there is a risk of panels flying off and injuring people. Therefore, cleanroom air handling units must first meet high strength requirements. Currently, common panel thicknesses are 30mm and 50mm, and some manufacturers have already launched high-end units with 60mm panels, which are expected to see widespread application in cleanroom environments.
1.2 Fan head height
Cleanrooms typically require at least three stages of filtration: coarse, medium, and high-pressure filters. The combined resistance of these three filters is approximately 700-800 Pa. Cleanrooms also generally employ centralized supply and return air systems to maintain the required positive and negative pressure regulation. Therefore, the duct resistance in cleanrooms is generally more than twice that of ordinary air conditioning systems. Overcoming this resistance requires the air handling unit’s supply fan to have sufficient head. Therefore, cleanroom air handling units typically use backward-curved airfoil fans or volute-less fans to achieve sufficiently high supply and return air head.
Under these conditions of high air volume and high head, the leakage rate of the unit is also a challenge. The lower the leakage rate of the cleanroom air handling unit, the more energy is saved for the customer, and the lower the operating costs.
1.3 High accuracy in temperature and humidity control
Unlike ordinary comfort air conditioners that prioritize personal comfort, cleanrooms require precise temperature and humidity control to meet specific process requirements. For example, the manufacturing of certain electronic products demands extremely strict temperature and humidity control, and medical and laboratory animal applications also have specific requirements for temperature and humidity precision. To achieve constant temperature and humidity, air handling units must possess at least cooling, heating, humidification, and dehumidification functions, and these functions must be precisely controlled. For instance, heat exchangers must use high-efficiency hydrophilic fins, and water flow must be controlled using proportional-integral (PI) control. Humidification output must also employ PI or PID regulation to achieve higher control accuracy.
1.4 Strict control of positive and negative pressure
Whether in electronics factories, isolation wards, pharmaceutical plants, or laboratory animal rooms, maintaining positive and negative pressure in cleanrooms is crucial to prevent the spread of dust and bacteria to other clean areas, and to prevent cross-infection caused by the spread of viruses and bacteria. This is especially true in medical settings where it is necessary to prevent the outward spread of radioactive dust, harmful gases, odors, and bacteria; accurate and effective control of positive and negative pressure is paramount. In practical engineering applications, industrial cleanrooms and general biological cleanrooms are maintained with positive pressure, while biological cleanrooms using toxic or harmful gases, flammable or explosive solvents, or other special requirements employ negative pressure control. To achieve accurate differential pressure control, cleanroom air handling units must have a low air leakage rate to ensure high control precision. Furthermore, centralized supply and return air distribution sections will be widely used in air conditioning units.
1.5 Possesses a good filtration system
Whether it’s an industrial cleanroom meeting process design requirements or a bioengineering cleanroom for medical, pharmaceutical, or laboratory animal applications, the common characteristic is the need for a dust-free environment, which requires a high-quality air filtration system. The degree to which cleanroom technology controls microorganisms and dust depends primarily on the performance of the filters. Cleanrooms generally require at least three stages of filtration: air handling units are equipped with coarse and medium-efficiency filters, and high-efficiency filters are installed at the air supply terminals. Air filters must be of high quality; once a leak occurs, achieving a clean environment becomes impossible. Besides being leak-free themselves, the sealing of the filters within the air conditioning unit is also crucial.
1.6 Adopting frequency conversion technology
The high air volume characteristic of cleanroom air conditioning systems leads to high energy consumption, prompting the widespread application of various energy-saving technologies. Among these, variable frequency technology (VFD) is widely used. Cleanroom air conditioning systems typically use at least coarse, medium (sub-HEPA), and HEPA filters. These filters increase in resistance over time. Unlike conventional air handling units where the fan speed is selected based on the filter’s calculated resistance, cleanroom air handling unit fan motors are selected based on the filter’s final resistance. Initially, the fan pressure is sufficient to overcome the system’s resistance and meet requirements. At this stage, using a VFD reduces fan speed, decreasing power consumption and achieving energy savings. As the system continues to operate, filter resistance increases, reducing airflow. Changes in airflow or static pressure in the ductwork are fed to the VFD, which increases fan speed to meet system airflow requirements. This also helps regulate positive and negative pressure within the room. Alternatively, by using a variable frequency fan, the unit’s speed can be reduced when the cleanroom is not in use, thus reducing the air volume of the blower, which is equivalent to using a standby fan.
1.7 Competitive stability and reliability
The rapid development of modern industry has resulted in industrial products characterized by small size and high value. If a cleanroom air conditioning unit malfunctions, it can lead to losses amounting to thousands of dollars, especially in medical cleanrooms. In surgical or delivery rooms, a malfunction could even be life-threatening. Therefore, air conditioning units must possess considerable stability and reliability. Every component of the air handling unit should be sourced from manufacturers with excellent quality management. For example, high-quality fans and motors with AMCA certification should be used inside the unit. The selection of filters, especially HEPA filters, is also extremely cautious; even a small pore can allow countless bacteria to enter, with unimaginable consequences. Furthermore, the production and testing of air handling units must strictly adhere to national standards.
These are common characteristics of cleanroom air conditioning. It is clear that cleanroom air handling units differ from ordinary air conditioning units.
2. Characteristics of Industrial Cleanrooms
2.1 High Cleanliness Requirements
Industrial cleanrooms need to meet process requirements. As product performance indicators continue to improve, the cleanliness of cleanrooms also needs to be continuously increased. This requires the use of ultra-high efficiency filters, which necessitates that air handling units be equipped with at least a coarse, medium, and sub-high efficiency filter system. This places higher demands on the strength of the units and requires a larger pressure head from the air handling unit’s blower.
2.2 Temperature control takes priority
Industrial cleanrooms generally require human operation, so air handling units should prioritize temperature control while ensuring adequate humidity control to meet the temperature and humidity requirements of personnel and thus guarantee product quality.
2.3 Meet the requirements of special processes
For example, some products require the use of chemical agents, so the unit needs to be corrosion resistant; others require the use of explosive gases, which requires the air handling unit to be explosion-proof and anti-static; some processes have very high requirements for vibration, so the air handling unit needs to have better vibration protection measures.
3. Characteristics of air conditioning systems in biological cleanrooms
3.1 A significant characteristic of air conditioning systems in biological cleanrooms is the prevention of cross-infection. This is manifested in the air handling units (HJUs). Specifically, HJUs used in biological cleanrooms are required to employ a full positive pressure design. This means placing the supply fan of the HJU at the very front, with the remaining functional sections in the positive pressure zone to prevent outside airflow from entering the HJU and causing cross-infection.
It is important to note that rotary heat recovery and plate heat recovery systems are not suitable for hospital settings. Although hospitals are high energy consumers, the working principle of rotary heat recovery, which involves exchanging exhaust and fresh air through a rotating wheel, can easily lead to cross-infection. In biological cleanrooms, heat pipe heat recovery and coil heat recovery technologies are widely used because they do not involve contact between fresh and exhaust air.
3.2 The second significant characteristic of air conditioning systems in biological cleanrooms is the prevention of secondary contamination. This characteristic is reflected in the following key points regarding air handling units:
3.2.1 The internal wall panels and internal components of the air handling unit should be kept smooth and resistant to corrosion from disinfectants. Stainless steel is recommended, and the right angles at the bottom of the unit should be rounded for easy cleaning.
3.2.2 Humidity control should be prioritized in air handling units to prevent bacterial growth on the surfaces of ducts and HEPA filters due to dampness.
3.2.3 Heat exchangers and filters should be placed in the positive pressure section. The fresh air valve should generally be closed immediately upon shutdown to prevent condensation on the internal surfaces of the unit. Even after the surface cooler is shut down, air should continue to be supplied to dry the water in the coils and condensate pan, allowing the internal wall temperature to gradually rise and preventing bacterial growth. Placing the filters in the positive pressure section also prevents them from becoming damp, effectively preventing bacterial growth.
3.2.4 The heat exchanger uses hydrophilic fins; the number of fins per inch should not be excessive, otherwise dust will easily accumulate on the heat exchange fins. A non-condensing coil is used. As long as the unit’s inlet water temperature is higher than the difference between the indoor dew point temperature and the heat transfer temperature difference, the heat exchange coil in the air conditioning unit can be guaranteed to operate in dry condition. This needs to be integrated with the air conditioning system to achieve, with fresh air handling the indoor humidity load.
3.2.5 Air handling units used in biological clean rooms generally do not use baffles, which requires the face velocity of the coil to be generally less than 2.5 m/s.
3.2.6 The electric heating element is made of stainless steel tubing – smooth, finless, corrosion-resistant, not prone to dust accumulation, and easy to clean.
3.2.7 Wet film humidifiers must never be used in hospital settings. Wet film humidifiers have very low initial investment and operating costs, and are widely used in civil buildings where humidification is needed but humidity control requirements are not high. However, since the humidification principle of evaporative cooling membranes relies on the exchange of air and water on the membrane, and a damp membrane provides an ideal environment for bacterial growth, it is absolutely unacceptable for biological cleanroom air conditioning systems, where secondary contamination must be strictly prevented. Therefore, evaporative cooling membranes are strictly prohibited in hospitals, pharmaceutical manufacturing facilities, and other biological cleanrooms. Dry steam humidification, electrode humidification, and electrothermal humidification, which do not produce condensate, are widely used in biological cleanroom air conditioning systems.
3.2.8 Fan coil units, wall-mounted units with integrated heat and cold sources, and floor-standing units are not suitable for use in operating rooms, delivery rooms, ICUs, infection isolation rooms, various laboratories, radiology departments, and similar locations. Fan coil units and wall-mounted units all have a condensate pan, and the damp environment inside is also conducive to bacterial growth.
3.3 The unit must have good sterilization and disinfection capabilities.
3.3.1 The air handling unit must have at least two stages of filtration. Filters can effectively remove airborne dust and block the spread of bacteria and viruses in the air conditioning system. High-efficiency filters, in particular, can achieve virtually 100% capture efficiency for airborne viruses and bacteria. Filters that effectively prevent bacterial growth can also be used, such as filters containing the antibacterial agent INTESPT or titanium dioxide filters. Filters must be replaced regularly.
3.3.2 Add an electronic sterilization device to the unit’s return air vent.
3.3.3 Coarse filters are generally disposable and should be discarded after a period of use.
3.3.4 The unit should be equipped with ultraviolet (UV) germicidal lamps. The sterilization mechanism of UV irradiation is to cause chemical changes in the nucleic acids, protoplasmic proteins, and enzymes within microorganisms, leading to their death. UV disinfection has a broad-spectrum bactericidal effect and can kill various microorganisms. UV germicidal lamps are generally installed in the downstream section of the air handling unit, such as the coil section and the air-facing side of the filter section.
3.4 High degree of automation. Because different departments in a hospital have varying usage times, required temperature and humidity levels, and different loads, the air conditioning system needs to be finely zoned, with independent control and adjustment for each room. This necessitates medical air handling units equipped with high-configuration control systems.
3.5 Preventing noise, equipment vibration, and high airflow from the air conditioning system from irritating patients. Since hospital patients have relatively weak immune systems, noise and equipment vibration can cause them distress and affect treatment effectiveness. Therefore, hospital air handling units require better vibration and noise reduction measures.
3.6 Pollution and cross-contamination caused by the production of special pharmaceutical products can seriously affect other medications, requiring strict separation. Examples include penicillin, contraceptives, radioactive drugs, vaccines, and blood products. This is particularly important when selecting air handling units. A thorough understanding of the production process is crucial, and any unreasonable air conditioning systems should be reported to the design institute and the owner.
3.7 The exhaust fan should be located at the end of the exhaust duct as much as possible to make the entire exhaust duct negative pressure and discharge pollutants such as harmful substances and radioactive aerosols. A HEPA filter should be installed at the exhaust inlet.