Clean room (clea nr oom is an airborne particle control room designed and used to minimize the intake, emission and retention of particles within the room, while allowing other parameters, such as temperature, humidity, to be controlled as needed. and pressure.
In such premises, the content pollutants in the air, on wall and ceiling surfaces should be kept to a minimum.
Specified particles may include materials such as dust, anesthetic waste gases, and micro-organisms.
Extremely clean indoor air can only be achieved by extracting indoor air and supplying filtered, conditioned expellant air.
In addition, as in the classical system, comfort parameters such as temperature, relative humidity, noise level, air pressure and velocity, as well as the minimum outdoor air flow must be controlled.
Technology clean rooms serves the following tasks:
- protection of products from contamination;
- protection environment from pollution;
- creating a protective environment for people in the room;
- protection of indoor occupants from human-borne microbes;
- protecting the environment from hazardous products;
- protection of the environment from microbes carried by humans.
A clean room requires clean atmosphere , clean gas, clean surfaces, clean equipment, clean products and clean technology.
No projects or investments should be made until the hygiene requirements for the cleanroom have been determined.
It is necessary to ensure guaranteed hygienic quality and maintain the required degree of cleanliness of the air in the room (not necessarily the highest possible).
High hygienic quality can be ensured by implementing an expensive protection project.
The basic approach should be to meet hygiene requirements where necessary, most inexpensive ways and with maximum efficiency, but only to the extent necessary to specific premises.
Implementation Affecting Options necessary conditions, can be divided into two groups: provisioning parameters comfort and hygiene.
The criteria for comfortable air parameters are:
- acceptable temperature range;
- acceptable moisture content;
- required flow rate of supplied air (l/s);
- allowable level noise.
These parameters are important for the assimilation of heat releases from external and internal sources, as well as for compensating for heat losses and for ensuring comfortable conditions in the room.
Criteria for hygienic air parameters:
- ensuring the concentration of microorganisms within the specified limits;
- removal of pollutants from the premises, such as escaping gases;
- control of air movement in the room.
The parameters for maintaining hygienic conditions are the concentration of microbes and polluting gases, as well as the movement of air between rooms.
In this regard, the concentration of pollutants should be at a minimum required level, air movement between rooms must be controlled.
However during design, consideration of these parameters in their totality should be carried out. For the assimilation of heat surpluses, providing required quality air, the amount of conditioned air should be checked, as well as the amount of displacing air required to keep the concentration of microorganisms in the room below a certain level.
Areas of application for cleanrooms
Cleanrooms are used in areas such as medicine, microelectronics, micromechanics and the food industry.
In medicine, operating rooms, drug preparation rooms, biochemical and genetic laboratories are cleared of particles and microorganisms.
Cleanrooms are used in microelectronics, space technology, thin film technology, the printed circuit industry and related areas where the removal of contaminants is required.
In the food industry from industrial premises both pollutant particles and microorganisms are removed.
Clean room with turbulent airflow
Terms used in cleanroom literature
living microorganisms. Bacteria, fungi and viruses fall into this category. Microorganisms can develop in the form of colonies in air, water and especially in cracks and on rough surfaces. The most common source of microorganisms is the human body, which spreads about 1,000 types of bacteria and fungi.
Contaminants other than microorganisms. Atmospheric substances and substances other than microorganisms are present in the atmosphere as a result of the action of wind, earthquakes and volcanic activity. These are usually referred to as dust or aerosol. This group includes smoke particles from industrial processes, building heating systems and vehicle exhaust emissions. The same group also includes particulate matter, which originates from the moving parts of machines in cleanrooms. In addition, as a result of the actions of people in a clean room, about 100,000 particles smaller than 3 microns are released into the air of this room.
Sterility. This is how you can characterize the situation in the room, in which products and devices are free of microorganisms.
Sterilization. A technique for destroying or killing microorganisms in products or devices.
HEPA filters (high efficiency particulate air filter - high efficiency aerosol filter). Such filters are a kind of high efficiency air filters. They are used directly in air handling units, as well as at the end points of the air supply to the room as the final purification stage. The efficiency of these filters for 0.3 µm particles ranges from 97.8 to 99.995%. Such filters are designed for rooms with a cleanliness class of 100-100,000.
ULPA filters (also known as ULTRA-HEPA). These are very effective special air filters. The efficiency of these filters for 0.3 µm particles ranges from 99.999 to 99.99995%. Such filters are designed for rooms with cleanliness class 1-100.
DOP test. Testing the effectiveness of HEPA filters in real conditions after installation.
Clean rooms with turbulent air flow. In these cleanrooms, conditioned air is supplied through HEPA filters located directly in the false ceiling. The air return openings are at floor level. This cleaning method is designed for rooms with a cleanliness class of 10,000-100,000 (Fig. 1).
Clean rooms with laminar air flow. In this method, a stream of air flowing at a constant speed carries the contaminants to the return air duct and then to the air handling unit. This method is suitable for rooms with cleanliness class 1, 10, 100, 1000
Clean rooms with laminar airflow
Airlock. At the entrance to the cleanroom, there must be an air lock that allows access to the room in accordance with current regulations. The airlock is a small chamber with two doors, which is supplied with conditioned air through two HEPA filters.
Cleanliness class. Depending on the type of production that must be carried out in a clean room, the class of cleanliness of this room is determined. Various standards are used to classify cleanrooms. Currently, Germany uses VDI 2083, France uses US 209 in AFNOR 44001, and England uses BS 5295.
In a cleanroom, all equipment and all systems (including air handling units, air ducts, duct equipment) must be able to be cleaned, replaced and after-sales service.
In rooms that require a high degree of sterility, three-stage filtration is used:
- First stage filter. Designed to keep the air handling unit clean, located in the inlet section of this unit. (Class F4-F5).
- Second stage filter. It is used as a final element for keeping the air duct clean. (Class F7-F9).
- Third stage filter. Placed at the entrance to a clean room to ensure hygienic conditions. (Class H13-H14).
- A hygienic air handling unit must, on the one hand, prevent the penetration of microorganisms and pollutants into the room, and, on the other hand, must prevent the formation and accumulation of foreign substances in its design.
- The systems must have a high degree of tightness, the proportion of air entering the room, bypassing the filter cassettes, must be very small.
- Another point in the system that is susceptible to microbial entry is the drain connection and drain line from the air handling system. At this point, a siphon system with two bends should be installed, which does not have a connection to the city sewer.
- To eliminate the need once again to open the door, a viewing eye must be installed in it, in addition, a lighting system must be provided.
- To prevent the accumulation of micro-organisms and contaminants, air handling units must have very smooth surfaces without cracks or undulations.
- Panel joints must use hygienic sealing elements to prevent the accumulation of contaminants in these places and facilitate maintenance procedures. In addition, differential pressure gauges should be used to enable visual control of the degree of clogging of the filters.
- Air ducts must have smooth surfaces and be made of galvanized steel, stainless steel and similar materials.
- The possibility of condensation is eliminated the right choice thermal insulation thickness. In the duct system, it is important to have a sufficient number of service openings with a good seal.
- Devices for measuring air flow parameters must have service openings with easy access. These devices must provide airflow and room pressure data, even when the filters are clogged.
Cleanroom components
Start-up procedures for cleanrooms. After completion of the testing procedures and commissioning, if the results of these procedures are positive, work can begin in the cleanroom.
The most important tests for a cleanroom are: duct testing for density, air handling units for flow, diffusers for temperature and humidity setpoints, pressure testing, and measurement of particulate matter. Instruments used for these purposes must be recalibrated before testing.
The outdoor air intakes of the air handling systems, exhaust dampers, rating plates, filter labels and all sections of the air handling system must be freely accessible, visually inspected and serviced.
Another important issue is the training of cleanroom personnel. The use of sterile clothing by staff is mandatory.
As with many engineering systems, regular procedures must be carried out in a clean room. Maintenance aimed at ensuring continuous operation without accidents and malfunctions. In order to maintain hygienic parameters at all times, the filters must be regularly checked for blockage before any problems occur in the system.
Air treatment systems for clean rooms .jpg)
INTECH company performs the whole range of works related to the design, supply of equipment and materials, as well as direct installation of engineering equipment complexes and clean room systems for heating, ventilation and air conditioning with a multi-stage, high-quality air filtration (purification) system. Using specialized climatic equipment for maintenance of clean rooms in industries:
- pharmaceutical industry;
- Microelectronics;
- The medicine;
- Biotechnology;
- Laboratories and scientific research;
- Aviation and space industry;
- Medical industry;
- food industry;
- Optics.
Purity classes
Room cleanliness class- these are clearly regulated requirements for the level of various kinds of impurities and particles in the air. Purity classes differ in the number of colony-forming bacteria per unit volume.
On the example of clean rooms of medical institutions, 3 classes of cleanliness are established:
- Premises with the first class of cleanliness must have the lowest concentration of bacteria - no more than 10 bacteria / m3. First-class facilities include operating rooms for transplants, complex orthopedic and cardiac surgery, intensive care and burns, leukemia therapy;
- The second class of cleanliness includes premises with a low level of microbial contamination - within 50-200 bact/m3. These are operating rooms for emergency operations, rooms for operating blocks (including corridors), maternity, prenatal wards, wards for premature and injured children;
- Premises of the third class have a concentration of bacteria of 200-500 pcs/m3. These are intensive care wards for people with heart diseases, newborns, sterilization, children's dressing and treatment rooms.
The task of the climate system for "Clean Rooms"
Technological requirements for ventilation and air conditioning systems for "clean rooms" are as follows:
- Reducing the spread of pathogens, which means removing air pollutants, supplying clean air, protecting the room from microbes and microparticles contained in the air, as well as preventing the entry of air from neighboring less "clean" rooms;
- Control of the required air parameters: temperature, humidity, mobility, as well as the concentration of harmful impurities that do not exceed the MPC;
- Elimination of the generation and accumulation of static electricity to prevent the associated risk of explosion.
Problem solving
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The task of ensuring cleanliness in the room is most effectively solved on the basis of a comprehensive approach that takes into account both the specific features of each particular room (space-planning characteristics, technological purpose, requirements for cleanliness and climatic parameters), and the features that characterize the room as an element of a set of rooms. This provision is reflected in the creation of cleanroom complexes, the main design principles of which are:
- ensuring the required design air exchange;
- preparation of supply air with the required parameters for humidity, temperature and microbiological purity;
- rational organization of air flows from cleaner modules to less clean ones;
- air distribution in modules with the organization of a given direction of its movement, taking into account the characteristics of the room and the technological process;
- highly efficient indoor air purification.
Design of the complex is determined by the specific purpose of clean rooms, their configuration and dimensions, acting regulatory requirements to air environment. AT general view the complexes offered by INTECH are made according to the modular principle and include the following functional systems and elements:
- air preparation, disinfection and distribution system;
- indoor climate control system.
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Raymond K. Schneider, Senior Cleanroom Consultant and CEO of Practical Technology, USA, Fellow of the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE)
The design of ventilation and air conditioning systems for clean rooms has a number of features. Below is an article by a well-known American cleanroom specialist, Mr. Raymond K. Schneider, which analyzes the requirements for ventilation systems for rooms of various cleanliness classes: from 1 to 9. The solutions proposed by the author, based on his great practical experience, deserve careful study and use where possible.
Air conditioning systems for clean rooms must supply purified air in a certain amount in order to maintain a given level of cleanliness in the room. Air is supplied to clean rooms in such a way as to prevent the formation of stagnant zones where dust particles can settle and accumulate. The air must also be conditioned in terms of temperature and humidity in accordance with the requirements for the parameters of the microclimate of the room. In addition, an additional amount of conditioned air is supplied to the room to create excess pressure.
This article discusses the design of air conditioning systems for clean rooms. In order to simplify the presentation of the material, the level of maintenance of cleanliness in the premises is divided into three categories: hard, medium and moderate (see table).
Air exchange
The calculated value of the supply of purified air is maximum for premises with a strict cleanliness regime and decreases as the requirements for purification decrease. Air exchange in rooms is usually expressed either in terms of air mobility in the room, or in terms of multiplicity (rpm / h).
Average indoor air mobility is usually used when the air is supplied through a filter ceiling. For many years, an air mobility of 0.46 m/s ± 20% was accepted as the highest level of purity. This was based on the first clean room designs carried out as part of the 1960–1970 space programs.
Recently, experiments have been carried out with lower velocities, which have shown that air mobility in the range of 0.35–0.51 m/s ± 20% is quite acceptable, depending on the type of activity and the installed equipment. The upper limit of air mobility corresponds to the high activity of personnel and the presence of dust-producing equipment. Lower values are accepted if there is little sedentary work and/or no dust-generating equipment.
Frequently knowledgeable customers with experience in cleanrooms will set low level air mobility values. And customers and novice designers, unaware of the feasibility of lower speeds, set air mobility at the upper end of the scale. There is no unambiguously defined average level of air movement or air exchange rate accepted in the industry for clean rooms according to this classification. The only exception is the FDA (Food and Drug Administration) air mobility value of 0.46±0.1 m/s for sterile areas in the pharmaceutical industry.
The standard air exchange values for clean rooms with medium and moderate air cleanliness are more common. For rooms with an average level of cleanliness, the recommended air exchange is between 30 and 60 rpm / h, while for a moderate level, air exchange can be reduced to 20 rpm / h. The designer chooses the air exchange value based on his experience and understanding of the dust generation in the production process. Recently, there has been a tendency to take lower values of air exchange; leading design and construction firms and prudent customers have successful experience in working with such parameters.
AT practical advice Microclimate Institute (IEST-CC-RP.012.1) has a table of recommended air exchange values for each cleanliness class; similar values were later published in ISO 14644-1, clause 4. These data are given in the table. Both documents are consistent with each other and represent the joint recommendations of designers, builders and users, proven by years of successful work. In all these documents, the responsibility for the choice of parameters rests with the "sellers" and "buyers" of clean rooms, so it is advisable to exercise some caution when using the above recommendations.
Picture 1.
Figure 2.
Filters
For many years, clean room technology has been developed to serve the microelectronics industry. The need for high efficiency air filters is dictated by the needs of this industry and related industries. The ULPA (Ultra High Purity) filter, which has an efficiency of 99.9995% on 0.12 micron particles, has been successfully used in harsh cleanrooms. Higher efficiency filters exist, but they are expensive and not widely used. Filters with 99.99% and 99.999% efficiency are available from several manufacturers; experience shows that they can also be used for hard mode.
HEPA (High Efficiency Purification) filters with 99.97% efficiency on 0.3 micron particles have been the workhorse of the clean room industry for many years. They are still widely used in the pharmaceutical industry, where the requirements for air purity are even more stringent.
When filters were laboratory tested with accurate particle counts, HEPA/ULPA filters were found to generally pass 0.1-0.2 microns. At the same time, the passport efficiency of filters for fractions of 0.12 and 0.3 microns was confirmed, and an even higher efficiency was found for particles that are larger and smaller than the indicated sizes. For the strict regime of purity standardization, when setting the filter efficiency, it is customary to indicate not the values of 0.12 and 0.3 microns, but the particle size of the fraction that is filtered worse than the others (MPPS). MPPS values vary slightly between filter manufacturers. Specifying the efficiency by the size of the worst filtered particles is considered by some designers and manufacturers to be the most convenient.
Most hard and medium clean rooms are equipped with filters in the ceiling. The filters can be grouped and connected to a common supply unit for easy installation in the ceiling, or they can be installed individually with individual supply ducts. This arrangement, resembling an inverted "T", forms a honeycomb structure under the ceiling. At the same time, the filters are carefully sealed in the housing to prevent the passage of uncleaned air. In addition, filters built into supply chambers are still used. However, the modular schemes that displace them make it possible to better ensure the regulation of air parameters and mobility.
Blocks "filter-fan" are widely used. In some designs, the filter is replaceable, in other cases, the entire unit is replaced at the end of its service life. Various standard sizes are offered for delivery for embedding in a honeycomb structure. The fans are equipped with electric motors designed for different voltages, which makes it possible to use various schemes power supply. Some complex systems controls provide the possibility of individual adjustment of each unit, registration of energy consumption, signaling of malfunctions of electric motors, regulation of groups of filter fans and changing the speed of rotation of fans according to the time of day. Blocks "filter-fan" are used for all classes of clean rooms.
Frontal air velocity for ceiling filters can be from 0.66 to 0.25 m/s, depending on the project. Since the system with cellular placement of filters of the “T” type occupies 20% of the ceiling area, the frontal speed of the filters of 0.51 m/s corresponds to an average speed in the working area of the room of 0.41 m/s.
Installing HEPA/ULPA filters directly in the ceiling of cleanrooms is dictated by the intention to minimize or eliminate the possibility of dust accumulation on any surfaces (for example, on the walls of air ducts) along the path of air from the filter to the cleanroom. The remote placement of HEPA filters is typical for moderate-mode clean rooms, since the amount of particles blown off the walls of the air ducts after the filters is within acceptable limits. The exception is when a standard air conditioning system, not certified for clean rooms, is converted for this purpose in accordance with ISO 14644. In this case, all air ducts after the filters must be thoroughly cleaned.
For moderate duty cleanrooms, fan boxes or mixboxes with HEPA filters on the discharge side are often used. At the same time, the frontal air velocity in the HEPA filters reaches 2.54 m/s, which corresponds to a greater pressure drop than with a ceiling installation. The aerodynamic resistance of a clean HEPA filter with a size of 600x600 mm is 375 Pa at a frontal speed of 2.54 m/s. With a ceiling installation, the frontal speed is 0.51 m/s, the aerodynamic drag is 125 Pa.
Air circulation in clean rooms
The air entering the clean room after being cleaned with HEPA and ULPA filters is practically free of suspended particles. The air supply to the room is made for a dual purpose. Firstly, the "dissolution" (decrease in concentration) of dust pollution arising from the presence of people and the performance of production processes. Secondly, the capture and removal of these contaminants from the premises.
There are three types of indoor air circulation:
1. Unidirectional ordered flow (previously called "laminar"), when the streamlines of all air jets are parallel.
2. Disordered flow (previously called "turbulent"), when the streamlines are not parallel.
3. Mixed flow, when in one part of the room the air jets can be parallel, but not in the other part.
Hard mode cleanrooms typically use unidirectional flow. This is achieved by installing HEPA / ULPA filters throughout the ceiling area and installing a perforated raised floor. The air moves vertically from the ceiling to the floor, is removed through the perforation into the exhaust chamber under the floor. The recirculated air is then fed back into the room through the peripheral recirculation ducts.
If the cleanroom is narrow (4.2-4.6 m), wall-mounted exhaust grilles installed at the bottom are used instead of a raised floor. Air is supplied from above and moves vertically to a level of 0.6–0.9 m, then the flow spreads towards the gratings. Such circulation is considered acceptable for rooms with a strict regime, especially in cases where there has been a conversion of the room into a clean room in the presence of dust in the upper zone.
In rooms with an orderly circulation, the placement of furniture and equipment affects the structure of the air flow. To reduce the impact of these items on the cleanliness of the room, it is necessary to place them in such a way that stagnant zones with dust accumulation do not form.
Random air movement is common in medium duty clean rooms. HEPA filters are placed evenly over the ceiling surface. The air flow is generally directed from top to bottom. However, the direction of individual jets is different and does not fit into a certain pattern. While the supply air contains practically no suspended particles, their appearance and accumulation in the working area of clean rooms depends on the amount of particles generated in the room itself; from reducing the concentration of dust due to air exchange; the intensity of entrainment of particles from the working area. In general, it can be said that the greater the air exchange, the cleaner the air in medium-mode rooms, however, the structure of the air flows in the room also plays a role.
The air removal scheme for rooms with disordered circulation is very important. In such rooms, wall-mounted exhaust grilles are widespread. They should be evenly distributed around the perimeter of the room. This requirement may conflict with the accepted layout of equipment along the walls. Where possible, equipment should be moved away from walls to allow air to flow behind it. It is also advisable to raise the equipment above the floor, placing it on a platform so that the air passes from below. In most cases, cleanroom designers aim to direct the air flow from the countertop to the floor and then to the low exhaust grilles. With this scheme, particles are removed from the room and sent to the filters, where they are captured. An exception may be such cases when particles of pollution are generated by equipment above the working area. Then some device should be used to catch the removal and particles at the top. In the general case, it is recommended to use a top-down air distribution scheme.
In rooms with a medium level of cleanliness, it is reasonable practice to limit the horizontal sections of airflow. The recommended values of horizontal sections are no more than 4.2–4.8 m. Thus, in a room with a width of no more than 8.4–9.6 m, it is permissible to install exhaust grilles along the perimeter of the walls. This limitation is dictated by the fear of secondary pollution during the deposition or other transfer of particles into working area from extended horizontal flows.
In wider rooms, it is customary to install exhaust grilles and air ducts in ducts mounted along the columns. If there are no columns in the room, vertical shafts are created from a suitable material.
In rooms of moderate cleanliness with remote installation of HEPA filters, standard ceiling air outlets of air conditioning systems can be used. The air circulation scheme is also similar to that adopted in air-conditioned rooms.
According to the “top-down” circulation scheme in practice for cleanrooms, the bottom installation of wall-mounted exhaust grilles is also recommended here. When the exhaust grilles are placed at the top, areas with a high concentration of suspended particles can form in the working clean area, especially during periods of intensive work. In the known cases of installation of ceiling exhaust grilles in moderate duty cleanrooms, the success was most likely due to the low level of particle generation in the room, rather than the efficiency of the air distribution system.
Mixed circulation is used when work is performed in the same room with critical and non-critical requirements for air purity. If it is impossible to ensure the performance of work with critical requirements in a separate room, then a common clean room with cleanliness zoning can be used. Zones are created by appropriate grouping of ceiling filters. In the zone with critical conditions for purity, the number of filters is greater, in the zone with non-critical conditions - less. In addition, the supply of fresh air can be carried out in such a way that it is first supplied through the air ducts to the critical zone, and then enters the rest of the room. Depending on the height of the clean room, a 0.6 m high plexiglass shelter or a plastic curtain that does not reach the floor by 304–457 mm can also be installed.
The direction of the exhaust air flows is regulated by the appropriate placement of the exhaust grilles in such a way as to prevent the transfer of contaminants throughout the room. The raised floor with the extract air collector installed below it will be in this case very efficient. However, the application of such a solution may be hindered by the limited budget of the customer, who chooses the mixed circulation zoned clean room design precisely because of its low cost.
The disadvantage of disordered air circulation in clean rooms is the appearance of areas with high dust content. Such areas may exist for a limited time, then disappear. This occurs due to the interaction of air flows resulting from production activities and disordered supply jets. Attempts have been made to reproduce unidirectional circulation by installing a false ceiling-air distributor and creating a zone of high pressure between the main and false ceiling. For this, perforated plastic or aluminum panels and a screen made of woven and non-woven materials.
As a result, an ordered unidirectional flow was formed in the room with velocities much lower than in clean rooms with a hard regime. The displacement effect created by the supply air flow prevents the formation of dusty areas and, in general, allows you to achieve more high level purity. The specified result, as noted above, is achieved at a lower air mobility than indicated in the standards for hard and medium cleanliness (Fig. 1).
Thermal load
The share of sensible heat in the heat load of clean rooms is typically over 95%. As a rule, year-round cooling is required, since the heat generated by the process equipment and the electric motors of the circulation fans enters the room. A small proportion of latent heat generation is generated by the presence of personnel. A unique project is developed for each clean room, so all the factors influencing heat load, should be carefully analyzed.
In rooms with strict and medium levels of cleanliness, a significant part of the supply air is not treated by air conditioners - this is recirculated air. The required sensible heat removal is carried out in the mixing and distribution chambers, where part of the total flow is cooled in surface heat exchangers and then returned to the general flow to the recirculation fans (Fig. 2). The inlet air temperature to severe cleanrooms can only be a few degrees lower than the exhaust air temperature due to the large inflow volume. This temperature difference makes it possible to use ceiling installation top-down HEPA/ULPA filters without compromising worker comfort.
In rooms with a moderate cleanliness regime, the requirements for air distribution in the room are in some cases the same as in ordinary refrigerated rooms. Thus, the temperature difference between supply and exhaust air can be 8–11 °C. In these cases, standard ceiling diffusers or other means are used to prevent unpleasant blast and ensure comfortable conditions in the room.
Outside air supply
The supply of outside air is necessary to compensate for the exhaust and exfiltration that always occurs in pressurized cleanrooms. Outdoor supply air is expensive, because before being supplied to clean rooms, it must not only be cleaned, but also subjected to temperature and humidity treatment. Since it is not possible to completely eliminate the supply of outside air, for reasons of overall economy and energy saving, its amount should be kept to a minimum.
The air pressure in clean rooms is usually increased relative to the surrounding rooms. As a rule, a pressure drop of 12 Pa is recommended. Higher overpressure causes whistling noise in cracks and difficulty opening doors. In blocks of clean rooms with different classes cleanliness, it is customary to maintain a pressure difference of 5 Pa between adjacent rooms, while a higher pressure is maintained in a room with a higher cleanliness class.
The amount of outside air is determined by summing the exhaust volume for all production processes and increasing the resulting multiplicity by 2 rpm/h. This semi-empirical value is a practice-proven calculated amount of air for the selection of air conditioning equipment. The actual amount of outside air will be variable, depending on door openings, leaks, and the actual operation schedule of the hood.
The outdoor air conditioner is designed to bring its parameters in line with the standards for clean rooms. This means that it must be possible to clean the air, preheat, cool, reheat, dehumidify and humidify.
In clean rooms with a strict regime, three stages of outdoor air purification are often done: preliminary - an ASHRAE filter with an efficiency of 30%, an intermediate filter with a 95% efficiency, and a final one - a HEPA filter. In clean rooms with a medium and moderate regime, as a rule, there are two stages of cleaning: preliminary (30%) and final (95%). From the name it is clear that the final filter is placed at the outlet of the air conditioner.
Preheating is necessary when the outside temperature drops below 4 °C in winter. If the dew point temperature of the air in the clean room is ≥5.6 °C, the surface heat exchanger cools and dehumidifies the supply air. Because workers in strict cleanrooms always wear overalls, the dry bulb temperature can be maintained below 19°C, with a minimum relative humidity for adjusting the regulators is 40%. The second heating is necessary in order to increase the temperature of the supply air after cooling and dehumidification in the heat exchanger. When calculating the amount of heat for the second heating, heat inputs from recirculation fans are taken into account. This is a significant value for clean rooms with a strict regime.
Reducing the surface temperature of the heat exchanger to the level required to keep the room dew point below 5.6°C can be difficult. When dehumidification of supply air below 40% RH is required, various desiccant agents are commonly used.
In the system described here, the outdoor air conditioner is loaded with latent heat and moisture in the room. It is assumed that the parameters of the supply air meet the requirements for the assimilation of latent heat emissions introduced by the room staff and moisture ingress through the clean room fences. It is also assumed that the latent heat load is more or less constant. These assumptions must be checked for each specific project. It is necessary to take into account the conditions in the rooms surrounding the clean room, the parameters of the outdoor climate, the possibility of moisture release from production processes in the room.
In small volume cleanrooms with little outside air demand, the recirculation air coolers in the mixing chambers discussed above can also be used to treat outside air. In this case, a mixture of outdoor and recirculated air is processed. The proportion between these supply air components is controlled by mixing valves depending on the pressure in the clean room. If the pressure drops, the outside air valve opens and the recirculation valve closes. The air from the mixing and distributing chambers is supplied to the circulation fans.
In moderate cleanrooms, the total supply air required can be close to the conditioned air flow. In this case, additional circulation fans are not installed; air is moved through the system only by the fans of one or more air conditioners.
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The new ISO classification of clean rooms is shown on the left. The classification according to US Federal Standard 209E in Anglo-American and metric units is also given. The "Recommendations" column contains three categories according to the classification of the author of this article. Note that "Class 100" can be assigned to a hard mode when the design provides for ordered circulation, or to an average mode if disordered circulation is designed for non-critical conditions. The two columns on the right give recommendations for indoor air mobility (ft/min) and air exchange (rev/h) for medium and moderate modes. |
conclusions
There is a tendency in cleanroom design regulations to assign the functions of a general expert to the designer, capable of fulfilling all the wishes of the customer (as far as he knows). The manuals usually use the expression "a matter of agreement between the buyer and the seller" in order to involve the customer in the decision-making process, since each developer can offer his own version of the project. The effectiveness of the design principle discussed in this article has been proven in practice; This approach, in the opinion of the author, makes it possible to harmonize technical requirements and the possibility of their implementation. These recommendations, like any others, must be adapted in each case to the specific conditions of use.
Reprinted with abridgements from the magazine ASHRAE.
Translation from English O. P. Bulycheva.
Scientific editing performed by Ph.D. tech. Sciences A. P. Inkov
Proper ventilation of cleanrooms is achieved by observing certain conditions in decoration and a thoughtful choice of equipment. A clean room is a room where the concentration of substances suspended in the air is monitored.
A room designed and built to minimize the entry and exit of particles, allowing control of temperature changes, humidity and, in special cases, pressure.
General ventilation requirements
Ventilation systems supply required amount air according to sanitary standards, remove harmful substances. Filter the input stream for achieving the desired cleanliness class maintaining the specified microclimate parameters.
For each factor air exchange volumes are estimated at the design stage. When a higher multiplicity of this parameter is requested to the detriment of cleaning, a recalculation is made to reduce it.
Why is it taken into account:
- Recovery time after contamination
- Air speed
- Temperature and Humidity
- Removal of harmful impurities
The main types of ventilation systems
Based on the requirements for the cleanliness class, the ventilation system for clean rooms is selected from the following types:
- Straight-through
- With recirculation
- Direct flow with heat recovery
- with local areas
- Two-level
The choice is justified by specific factors, taking into account capital costs and energy saving conditions. Local installations usually have a fan and can be located in the room itself or outside it. They are supplemented with HEPA filters, if necessary, chemical, odor neutralizing and others.
Direct flow system
The scheme is simple, air is supplied from the street, then it goes through all the main processing cycles. Economically not profitable due to high energy consumption and high costs for filtration consumables.
With recirculation
One-level system, includes air conditioning for clean rooms with air return from the cleaned area to the treatment. Energy consumption is average.
Direct flow with heat recovery
Skipable in this option through the filters, the air flow in a closed circuit returns heat to the premises.
Two-level
Requirements for cleanroom ventilation in this system justifies the best. If there are several air conditioners, as well as service rooms, there is a breakdown into the central one (only street air enters it) and recirculation air conditioners.
Local with local zones
Used to localize zones with high hygiene requirements. Most often, fan modules with filters are mounted, sometimes special recirculation installations.
Air balance
According to the norms, in technologically clean rooms, air ventilation should be used, for a balanced exchange, hoods, local and general exchange, filters are needed. Resource regulation takes place with valves. correcting air currents.
Cleaning systems in rooms that require an increased degree of disinfection of the atmosphere are multi-stage. A special table indicates the relationship between purity classes and the degree of filtration. Thinner models are protected at the entrance by large ones that will not let insects through.
The finishing barrier is mounted on the wall, ceiling of the clean area, this is required by the technology. As well as the fact that air ducts should not emit small particles, it is better to choose stainless steel.
To summarize, in the case of ventilation of premises there are standard solutions and individual ones. Only specialists can fully calculate which option to choose. Installation under the guidance of professionals will save time, nerves and possibly someone's health.
Construction video
Cleanroom ventilation is one of the most important tasks in maintaining the working environment. Why is ventilation so important? It is air purification that allows you to regulate the condition of the room, the norms of which are prescribed in GOST. There are several criteria according to which a room is assigned to one of nine cleanliness classes, each of which is characterized by the degree of air purification from impurities. Therefore, in technologically clean rooms, ventilation at several levels should be used.
What should be the air in a clean room?
Dust and bacteria are found in any air in the form of aerosol particles. Ventilation of clean rooms allows maintaining the maximum allowed amount of dust and bacteria for a given class of rooms.
draft, dry air or high humidity- enemies of the clean room. Therefore, the ventilation system regulates the state of the air, creating optimal conditions to work in this environment.
The air supply is regulated by automation, which means that there should be no pressure drops caused by the passage of air from one room to another. Thus, the sterility and tightness of the premises is maintained automatically.
The cleanroom air purification system is a complex automated group of filters. Clean room air filters are divided into coarse filters, fine filters and microfilters.
The air is filtered from coarse particles, finely cleaned, and then ultra-finely cleaned in microfilters. Thus, only air that meets the standards of GOST, and therefore free from dust and microorganisms by 99.9%, enters the room.
What is the mechanism of ventilation and air exchange?
In any room, sooner or later, impurities in the form of aerosol particles accumulate. A fresh portion of purified air enters the room in such a way that the flow of fresh air displaces impurities. This is called laminar flow as it is directed in one direction. Several of these flows create air exchange in the room. They are directed either parallel to each other, or, as is often the case in large rooms, in different directions so that the flows do not intersect. AT large rooms flows are regulated so that air enters directly into the working area. The air intakes are lower, "dirty" air moves towards them due to the created ventilation.
Supply and exhaust ventilation system The clean room also includes heat exchangers and an air humidifier. They create a microclimate that is comfortable for humans and maintains an optimal working environment.
Ventilation allows you to maintain constant values of temperature, humidity, eliminates dust and most microorganisms.
Tab. 2. Optimal scheme filter selection used in Switzerland for cleanroom classes according to ISO 14644-1 (GOST R ISO 14644-1)
To date, engineering practice has developed standard solutions, following which allows you to avoid inaccuracies and do without unnecessary capital and operating costs. These typical solutions apply to:
- principles of construction of ventilation and air conditioning systems;
- determining the necessary structure and parameters of the air conditioner;
- selection of the number of filtration stages and filter types;
- determination of the air exchange rate;
- ensuring the necessary temperature and humidity conditions in the room;
- creating thermal comfort for staff.
The experience of the Invar Clean Rooms Testing Laboratory during the certification of projects (DQ stage) and constructed clean rooms (IQ, OQ and PQ stages) also revealed typical errors.
Initial data when designing a ventilation and air conditioning system
Before starting the design, it is necessary to clearly formulate its purpose and determine the initial data. Errors and inaccuracies in this stage will lead to incorrect performance of all work. These source data include:
- requirements for air purity, and for clean rooms - setting the cleanliness class in accordance with GOST ISO 14644-1 or GOST R 52249;
- microclimate parameters for the technological process (temperature and humidity with tolerance limits);
- the number of employees in the premises;
- release of heat and moisture from equipment and processes;
- release of harmful substances;
- the area and height of the premises;
- technology requirements, based on the characteristics of technological processes and performed, applied materials and products;
- pressure differences between rooms and air flow rates (if necessary).
The structure of ventilation and air conditioning systems
Several types of air flows are involved in the ventilation and air conditioning system:
- exhaust - air leaving the room through the system forced ventilation. Part of the exhaust air (L in) can be removed directly into the atmosphere by local exhausts, part can be recirculated;
- external - atmospheric air, taken by the ventilation and air conditioning system for supply to the serviced premises, L n;
- supply - air supplied to the room by the ventilation and air conditioning system, L p;
- recirculation - air mixed with the outside and again sent to the ventilation system, L p;
- removed - air taken from the room and no longer used in it, L y.
Consideration should also be given to air leakage from rooms with high blood pressure(air exfiltration, L e) and air infiltration into the room with reduced pressure, L and. The simplest scheme ventilation and air conditioning is a direct-flow system, when 100% of outside air is supplied to the room (Fig. 1). This system is uneconomical because all the air entering the room passes full cycle preparation - from outdoor air parameters to the required cleanroom air parameters. This system is characterized by high energy consumption and reduced filter life.
where i is the room number. To a certain extent, the performance of this system can be improved by heat recovery (Fig. 2). Due to the recuperation, energy savings for heating up to 60% are achieved.
L n \u003d L p \u003d ΣL pi \u003d ΣL wi \u003d ΣL wi + L e, L y \u003d ΣL wi,
where i is the room number. Direct-flow systems, due to their uneconomical nature, are used only where they are necessary and where air recirculation is unacceptable (working with harmful substances, dangerous pathogenic microorganisms), Ch. 17 . Where possible, systems with recirculation are used, which makes it possible to reduce energy costs by several times compared to direct-flow systems. An example of a single-level system with recirculation is shown in fig. 3.
L in \u003d ΣL in i, L y2 \u003d ΣL in i,
L p \u003d L n + L p \u003d ΣL pk, L y \u003d L y1 + L y2 \u003d L in - L p + L y2 \u003d ΣL in i - L p - ΣL in i, L p \u003d L in - L y1,
where L vmi is the air flow rate of the local exhaust unit from the i-th room; L wi is the flow rate of air supplied to the air conditioner from the i-th room. In cold winter or hot summer conditions, as well as when serving clean rooms with several air conditioners, a two-level system is used. In it, outdoor air is prepared to certain parameters in a separate (central) air conditioner, and then it is supplied to recirculation air conditioners (Fig. 4).
Local filter-ventilation or recirculation installations (Fig. 5) are widely used to create zones with unidirectional air flow, for example, in operating rooms and other critical areas. The above diagrams give a general approach to the design of ventilation and air conditioning systems, they do not cover the whole variety of options for fundamental solutions, which in each case should be developed based on the task at the lowest capital and operating costs.
The types of air flows indicated above must be determined for each room and system as a whole. On this basis, the air exchange balance is calculated, the results of which are drawn up in the form of a table and applied to the schematic diagram of ventilation and air conditioning (Fig. 6). To regulate the balance of air exchange, it is advisable to install valves on the supply and exhaust.
The point of building an air exchange balance is to check that the total volume of air entering the room must be equal to the total volume of air removed from the room. Violation of this condition leads to the impossibility of providing the required pressure drops, difficulties in opening and closing doors, etc. For clean rooms, this plays a role special role, because it is necessary to maintain different pressure in different rooms.
In the air exchange balance table, the total flow of supply air and the total flow of exhaust air must be equal for each room (for each line of the table). For each cleanroom, the calculation of supply and exhaust air is performed, and air leaks are also taken into account (exfiltration - air leakage into rooms with lower pressure, air infiltration - air intake from a room with more high pressure). The main input data for the development of the design of the ventilation and air system for clean rooms:
- planning solutions indicating cleanliness classes and pressure drops;
- purpose of clean rooms (clean zones): protection of the product and process, protection of personnel and the environment;
- release of harmful substances;
- release of heat and moisture from the equipment;
- number of staff;
- characteristics of the climate of the construction area.
The outdoor air flow is calculated from the need:
- fulfillment of sanitary and hygienic standards;
- compensation for the removed air (both from individual rooms due to the operation of exhaust units, and removed through the air conditioning system);
- compensation for leaks due to pressure differences between clean rooms and the environment.
Outside air flow for the entire ventilation system is equal to the sum air flow for each room. The air consumption for an individual room is equal to the sum of the volumes of air removed by local exhaust units, and losses due to leaks. This amount should not be less minimum flow outdoor air according to regulations.
Supply air calculation for each room
The supply air performs the following functions:
- ensuring the required class of cleanliness;
- ensuring the requirements for microbiological purity of air where they are presented;
- supply of the required amount of outside air;
- removal of excess heat and moisture and maintaining the required parameters of the microclimate in the room;
- compensation for air leaks due to pressure drops.
The required air exchange rate is affected by all of the supply air functions listed above. For each of them, the required air exchange rate is determined and highest value included in the project. Let's take a look at each of these functions.
Cleanliness class
It is provided by multi-stage air filtration and selection of filters of the appropriate classes, setting the air flow rate (for unidirectional air flow), air exchange rate.
Air exchange rate
Sets the air flow rate for ISO class 6-9 cleanrooms (zones B, C, D). For zone A, the air flow is determined by the speed of the unidirectional flow. There are several approaches to determining the air exchange rate to ensure cleanliness:
- use of various recommendations, standards and regulations;
- calculation method.
Removal of excess heat and moisture
Technological equipment and personnel generate heat and moisture, which must be removed using a ventilation and air conditioning system. Ensuring the necessary microclimate with maintaining temperature and humidity - important condition ensuring the normal work of personnel in clean rooms. In addition, certain technological processes (for example, photolithography in the production of microcircuits) impose strict requirements on temperature and humidity.
Extraction system compensation
The total volume of exhaust air for this room. The quotient of dividing it by the volume of the room gives the air exchange rate necessary to compensate for the hoods.
Leak Compensation
The pressure difference between different rooms causes exfiltration (leakage) of air from the room through the gaps in the door porches and various kinds of leaks. The amount of leakage must be calculated for each room and taken into account in the air exchange balance. The air leakage must be compensated by an equal amount of outside air in the supplied supply air. Air infiltration must also be taken into account in the balance of air exchange, i.e. air intake from neighboring rooms.
Air exchange rates in general premises
In such rooms, the calculation of the air exchange rate is carried out in accordance with sanitary standards and according to the calculations of excess heat and moisture. Western countries use the following values air exchange rates (data from Airflow, England) for some rooms (Table 1).
Selecting Filter Types
Typically, air preparation systems for clean rooms are performed in three stages:
- first stage: medium efficiency type F filter to protect the air conditioner from pollution;
- second stage: high-efficiency filter type F to ensure cleanliness in the air ducts;
- third stage: HEPA or ULPA filter to guarantee high air quality directly to cleanrooms.
In addition, the use of a three-stage air filtration system guarantees a long service life for HEPA and ULPA filters. Recommendations for the optimal selection of filters are presented in Table. 2.
Typical errors
Purity classes
The most common misconception is the requirement to produce non-sterile medicines in clean rooms. It is generated by the notorious and illiterate OST 42-510-98 and previous documents of the same kind. Nowhere in the world is there a requirement to release non-sterile forms in clean rooms! The only document that gives specific data on the cleanliness of the supply air in the production of solid forms is the Guidelines of the International Organization of Pharmaceutical Engineers (ISPE).
It provides recommendations on the performance of final filters for various process steps. In world practice, these recommendations are widely used without specifying purity classes. Nobody forbids the use of cleanrooms, and many specify the production of solid forms in zones D, and liquid non-sterile forms - in zones C. But which way to choose - to use cleanrooms or simply limit yourself to a certain level of cleanliness of the supply air and the quality of the enclosing structure is up to you. customer.
This logic is followed by the EU GMP Rules (GOST R 52249) and US guidelines. If one wants to force a facility to apply an optional cleanliness class, we recommend a simple and effective remedy: legalize this coercion so that the costs of it are borne by the initiator himself. No arguments (such as “our “advanced” neighbors do this”) should not be taken into account.
The overestimation of cleanliness classes in sterile production is also widespread. There is one more factor to keep in mind. Other design organizations artificially inflate cleanliness classes and sizes of clean zones. The cost of the project and the fee of the performers directly depend on the cleanliness classes and the volume of costs. In the practice of the author, there was a project in which the release of particles by the staff was overestimated by 100 times!
Unreasonably stringent temperature and humidity requirements
There are, for example, requirements to maintain an air temperature of 22 ° C with an accuracy of ± 1 ° C and humidity within 45-50% without justification from the technological process. A simple expansion of the limits of regulation of microclimate parameters within the framework of existing standards can significantly simplify the entire system.
Unjustified use of once-through systems
Previously, under the conditions of a costly public financing mechanism, direct-flow systems were widely used, even where they were not needed. In world practice, air recirculation is used wherever it is permissible from the point of view of safety. Otherwise, recirculation heats the outdoor air in winter and cools it in summer, i.e. significant costs fly literally into the pipe.
Overestimation of the air exchange rate Incorrect choice of filters
Projects often include low filter classes (eg G3) in the first filtration stage. This increases the dust load on the downstream filters and shortens their service life.
Lack of a circuit diagram and a table of air exchange balances
It is impossible to judge the project without them. Their development is required. These errors are typical examples and do not exhaust the entire list of shortcomings encountered in practice.
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