Over the past ten years, abroad and in our country, the number of purulent-inflammatory diseases has increased due to infections that have acquired the name "nosocomial" (HAI) - as defined by the World Health Organization (WHO). According to the analysis of diseases caused by nosocomial infections, it can be said that their duration and frequency directly depend on the state of the air environment in hospital premises. In order to ensure the required microclimate parameters in operating rooms (and industrial clean rooms), unidirectional air diffusers are used. As the results of environmental monitoring and analysis of the movement of air flows have shown, the operation of such distributors can provide the required microclimate parameters, however, it negatively affects the bacteriological composition of the air. To achieve the required degree of protection of the critical zone, it is necessary that the air flow that exits the device does not lose the shape of the boundaries and maintain a straight line of movement, in other words, the air flow should not narrow or expand over the zone selected for protection, in which the surgical table is located.

In the structure of a hospital building, operating rooms require the greatest responsibility due to the importance of the surgical process and the provision of the necessary microclimate conditions for this process to be successfully carried out and completed. The main source of the release of various bacterial particles is directly the medical staff, which generates particles and releases microorganisms while moving around the room. The intensity of the appearance of new particles in the air space of the room depends on the temperature, the degree of mobility of people, the speed of air movement. The HBI, as a rule, moves around the room of the operating room with air currents, and the probability of its penetration into the vulnerable wound cavity of the operated patient never decreases. As observations have shown, improper organization of the ventilation systems usually leads to such a rapid accumulation of infection in the room that its level may exceed the permissible norm.

For several decades, foreign experts have been trying to develop system solutions to ensure the necessary conditions for the air environment of operating rooms. The air flow that enters the room must not only maintain the parameters of the microclimate, assimilate harmful factors (heat, smell, humidity, harmful substances), but also maintain the protection of selected areas from the possibility of infection entering them, and therefore ensure the required air purity in operating rooms. . The area in which invasive operations are carried out (penetration into the human body) is called the "critical" or operating area. The standard defines such a zone as an "operating sanitary protection zone", this concept means the space in which the operating table, equipment, instrument tables are located, and the medical personnel are located. There is such a thing as a "technological core". It refers to the area in which production processes are carried out under sterile conditions, this area can be semantically correlated with the operating room.

In order to prevent the penetration of bacterial contamination into the most critical areas, screening methods based on the use of air displacement are widely used. To this end, laminar air flow distributors have been developed with various designs. Later, "laminar" became known as "unidirectional" flow. Today you can find a variety of names for air distribution devices for clean rooms, for example, “laminar ceiling”, “laminar”, “clean air operating system”, “operating ceiling” and others, but this does not change their essence. The air distributor is built into the ceiling structure above the protected area of ​​the room. It can be of various sizes, it depends on the air flow. The optimal area of ​​such a ceiling should not be less than 9 m 2 so that it can completely cover the area with tables, personnel and equipment. The displacing air flow in small portions slowly flows from top to bottom, thus separating the aseptic field of the operating area, the area where the sterile material is transferred from the environment. Air is removed from the lower and upper zones of the protected room at the same time. HEPA filters (class H according to ) are built into the ceiling, which let the air flow through them. Filters only trap living particles without disinfecting them.

Recently, at the world level, attention has increased to the issues of air disinfection in hospitals and other institutions in which there are sources of bacterial contamination. The documents set out the requirements that it is necessary to decontaminate the air of operating rooms with a particle deactivation efficiency of 95% or more. Equipment for climate systems and air ducts are also subject to disinfection. Bacteria and particles emitted by surgical personnel enter the room air continuously and accumulate in it. In order to prevent the concentration of harmful substances in the room from reaching the maximum permissible level, it is necessary to constantly monitor the air environment. This control is carried out without fail after the installation of the climate system, repair or maintenance, that is, at the time when the clean room is used.

It has already become customary for designers to use ultra-fine unidirectional flow air distributors with built-in ceiling-type filters in operating rooms.

Air streams with large volumes slowly move down the premises, thus separating the protected area from the surrounding air. However, many specialists do not worry that these solutions alone are not enough to maintain the required level of air disinfection during surgical operations.

A large number of design options for air distribution devices have been proposed, each of them has received its application in a certain area. Special operating rooms among themselves within their class are divided into subclasses depending on the purpose according to the degree of cleanliness. For example, operating rooms for cardiac surgery, general, orthopedic, etc. Each class has its own cleanliness requirements.

For the first time, air diffusers for clean rooms were used in the mid-1950s. Since that time, the distribution of air in industrial premises has become traditional in cases where it is necessary to ensure reduced concentrations of microorganisms or particles, all this is done through a perforated ceiling. The air flow moves in one direction through the entire volume of the room, while the speed remains uniform - approximately 0.3 - 0.5 m / s. The air is supplied through a group of high efficiency air filters placed on the ceiling of the clean room. The air flow is supplied according to the principle of an air piston, which rapidly moves down through the entire room, removing harmful substances and pollution. Air is removed through the floor. This air movement can remove airborne contaminants from processes and people. The organization of such ventilation is aimed at ensuring the necessary cleanliness of the air in the operating room. Its disadvantage is that it requires a large air flow, which is not economical. For clean rooms of class ISO 6 (according to ISO classification) or class 1000, air exchange of 70-160 times / h is allowed. Later, more efficient modular-type devices came to replace them, having smaller dimensions and low costs, which allows you to choose a supply device, starting from the size of the protection zone and the required air exchange rates in the room, depending on its purpose.

Operation of laminar air diffusers

Laminar devices are designed for use in cleanrooms for the distribution of air of large volumes. For implementation, specially designed ceilings, room pressure regulation and floor hoods are required. When these conditions are met, laminar flow distributors will necessarily produce the required unidirectional flow having parallel streamlines. Due to the high air exchange rate, conditions close to isothermal are maintained in the supply air flow. Designed for air distribution in large air exchanges, the ceilings provide a low starting flow rate due to their large surface area. The control of changes in air pressure in the room and the result of the operation of the exhaust devices ensure the minimum dimensions of the air recirculation zones, here the “one pass and one exit” principle works. Suspended particles fall to the floor and are removed, so their recycling is almost impossible.

However, in operating room conditions, such air heaters work somewhat differently. In order not to exceed the permissible levels of bacteriological purity of the air in operating rooms, according to calculations, the air exchange values ​​are about 25 times / h, and sometimes even less. In other words, these values ​​are not comparable with the values ​​calculated for industrial premises. In order to maintain stable air flow between the operating room and adjacent rooms, the operating room is pressurized. Air is removed through exhaust devices, which are installed symmetrically in the walls of the lower zone. To distribute smaller volumes of air, laminar devices of a smaller area are used, they are installed directly above the critical zone of the room as an island in the middle of the room, and do not occupy the entire ceiling.

Observations have shown that such laminar air diffusers will not always be able to provide unidirectional flow. Since the difference between the temperature in the supply air jet and the ambient air temperature of 5-7 °C is inevitable, the colder air leaving the supply unit will fall much faster than a unidirectional isothermal flow. This is a common occurrence for ceiling diffusers installed in public areas. The opinion that laminars provide unidirectional stable airflow in any case, regardless of where and how they are used, is erroneous. Indeed, in real conditions, the speed of a vertical low-temperature laminar flow will increase as it descends to the floor.

With an increase in the volume of supply air and a decrease in its temperature relative to the room air, the acceleration of its flow increases. As shown in the table, thanks to the use of a laminar system with an area of ​​3 m 2 and a temperature difference of 9 ° C, the air velocity at a distance of 1.8 m from the outlet is increased three times. At the outlet of the laminar device, the air velocity is 0.15 m/s, and in the area of ​​the operating table - 0.46 m/s, which exceeds the permissible level. Many studies have long proved that with an increased speed of the supply flow, its “unidirectionality” is not preserved.

Lewis (Lewis, 1993) and Salvati (1982) analyzes of air control in operating rooms revealed that in some cases, the use of laminar units with high air velocities causes an increase in the level of air contamination in the area of ​​the surgical incision, which can lead to to his infection.

The dependence of the change in air flow rate on the temperature of the supply air and the area of ​​the laminar panel is shown in the table. When air moves from the starting point, the streamlines will run parallel, then the flow boundaries will change, there will be a narrowing towards the floor, and, therefore, it will no longer be able to protect the zone that was determined by the dimensions of the laminar installation. Having a speed of 0.46 m/s, the air flow will capture the inactive air of the room. And since bacteria are continuously entering the room, infected particles will enter the air stream leaving the supply unit. This is facilitated by air recirculation, which occurs due to the overpressure of air in the room.

To maintain the cleanliness of operating rooms, according to the norms, it is necessary to ensure an air imbalance by increasing the inflow by 10% more than the extract. Excess air enters adjacent, untreated rooms. In modern operating rooms, hermetic sliding doors are often used, then excess air cannot escape and circulates around the room, after which it is taken back into the supply unit using built-in fans, then it is cleaned in filters and re-supplied to the room. The circulating air flow collects all pollutants from the room air (if it moves near the supply air flow, it can pollute it). Since there is a violation of the boundaries of the flow, it is inevitable that air is mixed into it from the space of the room, and, consequently, the penetration of harmful particles into the protected sterile zone.

Increased air mobility entails intensive exfoliation of dead skin particles from open areas of the skin of medical personnel, after which they enter the surgical incision. However, on the other hand, the development of infectious diseases during the rehabilitation period after surgery is a consequence of the hypothermic state of the patient, which is aggravated by exposure to moving cold air flows. So, a well-functioning traditional laminar flow air distributor in a cleanroom can bring not only benefits, but also harms during the operation carried out in a conventional operating room.

This feature is typical for laminar devices with an average area of ​​​​about 3 m 2 - optimal for protecting the operating area. According to American requirements, the air flow rate at the outlet of the laminar device should not be higher than 0.15 m / s, that is, from an area of ​​\u200b\u200b0.09 m 2, 14 l / s of air should come into the room. In this case, 466 l / s (1677.6 m 3 / h) will flow, or about 17 times / h. Since, according to the normative value of air exchange in operating rooms, it should be 20 times / h, according to - 25 times / h, then 17 times / h is quite consistent with the required standards. It turns out that the value of 20 times / h is suitable for a room with a volume of 64 m 3.

According to current standards, the area of ​​the general surgical profile (standard operating room) should be at least 36 m 2 . However, higher requirements are imposed on operating rooms intended for more complex operations (orthopedic, cardiological, etc.), often the volume of such operating rooms is about 135 - 150 m 3. For such cases, an air distribution system with a large area and air capacity will be required.

If airflow is provided for larger operating theaters, this leads to the problem of maintaining a laminar flow from the outlet level to the operating table. Air flow studies were carried out in several operating rooms. In each of them, laminar panels were installed, which can be divided into two groups according to the occupied area: 1.5 - 3 m 2 and more than 3 m 2, and experimental air conditioning units were built, which allow you to change the value of the supply air temperature. In the course of the study, measurements were made of the speed of the incoming air flow at various flow rates and temperature changes; these measurements can be seen in the table.

Criteria for cleanliness of operating rooms

For the correct organization of circulation and distribution of air in the room, it is necessary to choose a rational size of the supply panels, ensure the normative flow rate and temperature of the supply air. However, these factors do not guarantee absolute air disinfection. For more than 30 years, scientists have been solving the issue of disinfecting operating rooms and offering various anti-epidemic measures. Today, the requirements of modern regulatory documents for the operation and design of hospital premises are faced with the goal of air disinfection, where HVAC systems are the main way to prevent the accumulation and spread of infections.

For example, according to the standard, the main goal of its requirements is decontamination, and it says that “a properly designed HVAC system minimizes the airborne spread of viruses, fungal spores, bacteria and other biological contaminants”, a major role in the control of infections and other harmful factors plays the HVAC system. B defines the requirements for room air conditioning systems, which state that the design of the air supply system should minimize the penetration of bacteria along with the air into clean areas, and maintain the highest possible level of cleanliness in the remainder of the operating room.

However, the regulatory documents do not contain direct requirements that reflect the determination and control of the effectiveness of decontamination of premises with various ventilation methods. Therefore, when designing, you have to engage in searches that require a lot of time and do not allow you to do the main work.

A large amount of regulatory literature has been published on the design of HVAC systems for operating rooms, it describes the requirements for air disinfection, which are quite difficult for designers to meet for a number of reasons. To do this, it is not enough just to know modern disinfection equipment and the rules for working with it, it is also necessary to maintain further timely epidemiological control of indoor air, which creates an idea of ​​the quality of HVAC systems. This, unfortunately, is not always observed. If the assessment of the cleanliness of industrial premises is based on the presence of particles (suspended matter) in it, then the cleanliness indicator in clean hospital rooms is represented by live bacterial or colony-forming particles, their permissible levels are given in. In order not to exceed these levels, regular monitoring of indoor air for microbiological indicators is necessary, for this it is required to count microorganisms. The collection and calculation methodology for assessing the level of cleanliness of the air environment was not given in any regulatory document. It is very important that the count of microorganisms should be carried out in the working room during the operation. But this requires a finished project and installation of an air distribution system. It is impossible to determine the degree of disinfection or efficiency of the system before starting work in the operating room; this is only established during at least a few operations. Here a number of difficulties arise for engineers, because the necessary research contradicts the observance of the anti-epidemic discipline of hospital premises.

Air curtain method

Properly organized joint work of air inflow and removal provides the required air regime of the operating room. To improve the nature of the movement of air flows in the operating room, it is necessary to ensure the rational relative position of the exhaust and supply devices.

Rice. 1. Air curtain performance analysis

Using both the area of ​​the entire ceiling for air distribution and the entire floor for extraction is not possible. Floor vents are unhygienic as they get dirty quickly and are difficult to clean. Complex, bulky and expensive systems are not widely used in small operating rooms. Therefore, the most rational is the "island" placement of laminar panels above the protected area and the installation of exhaust openings in the lower part of the room. This makes it possible to organize air flows by analogy with clean industrial premises. This method is cheaper and more compact. Air curtains acting as a protective barrier are successfully used. The air curtain is connected to the supply air flow, forming a narrow "shell" of air with a higher velocity, which is specially created around the perimeter of the ceiling. Such a curtain constantly works for exhaust and does not allow polluted ambient air to enter the laminar flow.

To better understand how an air curtain works, imagine an operating room with an exhaust fan installed on all four sides of the room. The influx of air that comes from the “laminar island” located in the center of the ceiling can only go down, while expanding towards the walls as it approaches the floor. This solution will reduce the recirculation zones and the size of the stagnation areas where harmful microorganisms are collected, prevent the room air from mixing with the laminar flow, reduce its acceleration, stabilize the speed and obtain a downflow overlap of the entire sterile zone. This contributes to the isolation of the protected area from the surrounding air and allows the removal of biological contaminants from it.

Rice. 2 shows a standard design of an air curtain having slots around the perimeter of a room. If you organize an exhaust along the perimeter of the laminar flow, it will stretch, the air flow will expand and fill the entire area under the curtain, and as a result, the “narrowing” effect will be prevented and the required laminar flow rate will be stabilized.

Rice. 2. Diagram of the air curtain

On fig. Figure 3 shows the actual air velocity for a properly designed air curtain. They clearly show the interaction of an air curtain with a laminar flow that moves uniformly. The air curtain avoids the installation of a bulky exhaust system on the entire perimeter of the room. Instead, as is customary in operating rooms, a traditional hood is installed in the walls. The air curtain serves as protection for the area around the surgical staff and the table, preventing contaminated particles from returning to the initial airflow.

Rice. 3. Actual velocity profile in the air curtain section

What level of disinfection can be achieved using an air curtain? If poorly designed, it will not bring more effect than a laminar system. You can make a mistake at high air speed, then such a curtain can “pull” the air flow faster than necessary, and it will not have time to reach the operating table. Uncontrolled flow behavior can pose a threat of contaminated particles entering the protected area from floor level. Also, a curtain with insufficient suction speed will not be able to completely block the air flow and may be drawn into it. In this case, the air mode of the operating room will be the same as when using only a laminar device. During design, you need to correctly identify the speed range and select the appropriate system. The calculation of the disinfection characteristics depends on this.

Air curtains have a number of distinct advantages, but they should not be used everywhere, because it is not always necessary to create a sterile flow during the operation. The decision on how much it is necessary to ensure the level of air disinfection is made jointly with the surgeons involved in these operations.

Conclusion

Vertical laminar flow is not always predictable, depending on the conditions of its use. Laminar panels, which are operated in clean industrial premises, often do not provide the necessary level of disinfection in operating rooms. The installation of air curtain systems helps to control the nature of the movement of vertical laminar air flows. Air curtains help to carry out bacteriological air control in operating rooms, especially during long-term surgical interventions and the constant presence of patients with a weak immune system, for whom airborne infections are a huge risk.

The article was prepared by A.P. Borisoglebskaya using materials from the ASHRAE journal.

Literature

1. SNiP 2.08.02–89*. Public buildings and structures.
2. SanPiN 2.1.3.1375–03. Hygienic requirements for the location, arrangement, equipment and operation of hospitals, maternity hospitals and other medical hospitals.
3. Guidelines for the organization of air exchange in ward departments and operating blocks of hospitals.
4. Guidelines for hygienic issues of design and operation of infectious diseases hospitals and departments.
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Photography of laminar flow

laminar flow- the calm flow of a liquid or gas without mixing. The liquid or gas moves in layers that slide against each other. As the velocity of the layers increases, or as the viscosity of the fluid decreases, the laminar flow becomes turbulent. For every liquid or gas, this point occurs at a certain Reynolds number.

Description

Laminar flows are observed either in very viscous liquids, or in flows occurring at sufficiently low velocities, as well as in the case of a slow fluid flow around small bodies. In particular, laminar flows take place in narrow (capillary) tubes, in a lubricant layer in bearings, in a thin boundary layer that forms near the surface of bodies when a liquid or gas flows around them, etc. With an increase in the velocity of this liquid, a laminar flow can some moment to go into a disordered turbulent flow. In this case, the force of resistance to movement changes sharply. The fluid flow regime is characterized by the so-called Reynolds number (Re).

When the value Re less than a certain critical number Re kp , laminar fluid flows take place; if Re > Re kp , the flow regime may become turbulent . The Re cr value depends on the type of flow under consideration. So, for a flow in round pipes, Recr ≈ 2200 (if the characteristic velocity is the cross-sectional average velocity, and the characteristic size is the pipe diameter). Therefore, for Re kp< 2200 течение жидкости в трубе будет ламинарным.

Speed ​​distribution

Velocity Averaging Profile:
a - laminar flow
b - turbulent flow

With laminar flow in an infinitely long pipe, the velocity in any section of the pipe changes according to the law V-V 0 ( 1 - r 2 /a 2 ), where a - pipe radius, r - distance from the axis, V 0 \u003d 2V sr - axial (numerically maximum) flow velocity; the corresponding parabolic velocity profile is shown in fig. a.

Friction stress varies along the radius according to a linear law τ=τ w r/a where τ w = 4μVav/a - friction stress on the pipe wall.

To overcome the forces of viscous friction in the pipe during uniform motion, there must be a longitudinal pressure drop, usually expressed by the equality P1-P2 = λ(l/d)ρV cf 2 /2 where P1 and P2 - pressure in k.-n. two cross sections at a distance l from each other λ - coefficient resistance depending on Re for laminar flow λ = 64/Re .

In fluid dynamics, laminar (streamlined) flow occurs when a fluid flows in layers without a break between the layers.

At low velocities, the fluid tends to flow without lateral mixing—neighboring layers slide past each other like playing cards. There are no transverse currents perpendicular to the direction of flow, eddies or pulsations.

In a laminar flow, the movement of fluid particles occurs in an orderly manner, along straight lines, parallel to the surface. Laminar flow is a flow regime with high momentum diffusion and low momentum convection.

If fluid flows through a closed channel (tube) or between two flat plates, either laminar or turbulent flow can occur, depending on the velocity and viscosity of the fluid. Laminar flow occurs at lower velocities that are below the threshold at which it becomes turbulent. Turbulent flow is a less ordered flow regime, with eddies or small packets of fluid particles, resulting in lateral mixing. In non-scientific terms, laminar flow is called smooth.

Yet, in order to better understand what a “laminar” flow is, it is better to see once what this “lamellar” flow looks like. Fluid moving and not moving is a very characteristic description of laminar flow. The flow is like a frozen jet, but it is enough to put your hand under this jet to see the movement of water (any other liquid).

Description:

The operating rooms are one of the most critical links in the structure of a hospital building in terms of the importance of the surgical process, as well as providing the special microclimate conditions necessary for its successful implementation and completion. Here, the source of release of bacterial particles is mainly medical personnel capable of generating particles and isolating microorganisms when moving around the room.

Hospital operating rooms
Air flow control

Over the past decades, in our country and abroad, there has been an increase in purulent-inflammatory diseases caused by infections, which, according to the definition of the World Health Organization (WHO), are called nosocomial (nosocomial) infections. An analysis of diseases caused by nosocomial infections shows that their frequency and duration are directly dependent on the state of the air environment in hospital premises. To ensure the required microclimate parameters in operating rooms (and industrial clean rooms), unidirectional air diffusers are used. The results of the air environment control and analysis of the movement of air flows showed that the operation of such distributors provides the required microclimate parameters, but often worsens the bacteriological purity of the air. To protect the critical area, it is necessary that the air flow leaving the device remains straight and does not lose the shape of its boundaries, that is, the flow should not expand or contract over the protected area where the surgical

The operating rooms are one of the most critical links in the structure of a hospital building in terms of the importance of the surgical process, as well as providing the special microclimate conditions necessary for its successful implementation and completion. Here, the source of release of bacterial particles is mainly medical personnel capable of generating particles and isolating microorganisms when moving around the room. The intensity of particles entering the room air depends on the degree of mobility of people, temperature and air velocity in the room. The HBI tends to move around the operating room with air currents, and there is always a risk of its penetration into the unprotected wound cavity of the operated patient. It is obvious from observations that improperly organized operation of ventilation systems leads to an intensive accumulation of infection to levels exceeding the permissible levels.

For several decades, specialists from different countries have been developing system solutions to ensure the conditions of the air environment in operating rooms. The air flow supplied to the room must not only assimilate various harmful substances (heat, humidity, odors, harmful substances), maintain the required microclimate parameters, but also protect strictly established zones from infections, that is, the necessary cleanliness of indoor air. The area where invasive interventions are carried out (penetration into the human body) can be called the operating area or "critical". The standard defines such a zone as an "operating sanitary protection zone" and means by it the space where the operating table is located, auxiliary tables for instruments and materials, equipment, as well as medical personnel in sterile clothing. In there is the concept of "technological core", referring to the area of ​​production processes in sterile conditions, which in meaning can be correlated with the operating area.

To prevent the penetration of bacterial contaminants into the most critical areas, screening methods through the use of a displacement air stream have become widely used. Laminar air flow diffusers of various designs were created, subsequently the term "laminar" was changed to "unidirectional" flow. Currently, you can find a variety of names for clean room air distribution devices, such as "laminar", "laminar ceiling", "operating ceiling", "clean air operating system", etc., which does not change their essence. The air diffuser is built into the ceiling structure above the protection zone of the room and can be of various sizes depending on the air flow. The recommended optimal area of ​​such a ceiling should be at least 9 m 2 in order to completely cover the operating area with tables, equipment and personnel. The displacing air flow at low speeds enters from top to bottom, like a curtain, cutting off both the aseptic field of the surgical intervention zone and the zone of transfer of sterile material from the environment. Air is removed from the lower and upper zones of the room at the same time. HEPA filters (class H according to ) are built into the ceiling structure, through which the supply air passes. Filters trap but do not decontaminate living particles.

Currently, much attention is paid all over the world to the issues of air disinfection in hospitals and other institutions where there are sources of bacterial contamination. The documents state the requirements for the need to decontaminate the air of operating rooms with a particle inactivation efficiency of at least 95%, as well as air ducts and equipment for climate systems. Bacterial particles emitted by surgical personnel continuously enter the room air and accumulate in it. To ensure that the concentration of particles in the indoor air does not reach the maximum permissible levels, it is necessary to control the air environment. Such control must be carried out after the installation of climate systems, maintenance or repair, that is, in the mode of an operated clean room.

The use of unidirectional flow air terminals with built-in ceiling-type ultra-fine filters in operating rooms has become commonplace for designers. Air flows of large volumes go down the premises at low speeds, cutting off the protected area from the environment. However, many specialists are unaware that these solutions are not enough to maintain the proper level of air disinfection during surgical operations.

The fact is that there are a lot of designs of air distribution devices, each of which has its own scope. The clean rooms of operating rooms within their "clean" class are divided into classes according to the degree of cleanliness, depending on the purpose. For example, general surgical operating rooms, cardiac surgery or orthopedic, etc. Each specific case has its own requirements for ensuring cleanliness.

The first applications of cleanroom air diffusers appeared in the mid-1950s. Since then, it has become traditional to distribute air in cleanrooms in cases where it is required to ensure low concentrations of particles or microorganisms in them, to be carried out through a perforated ceiling. The air flow moves through the entire volume of the room in one direction at a uniform speed, usually equal to 0.3–0.5 m/s. Air is supplied through a group of high-efficiency air filters placed on the ceiling of the clean room. The air supply is organized on the principle of an air piston moving downward through the entire room, while removing pollution. Air is removed through the floor. This air movement pattern helps to remove airborne contaminants from personnel and processes. This organization of ventilation is aimed at ensuring the cleanliness of the air in the room, but requires high air flow and is therefore uneconomical. For clean rooms of class 1000 or class ISO 6 (according to ISO classification), air exchange can be from 70 to 160 times/hour.

In the future, more rational devices of a modular type appeared, much smaller in size with low flow rates, allowing you to choose an air supply device based on the size of the protected area and the required air exchange rates of the room, depending on the purpose of the room.

Analysis of the operation of laminar air diffusers

Laminar devices are used in cleanrooms and are used to distribute large volumes of air, providing for the presence of specially designed ceilings, floor hoods and pressure control in the room. Under these conditions, the operation of laminar flow distributors is guaranteed to provide the required unidirectional flow with parallel current paths. The high air exchange rate contributes to maintaining close to isothermal conditions in the supply air flow. Ceilings designed for air distribution with large air exchanges, due to the large area, provide a small initial air flow velocity. The operation of floor-level extractors and room pressure control minimize the size of the recirculation zones, and the principle of "one pass and one exit" easily works. Suspended particles are pressed against the floor and removed, so the risk of their recirculation is low.

However, when such air distributors operate in the operating room, the situation changes significantly. In order to maintain acceptable levels of bacteriological purity of air in operating rooms, air exchange values ​​according to the calculation usually average 25 times / h and even less, that is, they are not comparable with the values ​​for industrial premises. To maintain the stability of the movement of air flows between the operating room and adjacent rooms, it is usually maintained at an overpressure. Air is removed through exhaust devices symmetrically installed in the walls of the lower zone of the room. For distribution of smaller volumes of air, as a rule, small-area laminar devices are used, which are installed only above the critical zone of the room in the form of an island in the middle of the room, instead of using the entire ceiling.

As observations show, such laminar devices will not always provide unidirectional flow. Since there is almost always a difference between the temperature in the supply jet and the ambient air temperature (5–7 °C), the colder air leaving the air handling unit descends much faster than an isothermal unidirectional flow. For ceiling diffusers used in public buildings, this is a common occurrence. There is an erroneous conventional wisdom that laminars provide stable unidirectional airflow regardless of where or how they are used. In fact, under real-world conditions, the velocity of a low-temperature vertical laminar flow will increase as it approaches the floor. The larger the volume of supply air and the lower its temperature relative to the room air, the greater the acceleration of its flow. The table shows that the use of a laminar system with an area of ​​3 m 2 with a temperature difference of 9 ° C gives an increase in air velocity by a factor of three already at a distance of 1.8 m from the beginning of the path. The air speed at the outlet of the supply unit is 0.15 m/s, and at the level of the operating table it reaches 0.46 m/s. This value exceeds the allowed level. It has long been proven by many studies that at overestimated inlet flow rates it is impossible to maintain its “unidirectionality”. The analysis of air control in operating rooms, carried out, in particular, by Salvati (Salvati, 1982) and Lewis (Lewis, 1993), showed that in some cases, the use of laminar installations with high air velocities leads to an increase in the level of air contamination in the area of ​​the surgical incision with subsequent risk of infection.

When the flow moves, at the starting point the air current lines will be parallel, then the flow boundaries will change, narrowing towards the floor, and it will no longer be able to protect the area defined by the dimensions of the laminar installation. At air speeds of 0.46 m/s, the flow will capture the stagnant air from the room. Since bacterial particles are constantly released in the room, contaminated particles will be mixed into the air flow coming from the supply unit, since the sources of their release are constantly operating in the room. This is facilitated by air recirculation resulting from the overpressure of air in the room. To maintain the cleanliness of the operating rooms, according to the standards, it is required to ensure an imbalance of air due to the excess of inflow over exhaust by 10%. Excess air is moved to adjacent less clean rooms. In modern conditions, airtight sliding doors are often used in operating rooms, there is nowhere for excess air to go, it circulates around the room and is taken back into the supply unit using fans built into it for further cleaning in filters and secondary supply to the room. The circulating air collects all polluted particles from the room air and, moving near the supply air flow, can pollute it. Due to the violation of the boundaries of the flow, air from the surrounding space is mixed into it and pathogenic particles penetrate into the sterile zone, which is considered to be protected.

High mobility contributes to intensive exfoliation of dead skin particles from unprotected areas of the skin of medical personnel and their entry directly into the surgical incision. On the other hand, it should be noted that the development of infectious diseases in the postoperative period is caused by the hypothermic state of the patient, which is aggravated by exposure to cold air flows of increased mobility.

Thus, a laminar flow air diffuser, traditionally used and effectively operated in a clean room, may be detrimental to operations in a conventional operating room.

This conversation is true for laminar devices with an average area of ​​​​about 3 m 2 - optimal for protecting the operating area. According to American requirements, the air flow rate at the outlet of the laminar panels should not exceed 0.15 m / s, that is, from 1 foot 2 (0.09 m 2) of the panel area, 14 l / s of air should enter the room. In our case, this will be 466 l / s (1677.6 m 3 / h) or about 17 times / h. According to the normative value of air exchange in operating rooms should be 20 times / h, according to - 25 times / h, therefore 17 times / h is quite consistent with the requirements. It turns out that the value of 20 times / h corresponds to a room with a volume of 64 m 3.

According to today's standards, the area of ​​​​a standard operating room (general surgical profile) should be at least 36 m 2. And for operating rooms for more complex operations (cardiology, orthopedic, etc.), the requirements are much higher, and often the volume of such an operating room can exceed 135–150 m 3. The air distribution system for these cases will require a much larger area and air capacity.

In the case of organizing air flow in larger operating rooms, there is a problem of maintaining the laminar flow from the exit plane to the level of the operating table. Several operating rooms have been used to study the behavior of airflow. In different rooms, laminar panels were installed, which were divided by area into two groups: 1.5–3 m 2 and more than 3 m 3, and experimental air conditioning units were installed, allowing you to change the temperature of the supply air. Multiple measurements of the incoming air flow rate were carried out at various flow rates and temperature drops, the results of which can be seen in the table.

Room cleanliness criteria

The right decisions regarding the organization of air distribution in operating rooms: the choice of a rational size of supply panels, ensuring the normative flow rate and temperature of the supply air - do not guarantee absolute air disinfection in the room. The issue of air disinfection in operating rooms was sharply raised more than 30 years ago, when various anti-epidemiological measures were proposed. And now the goal of the requirements of modern regulatory documents for the design and operation of hospitals is air disinfection, where HVAC systems are presented as the main way to prevent the spread and accumulation of infections.

For example, the standard considers decontamination to be the main goal of its requirements, noting: "a properly designed HVAC system minimizes the airborne transmission of viruses, bacteria, fungal spores and other biological contaminants", HVAC systems are given a major role in the control of infections and other harmful factors. B highlights the requirement for operating room air conditioning systems: “The air supply system must be designed in such a way as to minimize the penetration of bacteria into sterile areas along with air, and also maintain the maximum level of cleanliness in the rest of the operating room.”

However, regulatory documents do not contain direct requirements for determining and monitoring the effectiveness of disinfection for various ventilation methods, and designers often have to engage in search activities, which takes a lot of time and distracts from their main work.

In our country, there are quite a lot of different regulatory literature on the design of HVAC systems for hospital buildings, and everywhere there are voiced requirements for air disinfection, which, for a variety of objective reasons, are practically difficult for designers to implement. This requires not only knowledge of modern disinfection equipment and the correct use of it, but, most importantly, further timely epidemiological control of the indoor air environment, which gives an idea of ​​the quality of HVAC systems, but, unfortunately, is not always carried out. If the cleanliness of clean industrial premises is assessed by the presence of particles (for example, dust particles) in it, then the indicator of air purity in clean rooms of medical buildings is live bacterial or colony-forming particles, the permissible levels of which are given in. To maintain these levels, it is necessary to regularly monitor the air environment for microbiological indicators, for which it is necessary to be able to count them. The methodology for collecting and counting microorganisms to assess the purity of the air has not yet been given in any of the regulatory documents. It is important that the counting of microbial particles should be carried out in the operated room, that is, during the operation. But for this, the design and installation of the air distribution system must be ready. The level of disinfection or the efficiency of the system cannot be established before it starts working in the operating room, this can only be done under the conditions of at least several operational processes. For engineers, this presents great difficulties, since research, although necessary, is contrary to the order of compliance with the anti-epidemic discipline of the hospital.

air curtain

To ensure the required air regime in the operating room, it is important to properly organize the joint work of air inflow and removal. The rational interposition of supply and exhaust devices in the operating room can improve the nature of the movement of air flows.

In operating rooms, it is impossible to use both the area of ​​the entire ceiling for air distribution and the area of ​​the floor for its removal. Floor exhaust units are unhygienic as they get dirty quickly and are difficult to clean. Bulky, complex and expensive systems have not found their application in small-sized operating rooms. For these reasons, the most rational is the "island" arrangement of laminar panels above the critical zone with the installation of exhaust holes in the lower part of the walls. This makes it possible to model airflows similar to an industrial clean room in a cheaper and less cumbersome way. Such a method as the use of air curtains operating on the principle of a protective barrier has successfully proved itself. The air curtain is well combined with the supply air flow in the form of a narrow "shell" of air with a higher velocity, specially organized around the perimeter of the ceiling. The air curtain operates continuously for extraction and prevents the entry of polluted ambient air into the laminar flow.

To understand the operation of an air curtain, one should imagine an operating room with an exhaust fan arranged on all four sides of the room. The supply air coming from the "laminar island" located in the center of the ceiling will only go down, expanding towards the walls as it descends. This solution reduces the recirculation zones, the size of the stagnant areas in which pathogenic microorganisms collect, and also prevents the laminar flow from mixing with the room air, reduces its acceleration and stabilizes the speed, as a result of which the downward flow covers (locks) the entire sterile zone. This helps to remove biological contaminants from the protected area and isolate it from the environment.

On fig. 1 shows a standard design of an air curtain with slots around the perimeter of the room. When organizing the exhaust along the perimeter of the laminar flow, it is stretched, it expands and fills the entire zone inside the curtain, as a result of which the “narrowing” effect is prevented and the required laminar flow rate is stabilized.

From fig. Figure 3 shows the actual (measured) velocity values ​​that occur with a properly designed air curtain, which clearly demonstrate the interaction of a laminar flow with an air curtain, with a laminar flow moving evenly. The air curtain eliminates the need for a cumbersome exhaust system around the entire perimeter of the room, instead installing a traditional hood in the walls, as is customary in operating rooms. The air curtain protects the area immediately around the surgical staff and table, preventing contaminated particles from returning to the primary air stream.

After the design of the air curtain, the question arises of what level of disinfection can be achieved during its operation. A poorly designed air curtain will be no more effective than a traditional laminar system. A high air velocity can be a design error, as such a curtain will "pull" the laminar flow too quickly, i.e. even before it reaches the operating ceiling. The behavior of the flow cannot be controlled and there may be a risk of infiltration of contaminated particles into the operating area from floor level. Likewise, an air curtain with a low suction velocity cannot effectively shield a laminar flow and may be drawn into it. In this case, the air regime of the room will be the same as when using only a laminar supply unit. When designing, it is important to correctly determine the speed range and select the appropriate system. This directly affects the calculation of disinfecting characteristics.

Despite the clear advantages of air curtains, they should not be blindly applied. Sterile airflow generated by air curtains during surgery is not always required. The need to ensure the level of air disinfection should be decided jointly with technologists, who in this case should be surgeons involved in specific operations.

Conclusion

Vertical laminar flow can behave unpredictably depending on its mode of operation. Laminar panels used in cleanrooms generally cannot provide the required level of decontamination in operating rooms. Air curtain systems help to correct the nature of the movement of vertical laminar flows. Air curtains are the optimal solution to the problem of bacteriological control of the air environment in operating rooms, especially during long-term surgical operations and when there are patients with a compromised immune system, for whom airborne infections pose a particular risk.

The article was prepared by A.P. Borisoglebskaya using materials from the ASHRAE journal.

"... laminar air flow: an air flow in which the air velocities along parallel streamlines are the same..."

Source:

"ASEPTIC MANUFACTURE OF MEDICAL PRODUCTS. PART 1. GENERAL REQUIREMENTS. GOST R ISO 13408-1-2000"

(approved by the Decree of the State Standard of the Russian Federation of September 25, 2000 N 232-st)

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