The importance of proper ventilation in health care facilities cannot be overemphasized. HVAC design for health care facilities is all about providing a safer environment for patients and staff. This design differs from that of the conventional HVAC systems used in other building types.

This difference arises from:

⦁ The different temperature and humidity requirements for various areas and the accurate control of environmental conditions;
⦁ The need to restrict air movement in and between the various departments (no cross movement); and
⦁ The specific requirements for ventilation and filtration to dilute and reduce contamination in the form of odor.

These requirements demand very high quantities of outside air along with significant treatment of this ventilation air, including cooling, dehumidifying, reheating, humidifying, and filtration. HVAC particularly helps in controlling infection and contaminants in a healthcare setting.

HOW TO CONTROL INFECTION

The most effective means of controlling Infection, contaminants, odor, and indoor air pollution is through ventilation, which requires simultaneous control of a number of conditions:
⦁ Air change rates

⦁ Pressure gradient appropriate with the class of isolation

⦁ Appropriate air distribution in the compartments being air-conditioned.

⦁ High-quality air filtration, including absolute filtration

⦁ Precise temperature and humidity control ensuring maintenance of the intended microclimate

IMPACT OF VENTILATION RATE

Increasing ventilation rate is believed to reduce the cross-infection of airborne transmitted diseases by removing or diluting pathogen-laden airborne droplet nuclei. A higher ventilation rate can dilute the contaminated air inside the space more rapidly and decrease the risk of cross-infection.

Figure 1: Effect of Air Change per Hour on the Concentration of Airborne particles

 

A higher ventilation rate is able to provide a higher dilution capability to reduce the cross-infection. The use of higher ventilation rates also means a higher energy cost for mechanical ventilation. However, the recommended minimum ventilation rate for airborne infection isolation rooms is 12 air changes per hour (ACH).

Natural ventilation is able to deliver large ventilation rates with low energy consumption. Compared with mechanical ventilation, natural ventilation can provide much higher ventilation rates The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) is recognized as the world leader in establishing standards and training for the design of HVAC systems. The standards most frequently used in healthcare engineering are:

⦁ ASHRAE 170 — Ventilation of Health Care Facilities (building code in approximately 40 states)
The American Society of Hospital Engineers (ASHE) is part of the American Hospital Association. ASHE publishes guidelines for the operation and maintenance of health care facilities. Various clinical associations also publish guidelines that occasionally affect hospital HVAC design. ASHRAE is working to coordinate and incorporate the HVAC portions of these guidelines into Standard 170.

Table 1. Standard 170 room requirements

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⦁ ASHRAE 170 is on continuous maintenance, and addenda are issued several times a year. Table 1 is an excerpt from Standard 170. As shown, it calls out the pressurization, rate of dilution with outside air, supply air rate, whether or not the air may be recirculated, humidity range, and temperature range.

ISOLATION ROOMS

The infected patient can contaminate the environment. A single room with appropriate air handling and ventilation prevents direct or indirect contact transmission. It also reduces the risk of airborne transmission of microorganisms from a source patient to susceptible patients and other persons in hospitals. This is often termed “Isolation Room” in medical terminology.

There are two types of isolation rooms:

⦁ Airborne infection isolation (AII) refers to the isolation of patients infected with organisms spread via airborne droplet nuclei <5 µm in diameter.

⦁ Protective environment (PE) is a specialized area for patients who have undergone allogeneic hematopoietic stem cell transplant (HSCT), human
Immunodeficiency virus [HIV] infection, diabetes, cancer, chemotherapy, emphysema, or cardiac failure, etc.

Figure 2: Graphical presentation of a protective environment

 

HOW DOES ABOVE CLASSIFICATION AFFECT HVAC DESIGNER?

The differentiating factor between “AII” and “PE” rooms is the pressure relationships.
”PE” room is Positive pressure room & “All” room is Negative pressure room

⦁ POSITIVE PRESSURE ISOLATION ROOMS

Class P – positive pressure isolation rooms are set at positive pressure relative to ambient pressure, meaning that airflow must be from the “cleaner” area towards the adjoining space (through doors or other openings). This is achieved by the HVAC system providing more air into the “cleaner” space than is mechanically removed from that same space.

Figure 3: Positive Pressure Isolation Room

In the schematic diagram above, an airlock or anteroom is provided adjacent to the patient room. For a positive pressure room, air would flow from the isolation room to the anteroom and then to the corridor. Pressure control is maintained by modulating the main supply and exhaust dampers based on a signal from a pressure transducer located inside the isolation room.

 

⦁ NEGATIVE PRESSURE ISOLATION ROOMS

The basic principle of pressurization for microbial contaminant control is to ensure that air flows from less contaminated to more contaminated areas. The air in an open Class N room, for example, should flow from corridors INTO the isolation room to prevent the spread of airborne contaminants from the isolation room to other areas. The purpose of this design is to eliminate the spread of infectious contaminants and pathogens into the surrounding environment via the airborne route.

Class N is applicable to all infection isolation rooms where the patients known to or suspected to have infections are placed.

Figure 4: Negative Pressure Isolation Room

The schematic above shows the HVAC airflow arrangement for class N rooms. An anteroom designed to provide an “airlock” (no mix of air) between the infectious patient and the common space is placed adjacent to the patient room. The air would flow from the anteroom to the isolation room. Pressure control is maintained by modulating the main supply and exhaust dampers based on a signal from a pressure transducer located inside the isolation room.

 

TYPE OF HVAC SYSTEM – ISOLATION ROOMS AND CRITICAL EXAMINATION ROOMS

For the critical areas such as isolation rooms, intensive care units, and operating rooms, critical diagnostic and examination rooms, consider only the centralized HVAC system encompassing “all air systems.”

All air systems can be classified as single-zone, multi-zone, dual-duct, and reheat systems.

Single-zone systems: Single-zone systems serve just one zone having unique requirements of temperature, humidity, and pressure.

Figure 5: Single Zone System
For this type of system to work properly, the load must be uniform all through the space, or else there may be a large temperature variation.

Multi-zone systems: Multi-zone systems are used to serve a small number of zones with just one central air handling unit. The air handling unit for multi-zone systems is made up of heating and cooling coils in parallel to get a hot deck and a cold deck

Figure 6: Multi-Zone System

Zone thermostats control mixing dampers to give each zone the right supply temperature.

Dual-duct systems: Dual-duct systems are much like multi-zone systems, but instead of mixing the hot and cold air at the air handling unit, the hot and cold air are both brought by ducts to each zone, where they are then mixed to meet the needs of the zone.

Figure 7: Dual Duct Zone System

It is common for dual-duct systems to use high-pressure air distribution systems with the pressure reduced in the mixing box at each zone.

Reheat systems: Reheat systems supply cool air from a central air handler as required to meet the maximum cooling load in each zone.

Figure 8: Reheat System

Each zone has a heater in its duct that reheats the supply air as needed to maintain space temperatures. Reheat systems are quite energy-inefficient

 

TYPE OF HVAC SYSTEM – NORMAL PATIENT CARE ROOMS, ADMINISTRATIVE AND NON- CRITICAL AREAS

For the patient bedrooms and other non-critical areas, any one of the following HVAC systems can be used:

⦁ All air systems,

⦁ Terminal heating and cooling units, such as fan coil units or radiant ceiling panels, and

⦁ Radiant heating and cooling system.

The amount of outdoor air and how it is supplied to the occupied spaces would depend upon the type of HVAC system used. When the fan coil units or radiant ceiling panels are used, a central ventilation unit supplies conditioned air to the spaces. With this arrangement, the source of outdoor air being external to the principle cooling and heating equipment, it is possible to ensure the predetermined amount of outdoor air distribution to all the spaces.

HVAC CONTROL PARAMETERS FOR ISOLATION ROOMS

There are four main factors that affect the local concentration around a person in a room. These are:

⦁ The concentration of particles would tend to increase with the rate of production of particles in the room.
⦁ The proportion of supply and exhaust air quantity in relation to the size of the room.
⦁ The level of filtration of the supplied air will affect the ability of the ventilation system to dilute the room air particle concentration and
⦁ Air turbulence and air movement in the room can transport particles.

ROOM PRESSURE CONTROL

Building room pressurization is a critical factor to monitor in a hospital as it can greatly affect the controllability of the environment. If the building pressure is allowed to become negative due to supply filters being loaded, supply fans running too slow, or return fans running too fast, humid and dirty air can be drawn into the building through cracks and openings. This air is completely unconditioned and can provide several of the necessary ingredients to promote mold growth (e.g., moisture, more spores, and nutrients.)

Building room pressure gradient is achieved by controlling the quality and quantity of intake and exhaust air, maintaining differential air pressures between adjacent areas, and designing patterns of airflow for particular clinical purposes.

MONITORING OF ROOM PRESSURE

Dynamic pressure differential monitoring must take place in order to ensure the room is at the appropriate pressure. The two common methods of differential pressure control are
1) Flow tracking measurement & control and
2) Differential pressure measurement & control.

Figure 9: Flow Tracking Measurement & control Figure 10: Differential Measurement & control

Inflow tracking system, the exhaust, and supply flow rates “from” and “to” space are measured and controlled to produce a desired infiltration or exfiltration.

In a differential pressure system, the actual differential pressure between the isolation room and the corridor is taken by measuring the velocity of air induced through a hole in the envelope between the isolation room and corridor created by the differential pressure.

DIRECTIONAL CONTROL OF AIRFLOW

The design principle of pressurization control is to exhaust air from those areas which have the greatest contamination potential and allow air to be staged, or cascaded, from progressively cleaner areas. The figure below illustrates the basic principle of cascading airflows from clean areas to relatively contaminated areas.

Figure 11: Differential Airflow for Isolation Room

In the above diagram, a facility is depicted which has offices and isolation rooms, separated by corridors and other areas (storage rooms, labs). Air is supplied to the areas, usually in offices, maintained at the greatest positive pressure (marked with a ‘++’), and exhausted from the areas maintained at the greatest negative pressure (marked with a ‘- -‘). Transfer air (exfiltration/infiltration) is identified with blue arrows.

AIR DISTRIBUTION

In health care facilities (e.g., operating rooms, delivery rooms, catheterization laboratories, angiography rooms, HEPA-filtered rooms for immune-suppressed patients), the direction of air movement needs to be controlled. The air is introduced from ceiling registers on the perimeter and is returned or exhausted through registers located at least 6 inches above the floor. This arrangement provides a downward movement of clean air through the breathing and working zones to the contaminated floor area for exhaust.

The figure below shows the introduction of low-velocity air near the ceiling at the entrance of the room, flowing past the patient, and exhausted or returned close to the floor at the head of the patient bed. An airflow pattern is thus established, which helps to move microorganisms from the point of patient’s expulsion to the exhaust/return air terminal to prevent health care workers or visitors from inhaling the bacteria.

Figure 12: Room Air Distribution in the Isolation Room

Non-aspirating diffusers (typically perforated face) are recommended. These diffusers entrain large amounts of air, achieve good mixing, prevent updrafts, and provide a laminar flow of air that will flush the isolation room of unwanted airborne particles.

The diffuser should be placed away from the patient’s bed, preferably near the point where a health care worker or visitor would enter the room.

High-Efficiency Particulate Air (HEPA) Filters

HEPA filters have a minimum initial efficiency of 99.97% for removing particles 0.3 microns in size. This is a critical point as these filters are being used to remove mold and bacteria, typically 1 to 5 microns in size when airborne, as well as viral particles which are submicron in size

HEPA filters should be used:-

⦁ On the supply air distribution of the protective rooms.

⦁ On the return air of the infectious isolation rooms, the air is recirculated within the space in order to increase ACH while reducing the total exhaust requirements. Ideally, the infectious isolation rooms should be designed for 100% fresh air and exhaust.
⦁ On the exhaust of the infectious isolation rooms and local exhaust hoods.

TEMPERATURE & HUMIDITY CONTROL

Two essential components of conditioned air are temperature and humidity. After outside air passes through a low – or medium-efficiency filter, the air undergoes conditioning for temperature and humidity control.

Temperature Control

Control of temperature includes the operation of both heating and cooling systems to maintain temperature set points in the different areas of the building. Cool temperatures (68°F – 73°F) are usually associated with operating rooms, clean workrooms, and endoscopies suites. A warmer temperature (75°F) is needed in areas requiring greater degrees of patient comfort. Most other zones use a temperature range of 70°F – 75°F.

Temperatures outside of these ranges may be needed on limited occasions in limited areas depending on individual circumstances during patient care.
HVAC systems in healthcare facilities have either single-duct or dual-duct systems. A single-duct system distributes cooled air (55°F) throughout the building and uses thermostatically controlled reheat boxes located in the terminal ductwork to warm the air for individual or multiple rooms.

Humidity Control

Efforts to limit excess humidity and moisture in the infrastructure and on-air stream surfaces in the HVAC system can minimize the proliferation and dispersion of fungal spores and waterborne bacteria throughout the indoor air. Control of humidity includes the operation of both humidification and dehumidification systems to maintain a minimum and maximum humidity level in the facility.

 

Cleanroom describes a controlled environment where pollutants like aerosol, airborne bacteria, and dust are in small amounts. Cleanroom technology requires both technological and operational measures to avoid the potential risk of contamination of products.

The cleanroom has a controlled level of contamination that is determined by the number of particles per cubic meter at the defined particle scale. Outdoor- indoor air in a typical city setting produces 35,000,000 particles per cubic meter, 0.5 microns, and wider in diameter, equivalent to the cleanroom ISO 9 at the lowest point of cleanroom quality.

This article presents all you need to know about cleanroom technology. It’s usually best to start by understanding the difference between the conventional HVAC and cleanroom HVAC.

                                                                            

Difference between Conventional & cleanroom HVAC

Similar to standard HVAC, the HVAC of a cleanroom controls the temperature and the humidity to different levels of precision to create a comfortable environment. Along with comfort, cleanroom HVACs differentiate themselves from conventional systems by their increased air supply, airflow patterns, the use of high-efficiency filters, and room pressurization.
The increased air supply brings more air changes per hour with HEPA(High-Efficiency Particulate Air) filtered air circulating into the cleanroom many times per hour. In comparison, a conventional  HVAC system usually counts two to four air changes per hour, whereas in a cleanroom it can range anywhere from 15 to 250 or more.

Cleanroom HVAC designs require knowledge of regulations, cleanliness level guidelines, airflow, room pressurization, temperature control, humidity control, and accounting of the processes taking place inside.

Ventilation ducts are also different and require engineering knowledge. The HVAC must also maintain the appropriate pressure differential in order to prevent air from leaking from a less clean zone to a cleaner zone inside the cleanroom. A cleanroom differs from the conventionally ventilated room mainly in three ways.

⦁ Increased air supply: The increased air supply is an important aspect of particle control. Normal air-conditioning systems are designed for 0.5 to 2 air changes per hour, essentially based on the occupancy level or as determined from the building exhaust levels. In contrast, a cleanroom would have at least 10 air changes per hour and could be as high as 600 for absolute cleanliness. This large air supply in a cleanroom is mainly provided to eliminate the settling of the particulate and dilute contamination produced in the room to an acceptable concentration level.

⦁ The use of high-efficiency filters: High-efficiency filters are used to filter the supply air into a cleanroom to ensure the removal of small particles. High-efficiency filters used in cleanrooms are installed at the point of air discharge into the room. Room pressurization is mainly provided to ensure that untreated air does not pass from dirtier adjacent areas into the cleanroom.
⦁ Room pressurization: The cleanroom is positively pressurized with respect to the adjacent areas. This pressurization is done by supplying more air and extracting less air from the room than is supplied to it.

Classification of Cleanrooms

HVAC system design depends heavily on the required cleanroom classification according to ISO 14644-1. Although this standard relies on adopting a certain Air Change Number per hour (ACH), yet thermal loads need to be addressed in early stages of the project taking into consideration all internal and external heat sources.

It is important to understand that thermal load calculation is not only needed for equipment selection, but it is also required to understand load fluctuations, hence choosing the most feasible HVAC system from initial cost as well as operational cost point of view.

Table: ACH as per ISO Class

 

Cleanroom zoning

There are four types of clean zones in manufacturing sterilized pharmaceutical products. The grade is defined by the type of product and a part of the process which needs to be protected from contamination.The grades can be classified as A, B, C, or D.
⦁ A – Local zone, which is for operations that afford high risk for product quality, e.g. filling, closing, ampoule and bottle opening zones. Usually in such zones is used laminar airflow which provides similar velocity 0.36-0.54 m/s.
⦁ B – zone, which is used for aseptic preparation and fulfil.
⦁ C and D – are clean zones used for less responsible stages of manufacturing sterilized products.

Makeup Air and Building Pressurization

Typically many of the critical clean zones have their own dedicated air conditioning systems. While this is a good design strategy, many of the installations rely purely on the recirculation system without paying much attention to pressurization.
Without pressurization, gaseous contaminants can seep into these sensitive rooms through cracks in wall and ceiling joints, cable and utility penetrations, and spaces above drop ceilings and below raised floors.
Positive pressurization is the basis of assuring that uncontrolled and untreated air does not infiltrate the protected area. The ambient air used to provide the positive pressurization must be treated to ensure an environment free of both the gases and particulates.
The recommended minimum amount of positive pressurization gradient is 0.03” to 0.05” (~0.75 to 1.25mm) water column for cleanroom applications. This would generally equate to 3- 8% of gross room volume.
The amount of outside air required is a function of:
⦁ Equipment exhausts and exhaust through toilets, kitchen, pantry, battery rooms etc.
⦁ Leakage through pass through, conveyor openings, strip curtains, airlocks, door undercuts etc.
⦁ Duct leakage, wall and ceiling leakages
⦁ Level of positive pressurization required

The HVAC design must optimize the use of makeup air and minimize the uncontrolled air leakages while maintaining the controlled ventilation

AIR PRESSURE GRADIENT

Positive pressurization is maintained throughout the Cleanroom with the highest pressure being in the cleanest area, and it gradually decreases toward the less clean adjacent rooms. This reduces the chance of contaminants from dirty areas entering the cleaner room as the microorganisms will be pushed away by the pressure.

Figure 03: Air Pressure Gradient in Room

 

Static or active pressure control methods are used depending on the tolerances. Typical tolerance is ±0.01 inches wg. Some semiconductor cleanrooms require a precision of ±0.0025 inches wg. In high precision rooms, the control system must be responsive enough to maintain the differential pressure when doors are opened.

HOW TO CONTROL HUMIDITY IN YOUR CLEANROOM

Controlling the humidity in your cleanroom is crucial, not only to meet government and company specifications, but to protect the integrity of your processes and product. If the humidity is too high, bacterial growth can flourish, metal products or equipment can corrode, photolithographic degradation, condensation, and water absorption can occur. If the humidity is too low, static build-up and discharge can become an issue.

Many products manufactured and processed in a cleanroom environment are moisture-sensitive. For this reason, cleanroom specifications often include relative humidity (RH) control. These control points range from 35-65%RH for year-round operation. These RH levels generally are maintained in a narrow band ±2 % RH at temperatures below 70°F. The effects of higher humidity levels in close tolerance environments can be detrimental to product quality and production schedules.
Humidification is one of the environmental conditions usually specified for cleanroom operations. It is the process of increasing the water vapour content of the air. Common approaches to increase the humidity include:

Ultrasonic Humidifier:
An ultrasonic humidifier is a device that uses high-frequency sound vibrations to produce an extra fine water mist that is then expelled to add moisture to the room.

The ultrasonic generally has no filter factored into its design. It is considered safer in that there is no hot water present in the unit and therefore no risk of scalding since the unit does not heat the water in any way.

Disinfecting the humidifier becomes even more important than with a warm mist humidifier that does boil the water. Humidity influences a number of factors that could degrade overall cleanroom performance, including:
⦁ Bacteria growth
⦁ Personnel comfort zone
⦁ Static charge build-up
⦁ Metal corrosion
⦁ Moisture condensation
⦁ Photolithographic degradation
⦁ Water absorption

Two common approaches to humidity control are as below.

⦁ Air Conditioning

Air conditioning lowers the temperature of a surface exposed to the cleanroom airstream below the dew point of that airstream. Excess water vapour condenses, and the resulting air is dehumidified. The air must then be reheated to the proper control temperature and routed to the cleanroom. Standard refrigeration equipment can produce dew points of 40°F (4°C) on a reliable basis.

 

⦁ Desiccants

In a desiccant system, the process airstream passes through a desiccant medium. The desiccant adsorbs moisture directly from the airstream, and the resulting dehumidified air is routed to the cleanroom.
Desiccant dehumidifiers can produce dew points below 0°F (-18°C), a fivefold reduction in the air moisture beyond what can be achieved with standard HVAC-grade refrigeration systems.

Figure 04: Desiccant System in Clean Room

 

Supply air and exhaust (return) air

The location of the supply and exhaust (return) air grilles should take the highest priority when laying out the cleanroom.
The supply (from the ceiling) and return air grilles (at a low level) should be at the opposite sides of the cleanroom, to facilitate a “plug” flow effect.

Depending on the degree of cleanliness required, it is common for air systems to deliver considerably more air than would be needed solely to meet temperature and humidity design. Airborne particles can be organic or inorganic. Most contamination control problems concern the total contamination within the air.

Figure 05: Air Flow Pattern in Clean Room

 

Particles of varying sizes behave differently, as air moves through a room. Selection of the airflow patterns is a major step in cleanroom design. Because airflow is such an important aspect of particle control, the design of a cleanroom requires careful consideration of air motion and airflow patterns.
The general air patterns are:

⦁ Unidirectional (sometimes referred to as laminar flow) is an airflow pattern in which essentially the entire body of air within a confined area moves with uniform velocity and in a single direction with generally parallel airstreams. Clean rooms; class 100 and below have unidirectional airflow pattern.

Figure 06: Laminar flow pattern

 

⦁ Non-unidirectional airflow is not unidirectional by having a varying velocity, multiple pass circulation or nonparallel flow direction. Conventional flow clean rooms (class 1000 &10000) have non-unidirectional or mixed airflow patterns.

Figure 07: Unidirectional flow pattern

⦁ Mixed patterns combine some of each flow type.

Figure 08: Mixed Airflow pattern

 

HEPA Filter Setup Flow Evaluation

Cleanroom operation testing firms do cleanroom certification. These firms are designed with sophisticated and calibrated devices necessary for performing Cleanroom Performance Testing (CPT). Several evaluations are included by the cleanroom certification testing procedure; some are required, while others are not obligatory.

HEPA Filter Setup Flow Evaluation: HEPA cleanroom filters are highly powerful air particle filters, used to control microbes and pollutants and preserve environmental standards. The minimal efficacy revealed by means of a HEPA filter when examined using an aerosol is 99.97%. The HEPA setup filter flow test is conducted to find flows, damage and faults in the filters. The evaluation also verifies whether or not they are installed correctly and the integrity of the filters.

Both kinds of flow tests readily available for cleanroom HEPA filters are:

* Complete flow evaluation
* Scan flow evaluation

Figure 09: Performance Chart

 

Noise Criteria

Noise is one of the major issues in a cleanroom, and the designs usually require high degree attenuation and use of acoustic silencers. Cleanrooms is inherently noisy and requires close attention to noise control. This is due to the large requirements of airflow. Cleanroom noise can be attributed to three primary sources:
⦁ Fan noise
⦁ Airflow turbulence
⦁ Process equipment
Airflow noise is due to the turbulence that is typically generated by the introduction of discontinuities in the airstreams (such as elbows or transitions), which is more prominent at high velocities.

Cleanroom Ventilation Improvement with CFD Simulation

A well-designed cleanroom environment is necessary for activities performed in a controlled environment, containing a low level of pollutants—a critical requirement for many manufacturing, pharmaceutical, and scientific research applications.

Figure 10: CFD Simulation in Cleanroom

 

Hence, it is important to understand the role of HVAC principles in a cleanroom environment. One of the best ways to recognize this is by performing simulations. Simulation technology is a cost-effective and time-efficient tool which helps during the design.

There are several configurations available to get the desired airflow and recirculation patterns in order to remove internally generated contaminants.

⦁ Velocity Plot Comparison

Handling laminar and turbulent airflow principles might also be required in specific cases of cleanrooms, which can be analyzed using simulation. Laminar flow usually directs air downward with a constant velocity. This is usually applied over the entire ceiling to maintain a unidirectional flow

⦁ Contaminant Spread Comparison in a Cleanroom Environment

People working in a cleanroom environment face the risk of exposure to chemical vapours if there is any mishandling of chemicals. To approach this problem, experts assume a particular source of contamination and analyze how it spreads in the room. The ultimate aim is to obtain a flow to get rid of the contaminant by regulating it directly to the outlets.