Commercial Building Pressurization – Air Pressure
Why Building Pressurization Matters
In commercial construction, one often overlooked aspect of planning and design is keeping a building in a homeostatic state in terms to air pressurization. The reason for this is simply that, there are a lot of variables that can affect a building’s air pressure that can not be accounted for.
There exists many unaccounted for variables that affect a buildings air pressure which can cause it to function much differently than the way it was originally designed or engineered. Abnormal foot traffic patterns, causing doors to be opened more than anticipated is one of the more obvious ways. Natural degradation of building materials and/or poorly functioning HVAC systems is another; albeit this may take years to be noticible. Even something as simple as not changing air filters in the HVAC blowers can affect the pressurization of a building. In any case, the inability to create or maintain a homeostatic building pressurization can cause a multitude of issues.
Poor Building Air Pressurization = Poor Air Quality
Most buildings attempt to maintain an environment of positive air pressure. In short, this means there is greater or “higher” pressure inside the building than that of the outside atmospheric resting pressure. This serves a few basic functions. It ensures that dust, dirt and debris introduced into a building is “pushed” back out once a door or window is opened. You probably experience this when you enter a commercial building. In most commercial buildings, when the automatic door opens you typically feel a rush of air exiting the building. That air can be likened to deflating a balloon – the pressure in the balloon seeks to deflate to an area of lower pressure – so the balloon air rushes out.
This air moving into or out-of a building is due to the pressure differential. If the air pressure in the building is greater than the air pressure outdoors, it is called a “positive pressure room” or “positive air pressure building.” Conversely, if the pressure inside the building is less than the pressure outdoors, the room or building is said to have “negative pressure.” Generally it is not desirable to have negative pressure inside a building. The problem with negative pressure is that once a door or window is opened – any exterior air is permitted to enter the building. That unfiltered air brings with it pollen, dust, dirt, biological and chemical particulates, etc. These particles entering the building and affect the people working in the building and the general cleanliness of the building.
Filtration For Cleaner Air
Keeping the air fresh in a positive pressure building requires precise engineering. To keep a building pressurized requires air to be drawn into the building to create the positive pressure. Since it is not being pulled from undesirable sources like unfiltered air via doors and windows – it must be pulled from an outside source across a filtration system.
HVAC systems pull air from a fresh air duct and through various types of filters. The more fine the filter media the more force required to pull the air across it. So the more robust the HVAC system will need to be. This is like sucking a milkshake through a straw with a tiny diameter. Now, let’s imagine a person sucking the same milkshake through a very large straw. Same concept – more suction/blowing power is needed to pull more air through more filters or finer filters.
And how tight or leaky a building is will determine the amount of air per cubic foot per minute to inflate the building envelop (balloon). Some buildings require as little as 45 CFM per 1000 SqFt. Other, more drafty or leaky buildings may require up to 300 CFM. The amount of air needed to positively pressurize and maintain pressurization will initially affect the design of the building HVAC system, and after the building is completed, the energy costs. It is much better to seal up air leaks in a building than to use larger fans to ‘push’ more air into a leaky building to maintain a positive pressure.
Negative Pressure Areas
Even in the most strict of positive pressure room and building environments you will want areas of negative pressure. These include maintenance closets, bathrooms, food prep areas, areas where chemicals are used, etc. In these areas, it is important to create negative room or area pressure so the fumes exit these areas quickly. To create they localized areas of negative pressure exhaust fans can be used very effectively. Once installed they will function to relive the area of polluted air, and at the same time, allow an equal volume of clean air to constantly flow into the area.
It is important from an engineering perspective to refill the area of negative pressure with an equal volume of clean air. This air is called “make up air” – or “air that makes up for the deficit of air created by the exhaust.” The overall building HVAC system must be engineered and calibrated to account for these deliberate breeches in positive air pressurization.
Monitoring results
Because there are areas of positive and negative pressure inside a building it is not possible to measure “building pressure” at any one spot. Different rooms and areas have different pressures – some positive and some negative. It is much more important to think of local pressure. In a hospital, for example, it might be important to have negative pressure in patient’s rooms but positive pressure in lobby areas. It is critical, in fact even mandated by federal laws and local codes, to have negative pressure in areas of nuclear medicine. The system of maintaining positive and negative air pressure is a very complex process, more art than science in many cases. However, it is an important concept to understand. Just because a building “feels” like it has positive air pressure in no way means it is acting the way it should. This is where it is critical to ensure absolute certainty that a each area of a building’s air pressurization system is functioning properly. in fact, if it is not working properly – it can cost hundreds or even hundreds of thousands of dollars in operating costs, depending on the size of the building.
Simple yet effective systems exist that can either be integrated with existing building management and control systems and/or act as standalone monitoring systems for room air pressure. Some of these systems are capable of monitoring both negative and positive air pressure and in multiple rooms simultaneously. Even further, some systems allow for monitoring of temperature and humidity – both of which are complimentary variables in overall environmentally controlled rooms and especially in positive and negative air pressure rooms.
Installing many of these systems requires little or no technical experience and can be set up and running out of the box in a few minutes. The advantage of using some of the more reliable systems on the market is the ability to alerting one or more building maintenance personnel of issues before they are in a “no turning back” scenario. In such systems, authorized building personnel can receive alerts via test/SMS messaging, email alerts, and even phone calls. In essence, if there is too little pressure in a controlled environment, it means the company is spending more money in wasted energy to maintain positive pressure. If a system were put into place to monitor positive pressure, there would be no more doubt and even better – no more wasted money.
Keep this in mind next time you receive your heating and cooling bill. It may well be that lowering energy cost is a simple fix – but one you’ll be able to realize unless you are certain what your current environmental conditions are.
Preventing Mold In Hospitals
Mold in Hospitals?
Mold is a collective term which refers to fungi that grows in the form of multi-cellular thread-like structures called hyphae as opposed to fungi that exist as single cells are called yeasts. Some molds as well as yeasts cause disease or food spoilage while others are beneficial and play an important role in biodegradation or in the production of various foods, beverages, antibiotics and enzymes. Ironically, while most mold types are potentially harmful to patients, penicillin is a mold used in hospitals that is used to actually treat patients!
In a healthcare setting, it is important to prevent mold growth of any kind where possible; even the most harmless types can cause health issues for patients with serious illnesses and compromised immune systems. A patient with a compromised immune system can be especially at risk.
Although hospitals are generally considered to be clean and sterile environments, a closer look tells us there is actually a greater potential for problematic viral, fungal and bacterial threats due to the very nature of hospitals. Think of it this way, hospitals typically house patients with severe illnesses. These illnesses can be transferred to the garments, linens, walls and floors of the hospital. This is in addition to the obvious blood and other bodily fluids transferred from patient to hospital assets. Basically, there is a lot of fluids in one form or another being transferred from patients to the hospital itself. Where there are fluids, there is the potential for mold growth.
Keeping the hospital clean for patients and preventing cross-contamination is a full-time job, usually for several personnel – and can be a real challenge.
In all hospitals, heavy cleaning agents along with water as a dilutant are used to launder linens, clean surfaces and for general-purpose sanitizing. As you probably know, where there is water, there is a potential for mold growth. Additionally, some hospitals may be older structures with leaky roofs, and foundations; allowing for even greater potential for mold growth.
In any case, prevention is key. Once mold starts to grow, it can become a devastating and never-ending cycle of growth and remediation. Keeping mold growth to a minimum requires monitoring temperature and relative humidity effectively. It may also be beneficial to monitor and correct for undesirable differential air pressures in patient rooms and entire floors. A patient room with a positive air pressure will expel air along with any pollutants in the air every time the door is opened. So any mold spores quickly move in the hall ever time someone enters or leaves the room.
Sources of Mold
Mold growth can originate in places where standing water is present or in use such as bathrooms, showers, laundry areas and kitchens. Moisture, temperature and humidity are usually monitored in these locations. However, it is usually places that are not monitored effectively where mold can start growing and thrive.
Mold in ceilings
Most hospitals have drop-tile ceilings in rooms and hallways. These are usually made of mold-resistant materials and a general inspection of these tiles usually indicates a water or moisture problem because the ceiling tiles will have visible brown or discolored areas. These may appear as circular, and start from one corner or from the middle of the tile. Sources for the water leak may be the roof (if on higher floors) or from fresh water pipes, sprinkler systems or other plumbing. The problem is, by the time you can see evidence of water leaks it is too late to prevent mold growth. You are unfortunately just seeing the “tip” of the iceberg.
There are other instances where pipes may sweat because of temperature changes and relative humidity. This creates a humid environment which facilitates mold growth, even though there may be no visible sign of water leakage. HVAC and AC systems can be an especially common source for mold growth and increased relative humidity – heated air mixed with condensation; creating an environment that is relatively tropical. As a note, tropical climates are know to exponentially increase mold, bacteria and viruses.
It is advantageous and advisable for all hospitals to install temperature and relative humidity sensors in ceilings to constantly monitor humidity and temperature. Any temperature fluctuations can lead to greater condensation and relative humidity.
For many hospitals, installing a complete system would be a challenge because of having to install wire throughout a facility to a centralized location or trying to integrate it with an existing BACnet or other building management system. Fortunately, there are wireless systems that allow hospital administrators to monitor relative humidity, differential air pressure and temperature in ceilings throughout the entire hospital without construction costs. In fact, some systems even allow for automated email alerts, text/SMS alerts with automated phone calls to management personnel or maintenance staff long before a environment becomes too humid.
Can you prevent mold growth?
Mold can grow in obvious areas like hospital kitchens, bathrooms, showers, laundry, etc. Keeping these areas dry and properly ventilated is key. It requires more than just drying up the water; the room temperature may naturally increase the relative humidity – creating an environment for mold growth. An often overlooked way to combat this higher relative humidity is by using proper ventilation via exhaust systems and monitoring differential air pressure between common “wet” areas and dryer areas. Keeping a wet area under constant positive air pressure will allow the room to “push” the damper air to areas of dryer air. As the air mixes, it will become more homogeneous and stable.
If wet areas have negative air pressure, and inadequate exhaust, the air can become overly humidified and stale. The key takeaway is “Do you actually know for sure if you have adequate pressurization and ventilation between rooms?” If not, finding out the hard way can cost millions and even result in the potential loss of life.
While mold is less common in modern/new construction hospitals, it can still happen. Simple engineering and construction inconsistencies can allow for water to slowly trickle in from outdoors or allow for unwanted condensation in plumbing and HVAC systems. The problem is, no one really knows there is a problem – until there is a problem.
After all, how many hospital staff have the job description requiring them to physically inspect all plumbing, wet walls, and wet areas, ceilings, roof, etc.? Fortunately, there are automated systems that can monitor just about any area within fractions of a degree in temperature and relative humidity and provide advanced warning in the instance of environmental instability.
Note: If you are a hospital administrator and would like additional information on ways to keep mold and bacteria growth to a minimum with automated instrumentation, call (877) 241-0042 and ask for Rick Kaestner.
Laminar Air Flow Systems Explained
Laminar Air Flow Systems – General Terminology
Laminar air flow principles and Laminar air flow systems are commonly used in cleanrooms, positive and negative pressure rooms and other sensitive controlled environments. The best way to understand laminar air flow is to first differentiate between laminar air flow as a principle and laminar air flow in use as a system.
Laminar air flow: principle – Laminar air flow is a process where air is manipulated to force air to move at the same speed and in the same direction, with no or minimal cross-over of air streams in a given space.
“In laminar flow, the motion of the particles of the fluid is very orderly with particles close to a solid surface moving in straight lines parallel to that surface. Laminar flow is a flow regime characterized by high momentum diffusion and low momentum convection.” – Wikipedia
Laminar air flow systems – these are mechanical systems manufactured to provide laminar air flow in a large or small area to perform certain tasks in an environment of constant non-turbulent air flow.
Laminar air flow systems are usually cabinet-based, but may be designed to be a booth or work bench or even a room as well. Some common designs are:
- Vertical Laminar Flow Cabinets – Vertical Laminar Flow Cabinets function much like horizontal Laminar Flow Cabinets, however, in vertical setups the laminar air is directed vertically downwards onto the working area. The air can leave the working area via holes in the base. Vertical flow cabinets typically provide more protection for the booth/cabinet operator because the air can be directed away from the operator.
- Horizontal Laminar Flow Cabinets – Horizontal Laminar Flow Cabinets where air flow originates from above the work area but then changes direction and is processed across the work area in a horizontal direction. The constant flow of filtered air provides material and product protection.
- Laminar Flow Cabinets and Hoods – these are the housings manufactured to create the controlled air environment.
- Laminar Flow Benches and Booths – these are larger designs; typically designed for larger manufacturing processes.
Laminar air flow stations are built with respect to the type of process being performed. There are times when maintaining a very high level of cleanliness is desired, so a laminar air flow cabinet or bench may reside inside an existing cleanroom. Other times, a cabinet may be a standalone design in a facility without a cleanroom at all.
Laminar Flow Cabinets are typically designed to protect the working area only. They do not offer operator protection and are not suitable for working with bio-hazards.
Laminar Air Flow System Types and Uses
Depending on the goals of the company and operator of a laminar air flow system, there will exist certain “pro’s” and “con’s” of using one type over another. in fact, many larger facilities will take advantage of using both types.
Vertical Laminar Air Flow Systems
Let’s first look at some pros and cons of using a vertical laminar air flow system.
Pro’s
- The hood/unit is not as deep as horizontal units, and thereby requires less floor space.
- Best choice for compounding sterile products, consumables and pharmaceuticals.
- Safer because the controlled air is not blowing directly towards operator, standard sash provides barrier between process and operator’s face.
- The filter unit is located on top; which makes it easier to change without technical intervention.
- Typically has less turbulent effects from air striking larger objects and equipment.
- Less of a chance of cross-contamination between items positioned on the work surface.
Con’s
- While filters are easy to access, most systems require a generous overhead clearance; changing filters or servicing unit may require a step-ladder and altered construction to provide ample overhead clearance.
- Cannot place items (or hands) on top of other items: because or possible airflow obstruction.
- Potential for increased turbulent effect at the work surface because of air striking the work surface.
Horizontal Laminar Air Flow Systems
Now, let’s take a look at some pros and cons of using horizontal laminar air flow systems:
Pro’s
- Greatly reduced turbulent effect of air striking the primary work surface space.
- No sash: may make it easier to work and position equipment.
- Hands and gloves are generally less contaminating since they’re downstream of the primary work-space.
Con’s
- Changing the filter and/or servicing the unit requires the hood be removed/re-positioned and may require technical expertise.
- Large samples/objects may obstruct laminar air flow, and this may contaminate downstream objects.
- Horizontal flow blows fumes and/or particles in operator’s face.
In addition to the primary differences in functionality and operation of both vertical and horizontal laminar air flow systems there are many sub-types of hoods and booths that may fall under each primary type. These may include:
- Explosion proof laminar air flow systems
- Biological protection laminar air flow systems
- Ductless exhaust fume protection laminar air flow systems
- Biomedical laminar air flow systems
- Free-standing, portable, and built-in units
- Units with high heat, or UV exposure
- Clean benches
As a note of caution, neither vertical or horizontal laminar flow hoods are appropriate when operating in conditions where bio-hazards are present. Work with bio-hazards should only be carried out in an approved containment area. One such cabinet is a Class II, Type A2 Biosafety Cabinet; used in applications requiring Biosafety Level (BSL) 2 or 3 containment rating.
In essence, a laminar air flow system is a way of creating an ultra-clean work-space and environment for manufacturing a specific part of a larger process, or compounding a consumable/pharmaceutical compound. In a cleanroom or typical controlled air environment, there are too many factors that would negatively affect the process – the primary one being turbulence that can disturb processes on a microscopic level. Cleanrooms and controlled air environments can, in addition to improving the purity of a product, also protect operators and the general public.
Clean ratings for laminar flow systems, typically measured as an ISO class, varies, just as the rooms they are physically located in.
A large part of being able to control the environment inside of any laminar air flow system is the ability to control and monitor temperature, differential air pressure and relative humidity. Most modern units are manufactured with constant monitoring systems built-in to the laminar air flow unit. However, this is but one monitoring system. The drawback is potential instrument failure, and the fact that the builder of the laminar air flow system specializes in air flow, not necessarily in the monitoring and reporting of actual air conditions.
Many manufacturers of laminar air flow systems are turning to expert instrument manufacturers and creating OEM-based instrument inclusions. Some operators are sourcing secondary monitoring and alerting systems irrespective of the laminar air system manufacturer for additional protection. After all, the operator stands to lose the most in terms of product efficacy, safety and operator safety.
Air Filtration and Filtering In Cleanrooms – Part 2
Filter Media Rating
Air filters are commonly described and rated based upon their collection efficiency, pressure drop (or airflow resistance), and particulate-holding capacity. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) have developed standards 52.1-1992 and 52.2-1999 that classify filters in terms of “Arrestance” and “Efficiency”.
Standard 52.1-1992 measures arrestance, dust spot efficiency, and dust holding capacity. Arrestance means a filter’s ability to capture dust and describes how well an air filter removes larger particles such as dirt, lint, hair, and dust. The dust holding capacity of a filter is the amount by weight of standard dust that the filter will hold without exceeding the resistance 0.18 inch-w.c. for low-resistance filters, 0.50 inch-w.c. for medium-resistance filters and 1.0 inch-w.c. for high-resistance filters. Be aware that, arrestance values may be high; even for low-efficiency filters, and do not adequately indicate the effectiveness of certain filters for chemical or biological protection. Dust spot efficiency measures a filter’s ability to remove large particles, those that tend to soil building interiors. Dust holding capacity is a measure of the total amount of dust a filter is able to hold during a dust loading test. Dust arrestance can be expressed as
µa = 1 – Ca / Cb
Where
µa = dust arrestance
Ca = dust concentration after filter
Cb = dust concentration before filter
Since large particles make up most of the weight in an air sample, a filter could remove a fairly high percentage of those particles while having no effect on the numerous small particles in the sample. Thus, filters with an arrestance of 90 percent have little application in cleanrooms.
ASHRAE Standard 52.2-1999 measures the particle size efficiency (PSE). Efficiency measures the ability of the filter to remove the fine particles from an airstream by measuring the concentration of the material upstream and downstream of the device. If a supplier of filter only indicates efficiency as 95% or 99%, it does not really mean
anything unless it specifies the particle size range.
The ASHRAE Standard 52.2-1999 quantifies filtration efficiency in different particle size ranges and rates results as MERV (Minimum Efficiency Reporting Value) between 1 and 16. This numbering system makes it easier to evaluate and compare mechanical air filters and eliminates some of the confusion regarding the overall effectiveness of any type of a mechanical air filter on removing airborne particulates, especially those that are less than 2 microns in size. A higher MERV indicates a more efficient filter.
HEPA filters
HEPA stands for High Efficiency Particulate Air. The HEPA filters work on diffusion principle to remove particulate matter and are extremely important for maintaining contamination control. These filter particles as small as 0.3 µm (microns) with a 99.97% minimum particle-collective efficiency. This is remarkable considering that the outside air we breathe may contain up to 5 million suspended particles of dust, smog, and pollen in one cubic foot.
These filters typically use glass fiber media and are available in thicknesses of 6” and 12”. These have pressure drop of 1 inch- w.c. when clean and generally need to be replaced when the pressure drop exceeds 2 inch- w.c.
HEPA air filters are not MERV rated as they exceed the ASHRAE test protocol 52.2 used in determining the MERV ratings. In fact, HEPA air filters are the ONLY mechanical air filters that are tested and certified to meet a specific efficiency at a specific particle size. All HEPA air filters must meet a minimum efficiency of 99.97% at 0.3 microns.
ULPA filters
ULPA stands for Ultra Low Particulate Air. Growing market demand from advanced science and technology led to development of ULPA filters which provide a minimum of 99.999% efficiency (0.001% maximum penetration) on 0.3 micron particles for achieving better cleanliness classes and cleaner working environments. These are used for ultra- cleanrooms, where contamination levels have to be controlled at levels better than that which can be achieved with conventional HEPA filters.
Boron free ULPA filters of 99.9997% efficiency for particles down to 0.12 micron size for Class 10 and Class1 cleanrooms are specially used in electronic/semiconductors/ wafer manufacturing industries, where tolerance to contamination level above 0.12 micron is also very critical and not permitted.
Note that the text information for instance on the efficiency @ 99.97% and 99.997% of HEPA filters look similar but in reality the difference is not insignificant. A 99.97% efficient filter has a fractional penetration of 0.0003; while a 99.99% filter’s fractional penetration is 0.0001. This means that a 99.99% filter is three times more efficient in removing 0.3-micron particles.
Filter Testing
The efficiency of filter is of paramount importance and must be measured in an appropriate way. Typically the filters are shop tested and only provide the quality certification for required efficiency to the end user. But following installation, a check of the filter seals is recommended on a ninety-day basis, with a complete scan of the filters two times a year. There are five fundamentally different methods used to evaluate efficiency: (1) The Particle Count Method; (2) The Weight Method; (3) The Atmospheric Dust Spot Efficiency Method; (4) The Cold DOP Method and (5) The Hot DOP Method.
1) The Particle Count Method: In this method, actual particle count per unit volume of air is determined through microscopic analysis of the air sample. This procedure is extremely tedious and is susceptible to human error. The dust concentration must be quite low (or the sampling time must be unreasonably short) because the sample cannot be allowed to become too dense to count.
2) The Weight Method: The weight method indicates the weight of the dust removed by the filter as a percentage of the weight of dust in the air before filtering. The Weight Arrestance Test is a simple test which involves feeding a synthetic dust to a filter and rationing the weight of dust exiting the filter to the weight of dust originally fed into the filter. This method is very popular and easy to use. However, it has some shortcomings because weight measurements give predominantly the weight of the largest particles in the sample. Since small particles have little mass, this method offers almost no way of factoring small particle collection efficiency. Implications of the weight method are very important. Most, perhaps all, impingement-type filter manufacturers claim more than 80% efficiency for their products. They may be right, but only from one point of view. If the weight of the particulate matter collected by their filters is compared with the total weight of the particle samples from unfiltered air, they honestly obtain 80% efficiency or more by weight. Perhaps the filter traps only the 300 largest of the 300,000 particles actually in the air, but these 300 captured particles weigh enough to account for 80% of the total weight.
3) The Atmospheric Dust Spot Efficiency Method: Where small-particle efficiency is critical, the Dust Spot Test is often used. Here standard ambient air is passed through the test filter and the airstream has special test filters in front of and behind the test filter to monitor the presence of airborne particulate. Over time, both filters become soiled and are measured optically for relative soiling. These results are then translated into a filter efficiency rating. The justification for using such a test is that it is based on one of the observable effects of air pollution-the soiling effect. One drawback to the Dust Spot Test is that it uses atmospheric air. Because this air changes constantly, it is difficult to obtain repeatability for verification. As a result, many tests have to be run and the data averaged.
4) The Cold DOP Test: To overcome the drawback of the Dust Spot Test, the Cold DOP (Di-octyl Phthalate) test can be used. Cold DOP generators produce aerosol at room temperature, with particles ranging in size from 0.2 to 1.2 microns and with a mean diameter of 0.7 micron. The aerosol is introduced to the unit being tested and light scattering, due to particle concentration, is measured at the inlet and outlet of the unit. Because light scattering varies in direct proportion to particle concentration, the collecting efficiency of the unit can be expressed as a function of the difference in light scattering measured at the inlet and outlet at any given time.
5) The Hot DOP Test: In this test, DOP is evaporated by heat and condensed to form 0.3 micron particles with very little variation in size. This particle size is the most difficult for all kinds of air cleaners to collect and will normally produce a slightly lower efficiency on all kinds of air cleaning devices than the Cold DOP Method.
HEPA filters are tested using Hot DOP method. Here DOP is boiled and the vapor injected into the airstream in front of the test filter. As the vapor condenses back to ambient temperature, it forms very uniform droplets about 0.3 micron in diameter. By the use of light scattering instrumentation, upstream and downstream particle concentrations can be measured. In essence, if 10,000 .3 micron sized particles are blown into a HEPA air
filter, only 3 particles are allowed to pass through. Thus, you get the 99.97% at .3 micron rating. If you were to use the HEPA test on a 95% ASHRAE air filter they would be about 50% efficient on .3 micron sized particles once they loaded up with dust. So, HEPA air filters are at least 50% more effective at removing respirable sized airborne particles than any of the ASHRAE air filters on the market.
Installing HEPA/ULPA filters directly in the ceiling of the cleanroom is driven by the desire to minimize, if not eliminate, dust-collecting surfaces, such as the inside of ductwork, between the downstream face of the filter and the cleanroom. Remote mounting of HEPA filters is common in Less Stringent applications since the number of particles that can be contributed by ductwork downstream of the HEPA filters is small as a proportion of the amount that can be tolerated. An exception would be where a standard air– conditioning system with no cleanliness classification is being upgraded to support a cleanroom intended to carry a cleanliness rating per Federal Standard 209 or ISO Standard 14644. In that case, all ductwork downstream of the filter should be thoroughly cleaned.
The average HEPA filter, properly installed, and with frequent changes of the prefilter, should last from five to eight years. There are always unusual cases: filter used to capture hazardous particles or pathogenic organisms should, of course, be changed when they become unsafe for use. Otherwise, the resistance of the filter as indicated on a monometer or the air flow measured with a velometer is indications of need for a change.
Monitoring Relative Humidity in Controlled Environments
Relative Humidity and Controlled Environments
Relative Humidity (RH) is one of many important variables monitored and controlled in cleanrooms, negative and positive pressure rooms and generic controlled environments. Some specific types of controlled environments, such as USP 797 pharmaceutical-based rooms require close monitoring and control of relative humidity; other industries monitor and control humidity as a means of preserving product efficacy and quality.
Basically, relative humidity (RH) is the amount of moisture in the air compared to what the air can “hold” at the current temperature…
More precisely, relative humidity is the actual vapor density divided by the saturation vapor density multiplied by 100%. So, an example would be:
Readings from measuring instruments: temperature = 20.0 °C
Relative Humidity = 50%
In semiconductor cleanroom operations, relative humidity is held to much stricter standards. Typical values range from 30 to 50 percent with tolerances as narrow as ± 1 percent for some areas, such as photo-lithography. Ranges may be even less for deep ultraviolet processing (DUV).
An often overlooked capital expense within cleanroom operations is maintaining desired relative humidity ranges in makeup air. All too often, expenses associated with maintaining adequate filtration, and keeping low particulate counts are the focus of controlled environment design and operation, while variables such as temperature and relative humidity are considered. This can be a major miscalculation. If humidity gets out of control product integrity can also get out of control.
Why it matters
So, why go through all of the planning and expense to build and maintain a controlled environment and devote so much time to monitoring relative humidity? Quite simply because relative humidity affects not only room temperature (another important variable) but could degrade overall cleanroom performance and even potentially adversely affect the health and well being of cleanroom personnel and the general public.
Failure to maintain and control relative humidity could result in:
- Metal corrosion
- Static charge buildup
- Moisture condensation
- Photolithographic degradation
- Water absorption
- Bacteria growth
- Mold growth
- Pathogen growth
- Personnel comfort
- Temperature control
- Filtration effectiveness
- Fungi growth
- Prevent/Cause chemical reactions
- Affect adhesives and solvents
- Dirtier overall controlled environment
- Static electricity
Preventing Adverse Effects
In environments where relative humidity is at or above 60% it can be especially difficult to control or prevent the growth and contamination of:
- Bacteria
- Mold
- Fungii
- Viruses
Conversely, relative humidity lower than 30% can cause personnel discomfort such as:
- Dry and cracked skin
- Dryness in the eyes and mucous membranes
- Respiratory discomfort
- Electrostatic buildup and discharge
A compromise for both personnel comfort and product safety and efficacy is in the 40% to 55% range. In cases where more or less humidity is required that may result in personnel discomfort, additional safety procedures are taken to prevent electrostatic buildup and discharge. An example would be in semiconductor manufacturing.
Another consideration is in cleanroom operations where chemical reaction are taking place. Increases and decreases in relative humidity can result in undesired curing processes for epoxies and other chemical compositions – especially in the aviation industry or other manufacturing processes where adhesion is important to end-user safety.
Keeping Cleanrooms Clean
In controlled environments, and especially cleanrooms, cleanliness is the primary focus. Keeping particulate counts low is the name of the game. While expensive filtration devices and precisely engineered HVAC systems are important, noting on earth can completely eliminate all particulates from a given space. That’s right, with current physics, it is impossible. So, some particles will escape the filtration system and end up in the room; however, once in the room, relative humidity can affect the behavior of these particles in a process known as the Kelvin Condensation Effect (capillary condensation and adhesion).
When the relative humidity is higher, in the range of 60-70% or more, capillary forces creates a bonding bridge between a surface and containment; increasing particle adhesion to substances such as silicone. In some controlled environments, which require higher humidity to operate, micro-dimensional surfaces help to counteract this naturally occurring adhesive process.
Higher humidity can also create longer particulate airborne time. Denser air means particles of certain size can stay airborne longer, and thereby affect controlled room processes for longer periods of time.
Long-term Considerations
When designing and operating cleanrooms and controlled environments, one thing is for certain, no matter what you do, there will always exist the potential for unwanted dirt and particles which increases the risk to, not only personnel and the general public, but also to the processed going on in the room. The key is reducing this risk. In this case, it is very true “what you don’t know can hurt you.” Being aware of what environmental conditions, temperature and relative humidity, exist in your controlled environment at all times is critical.
There are dozens of environmental monitors on the market which measure relative humidity. Some of these instruments also monitor other variables such as temperature and differential pressure. The most important consideration is precision. In a critical/controlled environment, never purchase a product based on cost. Your instrumentation should be based on a proven track record of success, and already used by companies that have a lot to lose. Look at the instrument spec sheets, compare with similar products on the market. If possible, call the company and see if you get a live person. If the company does not have real people answering the phone during the sales process it is a pretty good bet that you will not get a real person on the phone when you need support. You may also want to ensure your instrumentation is easy to set up, install, and monitor.
Ease of Use
Chances are, if your instrument is too difficult to use and/or understand, it won’t be used at all. Think of a relative humidity monitor as a safeguard between your company, products, personnel and end users. Explore your options. If you already have a relative humidity monitor in place, make sure you have the best you can possibly afford; one that gives advanced warning in the case of potentially unwanted room conditions. Millions and millions of dollars are lost each year because of compromised controlled environments. This is completely avoidable if your cleanroom or controlled environment can provide advanced warnings to operators and personnel.
Air Filtration and Filtering In Cleanrooms – Part 1
HVAC SYSTEM DESIGN FOR CLEAN FACILITY
HVAC systems in cleanrooms are dramatically different from their counterparts in commercial buildings in terms of equipment design, system requirements, reliability, size and scale.
What differentiates cleanroom HVAC from conventional systems?
Cleanroom design encompasses much more than conventional temperature and humidity control. Typical office building air contains from 500,000 to 1,000,000 particles (0.5 microns or larger) per cubic foot of air. A Class 100 cleanroom is designed to never allow more than 100 particles (0.5 microns or larger) per cubic foot of air. Class 1000 and Class 10,000 cleanrooms are designed to limit particles to 1000 and 10,000 respectively. Reducing the number of particles present in a cleanroom to meet one of these classes can be very complicated. Conditioning air for a cleanroom differs from a normal comfort air conditioned space , in the following ways.
1. Increased Air Supply: Whereas comfort air conditioning would require about 2-10 air changes/hr, a typical cleanroom would typically require 20 – 60 air changes and could be as high as 600 for absolute cleanliness. The large air supply is mainly provided to eliminate the settling of the particulate and dilute contamination produced in the room to an acceptable concentration level.
2. The use of high efficiency filters: The use of high efficiency particulate air (HEPA) filters having filtration efficiency of 99.97% down to 0.3 microns is another distinguishing feature of cleanrooms. The HEPA filters for stringent cleanrooms are normally located at the terminal end and in most cases provide 100% ceiling coverage.
3. Room pressurization: The cleanroom is positively pressurized (to 0.05 in-wc) with respect to the adjacent areas. This is done by supplying more air and extracting less air from the room than is supplied to it.
There is much more into the design of cleanrooms in terms of details of technology of equipment, the type of filtration, efficiency, airflow distribution, amount of pressurization, redundancy, noise issues, energy conservation etc…etc…
FILTRATION SYSTEM
Any air introduced in the controlled zone needs to be filtered. Air filtration involves the separation of “particles” from airstreams. Their removal method is almost as diverse as the size ranges of the particulates generated. Understanding separation techniques requires an exact definition of what particles are. As particles become very small, they cease to behave so much like particles as they do gas phase molecules. It is difficult to tell whether such small particles are actually suspended in air (particles) or diffused throughout it (gas or vapor). The bottom boundary where particles act as true particles is about 0.01 micron. The normal theory of separation does not apply to particles below this size and removing them from air requires techniques reserved for gaseous materials. Particles above 0.01 micron are usually considered to be filterable.
All air entering a cleanroom must be treated by one or more filters. High-efficiency particulate air (HEPA) and ultra-low penetration air (ULPA) filters are the most common filters used in cleanroom applications.
Air filters are constructed of filter media, sealants, a frame, and sometimes a faceguard and/or gasket.
1) Media is the filtering material. Common types of media include glass fiber, synthetic fiber, non-woven fiber, and PTFE. High efficiency filters use sub-micron glass fiber media housed in an aluminum framework.
2) Sealant is the adhesive material that creates a leak-proof seal between the filter media and the frame.
3) Frame is where the filter media is inserted. It can be made from a variety of materials including aluminum, stainless steel, plastic or wood.
4) Faceguard is a screen attached to the filter to protect the filter media during handling and installation.
5) Gasket is a rubber or sponge like material used to prevent air leaks between the filter and its housing by compressing the two together.
Air enters the filter through the upstream side. It flows through the filter, contaminants are taken out of the air, and the ‘clean’ air exits through the downstream side. How
‘clean’ the air is on the downstream side depends on the efficiency of the filter.
Filtration Principles
Filtration of particles relies on four main principles: (1) inertial impaction, (2) interception, (3) diffusion, and (4) electrostatic attraction. The first three of these mechanisms apply mainly to mechanical filters and are influenced by particle size.
1) Impaction occurs when a particle traveling in the air stream deviates from the air stream (due to particle inertia) and collides with a fiber. Generally impaction filters can only satisfactorily collect particles above 10 microns in size and therefore are used only as pre-filters in multi-stage filtration systems. The higher the velocity of air stream, the greater is the energy imparted to the particles and greater is the effectiveness of the principle of impaction.
2) Interception occurs when a large particle, because of its size, collides with a fiber in the filter that the air stream is passing through. In this method, particles are small enough to follow the air stream. The particles come in contact with the fibers and remain “stuck” to the fibers because of a weak molecular connection known as ‘Van- der-Waals’ Forces.
3) Diffusion occurs when the random (Brownian) motion of a particle causes that particle to contact a fiber. Diffusion works with very small particles and works in HEPA and ULPA filters. The particles are so small that they move in a random motion causing the particle to acquire a vibration mode. Because of this vibration mode, the particles have a good chance of coming in contact with the fibers. The smaller the particle, the stronger this effect is. For large particles, over one micron in diameter, this filtration mechanism has virtually no effect.
In the order list above, the most critical areas lie between interception and diffusion. Impaction and interception are the dominant collection mechanisms for particles larger than 1 µm, and diffusion is dominant for particles smaller than 1 µm.
4) Electrostatic attraction, the fourth mechanism, plays a very minor role in mechanical filtration. If a charged particle passes through an electrostatic field, it is attracted to an oppositely charged body. Such charges can be generated and imparted to particles in an airstream in much the same way as static charges develop during the combing of one’s hair or just walking across a rug.
The typical electrostatic air filter is made from polyester or polypropylene strands that are supposedly charged as the air passes through them. Whether particle charges are induced by applying energy to a dirty airstream or occur naturally, they can be valuable tools in increasing air cleaning effectiveness.
Air Filtration and Filtering In Cleanrooms – Part 3
Terminal Filters
These filters are available in two types of constructions: (1) Box type and (2) Flanged type.
1) Box type filters are more suitable for housing within the ceiling slab cutout where removal of filter is from above. Whenever filter removal is not from above e.g. in case of filter being mounted in false ceiling, flanged type of filters is used.
2) With flanged type of filters, additional housing is required to facilitate the mounting of filters and transfer the load to false ceiling members. These housings can also be provided with an alternate arrangement to transfer the filter load to ceiling slab.
Aluminum / stainless steel slotted type protective grilles can be provided under the terminal filters. The housing and grilles should be epoxy/stove enamel painted.
Pre-filters to HEPA Filters
In order to prolong the service life of HEPA filters, pre-filters are recommended to filter out majority of particles above 1 micron. Pre-filters are normally mounted in a separate plenum with an access door after supply air fan discharge at an appropriate location. Normally flanged filters are used for mounting in such plenums.
It should be convenient to clean and replace these filters without disturbing the rest of the filtration system.
Pre-filters are available in various sizes with 6” and 12” thickness and with pressure drop in the range of 0.2” to 0.25” w.c. However, dust holding capacity of these filters is poor. For applications which require a filtration system with good dust holding capacity, bag type filters with fiberglass scrim cloth media are recommended. These can have efficiencies from 85% (down to 20 microns) to 99.97% (down to 5 microns).
For years, a value of 90 fpm (0.46 m/s) ±20% has been used to specify the airflow in the cleanest of cleanrooms. The primary objective is to maintain airflow in parallel flow streams that has two purposes: first, it needs to dilute particle concentrations that may have formed in the room due to personnel or process activity and second, to carry away particles or contaminants generated within the room. Although, higher air velocity is advantageous in particle removal/settlement, this will also result in over sizing of equipment that may be very energy inefficient, leading to higher energy costs.
Set velocity of 90 FPM! Is it Mandatory Requirement?
There is nothing called set velocity; the 90 fpm velocity is just a widely accepted practice. There is no scientific or statutory basis for this guideline. The figure 90 fpm velocity is purely derived from past practices over two decades and has become a common industry practice. In recent years, companies have experimented with lower velocities and have found that airflow velocity specifications ranging from 70 to 100 fpm (0.35 to 0.51 m/s) ± 20% could be successful, depending on the activities and equipment within the room. For example, in an empty room with no obstructions to the airflow, even the air velocities @70 FPM should remove contamination effectively. There is no single value of average velocity or air change rate accepted by the industry for a given clean-room classification. In general, the higher values are used in rooms with a greater level of personnel activity or particle-generating process equipment. The lower value is used in rooms with fewer, more sedentary, personnel and/or equipment with less particle-generating potential.
Airflow based on Air change rate (ACR)
Air change rate is a measure of how quickly the air in an interior space is replaced by outside (or conditioned) air. For example, if the amount of air that enters and exits in one hour equals the total volume of the cleanroom, the space is said to undergo one air change per hour. In addition to air change rate, air flow rate is measured in appropriate units such as cubic feet per minute (CFM) and can be calculated with this formula;
Air flow rate = Air changes x Volume of space/ 60
Air change rate is an indication of the air-tightness of a room, but it is difficult to pin down because it depends significantly on how the room is used, as well as the wind and temperature differentials experienced during the year. Even if the rate were determined with great precision with a blower-door test, there is no assurance the resultant value would apply under different conditions. The air change per hour criterion is most commonly used in cleanrooms of less stringent cleanliness. Intermediate cleanrooms are usually designed with hourly air change rates between 20 and 100, while less stringent cleanrooms have hourly air change rates up to 15. The designer selects a value based on his experience and understanding of the particle-generating potential of the process. This is a highly subjective process, which better than nothing, is not very scientific.
Higher ACR equate to higher airflows and more energy use. In most cleanrooms, human occupants are the primary source of contamination. Once a cleanroom is vacated, lower air changes per hour to maintain cleanliness are possible allowing for setback of the air handling systems. Variable speed drives (VSD) should be used on all recirculation air systems allowing for air flow adjustments to optimize airflow and/or account for filter loading. Where VSD are not already present, they can be added and provide excellent payback if coupled with modest turndowns. The benefits of optimized airflow rates are
1) Reduced Capital Costs – Lower air change rates result in smaller fans, which reduce both the initial investment from construction cost. A 20 percent decrease in ACR will result in close to a 50 percent reduction in fan size.
2) Reduced Energy Consumption – The energy savings opportunities are comparable to the potential fan size reductions. According to the fan affinity laws, the fan power is proportional to the cube of air changes rates or airflow. A reduction in the air change rate by 30% results in a power reduction of approximately 66%. A 50 percent reduction in flow will result in a reduction of power by approximately a factor of eight or 87.5 percent.
Designing a flexible system with variable air flow can achieve the objectives of optimized airflow rates. Existing systems should be adjusted to run at the lower end of the recommend ACR range through careful monitoring of impact on the cleanroom process (es).
Criteria for Selecting Citations and Studies for This Review:
Articles dealing with outbreaks of infection due to environmental opportunistic microorganisms and epidemiological- or laboratory experimental studies were reviewed. Current editions of guidelines and standards from organizations (i.e., American Institute of Architects [AIA], Association for the Advancement of Medical Instrumentation [AAMI], and American Society of Heating, Refrigeration, and Air-Conditioning Engineers [ASHRAE]) were consulted. Relevant regulations from federal agencies (i.e., U.S. Food and Drug Administration [FDA]; U.S. Department of Labor, Occupational Safety and Health Administration [OSHA]; U.S. Environmental Protection Agency [EPA]; and U.S. Department of Justice) were reviewed. Some topics did not have well-designed, prospective studies nor reports of outbreak investigations. Expert opinions and experience were consulted in these instances.
Pressurization In Commercial Buildings
Why Pressurization Matters in Commercial Buildings
Untreated outdoor air leaks into — infiltrates — a building when indoor pressure is less than the pressure outside. Control strategies typically strive to limit or eliminate infiltration as a means of minimizing HVAC loads and related operating costs. Infiltration isn’t always bad, however. During the heating season, for example, a small amount of dry outdoor air leaking into the building envelope discourages moisture from condensing there.
But excessively negative pressure causes problems. Uncomfortable drafts and stratification interfere with temperature control and may encourage odor migration. Outward- swinging doors become difficult to open, and inward-swinging doors fail to reclose, compromising security in addition to efforts to keep heating and cooling cost down.
Any amount of infiltration during the cooling season can raise the dew point within the building envelope, which increases the likelihood of microbial growth and structural deterioration. Infiltration of warm, moist air also affects occupied spaces by increasing the possibility of mold.
Conditioned indoor air leaks out of — exfiltrates from — the building when the pressure inside is greater than the pressure outside.
During the summer, exfiltration of cool, dehumidified indoor air benefits the building by keeping the envelope dry. But excessively positive pressure makes opening and closing doors difficult and creates noisy high-velocity airflow around doors and windows. It can also wreak havoc with temperature control by continuously leaking already conditioned air to the outside.
During the winter, even slightly positive pressure inside a building forces moist indoor air outside the building envelope. Moisture may condense on cold surfaces inside walls, hastening structural deterioration. Ideally, the net pressure inside the building relative to outside should range from slightly negative or neutral during cold weather (minimizing exfiltration) to slightly positive during warm weather (minimizing infiltration). Excessive building pressure, whether negative or positive, should be avoided.
Variables that may affect pressure
Preventing extreme building pressures, either positive or negative, is much easier said than done. In most structures, the indoor – outdoor pressure difference results directly from the combined effect of weather, wind, and operation of the mechanical ventilation (HVAC) system.
Exhaust airflow — which may be central or local, constant or variable — carries contaminants from the building. Local codes or industry standards define how much exhaust air must be removed from specific types of spaces (rest rooms, for example), regardless of pressure-related concerns or operating mode.
Weather: Like a column of water in a pipe, the weight of a column of air results in a “head” pressure that increases from the top of the column to the bottom. Described as hydrostatic pressure, more commonly known as “stack pressure,” the weight of the air column is affected by local barometric pressure, temperature, and the humidity ratio.
Temperature-related differences in indoor and outdoor air density create differences in pressure that can affect infiltration, exfiltration, and the direction of air movement within shafts and stairwells.
Relief airflow removes air from the building (again, either centrally or locally) to balance barometric pressure, temperature, and humidity ratio.
When indoor air is warmer than outdoor air, the less dense column of air inside the building results in a net negative pressure below the neutral pressure level (NPL) and a corresponding net positive pressure above it. Because all building envelopes contain unavoidable cracks and openings, this pressure difference allows outdoor air to enter the lower floors and indoor air to leave the upper floors. These leakage characteristics also encourage upward airflow — normal stack effect — within shafts and stairwells.
When indoor air is cooler than outdoor air, the column of air inside the building is more dense. The result is a net negative pressure at the top of the building and a corresponding net positive pressure at the bottom. Unless building pressure is controlled, outdoor air will infiltrate the upper floors while indoor air exfiltrates from the lower levels. The pressure difference also induces downward airflow in stairwells and shafts — reverse stack effect.
Putting it all together
As you can see, it is important to consider variables in addition to air pressure for maintaining correct negative or positive air pressure in commercial facilities. Knowing, controlling and maintaining temperature and humidity can positively or adversely affect pressurization, which has a big impact on heating and cooling costs. An independent monitoring solution that consistently and accurately monitors and alerts according to current room environmental condition is important. Instrumentation should easily display current conditions and alert personnel of changes in desired ranges in respect to temperature, relative humidity and room differential pressure.