By Compressed Air Best Practices
The NFPA 99 (National Fire Protection Agency) Standard for Healthcare Facilities (2005 Edition) is the current Code by which Healthcare facilities in the U.S. design their compressed air systems. The NFPA 99 Standard covers many requirements for medical gases, with compressed air being just a component of the Standard. To understand the complete scope of the Standard, including information on air compressor requirements, it can be purchased at http://www.nfpa.org/catalog/
Healthcare facilities have two separate compressed air systems, which fall under the Code of NFPA 99. They are the Medical Air and Instrumentation Air Systems. They are two completely independent systems which are not allowed, by Code, to be connected. Both systems have specific dew point and air quality requirements. Both systems are monitored by specific local and master alarm systems. This article discussed the Medical Air technologies used to meet the Standard.
Medical Air Systems
There are two types of Medical Air Systems. The definitions of Level 1 and Level 2 Systems are below from the NFPA 99 Code:
Level 1 Medical Piped Gas and Vacuum Systems. Systems serving occupancies where interruption of the piped medical gas and vacuum system would place patients in imminent danger of morbidity or mortality.
Level 2 Medical Piped Gas and Vacuum Systems. Systems serving occupancies where interruption of the piped medical gas and vacuum system would place patients at manageable risk of morbidity or mortality.
Medical air is produced by medical air compressors at 100 psig air pressure. This air is introduced into medical air dryers and filters at this pressure. The air is then pressure regulated down to 55 psig and distributed to occupancy areas for Level 1 or 2 applications. A pressure drop, in the piping distribution, is accounted for and the end use pressure is 50 psig.
Medical Air Dew point
Both Level 1 and Level 2 Medical Air have the same dew point specification. The NFPA 99 Standard states:
“The medical air dryer shall be designed to provide air at a maximum dew point that is below the frost point (0 C (32 F)) at any level of demand”.
The phrase “at any level of demand” is important. The NFPA 99 Standard goes on to state that dryers must be sized to deliver the specified dew point at peak calculated demand. Peak calculated demand, therefore, represents the full load conditions for which a dryer must be sized. This allows dew point quality to be maintained even when a crisis hits a hospital and all air consuming devices are in use.
The reality in hospitals is that actual demand is typically only 33% of peak-calculated demand. The estimate of 33% is unofficial, yet is agreed upon by many industry experts. This means that the air dryer, which must provide dew point at “any level of demand”, must be able to provide a dew point of 32 F at 33% load.
Medical Air Dew point Alarms
Medical air quality monitoring requirements are very specific in the NFPA 99 Standard. They are outlined as follows:
Medical Air Quality Monitoring. Medical air quality shall be monitored downstream of the medical air regulators and upstream of the piping system as follows:
Dew point shall be monitored and shall activate a local alarm and all master alarms when the dew point at system pressure exceeds +4°C (+39°F).
Carbon monoxide shall be monitored and shall activate a local alarm when the CO level exceeds 10 ppm. [See 126.96.36.199.4(2).]
Dew point and carbon monoxide monitors shall activate the individual monitor’s signal at all master alarm panels if the monitor loses power.
Medical air dew point, therefore, is monitored at both the dryer’s control panel (at 100 psig pressure) and downstream after the pressure regulators (at 50-55 psig pressure). Any presence of moisture, in the pipelines, will trigger the downstream alarms. There are different types of dryers used to eliminate the presence of moisture; refrigerated, membrane and desiccant-type dryers. All are capable of complying with the NFPA 99 Standard while facing different challenges to do so.
Refrigerated Air Dryers
Refrigerated-type compressed air dryers can provide pressure dewpoints which comply with the NFPA 99 specification of 32 F (0 C). They will first supply a pressure dew point of 38 F (3 C) at 100 psig pressure. Hankison refrigerated air dryers; a brand owned by SPX Corporation, have long been used for medical air. “When air with a 38 F (at 100 psig) pressure dew point is regulated down, the pressure dew point becomes 25.9 F at 55 psig and 24.1 F at 50 psig”, says Timothy J. Fox, Manager, Research and Development, SPX Dehydration & Process Filtration.
Refrigerated air dryers operate on the principle of using a refrigeration circuit to cool compressed air, in a heat exchanger, and provoke the condensation of moisture in the air. The condensed moisture is then separated, from the compressed air stream, by a moisture separator inside of the dryer. The compressed air now leaves the refrigerated air dryer at the design dew point of 38 F (3 C) at 100 psig pressure.
Moisture Separators in Refrigerated Air Dryers
The moisture separator, obviously, plays a critical role in the dryers’ ability to dry the air. A common problem, with some refrigerated air dryer designs, is that the moisture separator only performs, to a high degree of effectiveness (90-99+% moisture removal), when experiencing a full, 100% load. Moisture separators, in refrigerated air dryers, have varying designs and degrees of effectiveness – particularly under the partial load conditions of 33% commonly experienced in hospitals. This is why the NFPA 99 Standard specifically says “the dryer will provide the specified dew point at any level of demand”. There are many types of moisture separators used inside of refrigerated air dryers. A few are listed below:
- Mechanical Centrifugal Separator
- Velocity vs. non-velocity sensitive design
- Integrated/Heat Exchanger Separator
- Combination Filter/Separator
The traditional Centrifugal Separator can have a problem with a lack of air velocity at loads of 33%. These “velocity-sensitive” separators use centrifugal action to throw moisture droplets out of the air stream. Under partial load conditions like, 33% of load, there is insufficient air velocity to force the moisture droplets out of the air stream. The moisture droplets simply are re-entrained into the air stream and continue downstream of the dryer.
Some heat exchangers, inside of refrigerated air dryers, have a compartment which acts as a bulk separator of liquids. Effective at full loads, some of these separators have been known to see reduced performance efficiency, at reduced loads.
There are some designs, of mechanical separators, which do create enough turbulence to provide effective separation in low-flow conditions. These “non-velocity sensitive” designs were created with low-flow conditions in mind. Different compartments and chambers are added to the centrifugal action, to ensure the separation of the moisture droplets. Some manufacturers, like Domnick Hunter – a division of Parker Hannifin, will even offer an end user a third-party certification of effectiveness at low-flow conditions. “End users are glad to receive third-party validation from Lloyd’s Register, that the Domnick Hunter moisture separators perform at low load conditions”, says Tony Hergert, Market Development Manager at Domnick Hunter.
Another effective design is the “filter/separator”. This is a separator design which flows the air through a two-stage filter element. The air flows through the element, from the inside to the outside. The first stage is made up of perforated stainless steel and blocks the larger droplets of moisture. The second stage is made up of coalescing filter media and this is where the finer droplets are coalesced. This second stage is 99% effective at low-flow conditions, such as those seen in hospitals.
Membrane Air Dryers
New technical advancements, with membrane air dryers, are causing industry experts to re-examine this technology. Membrane air dryers can deliver customized dew points, to meet the NFPA 99 Standard, ranging from -20 F to +32 F. They also come in very small, “filter-like” housings, which make for easy installations. Historically, the negative factor about membrane air dryers has been that they used a lot of purge air to sweep the moisture out of the air stream. Recently, SPX Hankison introduced a new SweepSaver™ membrane air dryer product line, which reduces purge air requirements. The product uses a demand signal to open and close an integrated two-way valve to control the sweep air. This new purge control capability, combined with its’ inherent simplicity and compactness, makes membrane dryers a possible technology for the future.
Desiccant Air Dryers
Another type of compressed air dryer, the desiccant-type dryer, has become the dryer technology of choice in healthcare facilities. Over the past fifteen years, the desiccant air dryer has replaced the refrigerated air dryer. A NFPA 99 Committee Member, Mark Allen of BeaconMedaes, estimates that, “Medical air systems use desiccant air dryers 90% of the time”. The primary reason for this change was the installation of many refrigerated dryer designs, over the years, which do not have moisture separators which are effective at 33% load conditions. When dew point alarms became mandatory, in the NFPA 99 Standard, many hospitals began experiencing daily alarms. In most cases the alarms would activate overnight when there was very little demand for compressed air. During the day, with higher levels of demand, the alarms would work well.
Desiccant air dryer designs are more expensive than refrigerated air dryer designs- but they are not negatively affected by low-flow conditions. Compressed air passes through a bed of adsorbent material (normally activated alumina), which removes the moisture from the compressed air stream. Industrial desiccant air dryers are normally sized to produce dew points capable of -40 F and -100 F pressure dew points. The traditional design is a twin-tower design with two pressure vessels holding the adsorbent material. This design is very effective in providing the required dew point at all flow rates.
Some newer designs, of desiccant air dryers, have gained popularity in the healthcare industry. Some designs continue using the twin tower design but have been designed to provide a 14 F (-10 C) pressure dew point. An example of this is the BeaconMedaes Lifeline Dryer. Designing a twin tower desiccant air dryer to meet the NFPA 99 Standard for dew point has reduced the size of the towers and therefore the cost of the unit. These dryers also integrate dew point controls and alarms, required by NFPA, into the dryer package to further reduce system costs.
Another popular design is the “modular-type” desiccant air dryer. Designed to provide dew points of -4 F, these “modular” desiccant air dryers are very compact and easy to get through doorways and into elevators. This is often a significant issue for healthcare facilities who often have their compressed air equipment in areas with difficult access.
Medical Air Dew point Summary
Level 1 and Level 2 Medical Air Systems are required, by the NFPA 99 Standard, to provide a 32 F pressure dew point at supply pressure. Medical air is produced and dried at 100 psig pressure and then regulated down to 55 psig for distribution in the pipelines. Both local and master dew point alarms, per NFPA 99, will activate when pressure dew point exceeds 39 F. Refrigerated and desiccant air dryers are both technologies capable of achieving the pressure dew point requirement at the supply pressure of 50-55 psig. The low-flow characteristics of hospitals, normally 33% load, make it a challenging application for many refrigerated dryer designs due to issues with the moisture separators. As a result, the healthcare market has moved towards desiccant air dryers and is considering new technical advancements with membrane air dryers. These technologies are providing more reliable dew point performance, at partial loads, with new designs reducing cost and size issues experienced in the past.