Medical air systems are a vital element of all hospitals and most other healthcare facilities. The engineer must consider expense, capacity, physical size and weight, space limitations, and the availability of mechanical and electrical services when choosing a system for a particular project. It is important to coordinate equipment selections with the owner, as well as other engineering and architecture disciplines.
The first priority is life safety. Medical air is used for respiratory therapy and the calibration of medical devices for respiratory applications. Providing clean, oil-free air is mandatory. The medical air system should not be used to supply air for any other purpose (eg, hospital laboratory use) due to the potential for contamination of the distribution system. If a patient inhales medical air contaminated with oil from a failed compressor or nitrogen from a brazing purge, the consequences could be irreversible. In addition, shutdown of a pipeline or utility service should be coordinated with hospital staff to prevent accidental termination of service while patients are connected to the system. Engineers must be aware of the requirements before designing any medical gas system.
Distribution Systems
Medical compressed air systems must be designed to prevent the introduction of contaminants or liquids into the piping. Medical air systems must:
• be fed by cylinders, bulk containers, or medical air compressor sources; or reconstituted from USP oxygen and oil-free dry nitrogen
• meet medical air requirements
• contain no detectable liquid hydrocarbons
• contain less than 25 ppm of gaseous hydrocarbons
• contain 5 mg/m3 or less of permanent particles 1 micron or larger at normal atmospheric pressure.
In a typical fully operational healthcare facility, medical air is supplied by either a high pressure cylinder manifold system or a medical air compressor system. Multiple distribution systems are typically used in facilities that have very little demand for medical air. Medical air compressor plants are usually for larger installations.
Existing facilities may choose to upgrade their associated equipment and piping or add medical air plants as the facility expands. When selecting equipment for a new installation, the possibility of future expansion should be considered. To allow for future growth, it is good practice to be conservative when sizing a system.
Duplex Medical Air Compressor Supply Systems
An engineer generally has more options available when designing for a new facility than for a renovation or replacement project. Electrical and mechanical services can be more easily calculated, and chilled water, ventilation, and electrical services can be properly sized and located. The ideal schematic layout contains a well-ventilated, easily accessible mechanical room dedicated to medical gas equipment.
When selecting a medical air compressor for an upgrade, the engineer may have some issues due to mechanical utility inefficiencies (eg, poor chilled water quality, poorly ventilated mechanical space). The local power company may not support the pump layout, or poor access to equipment may require equipment parts to fail at significant cost increase. Thorough surveys of the surrounding mechanical space and utilities are imperative before determining the best type of compressor for the project.
It is a good idea to select more than one compressor type at the schematic design stage. You must develop a master plan showing existing demand and estimated additional capacity. The homeowner may want to get a cost estimate before making a final decision.
Compressor types
All medical air compressors must be able to supply compressed air that does not contain oil. This article is specifically about Level 1 hospital medical air systems.
There are three acceptable types:
• Oil-free compressors: These reciprocating compressors have no oil film on surfaces exposed to the air being compressed. They have oil in the machine and require separation of the oil containing section of the compression chamber by at least two seals. The interconnecting shaft and seals must be visible without disassembling the compressor.
• Oil-free compressors: These reciprocating or rotary-scroll compressors do not have oil in the machine. Lubrication is limited to seal bearings.
• Liquid Ring Pump – These rotary air compressor pumps have a water seal. It is recommended to use a heat exchanger to conserve the water in the seal.
Medical air compressor plants must be sized to meet the calculated maximum demand when the largest compressor is out of service. In an efficient design of a larger system (ie three pumps or more), each compressor is sized to handle an equal percentage of peak demand and create redundancy. There should never be less than two compressors.
accessory equipment
Several pieces of mechanical equipment accompany the medical air compressor system:
• Inlet: The compressor air inlet must be located outdoors, above roof level and at least 10 feet from any door, window, other doorway, or other opening. Inlets must be closed, protected and equipped with inlet filter silencers. These filters remove large amounts of particulates (microscopic particles of solid or liquid matter suspended in the air) and contaminants at the compressor inlet.
• Air reservoir: The function of the air reservoir is to store air and balance pressure variations. It should have a full size bypass as well as a manual and automatic drain to remove any accumulated condensate. It must meet the American Society of Mechanical Engineers (https://asme.org) Section 8 boiler and pressure vessel construction standards. Receiver sizing is based on system demand, compressor size, and compressor run times.
• Compressed Air Dryer: The dryer is used to remove water vapor from the air stream. At a minimum, it must be a valved duplex system to allow maintenance of one unit. The dryers must be of the double desiccant tower type, sized for 100% of the calculated load at design conditions. They must be rated for 32°F (0°C).
• Duplex Final Filters – These should be rated for 100% of system capacity, with a minimum efficiency of 98% at 1 micron or greater. The filter must be equipped with a visual indicator that shows the remaining useful life of the filter element.
• Medical Air Regulators: Regulators control the pressure of the air system. They should be sized for 100% of the maximum calculated demand of the system under design conditions. Pressure regulators should be set to provide the furthest outlet with 50 psig medical air.
• Alarm Sensors – A medical air compressor should have alarm sensors located nearby where hospital personnel can continuously monitor them. Typical alarms are for high pressure, low pressure, and other problems (eg, lead/lag pump operation, high temperature, high dew point, carbon monoxide). Additional alarm signals can be added based on compressor type and owner preference.
• Anti-vibration mounts: Must be provided for compressors, receiver and dryers, as required by the manufacturer.
Pipeline
Medical air lines are sized according to the calculated flow rate in cubic feet per minute (cfm). Compressed air piping is constructed of welded Type L copper rated for oxygen service. The pipe must slope towards the central plant, have drains at low points and must be valved and identified.
The flow rate for medical air outlets is typically 1 cfm. The pipeline flow rate is calculated by counting the number of connected medical air outlets and applying a usage factor. The flow rate of the total number of outlets is called the total connected load. Since not all outlets are normally used at the same time, a simultaneous use factor must be applied to reduce system flow. The fee is then applied to pipe sizes and compressors. The American Society of Plumbing Engineers (https://aspe.org/) has developed a table that quantifies the use of medical air in different areas of the hospital.
When the total connected load has been calculated and the usage factor applied, the main pipe and compressor set can be effectively sized and selected.
In short, engineers must take care when sizing and specifying medical air equipment to meet the needs of the healthcare facility and its patients. Before beginning a project, be sure to review the requirements related to medical gas systems in the latest codes. Both available information and technology develop daily, and it is the responsibility of the engineer to be informed.