Medical air systems are a lifeblood 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 architectural disciplines.
The first priority is the safety of life. Medical air is used for respiratory therapy and 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, the shutdown of a pipeline or utility 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 pipeline. Medical air systems must:
• Be powered by cylinders, bulk containers, or medical air compressor sources; or reconstituted from dry USP oxygen and nitrogen without oil
• meet the requirements of medical air
• do not contain detectable liquid hydrocarbons
• contain less than 25 ppm gaseous hydrocarbons
• contain 5 mg/m3 or less of permanent particles 1 micron or larger at normal atmospheric pressure.
In a typical operating healthcare facility, medical air is supplied by a high pressure cylinder manifold system or medical air compressor system. Multiple distribution systems are generally used in facilities that have very little demand for medical air. Medical air compressor plants are typically for larger facilities.
Existing facilities may choose to upgrade their associated equipment and plumbing 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 to him when designing for a new installation 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 design contains a well-ventilated and easily accessible mechanical room dedicated to medical gas equipment.
When selecting a medical air compressor for an upgrade, the engineer may run into some issues due to mechanical utility inefficiencies (eg, poor chilled water quality, poorly ventilated mechanical space). The local power utility may not support pump layout, or poor access to equipment may require breakdown of equipment parts at a significant increase in cost. Thorough studies 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 owner may want to get a cost estimate before making a final decision.
Types of compressors
All medical air compressors must be able to supply compressed air that is free of oil. This article is specifically about medical air systems for Tier 1 hospitals.
There are three acceptable types:
• Oil-free compressors: These reciprocating compressors do not have an oil film on the surfaces exposed to the air being compressed. They have oil in the machine and require the 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 have no oil in the machine. Lubrication is limited to the seal bearings.
• Liquid Ring Pump – These rotary air compressor pumps have a water seal. It is recommended to use a heat exchanger to conserve seal water.
Medical air compressor plants must be sized to meet the calculated peak 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 to create redundancy. There should never be less than two compressors.
accessory equipment
Various pieces of mechanical equipment accompany the medical air compressor system:
• Inlet: The air inlet of the compressor must be located outdoors, above roof level, and at least 10 feet from any door, window, other entrance, or other opening. Inlets must be closed, guarded, 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. Must comply with Section 8 Boiler and Pressure Vessel Construction Standards of the American Society of Mechanical Engineers (https://asme.org). 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 should be a valved duplex system to allow maintenance of one unit. 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% system capacity, with a minimum efficiency of 98% at 1 micron or better. 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 must be sized for 100% of the calculated maximum demand of the system at design conditions. Pressure regulators should be set to provide the furthest output with 50 psig medical air.
• Alarm Sensors – A medical air compressor should have alarm sensors located nearby where hospital staff 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: They must be provided for compressors, receivers and dryers, as required by the manufacturer.
Pipeline
Medical air lines are sized based on the calculated flow rate in cubic feet per minute (cfm). Compressed air piping is constructed of welded type L copper prepared for oxygen service. The pipe must be inclined towards the central plant, have drains in the low points and must be valved and identified.
The flow rate for medical air vents is typically 1 cfm. Pipe flow 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 rate is then applied to the 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 has been applied, the main piping and compressor package can be effectively sized and selected.
In summary, 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 most recent code requirements related to medical gas systems. Both the information available and the technology develop daily, and it is the responsibility of the engineer to be informed.