Engineering Forum. [Alexander Delli Paoli, Jr.

The HVAC Process

Alexander Delli Paoli, Jr.

Welcome to “Engineering Forum.” The first series of articles will discuss HVAC pro- This feature discusses applied technical principles cesses for warehouses, offices, laboratories, clean- associated with engineering topics that focus on use- rooms, containment facilities, and other manufac- fulness to practitioners in validation and compliance. turing environments. The series will also touch on We intend this column to be a resource for daily work specialized applications such as containment hoods, applications. biological safety cabinets, and dust collection systems. Engineering technology is a topic of great impor- Subsequent articles will address each of these topics in tance in contemporary pharmaceutical and medical greater detail, driving toward a working familiarity of device manufacturing. Validation and compliance HVAC systems and their typical challenges. professionals must become familiar with facilities, Reader comments, questions, and suggestions for equipment, utilities, and other systems at their sites. discussion topics are needed to help us fulfill the Global regulatory agencies require more than just column objective. Please send your comments and compliance to procedures in the modern manufac- suggestions to column coordinator Alex Delli Paoli turing organizations. They require understanding of at [email protected] or to journal managing processes and topics controlled by site procedures. editor Susan Haigney at [email protected]. Sound procedures, policies, and literally everything involved in regulated manufacturing must be based KEY POINTS on scientific principles whenever possible, must be The following key points are discussed: appropriate for risk, and must be continually moni- • Understand the reason your site needs different types tored and maintained. Outsourcing of manufacturing of HVAC systems and testing functions has further extended these • Recognize all the HVAC components and their responsibilities. purpose Engineering topics are not generally well known • Understand how all the components interact or understood beyond the engineering community. • Be aware of the physics of air: Personnel resources have been reduced, and indi- • Assure proper safety measures are in place vidual responsibility has increased. Those with lim- • A system is only as good as its maintenance and ited background and experience may find themselves compliance to design criteria. involved in areas with which they have little expertise. Further, the technical information supporting their INTRODUCTION areas may be esoteric and incomprehensible for Heating, ventilation, and (HVAC) sys- those not trained in the field. We intend to address tems are critical to pharmaceutical and medical device engineering topics with these considerations in mind. manufacturing. These systems comprise multiple com- These topics will be discussed clearly and in a mean- ponents that are integrated in a reliable process to create ingful way so that our readers will be able to under- a sustainable environment. stand and apply the principles discussed in daily work This discussion provides an overview of the process situations. used to bring air from the randomly variable mixture of

For more Author ABOUT THE AUTHOR information, Alex Delli Paoli, Jr., P.E., is managing director of Engineered Strategic Visions, Libertyville, IL, USA. go to Engineered Strategic Visions, Inc. (www.engineervisions.com) is an engineering consulting firm special- gxpandjvt.com/bios izing in project planning and management, manufacturing support, asset and energy management, [and other areas of expertise. He may be reached by e-mail at [email protected].

20 Journal of Validation Technology [Autumn 2011] ivthome.com Alexander Delli Paoli, Jr.

vapors and particles found in the atmosphere to a steady The HVAC process may be compared to a typical manu- state condition acceptable for the environment the system facturing process (see Figures 1 and 2). The manufacturing serves. The following will be discussed: process utilizes individual pieces of equipment to process • HVAC definition—the scope of HVAC and what active drugs and excipients into a final pharmaceutical general areas are encompassed product in commercial packaging. The HVAC process • The process of HVAC—HVAC systems include mul- utilizes multiple equipment components and operations tiple equipment that transforms variable air states to convert incoming air to processed air that meets facility to achieve target attributes needs. Most facilities require a broad scope of air quality. • Why process the environment?—the reasons that For example, a facility might include sterile product manu- air must be processed facturing, non-sterile product manufacturing, warehous- • Components and function of the (e.g., es, docks, offices, and other areas—each of which will the preheat coil and filters, cooling coil, return , have differing HVAC requirements. Further, there may be exhaust fan, and associated equipment) different requirements within a given area. For example, • Applications (e.g., warehouses, offices and records a non-sterile manufacturing area may require rooms with storage, laboratories, cleanrooms, containment facili- very low to process moisture sensitive drugs. ties, and general manufacturing areas). The HVAC system and its component subsystems must provide the required environmental conditions. HVAC DEFINITION HVAC is a hybrid engineering discipline based on HVAC Process Comparison to aspects of classical mechanical, electrical, and chemi- Manufacturing Process cal engineering practices. It includes knowledge of fluid The HVAC system process is generally similar to a product mechanics, machine design, and instrument control. It manufacturing process. Both convert input materials further includes a working knowledge of the physical by means of process parameters into a final product properties of several liquids and gases. Material compat- meeting required quality attributes. There are important ibility knowledge is also important for avoiding chemical differences that the HVAC system must overcome to interaction with dissimilar metals, incompatibility with accomplish its objectives. These include input material , and other deleterious effects due to cleaning considerations, process capability, and process changes. agents used in the facilities served by the HVAC units. Input materials. The manufacturing process converts Practical considerations such as physical layout and access drugs and inactive materials to pharmaceutical products. for maintenance along with a good sense of aesthetics The incoming materials should be well controlled by are also important to the discipline of HVAC. means of vendor approval and supply chain control. The HVAC started as a formal practice during the Indus- HVAC process converts air from the external environment trial Revolution when it was recognized that mines and to air possessing the quality attributes required for the factories needed to be ventilated (V) (i.e., air contain- respective areas in the site. The input air is uncontrolled, ing noxious fumes and particles needed to be replaced may have unexpected content (e.g., fumes, , with fresh breathable air). Heating (H) was also a need pollen, and insects) and has seasonal variation. Winter beyond the home to warm shops and factories air will have lower moisture content than summer air. Fur- in northern climates. Air conditioning (AC) became ther, the summer humidity may approach 100% in tropi- practical with the invention of the mechanical refrig- cal areas for extended periods during the rainy season. eration process. It was an obvious extension to H and Process capability. The manufacturing process V. Occasionally, R for is included when has defined process limits. These limits are usually not refrigerators and freezers are involved in a project. This exceeded because input materials are well controlled. In is typically shown as HVAC/R. contrast, the HVAC process may be challenged beyond the limit of design capabilities depending on environ- THE PROCESS OF HVAC mental variation. Traditional textbook explanations of HVAC systems Process changes. The manufacturing process is gen- often describe these systems as integrated pieces of erally well established and remains relatively constant equipment (e.g., air handler, fans, ductwork) having throughout the product lifecycle. The process is often individual functions. These descriptions ignore the pro- approved by regulatory agencies and is not able to be cess of HVAC to produce a “product”—air with desired changed without regulatory approval. In contrast, the temperature, humidity, particulate level, and other qual- HVAC process has significant seasonal variation requiring ity attributes. cyclical heating and cooling. These are significant process gxpandjvt.com Journal of Validation Technology [Autumn 2011] 21 Engineering Forum.

Figure 1: Manufacturing process. changes. Periodic checks to assure the HVAC systems are operating as intended are key to consistent operation. MANUFACTURING PROCESS API WHY PROCESS THE ENVIRONMENT? Excipient Excipient Any manufacturing, testing, or storage facility and their Excipient related processes strive for a steady state consistent envi- Manufacturing process unit operation ronment to deliver reliable products to customers. Even

Equipment #1 (IQ, OQ, PQ)* offices are expected to be consistent so as not to distract Equipment #2 (IQ, OQ, PQ) employees from their responsibilities or create office Equipment #3 (IQ, OQ, PQ) Excipient equipment failures. Excipient Manufacturing process unit operation HVAC Considerations Equipment #4 (IQ, OQ, PQ) The following are major parameters to consider when Manufacturing process unit operation looking at a process and the environment’s impact: Equipment #5 (IQ, OQ, PQ) Equipment #6 (IQ, OQ, PQ) • Temperature. Temperature is . This Bottles heat can be felt or sensed because it causes a rise or Caps Inserts fall in temperature. Packaging process unit operation • Humidity. Humidity is an indication of the level of

PRODUCT / PROCESS COMPLETED water vapor in the air. Humidity is typically expressed as relative humidity. HVAC and facility professionals * IQ=Installation qualication; OQ=Operations qualication; PQ=Performance qualication. utilize dew point to indicate moisture content. A psy- chrometric chart is a necessary tool for understanding the current state of the air and water vapor mixture (Figure 3). Humidity changes in controlled environ- ments usually contribute to the cooling load of a facil- ity. This load is called . Latent indicates a phase change from vapor to liquid, or visa versa. At the Figure 2: HVAC process. other end of the control demand is humidification. In Outside Air seasonally cooler climates, moisture must be added Return Air Fan Roughing Filters to air to increase humidity to within desired limits. Preheat Coil • Particulate contamination. Particle load may occur from the outside or from within the facil- Mix Air Streams, Preheat and Filtration ity. The wind and the season can play big parts in Filters contributing to the amount of particulate in the air Cooling Coil being introduced into an HVAC system. Internal Humidi er particulate loading can come from people, corrugate, facility materials of construction, and manufacturing Intermediate Filtration, Moisture Content Adjustment or testing contaminants. Particulate can be viable Supply Fan or non-viable. Both have their potential hazards to Re ned lters the facility environment. Motive energy imparted to air, additional ltration • Vapor contamination. Vapor contamination can Supply air distribution ductwork come from internal and external sources similar to In- heating coils particulate contamination. A site HVAC system near Supply delivery devices (diffusers, terminal lters) a highway can draw in detectable levels of hydro- carbons. A diesel engine inadvertently operating Trim air temperature, deliver and distribute to facility outside the intake of a HVAC system can bring odors and vapors into warehouses, offices, laboratories, Return air distribution ductwork and manufacturing areas served by the unit, caus- Collection devices (Fume hoods, etc.) ing discomfort or illness to the building occupants. Exhaust air ductwork Exhaust fan Improper amounts of outside ventilation air can cause carbon dioxide build-up with resulting oxygen HVAC PROCESS COMPLETED (cycle repeats) deprivation concerns.

22 Journal of Validation Technology [Autumn 2011] ivthome.com Alexander Delli Paoli, Jr.

• Pressure. Air pressure is assumed to be Figure 3: Psychrometric chart (courtesy of Climate Solutions, copy- incompressible in most HVAC applica- right Trane). tions. This assumption comes into ques- tion when elevated pressure or vacuum is needed in a process environment. Sites located in higher elevations must also not make the same assumptions that those living nearer to sea level make about sys- tem component selection. Most com- ponent capacities are de-rated at higher altitudes. • Safety. Safety is an important system capability often assumed or not adequate- ly considered. Fire protection consider- ations, adequate ventilation for personnel, fail-safe automatic controls, hazardous fume and particulate containment devices, and properly designed egress are among many facility safety concerns.

There are other factors that affect the way air is processed in an HVAC system, though one cannot analyze a single cubic foot of air to understand the impact of these design characteristics. Future articles in this series will address these factors in detail. The following are temperatures. The water is then returned to the water some of these factors: to take heat away from the facility. Open water • Ratio of outside air (OA) to return (recirculated) circuits require filtration to remove the same atmospheric air (% OA) contaminants to which air intakes are exposed. Both open • Amount of air moving through the HVAC system and closed water circuits require treatment additives. relative to the amount of air needed to heat or cool All HVAC systems using water in any form contain the facility additives to protect the piping and components from • Localized heat, humidity, and particulate issues temperature extremes, corrosion, and bioburden. This within a facility article will not discuss further detail about additives. It • Efficiency of air distribution and circulation within is important that validation and quality professionals a facility. become familiar with the additives in the systems at their site. The potential impact of additives should be Water Content in Air considered in any failure mode analyses performed. Water must also be included in any discussion about HVAC. Water is included because it is an effective heat AIR HANDLER transport medium at temperatures at and above 100∘C COMPONENTS AND FUNCTION (steam) as well as at temperatures approaching the freez- The central point of an HVAC system is the air handling ing point of 0∘C. unit (AHU) or air handler. Figure 4 provides a schematic Water is often used in closed circuits transporting heat drawing of an air handler. Air is shown moving from left between HVAC systems and remotely located equipment to right. This figure shows all the typical components such as water to produce and boil- used in an air handler. There are many variations in this ers to make hot water and steam. It is also used in open type of equipment. The system described in Figure 4 circuits to transport the rejected heat from water chillers to will serve as an effective starting point. Components large atmospheric heat exchangers called cooling towers. discussed include the following: Evaporative cooling is used in these towers by spraying • Preheat coil and filters water into an airstream within the tower. This sensible • Filters and latent cools the water to near dew-point • Cooling coil gxpandjvt.com Journal of Validation Technology [Autumn 2011] 23 Engineering Forum.

• Return fan Filters • Exhaust fan The air then continues through a finer set of filters to • Other components including ductwork, dampers, continue removal of finer particulates. Each successive sensors, and smoke detectors. set of filters is more efficient that the previous set. This is done to keep larger atmospheric dust particles from Preheat Coil and Filters plugging the finer downstream filter(s). Outside air (OA) and air returning (Return Air, RA) from the facility served by the unit are combined at Cooling Coil the back end of the AHU and filtered to create air The air next passes through a cooling coil. As it passes at some intermediate level of temperature, humid- through this , the air temperature drops to ity, and cleanliness (mixed air). Space is provided a prescribed temperature chosen so the air can effectively between components within the air handler to allow cool the facility it is serving. The air will likely get to its access by maintenance personnel. The air then passes dew-point temperature during the cooling process. At through a preheat coil (heat exchanger) to raise the this point and as it continues to cool, the air has become temperature of the air to a temperature that will not saturated with water vapor. Liquid water will form on the expose subsequent components to freezing hazards. cooling coil surfaces and run down the coil surfaces to a The preheat coil is not required in warmer climates drain pan. The water is then removed from the air handler where temperatures never go below the freezing point through a drain. The cool saturated air is then drawn of water. The preheat coil is usually heated by steam through a fan, which imparts the energy needed to gener- or -treated hot water. ate the pressure necessary to drive the air through all the Figure 4: Schematic diagram of a HVAC system.

24 Journal of Validation Technology [Autumn 2011] ivthome.com Alexander Delli Paoli, Jr.

air handler components and the ductwork distribution • The actual ductwork. Ductwork must allow minimal system. At this point, the air is heated by fan as it passes leakage to assure the intended volumes and condi- through the system. This raises the air temperature a few tion of air is delivered. Improperly constructed duct- degrees, bringing the air above its dew-point temperature work can be noisy, creating rattling or “tin canning”­ and avoiding condensation that could occur if the air when system adjustments happen. Above all, the was still saturated. At the other extreme, if the humidity ductwork must be of the correct size and made of of the air was very low as happens in cooler seasons in materials compatible with the vapor stream flowing northern climates, the dew-point temperature would within it. Typical ductwork is usually constructed on never be reached, and the HVAC control system could galvanized sheet metal. High levels of water vapor detect the need to add moisture to the environment. A or corrosive vapors flowing through an exhaust duct using steam or atomized water would then could dictate the need for an alternate material. achieve the desired humidity level by adding this vapor • Manual dampers for balancing airflow throughout into the air stream. the air distribution system. • Automatic dampers to control airflow in the system. Return Fan • Automatic control system sensors to measure tem- A return fan is also shown on the schematic. It is com- perature, humidity, airflow volume and air pressure mon to have an HVAC system with only one fan. Systems at key positions. operating in a compliance-oriented industry tend to have • Heating coils to warm air being delivered to specific more components that create more pressure drops as the rooms for individual temperature control. air passes through each. Rather than having one fan that • Life safety components such as smoke detectors, heat gets larger and larger with a greater pressure demand, it detectors, and fusible link fire dampers. is desirable to use two fans—a return fan and a supply • Terminal filters where a cleanroom type environ- fan. The return air fan draws the air through the return ment is desired. These filters are typically of high side of the ductwork distribution system. The supply fan efficiency, and they are the last item the air passes draws air from the outside and also draws the mixed through as it is introduced to the clean room. air stream through all the air handler components. The supply fan also drives the air through the supply duc- APPLICATIONS twork distribution system. The two-fan arrangement HVAC systems are present in every facility and have a also offers a push-pull configuration that is desirable in wide range of applications. Some applications might seem several applications. simple on the surface until regulatory implications and unintended consequences are discussed. The subtleties Exhaust Fan of each application may be touched upon in this article, When fumes, particulate, or other contaminants are but will be addressed in greater depth in subsequent generated in a facility, an exhaust system is required. The articles in this series. typical components are fan filters, cyclone separators, scrubbers, and a purpose-specific capture apparatus. Warehouses The apparatus can include fume hoods, biological safety The word warehouse may suggest images of a large boxy cabinets, and other custom-made enclosures all intended building with fork trucks running all around and truck to confine and conduct the contaminants to the exhaust dock doors routinely open to the atmosphere. Perhaps ductwork. Once in the ductwork, the contaminant-laden there is no issue ventilating the space with all that help air is treated in potentially many different ways. If the from nature, but it could be a real challenge to heat, cool, contaminant is not an environmental hazard such as and keep clean with the aforementioned openings to the simple particulate, the exhaust could go to the atmo- environment and potential sources of contamination. sphere with minimal processing. As the complexity and Most warehouses operating in a regulated environment hazardous nature of the exhaust air increases, differing are under much closer control. Truck docks are often types and degrees of filtration and other treatments can treated with close fitting dock door seals and localized be applied. heating, cooling, and filtration as a first defense to keeping the atmosphere out of the controlled environment. Pest The Rest of the System control measures are in place to keep rodents and flying Other system components are usually located in the insects from penetrating too far before being intercepted. supply and return ductwork, including, but not limited Ceiling–mounted propeller fans move warm air from the to, the following: underside of the roof down, creating a circulation pattern gxpandjvt.com Journal of Validation Technology [Autumn 2011] 25 Engineering Forum.

that draws cooler air up. This allows for a reasonably uni- ing manager should have a procedure for periodically form temperature distribution throughout the warehouse. verifying that the proper ventilation rate is being met. This is particularly important if the product being stored This could be done continuously with airflow measure- must reside within restricted ­temperature limits. ment by the control system, or by periodic physical test- Air handlers serving warehouses must have a first set and-balance reviews. In either method, a history record of filters to remove coarse atmospheric dust. A second should be maintained demonstrating a consistency across set of finer filters is often included in the air handler reviews. This discipline applies to all HVAC systems with to keep atmospheric dust fines from becoming visible a life-safety element, and not just office systems. deposits on the materials and products stored in the area. Heating and cooling coils will likely be present in Laboratories the air handler if the warehouse has operating limits for The basic laboratory HVAC system is similar to an office temperature and humidity. Humidity is important to HVAC system. Laboratories need a desk either in the control to keep the product packaging from developing lab or in a segregated area adjacent to the lab. Labora- mold or other environmentally introduced blemishes. tory facilities are usually served by an isolated HVAC The warehouse air handler will obviously need a supply system dedicated to the laboratory environment. While fan to drive the air through all its components and the the desks and offices for lab personnel are often on the supply distribution ductwork. A return fan is often not lab system, it is not considered good practice to expose included unless the HVAC system has return distribu- personnel in general office areas to laboratory risks (e.g., tion duct work. solvent fumes and particulates) that are controlled in the The amount of outside air the air handler is required laboratory facilities. to introduce to the warehouse facility is dependent on Many different types of laboratory environments exist. the amount of exhaust air required, the minimum ven- They can be wet or dry described by the type of activities tilation air required by local building codes, and the occurring in the facilities. Much of the electronics indus- number of gas-powered forklift trucks operating in the try uses dry labs where testing and assembly occurs. They building. Additional outside air may also be introduced have their own set of considerations for ventilation air to pressurize the warehouse building to form another and exhaust containment. Wet labs typically use liquids barrier against atmospheric contaminants. as the basis for their science. Sink and water sources are An exhaust system is definitely required over electric located throughout to source water and allow personnel forklift battery charging areas to capture hydrogen. Other to wash hands and have access to safety appliances such exhaust systems could also be needed to support ware- as emergency eyewashes and safety showers. house operations requirements to keep fumes, particu- Airflow in a laboratory is important. The following late, and other potential hazards from being introduced are potentially conflicting purposes to be addressed with into the general building environment. the lab HVAC system: • General circulation. Effective ventilation circulated Office and Records Storage throughout air supplies and returns must be distrib- HVAC systems serving office environments are usually uted across the lab in a way that promotes effective not captured in any product compliance regulations currents and room-air-change-turnover to avoid unless samples to be ultimately sold are brought into stagnant dead spots. office areas for testing or inspection. This could also • Cooling of lab instrumentation. The air distribution apply to documentation storage areas. The areas used for must deliver the correct quantity of air to the vicinity such functions could come under scrutiny, bringing the of heat-producing lab instruments to keep both the entire air handler into the qualification and validation equipment and the lab occupants operating properly. arena. Baring that situation, a typical office HVAC system • Avoidance of stray air currents near containment includes two sets of filters, preheat and cooling coils, a devices. Discharging air at relatively high velocities supply fan, supply and return air distribution systems, near a or other containment device can and a return fan. Exhaust systems are required in toilets cause eddy currents that negate the collection airflow and kitchens, and they may be required in break areas of the device and introduce contaminants into the and printing and reproduction areas. lab. Attention to this detail should obviously be a Ventilation is generally specified by the governing local high priority. building code. This can take the form of air changes of • Room pressurization versus containment. It is outside air per hour or cubic feet per minute of outside air desirable to keep some labs at a positive air pres- per person occupying the facility. Either way, the build- sure relative to adjacent areas to keep potential stray

26 Journal of Validation Technology [Autumn 2011] ivthome.com Alexander Delli Paoli, Jr.

contaminants from entering the laboratory work- The more a critical operation can be isolated in a space. This desire is often in conflict with life safety “mini” cleanroom environment, the less costly the requirements dictating that exit corridors and other operation can become. This is especially true where a evacuation routes should be pressurized to permit specific process must be conducted in a cleanroom while safe egress from the facility. There is no one answer requiring containment for some recognized hazard. to this conflict. The best answer is found when the Isolators are containment devices that can serve both lab owners, the facility management, and local loss needs. While the cost of the isolator may seem high, prevention authorities meet and come to a sensible the overall cost of the facility may make it a worthwhile compromise after evaluating all the risks. investment. This is particularly true when trying to adapt an existing facility and all its equipment and infrastruc- Cleanrooms ture to a new use. The HVAC demand in cleanroom facilities is significant. Often the amount of air being circulated has more to do Containment Facilities with room air change rate per hour than it does with the Containment facilities are unique HVAC system appli- heat load requirements. Air volume demands can range cations that work in conjunction with the facilities’ from a historical “conventional” cleanroom with 20 air physical layout, materials of construction, and operat- changes per hour to complete unidirectional cross-flow ing procedures to create an environment that isolates or down-flow airflow with room air changes occurring the object or process from the adjoining area. This is several times per minute. The type of cleanroom style typically more than a fume hood in a laboratory. It is dependent on several factors. includes facilities for potent drugs, biological hazards, The amount, source, and consequences of contamina- radiological hazards, explosion hazards, and many tion must be understood. If the particulate generator(s) other unique applications. can be isolated using containment devices, or if a critically clean area can be isolated in a pressurized device, there Non-Aseptic General is a good chance a more conventional approach to the Manufacturing Facilities cleanroom HVAC system can be taken. This helps mini- Many manufacturing areas have elements of the facilities mize the size and operating costs of the system. Often described herein. Temperature requirements, humidity the biggest contamination source in a cleanroom is its restrictions, microbial controls, and other design and human occupants. Minimizing the number of people operational requirements may be needed depending on in the room along with appropriate garments for those the type of product manufactured and associated pro- that remain can help lessen the clean-up demands on cesses. It is important to define the level of a controlled the HVAC system. If this is not possible and if the con- environment before designing or modifying an HVAC tamination control needs are great, it may be necessary process to serve it. to go to a more complex cleanroom design. Positive pressurization is usually desirable in clean- SUMMARY rooms to help keep the surrounding environment out. The reader should by now have related some aspect of Pressurization requires airflow through every door and this discussion to their individual experiences. This opening around the perimeter of the cleanroom, along article is an overview intended to bring all readers to a with unfound cracks and gaps that seem to occur in base level understanding of HVAC systems along with cleanrooms from time to time. The air flowing out of typical applications containing anecdotal information. the facility must be replaced with outside air, putting All the topics discussed will be revisited in greater a greater demand on the air handler to process that detail in subsequent articles. We will discuss specific air. This typically means more frequent filter changes, applications, the functional nature of system compo- higher dehumidification loads on the cooling coils, and nents, validation, maintenance, operational consistency higher demands on the site’s infrastructure supporting and sustainability. All readers are encouraged to submit the HVAC system. questions to be addressed as the details unfold. JVT Heat and humidity generation can be an issue in a cleanroom. Heat and humidity generation must be ARTICLE ACRONYM LISTING considered in conjunction with the placement of heat AHU Air Handling Unit generating equipment and the attire occupants must HVAC Heating, Ventilation, and Air Conditioning wear. These garments often increase the propensity for OA Outside Air perspiration. RA Return Air gxpandjvt.com Journal of Validation Technology [Autumn 2011] 27