Return-air system

Wisthaler et al. (2005) used PTR-MS to examine the products formed when ozone reacted with the materials in a simulated aircraft cabin, including a loaded high-efficiency particulate air (HEPA) filter in the return air system. 

Each floor of the building was divided into 2 air-conditioned zones, which were served by their respective AH Us, located at each end of the floor. Linear diffusers were employed for supply air and the return air system was based on the ceiling plenum concept. The floor plans for the 4th, 11th and 17th floors, in which the studies were carried out, are quite similar and a schematic representation is provided in Figure 10.7. 

Depending on the airborne contaminants in the return-air system it may be acceptable to use recirculated air, provided that HEPA filters are installed in the supply air stream to remove contaminants and thus prevent cross-contamination. The HEPA filters for this application should have an EN1822 classification of H13. 

The recirculating air stream is considered an integral portion of the life safety system therefore, in a hazardous occupancy cleanroom (such as a semiconductor wafer fab), the system should not be automatically shut down by the smoke detection system. In lieu of automatic shutdown, a manually operated remote switch is to be provided at an approved location outside the fabrication area (UBC 1202.2.5). Since such a detection could happen at any time, our recommendation for the location of the switch is in a continually occupied but relatively secure spot, such as the nearest Emergency Control Station (ECS). UBC 1202.2.5 also dictates that the return air system of one hazardous cleanroom area shall not be connected to another eleamoom oeeupancy. 

Outdoor air is generally less polluted than the system return air. However, problems with reentry of previously exhausted air occur as a result of improperly located exhaust and intake vents or periodic changes in wind conditions. Other outdoor contamination problems include contaminants from other industrial sources, power plants, motor vehicle exhaust, and dust, asphalt vapors, and solvents from construction or renovation. Also, heat gains and losses through the building envelope due to heat conduction through exterior walls, floor, and roof, and due to solar radiation and infiltration, can be attributed to effects from external sources. 

FIGURE 8.1 Model of a central recirculating system used for calculating the connection between contaminant concentrations, airflow rates, contaminant source strength, q, and air cleaner efficiency, rj. Cj p is the concentration in the supply (outside) air, c is the concentration in the room, c is the concentration in the returned air, (JaMot the total flow rate through the room, ic is the ratio between recirculated airflow rate and total air flow rate, T is the time constant for the room, and V is the room volume. 

One of the most common systems for cleaning air in homes, offices, schools, etc. is the room air cleaner. Figure 8.2 outlines a model of a local recirculating system. Usually these units are situated inside the room if they are small and movable (see Chapter 10). For the model it does not matter if the unit is placed inside or outside the room with the contaminant source, as long as the exhaust and return air openings are inside. 

Local ventilation in industry usually differs from the description above in that it is connected to a local exhaust hood (Chapter 10), which has a capture efficiency less than 100%. The capture efficiency is defined as the amount of contaminants captured by the exhaust hood per time divided by the amount of contaminants generated per each time (see Section 10.5). Figure 8.3 outlines a model for a recirculation system with a specific exhaust hood. Here, the whole system could be situated inside the workroom as one unit or made up of separate units connected with tubes, with some parts outside the workroom. For the calculation model it makes no difference as long as the exhaust hood and the return air supply are inside the room. 

As with all convective systems, warm air heating installations produce large temperature gradients in the spaces they serve. This results in the inefficient use of heat and high heat losses from roofs and upper wall areas. To improve the energy efficiency of warm air systems, pendant-type punkah fans or similar devices may be installed at roof level in the heated space. During the operational hours of the heating system, these fans work either continuously or under the control of a roof-level thermostat and return the stratified warm air down to occupied levels. 

With a supply system pressure of 1 kPa and another 250 Pa for the return air duct, the total fan energy of a central all-air system may amount to 12.5% of the maximum installed cooling load, and a much greater proportion of the average operating load. This power loss can only be reduced by careful attention to design factors. 

Return air for the HVAC system should not be supplied from the crawlspace. It is best to avoid routing return air ductwork through the crawlspace, but if it must be, then it should be thoroughly sealed with duct tape at a minimum. It should be understood, however, that duct tape may dry out and fall off. A better approach would be to use seamless ductwork in these areas. The use of floor joists and subflooring as three sides of a return air plenum should be avoided because of the difficulties encountered in sealing. If the space between the joists must be used, an alternative to ducts is to use a rectangular duct to fit the space. 

Note On large air handling unit systems, consideration should be given to bypassing the cooling coil with part of the return air to minimize the amount of reheat required. With coil bypass, pretreating the outside air for dehumidification may be required. 

Repeated pulsing can be used to provide efficient removal of non-condensable gases. The system is evacuated to a pressure lower than 50 mbar. Steam is then injected to return the system to atmospheric pressure. The system is then re-evacuated prior to injecting the steam for sterilization. This pulsing can be repeated several times to achieve improved air removal. 

A typical AHU fitted with a purging system is presented in Figure 10.6. The additional equipment includes an axial fan, air-duct connections, and motorized shutoff dampers and a control system which is part of the building automation system (BAS). Before each purging operation, the fresh air and purging dampers would be opened and the return air damper would be shut The AH U and the purging... 

Figure 2.6 illustrates the main components of an airhandler system, including fans, heat exchanger coils, dampers, ducts, and instruments. The system operates as follows Outside air is admitted by the outside air damper (OAD-05) and is then mixed with the return air from the return air damper (RAD-04). The resulting mixed air is filtered, heated, or cooled, and humidified or dehumidified as required. The resulting supply air is then transported to the conditioned zones (groups of offices) by the variable-volume supply fan station. 

During the industrial age, COz concentration in the ambient air increased from 280 to 360 ppm, and some predict that it could rise to 550 ppm if the use of fossil fuels continues. In addition to monitoring the atmosphere, air-quality-related measurements can also be used in heating, ventilation, and air conditioning (HVAC) systems to monitor the return air quality from occupied spaces. COz is also measured at emission points because some combustion equipment regulations limit, or probably will limit, allowable discharges. 

FIG. 11 -69 Typical central-station air-conditioning unit and control system. On a rising room wet-bulb temperature, the wet-bulb branch-line air pressure increases through the reverse-acting outdoor-air wet-bulb temperature-limit thermostat Ti to open gradually the maximum outdoor-air damper Di and simultaneously closr return-air damper Dg, then gradually open chilled-water valve Vi- On a rising room dry-bulb temperature, the dry-bulb branch-line air... 

There should be no failure of a supply air fan, return air fan, exhaust air fan or dust extract system fan. Failure can cause a system imbalance, resulting in a pressure cascade malfunction with a resultant airflow reversal. 

The airflow schematics of the two systems (Figs 24 and 25) indicate airhandling units with return air or recirculated air, having a percentage of fresh air added. Fig. 25 is a schematic diagram of an air-handling system serving rooms with horizontal unidirectional flow, vertical unidirectional flow and turbulent flow, for rooms A, B and C, respectively. 

There are numerous potential locations for filters in an HVAC system. The most common filter placement location is in the mixed airstream within an airhandling unit. Using that location results in filtration of both outside air and return air, and it protects coils and other downstream components from fouling. Filters could also be located directly in the outside airstream, at supply to individual spaces, on the return from individual spaces, in the common return, and on exhaust air (if there is concern about the consequences of contaminated exhaust). Stand-alone filtration devices that recirculate and clean air within a single zone might also be desirable. 

The earlier discussion of architectural design focused attention on the potential value of compartmentalization. However, the ability of HVAC airflows to distribute contaminants throughout a building, particularly when the system recirculates return air, is far greater than the ability of architectural barriers to retard... 

The use of an air-handling duct system to return air from the interior of a building to the air-handling unit. 

To make up the difference between the expected expander returned power and the required compressor input power. Meruit designed an air system using a commercially available positive-displacement compressor driven by an electric motor in tandem with Meruit s turbocompressor. Meruit designed a family of compressor and expander wheels with var5dng characteristics (specific speed, dimensional ratios, etc.) representing different compromises in providing performance compatible with the DOE specifications. After extensive evaluation, a compressor wheel was chosen to provide a desired pressure curve over a wide range of mass flows, and a turbine wheel was matched to it. The chosen compressor wheel is intended to trade peak efficiency for extended turndown performance. 

The test conditions should be in steady-state operation at fixed low return boiler water temperature or steady-state operation at fixed outlet and inlet boiler water temperatures. Boilers are classified based on 10 criteria output function, capacity and configuration storage, fuel configuration, boiler flue gas/intake air system technology, power, boiler/bumer configuration, boiler water temperature control, burner combustion control, and installation. 

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