Return air

Side and bottom supply return air duct openings (side x side)... 

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. 

Possible return air from the upper zone into the lower zone... 

Recirculation The process of returning air to the space from which it has been removed, with or without treatment. 

Return air Air that has been removed hom a space that is returned to the space either mechanically or naturally with or without treatment. 

The plant should be able to hold the room at the desired temperature when heat is not being dissipated in the room. The plant should also be able to raise the temperature of the room from cold under winter conditions (e.g. after a power failure or an outage). In considering the temperature to be achieved under test the comments given in test 2 of Section 29.9.6 also apply here. If hot water heating is used, full temperature elevation will not be achieved because high return air temperatures will reduce the coil output. For instance, where the design rise was 22°C above an external of —1°C 19°C, 29°C and 14°C are satisfactory with ambient of 5°C, 10°C and 15°C, respectively. 

Air entering the conditioning plant will probably be a mixture of return air from the conditioned space and outside air. Since no heat or moisture is gained or lost in mixing. 

Figure 25.1 Pre-heating of outside air and mixing with return air -process iines...

Example 26.3 Return air from an office picks up 90% of the input of 15 kW to the lighting fittings. Of this return air flow, 25% is rejected to ambient. What is the resulting heat gain from the lights ... 

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. 

Outdoor air is treated and pre-conditioned by a central water-to-air heat pump. The outdoor air is delivered into the return air plenum of the zone heat pump units. A total enthalpy wheel can reduce the energy consumed for treating outdoor air. 

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. 

PGDN was generated from a sample of Otto Fuel II by blowing air across a Pyrex reservoir of the compound to the return air duct of the air conditioner. Eighty percent of the air was recirculated. The concentration of PGDN in the air was monitored continuously by an infrared spectrophotometer and by a gas chromatograph fitted with an electron capture detector. The vaporized Otto Fuel II was 99% pure PGDN as measured by infrared analysis. 

Weaving Humiducts on Well Mater ACP-2, ACP-3. Figure 6 shows a weave room test wherein it was not practical to install low solids water. In this test, the atomizers were either operated normally or turned off for 30 to 45 minutes (ACP-2) or the looms stood and the atomizers were left on and turned off (ACP-3). The air-conditioning consists of humiducts operating on relatively high water hardness. The humiducts had good return air filtration but no mist elimination. 

Return air to air handling plant shall not be product contaminated. 

When sizing return air ducts, keep in mind that calculated room leakage rates are empirical and will vary depending on construction tolerances the acutal return air (R.A.) quantities may vary from theoretical design. Return ducts should be sized to handle at least 25% more than design airflow. Return air plenum designs should be avoided unless there is no potential for contamination problems. 

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. 

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