Air exchange efficiency

Application of the age of air concept can be justified by the fact that the content of contaminants found in the exhaust air normally rises from the value found in supply air entering the room. On its voyage through the room, the air is likely to pick up more contaminants the longer it stays in the room. This is a very simple assumption. It can be argued, however, that using the age of air concept is the best way to evaluate ventilation design for scenarios where little or no information is available on use of the room and locations and emission rates for heat and contaminant sources. 

In order to have effective exchange of air in important locations in a room, the age of the air in those locations should be low. The basis for comparison is the complete mixing scenario. That scenario gives the same age for any air volume selected in the room, identical to the nominal time constant for the ventilation airflow,. A steady-state scenario is assumed. See Sutcliffe for an overview of definitions related to age of air. The various air exchange efficiency indices are presented in Table 8.6. 

The average age of air for all air molecules in the complete room can be found by performing a step-up tracer gas experiment, and by measuring tracer gas concentration Q in the exhaust opening. The same procedure can be used for CFD simulations. The definition for average age of air in the room is 

For ideal displacement ventilation, e. Values for the other indices depend on the location of the zone in question. 

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Air exchange efficiency indices can be used for cases where no or little information on sources is available, whereas ventilation efficiency, which concerns workers, can be used where very detailed information is available on sources and activities. 

The concentration of indoor pollutants is a function of removal processes such as dilution, filtration, and destruction. Dilution is a function of the air exchange rate and the ambient air quality. Gases and particulate matter may also be removed from indoor air by deposition on surfaces. Filtration systems are part of many ventilahon systems. As air is circulated by the air-conditioning system it passes through a filter which can remove some of the particulate matter. The removal efficiency depends on particle size. In addition, some reactive gases like NOj and SOj are readily adsorbed on interior surfaces of a building or home. 

For supply inlets in rooms some performance measurements exist, such as air exchange and ventilation efficiencies (see Chapter 8). It is usually not possible to use these for local ventilation supply inlets, and for the moment there are no specific measurements to evaluate the influence of an inlet on contaminants. Some trials with comparison indices, which compare inhaled concentrations (or exposures) with and without a supply inlet, have been done. 

From nearly tight coverings and roofs only a very small flow rate of produced gases can be emitted. The emission by air exchanging is set to zero. Therefore the odour reduction efficiency of those coverings compared with an uncovered open manure surface is 95 to 100%. 40 points can be accepted, fig.4. 

Smoke (carbon) formation, which apparently is due to incomplete combustion of portions of the fuel-air mixture (i.e., rich combustion), also can pose a serious public relations problem at civilian airports and, by radiant-heat transfer from incandescent carbon particles, can shorten the endurance life of combustion-chamber liners and adjacent parts (0). Smoke would also constitute a serious problem in the case of automotive gas turbines, because accumulation of carbon and other nonvolatile fuel components on the intricate passages of the heat exchanger could reduce turbine and heat-exchanger efficiency by reducing heat-transfer rate and increasing the pressure drop across the... 

P = penetration efficiency (dimensionless) a = air exchange rate (h ) k = decay rate indoors (h 1)... 

The most common procedure for large-scale aerobic fermentations is to use deep tanks of a stirred liquid medium. Efficient air exchange is obtained by continuous pumping of sterile air into crossed or coiled perforated pipe placed at the bottom of the tank at rates of up to one volume of air per unit of medium volume per minute [62]. This method promotes aerobic growth throughout the medium, which accomplishes large volume production in a limited space. But it also has problems, which relate to the maintenance of sterility and occasionally from foam formation. Overall, however, submerged... 

If reservoirs and sumps can be tightly sealed so that all air exchanged between the atmosphere and the head-space can be directed through a single port, then high-quality filter breathers can be used to remove dust from incoming air at that port (vent). The quality of the filter (capture efficiency) should be no less than that of the oil filter in use. 

Hydrogen is an ideal fuel for gas turbines. Due to its rapid mixing with air, a smaller combustion chamber is sufficient and the efficiency is higher compared with conventional fuels. Gas turbines modified for liquid hydrogen operation yield an up to 10 % higher thermal efficiency and output compared with fossil-fueled turbines. For systems with advanced heat exchange, efficiencies of more than 50 % are estimated to be achievable. The remainder-free combustion is stable and favorable for lifetime and maintenance. Of disadvantage is NOX production. No particular difficulties are expected for a conversion of a stationary gas turbine to H2 fuel [51]. 

Processes carried out at a pH of 7-9 represent the most efficient use of this type of enzyme. The first industrial process involving this type of protease was in the bating of hides to remove the debris from the skin after liming, and to impart softness and air exchange to the finished leather. Although pancreatic bates are still in general use proteases derived from Aspergillus flavus-oryzae and Bacillus subtilis now supple ment them. 

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