Airflow rate

For heat transfer directly to solids, predictive equations give directly the volume V or the heat-transfer area A, as determined by heat balance and airflow rate. For devices with gas flow normal to a fluidized-solids bed,... 

A nominal resiilt of this techniqne is that the reqnired airflow rate and eqnipment size is abont two-thirds of that when evaporative cooling is not nsed. See Sec. 20 for eqmpmeut available. 

Air Flow Cartridge collectors are currently limited to low air flow capacity applications. Standard cartridge collectors are factory-built, off the shelf units. They may handle airflow rates from less than 0.10 to more than 5 standard cubic meters per second (smVsec) (("hundreds" to more than 10,000 standard cubic feet per minute (scfm)). 

Air-handling units equipped with heat recovery and sophisticated control of the key parameters of HVAC systems, such as temperature, airflow rate, and pressure difference... 

The main target levels inside a building are normally set for temperature, humidity, air velocity, airflow rate (and air distribution), and contaminant concentration.1 -... 

The airflow rate does not interfere with the building or the building materials, and usually not with the process. The air distribution may vary a lot, however, if the building and process layout and sometimes the building materials are not chosen with great care. Lamps, ladders, window shutters, cabins, process equipment, etc. are often placed where the ventilation designer did not expect any obstructions. Thus continuous cooperation between the different designers is necessary to prevent mistakes and to facilitate common solutions to this type of problem (see Chapter 8). 

Normally there is some connection between the airflow rate and noise and vibration generation. This could modify the building construction either to prevent spreading or to diminish the levels of noise and vibrations from the air-handling units. This naturally includes all parts of these units, i.e., fans, pumps, and valves (see Chapters 5 and 9). The demands on noise insulation also include the noise and vibrations from the process equipment, which often has a higher level of noise and vibration than the ventilation system. 

The influence of the building on the ventilation system, and indirectly on the target levels, can take many forms. There are, e.g., emissions from the materials that demand increased airflow rates and also door and window open-... 

In general, it is mandatory to have some kind of partition between different parts of a building because of fire risks and regulations. These partitions can easily change both the airflow rate and the air distribution if proper preventive measures are not taken. 

Local exhausts arc designed to capture air pollutants and heat at the source, and thus their location and the exhausted airflow rate should ensure sufficient capture velocity. 

Entrainment ratio is another jet characteristic commonly used in air distribution design practice. Specifically, it is used in analytical multizone models (see Chapter 8) when one needs to evaluate the total airflow rate transported by the jet to some distance from a diffuser face. Airflow rate in the jet, Q,., can be derived by integrating the air velocity profile within the jet boundaries ... 

Equations for airflow rate computation in compact, linear, and radial jets are presented in Table 7.13. 

TABLE 7.13 Airflow Rate through a Jet Cross-sectional Area... 

Thermal plumes above point (Fig. 7.60) and line (Fig. 7.61) sources have been studied for many years. Among the earliest publications are those from Zeldovich and Schmidt. Analytical equations to calculate velocities, temperatures, and airflow rates in thermal plumes over point and line heat sources with given heat loads were derived based on the momentum and energy conservation equations, assuming Gaussian velocity and excessive temperature distribution in... 

A point source has a convective heat output of 100 W (see Fig. 7, >2). Determine the airflow rate 1 m above the source. 

Calculate the airflow rate along an external wall with a surface temperature 3°C above room remperature, at a height of 4 meters above the lower edge of the surface. 

In reality, heat sources are seldom a point, a line, or a plane vertical surface. The most common approach to account for the real source dimensions is ro use a virtual source from which the airflow rates are calcu-lared " " see Fig. 7.64. The virtual origin is located along the plume axis at a distance on the other side of the real source surface. The adjustment of the point source model to the realistic sources using the virtual stmrce method gives a reasonable estimate of the airflow rate in thermal plumes. The weakness of this method is in estimating the location ol the virtual point source. 

The airflow rate from a heat source can then be calculated as half of the flow from a source with a heat emission of... 

Calculate maximum air velocity, airflow rate, and excessive temperature (relative to the ambient air temperature equal to 20 °C) in thermal plume above the heated cube (0.66 m x 0.66 m x 0.66 m) with convective heat production = 225 W, at heights of 2.0 m and 4.0 m above the floor level. Neglect temperature gradient along the room height. Compare the results with predictions made for the same case using CFD code (Fig. 7.80). 

FIGURE 7.79 Airflow rate in the plume above the cylinder of Example 7.S.S. 

There is a very good correspondence between the analytical and numerical results for temperature and velocity. The airflow rates differ, however, with a factor of 1.46 at a height of 2 m, whereas the correspondence at 4 m is very good. 

For nonenclosing hoods, the airflow rate that allows contaminant capture is called a target airfloitO The target airflow rate is proportional to some characteristic flow rate Qg that depends on the type of contaminant source ... 

For a buoyant source q can be equal to the airflow in the convective plume at the hood suction cross-section. For a dynamic source q can be equal to the airflow rate in the jet. 

An exhaust airflow rate lower than q results in reduced contaminantcapturing effectiveness. An exhaust airflow rate greater than q g results in excessive capturing effectiveness (Fig. 7.81). 

FIGURE 7.81 Hood performance for different exhaust airflow rates, (a) Target airflow rate q (b) Target airflow rate q < q. (c) Target airflow rate q > q. ... 

Numerical simulation of hood performance is complex, and results depend on hood design, flow restriction by surrounding surfaces, source strength, and other boundary conditions. Thus, most currently used method.s of hood design are based on experimental studies and analytical models. According to these models, the exhaust airflow rate is calculated based on the desired capture velocity at a particular location in front of the hood. It is easier... 

The airflow rate infiltrating and exfiltrating through each air leakage pass, Q , due to the combined effect of wind, stack, and mechanical ventilation system perfotmance can be calculated ftom the mass balance equation... 

The airflow rate Q, for each air leakage path is expressed with Eqs. (7.237), (7.242), and (7.243) using the infotmation on effective leakage area, CjA, and a pressure difference across the path. The total pressure acting on an opening from the outside is the sum of the pressure due to wind, gravity forces, and mechanical ventilation performance, and the static pressure inside the building results from Eq. (7.244). 

The airflow rate through an opening area with a height of dh can be calculated as... 

The infiltrating and exfiltrating airflow rates can be calculated using the following equations ... 

Natural ventilation design allows one to size the inlets, and outlets, / p based on their pressure loss characteristics, Cp, and on the airflow rate, G , required to maintain the occupied zone within desired limits. The reverse design procedure is commonly used to evaluate the airflow rate through the building given the sizes, characteristics, and locations of inlets and outlets and the heat load and characteristics of heat sources. 

The use of a natural ventilation system assumes temperature stratification throughout the room height. Air close to heat sources is heated and rises as a thermal plume (Fig. 7.105). Part of this heated air is evacuated through air outlets in the upper zone, and part of it remains in the upper zone, in the so-called heat cushion. The separation level between the upper and lower zones is defined in terms of the equality of and G, which are the airflow rate in thermal plumes above heat sources and the airflow supplied to the occupied zone, respectively. It is assumed that the air temperature in the lower zone is equal to that in the occupied zone, and that the air temperature in the upper zone is equal to that of the evacuated air,... 

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. 

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