AIRFLOW Factors Influencing Room Airflow
Air and contaminant movement and turbulent intensity in the ventilated space are affected by different external and internal forces, such as
• Supply air jets forced into the room by mechanical systems
• Free convection currents generated by air heating or cooling by surfaces (process equipment, external walls)
• Airflow in the vicinity of local exhausts (hoods) or general exhaust (due to negative pressure in the duct produced by mechanical systems)
• Airflow forced through intended and unintended openings in the building envelope, which depends on the pressure difference across the opening resulting from wind pressure on the building envelope, temperature difference between the indoor and outdoor air, and an imbalance in the mechanical exhaust ventilation system performance versus the mechanical air supply (positive or negative pressure building)
• Air currents produced by process equipment or moving people (e. g., high-speed rotating machines such as pulverizers, high-speed belt material transfer systems, falling granular materials, and escaping compressed air from pneumatic tools)
The airflow pattern and the scale of air currents in the room depend upon the types of sources and the energy introduced by each source, as well as the configuration and dimensions of the room. The energy of the predominant turbulent flow created by each source transfers into transverse turbulent pulsations, which convert large eddies into smaller eddies. This energy is finally converted into heat. Kinetic energy of air leaving the room through exhaust openings can be neglected. Typically, exhaust openings are protected by a grill, which does not let through large or medium-size energy-containing eddies.
The energy of large and medium-size eddies can be characterized by the turbulent diffusion coefficient, A, m2/s. This parameter is similar to the parameter used by Richardson to describe turbulent diffusion of clouds in the atmosphere.1 Turbulent diffusion affects heat and mass transfer between different zones in the room, and thus affects temperature and contaminant distribution in the room (e. g., temperature and contaminant stratification along the room height—see Chapter 8). Also, the turbulent diffusion coefficient is used in local exhaust design (Section 7.6).
Studies by Elterman show that turbulent diffusion coefficients in ventilated rooms outside jets and plumes can be described using the relationship2
A = Ce1/3/4/J, 17.28)
Where C can be evaluated from the equation
C = 0.25 ± A, (7.29)
Where A = confidence interval, which depends upon the required confidence probability, as shown in Table 7.9. In most cases, the average value C = 0.25 can be used in Eq. (7.28).
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TABLE 7.9 Coefficient C
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Characteristic length, / in Eq. (7.28), depends on the application; e. g., for local exhaust design / equals the characteristic hood dimension, and for room air distribution design with a temperature or contaminant stratification, / equals the room height.
Another important parameter used in Eq. (7.28) is e, which is the kinetic energy, Eroom, kg m2/s, dissipated in the mass of air, M, kg, in time t, si
E = Eroom = kg mVs = m^ 3Q.
M t kgs s
The total kinetic energy introduced by different sources can be calculated by summation of all sources,
E-room = /L + I Econv + Em o (7.31)
Contributing energies can be calculated using the following equations:-
• Kinetic energy introduced by supply air jet:
Ejet = 5PoQoV3 (7.32)
• Kinetic energy generated by convective heat source ( Wconv):
Ј — SWCQBVH (7 33)
Where Wconv is a convective component of the heat source, HT is the
Room height above the heat source, and Cp and T0 are the specific heat and absolute temperature of the room air, respectively.
• Kinetic energy from the moving objects, calculated from the body’s drag coefficient k, area A, velocity V, percent movement t, and the room air density p:
Јm. o. = kA V2p0? (7.34)
Posted in INDUSTRIAL VENTILATION DESIGN GUIDEBOOK