Air Curtains
When it is necessary to confine an air volume from the ambient environment and simultaneously have access for operators or machinery, plane air jets offer a possible and simple solution. Air jets (plane and round) are described in Chapter 7. This section describes plane air jets combined with exhaust openings. In principle, they are similar to the air jets described in Chapter 7 and Section 10.3, but the combination with an exhaust opening makes it necessary to consider the influence of the exhaust on the jet. Usually these curtains are used in large doors to shield the interior from the exterior when the door is open. For example, experimental results have shown that from the moment a door is opened, a short time interval, less than 1 minute, is sufficient to get complete development of the airflow through the door.1 An air curtain allows a reduction of the overall flow through the door. The principles and use of air currains are described in many textbooks.2-6 Some basics of air curtains are described here.
When using air curtains the edge effects are neglected and the flow is treated as two-dimensional. The different parts of a two-dimensional jet are sketched in Fig. 10.63.
The theory for plane jets is similar to descriptions of circular jets (see Section 7.4) and many derived equations describe both two-dimensional (plane) and three-dimensional (round) jets. The principle is to generate such high air velocity that a shield against pressure difference, temperature difference, and wind velocity is sustained. However, it is not possible to have complete separation by an air curtain. The main reason for this, is that the jet entrains air
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FIGURE 10.63 Plane air jet’s outline and development.
(from both sides) along its way and this entrained air is mixed inside the jet and transported together with the original supplied air. Other reasons are the extremely high velocities needed to stop oncoming high wind velocities and the transport of air through the curtain when vehicles or persons pass through the curtain.
10.4.2.3 Applicability of Sources
There are numerous possible applications for air curtains. For example, air curtains may be used to heat a body of linear dimensions (as used to move the fresh snow from the railway exchanges in Canada); to function as a partition between two parts of one volume; to function as a partition between an internal room and an external environment, that have different thermodynamic properties; and to shield an opening in a small working volume (see Section 10.4.6).
When used as an air curtain, the flow of the linear jet is blown across a doorway and an exhaust opening pulls the air. The supply air of the air curtain for a door could be either cold or warm, either placed on the inside or the outside, either blowing horizontally or vertically, and either blowing parallel to the opening or at a slight angle to the opening. Usually the curtain has an exhaust opening placed on the other side of the opening. This exhausted air could be circulated back to the supply opening or just transported away. All these alternatives mean that there are innumerable possible configurations for an air curtain. Some of these are shown in the following figures.
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Outlet plenum |
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CHAPTER 10 LOCAL VENTILATION Silencer |
Doorway
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Silencer |
Profile
Indoor
Outdoor
Air curtain System
Doorway
FIGURE 10.64 (a) Jet opening opposite to exhaust grill, (b) Jet opening adjacent to exhaust grill.
A jet blowing vertically downward, with the pull flow downward and upward, is shown in Figs. 10.64a and 10.646, respectively.
Air curtains with the jet vertically upward are shown in Fig. 10.65. The configurations differ in the internal or external placement of the air-handling
TABLE 10.10 Geometrical Parameters Describing an Air Curtain and Influencing Its Performance
I; H i Width and height of doorway a; I,, Initial width and length of linear jet
S] Thickness of the wall that delimits the surface of passage (doorway
Sy Distance between the axis of the initial section of the linear jet air curtain
Blade and the surface of the wall that delimits the doorway R; L, Thickness and width of the exhaust opening ct| Angle between the axis of the jet and the vertical one
A, Angle of divergence of the jet
H, Length of longitudal development of the jet
L; Distance between the surface of symmetry of the jet and the neighboring wall
Hi Height of the inside
Ss Distance between the axis of the opening of the supply device and the axis of
The grill
TABLE 10.11 Physical Parameters Influencing an Air Curtain
R Opening time of doorway
V Axial velocity of the jet
V: Velocity of the air outside normal to the symmetry surface of the linear jer
T, — Te Temperature difference between inside and outside
Tc Outside temperature
Tr Initial temperature of the jet
Pi Inside pressure
Pe Outside pressure
Pi Inside air density
Fie Outside air density
10.4.2.6 Theoretical Model and Experimental Researches
The influence of the geometrical parameters is not well described in the literature and the design of an air curtain is based on theoretical models of a plane turbulent jet.
One very good description is by Schlichting.7 Assuming both constant momentum and angle divergence of the jet, the velocity profiles are given by
U = ^ (1 — tanh2Tj) (10.81)
And
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(10,83)
Where U and V are the air velocities at distance Z in the Z and X directions, respectively; A is Reichardt’s constant and is equal to 7.67; and X and Z are the transverse and longitudinal coordinates of the jet, respectively. K is the kinematic momentum,
K = i (10.84)
3 5 Cr
Where Us is the centerline velocity at a fixed distance, S, from the outlet opening. The gas density should be included in these calculations, but is canceled out in the calculation of the kinematic momentum.7 These equations can be used to calculate the mean velocity, velocity distribution across the jet, total flow rate, and entrained flow rate, at a certain distance from the outlet.
Goodfellow2 has published the following expressions for these relations:
^-c = 2.5 (10.85)
U0 V z
Where Uz c is the centerline velocity at distance Z (m) from the opening, m s~1, U0 is the jet nozzle velocity, m s_1, and Iv is the nozzle width, m.
The velocity across the jet is
U2_ = e-(«x/z)1 (10.86)
«Z, c
Where Uzc is the centerline velocity at distance Z (m) from opening, m s’1, Uz Is the velocity at distance Z (m) from the opening and X (m) from the jet axis, m s-1, and A, a constant, is 6.52.
The mean velocity is given as
Uzm = 0.62m,.,, (10.87)
Where Uz<m is the mean velocity across the jet at distance?(m) from the outlet, m s_l, and U is the centerline velocity at distance Z (m) from the opening, m s_1.
The entrained air can be calculated as
& = 0.62 (z/w), (10.88)
“7 o
Where Qz is the airflow rate at distance Z (m), m3 s-1, Q0 is the airflow rate at the nozzle, m3 s_1, and W is the nozzle width, m.
The total amount of air moving at distance Z from the nozzle is thus equal to Qz. Of this flow rate, QQ comes from the nozzle and half of the remaining flow rate comes from either side of the plane jet.
This entrained flow rate is normally many times the original flow rate and it is the total flow rate that the receiving opening must be designed to exhaust. At the same time the entrained air must be available to both sides of the jet,
otherwise the jet will change direction. This is not a problem when there are large volumes on both sides of the curtain, e. g., if the curtain is at an exterior door for a large industrial hall, with separate supply and exhaust ventilation systems. If the curtain is placed at the opening of a small closed volume, e. g., a biological safety cabinet (Section 10.4.6.4), it is necessary to design the supply flow rate inside to include the entrained air in the curtain and the exhaust flow rate inside to include the total flow rate in the jet. Mostly the exhaust is designed still larger to exhaust some more air from the surroundings to increase the separating effect of the curtain.
These relations, which describe the velocity profile sketched in Fig. 10.63, have a similarity property, behind the form of the equations. Guttmark and Wignanski8 indicate that the similarity profile could be found up to a length ot 120 times the outlet opening w’idth.
There are also formulas for calculation of temperature and concentration distribution along and across an air jet. These are based on the similarity profile of the jet.9
Another procedure for design of an air curtain is proposed by Tamm10 Based on the Bernoulli equation. Recently Partyka11 proposed another procedure based on the model of Schlichting previously described.
The theoretical analysis could also be valid for nonisothermal jets assuming that the buoyancy is negligible. Grimitlyn, as reported by Hagstrom,12 Suggests a local Archimedes number defined as:
Ar, — gillf—Lei, (10.89)
Where G is 9.81 m s"2, 2 is the same as above, Uz is the velocity at distance z, T: Is temperature at distance 2, and Te is temperature outside.
He indicates that the buoyancy is negligible for the velocity field for an Archimedes number less than a critical value equal to Ar, cnt — 0.15.
Experimental laboratory results13 and results from a full-scale model1 Have shown the relation between the dispersed thermal power inside and the air temperature difference between the two sides of air curtain. The results shown in Fig. 10.67 are for different conditions. There are no other experimental data readily available, so caution is needed when applying these results to the design of an air curtain.
From the experimental results of Azzouz et al.1 it is possible to obtain ar, efficiency parameter of an air curtain:
V = = i _ ggil, (10.90)
<7 *«,0
Where 77 is efficiency, Qm 0 is the mass flow rate dispersed at the doorway without an air curtain, and L is the mass flow’ rate dispersed in presence of the air curtain.
Some of the available data are shown in Fig. 10.68 and these indicate that, even if with some uncertainty, r] decreases with an increase in the temperature difference—i. e., the higher the temperature difference, the less efficient the curtains are in separating the volumes. The same authors also claim that the efficiency can be as high as 78% with a thermal difference of 5 “C.1
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AT <K) FIGURE 10.67 Thermal power through air curtain versus thermal difference between the air curtain sides. |
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FIGURE 10.68 An air curtain’s efficiency versus the difference in temperature between indoors and outdoors.
10.4.2.7 Final Remarks
This short outline suggests that it is difficult to find optimal design parameters for air curtains. CFD may provide a more effective design method for air curtains (see Chapter 13). There are some published articles applying CFD to air jets, but comparison with experimental data is lacking.
Posted in INDUSTRIAL VENTILATION DESIGN GUIDEBOOK