Ann. occup. Hyg., Vol. 46, No. 4, pp. 383–393, 2002
© 2002 British Occupational Hygiene Society
Published by Oxford University Press
DOI: 10.1093/annhyg/mef048
Influence of Push Element Geometry on the Capture
Efficiency of Push–Pull Ventilation Systems in Surface
Treatment Tanks
F. MARZAL1*, E. GONZÁLEZ2, A. MIÑANA2 and A. BAEZA2
1Universidad
Politécnica de Cartagena, Departamento de Ingeniería Térmica y de Fluidos, 48 Paseo de
Alfonso XIII, 30203 Cartagena, Murcia; and 2Universidad de Murcia, Departamento de Ingeniería
Química, 30071 Espinardo, Murcia, Spain
Received 8 August 2001; in final form 2 January 2002
A full-scale installation which simulates a surface treatment tank provided with a push–pull
ventilation system has been designed. This study examines the influence of the geometry of the
push element on the capture efficiency of the system. It is observed that: (i) capture efficiency
increases with the number of holes because of the continuous curtain formed, the size of the
holes having no significant effect within the range studied (5–20 mm diameter); (ii) the push
element is best supported on the tank wall so that no air from outside penetrates below the emitting jets because in this way the impact of the curtain on the tank occurs earlier and losses are
less; (iii) the best results are obtained when the holes are directed downwards towards the tank
surface at an angle of between 22 and 45°.
Keywords: capture efficiency; open surface tanks; push–pull ventilation; tracer gas
ately falling, although disturbances may lead to
losses to the exterior.
3. The wall jet, which flows over the surface and
forms vortexes against the side walls (Curd,
1985; Katz et al., 1992).
4. The entrance to the exhaust hood, where recirculating currents may escape and so lead to substantial losses.
INTRODUCTION
Push–pull ventilation systems, as used in surface
treatment tanks, have complex fluid dynamics
(Heinsohn et al., 1986; Flynn et al., 1995; Robinson
and Ingham, 1996a; Marzal et al., 2001). Different
mechanisms can be seen to be operating, as identified
in Fig. 1, with the following being of note:
1. A semi-free curtain is emitted from the push
system. Continuous slots do not produce a
uniform emission flow (Robinson and Ingham,
1996b; Heiselberg and Topp, 1997), so it is more
common to use elements perforated with nozzles
or holes (Woods and McKarns, 1995; Rota et al.,
2001). A depression is produced underneath the
curtain and this generates a recirculating flow
(Klein, 1986).
2. When the curtain impacts on the tank surface, two
currents are produced: the above mentioned recirculation current and the main current, which, in
principle, rises to leave the tank before immedi-
Decreased capture efficiency caused by losses of
contaminant from the tank are produced by local
push flows which leave the tank and which are not
captured by the exhaust system. For this reason the
geometric design and operational conditions (push and
exhaust flows and tank temperature) play an important
role (Hughes, 1990; ACGIH, 1992; Robinson and
Ingham, 1996b; Rota et al., 2001).
One of the most important elements in the ventilation system is that which emits the push flow since
this conditions the initial geometry and fluid
dynamics of the current. The most important aspects
of this system are evaluated in this study using
sulphur hexafluoride as tracer gas (Niemelä et al.,
1991; Woods and McKarns, 1995; Bémer et al.,
1998; Maynard et al., 2000).
*Author to whom correspondence should be addressed.
383
384
F. Marzal et al.
Fig. 1. Flows involved in push–pull ventilation applied to surface treatment tanks.
Fig. 2. Overview of the pilot installation. 1, Tank; 2, exhaust system; 2a, exhaust hood; 2b, venturi; 2c, fan; 2d, chimney; 3, push
system; 3a, plenum chamber; 3b, venturi; 4, F6S generator; 5, control systems; 5a, infrared spectrophotometer; 5b, hiccough fans;
5c, flow meter; 5d, tank temperature control.
MATERIALS AND METHODS
An outline of the installation is shown in Fig. 2; the
immediate surroundings are such that any movement
of air does not exceed 0.1 m/s.
The tank, which is 1.80 m long and 1.60 m wide,
can be moved under the exhaust hood so that three
different lengths can be studied: 1.2, 1.53 and 1.80 m.
The surface is perforated by 460 holes connected to a
gas grid through which the tracer gas is emitted. Each
hole is fitted with a 0.3 mm needle to obtain uniform
emission, while a system of electric resistance heaters
heats the surface up to 100°C in a controlled and
uniform way.
The push system (Fig. 3) consists of a plenum
chamber with diffuser vanes, which distribute the air
pushed by the ventilator towards the emission slot.
For the push system we used interchangeable tubes
1.6 m in length and 50 mm in diameter closed at both
Capture efficiency of push–pull ventilation systems
Fig. 3. (A) Perforated tube used in the push system. (B) Plenum
chamber to which the perforated tube is attached (nozzle).
Table 1. Push elements
Diameter
of holes,
D (mm)
No. of
holes, N
Distance
Total open
between hole area,
centres, C*
S (mm2)
(mm)
5
105
15
2062
10
52
30
4084
15
35
45
4064
20
53
30
16650
ends and perforated with different numbers of holes
(which could be of different sizes), as depicted in
Table 1. The back is a continuous open slot (20 mm)
to facilitate the uniformity of jet speeds. The tube is
placed at the exit of the plenum chamber, its circular
profile permitting the angle of the emission jets to be
varied from 0° (parallel to the tank surface) to 45°.
The push flow rate, which is measured by a venturi,
can be regulated from 0.0025 to 0.4 m3/s by means of
an hiccough fan.
The exhaust hood has a 1.6 m wide rectangular
entrance, while the height can be varied from 0.15 to
0.60 m by means of a baffle (in this study a height of
385
0.3 m was used). The exhaust flow can be regulated
between 0.08 and 2.5 m3/s by an hiccough fan.
To determine the total and transverse linear efficiencies, as defined below, a sulphur hexafluoride gas
(tracer gas) generator was used (Fig. 4). This permitted
a constant flow of between 55 and 75 cm3/min to be
maintained by means of a pressure governor (1) and
needle valve (2), which was measured continuously
in a continuous flow meter (4). For the measurement
of total efficiency, the sulphur hexafluoride was
mixed with 20 l/min of air from a diaphragm pump
(8) and injected either into the gas grid under the tank
surface, as described, or directly into the exhaust
hood. To measure the transverse linear efficiency the
tracer gas is injected into the central part of the
narrow tube (1590 mm in length, 7 mm in diameter),
which is closed at both ends and perforated longitudinally with 100 holes of 0.3 mm diameter. In this
way, the tracer is emitted evenly along the whole
length of the tube, which is placed parallel to the push
in different positions over the tank (Fig. 5). As in the
previous case, the tracer can also be injected directly
into the exhaust hood.
The first measurement with the diffuser tube is
made at one end of the tank next to the push and then
at 10–15 cm intervals until a value close to 100% is
reached, i.e. when Cal ≈ CaT (see equation 2).
The tracer gas concentration was determined with
an infrared analyser (Bruel & Kjaer model 1302),
which measures samples taken immediately after the
venturi in the exhaust tube (Figs 2 and 5a), ensuring
that the tracer gas–air exhaust mixture is uniformly
mixed.
The total efficiency of the system, ET, is defined by:
ET = Ca/CaT
(1)
where Ca is the concentration of tracer measured in
the exhaust tube when the gas is injected from under
the tank and CaT is the concentration when the gas is
introduced directly into the exhaust tube.
The so-called transverse linear efficiency, El, is
defined by:
El = Cal/CaT
(2)
where Cal is the concentration of tracer measured in
the exhaust tube when the tracer gas is injected into
the diffuser tube placed over the surface tank (Fig. 5).
Both efficiencies are related by (Marzal, 1999):
L
ET =
El
∫ ----L- dl
(3)
0
where L is the total length of the tank and l is the
distance measured in the tank from diffuser tube to
push element.
386
F. Marzal et al.
Fig. 4. System for generating constant flow of sulphur hexafluoride. 1, Pressure governor; 2, needle valve; 3, safety valve;
4, continuous flow meter; 5, bubble flow meter; 6 and 7, ball check; 8, diaphragm pump for air; 9, pump pulsation absorber.
Fig. 5. Position of diffuser tube over the surface tank surface to determine transverse linear efficiency.
The results obtained are expressed by relating the
total capture efficiency with the parameter Qi2/S, i.e.
ET = f(Qi2/S)
(4)
where Qi is the initial flow push and S the total open
area of the push element used in each case. The above
parameter is the variable which determines the initial
momentum (M) of the push flow by means of:
M = ρQi2/S
(5)
where ρ is the air density, in this case taken to be
constant since the normal variations in environmental
temperature and pressure in the immediate surroundings do not have any significant effect on this param-
eter. For this reason, it is the same whether the initial
momentum or the parameter in equation (4) is used.
For the experimental design we started from the
hypothesis that the total efficiency of the ventilation
system depends in the first place on the initial
momentum of the jets and on the geometry of the
push element. This last variable can be defined from
various parameters: the diameter of the holes, D, the
number of holes, N, the distance between the centres
of the holes, C*, and the total open area, S. However,
only two of these are independent, given that:
S = NπD2/4
(6)
C* = W/N
(7)
and
Capture efficiency of push–pull ventilation systems
387
Table 2. Grouping of push elements providing approximately
the same open area
Group
1
2
3
N
S (mm2)
Smean (mm2)
5
105
2062
1985
10
26
2042
15
11
1944
20
6
1885
10
34
2670
15
16
2827
20
8
2513
10
52
4084
15
23
4064
20
13
4084
D (mm)
2670
4077
where W is the length of the push element, coinciding
with the width of the tank (1600 mm), which remains
constant. It is therefore only necessary to establish
the diameter and number of holes to define the geometry of the push element.
Fig. 6. Influence of the push geometry on total capture
efficiency. Group 1, Table 2.
RESULTS AND DISCUSSION
Influence of the push geometry
The first experiments were carried out grouping
push elements with the same total open area, as indicated in Table 2. In this way, for the same push flow
rate and group, all the jets together have the same
momentum.
The following experimental conditions were
applied.
Push element: Resting on tank
Initial angle of the jets parallel to tank
surface (0°)
Tank:
Length
1.53 m
Temperature
50°C
Exhaust:
Total flow rate
0.175 m3/s
Specific flow rate
0.0715 m3/m2/s
Height of slot
30 cm
The experimental results are depicted in Figs 6–8
after fitting by polynomial functions.
From the results indicated in Figs 6–8 the following
can be deduced:
•
•
•
The total efficiency depends on the initial momentum of the jets and the push element geometry.
For each group the influence of the geometry on
total efficiency becomes more evident as the
momentum increases. More explicitly, for a small
momentum there is no great variation in total efficiency but there is when momentum increases.
When comparing the different groups, a slight
increase in total efficiency is observed as the
total open area increases.
Fig. 7. Influence of the push geometry on total capture
efficiency. Group 2, Table 2.
•
•
Under the experimental conditions, the greatest
efficiencies are shown for the smallest momentums. The profiles show two distinct shapes: a
slight maximum or a continuous decrease.
The experimental design used does not permit
the influence of the diameter or number of holes
on the total efficiency to be determined.
This behaviour is confirmed for other conditions.
Thus Fig. 9 shows the results obtained for a tank
1.53 m long, a temperature of 95°C and an exhaust
flow of 0.248 m3/s, while Fig. 10 refers to a tank
1.80 m long, a temperature of 50°C and an exhaust
flow of 0.200 m3/s. In both cases, two push elements
of Group 3 were used (D 20 N 13 and D 10 N 52).
Note that the zones of maximum efficiency obtained
in the experiments depicted in Figs 9 and 10 are
wider than those of Fig. 8, due to the greater exhaust
flows.
388
F. Marzal et al.
Fig. 8. Influence of the push geometry on total capture
efficiency. Group 3, Table 2.
Fig. 10. Capture efficiencies in a 1.80 m tank at 50°C. Exhaust
flow 0.200 m3/s. The individual experiments, a and b, are
analysed by means of the transverse linear efficiencies of
Fig. 11.
Fig. 9. Capture efficiencies in a 1.53 m tank at 95°C. Exhaust
flow 0.248 m3/s.
Figure 11 shows the transverse linear efficiency
profiles obtained for a push flow of 0.02 m3/s
(0.100 m4/s2) in each of the two cases, as marked by
points a and b in Fig. 10.
The transverse linear efficiency of case a shows
that the losses are produced between 350 and 600 mm,
which is the impact zone of the jets over the tank and
where the wall jets are reorganized. In case b (D 20
N 13), on the oher hand, there are continuous escapes
of contaminant in the 0–600 mm band both through
the semi-free jets and in the impact and reorganization zones of the wall jet. We attribute this behaviour
to the distance separating the jets, which leads to a
feedback current escaping close to the zone nearest to
the push element (0–200 mm). On the other hand, the
smaller number of holes (N 13) leads to a greater
momentum in each jet so that impact on the tank is
Fig. 11. Transverse linear efficiency for a 1.80 m tank at 50°C.
Exhaust flow 0.200 m3/s. Push flow 0.02 m3/s (0.100 m4/s2).
more violent and the energy associated with each
impact current (initially lost from the tank) is that
much greater. Indeed, the ‘pneumatic screening
effect’ produced by the jets of the push element D 10
N 52 is more efficient than that of D 20 N 13 and
leads to a less intense impact.
In none of the cases studied does gas escape from
the rest of the tank surface or from the primary
exhaust element (600–1800 mm).
We then determined the individual influences of
the number and diameter of the holes on total efficiency, using some of the above described experiments (Figs 6–8) grouped according to Tables 3 and
4, where one parameter (D or N) is kept constant
Capture efficiency of push–pull ventilation systems
389
Table 3. Experimental design to determine the influence of
number of holes on total capture efficiency
Group
D (mm)
4
10
5
15
6
S (mm2)
N
20
26
2042
34
2670
52
4084
103
8090
11
1944
16
2827
23
4064
6
1885
8
2513
13
4084
53
16650
Table 4. Experimental design to determine the influence of
hole diameter on total capture efficiency
Group
7
N
11
N (mean)
12
13
8
26
~24
23
9
52
~52
53
10
105
103
104
D (mm)
S
(mm2)
15
1944
20
4084
10
2042
15
4064
10
4084
20
16650
5
2062
10
8090
while the other is varied. In addition to the previous
ones, two new experiments were carried out using the
following dimensions in the push elements (D 10
N 103 and D 20 N 53), with the other conditions kept
constant.
The results for each of the groups depicted in Table 3
are shown in Figs 12–14. It can be seen that total efficiency is slightly greater in the zone of low
momentum and that the parameter analysed has no
significant influence. On the other hand, in the zone
of maximum efficiency two sorts of behaviour can be
seen: when the number of holes is small the total efficiency decreases significantly, but if the number is
sufficiently high to form a continuous curtain the efficiency profiles coincide.
From the above, the positive effect of using a sufficient number of holes to form a continuous curtain is
evident.
Figures 15–18 show the results of experimental
groups 7–10 (Table 4). In general, it can be seen that
the diameter of the holes is not a determining factor.
Influence of height of the push element over the tank
The above described experiments were carried out
with the push element resting on the wall that forms
the width of the tank (10 cm above surface). In real
tanks the above height may represent the minimum
Fig. 12. Effect on total efficiency of number of holes in the
push element corresponding to Group 4 of Table 3. The profiles
have been constructed using data of experiments D 10 N 52 and
D 10 N 103.
Fig. 13. Effect on total efficiency of number of holes in the
push element corresponding to Group 5 of Table 3. The profiles
have been constructed using data of experiments D 15 N 16 and
D 15 N 23.
value necessary to prevent overflows caused by the
increased level after introduction of the pieces to be
treated.
In the present experiment the push element was
fitted with a device which permitted it to be raised to
a height of 20 cm above the tank surface.
When the push element is higher than the side wall
of the tank, a free zone is produced through which
air is incorporated into the lower part of the curtain
(Fig. 19), decreasing its deflection and increasing the
distance to impact. This behaviour is considered
unfavourable because of the greater concentration of
possible emissions in the impact and wall jet reorganization zones.
390
F. Marzal et al.
Fig. 14. Effect on total efficiency of number of holes in the
push element corresponding to Group 6 of Table 3. The profiles
have been constructed using data of push element D 20 N 53.
Fig. 16. Influence on total efficiency of diameter of holes in
the push element corresponding to Group 8 of Table 4. The
profile was constructed with the data of both experiments.
Fig. 15. Influence on total efficiency of diameter of holes in the
push element corresponding to Group 7 of Table 4. The profile
was constructed with the data of both experiments.
Fig. 17. Influence on total efficiency of diameter of holes in the
push element corresponding to Group 9 of Table 4. The profile
was constructed with the data of both experiments.
We thought it opportune to look more closely at these
hypotheses in three experiments described below.
Figure 20 shows the results obtained. A significant
decrease in total efficiency is evident as the height
above the tank surface increases, confirming the
hypothesis.
We also considered it of interest to evaluate the
losses in efficiency in the tank at two push element
heights by analysing the transverse linear efficiencies
in the following experiments.
Push element: Number of holes
53
Diameter of hole
20 mm
Angle of inclination 0°
Temperature
50°C
Exhaust:
Total flow
0.181 m3/s
Specific flow
0.0739 m3/m2/s
Slot height
30 cm
Height of the push element over tank
(a) Low, resting on tank wall
(b) Intermediate, 5 cm above tank wall
(c) High, 10 cm above tank wall
Push element: Number of holes
53
Diameter of hole
20 mm
Angle of inclination 0°
Push flow:
0.035 m3/s (0.074 m4/s)
Tank:
Length
1.80 m
Temperature
50°C
Capture efficiency of push–pull ventilation systems
391
contaminant is greater. Despite the fact that the initial
momentum is the same as in the previous case, the
curtain of air formed is more distorted and slower, so
that the escapes produced by impact cannot be reconducted by the air current towards the zone of influence of the exhaust. The result is a significant drop in
efficiency.
Fig. 18. Influence on total efficiency of diameter of holes in the
push element corresponding to Group 10 of Table 4. The profile
was constructed with the data of both experiments.
Total flow
0.199 m3/s
Specific flow
0.069 m3/m2/s
Slot height
30 cm
Height of the push element over tank
(a) High, 10 cm above tank wall
(b) Low, resting on tank wall
Exhaust:
The results obtained are shown in Fig. 21. It can be
seen that when the push element rests on the tank
wall (b) small losses occur in the impact and wall jet
reorganization zones (200–800 mm), leading to total
efficiencies of ∼98.7%. When the push element is in
the high position (a), total efficiency drops to 88.3%,
with continuous escape being evident along most of
the length of the bath (0–1200 mm). Impact occurs
later and in a zone where the concentration of
Influence of the angle of inclination of the holes in
the push element with respect to the tank surface
Figure 22 shows three exit configurations for the
jets, which contributes to a qualitative understanding
of the system as regards possible uncontrolled emissions in the impact and wall jet reorganization zones.
Different situations arise depending on the inclination angle (α). On the one hand, it is convenient that
the impact distance (x in Fig. 22) is minimum so that
the concentration of current in the zone of the impact
is also reduced, a situation which is favoured by a
larger inclination angle. However, this produces an
unfavourable increase in the impact angle (β) and the
angle corresponding to the rebound current. It therefore seems logical to use an angle α between the
two extremes represented in Fig. 22. To verify this
hypothesis we carried out experiments varying the
angle between 0 and 45° under the following conditions.
Push element: Resting on the tank
Number of holes
53
Diameter of hole
20 mm
Angle of inclination 0°
Tank:
Length
1.53 m
Temperature
50°C
Exhaust:
Total flow
0.181 m3/s
Specific flow
0.0739 m3/m2/s
Slot height
30 cm
Angle of inclination of push element 0, 15, 22, 30
and 45°
Fig. 19. Generic representation of push element. The jets receive an additional amount of air when the push element is not supported
on the tank wall.
392
F. Marzal et al.
Fig. 20. Influence of push element height above tank surface
on total efficiency.
Fig. 21. Transverse linear efficiencies obtained with the push
element in two positions: resting on the tank (low, b) and 10 cm
above the top of the tank (high, a).
Figure 23 shows the total efficiency profiles for 0,
22 and 45°. Those corresponding to 15° (very near
0°) and 30° (same results as 22°) are not shown. The
latter two (22 and 30°) produced the greatest efficiency.
To analyse the escape zone and mechanism
involved, two experiments were carried out and the
transverse linear efficiency profiles for the following
conditions are depicted.
The experimental results are shown in Fig. 24. It
can be seen that the horizontal position (0°) produces
continuous escape in the 0–800 mm zone through the
initial jets and in the impact and wall jet reorganization zones. Furthermore, escape occurs in the exhaust
(a factor not seen in the previous experiments) since
the variations in total efficiency occur at that end of
the tank where this element is situated. The best
results were obtained with an inclination angle of
45°, when a lower level of escape was detected in the
impact/reorganization zone (at 300–500 mm). Escape
was also detected in the exhaust unit, although at a
lower level than in the previous case.
Push element: Resting on the tank
Number of holes
53
Diameter of hole
20 mm
Push flow:
0.035 m3/s (0.074 m4/s2)
Tank:
Length
1.80 m
Temperature
50°C
Exhaust:
Total flow
0.199 m3/s
Specific flow
0.069 m3/m2/s
Slot height
30 cm
Angle of inclination of push element 0° and 45°
CONCLUSIONS
A large number of holes in the push unit is desirable so that the emission jets form a continuous curtain
near the exit. The diameter of the holes is of little
importance within the range investigated (5–20 mm).
Fig. 22. Push jet configuration depending on the exit angle, α.
Capture efficiency of push–pull ventilation systems
393
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Fig. 23. Influence of the inclination angle of the push element
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Fig. 24. Transverse linear efficiencies obtained with the push
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surface so that the wall jet forms as early as possible,
thus increasing efficiency. A gap between the tank
and push element should be avoided.
An inclination angle of between 22 and 45° is
better than 0° (parallel to the tank surface) because it
increases total efficiency.
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