Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a rain water roof out-
let for a building, comprising an outlet pipe leading from a
roof or some other collecting area to a discharge point, the
pipe having a mouth at its upper end, a trough connected to
the upper end of said pipe, said trough being wider than
the mouth of the pipe, the free upper edge of said trough
being located at a higher level than the mouth of said out-
let pipe, a lid provided with a closed top surface and
fastened above said mouth, said lid being wider than
the mouth but smaller than the trough, the lower edge of
said lid being located below the upper edge of the trough to
prevent the formation of a whirlpool in the outlet pipe
while the trough is filled with water, and a sieve extending
between said lid and the bottom of said trough, said sieve
surrounding the mouth of the outlet pipe and being provided
wtih perforations.
By means of such a construction of the roof outlet there
are provided at the mouth of the vertical pipe such conditions
that the flow of water in the vertical pipe takes place as
an airless flow, i.e. as a solid water flow across the entire
cross-section of the pipe, whereby the diameter of the vertical
pipe can be dimensioned essentially smaller for a specific
rain water quantity to be discharged per unit of time than
in conventional rain water roof outlets.
When the outflow of rain water occurs as a solid flow
the vertical pipe is thus completely filled with water and
the total difference in height from the roof to the discharge
point, which is normally located below the street level,
corresponds to the static pressure of airless water and can
be utilized for pressure losses produced in the rain
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water conduit and parts thereof.
The solid flow has the considerable advantage as
compared to other systems that even very wide roofs,
5.000 to 20.000 sq.m. and more, can be drained by
collecting the water amounts flowing to the roof outlets
into a horizontal conduit running centrally immediately
below the roof, said conduit extending downward only at
the gable of the building or at some other suitable point,
penetrating the ground level and ending at a discharge
point. Hereby the commonly used numerous vertical rain
water outlet conduits inside the building as well as the
manifolds positioned under the ground level and based on
discharge by gravitation can be entirely avoided, which
conduits and manifolds besides being expensive are also
bulky because they operate according to open flow and
the transport of water is carried out on the basis of the
inclination of the co~duits.
Because the ability of the said flow system to
carry water in an outlet conduit extending approximately in
plane with the roof is based on the pressure produced by
the difference in height between the roof and the
discharge point, the upper parts of the rain water outlet
conduit will be subjected to a static pressure which is
considerably lower than the air pressure, and the lowest
pressure will in general prevail at the point where the
horizontal collecting conduit running under the roof
changes its direction from horizontal to vertical. If said
height difference in height is more than approx. 10 m,
the rain water conduit is to be dimensioned so that at
the point of the lowest pressure the sum of the static
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pressure and air pressure is not smaller than the steam
pressure of the rain water (the static pressure is at the
point of the lowest pressure negative) because, if said sum
is smaller than said steam pressure, the water at said
point begins to vaporize and to form steam bubbles in the
conduit, wherefore the solid flow dimensions no longer
hold true. For this reason, the maximum allowable pressure
loss in the horizontal conduit portion is in practice
-about 10 m water column (about 1 bar).
Because the resistance causing said pressure losses
is composed of both the friction of water against the pipe
and the resistance of the pipe deformations (changes in
direction, diameter etc.) and especially of the roof outlet,
the resistance of the roof outlet should be made reasonably
small.
One typical way to keep the resistance of the roof
outlet small would be to enlarge the area of the sieve in
the roof outlet by enlarging the roof outlet while still
maintaining its capability to prevent entering of air into
the outlet. The enlargement of the diameter of the roof
outlet, however, results also in an increase in the depth
dimension of the outlet, and the installment of a
sufficiently big outlet in the roof becomes impossible.
On the other hand, the enlargement of the perforation area
of the sieve by means of increasing the size of the
individual holes is not possible because the sieve portion
must have a pipe protecting property such that all dirt
particles passing through the perforation of the sieve -
also pass through the rain water conduit itself without
clogging the conduit. Moreover, the enlargement of the
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total cross-sectional area of the perforations increases
the risk of air being carried along, which phenomen cannot
be allowed in a solid flow system. In addition, the sieve
portion still must have a sufficient strength against stresses
applied to the sieve, e.g., against treading.
The Swedish patent publication 362,678 discloses a roof
outlet, wherein the sieve above the outlet pipe is provided
with a perforation. When measured from the figures shown,
the relation between the total area of the perforations in
the sieve and the cross-sectional area of the outlet pipe
is about 2.2 and as small as about 1.85, respectively. The
structures shown in the Swedish patent publication result
in so big flow resistances, that larger diameters are used
for the pipes in order to keep the total flow resistance
within reasonable limits, which is an unecomonical solution,
as already described above.
It is the object of the present invention to provide a
rain water roof outlet, wherein the resistance of the outlet
has been reduced without interfering with the capability of
the roof outlet to proyide a solid outflow of water. This is
accomplished by means of a roof outlet according to the
invention, which is characterized in that the ratio of the
total area of the perforations in the sieve to the cross-
sectional area of the outlet pipe is between 2.5 and 3.5.
Experiments made with the roof outlet according to the
invention have proved that a flow resistance of about 5 per
cent of the flow resistance of the conduit portion subjected
to the lowest pressure can be allowed to the roof outlet
without any difficulty. In this way, the
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resistance of the roof outlet can be reduced to a
minimum, whereby the relation of the total area of the
perforations in the sieve to the cross-sectional area of
the outlet pipe has become decisive. The resistance number
of the roof outlet is calculated from the formula
(1) R = 0.5 + Rp ( ~ )
where
Fl = cross-sectional area of the pipe
Fo = area of the sieve perforations
Rp = resistance number of the sieve perforations at a
predetermined flow velocity through the sieve
perforations = 0.5 + 1.1 = 1.6.
The total flow resistance produced by the roof outlet
is
(2) ~p = R = v w
2g
where
v = flow rate in pipe
w = weight by volume of flowing fluid
g = acceleration
The most common flow rates in the pipe are about 4
m/s, whereby the above mentioned pressure loss (~p) sets the
limits to R. At the worst, the flow resistance of the pipe
portion subjected to the lowest pressure is 10,000 mm
water column, of which 5 per cent, i.e., the allowable flow
resistance of the roof outlet is 500 mm water column.
If R can be kept between the values of 0.63 to 0.75,
the result is very advantageous regarding the roof areas to
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be drained with one roof outlet as well as regarding the
required pipe diameters and, on the basis of formula (I),
the conclusion is that the relation between the perforation
area of the sieve and the cross-sectional area of the
pipe has to be from 2.5 to 3.5.
The invention will now be described in more detail
in the following with reference to the accompanying
drawing, where
- Fig. 1 is a schematic view of a rain water pipe
system of a building provided with roof outlets,
Fig. 2 is a diagram illustrating the pressure
distribution in the pipe as a function of the length of
the pipe, and
Fig. 3 is an axial section of one embodiment of a
roof outlet according to the invention.
The rain water pipe system shown in Fig. 1 of the
drawing comprises a number of roof outlets 2 positioned
on the roof of a building 1 and connected by means of
pipes 3 to a collecting pipe 4 leading via a vertical
pipe 5 to a collecting well 6.
In the diagram of Fig. 2, there is shown the
actual static pressure in the rain water pipe starting
from an outlet A
- = h _ v _ hf,
w 2g
where h = difference in height between the roof outlet and
the pipe,
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v = flow rate in the pipe,
hf = flow resistance,
w = weight by volume,
g = acceleration
at the roof outlet (A), at the junction of the connecting
pipe and collecting pipe (B), at the junction of the
collecting pipe and the vertical pipe (C) and at the
collecting wall (D). In the diagram, curve E shows the
-static pressure due to the difference in height between
the roof outlet and the pipe, curve F the flow resistance
of the pipe and curve G the static pressure less the
dynamic pressure. The vertical distance H between the
curves F and G indicates the actual static pressure of the
rain water pipe.
Fig. 3 shows a roof outlet comprising an outlet
pipe 7, the upper end of which is connected to a trough 8
which is wider than the mouth 7a of the pipe. The free upper
edge 8a of the trough is located at a higher level than the
mouth of the outlet pipe. A lid 9 with a closed top
surface is fastened above the mouth, said lid being wider
than the mouth but smaller than the trough. The edge of the
lid is located below the upper edge of the trough. The
lid prevents air from entering into the outlet pipe and,
accordingly, the formation of an air whirl when the water
level is located above the lid in the trough so that the
flow of water in the outlet pipe takes place as a solid
flow of water. To the lid is connected a sieve 10 surround-
ing the mouth of the outlet pipe and provided with
perforations 11. The relation of the area of the
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perforations 11 to the cross-sectional area of the outlet
pipe 7 is between 2.5 and 3.5.
Example
The diameter of the perforation 11 in the sieve 10
is chosen to be 12 mm because it has been established
experimentally that, by using perforations of said diameter,
all dirt particles passing through the sieve also pass
through the rain water conduit, wherefore there is no risk
-of clogging in the pipe. In a sieve provided with
perforations of this size and made strong enough to meet
the strength requirements, the relation between the total
area of the sieve and the total area of the perforations
in the sieve is about 3.
The diameter of the pipe 7 is 47 mm, whereby the
cross-sectional area of the pipe is 17.35 sq. cm. If the
relation Fo/Fl between the total area of the perforations
in the sieve 10 and the cross-sectional area of the pipe 7
is 3, the area of the perforations of the sieve is
52 sq. cm. and the total area of the sieve 156 sq.cm. The
height of the sieve will be, considering the distribution
of the perforations, 58 mm and the diameter 84 cm.
If the relation Fo/Fl is increased, e.g., to the
value 4, Fo will be 69.4 sq. cm. and the total area of the
sieve 208 sq. cm., whereby the sieve dimensions will
increase to the values height 70 mm (20 per cent) and
diameter 95 mm (13 per cent). The dimensions of the trough
8 then increase correspondingly and the installment of
such a trough on a roof will be much more difficult. On
the other hand, if the height of the sieve is kept
unchanged, the diameter thereof would increase to the value
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of 114 mm i.e., 36 per cent. For this reason, the area
relation between the perforations of the sieve 10 and
the pipe 7 must not be increased to exceed said upper limit.
On the other hand, a reduction of said relation
Fo/Fl below said value 2.5 will result in such a big
increase in the flow resistance of the outlet 2 itself that,
in order to keep the total flow resistance unchanged,
the increased outlet resistance must be compensated by
means of reducing the pipe resistance which is effected by
increasing diameter of the pipe. In one example, the
successive diameters of the horizontal collecting pipe
sections are, when the number of outlets connected
thereto is 5, as follows: 65.3 mmr 103 mm, 103 mm, 150 mm,
and 150 mm. If the relation Fo/Fl, for example, is 2.4,
the pipe diameters would have to be as follows to obtain
the same flow resistance: 68 mm, 103 mm, 103 mm, 150 mm,
and 150 mm. Thus, to compensate the increased outlet
resistance, the diameter of one section of the collecting
conduit would have to be enlarged.
The drawing and the accompanying specification are
only intended to illustrate the invention. In its details,
the roof outlet according to the invention may vary
considerably within the scope of the claims.
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