Air duct for distributing air in a greenhouse
Air duct for distributing air in a greenhouse, the air duct comprising a hollow main body extending along a longitudinal axis and at least one set of vents, wherein the main body has an outer surface and an inner surface, a first end and a second end at the opposite longitudinal ends of the main body, and a segment disposed between the first end and the second end extending over substantially the entire length of the main body, wherein the cross-sectional area of the main body in this segment decreases towards the second end, wherein the vents belonging to a respective set of vents are distributed at intervals over substantially the entire length of said segment, and wherein at least said segment of the main body is made from plastic.
1 . An air duct for distributing air in a greenhouse, the air duct comprising a hollow main body extending along a longitudinal axis and at least one set of vents, wherein the main body has an outer surface and an inner surface, a first end and a second end at the opposite longitudinal ends of the main body, and a segment disposed between the first end and the second end, wherein a cross-sectional area of the main body in said segment decreases over the whole length of this segment towards the second end, and wherein vents belonging to a respective set of vents are distributed at intervals over substantially the entire length (L 2 ) of said segment, wherein at least said segment of the main body is made of a substantially air-impermeable plastic film, and wherein the vents are configured as nozzles protruding from the outer surface of the segment.
2 . The air duct according to claim 1 , wherein the vents of a respective set of vents in said segment are arranged on a geodesic line when the air duct is used as intended.
3 . The air duct according to claim 1 , wherein the cross-sectional area (A) of the segment is configured to maintain uniform static pressure along the length of the segment.
4 . The air duct according to claim 3 , wherein the cross-sectional area is determined by equation (1):
A
A
L
=
{
(
1
+
fL
2
D
L
)
(
x
L
)
1
/
2
-
fL
2
D
L
x
L
}
2
(
1
)
and factor f used in equation (1) to calculate the change in cross-sectional area along the segment is additionally based on equation (2):
f
=
0.25
log
(
ε
/
D
3.7
+
5.74
Re
0.9
)
2
(
2
)
with
f=friction factor,
ε=absolute roughness coefficient,
D=cross-sectional diameter of the segment,
Re=Reynolds number.
5 . The air duct according to claim 1 , wherein vents belonging to a respective set of vents are equally spaced from one another, wherein the interval (d) between adjacent vents of a respective set of vents is between 5 mm and 100 mm, measured from the respective centers of the vents.
6 . The air duct according to claim 1 , wherein the at least one set of vents comprises two sets of vents, wherein the two sets of vents are arranged substantially mirror-symmetrical to a median longitudinal plane (P) of the air duct.
7 . The air duct according to claim 1 , wherein the nozzles each have a substantially circular air inlet opening with an inlet diameter (d 1 ) at the level of the main body's outer surface, a substantially circular air outlet opening spaced substantially parallel to said air inlet opening and having an outlet diameter (d 2 ), and a perpendicular distance (h) between the air inlet opening and the air outlet opening, wherein the air inlet opening is larger than the air outlet opening and wherein the ratio between the perpendicular distance (h) and the outlet diameter (d 2 ) is between 0.5 and 3.
8 . The air duct according to claim 1 , wherein the inner surface of each nozzle comprises a cylindrical region with a substantially constant inner diameter and a transition region in which the inner diameter increases towards the main body, wherein the inner diameter of the cylindrical portion corresponds to the diameter (d 2 ) of the air outlet opening and wherein the radius (r) of said transition region is between 0.05 and 0.5 of the outlet diameter (d 2 ).
9 . The air duct according to claim 1 , wherein at least the segment and the nozzles, are formed in one piece.
10 . The air duct according to claim 1 , wherein the main body comprises a plurality of segments which are connected to each other end-to-end in a sealed manner or are connectable to each other end-to-end in a sealed manner.
11 . The air duct according to claim 1 , wherein the air duct further comprises a hollow enclosing body having an outer surface and an inner surface, the enclosing body enclosing the main body at least partially in the direction of the longitudinal axis of the main body, wherein the main body and the enclosing body are connected to each other via at least some of the nozzles of the main body, wherein the nozzles protruding from the outer surface of the main body are open into vents in the enclosing body and/or extend through the vents in the enclosing body and protrude from the outer surface of the enclosing body in such a way that the interior volume of the main body is in fluid communication with the environment via the nozzles.
12 . The air duct according to claim 11 , wherein the enclosing body is comprised of plastic.
13 . The air duct according to claim 11 , wherein the nozzles are fixed to the enclosing body by means of at least one of a form-locking connection and a joining method.
14 . The air duct according to claim 11 , wherein the connection between each of the nozzles and the enclosing body is formed to be gas-tight.
15 . A greenhouse comprising at least one air duct according to claim 1 .
16 . A method for distributing air with an air duct according to claim 1 in a greenhouse, the method comprising the steps of:
providing conditioned air with at least one of pre-determined humidity and pre-determined temperature at the first end of the air duct;
conveying said conditioned air from the first end in the direction of the second end of the air duct, by means of an electrically powered ventilator arranged upstream of the air duct; and
supplying said conditioned air into the growing section of the greenhouse via the vents, whereby the air flow exiting through the vents is essentially the same between all vents.