WO2012169880A1

WO2012169880A1 - Greenhouse, air circulation system for a greenhouse and method for circulating air in a greenhouse

Info

Publication number: WO2012169880A1
Application number: PCT/NL2012/050329
Authority: WO
Inventors: Michel La Crois; Petrus Hubertus Emanuel Maria Malschaert
Original assignee: Maurice Kassenbouw B.V.
Priority date: 2011-05-13
Filing date: 2012-05-11
Publication date: 2012-12-13
Also published as: NL2006774C2

Abstract

The invention relates to a greenhouse comprising a plurality of cultivation gutters (3) for growing plants thereon, and an air circulation system (4) for circulating air in the interior of the greenhouse, which air circulation system comprises a plurality of individual air tubes (10) which each extend under a cultivation gutter, and pressure means (12) for forcing air through air tubes, wherein each of the plurality of air tubes is provided with passages distributed along the length thereof, through which air flows from the air tube into the interior of the greenhouse. Heating means (14) are provided in at least a number of air tubes, which heating means extend in the longitudinal direction of the air tube for heating air that flows past the heating means under the influence of the pressure means. A heating element (13) is disposed upstream of the heating means for heating air before it flows past the heating means. The invention further relates to an air circulation system for such a greenhouse and to a method for circulating air in a greenhouse.

Description

GREENHOUSE, AIR CIRCULATION SYSTEM FOR A GREENHOUSE AND

METHOD FOR CIRCULATING AIR IN A GREENHOUSE

DESCRIPTION

The present invention relates to a greenhouse comprising a plurality of cultivation gutters for growing plants thereon, and an air circulation system for circulating air in the interior of the greenhouse, which air circulation system comprises a plurality of individual air tubes which each extend under a cultivation gutter, over at least part of the length of the cultivation gutter in question, and pressure means for forcing air through said plurality of air tubes from respective upstream ends of said plurality of air tubes, wherein each of said plurality of air tubes has a circumferential wall provided with passages distributed along the length of the air tube, through which air flows from the air tube into the interior of the greenhouse in use.

The present invention further relates to an air circulation system suitable for circulating air in the interior of the greenhouse.

The present invention further relates to a method for circulating air in a greenhouse.

A known greenhouse comprises a number of cultivation gutters disposed inside the greenhouse for growing plants thereon, more specifically on substrate material provided on/in the gutters. The cultivation gutters are disposed parallel to each other in the greenhouse, extending in a longitudinal direction of the greenhouse over a significant part of the length of the greenhouse. Such cultivation gutters can have a length of a few hundred metres. On average, the length of a cultivation gutter is 100-150 m. The cultivation gutters are suspended from the roof structure of the greenhouse. The greenhouse further has an air supply system for supplying air into the interior of the greenhouse, i.e. supplying (fresh) outside air to the interior of the greenhouse, and/or for circulating air inside the greenhouse. The air supply system comprises a plurality of air tubes, which each extend in the longitudinal direction of a respective cultivation gutter, directly under a cultivation gutter, near the floor surface of the greenhouse. Provided in the wall of each air tube, distributed along the length thereof, are perforations that form passages for air. At an upstream end, usually at a short side wall of the greenhouse, several air tubes are connected to a connecting air tube that extends transversely to the longitudinal direction of the air tubes, so that air can be distributed over the air tubes in question, using one central fan, for thus supplying air to the interior of the greenhouse through said perforations. At their downstream ends, the air tubes are closed. Said known greenhouse further comprises a heating system comprising pairs of spaced-apart heating pipes extending in the longitudinal direction of the cultivation gutters, disposed in a passageway between two adjacent cultivation gutters, a small distance above the floor surface. Heated water is pumped through said heating pipes, thus heating the interior of the greenhouse under the influence of natural convection. Said heating system is also referred to as the lower net (Dutch: "ondernet").

A drawback of said known greenhouse is the fact that high water temperatures must be used for heating the greenhouse. Furthermore, the water capacity of the system is large and the transfer of heat takes place slowly. At present, low supply temperatures for the heating system are increasingly required. This is connected with the use of alternative energy sources, flue gas condensers for heat recovery, heat pumps and the like. A consequence of this is that the operative area of the heat pipes must be enlarged, for example by using pipes having a larger diameter, so as to maintain the heating capacity of the greenhouse.

It is an object of the present invention to provide a greenhouse comprising a simple and efficient air circulation and air heating system. This object is achieved with the greenhouse according to the present invention, which is characterised in that heating means are provided in at least a number of said plurality of air tubes, which heating means extend in the longitudinal direction of the associated air tube and over at least part of the length thereof, and which are configured for heating air that flows past the heating means under the influence of the pressure means, and in that a heating element is disposed upstream of the heating means, which heating element is configured for heating air before it flows past the heating means.

The forced air flow (in use) past the heating element and subsequently past the heating means in the air tube achieves that the transport of energy takes place more effectively and more rapidly than with the above-described known system. By blowing heated air into the interior of the greenhouse, using air tubes, the greenhouse is heated to the desired temperature much more rapidly than is possible with the above-described known system, which is based on natural convection. By integrating the heating means in the air tubes, a significant saving in space is realised. The heating element heats the air before it flows into the air tube. The heating means that extend in the air tube, preferably over at least substantially the entire length thereof, add further heat to the air. A consequence of this is that the air does not cool down, at least only minimally, or even becomes slightly hotter, while flowing through the air tube. Or, in other words, cooling of the air that would occur in the air tube if no heating means were present, and which would result in an uneven air temperature along the length of the air tube, and thus in the interior of the greenhouse, is effectively prevented by the heating means. Thus, an air circulation and heating system by means of which the temperature in the interior of the greenhouse can be evenly regulated is provided in a simple and very efficient manner. Quite preferably, the greenhouse comprises a separate heating element for each air tube in which heating means are provided, which heating element is disposed at the upstream end of the air tube.

Within the framework of the present invention, it is also possible for the heating element to be configured for cooling the air, or, in other words, the heating element is configured for influencing the temperature of the air before it flows past the heating means. The heating means may also be designed for cooling air that flows past, or, in other words, for influencing the temperature of the air flowing past.

In a simple but very efficient preferred embodiment, the heating means comprise a heating pipe through which a fluid, for example a liquid such as water, having a predetermined temperature can be forced under the influence of flow means, for heating or, alternatively, cooling the air flowing past.

It is advantageous in that regard if the heating means also comprise a further heating pipe, wherein the flow means are designed for forcing fluid in a first direction through the heating pipe and in a second, opposite direction through the further heating pipe. The use of the heating pipe and the further heating pipe, through which fluid flows in the opposite direction, achieves that the air flowing through the air tube is heated as evenly as possible along the length thereof and can flow out.

In connection with an even transfer of heat between the fluid and the air flowing past the heating means it is advantageous if the heating pipe and the further heating pipe extend in substantially parallel, spaced relationship in the air tube near the central axis thereof. In a simple, preferred embodiment, each of said plurality of air tubes is open at a first, upstream end thereof, and the pressure means comprise a circulation fan for each air tube at the location of said first end for drawing air from the interior of the greenhouse and forcing air through the air tube associated with the circulation fan.

Preferably, the heating element is disposed in the immediate vicinity of the circulation fan, preferably directly upstream thereof. In a simple but effective embodiment, the heating element is an air-water heat exchanger. By forcing water having a predetermined temperature through the heat exchanger, it is possible to control the temperature of the air flowing past (through) the heat exchanger for heating or cooling the air.

Preferably, the circumferential wall of the air tubes comprises a flexible foil material. Because of the presence of heating means in the air tubes, insulating the air tubes for the purpose of dealing with any temperature difference problems is not necessary.

In a very advantageous embodiment, the greenhouse further comprises a ventilation system provided with an inlet channel for introducing outside air into the interior of the greenhouse. As a result, in addition to air being circulated in the interior of the greenhouse, ventilation of the greenhouse can take place by means of the ventilation system that is provided separately from the air circulation system. In one embodiment, it is conceivable for an air inlet of one or more air tubes to be actually connected to the inlet channel, so that fresh air can directly flow into said at least one air tube.

Preferably, the greenhouse further comprises an outlet channel for exhausting air from the interior of the greenhouse to outside the greenhouse, wherein at least one of the inlet channel and the outlet channel comprises an associated fan for forcing air through the associated channel in question.

It is advantageous if the ventilation system comprises a first fan associated with the inlet channel for drawing in air from outside the greenhouse and a second fan associated with the outlet channel for drawing in air from the interior of the greenhouse. This makes it possible to set the capacity, expressed in cubic metres of air, of the air to be exhausted in relation to the capacity, expressed in cubic metres, of air to be supplied. The advantage of this is that it is possible to realise an overpressure in the greenhouse. Said overpressure prevents undesirable leakages from outside to the inside and prevents cold outside air from entering the greenhouse other than via the inlet channel, so that differences in temperature in the interior of the greenhouse are prevented or at least strongly reduced.

It is furthermore advantageous if the ventilation system comprises a heat exchanger in which the inlet channel and the outlet channel pass one another, which heat exchanger is designed for transferring heat between air from the outlet channel and air in the inlet channel.

In a simple, robust embodiment, the heat exchanger is of the plastic cross flow exchanger type.

The above-described ventilation system, which is provided separately from the air circulation system, can also be used advantageously in a greenhouse as described in the introduction. Accordingly, the present invention also relates to a greenhouse comprising a number of cultivation gutters provided in an interior space of the greenhouse for growing plants thereon, and an air circulation system for circulating air in the interior of the greenhouse, wherein the air circulation system comprises a plurality of individual air tubes which each extend under a cultivation gutter, over at least part of the length of the cultivation gutter in question, and pressure means for forcing air through said plurality of air tubes from respective upstream ends thereof, wherein each of said plurality of air tubes has a circumferential wall provided with passages distributed along the length of the air tube, through which air flows from the air tube into the interior of the greenhouse in use, wherein the greenhouse further comprises a ventilation system provided with an inlet channel for introducing outside air into the interior of the greenhouse.

The possibility of separate ventilation and recirculation thus provided has the advantage that this makes it possible to regulate the temperature and the air humidity in the interior of the greenhouse to a high degree. In addition, this obviates the need for a control valve for distributing the air in the primary and secondary air flows that is traditionally used in greenhouses. The controllability of the overall air system is thus made simpler and more efficient.

The ventilation system comprising a heat exchanger as described above can also be used advantageously in prior art greenhouses, which may or may not also comprise an air circulation system according to the invention as described above. Accordingly, the invention also relates to a greenhouse comprising number of cultivation gutters provided in an interior space of the greenhouse for growing plants thereon, and a ventilation system provided with an inlet channel for introducing air from outside the greenhouse into the interior of the greenhouse and an outlet channel for exhausting air from the interior of the greenhouse to outside the greenhouse, wherein the ventilation system comprises a first fan associated with the inlet channel for drawing in air from outside the greenhouse and a second fan associated with the outlet channel for drawing in air from the interior of the greenhouse, wherein the ventilation system comprises a heat exchanger in which the inlet channel and the outlet channel pass one another, which heat exchanger is designed for transferring heat between air from the outlet channel and air in the inlet channel.

The present invention also relates to an air circulation system suitable for circulating air in an interior space of a greenhouse according to the present invention as described above, said greenhouse comprising number of cultivation gutters provided in an interior space of the greenhouse for growing plants thereon, wherein the air circulation system comprises a plurality of individual air tubes which each extend under a cultivation gutter, over at least part of the length of the cultivation gutter in question, and pressure means for forcing air through said plurality of air tubes from respective upstream ends of said plurality of air tubes, wherein each of said plurality of air tubes has a circumferential wall provided with passages distributed along the length of the associated tube, through which air flows from the tube into the interior of the greenhouse in use, characterised in that heating means are provided in at least a number of said plurality of air tubes, which heating means extend in the longitudinal direction of the associated air tube and over at least part of the length thereof, and which are designed for heating air that flows past the heating means under the influence of the pressure means, and in that a heating element is disposed upstream of the heating means, which heating element is designed for heating air before it flows past the heating means.

The present invention further relates to a method for circulating air in a greenhouse, comprising the steps of:

- providing air tubes in a greenhouse comprising cultivation gutters for growing plants thereon, which air tubes each extend under a cultivation gutter, over at least part of the length of the cultivation gutter in question, wherein each of the air tubes has a circumferential wall provided with passages distributed along the length of the air tube, through which passages air flows from the air tube into the interior of the greenhouse in use,

- providing heating means in at least a number of said air tubes, which heating means extend inside the air tube, in the longitudinal direction thereof and over at least part of the length thereof,

- forcing air through said air tubes past the heating means, using pressure means, wherein the air is heated by a heating element upstream of the heating means, and wherein the heating means are heated to a predetermined temperature, wherein heat transfer takes place between the heating means and the air flowing past.

Advantages of the air circulation system and of the method according to the present invention are analogous to the above-described advantages of the greenhouse according to the present invention.

The present invention will be explained in more detail below by means of a description of a preferred embodiment of a greenhouse according to the invention, in which reference is made to the following figures, in which:

Figure 1 is a three-dimensional view of a part of a preferred embodiment of a greenhouse according to the present invention;

Figure 2 is a more detailed three-dimensional, partially transparent view, of a part of an air circulation system of the greenhouse of figure 1 ;

Figure 3 is a three-dimensional, partially transparent view of a ventilation system of the greenhouse of figure 1;

Figure 4 is a more detailed three-dimensional, partially transparent view of the air circulation system of the greenhouse of figure 1 ; and

Figure 5 is a sectional view along the line V-V in figure 1.

The preferred embodiment of a greenhouse according to the invention that is partially shown in figure 1 comprises a side wall 1 and a roof structure 2. The side wall 1 and further side walls (not shown) surround an interior space of the greenhouse. Provided in the interior of the greenhouse is an air circulation system 4, by means of which air can be recirculated in the interior of the greenhouse. The greenhouse further comprises a ventilation system 6, by means of which air can be exhausted from said interior space into the outside atmosphere, and by means of which also fresh outside air can be introduced into the interior of the greenhouse.

Disposed in the interior of the greenhouse are a number of cultivation gutters 3. The cultivation gutters 3 extend parallel to and spaced from each other in the longitudinal direction of the greenhouse over a significant part of the length thereof. Means by which the cultivation gutters 3 are provided in the greenhouse are not shown in the figures. Usually, cultivation gutters are suspended from the roof structure 2.

The air circulation system 4 comprises an air tube 10 provided under each of the cultivation gutters 3. The air tubes 10 extend over at least part of the length of the associated cultivation gutter 3, preferably over at least substantially the entire length thereof. Means by which the air tubes 10 are provided in the interior of the greenhouse are not shown in the figures. One possibility is to suspend the air tubes from the cultivation gutters. The air tubes 10 are cylindrical in shape, with an enveloping wall of a flexible foil material. Alternatively, the walls may be entirely or partially made of a rigid material. Each air tube 10 has a circulation casing 11 at the location of its first open end, with an air-water heat exchanger 13 forming a heating element provided at an inlet opening thereof, and a circulation fan 12 at the location of an outlet opening that connects to the open end of the air tube 10. The fan 12 can draw air from the interior of the greenhouse through the heat exchanger 13, which air is thus heated. The heated air then flows into the air tube 10. The air tube 10 is provided with a number of air passages or perforations (not shown in the figures) distributed along the length thereof, through which the air from the air tube 10 can flow into the interior of the greenhouse. A second end of the air tube 10 opposite the first end is closed. The overpressure generated by the fan 12 inside the air tube 10 is such that the air tube 10 retains its cylindrical shape, i.e. the shape of the wall of a flexible foil material remains unchanged.

Inside at least some of the plurality of air tubes 10 present in the greenhouse, preferably inside all the air tubes 10, there is provided a heating device 14 forming the heating means, which heating device extends over at least part of the length of the air tube 10, preferably over at least substantially the entire length thereof. See in particular figures 4 and 5. In the example shown in the figures, the heating device 14 comprises two heating pipes 16, 18, which are connected by a plate element 20. Means by which the heating device 14 is provided in the air tube 2 are not shown in the figures. A fluid, preferably water, having a pre-set temperature can be forced through the heating pipes, so that the temperature of the air flowing past can be maintained or said air can be heated (or, alternatively, be cooled, if desired). The heating pipe 16 is a supply pipe and the heating pipe 18 is a return pipe. That is, water heated by means not shown is forced through the pipe 16 and subsequently flows back through the return pipe 18. Heat from the water is transferred to the air flowing through the air tube 2 by convection, more specifically by forced convection on account of the presence of the fan 12. It is not relevant, for that matter, which one of the two heating pipes 16 and 18 is the supply pipe and which one is the return pipe. An alternative embodiment of the heating device 14 may be a single heating pipe, which passes through several air tubes 10, for example. Another alternative is to provide an electric heating device, for example. Within this framework it may also be considered to form heating means extending over at least part of the length of the air tube by placing a number of individual heating devices, which jointly form the heating means, distributed along the length of the tube. Yet another alternative is to provide a first heating pipe (or a combination of a supply pipe and the return pipe), which extends from a first end to, for example, halfway the air tube, and a second heating pipe (or a combination of a supply pipe and a return pipe), which extends from a second, opposite end to halfway the air tube, or which, in other words, extends in the remaining half of the air tube. The first and the second heating pipe jointly form the heating means in the air tube in that case.

To start a recirculation flow in the interior of the greenhouse, the fans 12 of the air tubes 10 are energised. Air is drawn in by the fans 12 and flows first through the heat exchanger 13 and then through the air tubes 10, past the heating devices 14 provided therein, and exits the air tubes via the passages distributed along the length of the air tubes 10. If the air temperature is to be increased, heated water can be introduced into the heat exchanger 13 and into the heating devices 14. If the water temperature is higher than that of the air flowing through the air tube 10, heat will be transferred from water in the pipes 16, 18 of the heating device to the air as a result of forced convection. As a result of the combination of the heat exchanger 13 and the heating device 14, air will flow at a constant temperature, or at least a sufficiently constant temperature, from the passages in the air tube 10 along the length of the air tube, since the air heated by the heat exchanger 13 flowing through the air tube 10 will not cool down, or at least will remain at about the same constant temperature or possibly heat up a little, as a result of the presence of the heating device 14. The use of a supply pipe 16 and a return pipe 18, through which heating water thus flows in opposite directions, achieves that the air flowing through the air tube 10 will be heated and flows out as evenly as possible, seen along the length of the air tube 10. The above also applies, if desired, if the air temperature in the interior of the greenhouse is to be decreased. In that case water having a lower temperature than that of the air in the greenhouse can be introduced into the heat exchanger 13 and the heating devices 14, as a result of which cooling of the air from the interior of the greenhouse can be effected.

The forced air flow through the heat exchanger 13 and past the heating device 14 achieves that the transport of energy takes place more effectively and quicker than with the heating systems that are currently used. By blowing heated air into the interior of the greenhouse, using air tubes 10, at a temperature that is to a high extent constant, the greenhouse can be heated to the desired temperature much more quickly than is possible with the above-described known system based on natural convection. Moreover, as a result of the presence of the heating devices 14 in the air tube 10 it is no longer necessary for the air tube 10 to be insulated so as to cope with any temperature difference problems.

The greenhouse comprises a ventilation system 6, independently of the air circulation system 4. Using the ventilation system 6, outside air can be introduced into the interior of the greenhouse and air can be exhausted from the interior of the greenhouse into the outside atmosphere. Exhaust fans are provided for discharging air. In the example shown in the figures, said exhaust fans are the exhaust fans 30 and 32. The number of exhaust fans is not relevant within this framework. Depending on the capacity of the greenhouse, a single fan might suffice, or more than two fans could be used. The fans 30 and 32 draw in air at a relatively high level in the greenhouse and force it through vertical air shafts 34 and 36, respectively, to a heat exchanger 40 which extends over at least part of the length of the wall 1 at the bottom of said wall. See also figure 3. Alternatively, individual heat exchangers separated from each other may be used. From the air shafts 34 and 36, air that has been drawn in flows into an inlet chamber 42 of the heat exchanger 40. From there the air flows diagonally through a heat exchanging unit 44, configured as a stack of plates, of the heat exchanger 40. Subsequently, the air can flow freely into the outside atmosphere via an outlet opening 46.

The ventilation system 6 also comprises a number of supply fans, among which the supply fans 50 and 52 that are shown in figure 3. Also with regard to the supply fans it applies that their number is not relevant within the framework of the invention. The supply fans 50 and 52 draw in fresh outside air and introduce it into a chamber 54 of the heat exchanger 40. From said chamber 54, said outside air can flow through the stack of plates 44 to the chamber 56, from where the air can flow freely into the interior of the greenhouse via the outlet opening 58. The exhaust path formed by the air shafts 34, 36 and the channel formed by the chamber 42, the stack of plates 44 and the outlet 46 is fully separate from the supply path formed by the chamber 54, the stack of plates 44 and the opening 58. In the stack of plates 44, supply air and exhaust air flow past one another, separated by the plates, whilst heat can be transferred between the supply air and the exhaust air.

The prevailing air humidity in the interior of the greenhouse can be influenced by controlling the amounts of supplied and exhausted air by means of the supply fans 50, 52 and the exhaust fans 30, 32. Because of the use of the above- described heat exchanger 40 and because it suffices to transfer only the latent heat from the exhaust air to the supply air, the capacity in cubic metres of air to be exhausted can be less than the capacity in cubic metres of air to be supplied. The advantage of this is that it is possible to realise an overpressure in the greenhouse. Said overpressure prevents undesirable leakages from outside to the inside and prevents cold outside air from flowing into the greenhouse by another route than via the heat exchanger 40, so that temperature differences in the interior of the greenhouse are prevented or at least strongly reduced.

Continuously exhausting and supplying small amounts of outside air has the advantage that part of the air in the interior of the greenhouse is continuously refreshed, so that the greenhouse is adequately ventilated at all times.

In the above-described preferred embodiment of the greenhouse according to the invention, an air recirculation system 4 and an air ventilation system 6 which operate independently of each other are used. The use of separate circulation and ventilation systems obviates the need for a control valve for distributing the air in the primary and secondary air flows that is traditionally used in greenhouses. The overall air system will thus be easier and more efficient to control.

Claims

1. A greenhouse comprising a plurality of cultivation gutters provided in an interior space of the greenhouse for growing plants thereon, and an air circulation system for circulating air in the interior of the greenhouse, which air circulation system comprises a plurality of individual air tubes which each extend under a cultivation gutter, over at least part of the length of the cultivation gutter in question, and pressure means for forcing air through said plurality of air tubes from respective upstream ends of said plurality of air tubes, wherein each of said plurality of air tubes has a circumferential wall provided with passages distributed along the length of the air tube, through which passages air flows from the air tube into the interior of the greenhouse in use, characterised in that heating means are provided in at least a number of said plurality of air tubes, which heating means extend in the longitudinal direction of the associated air tube and over at least part of the length thereof, and which are configured for heating air that flows past the heating means under the influence of the pressure means, and in that a heating element is disposed upstream of the heating means, which heating element is configured for heating air before it flows past the heating means.

2. A greenhouse according to claim 1 , wherein the heating means comprise a heating pipe through which a fluid can be forced under the influence of flow means.

3. A greenhouse according to claim 2, wherein the heating means also comprise a further heating pipe, wherein the flow means are configured for forcing fluid in a first direction through the heating pipe and in a second, opposite direction through the further heating pipe.

4. A greenhouse according to claim 2 and 3, wherein the heating pipe and the further heating pipe extend in substantially parallel, spaced relationship in the air tube near the central axis thereof.

5. A greenhouse according to any one of the preceding claims, wherein each of said plurality of air tubes is open at a first, upstream end thereof, and wherein the pressure means comprise a circulation fan for each air tube at the location of said first end for drawing air from the interior of the greenhouse and forcing air through the air tube associated with the circulation fan.

6. A greenhouse according to claim 5, wherein the heating element is disposed directly upstream of the circulation fan, and preferably it is an air-water heat exchanger.

7. A greenhouse according to any one of the preceding claims, wherein the circumferential wall of the air tubes comprises a flexible foil material.

8. A greenhouse according to any one of the preceding claims, further comprising a ventilation system provided with an inlet channel for introducing outside air into the interior of the greenhouse.

9. A greenhouse according to claim 8, wherein the ventilation system further comprises an outlet channel for exhausting air from the interior of the greenhouse to outside the greenhouse, wherein at least one of the inlet channel and the outlet channel comprises an associated fan for forcing air through the associated channel in question.

10. A greenhouse according to claim 9, wherein the ventilation system comprises a first fan associated with the inlet channel for drawing in air from outside the greenhouse and a second fan associated with the outlet channel for drawing in air from the interior of the greenhouse.

11. A greenhouse according to claim 9 or 10, wherein the ventilation system comprises a heat exchanger in which the inlet channel and the outlet channel pass one another, which heat exchanger is configured for transferring heat between air from the outlet channel and air in the inlet channel.

12. A greenhouse according to claim 1 1 , wherein the heat exchanger is of the plastic cross flow exchanger type.

13. An air circulation system for circulating air in an interior space of a greenhouse according to any one of the preceding claims, said greenhouse comprising a number of cultivation gutters provided in an interior space of the greenhouse for growing plants thereon, wherein the air circulation system comprises a plurality of individual air tubes which each extend under a cultivation gutter, over at least part of the length of the cultivation gutter in question, and pressure means for forcing air through said plurality of air tubes from respective upstream ends thereof, wherein each of said plurality of air tubes has a circumferential wall provided with passages distributed along the length of the air tube, through which passages air flows from the air tube into the interior of the greenhouse in use, characterised in that heating means are provided in at least a number of said plurality of air tubes, which heating means extend in the longitudinal direction of the associated air tube, over at least part of the length thereof, and which are configured for heating air that flows past the heating means under the influence of the pressure means, and in that a heating element is disposed upstream of the heating means, which heating element is designed for heating air before it flows past the heating means.

14. A method for circulating air in a greenhouse, comprising the steps of: