This invention relates to a form part for a ventilation ductwork system, comprising a substantial- ly integral duct wall surrounding a flow passage. As used herein, the definition "form part for a ventila¬ tion ductwork system" refers particularly to bends, branches, reducing pieces and other such form parts.

According to regulations concerning ventilation ductwork systems, such systems should comprise clean¬ ing doors at bends, branches and transformation points. In practice, however, it is almost always necessary to install such cleaning doors in the immediate vicinity of bends and branches, because there is no proper place for cleaning doors at such points themselves. Therefore the doors are constructed in the straight portions of the ventilation duct ad¬ jacent to the above-mentioned points. Furthermore, in most cases the doors are made in ventilation ductwork systems in situ during the installation thereof, which is both difficult and time-consuming. The air-tight¬ ness of a finished door is not usually very good, be¬ cause the accuracy in cutting a door opening in situ is not the best possible. The object of the present invention is to pro¬ vide a form part for a ventilation ductwork system, by means of which the above-mentioned problems associated with the installation of cleaning doors can be avoided. This is achieved by means of a form part according to the invention, which is characterized in that for forming a cleaning opening in the ventilation ductwork system a piece of the duct wall is detached, and that the detached piece of the duct wall is sup¬ ported by means of a gasket to the opening in the duct wall so as to form a cleaning door. In the form part

according to the invention, the cleaning door is form¬ ed before the form part is installed, which as such improves substantially the air-tightness and the oper¬ ating properties of the door. A particularly important feature of the form part according to the invention is that the cleaning door is formed by a piece cut off the duct wall of the same form part, so that the opening in the duct wall of the form part and the cleaning door fit perfectly together. In addition, substantial material savings can be obtained in that no separate piece of material is needed for the clean¬ ing door.

The gasket positioned between the door and the edges of the slightly larger opening in the duct wall can be fitted either only to the door edge or only to the edge of the opening, provided that the profile of the gasket is suitable. However, it is to be preferred that a gasket is provided both around the door edge and around the edge of the opening, whereby the gas- kets are preferably identical in profile and comprise a lip causing air-tightness under pressure. Further, it is to be preferred that the opening in the form part and the door detached therefrom are substantially round-cornered so that their edge gaskets ban be form- ed by continuous pieces of gasket tape having a uni¬ form profile. In this way, the door detached from the duct wall of the form part can be fitted air-tightly in the opening in the duct wall by extremely simple means - that is, by means of a gasket tape of one kind only. On the other hand, it can be ensured by means of the profile of the gasket tape that the door operates appropriately, i.e., that it will under no circumstan¬ ces thrust through the opening.

As to the operation of the door, it is further preferable to fasten the door to the form part by

means of a strap going over the door. The strap is fastened fixedly to the door and likewise fixedly but pivotably, e.g. by riveting, to the form part adjacent to the edge of the opening, and detachably, e.g. by means of a screw, bolt, clutch, or the like, to the form part adjacent to the opposite edge of the open¬ ing. In this way, the door can be opened and closed in a very simple manner. Being flexible and pivotable, the strap enables the door to be opened simply and ef- ficiently and, if required, to be displaced completely away from the opening.

In the following the invention will be de¬ scribed in more detail by way of example in light of illustrative embodiments with reference to the at- tached drawings, wherein

Figure 1 shows a 90° bend component according to the invention;

Figure 2 shows the bend component of Figure 1 when the cleaning door is closed; Figure 3 is a schematical cross-sectional view of the operation of the sealing of the joint between the cleaning door and the edge of the opening in the form part;

Figure 4 illustrates a T branch component ac- cording to the invention; and

Figure 5 illustrates the use of the embodiment of the invention shown in Figures 1 and 2.

In Figure 1, a form part 1 according to the in¬ vention, in this case a 90° bend, is provided with a door 3. The form part comprises a duct wall 2 out of which a round-cornered piece 3 forming the cleaning door is cut so as to form a cleaning opening. The simplest way of detaching the piece 3 is by flame cutting, whereby the cutting line will be very neat and, on the other hand, the detached piece will be

slightly smaller than the opening formed in the duct wall 2, because the width of the cutting line is about 1 to 2 mm. Gaskets 4 formed by continuous pieces of gasket tape of uniform profile are provided both around the edge of the door 3 cut off the duct wall 2 and around the edge of the opening in the duct wall. The profile of these gaskets is shown in more detail in Figure 3. The gaskets comprise a base portion 10 to be positioned on the door edge and on the edge of the opening in the duct wall 2, respectively, and a lip portion 5 projecting from the base portion. The base portion 10 is provided with a groove suitably corre¬ sponding to the thickness of the material of the door 3. When the base portion is fitted on the edge of the door 3 and pressed on the edge of the opening in the duct wall, the lip portion 5 is positioned against the duct wall. As the edge of the opening in the duct wall is provided with a similar gasket tape mounted in a reverse manner, the lip portions 5 are pressed against the base portion 10 of the gasket positioned around the edge of the door and around the opening of the duct wall, respectively, whereby the gap between the door and the opening in the duct wall will be covered by the lip portions 5 on both sides. This alone makes the joint very air-tight. Since the lip portion 5 is also suitably flexible, an overpressure or underpress¬ ure acting across the gap causes either one of the lip portions to be pressed even more tightly against the base portion 10 of the other gasket tape. This further improves the air-tightness of the joint. In addition, by choosing the dimensions of the base por¬ tion suitably, the door will in a way be wedged into the opening in the duct wall 2 so that the door is maintained reliably in place. In order to enable the door 3 to be opened and

closed, the door is fastened to the form part 1 by means of a strap 6 going over the door and fastened fixedly by means of rivets 7 to the door 3 and fixedly but pivotably to the form part 1 adjacent to the edge of the opening thereof. Such a fixed pivotable attach¬ ment is achieved by a single rivet 8. Of course, this kind of attachment could be achieved by means of a screw or bolt as well. One end of the strap 6 is fastened to the edge of the opening of the form part opposite to the rivet 8 in such a way that it can be detached for opening the door. In the embodiment of Figure 1, the strap is fastened detachably by means of a screw 9 arranged to be screwed into a hole 12 in a lug 12 fastened to the duct wall 2 e.g. by means of a rivet. In order to prevent the screw 9 from falling off, it is provided e.g. with an 0 ring of rubber act¬ ing as a counter piece after the screw 9 has been passed through a hole formed in the end of the strap 6. In the embodiment of Figure 4, the form part 1 comprising the door 3 is a T branch. As distinct from the embodiment of Figures 1 and 2, the cleaning door 3 is cut into a straight portion instead of a bent por¬ tion. In other respects, the structure and operation of the embodiment of Figure 4 correspond fully to those of the embodiment of Figures 1 and 2, wherefore the same reference numerals as in Figures 1 and 2 are used in Figure 4, too.

As shown in Figure 5, the ventilation ductwork system can be cleaned without difficulty in two direc¬ tions when the door 3 is dimensioned and positioned in a suitable way in the embodiment of Figures 1 and 2. Correspondingly, the cleaning can be carried out in three directions in the embodiment of Figure 4. It is to be seen that by means of the form part according to

the invention the number of cleaning doors required in the ventilation ductwork system can be substantially decreased. On the other hand, the installation can be carried out substantially more rapidly due to the fact that the cleaning doors are provided in the form parts prior to the installation.

The form part according to the invention has been described above by way of example by means of two specific embodiments only and it is to be understood that the idea of the invention can be applied to form parts in a ventilation ductwork system of any type; however, it is in connection with the above-described embodiments in particular that the invention offers particularly great advantages. As appears from the above, the basic idea of the invention is that the cleaning door is formed in a conventional form part having an integral duct wall by providing it with a cleaning opening of desired shape e.g. by flame cutting and by using the piece thus cut off as a cleaning door for closing the cleaning opening, whereby a suitable gasket tape is fitted around the edge of either the door or the opening or both of them. This way of manufacture ensures that the door and the opening provided in the form part fit perfect- ly together, which results in an extremely good sealing effect without affecting adversely the flow in the ductwork system or causing disturbing flow noise.