This invention relates to an air conditioning unit and, more particularly, to an air conditioning unit comprising a duct for conveying supply air through the unit, a duct for conveying exhaust air through the unit, a heat exchanger, particularly a recuperative heat exhanger, in which the suppl air duct and the exhaust air duct are linked with one another in heat-exchanging relationship and which comprises an exhaus air passage system including an exhaust air inlet at the top of the heat exchanger and an exhaust air outlet at the bottom of the heat exchanger, and a watering device including means for supplying water to the exhaust air passage system at the exhaust air inlet and means for collecting water draining off from the exhaust air outlet. Air conditioning units of this kind are illustrated and described in International Application PCT/SE89/00542.

As a result of the supply of water to the exhaust air passage system of the heat exchanger, the supply air is cooled. In the above-mentioned patent application features ar illustrated and described which aim, among other things, at improving the cooling of the supply air while at the same tim utilizing the supplied water for keeping the surfaces of the exhaust air passages clean.

In the air conditioning units described in this patent application, as well as in other, known air conditioning unit in which water is supplied to the exhaust air passage system, droplets of water tend to be entrained by the exhaust air and carried into that part of the exhaust air duct which is situa ted downstream of the heat exhanger. The problems resulting from this are readily appreciated.

The invention mitigates these problems and at the same time opens a possibility to design the air conditioning unit in a way that results in further advantages, as is explained below. By way of example, an embodiment of the air conditioning unit according to the invention is described in greater detai hereinafter with reference to the accompanying diagrammatic drawing, in which:

Fig. 1 is a vertical sectional view of the unit; and

Fig. 2 is a horizontal sectional view of the unit taken on line II-II of Fig. 1.

The air conditioning unit diagrammatically shown in Figs. 1 and 2 is intended for general ventilation of dwellings or office or industrial spaces and may be installed e.g. on the roof of the building containing the dwellings or spaces or in a suitable room within the building.

Outside air to be processed in the unit 10 so as to be suitable for use as supply air enters the unit through an intake 11 at one end of the unit and is transported substan¬ tially horizontally through a supply air duct, which is gene¬ rally designated by 12, and leaves the unit through the outlet of a supply air fan 13 at the other end of the unit.

While flowing through the unit 10 to the supply air fan 13 the air passes through a supply air damper (not shown) which is associated with the intake 11 but may also be independent thereof, a supply air filter 14, a secondary heat exchanger 15, a primary heat exchanger 16, a supply air humidifier 17 and a heater 18. The secondary heat exchanger 15, the supply air humidifier 17 and the heater 18, and possibly also the supply air filter 14, are components which may be omitted.

Both the primary heat exchanger 16 and the secondary heat exchanger 15 are plate heat exchangers made in accordance with International Application PCT/SE89/00542. For that reason, they are not shown in detail and they are not described in greater detail than is required for an understanding of the present invention; for a more detailed description of the design of the heat exchangers, reference is made to the just- mentioned patent application. As is disclosed in the just-mentioned patent application, each heat exchanger 15, 16 comprises a system of juxtaposed supply air flow passages, which are defined by vertical flat plates parallel to the longitudinal direction of the supply air duct and which are open towards the ends of the duct, and a system of exhaust air flow passages, which alternate with the supply air passages and are open upwardly and downwardly, whereby the flows of supply air and the exhaust air pass through the heat exchanger in cross-current and heat-exchang¬ ing relationship with one another.

At the inlet of the primary heat exchanger 16 and the outlet of the secondary heat exchanger 15, that is, at the t of the heat exchangers, each heat exchanger is associated wi a device 19 and 20, respectively, for supplying water to the exhaust air passage systems of the heat exchangers. Each of these devices is constructed as disclosed in International Application PCT/SE89/00542, and for a detailed explanation o the construction and operation of the devices reference is made to that patent application. The water supplied to the exhaust air passage systems serves to cool the supply air wh such cooling is required and to keep the surfaces of the exhaust air passages clean. It may also be used for improvin the resistance of the heat exchangers to fire.

As is explained in greater detail in the just-mentioned patent application, the water is supplied such that the en¬ tire, or practically the entire surface area of the plate faces which define the exhaust air flow passages is swept in the downward direction by a film of flowing water. This film of water is cooled in the exhaust air flow passages, because the water evaporates partially and in turn recuperatively cools the supply air flowing in the supply air flow passages

Beneath the heat exchanger 15, 16 there is a sump 21 whi is common to both heat exchangers and serves both to collect water flowing out of the exhaust air flow passages of the he exchangers and to convey the exhaust air leaving the exhaust air outlet of the primary heat exchanger 16 to the exhaust a inlet of the secondary heat exchanger 15.

Exhaust air passing through the air conditioning unit 10 enters the unit through an intake 25 positioned above the supply air fan 13 at one end of the unit and is then trans¬ ported through an exhaust air duct generally designated by 2 to the outlet of an exhaust air fan 27 positioned above the supply air intake 11 at the other end of the unit. The exhau air duct 26 comprises two substantially horizontal, aligned duct sections 26A, 26B between the adjacent ends of which there is a wall or barrier 28 including a bypass damper 29 means of which a smaller or greater portion of the exhaust may be passed direct from the first horizontal duct section 26A to the second horizontal duct section 26B.

Normally, a substantial portion of the exhaust air is passed from the first horizontal duct section 26A through the primary heat exchanger 16, the sump 21 and the secondary heat exchanger 15 to the second horizontal duct section 26B. Accordingly, the exhaust air duct 26 comprises, in addition to the two horizontal duct sections 26A, 26B, a pair of vertical sections, which are aligned with the supply air duct 12 and comprise the exhaust air passage systems of the heat ex¬ changers, and a further horizontal section, which includes the sump 21 and interconnects the vertical duct sections at their lower ends.

Several important advantages result from the above- described arrangement.

One advantage is that there is little risk of water being carried over to the second horizontal section 26B of the exhaust air duct 26, because entrainment of water in the vertical duct section adapted to accommodate or formed by the secondary heat exchanger 15 has to take place against the influence of gravity on the droplets while they are in that section. This advantage is achieved both when this section is formed by a secondary heat exchanger and when it only forms a passage between the sump 21 and the second horizontal exhaust air duct section 26B.

In addition, in case the just-mentioned vertical section of the exhaust air duct is formed by a secondary heat ex¬ changer 15 as is shown in the drawings, a significant improve¬ ment of the temperature efficiency of the heat-exchanging system is achieved without significant increase of the dimen¬ sions of the unit, compared with the case where the is no secondary heat exchanger. An increase of the temperature efficiency is often desirable, even though in some situations it may mean some danger of icing.

Alternatively, the total height of the unit may be reduced without consequent reduction of the temperature efficiency by a reduction of the height of the primary and secondary heat exchangers and a corresponding reduction of the height of the supply air duct 12.

If the secondary heat exchanger 15 is omitted, the verti¬ cal section of the exhaust air duct which connects the sump 21

with the second horizontal section 26B may be formed by a duc element positioned centrally in the supply air duct as is indicated in broken lines at 15' in Fig. 2.

The sump 21, which is coextensive in the longitudinal direction of the supply air duct 12 with the heat exchangers 15 and 16 and an intervening space as shown in Fig. 1, not only serves to collect water draining off from the heat ex¬ changers 15 and 16, but also forms a water reservoir for the water supply devices 19 and 20. Within the sump a substantial ly constant level of water is maintained in the manner and by the means described in International Application PCT/SE89/00542.

The first horizontal section 26A of the exhaust air duct 26 accommodates a burner 30, preferably a gas burner, which i used, when required, to increase the temperature of the ex¬ haust air before it reaches the primary heat exchanger 16. In this way, the supply air can be heated to the desired tempera ture before it leaves the primary heat exchanger. Known equip ment may be used for turning the burner 30 on and off and for controlling the fuel supply to the burner. For a more detaile description of the purpose of the burner and its disposition, reference is made to International Application PCT/SE89/00542.

At the downstream end of the second horizontal exhaust ai duct section 26B a recirculating air damper 31 is provided by means of which a smaller or larger portion of the exhaust air may be passed from this section of the exhaust air duct to th supply air duct upstream of the supply air filter 14 and the heat exchangers 15, 16. In the interest of simplicity, the means which are provided to this end in the exhaust air fan 2 are omitted.