In an air-conditioning plant in which an air supply fan supplies outdoor air to a room through an air supply duct, and in which an exhaust fan removes air from the room through a ventilating duct, it is common to adjust the room temperature by heating or cooling the supply air.

It is common to arrange at least one heat exchanger in the air supply duct in order to heat or cool the supply air. The heat exchanger is connected to a pipe system, through which water or other liquid is circulated for heat transfer between the air supply duct and a heat source or a refrigerating machine placed outside the air supply duct. It is known to arrange a refrigerating machine, so that to its evaporator or its condenser a heat exchanger may optionally be arranged in the air supply duct. Thereby is achieved that the refrigerating machine may be used both for cooling and for heating the supply air. It is a particular advantage of such an arrangement that the refrigerating machine may work as a heat pump and utilize the thermal energy, which is available at low temperatures, for heating the supply air to an adequate temperature before it is let out/flows in, as preheated supply air, at a forced flow rate caused by a suction fan in the room, where ventilation/exchange of air is to take place. This reduces the energy cost when heating during the winter time.

The energy cost for the heating of rooms in winter may also be reduced by recovering heat from the extracted air and transfer the heat to supply air. A simple way of doing this is to use recirculated air, but a drawback of this is that the exchange of air is reduced, and this limits the portion of extracted air, which can be recirculated.

However, heat-recovering elements do not have this drawback.

A heat recoverer works by a material, typically a metal (metal structure similar to steel wool), is alternatingly brought into contact with warmer extracted air and cooler supply air. The metal absorbs and stores heat from the extracted air for a period of time in order to subsequently

give off heat to colder supply air for a subsequent period. A construction much used comprises a rotating wheel which intersects the air supply duct and the ventilating duct, so that the wheel absorbs heat in the ventilating duct when intersecting it during rotation, and gives off heat in the air supply duct when intersecting this during continued rotation.

In a plant, where the refrigerating machine is arranged to work as a heat pump in order to heat supply air, it is common to arrange a heat exchanger to the evaporator of the refrigerating machine, in the ventilation duct. Thereby, the refrigerating machine may transfer heat from extracted air to supply air, the condenser of the refrigerating machine then having a heat exchanger arranged thereto in the air supply duct, as earlier mentioned.

In practice it is relevant to combine the use of a heat- recovering element and a refrigerating machine in an air- conditioning plant. Previously known solutions assume, among other things, that the evaporator of the refrigerating machine has a heat exchanger arranged thereto in the ventilating duct downstream of the heat-recovering element, whereas the condenser has a heat exchanger arranged thereto in the supply duct downstream of the heat-recovering element.

In this known arrangement extracted air first passes the heat-recovering element and then the heat exchanger, arranged to the evaporator of the refrigerating machine.

Correspondingly, the inflowing outdoor air first passes the heat-recovering element and then the heat exchanger, which is arranged to the condenser of the refrigerating machine.

However, this known arrangement has the disadvantage that heat available in extracted air cannot be fully utilized because water vapour in the extracted air condenses and freezes in the heat exchanger arranged to the evaporator of the refrigerating machine, if the extracted air is cooled towards zero degrees Celsius.

The object of the invention is to provide an improved arrangement by heat-recovering arrangements in air- conditioning plants, so that a larger portion of the thermal energy, which is brought to be given off from extracted air leaving the plant, can be utilized for heating inflowing outdoor air, which may thereby be supplied to the room as preheated supply air (in winter).

The object is realized through features as stated in the following description and subsequent Claims.

In an arrangement according to the invention a first heat exchanger, arranged to the evaporator of a refrigerating machine, is positioned in the ventilating duct upstream of the heat-recovering element of the heat-recovering arrangement. Thus, in contrast to known solutions, extracted air first passes the heat exchanger and then the heat- recovering element. A second heat exchanger, which is arranged to the condenser of the refrigerating machine, is placed in a known manner in the air supply duct downstream of the heat-recovering element.

Thereby is achieved that the extracted air may be cooled to a lower temperature than by known solutions. The reason is that heat exchangers of the type in question require a relatively great difference in temperature between extracted air and

heat exchanging surface, typically about five degrees Celsius, to work satisfactorily.

Thus, in order to cool the extracted air to five degrees, the cooling surface must be about zero degrees Celsius, and if further heat is to be drawn from the extracted air, the heat exchanging surface must be even colder. As mentioned, this causes water vapour in the extracted air to condense and freeze on the heat exchanging surface, and the heat exchanger will be clogged by ice in a short while. Of course it is possible to reduce the necessary temperature difference by increasing the heat exchanging surface, but the heat exchanger will then be more expensive and require more space.

A heat-recovering element is not as vulnerable as a heat exchanger with regard to icing and clogging through icing.

Firstly, a heat-recovering element may work at a relatively small temperature difference (difference between the temperature of outgoing waste air and the temperature of inflowing outdoor air), and secondly, ice from incipient icing, will be melted when the ice-covered surface of the rotating, heat-recovering wheel gets into contact with warm extracted air the next time. When the heat exchanger arranged to the evaporator of the refrigerating machine is placed upstream of the heat recoverer, the extracted air may be cooled towards zero degrees. Thereby a greater amount of heat than by known solutions may be drawn from the extracted air, and (in winter) be transferred to the supply air.

In the following the invention will be described in the further detail by means of an exemplary embodiment, and reference is made to the appended single figure in the

drawing, which shows a simplified block diagram of part of an air-conditioning plant.

In the figure the reference numeral 1 identifies a ventilating duct, in which a first (right hand) end 2 is arranged to a room not shown, in which the air is to be changed, whereas a second (left hand) end 3 is open to outdoor air. An air supply duct 4 is open to outdoor air at a first (left hand) end 5, whereas a second (right hand) end 6 is arranged to said room. An exhaust fan 7 is arranged to transport extracted air through the ventilating duct 1 in a direction from the first end 2 of the ventilating duct 1 to the second end 3 thereof. An air supply fan 8 is arranged to transport supply air through the air supply duct 4 in a direction from the first end 5 of the air supply duct 4 to the second end 6 thereof. A heat-recovering element 9 arranged to the ventilating duct 1 and the air supply duct 4 is arranged to transfer heat from the outgoing extracted air of the ventilating duct 1 to incoming supply air in the air supply duct 4.

In the ventilating duct 1, between the exhaust fan 7 and the heat-recovering element 9, i. e. upstream of the latter, there is arranged a first heat exchanger 10, which is arranged to exchange thermal energy between extracted air in the ventilating duct 1 and the evaporator of a refrigerating machine (not shown). Heat is transferred between the heat exchanger 10 and said evaporator by means of water or another liquid circulating in pipes 11,12.

In the air supply duct 4, between the supply fan 8 and the heat-recovering element 9, i. e. upstream of the latter, there is arranged, in a manner known in itself, a second heat

exchanger 13, which is arranged to exchange thermal energy between supply air of the air supply channel 4 and the condenser of said refrigerating machine. Heat is transferred between the second heat exchanger 13 and said condenser by means of water or other liquid circulating in pipes 14,15.

Techniques for connecting the evaporator of the refrigerating machine and the condenser to the two heat exchangers, 10 and 13 respectively, are well known to a person skilled in the art and are not described in further detail herein. For the same reason said refrigerating machine with evaporator and condenser is not included in the shown partial view of the block diagram of the air-conditioning plant either.

Maximum heat-recovery is achieved when the outdoor temperature is zero degrees Celsius or lower. The operating point of the refrigerating machine is set so, that extracted air flows out from the first heat exchanger 10 at a temperature which is sufficiently high to avoid icing in the heat exchanger 10, for example seven degrees Celsius. Heat which is thereby recovered from the extracted air, is transferred to the supply air through the condenser of the refrigerating machine, and the second heat exchanger 13.

The heat-recovering element 9 recovers further heat from the extracted air now cooled, and transfers recovered heat to the supply air. The extracted air may thereby be cooled towards zero degrees Celsius before it leaves the ventilating duct at the second end 3 of the ventilating duct 1.