The invention relates to a motor-operated fan for circulating a fluid in a heat-exchange plant-and to a process for cooling the motor for driving the motor-operated fan.
In heat exchange plants of large dimensions, in which very large quantities of heat are transferred, it may be necessary to use a motor-operated fan to circulate one of the exchange fluids.
In particular, in the plants for cooling the condenser associated with the turbine of a nuclear reactor, it is becoming common to use, as a replacement for the very tall cooling towers using natural circulation of air, shorter cooling towers in which are disposed refrigerating installations making it possible to cool the water of the condenser by circulation of atmospheric air. It is thus possible to use shorter towers which blend more easily into the landscape of the site of the nuclear power station.
In the case of cooling towers using natural circulation of air, a current of atmospheric air is produced by natural draft inside the tower and the water to be cooled is circulated, generally in a counter-current relative to the cooling air, in a divided form making it possible to assist the thermal contact. The cooling of the water is achieved mainly by evaporating a part of this water inside the cooling tower.
In certain cooling towers, cross-current circulation is employed, instead of counter-current circulation of water and of cooling air.
In the case of shorter cooling towers comprising refrigerating installations, these refrigerating installations each comprise a fan for circulating cooling air. For example, in the case of a nuclear power station rated at 1300 MWe, an assembly of eighteen refrigerating installations is used, comprising eighteen fans for circulating cooling air.
The fans may be arranged downstream of the zone of heat exchange between the cooling air and the water; the draft is then said to be xe2x80x9cinducedxe2x80x9d.
The fans can also be arranged -upstream of the heat exchange zone; one then speaks of a xe2x80x9cforcedxe2x80x9d draft.
Fans having, mechanical draft of cooling air must be driven by an electric motor which may be of the indirect drive or direct drive type. In the case of an indirect-drive fan, the revolving part of the fan comprising the blades for churning the air is driven by way of a gear train.
In the case of a direct-drive fan or motor-operated fan, the churning blades are directly integral with the rotor of the electric motor.
One type of electric motor which is particularly well suited to the case of a direct-drive fan is the disk-type motor whose rotor generally comprises permanent magnets fixed on one or more disks, cooperating with the coils of the stator.
The electric motors for driving motor-operated fans with high power ratings must be cooled during their operation.
The use of an auxiliary cooling device increases the installation and operating costs of the motor-operated fans and makes their design and use more complex.
Cooling the electric motor for driving a fan using cooling air circulated by the fan has the drawback of bringing into contact with the electrical parts of the motor, and in particular with the windings of the stator, a fluid which exhibits a certain amount of humidity and which may contain polluting substances.
The aim of the invention is therefore to propose a motor-operated fan for circulating a fluid in a heat exchange plant of large dimensions comprising a mobile part rotating about an axis of the motor-operated fan comprising at least one rotor of an electric motor for driving the motor-operated fan and fan blades integral with the rotor, and a rotating fixed part comprising at least one stator of the electric motor, it being possible to cool this motor-operated fan without using an auxiliary element and an external source of power.
With this aim:
a part at least of the electric motor comprising the stator is arranged in an enclosed chamber, and
the motor-operated fan comprises means for guiding a first fluid passing through one at least of the mobile part and fixed part of the motor-operated fan between a first zone with a first pressure of the first fluid outside the enclosed chamber on a first side of the blades of the motor-operated fan in the axial direction, and a second zone with a second pressure on a second side of the blades of the motor-operated fan outside the enclosed chamber, the second pressure being less than the first pressure and means for circulating a second fluid contained in the enclosed chamber, in thermal contact with the means for guiding the first fluid, in such a way as to cool at least a part of the electric motor of the motor-operated fan by circulation of the first and of the second fluids.
The invention also relates to the process for cooling the motor-operated fan.
To provide a proper understanding of the invention, a motor-operated fan according to the invention having direct drive comprising a disk-type motor will be described by way of example whilst referring to the appended figures.