FIELD OF THE INVENTION

The invention concerns a recovery (or recuperative) ventilation system. In particular the invention concerns a ventilation system, comprising a first air circuit which comprises an air drive module with a low pressure side and a high pressure side, for the creation of a first air flow from a first area, for example 'the open air', to a second area, for example the interior of a building or house, and a second air circuit for the transfer of a second air flow from the second area to the first area, furthermore, comprising a heat exchanger which is arranged for mutual exchange of the heat of the first and the second air flow.

BACKGROUND OF THE INVENTION

Ventilation in houses and/or buildings can take place in different ways. In hermetic buildings that comply with the requirements of NEN 2687 the ventilation system has mostly been applied mechanically. A reliable ventilation system implies balanced ventilation. Therein the supplied air quantity is equal to the removed air quantity and the air transport takes places through canals. If a balanced mechanical ventilation system gets enlarged with a heat recovery system (HRS) a large part of the energy in the removed air will be recovered and transferred to the supplied air. Removed air contains a quantity of water vapour which will condensate in case of cooling under the dew point. The condensate freezes at temperatures below 0°C. When freezing the HRS unit can get blocked fully or partially leading to a decrease in the flow of removed air or even to a total blocking. There is no more talk about balanced ventilation by then and the supplied air will not or only partially get heated in order to be added then to the area(s) at a too low temperature. This leads to disturbance of the ventilation and serious problems of comfort.

With an HRS unit the cold open air gets heated by the warm air that is being extracted from the house. The air flows are in balance, i.e. equally large. The warm extracted air has a higher moisture level than the open air. Heat exchangers through which heat exchange takes place according to the counter flow principle are inclined to slowly freeze already at temperatures of a few degrees below 0. The freezing occurs at the extracted air side of the heat exchanger (air from the house). The degree of freezing strongly depends on the moisture level of the extracted air. According to the counter flow principle the coldest air (from outside) gets through the arrival flow part of the heat exchanger (the counter flow part) in touch with the extracted air that leaves the heat exchanger and has been cooled down strongly by that. If, while cooling of the removed air, the dew point temperature gets underspent, condensation occurs. Combined with an

outside air flow < 0°C (and lower than the dew point temperature of the extracted air) freezing of the condensate will occur. At high levels of moisture the condensation already starts halfway the passage through the heat exchanger. This condensate is taken in downward direction by the air flow and the gravitation. At temperatures < 0°C this condensate will freeze as homogeneous mass. At low levels of moisture condensation and freezing occur at the same moment because of the small mass of the small particles of water (less water content). During the process white frost is being formed (layer of snow). At a relatively high level of moisture freezing occurs from the exit flow side of the heat exchanger (air to the outside) and is mostly visible from the lump of ice on the surface. At a relatively low level of moisture freezing occurs somewhere in the middle of the heat exchanger and is not visible at the exit side of the heat exchanger (extract side).

It is common knowledge that synthetic materials of which the heat exchanger is built up have a large cohesive capacity to retain moisture. From various researches (laboratory tests about performances of HRS units) it was found that the quantity of moisture (condensate) that remains 'hanging' on the wall of the heat exchanger is ca. 300-500 grams. This moisture is not removed by the air flow and enlarges the flow resistance between the plates of the heat exchanger, causing a change in the air distribution throughout the unit and with that a change in the heat exchange.

Most of the frost protections have a simple set-up and consist of one temperature sensor, placed in the removed air flow at the exit side of the heat exchanger. This sensor is placed at the coldest point in the removed air flow quite close to the heat exchanger. This coldest point has been located by measurements of the manufacturer. If the sensor detects a lower temperature than the regulated value a certain regulating action is being performed. Usually the volume flow of the supplied air is reduced, so that the removed air is being cooled down less and will leave the unit at a higher temperature. The eventually present ice can in this way slowly defrost and removed as fluid.

An often applied frost protection has a temperature sensor in the removed air, immediately behind the heat exchanger, on the by the manufacturer determined coldest place behind the heat exchanger. Nevertheless the dynamic behaviour of the (counter flow) heat exchanger has influence on the flow through the HRS system. At this coldest place the first condensation will occur. The water film that occurs in this way and that cannot be removed sufficiently fast causes locally a narrowing of the flow surface of the heat exchanger. The flow resistance thus increases at the place of the condensation and the air flow decreases. The temperature of the air is reduced locally still more because

of the imbalance in air flows while the total cooling diminishes (supplied air is not yet being influenced).

Many HRS units are equipped with constant flow ventilators that, when the resistance increases (for example at filter pollution, condensation of the water vapour in the unit and/or partial freezing of the unit), provide for a higher number of revolutions of the ventilators in order to keep the air quantity constant. This is accompanied by a considerable increase of the system noise (particularly during the night).

The air speed in the dry part of the heat exchanger will increase. First the heat exchange will increase at higher speeds and the formation of condensate will accelerate. In this way the area with higher (flow) resistance will enlarge as a snowball. The removed air (quantity does not change at a 'constant flow' regulation) gets in this way cooled in principle by a smaller effective heat exchanging surface. The temperature difference between the inlet and exit of the removed air becomes smaller. The exit temperature behind the heat exchanger finally increases. The local temperature of the frost thermostat will increase because of mixing of the local cold air with the warmer air flow. Measurements in the laboratory have shown that the temperature that is being measured by the frost thermostat is 5-lOK higher than in the unit (conclusion: unit frozen with T inside ⁼ -6°C; T frost thentiostat ⁼ +5 ⁰ C). The frost thermostat 'sees', some time after the activation of the regulation action by the thermostat, due to the dynamic behaviour of the heat exchanger a higher temperature than the regulated value, resets the regulation action, causing the return of the normal level of the quantity of supplied air. Next the extracted air will be cooled down stronger and places with condensate will freeze if the temperature of the supplied air is < 0 ⁰ C. This is possibly still detected by the frost thermostat causing an intermittent switch behaviour of the supplied air. The freezing is not being prevented by this behaviour.

By control of the frost protection the rotation speed of the supply air ventilator lowered. In this way the air balance gets disturbed and insufficient ventilation air is being transported. From this short description of the dynamic behaviour of the heat exchanger it becomes obvious that detection with one senor in the extracted air slowly reacts and does not prevent freezing of the unit.

SUMMARY OF THE INVENTION

The present invention aims to remove or reduce the disadvantages of the known ventilation systems, as the reduction of the effectiveness due to formation of condensation and/or freezing and a good, balanced functioning of the ventilation

system. To reach this the ventilation system is provided with a third air circuit for the transfer of a third air flow from a point in the second area ('inside') or in the first air circuit, at the high pressure side (exit side) of the air drive module, or in the second air circuit between that second area and the heat exchanger, to a point in the first air circuit, at the low pressure side (entree side) of the air drive module, between the first area ('outside') and the heat exchanger. In this way the relatively warm air can be taken back as much as needed to the entree side ('fresh air side ¹ ) of the heat exchanger, through the third air circuit that functions as bypass canal with an air flow from inside to outside, maintained by the air drive module in the first air circuit. AU this can be regulated for example in such a way that the supplied air originating from the open air to the heat exchanger and to possible other critical or 'sensitive' system locations just keeps above freezing point or just above the dew point temperature.

Preferably the ventilation system comprises control means that are arranged for the regulation of the third ('bypass') air flow in relation to one or more variable (measured) values which are in direct or indirect connection to the physical status of the first and/or second air flow. For example the control means can enlarge the flow of the third air flow when one or more of the variable values get(s) such a size that in the first or second air circuit for example due to condensation and/or freezing the flow gets threatened.

The control means may be arranged for the regulation of the third air flow in relation to the measured temperature at one or more locations in the first and/or second air circuit. An other variable measured value, which is (also) indicative for condensation or freezing danger, is the moisture at one or more locations in the first and/or second air circuit. Furthermore as an indicator the condition, for example the conductivity (under influence of an already formed layer of moisture or ice) of the surface that gets in contact with the concerned air flow, at one more locations in the first and/or second air circuit, can be considered. Under the control of such measured values the control means, as said, can increase or stimulate the flow of the third air flow if- by means of calculation or empiric research beforehand - from the size of one or more of those measured values it can be deducted that there is a threatening of formation of condensation or ice in the system - for example if the temperature at one or more locations in the first and/or second air circuit turns below a certain minimum value (for example ca. 0 ⁰ C) - and the control means can thus protect the system against the formation of condensation and/or ice.

Regarding the connecting of the third air circuit, this third air circuit is on the one hand

preferably connected to the first air circuit at a point — at the low pressure side of the air drive module - between the first area ('outside') and the heat exchanger, as the heat exchanger may be considered as most 'threatened' area. Notably when the second air flow gets, from the inside area, through the heat exchanger transported to the open air, the part of the second air circuit inside the heat exchanger will just be susceptible to problems of formation of condensation and/or ice as a result of the level of moisture of the inside air supplied to the heat exchanger combined with the larger heat exchanging surface of the heat exchanger. If now the temperature of the through the first air circuit of the heat exchanger supplied open air, meant to be heated by the inside air, is below the dew respectively freezing point, there is a chance that the damp inside air in the heat exchanger gets cooled to such a degree that due to condensation and/or ice formation in the heat exchanger the flow of the to be removed inside air stagnates, as already said. By means of the now proposed third air circuit this (too) cold open air flow can be mixed with relatively warm air, by which the undesirable condensation and/or ice formation can be prevented.

The point where the air must be taken in by the third air circuit can be chosen differently. Preferably this air gets taken in at a point in the first air circuit at the high pressure side of the air drive module — for the air flows from high to low pressure — between the second area ('inside') and the heat exchanger. The advantage here is that the take-in-point can be located constructively inside (for example the housing of) the ventilation system. Moreover the air is here relatively warm and dry. The take-in-point can however also, if desired, be connected directly to the second area, for example through a (separate) air intake in the inside area to be ventilated. Finally the take-in- point for the third air circuit can if desired be located at a point in the second air circuit between the second area (in this case the area to be ventilated) and the heat exchanger. This last option seems to have the disadvantage that compared to the first option the air taken in is rather damp. An advantage can however be that this air (the removed air of the area to be ventilated) is warmer, by which only few mixing with the (too) cold open air is required. The degree of moisture of the air taken in can if desired be reduced by means of for example moisture filter.

It is noticed that it is essential for the functioning of the invention that the (bypass) air through the third air circuit in all cases from 'inside' (the side of the area to be ventilated) to outside (the side of the open air) is. As the direction of the bypass air flow is determined by the pressure difference between both sides of the third air circuit, it is a condition in all configurations that the pressure from the third air circuit at the side of the area to be ventilated is higher than that at the other side.

It should be noticed that from US 5024263 a ventilation system is known that at first sight - due to the presence of bypass circuits - has much similarity with the system according to the present invention. This known system does however not intend to offer a solution for the said problem, in short and for simplicity indicated as 'frost protection", but intends an improvement in the balancing of the different volume flows. For that purpose the bypass circuit indicated with 'F' in US 5024263 has been connected to a first point in the 'fresh air circuit' between the 'open air' and the heat exchanger and a second point in the 'fresh air circuit', however at the other side of the heat exchanger. This second point is however located - unlike in the present invention - at the low pressure side of the air drive module - indicated with number 5 - in the 'fresh air circuit'. The result of this is - note the direction of the arrow - that an air flow through bypass circuit F is created from 'outside' to 'inside'. This is contrary to the air flow through the bypass circuit (III) in the present invention, which is always directed from 'inside' (relatively warm air) to 'outside' (danger of freezing), namely as a result of the fact that the take-in-point of the bypass circuit in the system according to the invention is located at the high pressure side of air drive module. In the prior art system the bypass circuit serves mainly for the balancing of air volumes, as the bypass circuit in the present invention serves for temperature increase of the air flow that is supplied to the heat exchanger.

Hereinafter an exemplary embodiment of the ventilation system according to the invention will be shown.

EXEMPLARY EMBODIMENT

Figure 1 shows a preferential embodiment of the invention.

Figure 2 shows a first alternative exemplary embodiment.

Figure 3 shows a second alternative exemplary embodiment.

Figure 4 finally shows very schematically in one figure the different exemplary embodiments and a reference to the prior art configuration from US 5024263.

The in figure 1 shown ventilation system comprises a first air circuit I, including means as canal parts, ventilator, inlet and exit etc., arranged for the transfer of a first air flow from a first area, the 'free open air' EXT, to a second area, the interior INT of a house or other building. The ventilation system comprises a second air circuit II, equally including means as canal parts, ventilator, inlet and exit etc., arranged for the transfer of a second air flow from the second area INT to the first area EXT. Furthermore the ventilation system comprises a heat exchanger X which is arranged for mutual exchange

of the heat of the first and the second air flow. The ventilation system comprises furthermore a third air circuit III, arranged for the transfer of a third air flow from the (hereinafter to be discussed) point that, seen from the heat exchanger X, is located at the side of the second area INT, to a point pi in the first air circuit I, located between the heat exchanger X and the first area EXT.

The ventilation system in figure 1 comprises furthermore control means, which comprise a control unit C and a flow regulator or valve V, that are arranged for the regulation of the third ('bypass') air flow in relation to one or more variable (measured) values which are in direct or indirect connection to the physical status of the first and/or second air flow. For this one or more measuring sensors have been connected to the control unit C which will be discussed hereinafter. These sensors can for example at one or more locations in the first and/or second air circuit measure the temperature and/or level of moisture and/or surface condition of the air canals and pass on to the control unit C. The control unit C can then, in dependency to the value of one or more variable (measured) values), direct the regulator valve V for, if needed, the regulation (opening, enlarging, reducing, closing) of the air flow from the third air circuit III. The control unit will through the directing of the regulator valve V enlarge the flow of the third air flow when one or more of the measured values get(s) such a size that there is a danger of condensation and/or freezing in the first or second air circuit by which the flow gets threatened.

In figure 1 the third air circuit III is connected on the one hand to the first air circuit I at the point pi in the first air circuit I, between the first area EXT and the heat exchanger X. On the other hand the third air circuit III is connected to a point p2 in the first air circuit I, between the second area INT and the heat exchanger X. In stead of this the third air circuit III could if desired also be connected directly to the second area INT, through an extra air intake point p3, as figure 2 illustrates. Finally the third air circuit can also be connected to a point p4 in the second air circuit II, between the second area INT and the heat exchanger X, as figure 3 illustrates. In this case a moisture filter MF can be added into the third air circuit III, as the air taken in from the second area is usually damp.

Finally figure 1 will be discussed in detail hereinafter. The applied temperature sensors will be introduced as Tl 1 , T 12, T21 and T22. Furthermore a moisture sensor RVl 1 can be used. AU sensors are connected to control unit C. The regulator valve V is equipped with a servomotor M. The air through the air circuits gets propelled by a couple of two ventilators F 12 respectively F22. F22.

There are different options:

1. Use of one temperature sensor, namely sensor T2 in the supplied air before the heat exchanger X: During underspending of a temperature regulated in advance in the control unit C the bypass valve V is opened from the control unit C. Heated air gets mixed up with cold open air. This mixing remains maintained until the temperature at the entree of the heat exchanger is higher than the in the control unit C regulated temperature and the bypass valve gets closed again. The control unit C is regulated in such a way that the from outside (EXT) supplied air must remain > 0 ⁰ C. In this embodiment the quantity of the recycled air depends on the pressure difference between the pressure side of the supply ventilator V22 and the entree side of the heat exchanger X on the one hand and on the other on the occurring flow resistances. During opening of bypass valva V the quantity supplied air will decrease after the system if the number of rotations of the ventilator is not adjusted. Adjustment of the number of rotations can take place by the ventilator - to be put in a higher switch mode from control unit C or by modulating regulation of the number of rotations on the basis of 'constant volume flow' principle.

2. Use of temperature sensor Tl 2 in the removed air after the heat exchanger X: Similar to option 1, however with the difference that on the basis of the removed air temperature, measured by T 12, a lower open air temperature, measured by Tl 1, can be admitted. The forst thermostat (control unit C) remains regulated at a switch temperature of 0°C.

3. Use of temperature sensor Tl 2 in the removed air after the heat exchanger X and temperature sensor Tl 1 and moisture sensor RVl 1 in the removed air before heat exchanger X:

For this embodiment the dew point temperature of the removed air is determined with the help of the sensors Tl 1 and RVl 1 and compared to the exit temperature, measured by T 12. If the temperature measured by Tl 2 is lower than the calculated dew point temperature and at the same time D 0 ⁰ C, the protection will be switched on due to the opening of bypass valve V by control unit C. The protection will always draw on if condensation occurs in the removed air and the removed air is 0°C. The open air temperature, measured by T21, can be < O ⁰ C.

4. Use of temperature sensor T21 in the supplied air before the heat exchanger X and temperature sensor Tl 1 and moisture sensor RVl 1 in the removed air before the heat exchanger:

For this embodiment the dew point temperature of the removed air is determined with the help of the sensors Tl 1 and RVl 1 and compared to the exit temperature

T21. If the temperature T21 is lower than the calculated dew point temperature and T21 at the same time D 0°C , the protection will be switched on by the control unit C due to the opening of bypass valve V. The protection will draw on at an open air temperature < 0°C at the moment that the difference between dew point temperature and the temperature measured by T21 becomes negative.

5. Use of temperature sensors Tl 1, Tl 2 and T21 and moisture sensor RVl 1 :

For this embodiment the dew point temperature of the removed air is determined with the help of the sensors Tl 1 and RVl 1 and compared to the exit temperature, measured by T12. If the temperature measured by T12 is lower than the calculated dew point temperature and the temperature measured by T21 is lower or equal to the dew point temperature of the exctracted air, the protection will be switched on due to the opening of bypass valve V. Through this way of regulation the protection can be regulated at a lower temperature than indicated at option 4.

It should be noticed that it is essential for the functioning of the invention that the

(bypass) air through the third air circuit in all cases is directed from 'inside' (the side of the area to be ventilated) to outside (the side of the open air). As the direction of the bypass air flow is determined by the pressure difference between both sides of the third air circuit, it is a condition in all configurations that the pressure from the third air circuit at the side of the area to be ventilated is higher than that at the other side. This applies also if, as in the said and discusssed exemplary embodiments, in the second air circuit also a (second) air drive module (ventilator F 12) has been applied. As the intake side pi of the bypass circuit III is connected in all cases (see figures 1, 2, 3) to the low pressure side of ventilator F22, the other end of the bypass circuit III (p2, p3 or p4) is always connected to a point where a higher pressure prevails.

In figure 4 the configurations of the figures 1, 2 and 3 have been drawn in the same way as in figure 1 of the prior art document US 5024263, namely with at the left the open air side EXT and at the right the inside air side INT. EXT and INT are connected with each other by means of the first air circuit I and the second air circuit II. Ventilator F22 - with low pressure side Lpres and high pressure side Hpres — produced the air flow from EXT to INT. In the second air circuit II the air flow from INT to EXT is maintained by ventilator F 12. By means of heat exchanger X the heat of both air flows is mutually exchanged. The ventilation system comprises a third air circuit III through which flows a third air flow.

In figure 4 the different optional configurations are proposed as these have been discussed here; moreover reference is made to the configuration of the prior art

document US 5024263.

In the configuration as discussed at figure 1 the third air circuit III is formed by a connection AA' between the points pi and p2. Please note well that the point p2 (though as well as the points p3 and p4) is located at the high pressure side Hpres of the ventilator F22, because of which - as point pi is located at the low pressure side Lpres of that same ventilator - an air flow through the 'bypass' circuit III is maintained that flows - for from high pressure to low pressure — in the direction (see arrow in circuit III) to point pi . This contrary to the prior art configuration, where the bypass circuit AA' at the side of INT (indicated as 'prior art') is connected to the low pressure side Lpres of the ventilator, because of which a part of the air gets diverted outside the heat exchanger ('shunt') and that air flow thus flows from point pi in the direction of INT, in other words in the same direction as the air flow through circuit I.

In the configuration that has been discussed at figure 2 the third air circuit III is formed by a connection AA' between the point pi and p3. Also point p3 (as well as point p2) is located at the high pressure side Hpres of ventilator F22 - the whole area INT is on that pressure level -, because of which an air flow is maintained through circuit III that flows from point p3 in the direction of point pi .

In the configuration that has been discussed at figure 3 the third air circuit III is formed by a connection AA' between the points pi and p4. Also point p4 (as well as the points p2 and p3) is located - through area INT - at the high pressure side Hpres of ventilator F22, because of which also in any case an air flow is maintained through circuit III that flows from point p4 in the direction of point p 1.