Cross-flow heat exchanger with bypass-valve

The invention relates to a cross-flow heat exchanger for use in a space heating system which is adapted for heating air by means of a burner;    which heat exchanger comprises two through-flow circuits physically separated from each other and thermally coupled to each other by means of a number of parallel walls (13),    which circuits are arranged in interwoven relation and through which flow, in mutual transverse direction, respectively <ul id="ul0001" list-style="none" compact="compact"><li>(1) an air flow from an outside air inlet to a discharge to the space for heating</li><li>(2) a gas flow from an inlet, for instance for hot flue gases coming from the burner, to a discharge for flue gases to the outside, or air coming from the space to a discharge therefor to the outside,</li></ul>    which walls are all substantially congruent and are held in mutual connection by construction means, which heat exchanger generally has a substantially prismatic form, the main direction of which parallel to the generating lines extends transversely of the main planes of the walls,    whereby in the prismatic form a smaller prismatic part serving as bypass circuit with the same section transversely of the main direction is free of walls and comprises a cavity bounded by two walls, which cavity forms part of the first through-flow circuit, in which cavity is arranged a valve which is displaceable between a closed position and an open position by means of control means coupled to the valve, which valve comprises a shaft rotatable by a drive, and bears a substantially plate-like element which extends substantially over the whole transverse dimension of said smaller prismatic part, extends between the side walls of the smaller prismatic part in the closed position of the valve, and is at a distance from at least one of these side walls in the open position of the valve.

Such a heat exchanger is known from the French patent document FR-A-2 626 969, which discloses a so called cross-flow heat exchanger with a bypass.

Using such a known heat exchanger can lead to a problem in a certain period of the year, such as the summer period, in-which the outside temperature is higher than the inside temperature. In that case the system with heat recovery will work reversely and the room will be heated relative to the outside temperature. Under circumstances this can be undesirable.

The present invention has for its object to improve upon the above known heat exchanger, and especially improving the operation during the summer period.

Therefore the present invention provides the control means are connected to a sensor for the outside temperature or the temperature of the air flow from the outside air inlet.

It is often found to be desirable for the first through-flow circuit to comprise a part through which not all cold outside air reaches the space for heating in preheated state. In certain conditions it may be desired to feed a fraction of the admitted outside air unheated to the space for heating. It is per se known for this purpose to incorporate a bypass through-flow circuit parallel to the first through-flow circuit of the heat exchanger.

A preferred embodiment has the special feature that the axis is placed symmetrically in relation to said side walls. This embodiment has the advantage that the air flowing along loads the plate-like element symmetrically, which element must of course be embodied symmetrically relative to the axis. The valve can hereby also be rotated with very little effort when air is flowing by, since no resistance is encountered by the air flowing by. In the stationary situation the drive with this structure is not under strain of rotation either.

In preference the cross-flow heat exchanger further has the special feature that the plate-like element is mounted rotatably at both its ends. Thus is avoided, particularly in the closed situation, that the plate-like element is placed under strain of bending by air flowing by.

A specific embodiment has the special feature that the drive comprises a motor with a reduction gearing. A smooth displacement of the plate-like element can hereby be realized, wherein the force can be sufficiently large under all conditions owing to the slow movement.

A simple embodiment has the special feature that the drive comprises two limit switches which respectively determine the open and the closed position of the valve. This structure has the advantage that both end positions can be determined with very simple, reliable and inexpensive means.

An alternative has the special feature that the drive comprises a stepping motor. A stepping motor enables in simple manner a possibly desired intermediate position. Such an intermediate position could in principle also be achieved with an ordinary motor. In that case however, position pick-up means must be used to signal the momentary position of the plate-like valve element to the control means, which respond thereto by comparing the momentary position with a nominal position set by a user or determined on the basis of program control and external signals.

A specific embodiment has the special feature that the control means are connectable for control purposes to an outdoor temperature sensor.

The system according to the invention is a system which is used in a space heating system with heat recovery. Contaminated inside air is replaced by fresh outside air, wherein exchange of thermal energy takes place between the heated inside air to be discharged and the still unheated, incoming outside air.

In known systems there occurs a problem in a period, for instance a summer period, when the outdoor temperature is higher than the indoor temperature. In this case the system with heat recovery will begin to operate in reverse, and the space in question is heated in relation to the outdoor temperature. In certain conditions this may be undesirable. In most comparable devices with heat recovery, the supply ventilation is simply switched off in these conditions. If for some reason this is not possible, such as in the case of strongly noise-charged outside walls which make it impossible for instance to open windows for natural ventilation, or because of personal circumstances such as allergies, a bypass circuit must be applied. This bypass could optionally be arranged outside the space heating system. When the bypass is switched on, the outside air will be re-routed round the space heating system with heat recovery in the active state of the bypass.

The present invention start from the principle that in the space heating system a cross-flow heat exchanger must be present which must itself be provided with the bypass system. This provides the possibility of a very simple structure.

In a preferred embodiment the heat exchanger according to the invention has the particular feature that the heat exchanger has the same general form as a known heat exchanger and is therefore interchangeable therewith.

The heat exchanger can also have the special feature that the prismatic part serving as bypass circuit is a separate unit. Such a relatively small prismatic part serving as bypass circuit forms a plug-in module and can be placed in a heat exchanger which is provided with an empty space adapted for this purpose in which said relatively small prismatic part fits.

In respect of both this embodiment and the embodiment described in the foregoing paragraph, attention must be drawn in this case to the fact that the bypass space causes a certain decrease in the effective dimensions of the heat exchanger. In practice however, it is found that heat exchangers in space heating systems are amply dimensioned such that this does not pose a problem.

One of the described embodiments is embodied such that the control means, which can comprise for instance a microprocessor or the like, are connectable for control to an outdoor temperature sensor.

The system can for instance have the following functional situations: <ul id="ul0002" list-style="none" compact="compact"><li>(1) Outside air temperature lower than 15°C; bypass closed; space heating system active. The supplied outside air is heated by the discharged inside air.</li><li>(2) Outside air temperature lower than 15°C or higher than 20°C; bypass open; space heating system not active. The supplied outside air is carried directly to the space without being heated.</li><li>(3) Outside air temperature lower than 20°C; bypass closed; space heating system active. The supplied outside air is heated by the discharged inside air.</li></ul>

Said levels of 15°C and 20°C are stated by way of example. The system can be designed for instance such that the user of the space heating system can adjust these temperatures.

The described adjustment can result in a lesser heating of the dwelling in the summer during the day (when it is warm outside) and a more rapid cooling of the dwelling during the night (when it is relatively cool outside).

The invention will now be elucidated with reference to the annexed drawing. Herein: <ul id="ul0003" list-style="none" compact="compact"><li>figure 1 shows a partly broken away perspective view of a part of a space heating system incorporating a cross-flow heat exchanger according to the invention;</li><li>figure 2 shows a perspective view of a known heat exchanger;</li><li>figure 3a shows a view corresponding with figure 2 of the heat exchanger of figure 1 in the situation where the bypass valve is closed; and</li><li>figure 3b shows a view corresponding with figure 3a in the situation where the bypass valve is fully opened.</li></ul>

Figure 1 shows a part of a space heating device 1. A cross-flow heat exchanger 2 is incorporated therein.

Via an inlet 2 cold outside air is fed to heat exchanger 2 via a first intake cavity 4. Via a first discharge cavity 5 the heated outside air, which has passed through heat exchanger 2 via a first through-flow circuit, is discharged by a fan 6 to an outlet 7 through which heated air is guided to the space for heating.

Via a second inlet 8 the air extracted from the space is fed via a second intake cavity 8 to a second through-flow circuit of heat exchanger 2 and discharged therefrom via a second discharge cavity 10 to an outlet 11 for discharge to the outside. Using device 1 there thus takes place an effective recovery of the heat present in the air to be discharged from the space.

Before discussion of the heat exchanger 2 according to the invention, reference is first made to figures 2, 3a and 3b.

Figure 2 shows a known heat exchanger 12. This is of the cross-flow type with two through-flow circuits which are physically separated from each other and thermally coupled to each other by means of a number of parallel walls 13, for instance of aluminium. These circuits are mutually interwoven and flow therethrough can take place in mutually transverse direction. The general direction of said two through-flow circuits is indicated on the right-hand end wall by means of two arrows 14, 15. The embodiment of this heat exchanger 12 is generally known. The walls 13 are held in mutual connection by means of construction means 16.

As shown in figure 2, heat exchanger 2 has a substantially prismatic form, the main direction of which parallel to the generating lines extends transversely of the main planes of the walls.

Heat exchanger 17 as according to figures 3a and 3b has the same generally prismatic form as heat exchanger 12 of figure 2. It is hereby fully interchangeable with this heat exchanger 12.

Heat exchanger 17 has the special feature relative to heat exchanger 12 that in the prismatic form a smaller prismatic form 18 with the same section transversely of the main direction is free of walls and thus comprises a cavity 18 bounded by two walls. This cavity 18 forms part of the first through-flow circuit which for heat exchanger 12 is designated with 14 in figure 2. In this respect the cavity is therefore bounded at the sides by two walls 19, 20. Arranged in cavity 18 is a plate 21 serving as valve body which is displaceable between a closed position as according to figure 3a and an open position as according to figure 3b. In the open position shown in figure 3b the valve is designated with 21'. Plate 21 is centrally mounted on its longitudinal ends in rotation bearings 31, 32 which together define a rotation axis which extends symmetrically relative to the prismatic part 18. Plate 21 is likewise placed symmetrically relative to the said axis. Plate 21 can thus be displaced between the closed position of figure 3a, in which the long side edges of plate 21 substantially connect onto walls 19, 20, and the open position, in which plate 21 extends in a plane extending more or less parallel to walls 19, 20. Rotation of plate 21 takes place with a drive 33 embodied in this case as stepping motor. Control thereof takes place by central control means, for instance comprising a microprocessor which is connected for instance to a space thermostat, an outdoor temperature sensor and control means which may comprise for instance a keyboard for operation by a user.

The air flow through cavity 18 is not carried along the surfaces of heat exchanger 17. As a result of this bypass cavity 18 a direct through-flow of outside air therefore takes place outside the heat exchanger.

A space heating system is generally known and applied on a large scale. Dwellings equipped with such a system must also remain ventilated by the intake fan during the summer. It is noted in this respect that it is not always desirable or even possible to open windows, for instance due to street noise. Adequate ventilation of the spaces in these dwellings is nevertheless necessary. For this purpose the valve 21 is placed in its position 21'. Supply of air with a considerably reduced flow resistance by means of fan 6 is thus also ensured in the summer. This also results in a considerable reduction in the required capacity of fan 6.

Attention is drawn to the fact that heat exchanger 17 according to figures 1 and 3 preferably has the same external form as the known heat exchanger of figure 2. Heat exchanger 17 can thus be used to replace the known heat exchanger 12.

Attention is further drawn to the fact that the invention also relates to a bypass unit which is adapted and intended for addition to a heat exchanger which is provided with a space for accommodating such a unit. This bypass unit according to the invention is specified in claim 10.