EP2406553A1

EP2406553A1 - A building ventilation air-to-air heat exchanger control

Info

Publication number: EP2406553A1
Application number: EP10750385A
Authority: EP
Inventors: Henning Bent Jensen
Original assignee: Exhausto AS
Current assignee: Exhausto AS
Priority date: 2009-03-10
Filing date: 2010-03-10
Publication date: 2012-01-18
Also published as: WO2010102627A1; EP2406553A4

Abstract

In heat exchangers for buildings certain atmospheric conditions cause condensation and freezing of the humidity in the exhaust air, thereby preventing heat exchanging action. Prior art remedies are to switch off or to supply heat from a heat source. Such solutions are not energy- efficient. Other remedies feed back a portion of the outside air that has just passed through the heat exchanger into the cold outside air at a feeding point upstream of the heat exchanger. According to the invention, the beginning of condensation and ice build-up is determined by a change in the pressure drop across the heat exchanger, said change opening a damper to permit a part of the heated outside air to return to the input end of the heat exchanger.

Description

A building ventilation air-to-air heat exchanger control

The invention relates to a procedure and a device to control an air-to-air heat exchanger for use in building ventilation, comprising a first channel system including a ventilator for transporting outside air as supply air into the building and a second channel system including a ventilator for exhausting inside air to the outside, the two channel systems being in such heat exchanging relationship that the heat energy from the warmest stream of air is transferred to the coldest stream of air and furthermore comprising pressure drop and temperature measuring means, in which in case of risk of condensation and ice build-up, a portion of the supply air that has just passed through said first channel system is fed back into the cold outdoor air at a feeding point upstream of said heat exchanger.

The following terms are used in the description of the invention:

outdoor air is the fresh air surrounding the building, which passes through a first channel system and becomes supply air for the inside of the building after having been heated in a heat exchanger by the extract air from the inside of the building that passes through a second channel system and is changed into a colder stream of exhaust air that is ejected into the surroundings of the building. The terms balanced flow and unbalanced flow refer to the pressure conditions inside the building and are well-known in the art and need no further definition.

It has been realized for a long time that an arrangement of the kind defined above is able to reduce the energy costs of providing fresh but somewhat heated outside air to housing. However, precisely in buildings the composition of the inside air comprises considerable amounts of moisture, which may reach the dew point inside the second channel system under certain conditions of outside temperature. This means that water condenses in the second channel system and has to be drained. Under more severe outside temperatures, the water does not drain but freezes solid inside the second channel system where the outside air is the coldest. This creates a partial block of the second channel (which at this point is divided into many smaller channels inside a heat exchanger), and a number of the smaller channels are partially blocked, reducing the cross section available for exhausting air. Unless the loss of heat is stopped, the whole air transport system is blocked, because the heat loss is progressive.

Prior art solutions comprise the use of external heat sources to heat the heat exchanger locally, intermittent operation of the heat exchanger to permit thawing of the ice formed, and passing inside air with the higher temperature through the first channel system in order to melt the ice by providing a conduit that bypasses the first channels. In principle the inside air may also be passed backwards through the first channel, i.e. against the normal direction of flow. Relevant examples of prior art may be found in the following patent texts: US 6,983,794 describes an intermittent prevention of intake of cold air. US 5,497,823 describes re-circulation of exhaust air and overdriving of ventilators to create more waste heat for defrosting the heat exchanger intermittently. US 3,980,129 describes intermittent reversal of flow of the moisture-laden exhaust air. JP61062743 describes an electric heater in the air intake.

The solution that is most similar to normal operation of the heat exchange system is that of passing the relatively warmer extract air through the first channel system. However, it means that the pressure distribution inside the housing is disturbed, because there is a period in which there is more air going out than going in, unless means are provided for controlling it. Furthermore, the extract air also contains contaminants in the form of airborne dust, odours, possibly tobacco smoke, and this will contaminate the channels that normally carry relatively cleaner outdoor air.

Further known solutions comprise enabling a portion of the outdoor air that has just passed through the first channel system in heat exchanging relationship with the second channel (the supply air) to be fed back into the cold oixtdoor air at a feeding point upstream of the heat exchanger. Such solutions may be found in WO2006/071117 and DE 199 37 137.

The temperature measured at a point at the output (coldest) end of the heat exchanger connected to the second channel system may be used as an indicator of ice forming and start a cycle of re-heating. However various flow conditions may make this measurement too local for efficient control of the re-heating flow. If re-heating is instituted when it is not needed, the efficiency of the installation falls and unbalanced flow may be caused when it is not necessary. It is hence not a simple matter to control re-heating efficiently.

According to the invention these problems are avoided by a construction that is particular in that at least the flow resistance in the heat exchanger part of said second channel system is used to determine the formation of ice as an input parameter for controlling the amount of said supply air fed back to the stream of outside air. The flow resistance is directly related to the blocking of the narrow channels of the heat exchanger due to ice formation, but also at some volume speeds already discernible when condensation occurs in the form of droplets that are subsequently drained.

An advantageous embodiment of the invention is particular in that the flow resistance is determined as the pressure drop across said heat exchanger by means of a differential pressure measurement. The pressure drop is directly related to the pressure drop across the heat exchanger, but it is not linear, because the flow conditions may be dependent on the transport velocity of the air. For this reason a further advantageous embodiment is particular in that said differential pressure measurement is performed at two transport capacities of said ventilator of said second channel system. In practice, such measurements are made before and after having briefly increased the speed of rotation of the ventilator as well as during the brief increase. The three measurements thereby obtained will indicate the degree and type of ice formation and is hence capable of better and faster determination of the need for re-heating.

A further advantageous embodiment is particular in that the flow resistance measurement is combined with a temperature measurement at the output of the heat exchanger part of said second channel system to determine the formation of ice. A temperature measurement on its own might be erratic because of local ice formation forcing a fixed indication of the melting point of ice, irrespective of the extent of the ice formation. However, in combination with the flow resistance measurement it forms a useful alert signal that suitably weighted enters into the decision process for the activation of re-heating. An apparatus to perform the invention is particular in that it further comprises means for measuring the flow resistance across the heat exchanger in the second channel system, means for classifying the measurements, and means for controlling said damper to direct flow through said third channel when said classification indicates a condition of condensation and ice build-up, until said measurement means indicate that said condition no longer exists. This is essentially a control mechanism that attempts to re-establish the flow conditions that occurred before ice formation started.

An advantageous embodiment of the apparatus is particular in that means are provided for measuring the volume flow in said third channel. This information is important to control the ventilator in the first channel system, because essentially it is now transporting part of the same air twice, which reduces its net yield. This is further elaborated in a further embodiment, in which the balance of the flow and the pressure inside the building are controlled.

According to a further embodiment several heat exchanger units are used and it is particular in that one each of said third channel and associated measurement, damper, and control means is associated with each heat exchanger in said first and second chamiel systems. This solution relates to the fact that the ice formation may occur in any of the involved heat exchangers and that there is no need to perform re-heating in those that are not affected.

A further advantageous embodiment is particular in that two heat exchangers are mounted in tandem in such a manner that there is a space constituting the third channel between them. This is a compact construction, using the components themselves as elements that create parts of the third channel, which permits short and space saving air passages.

The invention will be explained in more detail in the following, with reference to the drawing, in which Fig. 1 shows a longitudinal section of an air-to-air heat exchanger, indicating flow directions and temperatures for a standard operation comprising balanced flow, as well as the temperature distribution in the second channel of the heat exchanger body,

Fig. 2 shows the same for a normal condition with a balanced flow in the case of freezing outdoors temperatures,

Fig. 3 shows the same as Fig. 2, but for an imbalanced flow,

Fig. 4 shows the same features for an embodiment of the invention for use with imbalanced flow,

Fig. 5 in relation to a somewhat higher outside temperature shows the same features for an embodiment of the invention for use with a balanced flow,

Fig. 6 shows the principle of measurement applied to the heat exchanger, and

Fig. 7 shows a practical embodiment of the invention.

In Fig. 1 is shown the well-known behaviour of an air-to-air heat exchanger in the case of an outdoors air having a temperature of 5 ⁰C and a relative humidity RH of 50%. The arrangement is such that Outside, cold, fresh air Al is injected into the building after heat exchange as Supply air Bl, while Extract air A2 from the building becomes Exhaust air B2 after having exchanged heat in the heat exchanger.

The indoors air is 22 ⁰C, and the heat of this air is used to heat the outdoors air to 19 ⁰C, whereby the exhaust air temperature drops to 8 ⁰C. In Fig. Ia is shown the isotherms of Channel 2 carrying the exhaust air corresponding to this situation, which display a gradient of smoothly falling temperatures towards the outdoors facing side of the heat exchanger. There is equilibrium, and the humidity of the indoors air of 30% RH does not create any problem. All isotherms are marked in °C.

The conditions have changed materially in Fig. 2, because the outdoors air is now at -12 ⁰C, and even the low humidity of 30 % RH of the indoors air creates problems when the temperature drops this much, which may be seen from Fig. 2a, where the isotherms clearly show the freezing at the outward-facing part of the heat exchanger.

With an unbalanced flow as in Fig. 3, the situation is only marginally better, and there is still a considerable cross section of the heat exchanger that is blocked.

Rather than using the traditional remedies of local heating or pausing the service of the heat exchanger, according to the invention a part of the air that has been heated by passing from the outside to the inside is returned upstream of the heat exchanger by means of the flow channel R, as shown in Fig. 4, which shows a flow distribution that represents an imbalanced flow. This is so efficient that according to the diagram of isotherms in Fig. 4a, there is a minimal area, almost diminishing to a point where the temperature drops to 0 °C, which might entail condensation but not freezing.

hi Fig. 5 is shown a different outside temperature and the use of balanced flow via suitable control of the ventilator 1. Again, freezing is prevented by feeding part of the heated air back into the input of the heat exchanger, and from Fig. 5 a it is seen that the temperature drops to 0 ⁰C even at balanced flow, which might entail condensation but not freezing.

It should be noted that the above examples are only points on a continuum of conditions with several variables in which a heat exchanger is intended to function efficiently.

In Fig. 6 is shown the principle of measurement applied according to the invention. The pressure drop ΔP across the 2-2 channel system of the heat exchanger provides information on the flow resistance due to ice formation. It also shows that the preferred heat exchanger is a counter- flow heat exchanger. It is also shown that the measurement of the flow ΔV through the third or re-heating channel R is measured by two pressure measurements related to a Venturi RL. One measurement is performed by a pressure sensor in the full cross sectional area of the third channel, and the other is performed by measuring the pressure at the narrow region of the Venturi structure in the channel. As the volume flow of the ventilator of the first channel system is known (via a differential pressure measurement across the ventilator itself or via calibration of the power consumption) it is also known that the third channel volume flow is removed from the effective supply of air to the building, and it is hence possible to compensate. The control system that is not shown controls the damper D and the respective velocities of the ventilator 1 and ventilator 2 in accordance with the inputs of ΔP and ΔV.

In Fig. 7 is shown a practical embodiment using an array of two counter-flow heat exchangers in parallel. Only one side of each of the heat exchangers 4 is shown, namely that which receives the stream of cold outside air. The outside air 10 enters via a particle filter 3 into the visible space that acts as a manifold for the two heat exchangers when the space is closed by a wall that has been removed for clarity between the viewer and the array. The cold air 10 is heated by the exhaust air that passes through the other channel in each heat exchanger, and the power to cause the airstreams is provided by a supply ventilator 5 (also known as ventilator 1) and an exhaust ventilator 6 (also known as ventilator T). A part of the heated air is diverted on the pressure side of the supply ventilator 5 as indicated by the smaller air stream 12 (as compared to the larger air stream 11 that enters the building as fresh, preheated air). The amount of air in the air stream 12 is controlled by a damper 13 and is fed through the channel 14 and to the Venturi structure 15 to the above-mentioned manifold where it is mixed with the incoming air 10. At point A in the supply channel 14 the pressure is measured for comparison with the pressure at point B in the Venturi structure in order to obtain the flow. For clarity, because the layout of the unit is three-dimensional, only the air stream in channel 1 from the outside to the inside of the building is shown. However, channel 2 carries the exhaust air from port 2, through the heat exchanger array where it heats the incoming air and to the exhaust port.

To sum up: in heat exchangers for buildings certain atmospheric conditions cause condensation and freezing of the humidity in the exhaust air, thereby preventing heat exchanging action. Prior art remedies are to switch off or to supply heat from a heat source. Such solutions are not energy-efficient. Other remedies feed back a portion of the outside air that has just passed through the heat exchanger into the cold outside air at a feeding point upstream of the heat exchanger. According to the invention, the beginning of condensation and ice build-up is determined by a change in the pressure drop across the heat exchanger, said change opening a damper to permit a part of the heated outside air to return to the input end of the heat exchanger.

The foregoing description of the specific embodiments will so fully reveal the general nature of the present invention that others skilled in the art can, by applying current knowledge, readily modify or adapt for various applications such specific embodiments without undue experimentation and without departing from the generic concept, and therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. The means, materials, and steps for carrying out various disclosed functions may take a variety of forms without departing from the invention.

Thus, the expressions "means to ... " and "means for ...", or any method step language, as may be found in the specification above and/or in the claims below, followed by a functional statement, are intended to define and cover whatever structural, physical, chemical, or electrical element or structure, or whatever method step, which may now or in the future exist which carries out the recited functions, whether or not precisely equivalent to the embodiment or embodiments disclosed in the specification above, i.e., other means or steps for carrying out the same function can be used; and it is intended that such expressions be given their broadest interpretation.

Claims

PATENT CLAIMS

1. A procedure to control an air-to-air heat exchanger for use in building ventilation, comprising a first channel system (1-1) including a ventilator (1) for transporting outside air as supply air into the building and a second channel system (2-2) including a ventilator (2) for exhausting inside air to the outside, the two channel systems being in such heat exchanging relationship that the heat energy from the warmest stream of air is transferred to the coldest stream of air and furthermore comprising pressure drop and temperature measuring means, in which in case of risk of condensation and ice build-up, a portion of the supply air that has just passed through said first channel system (1-1) is fed back into the cold outdoor air at a feeding point upstream of said heat exchanger characterised in that at least the flow resistance in the heat exchanger part of said second channel system (2-2) is used to determine the formation of ice as an input parameter for controlling the amount of said supply air fed back (R, 12) to the stream of outside air.

2. A procedure according to claim 1, characterised in that the flow resistance is determined as the pressure drop across said heat exchanger by means of a differential pressure measurement.

3. A procedure according to claim 2, characterised in that said differential pressure measurement is performed at two transport capacities of said ventilator of said second channel system (2-2).

4. A procedure according to claim 1, characterised in that said flow resistance measurement is combined with a temperature measurement at the output of the heat exchanger part of said second channel system (2-2) to determine the formation of ice.

5. An apparatus for performing the procedure of claim 1, comprising first and second ventilators (1,5; 2, 6) for transporting air in said first and second channel systems (1-1; 2-2) respectively, at least one heat exchanger associated with the two channel systems, a third channel (R, 12) for air transport comprising a damper (D, 13) and connected between the output of the first channel after passage of the heat exchanger and back to the input of the first channel immediately before entering the heat exchanger, characterised in that it further comprises means for measuring the flow resistance across the heat exchanger in the second channel system, means for classifying the measurements, and means for controlling said damper (D, 13) to direct flow through said third channel when said classification indicates a condition of condensation and ice build-up, until said measurement means indicate that said condition no longer exist.

6. An apparatus according to claim 5, characterised in that means (RL) are provided for measuring the volume flow in said third channel.

7. An apparatus according to claim 6, characterised in that the means for measuring the flow are used for controlling the balance of the flows provided by the ventilators (1, 2) for the first and second channels respectively in order to balance the flow and the pressure inside the building.

8. An apparatus according to claim 5, in which more than one heat exchanger is associated with said first and second channel systems, characterised in that one each of said third channel and associated measurement, damper, and control means is associated with each heat exchanger in said first and second channel systems.

9. An apparatus according to claim 5, characterised in that two heat exchangers are mounted in tandem in such a manner that there is a space constituting said third channel between them.