A HEAT EXCHANGER, A HEAT SINK AND A HEAT EXCHANGE SYSTEM

FIELD OF THE INVENTION

The invention relates to heat exchangers and heat sinks. In particular, the invention relates to heat exchangers and heat sinks for use in heat recovery ventilation systems such as those used in the ventilation of buildings.

BACKGROUND OF THE INVENTION

Heat exchangers may be used as stand-alone systems or for enhancing the efficiency of Heating, Ventilating and Air Conditioning (HVAC) systems, Heat Pumps, other Heat Recovery Ventilation (HRV) and Energy Recovery Ventilation (ERV) systems.

Currently the majority of Heat Recovery Ventilation systems utilise air-to-air heat exchangers that are of a plate-type cross-flow design, such that the flow paths of the two air streams (the inflow stream and the outflow stream) are at 90° to each other. While convenient from a construction standpoint, this configuration results in less than optimal performance, with the maximum theoretical thermal efficiency being 75%. In order to obtain the best possible performance from the cross-flow configuration, these designs typically also have square plates. This results in an overall size that is difficult to package and may make for extreme difficulty in placing the unit into the very confined spaces typical of a building services environment.

Cross-flow type exchangers can be prone to fan and airflow induced plate vibration; resulting in an unpleasant drumming noise. Due to the short flow-path, cross-flow type heat exchangers also typically require a larger heat transfer surface area than counter-flow heat exchangers. Relative to counter-flow units, this results in an increase in pressure drop across the exchanger plate pack, such that the rating and power consumption of the circulation fans is increased. Counter-flow exchangers also have a better theoretical maximum efficiency of

Most Heat Recovery Ventilation Systems draw in fresh air at whatever temperature it happens to be at, with no attempt to pre-heat or cool the air to a more moderate temperature. This may place a higher than desired load on the heat exchanger and result in inflow air temperatures being somewhat less than desired.

Many Heat Recovery Ventilation Systems are fitted with various types of frost protection. A common method used is a temperature sensor which, on detecting a threat of the unit icing, either turns in an integrated heater or causes warm air to be circulated through the system. While effective, both methods result in undesirable use of energy.

The thermal efficiency of Heat Recovery Ventilation Systems is frequently challenged by a lack of balance between the extract and charge air. Aside from airflow imbalance, the efficiency of the system may be further challenged by the relative humidity of both airflows and this can have a very significant impact on overall thermal performance.

Furthermore, the overall performance of Heat Recovery Ventilation Systems may also be compromised even by the heat exchanger, given that there will be times when it is desired to deliver air directly from the attic space without having it heated and/or cooled by the heat exchanger.

Heat Recovery Ventilation Systems are typically arranged to deliver fresh, dryer air to individual rooms while extracting damp, stale air from a different position in each of these rooms. While effective in maintaining air freshness and low humidity, the lower or higher than desired temperature of the air in at least some of the rooms, will result in a lower overall system efficiency.

By nature of their operation, moisture condenses in Heat Recovery Ventilation Systems. This makes such systems prone to health threatening diseases such as Legionnaires' disease and Stacchybotris contamination. Currently this

Most heat exchangers are made from expensive conductive materials such as metal sheet, resulting in high fabrication cost. Heat exchanger plates are also generally of a relatively high surface roughness, causing larger pressure drops across the plates than could be achieved with smooth heat exchanger plates.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a high performance, simple, reliable and low cost Heat Recovery Ventilation System that will improve upon the attributes of currently available designs or at least provide the public with a useful choice.

In a first aspect the invention provides a counter-flow heat exchanger including: a first sheet of fluted plastic material having a plurality of inflow channels formed therein; and a second sheet of fluted plastic material having a plurality of outflow channels formed therein; wherein the first and second sheets of fluted plastic material are: arranged adjacent to each other with the inflow and outflow channels being parallel to each other; and offset in a direction parallel to the inflow and outflow channels, such that a first end of the inflow channels is offset from a first end of the outflow channels.

In a second aspect the invention provides a heat sink including: a plurality of flow channels; and a heat sink material arranged adjacent to the flow channels; wherein at least some of the flow channels are formed from a plastic twin-wall material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic drawing of a Heat Recovery Ventilation System according to one embodiment; Figure 2 shows a heat exchanger plate pack; Figure 3 shows an alternative heat exchanger plate pack;

Figure 3B shows a further alternative heat exchanger plate pack; Figure 4 shows a heat exchanger shell; Figure 5 shows a heat sink shell;

Figure 6 shows a plenum at the input end of a set of flow channels; Figure 7 is a schematic diagram of a geothermal duct;

Figure 8 is a graph of showing ground surface temperatures and below- ground temperatures throughout the year;

Figure 9 is a schematic diagram of a night-sky radiation unit; and Figure 10 shows a noise-reduction plenum.

DETAILED DESCRIPTION

Figure 1 is a schematic drawing of the Heat Recovery Ventilation System according to one embodiment. Air enters the system from the outside of a building via a fresh air filter 1 , which is preferably a filter for removing particulates from the inflow air, such as a HEPA filter. The inflow air then passes through a chamber 2, in which a UV-C sterilizing lamp (not shown) is mounted. UV-C rays are capable of neutralizing or even destroying many dangerous organisms including viruses and bacteria. Other sources of radiation suitable for destroying harmful organisms may also be suitable.

Next, the inflow air passes through a heat sink unit 3, which is described in detail below. The air flow through the system is created by a pair of fans 4, 4a. The inflow fan 4 operates to create airflow into the building while the outflow fan 4a operates to create airflow out of the building.

A heat exchanger 5 allows the exchange of heat between inflow and outflow air streams without mixing of those streams. The heat exchanger 5 is also described in detail below.

Both the heat sink 3 and the heat exchanger 5 are formed of a plastic twin-wall material. This material is preferably a fluted material, such as flute board or corrugated plastic board. This is a commonly available material which is often

material, which is easily made and which can easily be adapted to form the devices disclosed herein. Although plastic has very low thermal conductivity and therefore might not be thought suitable, it has been found that the drop in efficiency caused by the very thin very low conductivity walls is in fact very small. Therefore, use of this material allows for significant cost savings with minimal effect on performance.

Furthermore, plastic twin-wall material generally provides a very smooth surface, with offers minimal resistance to airflow over its surface. This causes a lesser pressure drop over the heat exchanger plates than with conventional materials.

The fans 4, 4a providing the air movement may be installed as an integral part of the heat exchanger or as separate items. In the case of the latter, they may be mounted upstream or downstream of the heat exchanger 5. Preferably the inflow fan 4 is mounted upstream of the heat exchanger while the outflow fan 4a is mounted downstream of the heat exchanger 5. This means that both fans are mounted to the "outside" of the heat exchanger 5. This is advantageous because the heat exchanger 5 assists with noise attenuation. The fans may be individually or jointly speed controlled so as to provide optimal airflow for the particular application. Any suitable controller may be used, including rheostat, "chopper" or frequency conversion controllers.

Each fan may be mounted to a supporting structure using a flexible strap or cord. This assists in preventing noise transmission to the inside of the building.

Ducting 6 is provided for air streams flowing into the building from the heat exchanger 5 and out of the building to the heat exchanger 5. Preferably this ducting is insulated to prevent heat loss from the air streams. Ducting 7 is provided for the air stream leaving the building from the heat exchanger 5. The outflow air passing through this ducting 7 is simply reject air which is serving no further useful purpose, so the ducting 7 need not be insulated.

A number of ducting branches are created within the building, using ducting

distributed through the building. Similarly, a number of outflow vents 9a may also be distributed through the building. Preferably only a single output vent 9b is provided, outside the building, for expelling the reject air.

The heat exchanger is preferably tilted from the horizontal such that any condensate collects at its lowest point. Preferably this lowest point is at the "cold" end of the heat exchanger. To ensure frost protection, a plastic condensate drain conduit 10 is fitted at this lowest point. This conduit is rigid enough to enable installation with a continuous fall to the outside of the building. Preferably it is arranged at an angle of at least 5° to the horizontal. It is also large enough in diameter to allow the passing of reject stale air from the heat exchanger, thus ensuring that the still slightly warm air will prevent icing of any water that may accumulate in the condensate drain tube. Preferably the conduit has an internal diameter of at least 10 mm.

In use, stale, warm, moisture-laden outgoing air enters the heat exchanger 5 via outflow vents 9a, insulated ducting 6 and through a filter (not shown). This air gives up its heat in the heat exchanger 5 to fresh inflow air travelling in the opposite direction.

The stale outflow air should preferably be from warmer sources such that the system usefully contributes to maintaining warmth in the building. It may be advantageous however, to extract very moist air from bathrooms and laundries and in such instances, air movement would preferably be controlled with either a thermostatically controlled fan or louvre or thermo-actuated (wax motor) type louvre.

In this embodiment, the wet area (laundry, bathroom, en suite, spa room etc) is fitted with a humidity sensor. When the humidity exceeds a certain set point, the sensor causes an extraction fan with electrically operated louvres, to start-up. The louvres open and the fan sucks moist air, via insulated ducting, into the HRV heat exchanger 5. As an alternative, the system may use only an electrically operated louvre, in which case the HRV exhaust fan 4a would provide the

In the preferred embodiment incoming fresh air is drawn through the heat sink 3 prior to it entering the heat exchanger 5. This reduces the load on the heat exchanger. Should the fresh air have a higher than desired temperature, the heat sink removes some of this and stores it. If the temperature of the fresh air later becomes lower than desired, the heat sink releases heat to the fresh air supply, thereby reducing the load on the heat exchanger and improving the overall system efficiency.

A thermostat 11 and a controller 12 are also provided. The controller 12 may control the speeds of the various fans, as well as any louvres, bypass systems etc. The combination of the thermostat 11 and controller 12 allows for monitoring of the temperature of the inflow air and the outflow air and for control of the system based on the temperatures and/or temperature difference. Various methods of control are described below.

The invention may include provision for the insertion of an air-quality sensor or sampling tag that would enable the verification of air quality in respect of particulates or dangerous organisms such as Stacchybotris, for example.

Figure 2 shows a heat exchanger plate pack, as used in the heat exchanger 5 of Figure 1. Fresh inflow air enters at 20 and exits at 21 while reject warm air enters at 22 and exits at 23. This drawing shows the fluted plastic material which forms inflow channels 24 of generally rectangular cross section. A number of baffles or spacers 25 are attached to the top of the plastic fluted material to create the outflow path. These may also be formed from cheap, plastic material. This configuration allows a number of identical plate packs to be stacked together. The inflow and outflow air streams are separated by the combination of the twin-wall Flute-board material and the baffles or spacers as shown. All elements are preferably joined by a suitable glue to create an air-tight join and to allow heat transfer between elements.

In this configuration, the inflow and outflow airstreams travel in opposite

configuration. This allows long, narrow plates to be used, as opposed to the square plates generally used in cross-flow systems.

Figure 3 shows an alternative design of plate pack for use in a heat exchanger 5, having slightly different airflow paths. Fresh inflow air enters at 30 and exits at 31 while reject warm air enters at 32 and exits at 33. Both the inflow channels 34 and the outflow channels 35 are formed from twin-wall Flute-board material.

Again, baffles or spacers 36 are provided in order to separate the inflow and outflow air streams. Again, a number of these plate packs can be stacked together.

The plate pack in Figure 3 includes a first sheet 37 and a second sheet 38 of fluted plastic material. The first and second sheets may be identical and are arranged in an offset manner. This forms an offset region 39, 40 at each end of the plate pack. The baffles or spacers 36 are positioned in each offset region to form a chamber or plenum 41. Thus inflow air 30 enters the chamber or plenum 41 , where it may flow in a turbulent manner. The flow through the inflow channels 34 is more equally divided between the channels due to this turbulent flow. Within the channels 34, the flow may be laminar.

In Figure 3, the upstream end of the inflow channels and the downstream end of the outflow channels are offset, and the downstream end of the inflow channels and the upstream end of the outflow channels are offset.

The chambers 41 each open to the side of the plate pack. That is, each chamber opens in a direction substantially transverse to the length of the flow channels.

A number of first plates and a number of second plates may be stacked together in alternating order.

This configuration provides an inflow flow path and an outflow flow path which are substantially the same. This means that the pressure drop across the inflow path is substantially the same as the pressure drop across the outflow path, which

makes it easier to create a desired flow rate in each path. This makes 'balancing' the flows easier, leading to higher efficiency operation.

This configuration is also relatively simply to assemble, requiring less assembly time than some alternative configurations.

One problem with the use of fluted material is that it tends to have a surface which is rippled transverse to the lengths of the channels. These ripples tend to impede the flow of air across the surface, which, results in air tending to flow through channels closer to the opening of the chamber 41. Thus inflow air entering the heat exchanger at 30 is impeded from traveling across the heat exchanger plate pack by ripples on the surface of the chamber 41. This air therefore tends to flow through channels closer to the edge 41a of the plate pack (effectively taking the path of least resistance). This results in uneven pressure across the different channels 34, and therefore decreased efficiency.

The embodiments shown in Figures 3A and 3B are intended at least to make some improvement to this uneven pressure.

Figure 3A shows a further embodiment, similar to that of Figure 3. In this embodiment the face 42 of the first sheet 37 is cut at an angle α. Similarly, the face 43 (indicated by a dotted line in Figure 3A) of the second sheet 38 is cut at an angle α. The angle α may be between about 20 and 45°, preferably around 23°.

The angled sheets of Figure 3A may provide a more even pressure across the inlet ends of the flow channels, compensating for the rippled surface of the fluted material and thereby improving the evenness of pressure drop and therefore flow across the different channels.

Figure 3B shows another embodiment of a plate pack, including a number of first sheets 43 of fluted plastic material and a number of second sheets 44 of fluted

plastic material. In the drawing, three first sheets 43 and three second sheets 44 are shown, although any suitable number of sheets may be used.

Each first sheet 43 is simply a rectangular sheet of fluted material. Each second sheet 44 is cut at an angle β. Baffles 45 are used to separate the inflow and outflow air and to form chambers 46, 50 at each end of the channels 47 in the first sheets 43.

Again, the shape of the chamber 46 helps to create a more even pressure across the different channels 47.

In this embodiment inflow air entering at 48 exits the heat exchanger at 49. Air entering a channel close to the opening of the chamber 50 then exits into the narrow part of the chamber 46. This further assists in equalizing the pressure across the different channels.

The angle β may be in the range about 20 to 45°, preferably around 23°.

A heat sink may be formed in a similar fashion to the heat exchanger plate pack shown in any of Figures 2, 3, 3A and 3B. However, the outflow paths are filled with a suitable heat sink material, leaving only a flow path for inflow air, through the channels in the fluted material. Preferably the heat sink material is a phase change material, although high heat capacity materials may be suitable. Several suitable phase change materials have been identified, including Glauber's Salt, Slack Wax and Calcium Carbonate along with various other proprietary phase change materials that have been specifically developed for functionality in the 18-

35°C operating range. Other suitable materials may also be used.

The phase change material is provided in voids between the layers of fluteboard. Suitable phase change materials are generally semi-solid (usually a salt) at lower temperatures and change phase at higher temperatures. This means that most suitable materials are liquid at higher temperatures, so must be contained to avoid leakage.

Preferably both the heat exchanger 5 and the heat sink 3 are formed in long but narrow plate packs as shown in Figures 2, 3, 3A and 3B, from Flute-board or similar, extruded plastic twin wall sheeting. A shell is provided to enclose the plate packs and the whole is preferably dimensioned such that it may be installed through a standard ceiling assess-way. Preferably it has a cross-section of 420mm by 320mm or less. Preferably the length of the heat exchanger is at least three times its width, to provide good efficiency in a compact package. The shell is preferably manufactured from a material having low thermal conductivity such as rotationally moulded low density Polyethylene. Nozzles allowing connection of ducting to the heat exchanger or the heat sink can be integrally moulded in the shell. A pliant material such as soft rubber, foam or Silicon mastic is placed either side of the side entry nozzles to ensure that the air streams remain separated.

The heat sink may also have a cross-section of 420mm by 320mm or less. Preferably the length of the heat sink is at least three times its width, to provide good efficiency in a compact package.

Figure 4 shows the heat exchanger shell 55, including integrally formed nozzles 56. Figure 5 illustrates the heat sink shell 57 which is similar to the heat exchanger shell in that charge air enters at 58 and exits at 59. The difference between this vessel and the heat exchanger is that the reject air side is filled with a suitable phase change material and it therefore includes two, rather than four, nozzles.

Figure 6 is a plan view of a set of flow channels 60 formed from fluted plastic material. Ducting 61 is connected to the flow channels 60 via a chamber or plenum 62. This plenum is dimensioned and arranged such that air entering the plenum from the ducting 61 creates turbulence within the plenum 62, creating a substantially equal pressure distribution across the various flow channels 60. A similar plenum is also provided at the downstream end of the flow channels 60. This means that airflow is substantially equally distributed across the different channels, allowing all channels to contribute to the efficiency of the heat

The simple rectangular or triangular plenums shown in the Figures compare very well to a more intricate plenum with a gradual change in shape from the round ducting to the rectangular shape formed at the openings of the flow channels. In fact, the rectangular plenums have been found to exhibit a much lower temperature differential across the flow channels, indicating that they more effectively equalize pressure for even flow in the different channels.

The flow area in the ducting should match the flow area in the plenum. So if we have a ducting inlet with an internal diameter of 0.2m, this is a flow area of 0.03142m ² . The same inlet has an internal circumference of 0.628m. Assuming the width of the plenum is the same as the diameter of the inlet, the height of the plenum should be 0.03142/0.628 = 0.05m.

Similar plenums may be used in the heat sink.

Figure 7 is a schematic view of a geothermal duct which may be used in one embodiment. A geothermal duct 70 penetrates the foundation 71 of a building and lies under the ground surface 72, below the frost level. The geothermal duct 70 is connected to the system of Figure 1 such that inflow air exchanges heat with the geothermal duct. This allows advantage to be taken of the difference between air temperature and below ground temperature, as shown in the graph of Figure 8. This graph shows the ground surface and 1.5m below surface temperatures for the year, in New Zealand. The graph of course varies depending on location, but generally the temperature below the frost line is greater than the surface temperature in the winter months and less in the summer. Therefore, during the winter, this geothermal duct operates to warm inflow air.

Inflow air may be drawn from a warm space, such as a roof cavity in order to minimize the load on the heat exchanger.

Other sources of heating and cooling can be added to the system as desired.

source of cooling. On clear nights, night-sky radiation can result in temperatures at or near a conductive surface being as much as 11 ⁰ C cooler than the ambient air temperature. This low temperature can be harnessed on warm, clear summer evenings to help cool the inside of the building.

Figure 9 shows the night-sky radiation unit 90 mounted just below a conductive roof 91 (shown in the drawing as a corrugated metal roof, but any conductive surface may be suitable). The night-sky radiation unit 90 includes a tray collector 92 which may be attached across two roofing joists 93 or otherwise mounted close to the roof 91. A duct 94 connects to the tray collector to transfer the cool temperatures from the collector tray to the main part of the heat exchange system.

Figure 10 shows a noise reduction plenum 100, which may be installed to the inside of the heat exchanger and fans (i.e. inflow air passes through the fan and heat exchanger before entering the noise reduction plenum). A similar plenum may be used for the outflow air. The plenum 100 includes an inlet 101 and a chamber body 102. The chamber body is shaped to provide anechoic surfaces.

Alternatively, the chamber could be lined with a suitable noise attenuating material. In any case, noise from the fan(s) and any other moving parts is attenuated in the noise reduction plenum 100 so as to prevent this noise from reaching the interior of the building. The plenum 100 includes one or more outlets 103. The plenum shown also acts as a manifold, having three outlets for distributing inflow air to different parts of the building.

The noise reduction plenum 100 could also act as a dust cyclone, thereby prolonging the life of a replaceable particulate filter. -

To ensure that the system operates at optimal efficiency, regardless of air-flow rate and humidity, the heat exchanger would preferably be fitted with resistance temperature detectors (RTDs) or similar sensors at two or more nozzles. These sensors would provide feedback to an e-PROM or similar device having programmed logic that would analyse the temperature differentials of the air

differential vary a pre-set amount, the delivery fans would be either slowed or sped up to create optimal thermal efficiency.

The controller (such as an e-PROM or similar device) may be arranged to control operation of the fan(s), as well as any other flow control devices, such as valves etc. Inputs other than the temperature readings may be used, such as signals from any sensors, manual input of desired temperature ranges or other operational characteristics etc.

The controller may be programmable. For example, the controller may be programmable to set a first temperature for certain hours of the day and a second temperature for other hours of the day. So, a warmer temperature could be set for the daytime and a cooler temperature for nighttime.

The controller may be programmable over a communications link, such as a dial- up or wireless link.

The controller may be arranged to prevent air flow through the heat exchange system under certain conditions, such as certain temperature conditions.

The system would preferably be installed (i.e. the inflow and outflow vents would be arranged) so as to only draw stale air from those rooms that are as close as possible to the desired temperature and deliver fresh supply air to rooms that are cooler than desired. By this means, the overall system efficiency will be improved while the air circulation will typically be from one part of a building to another, rather than within individual rooms. This will create a 'whole-of-building' rather than individual room air flow and thereby make the occupied space as temperate as possible.

During warmer periods, the system may preferably be automatically controlled with a thermostat that is set to turn off the delivery air fan. Meanwhile, the reject air fan continues to operate and continuously extract warm air from the building. To assist with airflow in this mode, a small window on the cool side of the building

In another embodiment of the design, during warmer periods a diverter flap in the fresh delivery air fan may be operated to draw air from outside the building. In this case, the thermostat would automatically turn the reject air fan off. Although not quite as effective as the former method of providing summer cooling, this embodiment does allow filtering of the delivery air whilst also holding the building under a positive pressure.

A system comprising both the heat exchanger and heat sink may be installed. However, the heat exchanger and the heat sink are also capable of being installed separately. The system may be a new component or retrofit for installation into an existing ventilation or HVAC system.

The heat exchanger described above has achieved up to 95% true thermal efficiency at a pressure drop of between 20 and 60 Pa. Due to the very thin walls of the plastic twin wall material and the low conductivity of air, the reduction in efficiency compared to an exchanger using aluminium plates is only about 3%.

However, the twin wall plastic material gives a cost saving of about 80% over aluminium, and a weight reduction also of about 80%.

The flow channels in the heat exchanger may be formed by the substantially undeformed fluting of the plastic twin wall material.

While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in detail, it is not the intention of the Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of the

Applicant's general inventive concept.