GB2563056A

GB2563056A - Heat exchanger

Info

Publication number: GB2563056A
Application number: GB1708769.3A
Authority: GB
Inventors: Ridley Anthony
Original assignee: TRANSP FOR LONDON
Current assignee: TRANSP FOR LONDON
Priority date: 2017-06-01
Filing date: 2017-06-01
Publication date: 2018-12-05
Also published as: WO2018220343A1; GB201708769D0

Abstract

A heat exchanger having an air duct 303-306 and a heat exchange fluid (coolant) duct 315 for heat exchange between air and a heat exchange fluid respectively. The heat exchanger comprises a means 317 for generating an air flow of environmental air through the air duct. The heat exchange fluid duct does not obstruct air flowing through the air duct. This reduces the extent to which dust entrained in the air flowing through the air duct is able to clog the heat exchanger. The air flow generating means may comprise no moving parts, such as a fan, exposed to the environmental air flowing through the air duct. The air flow generating means may deliver compressed air to the air duct and generate an airflow which draws the environmental air through the air duct. The air flow generating means may be a Coanda nozzle or an air amplifier. A panel 302 may comprise at least two said heat exchangers. The panel or heat exchanger may be curved to conform to the wall of a tunnel. The heat exchanger is intended for controlling the air temperature in confined spaces which may be dusty, such as underground railway systems.

Description

Heat exchanger

The present invention relates to heat exchangers, and particularly to heat exchangers for cooling air.

In confined spaces it is sometimes necessary to provide a temperature controlled environment. A particular problem arises where the confined space is dusty. Dust which is entrained in the air can clog existing systems for controlling the temperature of the air. Over time this can reduce the efficiency of the air cooling system, leading to higher operational costs and/or lower performance.

Underground railway systems are one example of a dusty environment in which control of the air temperature is desirable. Conventionally, air cooling has been provided in the platform areas of the tunnels in underground railway systems by Air Handling Units (AHUs). Figure 1 shows a schematic cross section of an existing AHU (1). Hot air (2) from the platform is drawn through the AHU by a fan (3) situated at the rear of the unit. Air entering the unit passes through a filter (4) and then through a cooling zone (5). In the cooling zone the hot platform air contacts pipes (6) which carry a flow of cooling fluid (7). Energy is transferred from the hot air to the cooling fluid in the pipes, raising the temperature of the cooling fluid, and lowering the temperature of the air flowing through the unit. The flow of the cooling fluid then carries that energy away from the airflow, to a device (8) for reducing the temperature of the cooling fluid, such as a chiller or condenser. The cooled air (9) then exits the unit via the fan and is returned to the platform.

Figures 2a-d are four photographs showing the effect of a dusty environment on existing AHUs. The photographs show the progressive build-up of dust on the front face of an AHU over a time period of around 5 months. Figure 2a shows the surface of the cooling pipe (6) starting to foul after an initial operating period. Figure 2b shows fibres caught on the fins (10) after a further operational period of 43 days. Figure 2c shows bridging of dust build-up on the coil/fin shoulders after a further operating period of 2 months, 20 days. Figure 2d shows substantial clogging occurring on the surface of the cooling pipes (6) and bridging the fins (10) after a further operating period of 27 days. The clogging reduces the performance of the AHU. This leads to high energy usage for the same cooling output and/or high maintenance costs associated with repeated cleaning of the unit.

It is desirable that systems for controlling the air temperature are energy efficient and low cost to run, reguire little maintenance and are cheap to manufacture and install. It is amongst the objects of the invention to address these reguirements.

In a first aspect of the invention, there is provided a heat exchanger comprising;

a first conduit through which air may be flowed, the first conduit having an inlet and an outlet, a second conduit through which a heat transfer fluid may be flowed, and a means for generating an air flow through the first conduit so that environmental air enters the first conduit through the inlet and exits through the outlet, wherein the first and second conduits are arranged so that heat may transfer between air flowing through the first conduit and a heat transfer fluid flowing through the second conduit.

Environmental air means any air which is in the vicinity of the heat exchanger. This environmental air is the air whose temperature is intended to be controlled using the heat exchanger. When dust is entrained in the environmental air, this gives rise to some of the problems solved by the invention. In the context of underground railway systems, the environmental air may be the warm and dusty air present in a tunnel or platform.

Preferably the second conduit does not present an obstacle to air flowing through the first conduit. This means that the path of air flowing through the first conduit is not substantially altered by the presence of the second conduit; for example, preferably, air flowing through the first conduit is not be forced to flow around tubes containing the second conduit. Preferably the second conduit does not present any surfaces inside the first conduit on which dust entrained in environmental air flowing through the first conduit can gather. These features enable the heat exchanger to run at a high efficiency and with low maintenance over a long period of time in dusty environments. The features achieve this object by ensuring that dust which is entrained in the environmental air does not encounter obstacles or surfaces on which it can settle and thereby clog the first conduit.

Preferably the means for generating an air flow through the first conduit comprises no moving parts which are exposed to environmental air flowing through the first conduit. Preferably the means for generating an airflow in the first conduit does not comprise a fan which is exposed to environmental air flowing through the first conduit. These features enable the exchanger to be run without maintenance over long periods of time since moving parts such as fans are most likely to be clogged when they are exposed to dust entrained in the environmental air. Any moving parts such as fans or pumps may be isolated from dusty environmental air flowing through the first conduit.

The means for generating an air flow through the first conduit may be adapted to deliver compressed air to the first conduit. This provides an airflow which efficiently draws environmental air through the first conduit, without reguiring any moving parts to be placed in dusty environmental air flowing through the first conduit. The compressed air may also originate from a non-dusty, isolated, or separate source to the environmental air. This prolongs the life of any compressor used to compress the air. The term 'compressed air' means any air which is at a pressure which is higher than the atmospheric pressure surrounding the heat exchanger.

The means for generating an air flow through the first conduit may be arranged so that air flow which it generates draws environmental air through the first conduit. This enables dusty environmental air to be drawn through the first conduit without the need for the means for generating the airflow to interact directly with the dusty environmental air.

The means for generating an air flow through the first conduit may be arranged centrally in the inlet of the first conduit. This increases the efficiency with which environmental air is drawn through into the first conduit.

The means for generating an air flow through the first conduit may comprise a nozzle. This provides a directed air flow in the first conduit so that environmental air is drawn efficiently into the first conduit. Suitable nozzles are available from Meech International of 2 Network Point, Range Road, Whitney, Oxfordshire, 0X29 OYN, www.meech.com, under the product code A848009. These nozzles may be described as energy saving nozzles.

The means for generating an air flow through the first conduit may comprise a Coanda nozzle or an air amplifier. These types of means for generating airflow draw a large amount of air into the first conduit whilst only using a small amount of air themselves. That is, they provide high volume flow rates. Air amplifiers are available from Meech International, of the address stated above, under the product code Al 5000.

The means for generating an air flow through the first conduit may be adapted to generate an air flow in the first conduit having a speed of greater than 7 m/s. This level of air flow provides efficient heat transfer characteristics between coolant fluid in the second conduit and environmental air flowing through the first conduit. It also reduces the chance of any dust settling in the first conduit. The means for generating an air flow in the first conduit may also comprise a means for varying the speed of air flow in the first conduit. This may comprise a means for varying the pressure of compressed air supplied to the means for generating the air flow. The heat transfer rate between coolant in the second conduit and environmental air in the first conduit will be dependent upon the air speed in the first conduit. Varying the speed of the air flow in the first conduit can therefore be used to increase or decrease the cooling power of the heat exchanger. The means for varying the speed of the air flow in the first conduit may be linked to a controller for controlling the speed of the air flow, in response to conditions in the environment surrounding the heat exchanger, The controller may comprise a sensor for determining the temperature of environmental air.

The means for generating an air flow through the first conduit may be adapted to generate a volume flow rate of between 1:2 and 1:100. These ratios specify 'the amount of air supplied by the means for generating air flow' to 'the amount of environmental air drawn through the first conduit by the air flow'. These high volume flow rates mean that only a small amount of air must pass through the means for generating airflow to draw a large amount of environmental air through the first conduit. If only a small amount of air must pass through the means for generating air flow, the running costs of the heat exchanger are kept low. A typical volume flow rate used in the invention may be 1:25.

The cross section of first conduit may be constant over its length. This assists in providing a uniform air flow in the first conduit, so that dust from the environmental air flowing through the first conduit is less likely to settle and clog the first conduit.

The second conduit may be substantially parallel to the first conduit. This assists in obtaining efficient heat transfer between the first and second conduits. It also assists in providing a uniform air flow in the first conduit, so that dust from environmental air flowing through the first conduit is less likely to settle and clog the first conduit.

The first and second conduits may share a wall. This assists in obtaining efficient heat transfer between the first and second conduits.

The first and second conduits may be formed from a metal such as aluminium. This assists in obtaining efficient heat transfer between the first and second conduits. Other metals which have high heat conduction properties may also be used.

The heat exchanger may further comprise a pump for generating a flow of heat transfer fluid in the second conduit. This assists in obtaining efficient heat transfer between the first and second conduits, and removing hot heat transfer fluid from the vicinity of the first conduit.

The means for generating an air flow through the first conduit and the pump for generating a flow of heat transfer fluid in the second conduit may be arranged so that the direction of flow of any heat transfer fluid present in the second conduit is opposite to the direction of flow of any air present in the first conduit. This assists in obtaining efficient heat transfer between the first and second conduits.

The heat exchanger may have four first conduits through which air may be flowed, grouped around a single second conduit through which a heat transfer fluid may be flowed. This assists in obtaining efficient heat transfer between the first and second conduits.

The means for generating an air flow in the first conduit may;

be fed by air from a location which is isolated from the environmental air which may enter and exit the first conduit, and comprise a duct to deliver an air flow to the first conduit.

This allows non-dusty air to be used as the air for generating the air flow in the first conduit, thereby prolonging the life of any moving components, for example a compressor, used to generate the air flow.

The means for generating an airflow in the first conduit may comprise a fan or compressor,

In a second aspect of the invention there is provided a panel comprising at least two heat exchangers as described herein, side by side. This enables a large amount of environmental air to be processed at once.

The panel or heat exchanger may be curved to conform to the wall of a tunnel. This maximises the surface area over which heat may be transferred between the first and second conduits.

The panel or heat exchanger may further comprise a mount to enable attachment of the panel to a wall.

The first conduit may be provided with a fin or fins. These fins, whilst not presenting an obstacle to air flowing through the first conduit, increase the area of the wall of the first conduit which is in contact with environmental air. This provides efficient heat transfer.

The heat transfer fluid may be water and may be contained in a closed circuit which comprises a means for reducing the temperature of the fluid, such as a chiller or condenser. Other heat transfer liguids or gases may also be used.

Embodiments of the invention will now be described with reference to the figures of the drawings, in which:

Figure 1 shows a schematic view of a prior art air handling unit.

Figures 2a-d show photographs of dust build-up over time on a prior art air handling unit.

Figure 3 shows a partial cross sectional view of a portion of a heat exchanger according to an embodiment of the invention.

Figure 4 shows a cross sectional plan view of a portion of the heat exchanger shown in figure 3.

Figure 5 shows a cross sectional view of an air amplifier and the associated air flow for use in an embodiment of the invention.

Figure 6 shows a perspective view of a portion of the air inlet end of a heat exchanger unit according to an embodiment of the present invention.

Figure 7 shows a perspective view of a portion of the air outlet end of a heat exchanger unit according to an embodiment of the present invention.

Figures 1 and 2 are discussed above and relate to prior art air handling units.

Figure 3 shows a portion of a heat exchanger 301. The heat exchanger has a panel 302 provided with air ducts 303-306 having open ends which form air inlets 307-310. The air duct 308 is shown in cross section. The panel is made from aluminium. Each of the air ducts extend from their inlet to a corresponding outlet 311-314. The air ducts have a generally rectangular cross section across their whole length. Running parallel to the duct 304 are coolant ducts 315 and 316. These coolant ducts have a circular cross section along their whole length and are separated from the air duct 304 by a thin aluminium wall. A cross sectional view of the structure of the air ducts and coolant ducts is shown more clearly in figure 4. At the inlet ends of the air ducts, Coanda nozzles 317 are provided. These nozzles are obtained from Meech International of 2 Network Point, Range Road, Whitney, Oxfordshire, 0X29 0ΥΝ, wwwjmeecm&am., under the product code A848009. The nozzles are centrally located in the inlets of the corresponding air duct. The nozzles are supplied with compressed air by hoses 318 which are connected to a compressor (not shown). The nozzle located in the air duct 306 is not shown in figure 3, for clarity, but is shown in figures 4 and 5.

In use, the nozzles deliver a compressed air stream into the inlet of their corresponding air duct. This compressed air stream draws hot air surrounding the heat exchanger unit through the inlet to provide an airflow shown by the arrows in air duct 304.

Coolant water 319 is introduced by a duct (not shown) into inlets 320 of the coolant duct which are situated adjacent to the outlet 312 of the air duct 304. The coolant water flows from the inlets 320 towards the outlets 321 where it exits the duct and is carried away by a further duct (not shown). As the coolant water flows through the coolant duct, energy is transferred from the hot air (which has been drawn into the air duct by the compressed air stream) to the coolant water via the aluminium walls of the air and coolant ducts. The temperature of the air flowing through the air duct therefore decreases as it flows through the duct until it exits the duct and is returned to the surroundings of the heat exchanger as cool air 322.

Figure 4 shows a cross sectional plan view of a portion of the heat exchanger shown in figure 3. The aluminium panel 302 has a honeycomb type structure with the air ducts 303306 arranged side by side. A second row of air ducts 323-326 are arranged parallel to the air ducts 303-306. The second row of air ducts share walls with the first row of air ducts.

The nozzles 317 are centrally located in each of the air ducts. The coolant ducts 315, 316 and 327 are each surrounded by four air ducts. The coolant ducts share portions of their walls with four air ducts.

Figure 5 shows a cross sectional view of a Coanda nozzle 317 taken along the line A-A in figure 4. The air duct 306 in which the nozzle is located, and the coolant duct 327 are also shown. The nozzle is located in the centre of the inlet 310 and is fed compressed air by a duct (not shown) which connects the nozzle to a compressor (not shown). The compressor may take its air from a source which is remote and/or isolated from the air in the immediate vicinity of the heat exchanger. Compressed air enters the nozzle where it is throttled by narrow passageways 328. This increases the velocity of the air. The air then leaves the narrow passageway and adheres to the tapered surfaces 329 to form a focussed air stream 330 which is central in the air duct 306. The air stream 330 draws hot environmental air 331 through the inlet 310 and into the air duct 306 to create a combined air flow in the air duct. Coanda nozzles are available from Meech International of 2 Network Point, Range Road, Whitney, Oxfordshire, 0X29 0ΥΝ, www.meech.com, under the product code A848009.

Figure 6 shows a lower end of a portion of a heat exchanger. The heat exchanger comprises a panel 601 with a honeycomb structure having air ducts 602-604. Hoses 605 for taking warm coolant water away from the coolant ducts are attached to the outlets of the coolant ducts. The water is delivered by the hoses to a cooler (not shown) where the temperature of the water is reduced so that the water can be recirculated through the coolant ducts. Compressed air nozzles 606 are supported in the centre of the inlets to the air ducts by bracket 607. The bracket is sandwiched between the panel 601 and the nozzles 606 and is bolted to the panel. The hoses 607 are connected to a compressor (not shown) and supply compressed air to the nozzles 606.

Figure 7 shows a portion of the air outlet end of the heat exchanger shown in figure 6. The air ducts 603 of the panel 601 have outlets through which hot air can exit the ducts. Hoses 608 are bolted to the coolant ducts and supply cool water to the coolant ducts. The cool water flows through the coolant channels and exits the panel via the hoses 605 shown in figure 6. Some of the air ducts may be provided with fins 609 which extend from the wall of the air duct which is shared with the coolant duct. The fins extend along the length of the air duct and provide a large surface area for contact with the air flowing through the air duct. This enhances heat transfer between air in the air duct and coolant in the coolant duct.

Claims

1. A heat exchanger comprising;

a first conduit through which air may be flowed, the first conduit having an inlet and an outlet, a second conduit through which a heat transfer fluid may be flowed, and a means for generating an air flow through the first conduit so that environmental air enters the first conduit through the inlet and exits through the outlet, wherein the first and second conduits are arranged so that heat may transfer between air flowing through the first conduit and a heat transfer fluid flowing through the second conduit, and wherein the second conduit does not present an obstacle to air flowing through the first conduit.

2. A heat exchanger according to claim 1 wherein the means for generating an air flow through the first conduit comprises no moving parts which are exposed to environmental air flowing through the first conduit.

3. A heat exchanger according to either preceding claim wherein the means for generating an airflow in the first conduit does not comprise a fan which is exposed to environmental air flowing through the first conduit.

4. A heat exchanger according to any preceding claim wherein the means for generating an air flow through the first conduit is adapted to deliver compressed air to the first conduit.

5. A heat exchanger according to any preceding claim wherein the means for generating an air flow through the first conduit is arranged so that air flow which it generates draws environmental air through the first conduit.

6. A heat exchanger according to claim 5 wherein the means for generating an air flow through the first conduit is arranged centrally in the inlet of the first conduit.

7. A heat exchanger according to any preceding claim wherein the means for generating an air flow through the first conduit comprises a nozzle.

8. A heat exchanger according to any preceding claim wherein the means for generating an air flow through the first conduit comprises a Coanda nozzle or an air amplifier.

9. A heat exchanger according to any preceding claim wherein the means for generating an air flow through the first conduit is adapted to generate an air flow in the first conduit having a speed of greater than 7 m/s.

10. A heat exchanger according to any preceding claim wherein the means for generating an air flow through the first conduit is adapted to generate a volume flow rate of between 1:2 and 1:100.

11. A heat exchanger according to any preceding claim wherein the cross section of first conduit is constant over its length.

12. A heat exchanger according to any preceding claim wherein the second conduit is substantially parallel to the first conduit.

13. A heat exchanger according to any preceding claim wherein the first and second conduits share a wall.

14. A heat exchanger according to any preceding claim wherein the first and second conduits are formed from a metal such as aluminium.

15. A heat exchanger according to any preceding claim further comprising a pump for generating a flow of heat transfer fluid in the second conduit.

16. A heat exchanger according to claim 15 wherein the means for generating an air flow through the first conduit and the pump for generating a flow of heat transfer fluid in the second conduit are arranged so that the direction of flow of any heat transfer fluid present in the second conduit is opposite to the direction of flow of any air present in the first conduit.

17. A heat exchanger according to any preceding claim wherein there are four first conduits through which air may be flowed, grouped around a single second conduit through which a heat transfer fluid may be flowed.

18. A heat exchanger according to any preceding claim wherein the means for generating an air flow in the first conduit is;

fed by air from a location which is isolated from the environmental air which may enter and exit the first conduit, and comprises a duct to deliver an air flow to the first conduit.

19. A heat exchanger according to any preceding claim wherein the means for generating an airflow in the first conduit comprises a fan or compressor.

20. A panel comprising at least two heat exchangers according to any of claims 1-19 side by side.

21. A panel according to claim 20 or a heat exchanger according to any of claims 1-19 wherein the panel or heat exchanger is curved to conform to the wall of a tunnel.

22. A panel according to either of claims 20 or 21 or a heat exchanger according to any of claims 1-19, wherein the panel or heat exchanger further comprises a mount to enable attachment of the panel or heat exchanger to a wall.