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1. (WO2017138820) IMPROVEMENTS IN AND RELATING TO AIR CONDITIONING
Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

Improvements in and Relating to Air Conditioning

TECHNICAL FIELD

The present invention relates to improvements in and relating to air conditioning. In particular, it relates to the control of temperature and humidity within a climate controlled space.

BACKGROUND ART

Climate control systems are used to control the temperature and relative humidity of an enclosed environment, for example in a home, vehicle or working environment. Humans and other animals require particular levels of temperature and humidity in order to feel comfortable and to remain healthy.

Humans are sensitive to humidity because the human body uses evaporative cooling as the primary mechanism to regulate temperature. Perspiration evaporates from the skin more slowly under humid conditions than under dry conditions. Because humans perceive a low rate of heat transfer from the body to be equivalent to a higher air temperature the body experiences greater discomfort at high humidity than at lower humidity, given equal temperatures, during periods when cooling is required.

Climate control typically relies on heating ventilation and air conditioning (HVAC) systems, the objective being to maintain the relative humidity within a comfortable range— low enough to be feel comfortable but high enough to avoid problems associated with very dry air whilst also maintaining a comfortable ambient temperature.

HVAC systems typically provide two modes of operation, heating and cooling. When operating in cooling mode, the system is referred to as an air conditioner. When operating in a heating mode the system is referred to as either reverse cycle air conditioner, or a heat-pump.

One benefit of HVAC systems is that when they operate in air conditioning mode they extract moisture from the air. This action is typically referred to as dehumidification. Dehumidification occurs due to the dry bulb temperature of the air passing through the indoor heat exchanger decreasing to the point where it is below the wet-bulb temperature. This results in the water vapour in the air condensing on the indoor coil, and therefore being removed from the air. The condensed water is allowed to drain away from the system.

In some applications, dehumidification can be excessive, leading to a relative humidity which is too low. This to some extent can be controlled by increasing the air-flow through the indoor coil which results in the dry bulb temperature being maintained at a higher temperature.

In some applications however, the internal (within the air conditioned space) moisture loading is significant, and is at times unable to be suitably dehumidified as a by-product of the cooling mode of an HVAC system. In many instances, this is a result of an air conditioning system being sized during selection based on near maximum outdoor sensible conditions. Under these conditions, the heat removal capabilities of the system will exceed the internal moisture loading of the internal space, resulting in very dry air. However, these conditions only infrequently occur. Much more commonly, the system is being operated in part load (cooler) outdoor conditions, with a high indoor humidity level.

In these instances, attempting to control the humidity of the indoor space using an air conditioner leads to an indoor temperature that is too low for human comfort. Therefore, a combination of dehumidification and heating is required in order to achieve a desirable low humidity and a comfortable temperature.

Reheat systems are known, these systems reheat the air-conditioned air prior to the air being discharged into the space being climate controlled. This enables moisture removal without depressing the dry-bulb temperature.

One method for reheating is supplementary heating. This method relies on externally provided heat to be fed through a reheat coil. The air-conditioned air is blown over the reheat coil prior to being discharged into the climate controlled environment. Typically a water sourced reheat coil is used in which water which has been heated by a boiler, or which has recovered waste heat from another cooling system. In the latter example, the reheat coil is termed a reclaim coil. A reheat coil heated via supplementary heating is considered inefficient, and has been legislated against in a number of jurisdictions. A reclaim coil is dependent on an available heat source at the time reheat is required.

A further method for reheating is to reverse cycle one unit of a multiple unit air-conditioning system. This allows dehumidification by the systems that are in cooling mode, whilst the ambient temperature is raised by the unit that is operating in heating mode. This approach is also considered inefficient. The advantage of the reverse cycle approach is simplicity, and the reduction in fan power needed to overcome the added resistance to air flow of a reheat / reclaim coil.

Hot gas bypass is another method that is used, such as that described in US patent

US5752389. Hot gas bypass is presently the preferred solution for reheating. This method intercepts the high pressure line from the compressor, diverting hot, high pressure, refrigerant vapor directly to a reheat coil, where the refrigerant releases its thermal energy and condenses. An electrically activated solenoid valve controls the proportion of gas diverted to the reheat coil, and the proportion that flows to a standard condenser. The advantage of a hot gas bypass method is that all compressors in a multiple unit system remain in cooling mode, and therefore are dehumidifying the air passing across the evaporator. The heat used to reheat the ambient air is a byproduct of the air conditioning system. As a result, this system is typically more efficient than a reverse cycle approach. The disadvantage of hot gas bypass is the increase in refrigerant charge required, the complexity, and therefore cost of the piping required, and the potential for compressor oil to be captured in the reheat coil when the reheat system is not in use leading to premature compressor failure.

It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.

All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.

Throughout this specification, the word "comprise", or variations thereof such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

SUMMARY OF THE INVENTION

The present invention provides a system in which a heat pump/air conditioning unit includes a reheat system. I n a traditional heat pump/air conditioning system a first heat exchanger is located in a space to be air conditioned. A second heat exchanger is located externally to the space to be air conditioned, generally in an open outdoor space. Thermal energy is transferred between the first and second heat exchangers using known methods for pumping thermal energy. When used in an air conditioning mode heat is absorbed by the first heat exchanger pumped to the second heat exchanger and is rejected to the outdoor space. One desirable use

of this air-conditioning mode of operation is to dehumidify the air in the space being air conditioned. However, dehumidification of the air in an already cold space can result in the temperature in that space becoming uncomfortably cold. As a result it becomes necessary to reheat the cold air generated by the first heat exchanger. The present invention does this in a novel and inventive way by transporting thermal energy by way of a thermal energy transport circuit to a third heat exchanger. The third heat exchanger is positioned within an airflow from the first heat exchanger, thereby heating the cold air.

Transporting thermal energy in the manner of the present invention provides a number of advantages including one or more of the following: the refrigerant volume is minimized, saving cost and reducing complexity of the circuit; compressor oil is prevented from becoming trapped in the reheat circuit when the reheat circuit is not active; the risk of refrigerant leaks is reduced; and greater control is provided over the amount of reheat that can be provided.

According to one aspect of the present invention there is provided a reheat air conditioning system, the system including:

· a first heat exchanger located in a space to be air conditioned;

• a second heat exchanger located externally to the space to be air conditioned;

• a third heat exchanger positioned within the space to be air conditioned and within an air flow path from the first heat exchanger;

• a thermal energy pumping circuit configured to pump thermal energy from the first heat exchanger to the second heat exchanger, and

• a thermal energy transport circuit configured to transport thermal energy from the

thermal energy pumping circuit to the third heat exchanger.

According to a further aspect of the present invention there is provided a reheat system for use with an air conditioning system having a first heat exchanger located in a space to be air conditioned, a second heat exchanger located externally to the space to be air conditioned, and a thermal energy pumping circuit configured to pump thermal energy from the first heat exchanger to the second heat exchanger, the reheat system including:

• a reheat heat exchanger configured to be positioned within the space to be air

conditioned and within an air flow path from the first heat exchanger, and;

· a thermal energy transport circuit configured to transport thermal energy from, the

thermal energy pumping circuit to the reheat heat exchanger.

According to a further aspect of the present invention there is provided a method of air conditioning a space using a reheat air conditioning system, the reheat air conditioning system including a first heat exchanger located in a space to be air conditioned, a second heat exchanger located externally to the space to be air conditioned, a third heat exchanger positioned within the space to be air conditioned and within an air flow path from the first heat exchanger, a thermal energy pumping circuit configured to pump thermal energy from the first heat exchanger to the second heat exchanger, and a thermal energy transport circuit configured to transport thermal energy from the thermal energy pumping circuit to the third heat exchanger, the method including the steps of:

a) absorbing thermal energy from the space to be air conditioned through the first heat exchanger;

b) pumping thermal energy from the first heat exchanger to the second heat exchanger by way of the thermal energy pumping circuit;

c) releasing thermal energy from the second heat exchanger; and

d) transporting thermal energy from the thermal energy pumping circuit to the third heat exchanger by way of the thermal energy transport circuit,

the method characterized in that, in use, the air that is cooled by the first heat exchanger becomes dehumidified prior to flowing over, and being heated, by the third heat exchanger.

In some embodiments the method may include the additional steps of using a control system to:

e) measure the temperature of the air in the space to be air conditioned

f) compare the measured temperature to a desired setpoint

g) adjust the transport rate of thermal energy from the heat pumping circuit to the third heat exchanger.

In some embodiments the first heat exchanger may be a single suitably scaled heat exchanger, or may comprise a plurality of heat exchangers connected in a parallel and/or series

arrangement to achieve a designed level of thermal coupling.

The first heat exchanger may take a number of forms without departing from the scope of the present invention, non-limiting examples of which include plate, plate fin, pillow plate, radiator and coil heat exchangers. The first heat exchanger is configured to facilitate the flow of thermal energy from air within the space to be air conditioned to the medium used by the thermal energy pumping circuit.

Preferably the first heat exchanger includes a forced convection fan configured to direct a flow of air from the space to be air conditioned and over/through the first heat exchanger.

Preferably the flow of air over/through the first heat exchanger flows over / through both the first heat exchanger and the third heat exchanger.

The medium used by the thermal energy pumping circuit for transporting thermal energy around the thermal energy pumping circuit will vary depending on the design of the thermal energy pumping circuit. For example solid state thermal energy pumping circuits such as peltier or electromagnetic circuits may rely on heat flow from a heatsink to a heat transportation fluid. Thermal energy pumping circuits that use vapour compression or phase change technologies may comprise a refrigeration fluid such as hydroflurocarbons, hydrocarbons, CO2 or NH3 as the thermal energy transport medium.

The space to be air conditioned will typically comprise an enclosed commercial or living space, such as one or more areas or rooms of a building or a house. Typically such a space will require both humidity and temperature regulation in order to achieve a preferred level of comfort.

In some embodiments the second heat exchanger may be a single suitably scaled heat exchanger, or may comprise a plurality of heat exchangers connected in a parallel and/or series arrangement to achieve a designed level of thermal coupling.

The second heat exchanger may take a number of forms without departing from the scope of the present invention, non-limiting examples of which include plate, plate fin, pillow plate, radiator and coil heat exchangers. The second heat exchanger is configured to facilitate the flow of thermal energy from the medium used by the thermal energy pumping circuit to an environment external to the space to be air conditioned.

The environment external to the space to be air conditioned (i.e. where the second heat exchanger is located) may vary without limitation, examples of suitable environments external to the space to be air conditioned include, but should not be limited to, an open outdoor space, a fluid body such as a lake, river or water reservoir or the like.

The third heat exchanger may take a number of forms without departing from the scope of the present invention, non-limiting examples of which include counter current and parallel flow fluid to fluid, heat sink to fluid, shell and tube or tube in shell heat exchangers or the like. The third heat exchanger is configured to facilitate the flow of the thermal energy transferred from the thermal energy pumping circuit to the air flow path from the first heat exchanger.

The air flow path from the first heat exchanger may be a natural convection flow, or may be a forced air flow, for example fan forced air. That is, the reheat air conditioning system may

comprise a fan configured, in use, to cause air to flow from the first heat exchanger along the air flow path to the third heat exchanger.

Preferably the air flow through the first heat exchanger and third heat exchanger is provided by the same fan or fans.

Preferably the thermal energy transport circuit transfers thermal energy by way of a fluid flow.

Preferably the thermal energy transport circuit includes a fourth heat exchanger thermally coupled with the thermal energy pumping circuit. The fourth heat exchanger is configured to facilitate the flow of thermal energy from the thermal energy pumping circuit to the transport medium used in the thermal energy transport circuit whilst providing fluid isolation between the thermal energy pumping circuit and the thermal energy transport circuit.

In some embodiments the fourth heat exchanger may be a single suitably scaled heat exchanger, or may comprise a plurality of heat exchangers connected in a parallel and/or series arrangement.

The composition of the medium used in the thermal energy transport circuit may vary without departing from the scope of the present invention, examples of suitable transport mediums include, but should not be limited to, glycol, water, brine, oil or the like. Alternatively, a phase change medium, such as a refrigerant could be used as the thermal energy transport medium.

In preferred embodiments the thermal energy pumping circuit and the thermal energy transport circuit are fluidly isolated from one another.

In preferred embodiments the thermal energy transport circuit is a hydronic circuit.

In preferred embodiments the thermal energy transport circuit includes a pump.

Preferably the pump is a variable speed pump. A variable speed pump allows the rate of thermal transport to be varied, thereby adjusting the temperature of the third heat exchanger.

In preferred embodiments the thermal energy transport circuit is substantially isobaric in operation.

Isobaric operation provides thermal energy transport at substantially constant pressure.

Constant pressure transport of thermal energy provides high efficiency transport of thermal energy when a high temperature differential is present.

In preferred embodiments the reheat air conditioning system comprises a control system for controlling operation of the reheat air conditioning system, the control system comprising one or more processors configured to execute instructions to cause the control system to control

operation of the reheat air conditioning system. It will be appreciated that the control system operates to regulate the flow of thermal energy in the thermal energy transport circuit.

Preferably the control system includes a microprocessor configured to receive a measure of a temperature of the space to be air conditioned and to adjust the rate of thermal energy transport to the third heat exchanger so as to raise or lower the temperature of the space to be air conditioned to a desired level. It will be understood that the reheat air conditioning system may comprise a temperature sensor configured to sense the temperature of the space to be air conditioned and provide the measure of the temperature of the space to be air conditioned to the control system.

In some embodiments control of the rate of thermal energy transport to the third heat exchanger may be by way of, but should not be limited to, a thermostatic switch such as a bimetallic switch, a two wire thermostat, or a digital thermostat.

Preferably, the control system is configured to control the rate at which the thermal energy is pumped through the thermal energy pumping circuit. More preferably, the control system is configured to control the rate of operation of the pump.

Preferably, the reheat air conditioning system comprises a second heat exchanger fan configured to cause air to flow through the second heat exchanger in use. The control system may be configured to selectively control the rate of operation of the second heat exchanger fan.

Preferably the reheat air conditioning system comprises a flow reverser provided between the first and second heat exchangers, the flow reverser being operable in use to control the direction of flow of thermal energy in the thermal energy pumping circuit. More preferably, the flow reverser is a reversing valve.

Preferably the reheat air conditioning system comprises one or more flow restrictors provided between the second heat exchanger and the first heat exchanger, wherein the one or more flow restrictors are configured to restrict the flow of thermal energy in the thermal energy pumping circuit to the second heat exchanger. More preferably, the one or more flow restrictors are selectively operable to prevent thermal energy flowing to the second heat exchanger.

More preferably, the one or more flow restrictors are provided between the second heat exchanger and the flow reverser.

In certain embodiments, the one or more flow restrictors comprises first and second valves connected in parallel between the first and second heat exchangers.

Preferably, the control system is configured to selectively control one or more components of the reheat air conditioning system to control the degree of cooling and/or humidification in the space to be air conditioned.

More preferably, the control system is configured to selectively take any one or more of the following actions to control the degree of cooling and/or humidification in the space to be air conditioned:

1. Control the rate of operation of the pump;

2. Control the rate of operation of the second heat exchanger fan;

3. Alter the flow of thermal energy in the thermal energy pumping circuit to the second heat exchanger; and

4. Alter the flow of air along the air flow path between the first and third heat exchangers.

According to a further aspect of the invention there is provided a method of controlling a reheat air conditioning system to air condition a space, the method comprising:

pumping thermal energy around a thermal energy pumping circuit from a first heat exchanger to a second heat exchanger, the first heat exchanger being located in the space to be air conditioned and the second heat exchanger being located externally to the space to be air conditioned; and

pumping thermal energy around a thermal energy transport circuit from the thermal energy pumping circuit to a third heat exchanger.

Preferably the method comprises:

measuring the temperature of the air in the space to be air conditioned;

comparing the measured temperature to a desired setpoint; and

adjusting the transport rate of thermal energy from the heat pumping circuit to the third heat exchanger.

More preferably, the method comprises selectively taking any one or more of the following actions to control the degree of cooling and/or humidification in the space to be air conditioned:

1. Control the rate of operation of the pump;

2. Control the rate of operation of the second heat exchanger fan;

3. Alter the flow of thermal energy in the thermal energy pumping circuit to the second heat exchanger; and

4. Alter the flow of air along the air flow path between the first and third heat exchangers.

Preferred embodiments of the present invention provide a number of advantages over the prior art, non-limiting examples of which include:

• greater efficiency than traditional reheat systems,

• a reduction in refrigerant requirement when compared to systems that use refrigerant bypass,

• Suitability for use with moderately flammable thermal energy transport fluids due to lower refrigerant change requirements, and less refrigerant piping,

• Efficient operation under heating, cooling and reheat modes,

• Prevention of compressor oil becoming trapped in the reheat circuit,

• Reduced complexity in piping, and

• Increased control of the amount of reheat required, as the transport of energy between the heat-exchanger and the reheat coil can be more precisely controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present invention will become apparent from the ensuing description which is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 shows a reheat system in accordance with one preferred embodiment of the present invention;

Figure 2 shows a reheat system in accordance with a further preferred embodiment of the present invention; and

Figure 3 shows a reheat system in accordance with a further preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Unless the context clearly requires otherwise, certain terms used in this specification should be understood to have meanings as follows:

Reheat system - a system that provides heat to the relatively cool airflow exiting an air-conditioning system. Reheat systems provide control over both humidity and temperature.

Thermal energy pumping circuit - a circuit that moves thermal energy in a direction opposite to natural heat flow, for example, by absorbing heat from a region of lower thermal energy and releasing it to one of greater thermal energy. It will be understood that, where "pump" or "pumping" are used as verbs or adjectives, this is not a requirement for a pump (noun) as a physical device as it would be traditionally known to the skilled addressee to be present. Other mechanisms for energy transport may be used.

Thermal energy transport circuit - a thermal energy transport circuit transports thermal energy in the direction of natural heat flow, i.e. from a region of higher thermal energy to a region of lower thermal energy. Transport may be by way of natural convection flow or may be assisted by pumps or the like.

Air conditioning system - An air conditioner provides cooling and humidity control. An air conditioning system may be incorporated in a dual mode heat pump, an HVAC system or may be standalone.

Heat exchanger - a device used to thermally couple at least one fluid with a heat source, allowing thermal energy to be exchanged by way of natural flow of thermal energy.

Hydronic circuit - a system that relies on water, or water based mixtures (such as brine, glycol etc) as a transport medium.

With reference to figure 1 there is shown a reheat air conditioning system as generally indicated by designator 1. The system includes a first heat exchanger 2 that is located in a space 3 which is to be air conditioned. The space 3 is a closed space such as a room in a house. A second heat exchanger 4 is located externally to the space to be air conditioned. A thermal energy pumping circuit comprising pipes 8a to 8f, compressor 9 and expansion valve 10 is configured to pump thermal energy from the first heat exchanger 2 to the second heat exchanger 4. It will be appreciated that the compressor 9 may operate to pump thermal energy around the thermal energy pumping circuit.

The thermal energy pumping circuit shown in Figure 1 is shown as a typical vapor compression system. In operation pipe 8a supplies refrigerant in vapor form to compressor 9. The compressor increases both the temperature and the pressure of the vapor and discharges the hot high pressure vapor through pipe 8b. The hot high pressure vapor travels through pipe 8 c to second heat exchanger 4 where thermal energy is discharged, causing the high pressure

vapor to condense. The high pressure cooled liquid passes along pipe 8d to expansion valve 10. Expansion valve 10 results in the low pressure liquid evaporating in first heat exchanger 2 as thermal energy is absorbed.

The reheat air conditioning system 1 also includes a third heat exchanger 5 positioned within the space 3 and within an air flow path 6 from the first heat exchanger 2. The air flow path 6 is produced by first heat exchanger 2 fan 7. Pipes 11a and 11 b connect the third heat exchanger 5 to pump 12 and heat exchanger 13. In operation pump 12 circulates a thermal transport fluid along pipe 1 1a, through third heat exchanger 5, along pipe 11 b and through coil 13a of heat exchanger 13. Thermal transport fluid passing through coil 13a is in thermal coupling with the refrigerant passing through coil 13b of the heat pumping circuit. The thermal coupling between coil 13a and coil 13b results in the thermal energy of the thermal transport fluid being raised and the thermal energy of the refrigerant dropping. The thermal transport fluid releases the absorbed thermal energy as it passes through heat exchanger 5, thereby heating the airflow 6.

Referring to Figure 2 there is shown a further preferred embodiment of a reheat air conditioning system, as generally indicated by designator 100. The system 100 includes a first heat exchanger 102 that is located in located in a space 103 which is to be air conditioned. The space 103 is a closed space such as a room in a house. A second heat exchanger 104 is located externally to the space to be air conditioned. A thermal energy pumping circuit comprising pipes (shown as lines connecting components), compressor 109 and expansion valve 121 is configured to pump thermal energy from the first heat exchanger 102 to the second heat exchanger 104. The thermal energy pumping circuit also includes a flow reverser 115 configured to reverse the flow of refrigerant allowing the system to operate in both heating and cooling modes of operation. Flow reverser 115 may take the form of a reversing valve in certain forms of the invention. In other forms, other suitable mechanisms for reversing the flow of refrigerant may be used.

The second heat exchanger 104 and a fourth heat exchanger 113 are configured as a parallel circuit, refrigerant vapour passes from the first heat exchanger 102 to compressor 109 before passing through the parallel combination of second heat exchanger 104 and fourth heat exchanger 113, with a second expansion valve 120 provided in the circuit between the fourth heat exchanger 1 13 and first heat exchanger 102. In preferred embodiments, the fourth heat exchanger is a hydronic heat exchanger and will be referred to as such in the ensuing description.

The hydronic heat exchanger 113 is connected in a thermal energy transport circuit between the thermal energy pumping circuit and third heat exchanger 105 and is configured to facilitate the transfer of thermal energy therebetween. Pump 112 is provided as part of the thermal energy transport circuit to cause thermal transfer around the circuit, for example by pumping a thermal transfer medium around the circuit. Examples of suitable thermal transport mediums include, but are not limited to, glycol, water, brine, oil or the like. Alternatively, a phase change medium, such as a refrigerant could be used as the thermal energy transport medium in the thermal energy transport circuit. The thermal energy transport circuit between the hydronic heat exchanger 113 and second heat exchanger 105 is fluidly isolated from the thermal energy pumping circuit by which hydronic heat exchanger 113 is connected to both first heat exchanger 102 and second heat exchanger 104.

Reversing valve 115 switches the system between heating mode and cooling mode by switching the direction of flow of the refrigerant. Both the second heat exchanger 104 and hydronic heat exchanger 113 have the capacity to fully condense the compressed vapour in cooling mode, however the hydronic heat exchanger 113 can only transport thermal energy to the third heat exchanger 105 if pump 112 is operating. It should be noted that, in heating mode, the first heat exchanger 102 also has the capacity to fully condense the compressed vapour.

When reheat capacity is not required, the pump 112 is not activated and the hydronic heat exchanger 113 reaches a temperature which prevents further refrigerant phase change. The second heat exchanger receives the refrigerant vapour, and condensation occurs, with or without fan 116 providing forced air flow.

When reheat is required, pump 112 is activated, causing a flow of thermal transport fluid through the hydronic heat exchanger to the third heat exchanger 105. The rate of fluid flow provided by pump 112 controls the rate that thermal energy is transferred from hydronic heat exchanger 113 to third heat exchanger 105.

Maximum reheat independent of the external ambient conditions can be provided by preventing the flow of refrigerant to the second heat exchanger, thereby ensuring that the available thermal energy is available to hydronic heat exchanger 113. An example of how this may be achieved in certain embodiments of the invention is described below with reference to Figure 3.

The operation of the first heat exchanger 102 is unaffected by the balance of thermal energy flow between the hydronic heat exchanger 113 and the second heat exchanger 104.

The balance between sensible and latent heat removed from the air can be controlled to some extent by adjusting the volume of airflow through the first heat exchanger 102 and the third heat exchanger 105 by control of the speed of fan 107.

When operating in reverse cycle mode, the third heat exchanger is effectively isolated from the circuit, and has no influence on system operation.

The refrigerant flow between the expansion valves 120, 121 adjusts based on the suction superheat being achieved in the low pressure circuit. The speed of the fans 107, 116 is controlled by a control system based either on condensing temperature or compression ratio.

Figure 3 is an illustration of a reheat air conditioning system 100 in accordance with another embodiment of the present invention. The reheat system 100 shown in Figure 3 is similar to the reheat system shown in Figure 2 and it will be understood that the description of the system of Figure 2 applies to the system of Figure 3. For that reason the ensuing description will describe only those features of the reheat system 100 in Figure 3 that differ from those of the system in Figure 2.

In the reheat system 100 of Figure 3, one or more flow restrictors are provided in the thermal energy pumping circuit between the flow reverser 1 15 and the second heat exchanger 104. The flow restrictors are configured to restrict the flow of thermal energy in the thermal energy pumping circuit to the second heat exchanger 104. The flow of thermal energy may be sufficiently restricted that substantially no energy is transferred to the second heat exchanger 104 in some modes of operation. The control system may be configured to selectively control the flow restrictors to control the level of restriction of the flow of thermal energy.

In the embodiment of Figure 3, the flow restrictors take the form first valve 301 and second valve 302 connected in parallel between the second heat exchanger 104 and the flow reverser 115. The valves are able to be selectively opened and closed by the control system subject to the heating / cooling / humidification requirements of the situation. It will be appreciated that other mechanisms for restricting the flow of thermal energy in the thermal energy pumping circuit may be provided in other embodiments of the invention.

In one embodiment, first and second valves 301 and 302 are one-way valves that allow the heat conducting medium (e.g. refrigerant) in the conduits of the thermal energy pumping circuit to flow in one direction through the valves only.

First valve 301 allows the heat conducting medium to flow from the flow reverser 1 15 to the second heat exchanger 104 when the valve is open but does not allow the heat conducting medium to flow from the second heat exchanger 104 to the flow reverser 115. In such a flow, the heat conducting medium is at high pressure and temperature so a conduit with a relatively small diameter may be used. First valve 301 may therefore take the form of a solenoid valve in some embodiments of the invention, as solenoid valves operate effectively and are cost effective at this size.

In an alternative embodiment, first valve 301 may take the form of a bi-directional valve, for example a bi-directional solenoid valve or a motorised ball valve.

Second valve 302 allows the heat conducting medium to flow from the second heat exchanger 104 to the flow reverser 1 15 when the valve is open but does not allow the heat conducting medium to flow from the flow reverser 115 to the second heat exchanger 104. In such a flow, the heat conducting medium is at low pressure and temperature so a conduit with a relatively large diameter may be used. To control the flow in a relatively large diameter conduit, solenoid valves are typically expensive so a check valve may be used instead. In addition, since a check valve is not powered, the reheat system 100 will continue to operate in many conditions even if the first valve 301 fails. In some embodiments the first valve 301 is configured to adopt an open configuration in the event of a malfunction. This provides reheat system 100 with an advantage over some hot gas bypass systems, which require check and solenoid valves to function correctly for effective operation and to avoid heat conducting medium being trapped in the system. However these valves can become unreliable over time, causing problems with such systems.

In use, the first valve 301 may be selectively closed by the control system, reducing or preventing the flow of the heat conducting medium to the second heat exchanger 104, depending on the extent to which first valve 301 is closed. This means all or a greater proportion of the thermal energy in the thermal energy pumping circuit is driven to the third heat exchanger 113, providing a greater reheat capability to the reheat system 100.

The reheat system 100 of Figure 3 may be controlled by a control system (not shown). The control system may be any system that is configured to control operation of the reheat system to effect heating, cooling and/or air conditioning of the indoor space. In preferred embodiments, the control system comprises one or more processors configured to execute instructions to cause the control system to control operation of reheat system 100. The instructions may be provided, for example, by way of processor-readable media. The control system may be connected to, or in communication with, all or some of the components of reheat system 100 shown in Figure 3 in order to affect their operation. The control system is configured to receive user inputs in order that, for example, set parameters under which the reheat system 100 operates and to control operation of the reheat system accordingly. The control system may also receive signals from sensors that sense conditions within which the reheat system 100 is operating, and control the reheat system accordingly. For example, a temperature sensor and/or a humidification sensor may be provided in the indoor space to provide signals to the control system, which controls the reheat system to provide the desired level of heating / cooling / dehumidification accordingly.

A number of different mechanisms are available in the reheat system of Figure 3 to increase the amount of reheat available. Exemplary options are:

1. Control the rate of operation of pump 112. An increased amount of pumping of thermal energy in the thermal energy transport circuit increases the amount of reheat delivered to the indoor space.

2. Control the rate of operation of the fan 116 of the second heat exchanger 104. Reducing the rate of operation of fan 116 results in more thermal energy being driven to the hydronic heat exchanger 113, increasing the amount of reheating. Stopping the outdoor fan completely may maximise this effect.

3. Alter the flow of thermal energy in the thermal energy pumping circuit to the second heat exchanger 104. As explained above, depending on the extent to which first valve 301 is closed, a greater proportion of the thermal energy in the thermal energy pumping circuit is driven to the third heat exchanger 113 (all of the thermal energy in the case the first valve 301 is closed), providing a greater reheat capability to the reheat system 100.

4. Alter the flow of air along the air flow path between the first and third heat exchangers

102, 105. By reducing the rate of air flow in the indoor space the cooling capacity of the reheat system is applied to a smaller volume of air so it is cooled to a lower temperature. Since the amount of cooling is reduced, but the volume of air that is cooled is cooled more, drier air is produced. This may be useful under conditions of low relative humidity but may not be always permitted by the airflow requirements of the building. Such building requirements (and any other building compliance requirements) may be provided to the control system, for example in the form of rules or the like, such that the control system controls the operation of the reheat system to meet the building requirements. In some embodiments, the reheat system comprises a CO2 sensor configured to measure the CO2 concentration of the air in the indoor space and the control system is configured to control airflow based on the CO2 concentration measurement, for example to increase the air flow if the CO2 concentration is higher than a predetermined threshold.

The control system may be configured to perform any one of more of these options to control the amount of reheat supplied by reheat system 100 to the indoor space. In certain

embodiments, the control system is configured to take the four specified actions sequentially as greater amounts of reheat are required.

In some embodiments, if reheating is required, the control system advances further through the above options the cooler and/or windier the environmental conditions around the second heat exchanger 104.

One advantage of those embodiments including a hydronic heat exchanger in the manner described is that this heat exchanger operates under very stable, consistent conditions (it operates at high pressure with hot transport medium) so is little directly affected by the outdoor or indoor conditions in which the system is operated.

The invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, in any or all combinations of two or more of said parts, elements or features.

Where in the foregoing description reference has been made to integers or components having known equivalents thereof, those integers are herein incorporated as if individually set forth.

It should be noted that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be included within the present invention.