The invention relates to an air handling system for a system space, a controller and a method for controlling the temperature and humidity of a system space. In particular, the system and method may be suitable for controlling a temperature and a humidity within a system space such as a data centre or server room or telecommunications facility or any other enclosed space.

There is a need for cost-effective, low-energy cooling of buildings and facilities such as system spaces in the Information Technology (IT) sector such as data centres, server rooms and spaces housing telecommunication equipment. Such IT equipment or apparatus, which may include computing, telecommunications and other types of equipment, generates heat during normal operation and needs to be appropriately cooled. This may conventionally involve a re-circulating ventilation system cooled by a refrigeration unit, or a ventilation system drawing in ambient air, which may for example be assisted by refrigeration or by evaporative cooling.

In order to reduce the cost of cooling IT facilities, there is a desire to locate new data centres and the like in regions with cold climates, such as northern Europe, Canada and Scandinavia. In such regions, ambient air may be usable for cooling for a large proportion of the year, with additional cooling systems as a back-up to be switched on when ambient temperatures become too high.

System spaces for housing IT equipment are conventionally designed to meet

predetermined standards for cooling, such as the ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) standards illustrated in Figure 1 , which set out standard-compliant temperature and humidity ranges and recommended temperature and humidity ranges for system spaces housing IT equipment.

While cold ambient air is useful for cooling, the quantity of moisture that can be held by a given quantity of air varies with the temperature of the air, such that cold air can hold much less moisture than warm air. This means that when ambient air is cold, its humidity may be below the allowable or recommended standards. In this case, incoming ambient air must be humidified before delivery to a system space, as sparks can be generated by electronic equipment if the air is too dry. This requirement is conventionally fulfilled by artificial humidifiers or misting systems, which greatly increases the cost, complexity and energy consumption of the system as a whole.

It is desirable to use evaporative cooling systems for controlling the temperature within system spaces, as they are much more energy-efficient than refrigeration-based air- conditioning systems. However, such cooling systems are typically located outside the system space. One reason for this is that they are bulky, and occupy a significant amount of space, because effective evaporative-cooling systems require relatively low air velocities to maintain high coefficients of performance (COP). The designer of a system space such as a data centre therefore prefers to maximise use of the available space by installing electronic equipment within the system space rather than evaporative cooling systems. Also, any noise or heat generated by fans or other parts of the evaporative cooling system are undesirable within the system space. Further, an evaporative cooler requires a water supply, which is undesirable in a system space containing valuable electronic equipment due to the possibility of flooding.

GB2540139 discloses a combined cooling and humidification system in which a plurality of evaporative coolers are controllable independently of one another. This allows the system of GB2540139 to deliver a desired net air flow to the system space by combining a plurality of air flows delivered by different evaporative coolers at different temperatures and/or humidities and/or flow rates.

Summary of the Invention

The invention provides an air handling system for a system space, a controller for an air handling system, and a method for controlling the temperature and humidity of a system space as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent subclaims.

In a first aspect, the invention may provide an air handling system for a system space, such as a data centre or server room or telecommunications facility or any other enclosed space.

The air handling system may alternatively be termed a combined ventilation, cooling and humidification system, or a cooling system. The system comprises a first air handling unit a first air handling unit controllable to provide a first air flow at a target temperature and a first humidity. The system further comprises a second air handling unit controllable to provide a second air flow at the target temperature and a second humidity.

A controller, or system controller, is programmed to control the first air handling unit and the second air handling unit.

The first air flow and the second air flow are mixable to provide a mixed air flow for delivery to the system space, the mixed air flow having the target temperature and a humidity between the first humidity and the second humidity.

By combining first and second air flows to form a mixed air flow, the system of the present invention may advantageously allow the delivery of a net“mixed air flow” to the system space, the temperature and/or humidity of which would not be achievable by a single second air handling unit.

The system space preferably contains heat-emitting equipment, which heats up the temperature of air within the system space. Thus, air that is recirculated from the system space is at a higher temperature than the temperature of ambient air from outside the system space. The target temperature is below the temperature of the air in the system space, so that the mixed air flow cools the system space.

The system aims to provide a mixed air flow at a desired temperature for delivery to the system space, so that the air flow delivered into the system space cools the heat-emitting equipment in the system space. Once delivered into the system space, the mixed air flow will flow over and through the heat-emitting equipment, heating up to a higher temperature before being recirculated for cooling or exhausted out of the system space.

Unlike the prior art disclosure of GB2540139, the present system prepares two air flows at the same target temperature, before mixing those air flows to arrive at a mixed air flow.

In GB2540139, the air flows produced by different coolers were at different temperatures and humidities.

Figure 2 of this application replicates Figure 9 of GB2540139, which is a psychrometric chart illustrating the operation of the system of GB2540139 when the ambient air temperature is very cold, at about -7°C. In GB2540139 a cooler operating in ventilation mode delivers air to the system space at quality A (the temperature and humidity at point A), a cooler operating in recirculation mode without evaporative cooling delivers air with quality B, and a cooler operating in

humidification mode (or adiabatic recirculation mode) delivers air with quality C. A cooler operating in attemperation mode (without humidication) delivers air having any quality on a straight line on the psychrometric chart between points A and B, depending on the relative flow rates of ambient air and recirculated air entering the cooler.

GB2540139 enables a user to control the mixing of multiple air flows with different temperatures and humidities (at A, B and C, for example), to arrive at a mixed air flow with desired properties D.

The inventor of the present system has realised, however, that the mixing of different- temperature air flows in GB2540139 can lead to unstable temperatures in the mixed air flow, particularly at the commencement of the cooling process. In GB2540139 colder air from streams C and/or A are mixed with warmer air from stream B to form a mixed air flow. The inventor has found that there are inherent practical difficulties with mixing air flows at different temperatures to form a single air flow with a stable, consistent temperature.

Particularly when the system is starting up, the temperature of the mixed air flow may be inconsistent due to uneven mixing of the hot and cold air flows. Uneven mixing of the mixed air flows may mean that different areas of the system space experience different temperatures.

In the present invention, the air handling units are controlled to provide the first and second air flows at the same target temperature before they are mixed together. This means that the air flows combined to form the mixed air flow are already at the same temperature, but different humidities. In this situation, all of the air entering the system space is at a controlled temperature. The inventor has found that air flows with the same temperature may advantageously mix more evenly than air flows at different temperatures, leading to a mixed air flow with more consistent properties. This may advantageously improve the stability of the mixed air flow properties, particularly shortly after the system has been started up.

Uneven mixing of the incident air flows may also be of lesser importance to IT equipment in the system space when the first and second air flows are both at the target temperature, as even if the mixed air flow is not perfectly mixed, all air entering the system space is at the desired target temperature. IT equipment is typically less sensitive to differences in humidity than to differences in temperature. If air is delivered to the system space at the target temperature, but with a variation in humidity, the humidity-related failure rates of IT equipment may therefore be advantageously reduced compared to temperature-related failures that may be experienced when air is delivered to the system space at the wrong temperatures.

Thus, by providing multiple air flows at the same target temperature before combining them to form a mixed air flow, the system of the present application advantageously provides an improvement over the disclosure of GB2540139.

The properties of the mixed air flow are a“net” combination of the properties of the air flows that are combined to form the mixed air flow. Thus, when a first air flow having a first, low, humidity, is mixed with a second air flow having a second, higher, humidity, the resulting humidity of the mixed air flow will be between the first humidity and the second humidity. If the first and second air flows are mixed in a 50:50 ratio, for example, the mixed air flow will have a specific humidity half way between the first and second humidities.

The first air handling unit may be configured to receive a flow of ambient air from outside the system space, and may comprise a recirculation damper for mixing the flow of ambient air with a first flow of recirculated air recirculated from the system space.

The second air handling unit may be configured to cool and humidify a second flow of recirculated air recirculated from the system space.

The controller, or system controller, may be programmed to control the first air handling unit to mix the flow of ambient air with the first flow of recirculated air to provide a first air flow at a target temperature and a first humidity, and/or to control the second air handling unit to cool and humidify the second flow of recirculated air to provide a second air flow at the target temperature and a second humidity.

The first air handling unit is preferably configured to draw in a flow of ambient air from outside the system space. The first air handling unit may also comprise a recirculation damper for mixing the flow of ambient air with a first flow of recirculated air recirculated from the system space. The recirculation damper may thus enable the first air handling unit to raise the temperature of the incoming ambient air by mixing it with hotter recirculated air that has been heated up in the system space.

The recirculation damper may be a variable damper controllable by the controller. The position, or openness, of the damper may be controlled by the controller to alter the resistance presented to air flow through the damper, and thus to control the quantity and flow rate of recirculated air that flows through the recirculation damper.

The first air handling unit is preferably configured to draw ambient air into the air handling unit through an inlet, and may additionally comprise an inlet damper controllable to control the passage of ambient air through the inlet.

The controller may be configured to control the recirculation damper, and optionally the inlet damper, to control the proportions of ambient air and recirculated air that combine to form the first air flow. By controlling the mixing proportions of ambient and recirculated air, the temperature of the first air flow may be controlled to maintain the target temperature.

The controller may be responsive to a temperature and/or humidity of air within the system space, or within the air handling units. The controller may be responsive to a temperature and/or humidity of ambient air outside the system space.

An air handling unit configured to provide a flow of cooling air to a system space may be termed a“cooler”.

In a preferred embodiment, the first air handling unit may be a ventilation unit, which may be termed a ventilation cooler or a fresh-air cooler. The first air handling unit may comprise no cooling means to reduce the temperature of the flow of ambient air drawn into the air handling unit.

In other embodiments, the first air handling unit may be an evaporative cooler configured to optionally cool and humidify the flow of ambient air, or the combined flow of ambient and recirculated air.

The second air handling unit may be an evaporative cooler. The second air handling unit may comprise a humidification loop for cooling and humidifying recirculated air.

Evaporative cooling involves evaporating water into an air flow, so that the water obtains its latent heat of evaporation from heat energy in the air flow and evaporates, cooling the air in the process. Air cooled by evaporative cooling is therefore also humidified. Evaporative cooling may be performed by passing air through a wet evaporative-cooling element or pad, or alternatively by spraying or misting water droplets into an air flow.

The second air handling unit may be configured to evaporatively cool and humidify the second flow of recirculated air to a temperature below the target temperature. For example, the second air handling unit may be configured to evaporatively cool the second flow of recirculated air to near its wet bulb temperature. In order to raise the temperature of the cooled air to the target temperature, the second air handling unit may be configured to mix the cooled flow of air with a third flow of recirculated air recirculated from the system space to form the second air flow at the target temperature.

The second air handling unit may comprise an evaporative-cooling chamber comprising an evaporative-cooling means, such as an evaporative-cooling element, wetted media or a spray humidifier. The second air handling unit-cooling chamber may be coupled to deliver air to an air-mixing chamber, the air-mixing chamber having an outlet through which air is deliverable to the system space. The second air handling unit may further comprise a humidification damper for controlling the passage of the second flow of recirculated air from the system space through a humidification inlet into the evaporative-cooling chamber.

In a preferred embodiment the second air handling unit also comprises a bypass damper for controlling the third flow of recirculated air from the system space through a bypass inlet into the air-mixing chamber.

The humidification damper and the bypass damper are both variable dampers, the positions of which may be controlled by the controller to alter the resistance presented to air flow through the dampers, and thus to control the quantity and flow rate of air that flows through the dampers.

Thus, in use, the second flow of recirculated air may be channelled through the

humidification damper into the evaporative-cooling chamber, so that it flows through the evaporative-cooling element and in the process is evaporatively cooled and humidified. After passing through the evaporative-cooling element, the cooled and humidified air passes into the air-mixing chamber. If the air has been evaporatively cooled to a

temperature below the target temperature, the bypass damper is partially opened to allow the third flow of recirculated air to pass into the air-mixing chamber. In the air-mixing chamber, the hotter third flow of recirculated air mixes with the cooled and humidified air, raising the temperature of the resulting mixture to the target temperature.

The second air handling unit preferably comprises a fan configured to draw air through the evaporative-cooling chamber and the air-mixing chamber, and to force the air from the air mixing chamber out of the outlet as the second air flow. The fan may be a variable-speed fan.

The controller is preferably responsive to temperature and humidity sensors to control the openness of the humidification damper and the bypass damper, to control the proportions in which the cooled and humidified air is mixed with the third flow of recirculated air, and thus the temperature and humidity of the second air flow. The control configured to control the speed of the fan to control the flow rate of the second air flow out of the second air handling unit.

The controller is preferably programmed to control the humidity of the mixed air flow by controlling the proportions in which the first air flow and the second air flow are mixed to form the mixed air flow. As the specific humidity of the mixed air flow is the net result of the specific humidities of the first and second air flows, the humidity of the mixed air flow may be controlled by altering the proportions, or the mass balance, in which the first and second air flows are mixed.

When the second air flow contains air that has been cooled and humidified, the second air flow typically has a higher humidity than the first air flow.

As the first and second air flows are already at the target temperature, the mixing proportions of the first and second air flows may advantageously be controlled to alter the humidity of the mixed air flow without also affecting its temperature. This advantageously decouples the temperature control of the mixed air flow and the humidity control of the mixed air flow. This may significantly simplify the control of temperature and humidity compared to mixing multiple air flows with different temperatures and different humidities, as in GB2540139, where the net temperature and humidity properties of the resulting mixed air flow are linked together.

Each air handling unit preferably comprises a fan configured to draw air through the air handling unit. In a preferred embodiment the controller is programmed to control the speed of each fan in the system. The controller may be programmed to control the relative speeds of different fans to control the relative flow rates of the first air flow and the second air flow. By controlling the flow rates of individual air flows, the controller may control the proportions in which the first air flow and the second air flow are mixed to form the mixed air flow. This in turn controls the mass balance of the lower-humidity first air flow and the higher-humidity second air flow in the mixed air flow, and therefore controls the humidity of the mixed air flow.

Each air handling unit may comprise one or more dampers configured to restrict the passage of air through the air handling unit. In a preferred embodiment, the air handling units may comprise outlet dampers arranged to control the passage of the first air flow and the second air flow, respectively, out of the air handling units. Alternatively, the

recirculation, humidification and bypass dampers may be controllable to restrict the passage of air through the air handling units.

The controller may be programmed to control the one or more dampers to control the flow rates of the first air flow and the second air flow out of the air handling units, to control the proportions in which the first air flow and the second air flow are mixed to form the mixed air flow. This in turn may control the humidity of the mixed air flow.

In a preferred embodiment, the system may comprise more than just the two air handling units. The system may, for example, comprise two or more first air handling units and/or two or more second air handling units controllable by the controller.

In a preferred embodiment, the system may comprise one second air handling unit and two, or three, or four first air handling units, in which the first air handling units are ventilation coolers and the second air handling unit is an evaporative cooler.

The system may comprise a plurality of second air handling units.

The system may comprise a plurality of first air handling units. The first air handling units may be ventilation coolers.

The numbers and relative proportions of second air handling units and first air handling units in the system may be selected according to the ambient temperature and humidity conditions in the location where the system space is located. In a preferred embodiment, the system is configured so that the mass flow of the mixed air flow comprises 75% first air flow (ambient air mixed with recirculated air) and 25% second air flow (cooled and humidified). This may be particularly suitable for system spaces situated in locations with cold weather conditions, for example those located in the Arctic Circle. An exemplary system may comprise multiple air handling units in a ratio of three first air handling units to one humidifying second air handling unit, so that the first air flow and the second air flow may easily be mixed in a ratio of 3:1. The controller may control the air handling units to operate differently in different weather conditions, but such a system may advantageously provide a mixed air flow with desired properties across the range of weather conditions experienced in such a location.

In a preferred embodiment, each air handling unit is operable in two or more different operating modes to produce air flows with different properties, and the controller is programmed to control the operating mode of each air handling unit to produce first and second air flows at the target temperature.

The different air flows delivered by different air handling units may be mixed to form the mixed air flow before delivery to, or within, the system space.

Preferably the system is configured to mix, or to combine, the first air flow and the second air flow to form the mixed air flow. For example, the system may be configured so that the first and second air flows are directed into a supply chamber in which the flows are mixed before delivery to the system space. The system may comprise a mixing means, for example an air flow manifold through which the first and second air flows may pass before delivery to the system space.

The air handling units may be configured so that the first and second air flows are directed into a supply plenum chamber, or a supply corridor, so that the air flows mix to form the mixed air flow in the supply plenum. The supply plenum, or the air handling units, may be configured so that the first and second air flows are delivered to a confined space, or to a shared channel, in which the velocity of the air flows causes them to mix together to form the mixed air flow.

The system may comprise one or more supply fans arranged between the supply plenum chamber and the system space, the supply fans being configured to draw air from the supply plenum chamber into the system space. The air handling units may be constructed as similar modular units. Preferably the second air handling unit and the first air handling unit are housed in identical housings. Preferably the air handling units are suitable for installation inside the system space.

The system may comprise a third air handling unit, controllable to provide a third air flow at the target temperature and a third humidity. The third air handling unit may be configured to evaporatively-cool a flow of ambient air from outside the system space, and comprising a recirculation damper for mixing the flow of cooled ambient air with a flow of recirculated air recirculated from within the system space.

The third air handling unit may have the same structure as the first air handling unit and/or the second air handling unit.

The controller may be programmed to control the third air handling unit to deliver a third air flow at the target temperature and a third humidity. The third air flow may be mixable with the first air flow and the second air flow to form the mixed air flow. The humidity of the mixed air flow may then have a humidity that is an aggregate combination of the first humidity, the second humidity and the third humidity. In warm or hot weather, where the temperature of the incoming ambient air is higher than the target temperature, the third air handling unit may be controllable to act in a cooling mode by evaporatively-cooling incoming ambient air. In this mode, the third air handling unit may evaporatively-cool the incoming ambient air to a temperature below the target temperature, before heating the air to the target temperature by mixing it with recirculated air.

According to a second aspect, the invention may provide a controller programmed to control a first air handling unit to provide a first air flow at a target temperature and a first humidity, and to control a second air handling unit controllable to provide a second air flow at the target temperature and a second humidity, so that the first air flow and the second air flow mix to provide a mixed air flow at the target temperature and a humidity between the first humidity and the second humidity.

The controller may be programmed to control the first air handling unit to mix a flow of ambient air from outside the system space with a first flow of recirculated air recirculated from the system space to form a first air flow at a target temperature and a first humidity, and/or to control an second air handling unit to cool and humidify a second flow of air recirculated from the system space to form a second air flow at the target temperature and a second humidity, so that the first air flow and the second air flow mix to provide the mixed air flow at the target temperature and a humidity between the first humidity and the second humidity.

The controller may be a controller for a system according to the first aspect of the invention.

The controller may be programmed to control one or more additional air handling units to provide additional air flows at the target temperature, so that the additional air flows mix with the first and second air flows to provide the mixed air flow at the target temperature.

The controller may be a system controller operatively linked to built-in controllers of the air handling units, such that the system controller may control the controllers of the individual air handling units according to a predetermined control protocol.

The controller may advantageously control any fans, variable dampers and water supplies of the air handling units.

According to a third aspect of the present invention there may be provided a method for controlling the temperature and humidity of a system space, comprising the steps of:

controlling a first air handling unit to provide a first air flow at a target temperature and a first humidity, controlling a second air handling unit to provide a second air flow at the target temperature and a second humidity, and mixing the first air flow and the second air flow provide a mixed air flow for cooling the system space, the mixed air flow having the target temperature and a humidity between the first humidity and the second humidity.

The method may comprise the steps of mixing a flow of ambient air from outside the system space with a first flow of recirculated air recirculated from the system space, to form the first air flow at a target temperature and a first humidity; and/or cooling and humidifying a second flow of recirculated air recirculated from the system space, to form the second air flow at the target temperature and a second humidity

The method may comprise the step of mixing the first air flow and the second air flow in controlled proportions to control the humidity of the mixed air flow.

A first air handling unit may be controlled to form the first air flow, and an second air handling unit may be controlled to form the second air flow. Each air handling unit may comprise a fan configured to draw air through the air handling unit, and the relative speeds of the fans may be controlled to control the flow rates of the first air flow and the second air flow, to control the proportions in which the first air flow and the second air flow are mixed to form the mixed air flow. This in turn may control the humidity of the mixed air flow.

Each air handling unit may comprise one or more dampers configured to restrict the passage of air through the air handling unit. The positions of the dampers may be controlled to control the flow rates of the first air flow and the second air flow, to control the proportions in which the first air flow and the second air flow are mixed to form the mixed air flow. This in turn may control the humidity of the mixed air flow.

The method may be a method for controlling a combined ventilation, cooling and humidification system according to the first aspect of the invention.

In a further aspect there may be provided a controller programmed to carry out the method of the third aspect.

In a fourth aspect there may be provided a combined ventilation, cooling and humidification system for a system space, comprising:

a first air handling unit configured to receive a flow of ambient air from outside the system space, the first air handling unit comprising a recirculation damper for mixing the flow of ambient air with a first flow of recirculated air recirculated from the system space;

a second air handling unit configured to cool and humidify a second flow of recirculated air recirculated from the system space; and

a controller programmed to control the first air handling unit to mix the flow of ambient air with the first flow of recirculated air to provide a first air flow at a target temperature and a first humidity, and to control the second air handling unit to cool and humidify the second flow of recirculated air to provide a second air flow at the target temperature and a second humidity,

in which the first air flow and the second air flow are mixable to provide a mixed air flow for cooling the system space, the mixed air flow having the target temperature and a humidity between the first humidity and the second humidity.

Features of the combined ventilation, cooling and humidification system may be as described above in relation to the first aspect of the invention. In a fifth aspect there may be provided a method for controlling the temperature and humidity of a system space, comprising the steps of:

mixing a flow of ambient air from outside the system space with a first flow of recirculated air recirculated from the system space, to form a first air flow at a target temperature and a first humidity;

cooling and humidifying a second flow of recirculated air recirculated from the system space, to form a second air flow at the target temperature and a second humidity; and mixing the first air flow and the second air flow to provide a mixed air flow at the target temperature and a humidity between the first humidity and the second humidity.

Features of the method may be as described above in relation to the third aspect of the invention.

Features described above in relation to the first aspect of the invention may equally be applied to any of the second, third, fourth and fifth aspects of the invention.

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

Figure 1 shows a psychrometric chart illustrating recommended and allowed conditions set out in the ASHRAE TC 9.9 2011 Thermal Guidelines for Data Processing Environments - Expanded Data Center Classes and Usage Guidance;

Figure 2 shows a psychrometric chart illustrating the operation of the prior art system of GB2540139;

Figure 3 is a schematic diagram of a ventilation cooler usable in an embodiment of the invention;

Figure 4 is a schematic diagram of an second air handling unit usable in an embodiment of the invention;

Figure 5 is a schematic diagram of an second air handling unit usable in an embodiment of the invention; Figures 6 and 7 show the cooler of Figure 5 in ventilation mode and humidification mode, respectively;

Figures 8 and 9 show psychrometric charts illustrating the operation of the invention;

Figures 10 and 11 are perspective views of a system incorporating a plurality of air handling units embodying the invention.

Figure 1 illustrates ASHRAE (American Society of Heating, Refrigeration and Air- conditioning Engineers) standards for standard-compliant temperature and humidity ranges and recommended temperature and humidity ranges for system spaces housing IT equipment.

Figure 2 illustrates the operation of the prior art system of GB2540139. As discussed above, the system of GB2540139 mixes multiple air streams having different temperatures and humidities, in order to arrive at a mixed air stream with the desired properties for delivery to a system space.

In a first preferred embodiment, the system of the present invention comprises a first air handling unit which is a ventilation cooler 2 as shown in Figure 3. The ventilation cooler 2 is arranged to provide a first air flow at a target temperature. The system space, or the portion of the system space requiring cooling, may be termed a room load, and may contain IT equipment (not shown).

The ventilation cooler 2 comprises a first, ambient-air, inlet 6 for admitting a flow of air from the surrounding environment, through an ambient-air inlet damper 8 into a ventilation chamber 10. A supply fan 16 at an outlet 18 from the ventilation chamber draws air through the cooler and delivers it to the system space 4.

The ventilation cooler 2 comprises a recirculation channel 20 arranged to allow a flow of recirculated air from the system space (room load) to a recirculation inlet 23 into the ventilation chamber. Recirculated air passing through the recirculation inlet 23 passes through a recirculation damper 24 mounted across the recirculation inlet.

An exhaust outlet 28 leads from the system space, through an exhaust damper 30, for exhausting air from the system space. An exhaust fan 32 is arranged to drive exhaust air through the exhaust outlet. The exhaust outlet 28 is typically remote from the cooler, for example at a far end of the system space.

A controller (not shown) controls the openings of the ambient-air inlet damper 8, recirculation damper 24 and the supply fan 16. The controller also controls the exhaust damper 30 and the exhaust fan 32.

In addition to the ventilation cooler 2, the system also comprises an second air handling unit 200 arranged to provide a second air flow at the desired temperature. A“humidification cooler” embodiment of the second air handling unit is shown in Figure 4. The second air handling unit 200 is configured to cool and humidify a flow of recirculated air recirculated from the system space 4.

The second air handling unit 200 comprises an evaporative-cooling chamber 210 comprising one or more evaporative-cooling elements, or pads, 212 through which air may flow into an air-mixing chamber 214. A supply fan 216 at an outlet 218 from the air-mixing chamber draws air through the cooler and delivers it to the system space 4.

The second air handling unit 200 comprises a recirculation channel 220 arranged to allow a flow of recirculated air from the system space 4 (room load) to a humidification inlet 223 into the evaporative-cooling chamber and to a bypass inlet 226 into the air-mixing chamber. Recirculated air passing through the humidification inlet 223 passes through a

humidification damper 224 mounted across the humidification inlet. Recirculated air passing through the bypass inlet 226 passes through a bypass damper 222 mounted across the bypass inlet.

The same controller that controls the ventilation cooler 2 also controls the second air handling unit 200. The controller controls the openings of the humidification and bypass dampers 224, 222 and the supply fan 216.

In the air handling system of the present invention, the ventilation cooler 2 operates to provide a first air flow at a target temperature, while the second air handling unit 200 operates to provide a second air flow at the same target temperature. The ventilation cooler and the second air handling unit are configured to deliver the first and second air flows into a mixing area (not shown) where the air flows mix to form a mixed air flow at the target temperature. The mixed air flow is then delivered to the system space 4. The controller is programmed to control the ventilation cooler 2 to provide the first air flow at a target temperature. The target temperature is programmed to be an allowed or recommended temperature for the system space, for example a temperature falling within the recommended temperature range of ASHRAE recommended conditions.

In locations where ambient conditions are relatively cold throughout the year, the temperature of ambient air may advantageously be reliably below the maximum

recommended or allowable temperature for the system space. The temperature of the ambient air can therefore be assumed to be below the target temperature.

In use, the controller controls the ventilation cooler 2 to draw in a flow of ambient air from outside the system space through the first inlet 6. As the temperature of the ambient air is below the target temperature for delivery to the system space, the controller controls the recirculation damper 24 to allow a flow of recirculated air recirculated from the system space into the ventilation chamber, where it mixes with the incoming ambient air to form the first air flow. As the recirculated air has been heated inside the system space before it is recirculated, the recirculated air has a temperature above the target temperature. Mixing the recirculated air with the ambient air therefore raises the temperature of the ambient air flow. The controller controls the relative openings of the ambient-air inlet damper 8 and the recirculation damper 24 to control the relative proportions of ambient air and recirculated air that are mixed. The mixing proportions are controlled so that the first air flow is at the target temperature. The humidity of the first air flow is not actively controlled by the controller, so the first air flow has a first humidity between the humidity of the ambient air and the humidity of the recirculated air.

At the same time, the controller controls the second air handling unit 200 to provide the second air flow at the same target temperature. In order to do this, the controller controls the humidification damper 224 to allow a flow of recirculated air to pass into the

evaporative-cooling chamber. The controller also controls a water supply (not shown) to supply water to the evaporative-cooling pad 212, so that the air in the evaporative-cooling chamber is drawn through the evaporative-cooling pad and evaporatively-cooled. During cooling, the air is naturally humidified. The controller preferably controls the flow rate of the air and the water supply so that the cooled air is cooled to at or near its wet bulb

temperature, which may be below the target temperature. If the cooled air has a temperature lower than the target temperature, the controller controls the bypass damper 222 to allow a further flow of recirculated air to pass into the air-mixing chamber 214. In the air-mixing chamber, this recirculated air is mixed with the cooled and humidified air to form the second air flow. As the temperature of the recirculated air is above the target temperature, the recirculated air raises the temperature of the cooled air flow. The controller controls the relative openings of the humidification damper 224 and the bypass damper 222 to control the relative proportions of cooled and humidified air and recirculated air that are mixed. The mixing proportions are controlled so that the second air flow is at the target temperature. The second air flow ends up with a second humidity between the humidity of the cooled and humidified air and the humidity of the recirculated air.

The fans 16, 216 force the first air flow and the second air flow out of the air handling units, where they mix by convection to form a mixed air flow that is then delivered to the system space 4. The mixed air flow is formed at the target temperature, while the mixed air flow ends up with a humidity between the first humidity and the second humidity. The controller controls the humidity (which may be expressed in terms of relative humidity or specific humidity) by controlling the mixing proportions of the first air flow and second air flow.

The temperature and humidity of the mixed air flow are controlled by the controller to be properties desired for delivery to the system space 4.

The humidifying effect of the second air handling unit 200 provides a high humidity air flow for mixing with the lower-humidity first air flow. This is particularly advantageous in cold ambient conditions, where the humidity of the incoming ambient air is too low for delivery directly to the system space. Providing the high-humidity second air flow therefore allows the overall humidity of the mixed air flow to be raised to a desirable level for delivery to the system space.

Preferred embodiments of the air handling system comprise more than two air handling units. The system may comprise a plurality of ventilation coolers and only one evaporative cooler, or a plurality of evaporative coolers and only one ventilation cooler. Alternatively the system may comprise a plurality of evaporative coolers and a plurality of ventilation coolers. In a preferred embodiment, the first air handling unit may also be an evaporative cooler.

For example, the first air handling unit may be an evaporative cooler controllable to operate in a ventilation mode. Thus, all of the coolers in the system may be evaporative coolers, the coolers being operable in different modes to provide a plurality of mixable air flows at the target temperature.

A preferred embodiment of a second air handling unit 300 usable with the present system is shown in Figure 5.

The cooler 300 comprises a first, ambient-air, inlet 306 for admitting a flow of air from the surrounding environment, through an ambient-air inlet damper 308 into an evaporative- cooling chamber 310. The evaporative-cooling chamber comprises one or more evaporative-cooling elements, or pads, 312 through which air may flow into an air-mixing chamber 314. A supply fan 316 at an outlet 318 from the air-mixing chamber draws air through the cooler and delivers it to the system space 4.

The cooler 300 comprises a recirculation channel 320 arranged to allow a flow of recirculated air from the system space to a humidification inlet 323 into the evaporative- cooling chamber and to a bypass inlet 326 into the air-mixing chamber. Recirculated air passing through the humidification inlet 323 passes through a humidification damper 324 mounted across the humidification inlet. Recirculated air passing through the bypass inlet 326 passes through a bypass damper 322 mounted across the bypass inlet.

The cooler 300 is controllable to act in a ventilation mode, as shown in Figure 6, by closing the humidification damper 324 and not supplying water to the evaporative-cooling pad 312. In this mode, the cooler 300 operates similarly to the ventilation cooler 2 discussed above.

In warm or hot weather, where the temperature of the incoming ambient air is higher than the target temperature, the cooler 300 is also controllable to act in a cooling mode by closing the humidification damper and supplying water to the evaporative-cooling pad 312. In this mode, the cooler 300 evaporatively-cools the incoming ambient air to a temperature below the target temperature, before heating the air to the target temperature by mixing it with recirculated air.

The cooler 300 is alternatively controllable to act in a humidification mode, as shown in Figure 7, by closing the ambient-air inlet damper 308 and supplying water to the evaporative-cooling pad 312. In this mode, the cooler 300 operates similarly to the second air handling unit 200 discussed above.

The same cooler 300 may thus be controlled to act as a ventilation cooler or an second air handling unit. The system may thus comprise a plurality of second air handling units operable in ventilation or evaporative-cooling modes. All coolers in the system may be second air handling units, but the controller may control different coolers to operate differently, so that they provide air flows at the target temperature but at different humidities.

The control protocol of the system is illustrated on the psychrometric chart of Figures 8 and 9. Ambient air drawn into a cooler from outside the system space is shown at point AA on the psychrometric chart. Heated air being recirculated from within the system space is shown at point BB on the psychrometric chart. The properties of the air cooled and humidified by the second air handling unit 200, 300, before mixing with recirculated air, are shown at point CC. The properties of the first air flow are shown at point DD, while the properties of the second air flow are shown at point EE. The properties of the mixed air flows are shown at point FF.

The first air handling unit 2, 300, draws in ambient air at AA and mixes it in controlled proportions with recirculated air BB to form the first air flow with properties DD. The mixing proportions are controlled so that the temperature of the first air flow is the target temperature T, while the humidity of the first air flow is between the humidities of the ambient air at AA and the recirculated air at BB.

The second air handling unit 200, 300, draws in recirculated air at BB and evaporatively- cools and humidifies the recirculated air to point CC. As this temperature is below the target temperature T, the cooler mixes the humidified air in controlled proportions with more recirculated air BB to form the second air flow with properties EE. The mixing proportions are controlled so that the temperature of the second air flow is the target temperature T, while the humidity of the second air flow is between the humidities of the humidified air at CC and the recirculated air at BB.

The controller controls the relative speeds of the fans in the first air handling unit and the second air handling unit to control the relative quantities of the first and second air flows that are delivered out of the coolers. Once delivered out of the coolers, the first and second air flows are mixed to form a single mixed air flow. The properties FF of the mixed air flow arise as a composite of the properties of the first and second air flows. Thus, the mixed air flow retains the target temperature T, but ends up with a humidity part-way between DD and EE. The position of the mixed air humidity between DD and EE is controlled by controlling the mixing proportions of the first and second air flows.

In the exemplary embodiment shown in Figure 9, the first air handling unit 2, 300 mixes ambient air and recirculated air in a ratio of 33% ambient air : 67 % recirculated air to arrive at point DD. The second air handling unit 200, 300 mixes humidified air and recirculated air in a ratio of 82% humidified air : 18 % recirculated air to arrive at point EE. The controller mixes the first and second air flows in a ratio of 77% first air flow : 23% second air flow to arrive at the desired properties FF.

The exemplary system of Figures 10 and 11 comprises two ventilation coolers 2 (first air handling units) and one second air handling unit 300. Each cooler is preferably constructed as a modular unit so that a plurality of coolers may be installed within a system space. In that case, a row 50 of coolers 2, 300 may advantageously be installed, so as to set up a consistent circulation path for air within the system space. Examples are illustrated in Figures 10 and 11.

The plurality of coolers 2, 300 advantageously form a combined ventilation, cooling and humidification system, which is controlled by a controller (not shown), using the modes of operation described above.

The controller can control the supply fan, the variable dampers and the pump of each second air handling unit, in response to sensors measuring parameters such as a system- space air temperature and humidity and ambient air temperature and humidity, in order to deliver the mixed air flow to the system space at a desired temperature, humidity and flow rate.

Depending on the temperature and humidity of the ambient air, the controller may control a plurality of coolers to operate in the same way so that those coolers deliver air at the target temperature and a first humidity while other coolers deliver air at the target temperature and a second humidity. Any coolers that are not required at any time may be switched off, preferably by closing at least the first damper and turning off the supply fan and water supply. By controlling each cooler independently of the other coolers, the controller can advantageously control the system to operate in a modular fashion. This may be particularly advantageous when the temperature and/or humidity of the ambient air is such that two coolers cannot deliver air to the system space with both a suitable temperature and a suitable humidity.

One such situation may be in very cold weather, when the ambient air is too cold and too dry for delivery directly to the system space. For delivery to the system space, the ambient air must therefore be warmed to an appropriate temperature, and humidified to an appropriate humidity. However, in an evaporative cooling system, air cannot be humidified without cooling it further, and this is not possible with extremely cold air, as cold air can only hold a small amount of water (corresponding to a low specific humidity). To solve this problem, a first air handling unit may operate in a ventilation mode, so as to deliver a first air flow warmed to the target temperature, and a second cooler may operate in a humidification mode, so as to deliver a second air flow of humidified recirculated air at the target temperature. A third cooler may optionally operate to deliver a third air stream at the target temperature. By controlling the flow rates of these air streams and mixing the air streams before they are delivered to the portion of the system space which requires cooling, the controller may deliver a net air flow to the system space that is at a desired temperature and humidity.

As described above, temperature and humidity are naturally coupled in systems which employ evaporative cooling. While this is usually not a problem for facilities in temperate climates, it may potentially be problematic in extremely cold regions. By providing a modular system comprising coolers operable to provide separate air flows at the same target temperature but with different humidities, the system of the present invention can effectively decouple temperature and humidity considerations in an improved way.

In a preferred embodiment, the system space comprises a cooling channel 52 into which controlled temperature and/or humidity air is delivered from each of the plurality of coolers 2, 300, as shown in Figures 10 and 11 , in order to mix air streams having the same temperature but different humidities prior to delivery into the portion of the system space requiring cooling. Air streams delivered into this channel form mixed air at the target temperature and a desired humidity before flowing into the portion of the system space which requires cooling. The system is advantageously in a system space housing IT equipment. In such a system space, air heated by the IT equipment tends to rise towards the ceiling, or towards an upper portion, of the system space. This warmed air may then be drawn into the recirculation inlet 38 of the cooler. Cooled, or temperature-controlled, air is then delivered by the cooler to a lower portion of the system space, where it cools the IT equipment (typically mounted on the floor). The heated air rises within the system space and can be re-circulated through the cooler. Thus, by arranging the cooler to draw air downwardly and to deliver cooled air to the lower portion of the system space, an air-circulation path can be set up within the system space, as illustrated in Figures 10 and 11. This reduces the energy consumption of the cooler, and increases its cooling effect, because the air within the system space and in the cooler is flowing in the same direction around the circulation path. Minimising the energy consumption of the cooler is important because the cooler is housed within the system space, and so its own energy consumption may

disadvantageously contribute to heating within the system space.

IT equipment in a system space is typically arranged in a series of parallel aisles 54. In order to set up the desired circulation path through the coolers, as described above, the coolers should be arranged at the end of an aisle, so that air circulation can be set up within, or along, the aisles.