This disclosure is directed to systems and methods for removing heat from recirculated air from an enclosed space, particularly from a data center, in an efficient and effective manner. 
Data centers include data servers and other ancillary equipment.  The data centers draw significant amounts of power and generate significant amounts of heat.  Cooling the data centers, particularly as they have increased in size and electronic capacity, has become an increasing challenge.  Cooling the data centers economically is an even greater challenge. 
Computer servers have typically been cooled by air-conditioned, refrigerated air that is delivered into the data centers by all available means, including through the elevated flooring, to individually cool each computer server.  The requirement for providing 55° F. to 65° F. refrigerated cold air was understood industry wide based on the need to minimize the potential for hot spots within the data servers.  Providing the required amount of refrigerated cold air, as the server banks increased in size, proved increasingly costly and complex. 
Creative air management methodologies have been undertaken to remove the generated heat in a more efficient manner.  These include construction of specific air pathways with segregated, enclosed cold air supply aisles (“cold aisles”) and separate hot air return aisles (“hot aisles”) within data centers in an attempt to keep from commingling cold air with hot air in an open space.  Such construction aids in providing necessary cooling capacity more economically and reduces the potential for server hot spots. 
A significant change to the parameters for cooling data centers was realized when the industry determined that wanner server inlet conditions for Class I and II data centers was acceptable.  That, combined with advances in hot and cold aisle containment systems and designs, permits data center cooling units to deliver warmer air to cool the servers than previously thought possible.  In 2008, an expanded envelope for air conditions entering the servers in data centers was approved, bringing the maximum recommended server inlet temperature up to 80.6° F. The above change in recommended inlet air temperature for the data servers caused engineers to seek new energy efficient solutions to manage the thermal loads in the data centers. 
An objective is to provide inlet air to the servers at a temperature that is within the recommended range while consuming the least amount of total power.  A number of methodologies have been attempted.  Some of these methodologies are discussed briefly below.  Prior to a discussion of any individual methodology, however, it should be noted that any air cooling solution for data centers should also provide for humidity control of the space in order to limit the negative effects of extreme humidity deviations within the space.  Also, the air flow cannot be on a completely closed loop because, even in the cleanest data center environment, operation of the data servers themselves creates certain byproducts that need to be diluted with some minimum level of ventilation air and/or filtered out of the air flow on a routine basis. 
The following is a non-exhaustive list of methodologies that have been attempted. 
Air-Side Economizers (ASEs) simply deliver outdoor air, mixed with return air when required to achieve the target air delivery temperature, when ambient conditions favor free cooling, through the data center.  A shortfall of ASEs, however, is that they cannot generally be used under hot or humid ambient conditions.  Thus, un-augmented ASEs provide little ability to reduce the necessary size of supplemental refrigeration-based cooling and offer no reduction in peak electrical load.  Use of ASEs also often leads to wide swings in space humidity unless humidification control systems are employed. 
Direct Evaporative Cooling (DEC) can be used to complement ASEs to extend the operating envelope of ambient conditions for the ASEs. 
Separate Water-Side Economizers may be used in which cooling towers or fluid coolers are employed in a variety of design configurations to reject some or all of the heat from the data center in combination with refrigeration-based cooling. 
Various combinations of the above methods have been attempted. 
Finally, Liquid-Cooled Enclosures, sometimes referred to as in-row or in-rack cooling, may be provided in which cool water produced from a combination of chillers and cooling towers, or otherwise by refrigerants, is pumped to heat changers or cold plates installed directly within server racks. 
While these systems result in energy savings compared to traditional 100% refrigeration-based approaches, each is less than ideal.  Among the shortfalls of these systems are: (1) the need to include substantial pre-filters and post filters, particularly on systems implementing ASEs; (2) the need to modulate damper systems and relief fans or barometric dampers and pressure controls on any system that introduces outdoor air directly into the data center; (3) ongoing extensive reliance on chilled water and/or condenser water piping or refrigerant piping; (4) redundant pumps; and (5) other additional support systems.  These systems are often maintenance intensive.