Patent Publication Number: US-8978169-B2

Title: Protective clothing ensemble with two-stage evaporative cooling

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of and claims the benefit of priority under 35 U.S.C. 120 to U.S. patent application Ser. No. 13/481,292 filed on May 25, 2012, which is a continuation-in-part of and claims the benefit of priority under 35 U.S.C. 120 to International Application Number PCT/US11/30478 filed on Mar. 30, 2011, which claims priority to U.S. provisional patent application Ser. No. 61/319,070 filed on Mar. 30, 2010, all of which are expressly incorporated by reference herein. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used and licensed by or for the United States Government. 
    
    
     BACKGROUND OF THE INVENTION 
     Warriors, first-responders, and industrial workers are examples of personnel who may perform physically-demanding tasks with high rates of metabolic energy expenditures and metabolic heat production. These personnel may be equipped with protective clothing, for example, chemical, biological, radiological, nuclear, and explosive protective clothing, combat clothing, or other individual protective clothing ensembles. Normal mechanisms of dissipating excess metabolic heat, for example, through evaporative cooling in warm and hot environments, may be compromised by the insulation and resistance to water vapor permeation of known protective ensembles. Known protective clothing may increase metabolic heat production due to the metabolic cost of carrying and using the ensemble, and compromise metabolic heat loss by impeding evaporative cooling and dry heat dissipation through conduction, convection, and radiative heat loss. Reducing the thermal burden imposed by protective ensembles has long been, and continues to be, an important need for designers, manufacturers, and users of protective clothing. 
     Active cooling systems for protective ensembles are known. Active microclimate cooling systems may be thermoelectric systems, or compressor-based systems with a coolant that is circulated in tight-fitting vests, or, perhaps, blower systems that pass filtered outside air over the body and exhaust the air outside the protective suit. Compressor-based or thermoelectric systems may be power hungry, may be expensive, and may be heavy in weight. Air blower systems may be lighter in weight and more comfortable than compressor-based systems, but may be noisy, may have relatively high heat signatures (i.e., may be detected by infrared sensors), may require intake filtering of the air, and may have variable performance, depending on air inlet temperature and humidity. Air blower systems may be impractical in a chemically, biologically, and/or radiologically contaminated environment where filtering a large volume of inlet air may require a large filter capacity. 
     A long-felt but unsolved need has existed, and continues to exist, for lighter weight, more energy-efficient methods and apparatus to help reduce the thermal load of personnel equipped with protective clothing ensembles. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a protective garment for an animate being. The protective garment may include an impermeable inner layer. A reservoir may be disposed interior to the inner layer, for collecting sweat from the animate being. The garment may include a pump for moving the sweat from the reservoir to a location external to the inner layer. The animate being may be a human. 
     The sweat collected in the reservoir may be unevaporated liquid sweat, and/or liquid sweat that has exuded or been excreted from the animate being, evaporated, and condensed on the inner layer. The pump may be disposed interior to the inner layer. 
     The garment may include a distribution system located external to the inner layer, for distributing the sweat on an exterior of the garment. Inlet tubing may have one end in fluid communication with the reservoir and another end connected to an inlet of the pump. Outlet tubing may have one end connected to an outlet of the pump and another end that passes through the inner layer. The outlet tubing may be operatively connected to the distribution system. 
     The garment may include an external reservoir disposed exterior to the inner layer and fluidly connected to the internal reservoir. The external reservoir may supply water to the internal reservoir for distribution inside or outside of the inner layer. 
     The distribution system may include wicking material and/or at least one fluid conduit. The distribution system may include at least one fluid conduit in fluid communication with the outlet tubing, and wicking material adjacent to at least one fluid conduit. The wicking material may be an external layer of the garment. 
     Another aspect of the invention is a method. The method may include providing an animate being with a protective garment and collecting sweat from the animate being in a reservoir. The method may include pumping the sweat to an exterior of the garment. The collected sweat may include sweat that has condensed on an inner layer of the garment. The collected sweat may include unevaporated sweat. 
     The method may include, after pumping, distributing the sweat on an exterior of the garment. The method may include, after distributing, evaporating the sweat from the exterior of the garment. 
     Water from a reservoir that is external to the inner layer of the garment may also be collected in the reservoir that collects sweat. One or both of water and sweat may be pumped to the exterior of the garment or distributed between the inner layer and the animate being. 
     The invention will be better understood, and further objects, features and advantages of the invention will become more apparent from the following description, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
         FIG. 1  is a schematic side view of one embodiment of a protective garment. 
         FIG. 2  is an enlarged, schematic, sectional view of portion “A” of  FIG. 1 . 
         FIG. 3  is a schematic, cutaway, side view of one embodiment of a boot or shoe having a pump. 
         FIG. 4  is a schematic, cutaway, side view of one embodiment of a pump located near an elbow of a human. 
         FIG. 5  is a schematic front view of one embodiment of a pump located on a torso of a human. 
         FIGS. 6A and 6B  are schematic fluid flow diagrams of a protective garment. 
         FIGS. 7A and 7B  are side and ends views, respectively, of one embodiment of tubing for a distribution system. 
         FIGS. 8A-B ,  9 A-B, and  10 A-B are graphs of core temperature ( FIGS. 8A ,  9 A,  10 A) and physiological strain index ( FIGS. 8B ,  9 B,  10 B) versus time for varying temperature and humidity conditions, with (ACP2E) and without (MOPP-4) two-stage evaporative cooling. The physiological strain index (PSI) is a measure of thermal/work strain expressed on a scale of 1 to 10. Increases in heart rate and body temperature result in increased PSI levels.  FIGS. 8A-B ,  9 A-B, and  10 A-B were generated using thermal-physiological modeling based on principles of physics and physiology. 
         FIG. 11  is a schematic fluid flow diagram of a protective garment that includes an external reservoir. 
     
    
    
     DETAILED DESCRIPTION 
     A two-stage evaporative cooling process and protective overgarment may reduce overheating and heat illness experienced by those who wear protective garments such as hazardous material suits. The cooling process and overgarment may be suitable for animate beings, in particular, humans. A first stage of evaporative cooling may include evaporation of sweat from the skin of a human, or evaporating sweat from an undergarment that is worn next to the skin. The undergarment may have multiple layers. The sweat vapor may condense on an interior surface of an inner, impermeable layer of the loose-fitting protective garment. 
     As used herein, “impermeable layer” means a layer of a garment that is at least impermeable to water vapor and water. Preferably, the impermeable layer may also be impermeable to a range of chemical, biological, and other types of hazards. Different chemical, biological, or other types of hazards may require the selection of varying materials for the impermeable layer. Examples of materials for impermeable layers of protective garments are well-known in the field of hazardous materials protection. Such materials may include PTFE (polytetrafluoroethylene, e.g., TEFLON®), Dupont™ Tychem® TK, impermeable Dupont™ Nomex®, Gore® CHEMPACK® Ultra Barrier, or other impermeable materials, such as cotton or nylon fabric coated with polyvinyl chloride (PVC), polyurethane (PU), or rubber. 
     A second stage of evaporative cooling may occur on the exterior surface of the protective garment, exterior of the impermeable layer. The second stage of evaporative cooling may help dissipate the heat of condensation generated on the interior surface of the impermeable layer. The second stage of evaporative cooling may include pumping condensed sweat from inside the garment to the exterior of the garment and then distributing the condensed sweat on the exterior surface of the garment for re-evaporation. The second stage of evaporative cooling may include pumping unevaporated sweat from inside the garment to the exterior of the garment and then distributing the unevaporated sweat on the exterior surface of the garment for evaporation. In some embodiments, the second stage may include pumping water from inside the garment to the exterior of the garment and then distributing the water on the exterior surface of the garment for re-evaporation. 
       FIG. 1  is a side view of one embodiment of a protective garment  10 . Protective garment  10  may be a unitary garment, or may have separate top (jacket) and bottom (pants) portions. Protective garment  10  may include removable gloves  12 . Or, gloves  12  may be integral with garment  10 . Protective garment  10  may include removable shoes or boots  14 . Or, shoes or boots  14  may be integral with garment  10 . Apparatus and methods for seals  22  around removable boots  14  and removable gloves  12  are known in the art. The degree of integrity of the sealing method that is required for boots  14  and/or gloves  12  depends on the nature or level of the chemical, biological, or other threat. As is known in the art, the composition of garment  10  may be different for different areas of garment  10 . For example, the composition of boots  14  and/or gloves  12  may differ from the composition of the remainder of garment  10 , particularly if boots  14  and/or gloves  12  are separately removable from the remainder of garment  10 . 
     In the embodiment of  FIG. 1 , garment  10  includes an integral head covering  16 . Head covering  16  may include a transparent viewing portion  18 . Respiration may be variously accomplished via a backpack re-breather, a self-contained breathing apparatus, or a tethered system where air is supplied via a hose (not shown), as in the U.S. Army&#39;s Self-Contained Toxic Environment Protective Outfit (STEPO). Excess pressure may be released via one or more one-way exhaust vents  20 . Or, in lieu of integral head covering  16 , a gas mask with or without other head covering may be used. In some embodiments, water in expired breath may be condensed, captured and re-evaporated. 
     Garment  10  may be an overgarment, that is, the outermost component of a clothing ensemble. As such, garment  10  may be sized to be generally loose-fitting on the wearer of the garment, for example, to allow freedom of movement or to provide ample space for undergarments. Undergarments are not required with garment  10 , but may be used. For example, a T-shirt and shorts may be worn under garment  10 . For military use, an Army Combat Uniform (ACU) worn with undershirt and underpants may be worn with or without armor under garment  10 . Other types of garments may be worn under garment  10 . In general, garment  10  may not be pre-tensioned against the wearer, in contrast to elasticized, tight-fitting garments. But, in some embodiments of garment  10 , selected pre-tensioning may be used for protective purposes, for example, elastic sleeve cuffs, leg cuffs, neck band, etc. 
       FIG. 2  is an enlarged, schematic, sectional view of portion “A” of  FIG. 1 . In  FIG. 2 , a human  24  has an outer skin  26 . Optionally, an undergarment  28  may be juxtaposed with skin  26 . An air gap or space  30  may be adjacent undergarment  28 , or, if undergarment  28  is not present, air gap  30  may be adjacent skin  26 . Garment  10  may be disposed adjacent air gap  30 . The width of air gap  30  may vary on different areas of human  24  as human  24  moves around and/or changes position. At some times, in some areas of the human&#39;s body that are flexed (e.g., elbows, knees) or are supporting the weight of garment  10  (for example, shoulders), the width of air gap  30  may approach or become zero. 
     Garment  10  may include an impermeable, inner layer  32  having an inner surface  34  contiguous with air gap  30 . Garment  10  may include a moisture wicking, outer layer  36  disposed opposite impermeable inner layer  32 . Garment  10  may have an exterior surface  38 . Wicking outer layer  36  may be a wicking fabric, such as polyester, for example. Wicking fabrics may be non-absorbent. Wicking fabrics may include a system of fibers that work like capillaries to carry water. Wicking fabrics may have surface texture, for example, puckers in the fabric may increase the surface area and enhance evaporation. Wicking outer layer  36  may also be a surface treatment, for example, a liquid or spray that may be applied to an outer surface of impermeable inner layer  32 . The surface treatment may be a surfactant (e.g., Woolite®) that decreases water surface tension and promotes wetting of fabric. 
     In some embodiments of the invention, a semi-impermeable layer may be substituted for impermeable layer  32 . The semi-impermeable layer may be at least impermeable to liquid water, but semi-impermeable to water vapor, such as a GORE-TEX® type of material. 
     Human  24  may excrete or exude liquid sweat  40  from skin  26 . If no undergarment  28  is present, liquid sweat  40  may evaporate directly from skin  26 , pass through air space  30  as sweat vapor, and condense on inner surface  34  as condensed sweat  42 . If undergarment  28  is present, liquid sweat  40  may pass through undergarment  28 , evaporate from undergarment  28 , pass through air space  30  as sweat vapor, and condense on inner surface  34  as re-condensed liquid sweat  42 . In either case, skin  26  may be directly or indirectly cooled by evaporation of liquid sweat  40 . 
     As will be described in more detail below, condensed sweat  42  may be collected and transported to wicking outer layer  36 . In addition or alternatively, liquid sweat  40  that may not have evaporated may be collected and transported through impermeable inner layer  32  to wicking outer layer  36 . On or in wicking outer layer  36 , the transported sweat  44  may evaporate from external surface  38  of garment  10 . Evaporation of transported sweat  44  from external surface  38  may cool wicking outer layer  36 , thereby indirectly cooling impermeable inner layer  32 , air space  30 , and human  24 . It should be noted that, in some embodiments of garment  10 , wicking outer layer  36  may be included only in selected areas of garment  10 . For example, wicking outer layer  36  may be included on areas of garment  10  that are near to areas of human  24  which exhibit the greatest increases in sweat rate when the core temperature of human  24  increases. Such areas of higher sweat rates in human  24  may be, for example, the head, torso, arms, and upper legs. 
       FIG. 3  is a schematic, cutaway, side view of one embodiment of a boot  14 . Boot  14  may be made integral with garment  10  or may be removable separately from the remainder of garment  10 . Boot  14  may include a pump  50 , a forward insole  52 , and a rear insole  54 . Forward insole  52  and rear insole  54  may include pores  56 . Condensed sweat  42  from inner surface  34  of garment  10  and/or unevaporated sweat  40  may accumulate in boot  14  and pass through pores  56  into a bottom area  58  of boot  14 . From bottom area  58 , the accumulated sweat may enter inlet tubing  60  and thence reservoir  62 . A check or one-way valve  64  may be disposed in inlet tubing  60  to prevent flow from reservoir  62  into bottom area  58 . Reservoir  62  may be, for example, an elastic or flexible bladder. Rear insole  54  may be, for example, an elastic membrane. 
     The intermittent force of the heel of human  24  on rear insole  54  and reservoir  62  may pump collected sweat from reservoir  62  through an outlet tubing  66  and, ultimately, through impermeable inner layer  32  to wicking outer layer  36 . A check valve or one-way valve  64  may be disposed in outlet tubing  66  to prevent backflow into reservoir  62 . A quick-disconnect coupling  68  may be included in outlet tubing  66 , particularly if boot  14  is a removable type boot. 
       FIG. 4  is a schematic, cutaway, side view of one embodiment of a pump  70  located near an elbow  72  of human  24 . Pump  70  may include an elastic or flexible bladder  74  for containing unevaporated and/or condensed sweat. Bladder  74  may be connected to inlet tubing  76  and outlet tubing  78 . A flexible shaft  80  may have one end fixed to upper arm  82  with adjustable strap  84  and another end that extends toward lower arm  86  and bears on bladder  74 . Inner surface  34  of impermeable layer  32  may include a reservoir  88  for collecting unevaporated sweat and/or sweat that has condensed on inner surface  34 . Reservoir  88  may be in the form of, for example, a flexible, semi-rigid, or rigid gutter  87  with one end  89  fixed to surface  34 . Gutter  87  may extend circumferentially (partially or completely) around the inner surface  34  of a sleeve  90  of garment  10 . Gutter  87  may be made of, for example, a plastic material covered with a waterproof fabric. 
     Movement of elbow joint  72  may cause pump  70  to transport accumulated sweat from reservoir  88  via inlet tubing  76  to outlet tubing  78  and, ultimately, through impermeable inner layer  32  to wicking outer layer  36 . Check valves  64  may be disposed in inlet tubing  76  and outlet tubing  78 . A quick-disconnect coupling  68  may be included in outlet tubing  78  to facilitate set-up of garment  10  and to provide an option to use or not use pump  70 . 
       FIG. 5  is a schematic front view of one embodiment of a pump  92  located on a torso  94  of human  24 . Pump  92  may be located on the lower chest so that inspiration movements of human  24  may cause elastic bladder  96  to decrease in volume. Bladder  96  may be attached to human  24  using, for example, an adjustable strap  98  that may extend around torso  94 . A reservoir  100  may be disposed on an inner surface  34  of impermeable layer  32 . Reservoir  100  may be in the form of, for example, a flexible, semi-rigid, or rigid gutter  101  with one end  102  fixed to surface  34 . Gutter  101  may extend circumferentially (partially or completely) around the inner surface  34  of a torso portion  104  of garment  10 . Gutter  101  may be made of, for example, a plastic material covered with a waterproof fabric. Inlet tubing  106  may extend from pump  92  to an opening  103  in gutter  101 . 
     Breathing movements of human  24  may cause pump  92  to transport sweat from reservoir  100  through opening  103  and inlet tubing  106  and then to outlet tubing  108  and, ultimately, through impermeable inner layer  32  to wicking outer layer  36 . Check valves  64  may be disposed in inlet tubing  106  and outlet tubing  108 . A quick-disconnect coupling  68  may be included in outlet tubing  108  to facilitate set-up of garment  10  and to provide an option to use or not use pump  92 . In lieu of pump  92 , one or more downspouts in the form of tubing  105  (internal to torso portion  104  of garment  10 ) may carry contents of reservoir  100  to bottom area  58  of boot  14  or to reservoir  62  in boot  14 . 
     As discussed above, pumps  50 ,  70 , and  92  may be powered by the natural movements of human  24  that may occur while performing a task. “Natural body movements” are not movements of human  24  that are consciously and specifically directed to only actuating a pump. One or more of pumps  50 ,  70 ,  92  may be used in various combination and numbers. For example, a multiplicity of pumps may be arrayed circumferentially around elbows, knees, waist, shoulder, underarm, hip and other areas such that the action of bending at these locations may result in bladder compression and fluid output, and straightening at these locations may result in bladder re-expansion and fluid intake. Other pumps, such as battery-powered pumps or hand pumps may be used. The sweat may be pumped by the pump or pumps through the outlet tubing and through impermeable inner layer  32  to wicking outer layer  36 . From wicking outer layer  36 , the sweat may be distributed on external surface  38  of garment  10  and evaporated to thereby cool garment  10 . 
     Outlet tubing from each pump, for example, outlet tubing  66 ,  78  and  108 , may be joined together before piercing impermeable layer  32 . Or, each outlet tubing may independently pierce impermeable layer  32 .  FIG. 6A  is a schematic flow diagram of a garment  10  having two pumps  50 , two pumps  70  and one pump  92 . Outlet tubing  66 ,  66 ,  78 ,  78 , and  108  from each of the respective pumps may join an outlet header or manifold  110 . Header  110  may pierce or pass through impermeable layer  32  at an opening  112 . Opening  112  may be sealed around header  110 . Check valves  64  may be used to prevent backflow.  FIG. 6B  is a schematic flow diagram of a garment  10  having two pumps  50 , two pumps  70  and one pump  92 . Outlet tubing  66 ,  66 ,  78 ,  78 , and  108  from each of the respective pumps may independently pass through impermeable layer  32  at multiple openings  112 . Openings  112  may be sealed around each outlet tubing. Check valves  64  may be used to prevent backflow. Outlet tubing from the pumps and/or outlet header  110  may be fastened to inner surface  34  of impermeable layer  32 .  FIGS. 6A and 6B  are exemplary only. The number of pumps used may be one or more. 
     At opening  112  or openings  112 , sweat flowing in the outlet tubing or outlet header may flow into a distribution system for distributing the sweat on or in the outer wicking layer  36 .  FIG. 1  shows a distribution system  114  that may include a plurality of tubes with holes or perforations. The holes may allow the sweat to flow into wicking layer  36 . The cross-section of the tubing that forms distribution system  114  may be circular, semi-circular or some other cross-section. 
       FIGS. 7A and 7B  are side and ends views, respectively, of one embodiment of tubing  116  for distribution system  114 . Tubing  116  may have a semi-circular cross-section. Tubing  116  may include openings  118  for the passage of liquid sweat from tubing  116  to wicking layer  36 . A flat side  120  of tubing  116  may face inward toward human  24 . Tubing  116  may be disposed so as to lie on top of wicking layer  36 , or be partially or completely embedded in wicking layer  36 . Wicking outer layer  36  may also be a surface treatment, for example, a liquid or spray that may be applied to an outer surface of impermeable inner layer  32  thereby enabling the outer surface to wick, spread, and/or distribute water over regions of the outer surface. 
     In  FIG. 1 , outlet header  110  ( FIG. 6A ) may exit layer  32  at opening  112  (shown in dashed line) in the neck area and may fluidly communicate with tubing  116   a  disposed around the bottom of head covering  16 . A vertical tubing  116   b  may lead to a tubing  116   c  that may be arranged circularly or circumferentially (partially or completely) around the top of head covering  16 . A check valve (not shown) may be included in vertical tubing  116   b  to prevent backflow. A tubing  116   d  may extend from tubing  116   a  down sleeve  90  of garment  10 . A tubing  116   e  may extend from tubing  116   a  down torso portion  104  of garment  10  to a waist tubing  116   f . Waist tubing  116   f  may be arranged circumferentially (partially or completely) around garment  10 . Vertical leg tubing  116   g  may extend from waist tubing  116   f  to a circumferential thigh tubing  116   h . Of course, tubing  116  may be arranged in many different ways on the exterior of garment  10 . In addition, garment  10  may include plumbing and valves configured to distribute harvested sweat to hotter surfaces where sweat evaporation may occur most effectively. Toxic environments of microbes, viruses and tiny insects, etc., may require check valves with enhanced sealing features. Such check valves may require higher opening pressures. Higher opening pressures may be supplied by, for example, a piston or pump driven by a battery-operated, electric motor or solenoid. 
     Wicking layer  36  may receive liquid sweat that may exit openings  118  in the network of tubing  116  that forms distribution system  114 . Wicking layer  36  may be present wherever impermeable layer  34  is present, or may be selectively used. In  FIG. 1 , wicking layer  36  is shown with Xs and may be present in areas near tubing  116   a - h.    
     In some embodiments, garment  10  may include one or more external reservoirs  122  ( FIG. 1 ). In  FIG. 1 , the locations and sizes of external reservoirs  122  on garment  10  are exemplary only. External reservoir(s)  122  may be of varying capacity. An example of a capacity for external reservoir  122  is 2 liters. External reservoir  122  may be made of any material capable of holding water, for example, plastic or rubber. Reservoir  122  may be flexible or rigid. Reservoir  122  may be attached to the outer surface of garment  10  using, for example, straps or hooks. Reservoir  122  may contain water  124  and may include a fill opening for adding water therein. External reservoir  122  may be disposed exterior to impermeable layer  32  ( FIG. 2 ). External reservoir  122  may be fluidly connected to one or more reservoirs located interior to layer  32 , for example, internal reservoirs  62  ( FIG. 3 ),  88  ( FIG. 4 ), or  100  ( FIG. 5 ).  FIG. 11  is a schematic fluid flow diagram of garment  10  showing external reservoir  122  connected by tubing  126  to, for example, interior reservoir  62 . Flow of water  124  from reservoir  122  may be controlled by, for example, a valve  128 . 
     Thus, one or more of liquid sweat  40  ( FIG. 2 ), condensed sweat  42  ( FIG. 2 ) and water  124  (from reservoir  122 ) may be pumped from reservoirs internal to layer  32  to outlet tubing. As an example, in  FIG. 11 , the contents (which may be one or more of liquid sweat  40 , condensed sweat  42 , and water  124 ) of interior reservoir  62  may be pumped through outlet tubing  66 . In addition to pumping the contents of reservoirs internal to layer  32  to external distribution system  114  ( FIG. 1 ), some or all of the contents of the internal reservoirs may be redistributed in space  30  or on undergarment  28  ( FIG. 2 ) for re-evaporation, which may enhance cooling. Distribution in space  30  or on undergarment  28  may be helpful, for example, when garment  10  is initially donned, when the user is under-hydrated, or when the user is not sweating adequately. Inadequate sweating may result from, for example, medications taken by the user of garment  10  to resist the neurotoxic effects of chemical agents. 
     Redistribution in space  30  or on undergarment  28  may be accomplished by providing one or more fluid exit ports  130  ( FIGS. 6A and 6B ) in one or more outlet tubes or headers, such as outlet tubes  66 ,  78 ,  108  and header  110 . Fluid exit ports  130  may include mini or micro nozzles for spraying one or more of liquid sweat  40  ( FIG. 2 ), condensed sweat  42  ( FIG. 2 ) and water  124  onto undergarment  28  and/or in space  30 . Ports  130  may be sized such that a portion of the flow through the outlet tubes or headers is redistributed on skin  26  and a portion of the flow is transported to external distribution system  114 . Alternatively, separate outlet tubings may be provided from the internal reservoirs for each of: (1) flow to the external distribution system  114 ; and (2) flow to be redistributed on undergarment  28  and/or in space  30 . 
     The maximum perspiration rate for a human may be about 1.5 liters per hour. The size and capacity of the reservoirs, pumps, bladders, inlet tubing, outlet tubing, outlet headers, and distribution system tubing may be determined, for example, from the maximum perspiration rate and the number and location of pumps used. 
     Two-stage evaporative cooling garment  10  may be more efficient under certain temperature conditions. For example, garment  10  may be particularly effective for cooling when the ambient (external to garment  10 ) wet bulb temperature is less than the temperature of impermeable barrier  32 , and the temperature of impermeable barrier  32  is less than the temperature of skin  26  ( FIG. 2 ). Even when temperature and humidity conditions may be less than optimal for functioning of garment  10 , garment  10  may, nevertheless, provide important advantages. For example, removal of sweat condensate  42  and/or unevaporated sweat  40  from the interior of garment  10  reduces humidity in air space  30 , thereby enhancing first-stage evaporation (from skin  26  or undergarment  28 ). Also, condensed sweat  42  and/or unevaporated sweat  40  that may accumulate inside a garment may cause skin  26  of human  24  to become very soft and perishable. Removal of the sweat helps reduce damage to skin  26 . 
     Thermal physiological modeling of two-stage evaporative cooling indicates that physiological heat strain may be reduced.  FIGS. 8A-B ,  9 A-B, and  10 A-B are graphs of core temperature ( FIGS. 8A ,  9 A,  10 A) and physiological strain index (PSI) ( FIGS. 8B ,  9 B,  10 B) versus time, with and without two-stage evaporative cooling. PSI reflects thermal-work strain (i.e., increases in both body temperature and heart rate). The graphs were created from a computer simulation of a human walking while clad in two different ensembles and breathing filtered outside air. One ensemble is a MOPP-4 (Mission Oriented Protective Posture-Level 4) suit without two-stage evaporative cooling. A second ensemble (labeled as ACP2E) is a two-stage evaporative cooling garment  10  with a standard U.S. Army Combat Uniform (ACU) as undergarment  28 . In  FIGS. 8B ,  9 B, and  10 B, a physiological strain index (PSI) of  10  corresponds to maximum permissible core temperature (T C ) and heart rate (HR). In practice, a maximum PSI of 8 is more desirable. 
       FIGS. 8A-B  assume no direct sunlight, ambient temperature of 20 degrees C., relative humidity (RH) of 50%, and a dew point of 9.5 degrees C.  FIGS. 9A-B  assume no direct sunlight, ambient temperature of 25 degrees C., relative humidity of 38%, and a dew point of 9.5 degrees C.  FIGS. 10A-B  assume no direct sunlight, ambient temperature of 30 degrees C., relative humidity of 28%, and a dew point of 9.5 degrees C. Compared to the MOPP-4 ensemble, the ACP2E ensemble shows substantially extended safe exposure times and a reduction in PSI (i.e., thermal-work strain) of about 40% ( FIG. 8B ), 35% ( FIG. 9B ), and 25% ( FIG. 10B ), respectively. 
     Tests were conducted with a stationary sweating thermal manikin wearing a commercially available chemical protection suit (Blauer Multi-threat Ensemble, Blauer Manufacturing Company, Boston, Mass. 02215). The chemical protection suit was modified for water distribution on its outer surface. The modification included a thin wicking fabric bib and related tubing to distribute water over chest, abdomen and groin areas. The wicking bib system provided an evaporating water surface over about 27% of the suit area. In a climate chamber environment of 95° F. and 40% RH, the wicking bib system increased cooling by 119 watts, compared to cooling without the bib. With 80% of the suit wet, the potential cooling increase is estimated to be about 340 watts. The manikin tests further demonstrate the cooling capability of the two-stage evaporative cooling apparatus and method. 
     The simulation results of  FIGS. 8B ,  9 B, and  10 B and the manikin test results indicate that, at least for the test conditions, the ACP2E garment enables unlimited safe exposure times, compared to safe exposure times of about 180 minutes ( FIG. 8A ), 130 minutes ( FIG. 9A ), and 110 minutes ( FIG. 10A ) for the MOPP-4 ensemble. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention, as expressed in the appended claims.