SUPERCRITICAL AIR MOBILITY PACK (SCAMP®) DESCRIPTION
The SCAMP® Self Contained Breathing Apparatus (SCBA) was originally developed under NASA Kennedy Space Center (KSC) SBIR Phase I and II contracts to produce a compact breathing apparatus which delivers air independent of tank attitude and eliminates oxygen enrichment during tank loading and storage. The KSC rescue personnel in a launch pad emergency must be able to crawl through a 20" (51 cm) square opening. Therefore a compact air system is needed which has a real lifetime in excess of 30 minutes and a backpack thickness of less than 5 1/2 inches (14 cm).
The Air Force later provided SBIR Phase I and II support to use the supercritical cryogenic storage of air in an SCBA to provide personnel cooling as well as breathing air in a closed firefighter's suit. The cooling provided by the cryogenic air flow will range from 100 to 800 watts depending on air flow rate. The cooling that is provided for the person will be heat absorbed by the cryogen exiting the tank. Because the person will breathe more as they exert more effort, the air flow rate which provides cooling will increase at approximately the same rate as the cooling requirements. These systems are shown in Figure 1.
The use of supercritical cryogenic air in the SCAMP came from the technology developed for the life support and fuel cell support systems on both the Apollo lunar program and the space shuttle. This particular fluid recovery system which works in the low-gravity of space will produce a flow at any attitude of the supply tank in the gravity of earth. The SCAMP® SCBA system application leads to a use of space technology for groundbased purposes at NASA, and the potential of use for commercial purposes in emergency services, industry, and recreation.
The heart of the SCAMP® SCBA system (Fig. 1) is the dewar which contains the supercritical air. The SCAMP® SCBA one-hour dewar is approximately the size of a standard 30 minute SCBA compressed air bottle but stores twice the quantity of air due to the high density. A two hour unit increases the dewar diameter and thus backpack thickness by about 40%. The majority of the weight increase of 20% is air. The top of the dewar has a plumbing manifold with disconnects, relief valves, quantity sensor readout, and pressure gauge. The quantity sensor is a capacitance type as the single phase storage will not have a liquid-vapor interface. The external fin heat exchangers on the dewar provide the heat for expulsion of the air from the dewar. For use the dewar is placed in a backpack which contains quick disconnects for the dewar and mask, the pressure regulator(s), and the fluid conditioning heat exchangers. The backpack provides an interface between the user and the dewar and is used to protect the extended components. A special loading system is used to hold the dewar during filling and to cool the air from the ambient temperature provided by the compressor/purifier down to cryogenic temperatures (about 90 K) using liquid nitrogen.
By maintaining the air in the supercritical condition the air does not become liquid at any time during the operation of the SCAMP dewar. This has several advantages on the operation of a cryogenic air system. The first of these is that when the fluid is supercritical it is always a single phase fluid. This single phase means that the air state is identical at any point in the pressure vessel (PV) and thus the dewar may be operated in any attitude and still provide the same delivery of air to the mask. The second is that the air does not change composition when it is stored for a long period of time, as it does when liquid (two-phase) air is used. When liquid (two-phase) air is stored, the natural heat load from atmospheric temperature to cryogenic causes the more volatile nitrogen to boil off first and thus leaves an oxygen enriched liquid in the tank. The single phase fluid loses oxygen and nitrogen in proportion; since there is no separation effect in a compressed gas the boil-off will not change the mixture ratio of the stored or supplied air. The supercritical cryogenic fluid expands to fill the bottle that it's stored in and thus delivers from a single point exit in any bottle attitude.
There is a penalty exacted for the increased use time or decreased bulk with the cryogenic storage in that the air bottle must be refilled on a regular basis (every 24 hrs) if it is to be ready for immediate use.
Breathing Only Dewar
Breathing Only Pack
Figure 1: Supercritical Air Mobility
Pack Breathing, Cooling, and Loading Systems
The loading system for use with the SCAMP dewar is as shown in Figure 1. The loading system requires a supply of clean, dry air. This may be from a cascade system of high pressure bottles or from a compressor which is used to fill any SCBA. The grade of air depends upon the purification system which is built into the compressor and should be a minimum of Grade D, or preferably Grade E. The air pressure into the loading system should be about 1000 psi (7 MPA or 70 atm) to allow for an adequate airflow. This keeps the loading process in a purely supercritical regime and thus does not allow the compressed air to separate or change composition during the dewar loading. A source of liquid nitrogen is required to cool the air to cryogenic temperature as it is loaded into the dewar.
After loading, the SCAMP dewar may be removed from the loading system, and either stored separately or placed in the backpack. For use in the KSC environment, which requires breathing air only, the SCAMP dewar would be mounted in a backpack as shown in Figure 1. The backpack provides a mounting interface to the user as well as providing protection for the dewar and hardware.
During the development of the SCAMP SCBA for NASA it was obvious that there was an opportunity to provide cooling for the user as well as breathing air. The source of the cooling for the user will be the heat exchangers which provide the heat for conditioning the air to breathing temperature. In these heat exchangers the fluid is brought from cryogenic temperature to just below ambient temperature by the addition of heat from some source. In the NASA/KSC (breathing air only) configuration this heat is supplied by free convection from the atmosphere; in the cooling configuration this heat will be supplied by the body of the user. Since the air temperature, into the heat exchangers from the dewar, starts at near liquid nitrogen temperature (-160°C, 90 K or -300° F), there is a large amount of cooling available for the human body. This will provide cooling to the user, while supplying the heat which is required by the SCAMP dewar. Maximum cooling will be necessary in a hot environment and fire conditions whereas in cold weather the user may become too cold when the activity level is low.
The cooling is transferred from the cryogenic air as it flows from the dewar to provide breathing air and is transferred from the heat exchanger to the person using a non-toxic antifreeze-water solution.
The total energy which a person can produce metabolically and which must therefore be removed by a coolant system depends upon both the person and the activity being performed. Goldman gives values of metabolic heating for firefighters as shown in Figure 2. The heat energy which must be removed by the coolant, during rest and exercise, is the metabolic heat release reduced by work performed and modified by environmental conditions. Human physiology gives a relationship between air consumption and energy produced as a metabolic energy production rate of 284.3 Btu/lbm (183.6 watt-hr/Kg) of air used. For a NIOSH rated air use of 40 SLM (2.932 Kg/hr, 6.466 lbm/hr) this corresponds to a metabolic energy release of 537.3 watts.
The summary of the cooling capability of the SCAMP SCBA and the cooling deficit is shown in Figure 3. This shows the cooling deficit with and without cooling. By 20 minutes the SCAMP system has over-cooled the user by about 20 Btu/lbm/hr (32.6 kcal at 40 SLM) and at the end of the hour the user has been under-cooled by approximately the same amount. For a 180 lb (82 Kg) user with the rated consumption of 40 SLM this will amount to less than one-half (0.5) degree Celsius (one (1) degree Fahrenheit) rise in the core body temperature of the user in the hour. Without cooling, the user will nominally have a cooling deficit of 83.3 watts for each one lbm/hr air of flow rate. For the NIOSH rated flow of 40 SLM this totals 539 watts or summed over the hour the user has generated 483.5 kcal (1,838 btu) which must be stored (temperature rise of 5.9°C) or dissipated. The final analysis shows the SCAMP cooling removes 94% of the metabolic energy produced with a final deficit of 32.6 kcal.
Higher or lower use rates of the breathing air will change the rate at which heat builds up in the user's body but not the final cooling deficit. If the user breaths at 80 SLM the deficit rate will double by shortening the time period of Figure 4 to 30 minutes but the final cooling deficit will remain 32.6 kcal. Only by doubling the lifetime using a NIOSH two hour version of the SCAMP would the cooling deficit
Figure 2: Heat Production in Fire
Figure 3: Cooling Deficit Compared to
be doubled. This is the cooling deficit experienced in an impermeable suit in a 15 minute time period at a moderate activity level (40 SLM air use). This SCAMP cooling is far better than any of the current phase change material, such as ice or blue ice devices in which the coolant material is stored in the user's vest and receives heat either by circulation or conduction. Those units are limited both in cooling rate and total absorption of energy. In addition, the phase change units add to the weight of the systems which the user must carry.
The SCAMP cooling system is an outstanding improvement in the life support field. Because the SCAMP cooling source is the same air which is being used for breathing, nothing additional is required to provide the cooling. The SCAMP backpack with cooling will be approximately the same weight as without cooling due to reduction of heat exchangers, but increases in weight of mechanisms such as the pump and coolant reservoir. Weight of a two-hour system will not increase proportionally to the weight of air, because of the fixed weights.
The backpack consists of a laminated composite case to carry and protect the mounting bridge, heat exchangers, and other components for body cooling giving a total weight for a one hour rated (40 SLM) of approximately 25 lbs (11 Kg). This is lighter than the current high pressure one-hour systems when combined with a phase change cooling system, as well as being both thinner and more compact. A two hour SCAMP bottle has also be calculated. The projected weights for these configurations are given in Table I. The most certain of the weights are the current one-hour SCAMP SCBA because the prototypes have been built and weighed. Credit is given in this table for future weight savings based on designs in progress. The weights given are a combination of fixed numbers for the fluid, and calculated numbers for the pressure vessel and outer shell portion of the dewar and for the backpack and components.
The cooling system will keep a user's body temperature within one degree Celsius of normal for a period of four hours at rest, two hours at a moderate activity level, or one hour at a high activity level without having to resort to exterior sources of cooling. This is also the approximate limit of air which can be carried (at the current time) without exceeding the 35 pound (16 Kg) backpack weight limit imposed by military standards and NIOSH. To minimize dehydration, provide for maximum comfort, and to allow a person to stay in a hazardous or contaminated area for the longest possible time this cooling system to limits sweating due to the reduction of the skin temperature with the cooling . Placing the cooling suit next to the skin or separated only by underwear achieves the maximum rate of cooling; however wearing the cooling suit over normal clothing is still effective, particularly with high environmental heat loads.
TABLE 1-I: Comparative Weights of Different Configurations
LIFETIME DEWAR PACK FLUID TOTAL
1 18 10 7 35
11 7 7 25
7 11 7 25
BREATHING/COOLING 2 10 11 14 35
The current configuration of a cooling suit is shown in Figure 1. The SCAMP upper body loop is the higher capacity loop and provides the cooling for the torso and head . The lower body loop will be the primary coolant for the legs and hips. The body cooling suit is connected to the backpack with stainless steel braid covered teflon lines. For lower levels of cooling the suit may be reduced to the upper body or even just the torso. This covers the region of maximum heat production at rest and during low levels of activity. The standard suit has a nominal capacity of 275 watts, but a special suit has been developed for the U.S. Air Force which has a capacity of 600 watts. The possible cooling suit configurations are as various as the needs for cooling.
When Niwot Technologies first described the SCAMP to marketing it was seen as being valuable in the commercial market. A need was seen in the emergency services for a lighter and/or a longer lifetime SCBA. The longer time provides a greater margin of safety so that a First Responder, firefighter or haz-mat worker can work longer and still have a quarter hour of air after the low level alarm. This would aid in both the reduction of damage and increasing the possibility of life saving operations. The addition of body cooling is essential in both the haz-mat industry where the worker is severely limited in the time which can be spent in a Class A suit. Class B suits are less confining but are still impermeable and confining. Fire suits are yet less confining but are highly insulating and the environment is often extremely warm. In all of these emergency response uses the presence of body cooling will be beneficial in the reduction of the debilitating features of heat stress. Recovery time will be reduced from several hours for increased body core temperature to basically the time to recover from the fatigue of exercise.
The SCAMP can be used anywhere that a standard compressed air SCBA can be used. However its unique capabilities will be as a lower profile system (KSC use) where space or clearance distance is critical or as a longer life unit for particular situations. First Responder applications in large or high-rise buildings, in tunnels, and on shipboard are seen as particular applications of a two hour or greater bottle. The SCAMP with cooling may be used in Haz-Mat working and cleanup to reduce the number of times a worker will have to leave the work area, and reduce the amount of equipment which must be decontaminated. For the military use, sealed chemical warfare suits need to include both air supply and cooling. Also there might be a particular application for damage control where the area is large and contains toxic and radioactive materials.
There appears to be a special and extremely important use in the field of counter-terrorism. Decontamination of chemical and biological sites could be done much more rapidly and efficiently with much less risk to the personnel using SCAMP than with existing self-contained breathing apparatus. The time which can be spent in protective clothing is normally now limited to approximately 20 minutes due to heat exhaustion and dehydration. With SCAMP the time will be more than doubled, and the user will be in better physical condition at the end of the time period.
There will be a major use in industrial applications where high temperature work areas are is a problem such as nuclear facilities, chemical plants, and foundries.
Multiple domestic and foreign patents on this technology have been applied for and are pending. Three U.S. patents (5,582,016; 5,709,203; and 6,089,226) have already been granted and one is pending.