Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R43-OH-009191, 2009 Jan; :1-41
Wearing level A or B protective suits, the HAZMAT worker is particularly vulnerable to heat stress which results in reduced worker productivity and increased injuries, illnesses, and even death. The program's overall goal is to develop a high-performance personal cooling system, optimized to safely protect workers from heat stress, fatigue, and injury. Presently, existing systems are too large, heavy, and inefficient which precludes wide user acceptance. Existing control systems limit the efficiency of these systems because delivery of cooling to the user is suboptimal, resulting in overcooling or undercooling the user. This Phase I effort demonstrated the feasibility and benefits of using several new control approaches with a mobile personal cooling system based upon miniature vapor-compression (VC) refrigeration technology. Benefits include delivery of cooling that is more effective for removing heat from the user's body and increases the overall efficiency of the system - enabling longer mission durations and a smaller, more lightweight unit. Different control approaches were tested using a mobile cooling system which delivered cooling to a thermal manikin by means of a liquid-cooled tubesuit vest. Continuous control (CC) approach was used as a baseline - delivering constant water flow at a fixed supply temperature of 55°F to the vest and maintaining a manikin 'skin' temperature of 82°F. The manikin temperature of 82°F is consistent with previous physiological tests by other researchers using the original CC approach and mobile cooling system. 82°F represents an actual skin temperature at which the skin is overcooled, resulting in severe vasoconstriction which causes significantly increased thermal resistance within the body and a reduction in actual body heat removal by the cooling garment. Tests were conducted for increasing manikin skin temperatures of 82°F (baseline) and 92 and 95°F to imitate gradual reductions in overcooling, skin vasoconstriction, and body thermal resistance. Corresponding to increases in manikin temperature, water delivery temperature setpoints were 55 (baseline), 65, and 68°F, respectively, to result in a constant manikin heat rejection. The results demonstrate that, by avoiding overcooling of the user along with a new control approach called alternating flow control (AC), up to 54 percent increase in system efficiency can be achieved. The AC control approach changes the direction of water flow through the vest cooling system every 2 minutes. The increased efficiency reduces power draw by 35 percent; thus, for a given battery energy capacity, mission duration increases by 54 percent. Additionally, compressor speed dropped by 39 percent. Similar gains, but to a lesser degree, can also be achieved using the continuous control approach (CC) for the higher manikin skin temperatures. Results from a third approach, pulsed control (PC) -- which turns the cooler on and off every 2 minutes -- were inconclusive. Designers can use these findings to optimize the system and improve its acceptability to workers. The findings improve performance of the system while, at the same time, reduce system requirements so that smaller, more lightweight and efficient systems may be developed. To achieve these gains over a range of operating conditions, a greater level of control resolution (i.e. besides one or two water delivery setpoints) and better feedback from user (i.e. manual or automatic from physiology) is needed so that overcooling (resulting in low temperatures) can be reduced or eliminated. In Phase II of this program, the control approaches will be further developed through physiological testing of HAZMAT workers, and a smaller, more lightweight cooling system will be developed, optimized, and integrated within the HAZMAT PPE. It is anticipated that the NIOSH National Personal Protective Technology Laboratory (NPPTL) will be involved in much of this effort, since increasing human performance and safety and developing fully-integrated, intelligent ensembles for workers is one of the laboratory's strategic objectives.
Mr. Glenn Deming, Aspen Systems, Inc., Advanced Thermal Division 24 St. Martin Drive Marlborough, MA 01752
Aspen Systems, Inc., Marlborough, Massachusetts