1. Field of the Invention
The present invention relates to powered air-purifying respirators (PAPRs). More particularly, the invention relates making PAPRs safe for use in explosive atmospheres.
2. Description of Prior Art
Air purifying respirators have an air-purifying filter, cartridge, or canister that removes specific air contaminants by passing ambient air through the air-purifying element. Powered air-purifying respirators (PAPRs) are air-purifying respirators that use a battery (preferably a rechargeable battery) to supply power to a blower to force ambient air through an air-purifying element to a headpiece. The headpiece forms a protective barrier between the user's respiratory tract and the unfiltered ambient air. The objective of such respirators is to protect the health of the user and to control diseases caused by breathing air contaminated with harmful dusts, fogs, fumes, mists, gases, smokes, sprays or vapors.
However, dusts and gases often create an explosive atmosphere, in addition to the respiratory issues described above. Equipment for such explosive atmospheres must be designed, installed, operated and maintained according to certain additional standards and regulations. Battery powered equipment, such as a PAPR, is of particular concern in areas that have an explosive atmosphere because batteries are a potential source of ignition energy in an explosive atmosphere. This is because batteries produce power by chemical reactions that are capable of delivering large amounts of energy in relatively short periods of time. For instance, a standard AAA-size 1.2 volt Ni-Cad battery stores about 250 milliamp hours of energy (about 1080 joules). Such a battery is capable of discharging around 6.5 amps at 1.2 volts (7.2 joules/sec) when short circuited, which is more than enough energy to ignite an explosive atmosphere. Thus, battery powered equipment for use in explosive atmospheres must have designs that address the battery as a potential source of ignition energy.
Historically, PAPRs have been made safe for use in explosive atmospheres by using a resistor circuit in series with the battery to limit the current (power) flow to a level lower than required for creating a spark capable of igniting the explosive atmosphere. Even in a short circuit, the resistor circuit would limit the current (power) flow from the battery to a level less than required to create an ignition of the explosive atmosphere.
However, while the resistor circuit does provide a solution for making PAPRs safe, several problems exist with the use of such circuits. For example, the resistors consume power even under normal operation of the PAPR, and, thus, reduce the length of time that the battery pack can power the blower before requiring recharging or replacement. Further, the resistor circuit drops the voltage available to the blower motor, which for a DC motor will decrease the speed of the motor and the volume of air forced through the air-purifying element to the headpiece. Still further, it is desired to use batteries that will supply power to a PAPR for a reasonable amount of time, such as four hours, to reduce the frequency of battery changes. Such batteries are larger and have a larger amp-hour capacity, also increasing the physical size of the resistors needed to limit the power flow. The increased size of the resistors poses problems for designers that wish to combine the battery and the resistor circuits in a single battery unit or “battery pack.” Even further yet, increases in the power capacity of the batteries and the size of the resistors also results in increased heat that must be dissipated by the battery and the battery pack during a short circuit event, which also poses additional design challenges. Thus, there is a need for an improved solution for making PAPRs safe for use in explosive atmospheres.
Further, any improved solution for making PAPRs safe for use in explosive atmospheres must also provide sufficient power to properly start and operate the blower of the PAPR.