For more than 100 years, scientists and engineers have been actively involved in the design of self-contained breathing apparatus.
Two series of self-contained breathing apparatus are widely used in fire fighting, open circuit and rebreathers. In an open system, each exhaled breath is discharged into the atmosphere. A rebreather or closed-circuit device recovers the user’s breath, removes carbon dioxide and increases oxygen. Due to their efficiency, rebreathers are light in weight, small in size and long lasting.
The open-circuit breathing system consists of an air supply device, a pressure reducer/demand valve, an exhalation valve and a mask. The air supply in an open circuit system is usually compressed air. The air volume per breath is supplied through the pressure reducer/demand valve, and discharged into the ambient atmosphere after being inhaled.
All rebreathers include a breathing bag as a reservoir for the user’s breath. Because the rebreather removes the carbon dioxide produced by the user and replenishes the oxygen he consumes, the gas breathed is almost 100% oxygen.
Provides three equipment designs for oxygen replacement and carbon dioxide removal: chemical oxygen, cryogenic and compressed oxygen.
The chemical oxygen type device uses a chemically generated oxygen source. The water exhaled by the user activates the superoxide filter, releasing oxygen and forming alkaline salts. This oxygen reaches the user through the rebreather bag. The alkali produced by this chemical reaction removes the next exhaled carbon dioxide and adds more oxygen. Since this reaction cannot be accurately controlled, the device is designed to produce more oxygen than is required for metabolism. This excess oxygen is discharged into the ambient air through the discharge valve.
The main advantage of this simple equipment design is the low initial cost. However, there are some disadvantages. Starting a chemical reaction at low temperatures is difficult and sometimes impossible. The unit cost of chemical cartridges is high. What makes this problem more complicated is that once a chemical reaction starts, it cannot be interrupted. Regardless of need, the entire chemical charge must be used or discarded.
In low-temperature closed systems, liquid oxygen is used. In this very complex system, the exhaled carbon dioxide is removed by freezing, and the low-temperature radiator is provided by liquid oxygen, some of which enters the breathing bag. This extremely complex and expensive system has never achieved commercial success. However, cryogenic gas storage has been widely used in open systems.
The third type of closed circuit system is the compressed oxygen design. In this type of rebreather, the oxygen stored in the cylinder passes through the pressure reducer into the breathing bag, from which the required amount of oxygen is inhaled.
The exhaled gas passes through the carbon dioxide absorber. Here, the carbon dioxide in the user’s breath is removed, and the unused oxygen flows into the breathing bag. Fresh oxygen is added, and the updated breathing gas is delivered to the user and continues to circulate. The simplicity, robustness, and low cost of reuse of such devices have made compressed oxygen respirators popular for many years.
In 1853, Professor Schwann designed a compressed oxygen respirator for a competition held by the Belgian Academy of Sciences. Schwann seems to be the first to realize the potential of rebreathers used in mines and fire departments. At the turn of the century, Bernhard Draeger of Lübeck, Germany designed and manufactured a rebreather. In 1907, the Boston and Montana Smelting and Refining Company purchased five Draeger rebreathers, which were the first devices used in the country. Rebreathers have been widely used in fire services for more than 25 years.
In the past 70 years, many improvements have been made to rebreathers. Through the strict regulations and controls of NIOSH and MESA, today’s devices are more reliable than ever.
Post time: Dec-03-2021
