within a few degrees of ambient air temperature. Further temperature reduction results from expansion at the turbine. Condensation is removed in the water separator. System Operation Regulated hot bleed air from the bleed-air supply subsystem enters the refrigeration unit heat exchanger, where it is cooled to within a few degrees of ram-air temperature. The cooled high- pressure bleed air enters a radial flow turbine, where it expands to approximately cabin pressure. The power output of the expansion turbine drives an axial-flow cooling air fan. A substantial temperature drop occurs in the expansion of high- pressure air to cabin pressure (165 psi bleed-air to 15 psi cabin air), which results in air temperatures well below ram-air temperature. Depending upon the cool air temperature and dew point, a portion of the water vapor in the air condences as small droplets. A water separator is installed downstream from the turbine discharge to remove between 50 and 70 percent of the moisture in the cooled air. If the turbine discharge air is also below 32°F, the water vapor condenses as ice crystals. Potential icing and blockage are eliminated by the nonice and low-limit control valve, the ice screen, and the mixing muff. The nonice and low-limit control valve senses any pressure drop through the ice screen. If ice accumulates, the nonice and low-limit control valve admits turbine bypass air into the mixing muff to increase air temperature above icing conditions. Ram cooling air for the heat exchanger flows through the heat exchanger core. The turbine shaft drives the fan, which pulls the ram air through the heat exchanger and discharges it overboard through the heat exchanger exhaust duct. Components There are nine basic components in the refrigeration subsystem. Each of these components is discussed in the following paragraphs. The relationship of the items is shown in figure 4-6. TURBINE AND FAN ASSEMBLY.— The turbine and fan assembly (fig. 4-7), which is mounted in the heat exchanger upper plenum (7), is a removable component of the refrigeration package. High-pressure, partially cooled bleed air drives the turbine, which is mechanically coupled to an axial flow fan. The fan is used to impel ram air through the heat exchanger and an overboard exhaust duct. Pressure reduction and final heat loss occur as a result of energy loss and expan- sion of bleed air as it passes through the turbine. Wool wicks, with one end submerged in MIL- L-23699 oil, transmit lubricant to the bear- ings supporting the common shaft of the turbine and fan assembly. A sight gauge on the turbine housing is used to check the oil level. Two overtemperature indicators are installed on the turbine. Each sensor probe head holds down a spring-loaded pop-up stem with an eutec- tic solder alloy. If the air in the passage reaches the melting point for the solder alloy, the indicator head pops up and stays exposed to alert maintenance personnel that the cooling turbine has been exposed to an excessive temperature level and needs to be replaced. The probe in the turbine inlet is set to trip at 217°±10°F, and the probe in the fan inlet will trip at 450°±10°F. Obstructions or collapse of the ram air inlet duct is the most likely cause of actuating this indicator. HEAT EXCHANGER CORE, UPPER AND LOWER PLENUMS.— The heat exchanger lower plenum (2) contains the ducting for the ram air inlet and outlet. Cooling ram air (4) flows into the lower plenum and through the heat exchanger core (3) and out through the overboard exhaust duct (6) to the upper plenum, or to the ram air augmentation subsystem (5). The heat exchanger upper plenum, which sup- ports the turbine and fan assembly (1), is mounted on the opposite side of the heater core. Ram air drawn through the heater core for cooling pur- poses is diverted to the heat exchanger exhaust duct through the heat exchanger upper plenum. The heat exchanger core is the air-to-air heat sink, and it uses ram air to cool the bleed-air supply. NONICING AND LOW-LIMIT CONTROL MODULATING VALVE.— The nonice and low- limit control valve maintains conditioned airflow through the water separator by adding bleed air at the mixing muff to prevent water separator 4-10