AIR-CONDITIONING SYSTEM

Learning Objective: Identify the operating principles and components of an airconditioning system.

The air-conditioning system consists of two subsystems: refrigeration and cabin temperature control. These subsystems and their components are discussed in the following paragraphs.

REFRIGERATION SUBSYSTEM

The refrigeration subsystem, shown in figure 4-6, consists of two physically separated packages: refrigeration and cabin air/water separator. The refrigeration subsystem operates on bleed air, which is temperature and pressure regulated. Passage through the ram air-cooled heat exchanger reduces the bleed-air temperature to

Figure 4-6.-Refrigeration subsystem schematic. 4-9

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 highpressure 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 highpressure 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 32F, 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.