IMPLICATIONS FOR THE TRAINING OF RESEARCHERS

LEARNING OBJECTIVE: Recognize the components and operating principles of air cycle air-conditioning systems (ACS).

Most naval aircraft are designed with an air cycle ACS because it is efficient for the weight and space required and is relatively trouble-free. The name air cycle or air-to-air comes from the principle of cooling the air without the use of refrigerants by compression and expansion of bleed air. The P-3 air cycle ACS is an example of this type of system.

DESCRIPTION

The P-3 air-conditioning system is comprised of two independent air cycle cooling systems of identical capacity, each with its own temperature control system, and fresh air sources. Fresh air sources are comprised of two engine-driven compressors (EDCs) and the air multiplier package (AMP).

Fresh Air Sources

In order for the air-conditioning system to function, air at the proper temperature and flow volume must be available. The fresh air sources are the EDCs and the AMP. The EDCs are single-stage compressors with fully automatic controls. They supply air to the air cycle cooling systems during flight and are operable only when no. 2 and no. 3 engines are running. The EDCs also serve as a secondary air source for the air cycle cooling systems during ground operations. The AMP is the primary air source for the air cycle cooling systems only during static ground operations when the auxiliary

power unit (APU) is running. The AMP interacts with the APU to such an extent that it is referred to as the APU/AMP combination.

EDCs

The P-3 aircraft has two EDCs (fig. 6-1), to supply air to each of the two air cycle cooling systems. During EDC operation, there is no interconnect between the flight station and cabin systems until well downstream in the air distribution and exhaust system. The no. 2 engine EDC supplies air to the right (flight station) air cycle cooling system and the no. 3 EDC supplies air to the left (cabin) air cycle cooling system. The duct crossover is in the APU compartment and allows the ducts some flexibility for expansion. The EDC is mounted to a drive pad on the left side of the engine reduction gearbox assembly. The EDCs are adjusted for a maximum power requirement of 81 horsepower (hp) to deliver 60 pounds of air per minute at sea level.

APU/AMP

The APU/AMP combination supplies air to the air cycle cooling systems during ground operation only. It

6-3

Figure 6-1.—EDC view from left.

serves as a single source of air with a flow rate equal to that supplied by the two EDCs. With the engines operating at normal revolutions per minute (rpm), each EDC supplies air to its respective air cycle cooling system at the rate of approximately 60 pounds per minute (lb/min). The APU/AMP combination supplies air to a duct common to both air cycle cooling systems at a rate of approximately 125 lb/min. The air volume divides in the air cycle cooling system interconnection duct, with half going to the flight station air cycle cooling system and half to the cabin air cycle cooling system.

An air-conditioning system that employs a single source of air to supply two air cycle cooling systems that operate at different back-pressures will have air flow problems unless a control is added to balance airflow. If airflow is not properly balanced, the air cycle cooling system with the lower back-pressure (as the result of more air bypass) will rob air from the unit with the higher back-pressure. Two flow-limiting venturis

are used to balance airflow when the APU/AMP combination is the air supply source. Figure 6-2 shows an AMP installation.

COMPONENTS

Components include a heat exchanger package, turbine refrigeration unit, water separator, water spray system, and a flow-limiting venturi.

Heat Exchanger Package

The function of the heat exchanger package (fig.

6-3) is to reduce the temperature of the supply air furnished by the EDC or AMP. Two heat exchanger packages, each consisting of a primary and secondary section, electric fan assembly, check valve, and ram air duct check valve are installed on each side of the nose wheel well. The left heat exchanger package supplies air for the cabin systems and the right package cools flight station air.

6-4

Figure 6-2.—AMP installation.

The heat exchanger unit is constructed of a brazed core, which contains a series of metal plates separated by layers of fins that form a passage for cooling air and separate passage for supply air.

During ground operation, the fan assembly installed on the heat exchanger forces ambient air through the heat exchanger and overboard. A check valve, installed at the fan outlet, directs ambient airflow in the heat exchanger. Another check valve, located in

the ram air inlet duct, closes, preventing ambient air from spilling overboard through the ram air duct check valve. The heat exchanger check valve closes, and the ram air is used to cool the supply air.

Turbine Refrigeration Unit

Each air cycle cooling system has a turbine refrigeration unit (fig. 6-4) installed on each side of the

6-5

Figure 6-3.—P-3 air-conditioning system schematic diagram.

Figure 6-4.—Turbine refrigeration unit.

aircraft nose wheel well. The refrigeration unit, along with the secondary section of the heat exchanger, lowers the temperature of supply air so that it may be used for aircraft cooling. The refrigeration unit consists of a rotating assembly and a housing assembly. The rotating assembly mounts a turbine scroll and a compressor scroll, enclosing the rotating assembly. The bottom of the bearing support housing forms a sump for lubricating oil. A sight gauge is provided in the sump for determining the level of lubrication oil. Each of the three sections formed by the housing assembly is sealed to prevent air and oil leakage.

Compressed supply air, after it has passed through the primary section of the heat exchanger, is ducted into the compressor section of the turbine where it is further compressed as it passes through the compressor scroll.

The compressed air, with a temperature slightly above that of ambient air, is then routed to the secondary section of the heat exchanger, where it is cooled to a lower temperature. Returning from the heat exchanger, the compressed air enters the turbine scroll, expanding as it flows from the nozzle through the turbine wheel to the outlet duct. As the air is expanded, it drives the turbine wheel at high speed. Mechanical energy, which is extracted from the air to drive the turbine wheel, is transmitted to and absorbed by the compressor wheel.

This mechanical energy reduces the supply air pressure

and temperature to the point where the air becomes usable for aircraft cooling.

Lubrication of the rotating assembly bearings is accomplished by an air-oil mist. Oil is absorbed by wicks, which extend from the oil sump to the shaft of the rotating assembly. Oil is distributed on the rotating assembly shaft as a result of capillary action in the wicks. Rotation of the shaft causes the oil to diffuse into an air-oil mist. The action of the oil slingers causes the air-oil mist to pass through the bearings, providing lubrication.

Water Separator

The water separator (fig. 6-5) removes moisture from the air before it is distributed within the aircraft.

Two water separator units are installed in the APU compartment. The cabin system unit is located in the aft upper left section of the APU compartment and the flight station unit is located in the forward upper right section.

The water separator consists of a condenser assem-bly and a collector assemassem-bly. The condenser assemassem-bly is a coalescer. An ice-limiting sensor is installed in the inlet section of the unit to protect against icing and a check valve is installed in the water separator outlet to prevent reverse airflow through the unit.

6-6

Figure 6-5.—Dual check valve, water separator, and ice-limiting sensor.

As supply air passes through the coalescer, moisture particles are condensed into droplets. After the air has passed through the coalescer, hundreds of small vanes create a swirling motion of the air and the airborne water droplets. This swirling motion centrifuges most of the water droplets from the air into the coalescer sump, where the water accumulates and drains overboard. The air, relieved of approximately 70 percent of its moisture, is then ducted into the aircraft and distributed.

Water Spray System

The water spray system increases basic cooling capacity of the air cycle cooling system by spraying water separator discharge water into the ram air, cooling it by evaporation before the ram air passes through the heat exchanger’s secondary section.

Flow-Limiting Venturi

Each air cycle cooling system has a flow-limiting venturi installed in the left and right sides of the APU

compartment in the air distribution duct between the AMP and the EDC air ducts. The venturi is sized to limit airflow to 67 lb/min from the AMP to the air cycle cooling system to ensure proper flow division and to prevent excessive flow through the aircraft during the heating mode. It functions to limit flow through the refrigeration unit in the event the other refrigeration unit is operating at a different bypass setting; that is, one refrigeration unit is in maximum cooling while the other is modulated toward heating. A check valve is located in the outlet of each venturi to prevent reverse EDC airflow through the venturi.

CABIN AND FLIGHT STATION