-- Crew pressure low.
When this indicator lights, it
indicates the crew compartment positive pressure has
dropped below 0.06 psi.
Introduction to the Air Cycle System (fig. 1-3).
The air cycle system takes bleed air from the P3 stage of the AGT-
1500 engine at approximately 700,F and between 15 and 200 psi. The
air system provides filtered air through the following major
Prioritization Regulating Valve.
This valve is a dual
pressure regulating and shut-off type valve (sometimes
called the "bleed air shut-off valve"). This valve reduces
the pressure of the bleed air to 44 ƒ5 pounds per square
inch gage (psig) at tactical idle or 16 ƒ3 psig when the
output of the engine falls below 400 shp.
Precooler. Following the main path of flow, the bleed air
enters the precooler.
The precooler partially cools the
bleed air using cooling (ambient) air as a heat sink, this
is the first stage of cooling.
Pressure Regulating Valve.
This is a single pressure
regulating valve. This valve reduces the pressure to 35 ƒ3
This uses a flow limiting venturi to limit the
amount (cfm) of air flow to the system by reducing the size
of the duct.
The reduced area of the duct is called the
The diameter of the throat determines the amount
of air that passes to the system. The restricting orifice
within the venturi establishes an air flow of 200 cfm.
to the heat exchanger. The heat exchanger further reduces
the temperature of the bleed air, the second stage of
cooling. The heat exchanger reduces the temperature of the
bleed air by passing cooling air across the fins, much the
same as the fins of a radiator used to cool the water in an
The temperature difference between the
cooling air and the bleed air allows cooling to take place.
(Heat always flows from hotter to colder.)
Condenser. The bleed air moves from the heat exchanger to
functions the same as the precooler and heat exchanger; but
use turbine air for cooling. Since air