The
two air condition packs provide “conditioned” air (temp and moisture) to the
air condition system. They are supplied with bleed air from the bleed manifold,
normally from the APU or the high pressure compressor of the engines. Hot air
flowing into the packs has a temperature of ± 199 – 227°C, controlled by the
engine BASOV and the pre-cooler. To get to the normal temperature range of 18 –
30°C of the AC system, it needs to be cooled which is accomplished by the two packs.
Let’s
look at the flow of air through a pack explaining the components the air
passes. Air enters the pack through the PACK FLOW CONTROL VALVE and can go in
two directions, into the cooling circuit or it by-passes the so called ACM (air
cycle machine) circuitry.
The
cooling cycle starts at the “primary” (air–to–air) heat exchangers that cools
the air. The heat exchanger works like a car radiator but exchanges hot bleed
air to the ram air duct airflow. On the ground by the way, duct airflow is
created by a fan which is driven off the ACM. Next the air “hits” the
compressor of the ACM which turns the compressor and turbine resulting in an
increase of pressure and temperature. Another “secondary” heat exchanger cools
the air again, where after the following components in the manifold are to
“create” and extract water from the air. A “secondary” water extractor drains
water from the manifold, a re-heater pre cools the air before it enters the
condenser and warms the air (from the 2nd water extractor) before it enters the
turbine to increase efficiency of that turbine operation. The condenser creates
water droplets in the air where after the “primary” water extractor removes
water by creating a swirling motion, “centrifuging” that water to the outer
collector wall where it is collected and relieved in the ram air duct adding in
cooling at the heat exchangers. The last component of the pack is the expansion
turbine which can cool the air to below the freezing level as a function of
very fast expansion (extracting energy) of the air. Finally the air is mixed
with by-pass air representing the pack control requested output temperature.
The
pack is protected against a request of a too high demand of cool air, thereby
overloading that pack. This is accomplished by several temperature sensors
detecting over temperatures in the cooling cycle causing the pack to trip off
line. If this happens, it closes the pack flow control valve indicated by the
PACK OFF indication on the bleed panel together with a MASTER CAUTION light
(AIR COND). The solution to this problem is to let the pack cool down and
selecting a warmer temperature before resetting and “unloading” the tripped
pack or else the overload/overtemp will occur again.
Water
is extracted from the air to protect the avionics in the aircraft against
moisture, to prevent mold to form and against oxidation of metal components.
