Crossfeed ducts are installed between the distribution head and the main supply ducts. These are made of aluminum and contain noise-attenuators. The main supply duct to the cockpit is made of aluminum at its interface with the hot trim-air system. An electrically operated back-up flap is installed in this duct. This flap makes sure that sufficient fresh air is delivered to the cockpit in case of a pack 1 failure (Ref. 21-51-00). Noise-attenuators are installed downstream of the hot trim-air interface. The mixing unit and crossfeed ducts are insulated with glasswool and a jacket made of a Hypolon material.
 **ON A/C ALL
 F. Air Recirculation Cabin air from the underfloor area is mixed with conditioned air. This increases the amount of air which is blown into the distribution system. The two recirculation fans 14HG (15HG) installed one each side of the mixing unit do this. Cabin air is drawn through two recirculation filters 4012HM (4013HM) with their appropriate housings 4010HM (4011HM) and blown through two check valves 4020HM (4021HM) into the mixing unit.
 4.
 Power Supply

____________
 (Ref. Fig. 005)

 5.
 Component Description

_____________________
 A. Cabin Recirculation Fans
 (Ref. Fig. 001, 006)

R **ON A/C 001-049, 051-099, 101-149, 151-199, 201-299, 401-499,
 The cabin recirculation fans 14HG (15HG) are installed in line with the recirculation ducts. They are powered by a three-phase six-pole induction motor that drives a fan-wheel which has high-efficiency blades. The fans will operate continuously at about 7700 rpm when supplied with electrical power. Thermo switches are installed inside the stators of the recirculation fans. If the temperature of the stators gets to 160 DEG.C
 (320.00 DEG.F) +/- 8 DEG.C (46.40 DEG.F), the thermo switches isolate the electrical supply to the fans. The recirculation fans are installed on vibration-damper mountings, these prevent damage to the aircraft structure, due to fan vibration. Arrows on the fan casing show the direction of airflow through the fan and the direction that the impellor rotates.

 Cabin Fans - Schematic Figure 005
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 Recirculation Fan Figure 006
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 **ON A/C 301-399,
 The cabin recirculation fans 14HG (15HG) are installed in line with the recirculation ducts. They are powered by a three-phase six-pole induction motor that drives a fan-wheel which has high-efficiency blades. The fans will operate continuously at about 7700 rpm when supplied with electrical power. Thermo switches are installed inside the stators of the recirculation fans. If the temperature of the stators get to 180 DEG.C
 (356.00 DEG.F) +/- 8 DEG.C (46.40 DEG.F), the thermo switches isolate the electrical supply to the fans. The recirculation fans are installed on vibration-damper mountings, these prevent damage to the aircraft structure, due to fan vibration. Arrows on the fan casing show the direction of airflow through the fan and the direction that the impellor rotates.
 **ON A/C ALL
 B. Check Valves (Ref. Fig. 001) A check valve 4020HM (4021HM) is installed downstream of each cabin recirculation fan 14HG (15HG). Two semicircular flaps are installed on a hinge-bar and a spring holds the semicircular flaps in the closed position. Airflow from the cabin recirculation fans 14HG (15HG) will lift the semicircular flaps from their seats. This will permit air to flow through the check valves 4020HM (4021HM) into the ducts. Airflow in the opposite direction through the check valve will push the semicircular flaps back onto their seats which will stop the airflow. An arrow on the check valve casing shows which way air will flow through the check valve.
 C. Recirculation Filter (Ref. Fig. 001) Two recirculation filters 4012HM (4013HM) are installed, one upstream of each recirculation fan 14HG (15HG). Each consists of a multi-layer glassfiber filter-cartridge, installed inside a perforated Carbon Fiber Reinforced Plastic (CFRP) cylinder. The complete filter unit is installed in an open frame type housing 4010HM (4011HM) made of aluminum. The filter unit is secured in position with an adjustable flange at one end of the housing.
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 6. Operation/Control and Indicating
________________________________
 (Ref. Fig. 007)

 A. Operation of Cabin Recirculation Fans
 (Ref. Fig. 005)

 (1) Normal Operation On the overhead panel 22VU, the CAB FANS pushbutton switch 4HG operates the cabin recirculation fans 14HG (15HG). The switch supplies 28 V DC from the normal busbar 101PP (204PP) through two circuit breakers 2HG (11HG) to two power relays 5HG (6HG). The power relays energize the cabin fans with 115 VAC from the normal busbar 1 and 2 101XP (202XP) through the two circuit breakers 1HG (3HG).
　
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