Refrigeration Systems

Refrigeration Systems: The individual components of a refrigeration system can usually be removed and installed separately. However the Maintenance Manuals appropriate to the system and aircraft should always be referred to before attempting such work. Some of the general precautions applicable to closed circuit Vapour Cycle Systems are as follows:­

(a)       Gloves and goggles should be worn when handling liquid refrigerants which can be harmful to the skin and eyes.

(b)       Before charging a newly installed system, or recharging a system which has been partly disconnected, all air should be evacuated in the manner prescribed in the relevant Maintenance Manual.

(c)       While refilling is in progress, care should be taken to ensure that refrigerant used is of the specified type, and quantity, and that all precautions recommended by the manufacturer are observed.

NOTE: The Refrigerant used in Vapour Cycle Cooling Systems, usually contains a specific amount of oil to lubricate the compressor bearings. Therefore, in order to maintain the correct ratio of constituent parts, reference should be made to the relevant Maintenance Manual for the correct volume of oil to be added.

Temperature Control System Components

(a)       The temperature control of complex air conditioning systems is usually accomplished either electrically or electronically. Consequently the following precautions are normally adopted when installing such equipment.

(b)       As temperature-sensing elements are positioned so that they will be directly affected by the changes in duct and cabin air temperatures. Therefore, care should be taken to ensure that elements sensing cabin air temperature are not shielded by loose upholstery, and are protected if paint spraying or similar operations are performed in their vicinity.

(c)       The damping effect of shock absorbers and anti-vibration mountings which may provide support for electronic amplifiers and similar sensitive equipment, should be checked by hand after installation.

(d)       Cables interconnecting components must be of the rating specified by the manufacturer and all connections must be clean and securely made.

(e)       When installing control units, care should be taken that such controls as pre-set potentiometers and fine adjustment resistors are not disturbed.

(f)        On completion of the installation of a component, sensitivity tests and final balance adjustments should be carried out in accordance with the procedure laid down for the specific aircraft system. Tests of the overall controlling function should also be made by selecting various temperature settings and noting that the actuators controlling such components as heat exchanger cooling air flaps, by-pass valves, etc., move in the appropriate directions.

2.2.9 Valves

(a)       Mechanically and electrically-operated valves are employed in the various types of heating, ventilating and air conditioning systems and therefore Maintenance Manuals should always be referred to for the appropriate installation procedures. The details given in the following paragraphs are of a general nature.

(b)       All valves should be inspected before installation for cleanliness, signs of damage and freedom of movement. Functional checks should be made on electrically-operated valves, e.g. spill valves, by-pass valves and choke valves to ensure that limit switches are correctly adjusted at the extremes of valve travel.

(c)       Valves are often marked with arrows to indicate the direction of flow and particular care is necessary to ensure that the valve is installed in correct relation to flow.

(d)       The attachment of valves to their respective mountings and duct sections must be secure and torque loadings strictly observed.

(f)        Electrical connections to actuators and to position indicators where fitted, should be checked against the relevant wiring diagrams, and plugs, sockets and terminal screws checked for security.

(f)        On completion of the installation of a valve, an in-situ functional test should be carried out in accordance with the procedure specified in the relevant Component and Aircraft Maintenance Manual.

2.2.10 Distribution Systems

The methods of installing ducting and other components of distribution systems depend on the type of air conditioning system and reference must, therefore, always be made to the relevant Aircraft Maintenance Manual and the procedures specified carried out.

The following summary serves as a guide to some important aspects common to installation procedures:­

(a)       Ducting should be inspected externally and internally for cleanliness, signs of damage and security of end fittings.

(b)       Lagging, where fitted, should be inspected to ensure freedom from tears, damage and evidence of deterioration.

(c)       When fitting ring clamps, the sealing rings must be correctly positioned between duct and fittings and the fittings should abut each other squarely before the clamps are tightened.

(d)       Ring clamps should be torque-tightened to the loads specified; the loadings should be rechecked after the engine run following installation.

(e)       Ducts made from fibreglass, plastic and reinforced plastic should not be subjected to any weight or load during installation, and the straps or clamps attaching the ducts to support brackets should not be over tightened.

(f)        After replacement of a duct, the disturbed joints should be checked for leakage.

(g)       Where specified, ducts must carry identification labels.

(h)       When assembled on ducts, rubber sleeves should be in a free condition, i.e. they should not be twisted, stepped or collapsed.

(i)         Bedding tape or metal clips must be fitted between rubber sleeves and adjustable clamps to prevent damage to the sleeves when tightening the clamps. Expansion bellows, sliding clamps or gimbal mountings where installed, should be checked for full and free movement.

(j)         Electrical bonding leads must be properly secured.