Elimination of vibrations and noise in air conditioning and heating systems is of utmost importance to ensure a quiet operation. Equally critical is to ensure that the correct fabric is used based on the environment where the equipment is installed, says Ravi Wadhwani.

Flexible duct connectors are fabricated by roll forming a fiberglass-based coated fabric between two strips of sheet metal. A dual fold ensures that the fabric remains tightly fixed to the steel, enabling it to withstand high pressures. Using rivets, one part of the sheet metal is fixed to the mouth of the equipment, and the other part is fixed to the ducting. All duct connectors should be rated Class 1 as per ASTM E 84 test for fire and smoke, and should meet the requirements of NFPA 701 for surface spread of flame. Depending on the applications, higher specifications fabrics should be rated Class 1 as per BS 476, Part 7 and Class O as per BS 476, Part 6 tests for fire spread.

A vinyl fabric is suitable to be used in clean air applications and is ideal for connecting to fan coil units, and air handling units. Toxic fumes in corrosive environments will damage the vinyl, and therefore neoprene-coated fiberglass fabric is recommended for this type of installation. Smoke extract fans are rated to run for two hours at a temperature of 300°C. Using a connector which does not match this specification is very dangerous, as the fabric will melt due to the heat generated by the fire, and this will render the fan useless. A higher grade of silicon-based fiberglass fabric can withstand 300°C for two hours making it the ideal fabric to be used for high temperature applications.

A common problem experienced in duct connector installations is the condensation at the joints, in areas like the Middle East where the temperature difference is too high. The contractors would insulate the ducting till the connector, and keep the fabric part exposed. This will lead to sweating, and will affect the performance of the unit. In an effort to control condensation, some installers fix insulation over the fabric resulting in stiffening of the fabric. This renders the fabric ineffective to control the noise and the vibrations. A proper method is to use an insulated duct connector, which has a filling of 25mm thick fiberglass sandwiched between two layers of fabric. This ensures that the flexibility of the fabric is maintained, while eliminating condensation.

The writer is Business Manager – HVAC at Hira Industries. He can be contacted at ravi@rhira.com.