Air Terminal Units in HVAC

17. Air Terminal Units

17.1   Bypass Air Terminal Unit

17.1.1  General

• Relief type terminal unit shall specifically shall designed to provide multi-zone temperature control for constant volume air distribution systems.
• Automatic zone control is obtained by changing the air volume supplied to the various spaces to be conditioned, in direct proportion to cooling load requirements.
• When a room thermostat becomes satisfied, by-passed room supply air is relieved to the ceiling return air plenum or return duct system based on the design ductwork configuration.
• Air quantity and static pressure in the entire air distribution system remains constant at all times regardless of the volume of air being supplied to individual zones.

17.1.2  Construction

• The casing for bypass terminal unit shall be fabricated from zinc-coated heavy gauge steel. Internal surfaces shall be acoustically and thermally insulated as required with glass fiber, surface-coated to prevent erosion, insulation material shall meet NFPA 90A and UL181 safety requirements.
• The internal damper assembly shall be constructed of exactly dimensioned extruded anodized aluminum components and shall be engineered to provide even distribution of air flow through the unit at a low and consistent static pressure drop.
• Damper blades made of extruded ribs which key into grooved shaft for permanent fix of damper position.
• The bypass terminal unit shall have discharge openings, factory formed for easy low velocity duct work connection.

17.1.3  Room Temperature Controller

• Room temperature controller shall be electronic modulating type with built-in sensor and set point potentiometer for controlling the air relief damper unless otherwise noted, controller shall be mounted at 1.7 m above floor level, on a terminal conduit box located in a draft-free position shielded from direct sunlight and away from any heat or supply air stream sources as indicated on the drawing.
• The thermostat shall be connected into the central circuit of the air relief damper.

17.2   Variable Air Volume (VAV) Terminal Unit

17.2.1  General

• VAV unit shall be factory assembled, variable volume pressure independent type, with aluminum volume controller and pressure velocity sensor.
• Noise produced one meter from room outlet shall not exceed room desired NC level.
• Air leakage shall not exceed 2 % in fully closed position of units maximum rated capacity at 0.75 kPa static press

17.2.2  Construction

• The casing for VAV Box shall be fabricated from minimum of 22 gauge galvanized steel. Internal surfaces are acoustically and thermally insulated with fiberglass, polyester film linings over acoustical fill and not less than 25 mm thick with a density not less than 24 kg/m3. In addition insulation material shall meet NFPA 90A and UL181 safety requirements.
• VAV control damper shall be factory calibrated damper assembly consisting of a swing damper controlled by an internally mounted inflatable non-metallic regulator bellows and externally mounted electric control actuator which act in response to the space thermostat.
• The control air damper assembly shall be constructed from heavy gauge steel with solid shaft rotating in standard bearings. Damper shaft shall be marked on the end to indicate damper position. Damper blade shall incorporate a flexible gasket for tight air flow shutoff and operating over a full 90 degre The unit shall be equipped with an amplified linear averaging flow probe located horizontally across the inlet. The sensor shall provide a differential pressure signal amplified equal to 3 times the velocity pressure with an acceptable accuracy level.
• Nylon type bearing for free operation noise and no lubrication.

17.2.3  Temperature Controller. Digital space thermostat with brushed aluminum cover and space visible temperature indicator in degree Celsius. In addition the controller shall be mounted at 1.7m above floor level, unless otherwise noted on a terminal conduit box located in a draft- free position shielded from direct sunlight and away from any heat or supply air stream sources as indicated on the drawing.

17.3   Constant Air Volume (CAV) Terminal Unit

17.3.1   General

• The CAV shall be single duct terminal box for constant room air supply volume combined with VAV primary air control having high turn down by use of a multi-point sensing grid, erosion resistant to approx. 20 m/s, with circular inlet spigot suitable for inside diameter circular duct sizes, outlet suitable for fixing of angle flange or slide on flanges, inspection cover sealed by a quick release clamping bar.
• Constant air volume boxes: provide separate constant air volume boxes these boxes shall have different characteristics, items to be tested are:

(i)         Flow with bypass to space.

(ii)        Flow with bypass ¾ open to space.

(iii)        Flow with bypass ½ (50%) open to space.

17.3.2  Construction

• Single blade control damper mounted in the circular duct behind the flow measuring grid. The closed damper shall have shut off leakage at 500 Pa inlet pressure of less than 0.5% of rated flow. Evoprene damper seal, thermoplastic elastomer compound seal shall be suitable for temperature up to 50° C.
• Constructed of minimum 1 mm thick galvanized sheet steel casing, lining of attenuator and regulator section with minimum 50 mm thick rockwool with a density 40 kg/m3, erosion protected with laminated glass fiber securely fixed to substrate. The access door also lined with minimum 30 mm thickness of same rockwool material as per NFPA 90A material requirements.
• c. Reheat coil (where required) shall be fixed to box discharge as an integral unit complete with controls, to heat the volume flows. Element consist of 80/20 nickel chrome wire in stainless steel tube filled with magnesium oxide. Attenuator provided to fit box or reheat coil, to reduce air regenerated noise, with angle flange or slide on flanges on both ends.

17.3.3  Temperature Controller

• Volume flow rate suitable for pressure independent control sensed by a star-shaped multi-point, multi-arm averaging sensor measuring pressure differential from 5 Pa upwards. Sensor hub in plastic, sensor tubes in aluminum. External pressure tapings located in an accessible position with tight fitting end caps. Calibration graphs and constants to be provided to relate volume flow in L/s to the measured pressure differentials.

17.4   Execution

17.4.1  Install all boxes as indicated, and in accordance with manufacturer’s installation instructions.

17.4.2  Install each box level and accurately in position indicated in relation to other work, and maintain sufficient clearance for normal service and maintenance, but in no case less than that recommended by manufacturer.

17.4.3  Connect ductwork to terminal boxes in accordance with manufacturer instructions.

17.4.4  Upon completion of installation and prior to initial operation, test and demonstrate that terminal boxes and duct connections are leak tight.

17.4.5  Repair or replace air boxes and duct connections as required to eliminate leak, and retest to demonstrate compliance.

17.4.6  All Air terminal units shall be set to air flow indicated in design drawings.

17.4.7  Any cut edges of fiberglass exposed to the air stream shall be coated with NFPA 90A approved sealant.