This article describes the minimum requirements for central station air handling units, their accessories and controls. Central station air handling units include indoor and outdoor units with chilled water or DX cooling coils, electric resistance or hot water heating coils, fan and motor, and filters.
Central Station Air Handling Units
An Air Handling Unit (AHU) is used to re-condition and circulate air as part of a heating, ventilating and air–conditioning system. The basic function of the AHU is take in outside air, re-condition it and supply it as fresh air to a building. All exhaust air is removed, which creates an acceptable indoor air quality.
- Product data shall be submitted for each type of modular air handling unit indicated, including the following:
- Certified fan performance curves with system operating conditions indicated
- Certified dimension drawings
- Certified fan sound power ratings
- Certified coil performance ratings with system operating conditions indicated e. Motor ratings, electrical characteristics, and motor and fan accessories
- Material gages and finishes
- Filters with performance characteristics
- Dampers, including housings, linkages, and operators
- Shop Drawings shall show details of the following:
- Wiring diagrams indicating power, signal and control wiring. Wiring diagrams shall differentiate between manufacturer installed and field installed wiring.
- Manufacturers’ certified cut sheets indicating unit dimensions, weights, connection points
- Operation, installation and maintenance data shall be submitted for central station air handling units to including operation, installation and maintenance manuals.
Quality Assurance Check of Air Handlers
- Air handling unit fan section sound power level ratings shall comply with AMCA 301. Fans shall be tested in accordance with AMCA 300 and shall bear AMCA certified sound ratings seal.
- Fans shall be factory tested for flow rate, pressure, power, air density, rotation speed and efficiency in accordance with AMCA 21
- Air handling units and their components shall be factory tested in accordance with ARI 430 and labeled by A
- Central Station air handling units and components shall be designed, fabricated, and installed in compliance with NFPA 90A and NFPA 90B.
- Electrical components, devices, and accessories shall be listed and labeled as defined in NFPA 70, Article 100, by a testing agency acceptable to authorities having jurisdiction, and marked for intended us
Frame and Casing in Air Handling Unit
a. Casing materials shall be formed and reinforced single wall insulated panels, fabricated to allow removal for access to internal parts and components, with joints between sections seal
b. Casing shall galvanized steel, steel gage as required for unit to withstand expected unit operating pressures.
c. Floor plate shall be galvanized steel, 5 mm thick.
Insulation of in Air Handling Unit
- a. Insulation materials shall comply with ASTM C 1071 with coated surface exposed to airstream to prevent erosion of glass fibers.
- b. Insulation thickness shall be 25 mm minim
- c. Insulation thermal conductivity (k-Value) shall be 0.037 W/(m·K) at 24° C mean temperatur d. Insulation fire hazard classification shall be a maximum flame-spread index of 25 and smoke-developed index of 50, when tested according to ASTM C 411.
- e. Insulation adhesive shall comply with NFPA 90A or NFPA 90B and ASTM C 916.
- Insulation mechanical fasteners shall be galvanized steel, suitable for adhesive attachment, mechanical attachment, or welding attachment to duct without damaging liner when applied as recommended by manufacturer and without causing leakage in cabinet.
- g. Insulation shall be factory applied with adhesive and mechanical fasteners to the internal surface of section panels downstream from and including the cooling coil section.
Access Doors and Panels
Access Panels and Doors shall be fabricated from the same materials and finishes as cabinet, complete with hinges, latches, handles, and gaskets. Inspection and access panels and doors shall be sized and located to allow periodic maintenance and inspections. Access panels and doors shall be provided in the following locations:
a. Fan sections shall be provided with inspection and access panels or doors. b. Auxiliary access section shall be provided with door
c. Coil sections shall be provided with inspection panel
d. Damper sections shall be provided with inspection and access panels or door
e. Filter sections shall be provided with inspection and access panels or doors to allow periodic removal and installation of filters.
Condensate Drain Pans
a. Condensate drain pans shall be formed sections of stainless steel sheet complying with requirements in ASHRAE 6 Pans shall be fabricated with slopes in two planes to collect condensate from cooling coils (including coil piping connections and return bends) when units are operating at maximum catalogued face velocity across cooling coil.
b. Pans shall be double wall construction with the space between walls filled with insulation and sealed moisture tigh
c. Pan shall have drain connections provided on the side of the unit as specifi
Fan Section in Air Handling Unit
Fan section construction shall be belt driven centrifugal consisting of housing, wheel, fan shaft, bearings, motor, drive assembly, and support structure and equipped with formed steel channel base for integral mounting of fan, motor, and casing panels. Fans shall be mounted with vibration isolation.
Centrifugal fan housings shall be formed and reinforced steel panels to make curved scroll housings with shaped cutoff, spun metal inlet bell with access doors or panels to allow entry to internal parts and components.
a. Panel bracing shall be steel angle or channel iron member supports for mounting and supporting fan scroll, wheel, motor, and accessories.
b. Performance Class shall be AMCA 99 pressure Class I, II or III depending on system pressure which is served by the uni
c. Horizontal flanged split housing shall be of bolted constructio
Fan assemblies shall be statically and dynamically balanced and designed for continuous operation at maximum rated fan speed and motor horsepower.
Backward inclined fan wheels shall be of steel or aluminum construction with curved inlet flange, backplate and backward inclined blades welded or riveted to flange and backplate; cast iron or cast steel hub riveted to backplate and fastened to shaft with set screws.
Forward curved fan wheels shall be black enamel, galvanized steel or aluminum construction with inlet flange, backplate, and shallow blades with inlet and tip curved forward in direction of airflow and mechanically secured to flange and backplate; cast steel hub shall be swaged to backplate and fastened to shaft with set screws.
Airfoil fan wheels shall be steel or aluminum construction with smooth curved inlet flange, heavy backplate, and hollow die formed airfoil shaped blades continuously welded at tip flange and backplate; cast iron or cast steel hub riveted to backplate and fastened to shaft with set screws.
Fan wheel coating shall be powder baked enamel or as required by the specific project.
Shafts shall be statically and dynamically balanced and designed for continuous operation at maximum rated fan speed and motor horsepower, with final alignment and belt adjustment made after installation.
- Shafts shall be turned, ground, and polished hot rolled steel with keyway and shipped with a protective coating of lubricating oil.
- Shafts shall be designed to operate at no more than 70 percent of first critical speed at top of fan’s speed range.
Bearings shall be one of the following types:
Pre-lubricated and sealed shaft bearings shall be self-aligning, pillow block type ball bearings with an ABMA 9 L10 life of 120,000 hours.
Grease lubricated shaft bearings shall be self aligning, pillow block type, tapered roller bearings with double locking collars and two piece, cast iron housing, with an ABMA 11 L10 life of 120,000 hours.
c. Grease lubricated shaft bearings shall be self aligning, pillow block type, ball or roller bearings with adapter mount and two piece, cast iron housing with an ABMA 9 L10 life of 120,000 hours or ABMA 11 L10 life of 120,000 hours as applicabl
- Belt drives shall be factory mounted, with final alignment and belt adjustment made after installation with 1.2 service factor minimum based on fan motor.
- Pulleys shall be cast iron or cast steel with split, tapered bushing; dynamically balanced at factory.
- Motor pulleys shall be adjustable pitch for use with 5 HP motors and smaller; fixed pitch for use with motors larger than 5 HP. Pulleys shall be selected so pitch adjustment is at the middle of adjustment range at fan design conditions.
- Belts shall be oil resistant, non-sparking, and non-static; matched sets shall be provided for multiple belt drives.
- Belt guards shall be fabricated to OSHA/SMACNA requirements, 2.7 mm thick, 20 mm diamond mesh wire screen welded to steel angle frame or equivalent; prime coated.
- Motor mount shall be adjustable for belt tensioning.
Discharge dampers where required shall be heavy duty steel assembly with channel frame and sealed ball bearings, and opposed or parallel blades constructed of two plates formed around and welded to shaft, with blades linked out of airstream to single control lever.
- a. Motor torque characteristics shall be sufficient to accelerate driven loads.
- b. Motors shall have a service factor of 1.15 for poly-phase motors and 1.35 for single-phase motors.
- c. Motor construction shall be NEMA MG 1, general purpose, continuous duty, Design B mounted on an adjustable bas Unit location or project requirements may change motor construction.
- d. Energy efficient motors shall have a minimum efficiency as scheduled according to IEEE 112, Test Method B. If efficiency is not specified, motors shall have a higher efficiency than “average standard industry motors” according to IEEE 112, Test Method B
Coil sections shall have common or individual, insulated, galvanized steel casings for heating and cooling coils. Coil section shall be designed and constructed to facilitate removal and replacement of coil for maintenance and to ensure full airflow through coils.
- a. Water coils shall be continuous circuit coil, self-draining or cleanable coil fabricated according to ARI 410.
- b. Piping connections shall be threaded or flanged. c. Coil tubes shall be copper.
- d. Coil fins shall be aluminum or copper with fin spacing of 1.42 mm maximum. e. Fin and tube joint shall be mechanically bon
- f. Headers shall be cast iron with cleaning plugs and drain and air vent tappings, seamless copper tube with brazed joints, prime coated or fabricated steel with brazed joints, prime coated.
- g. Frames shall be galvanized steel chann
- h. Coils shall be design tested and rated according to ASHRAE 33 and ARI 410. Working pressure rating shall be 1380 kPa, 163° C unless project requirements dictate otherwise.
- i. Coils shall be tested 2070 kPa and to 1380 kPa underwater
Refrigerant Coils in Air Handling Unit
- a. Coil shall be designed and fabricated in accordance with ARI 410, connected with soldered or brazed fittings.
- b. Coil tubes shall be copper.
- c. Coil fins shall be aluminum or copper with fin spacing of 1.42 mm maximum. d. Fin and tube joint shall be mechanically bon
- e. Suction and distributor shall be seamless copper tube with brazed joints. f. Frames shall be galvanized steel chann
- g. Coils shall be design tested and rated according to ASHRAE 33 and ARI 41 Working pressure rating shall be 2070 kPa.
- h. Coils shall be tested 3105 kPa and to 2070 kPa underwater.
Filter Section of Air Handling Unit
Filter section shall be provided filter with holding frames arranged for flat or angular orientation, with access doors on both sides of unit. Filters shall be removable from at least one side. Units with widths larger than 1.83 m shall have filters removable from both sides.
Refer to SES H01-E01 for filter media requirements.
Damper Section of Air Handling Unit
Mixing boxes shall be provided with parallel blade galvanized steel dampers mechanically fastened to steel operating rod in reinforced in a galvanized steel cabinet. Operating rods shall be connected with common linkage and interconnect linkages so dampers operate simultaneously.
Combination filter/mixing boxes shall be provided with parallel blade galvanized steel dampers mechanically fastened to steel operating rod in reinforced, galvanized steel cabinet. Operating rods shall be connected with common linkage and interconnect linkages so dampers operate simultaneously. Cabinet support members shall hold pleated, flat permanent or throwaway filters. Refer to SES H01-E01 for filter media requirements. Hinged access panels or doors shall be provided to allow removal of filters from both sides of unit.
Damper leakage rate shall not exceed 2 percent of air quantity at 10 m/s face velocity through damper and 1000 Pa pressure differential in accordance with AMCA 500.
Outside air dampers shall be low leakage, double skin, airfoil blade galvanized steel dampers with compressible jamb seals and extruded vinyl blade edge seals. Blade arrangement shall be opposed blade with steel operating rods rotating in stainless steel sleeve bearings mounted in a single galvanized-steel frame, and with operating rods connected with a common linkage. Leakage rate shall not exceed 0.22 L/s per sq. m at 250 Pa and 0.4 L/s per sq. m at 1.0 MPa.