Orange County NC Website
Browse       Search
DocuSign Envelope ID: 9826C043- 4C32- 49CD- A4CC- DC28ABAB7CE7 <br />SECTION 238147 — DEDICATED OUTDOOR AIR SYSTEM <br />D. Each coil shall be factory leak tested with high - pressure air under water. <br />2.010 SUPPLY FAN <br />A. Supply fan shall be a single width, single inlet (SWSI) airfoil centrifugal fan. The fan wheel shall <br />be Class II construction with fan blades that are continuously welded to the hub plate and end rim. <br />The supply fan shall be a direct drive fan mounted to the motor shaft. Belts and sheaves are not ac- <br />ceptable due to the additonal maintenance. <br />B. All fan assemblies shall be statically and dynamically balanced at the factory, including a final trim <br />balance, prior to shipment. <br />C. Supply fan and motor assembly combinations larger than 8 hp or 22" diameter shall be internally <br />isolated on 1" deflection, spring isolators and include removable shipping tie downs. <br />D. The fan motor shall be a totally enclosed EC motor that is speed controlled by the rooftop unit <br />controller. The motor shall include thermal overload protection and protect the motor in the case of <br />excessive motor temperatures. The motor shall have phase failure protection and prevent the motor <br />from operation in the event of a loss of phase. Motors shall be premium efficiency. <br />E. The supply fan shall be capable of airflow modulation from 30% to 100% of the scheduled de- <br />signed airflow. The fan shall not operate in a state of surge at any point within the modulation range. <br />2.011 VARIABLE AIR VOLUME CONTROL <br />A. The unit controller shall proportionally control the Electronically Commutated Motors (ECM) on <br />the supply and exhaust fans. The supply fan shall be controlled to maintain an adjustable duct pres- <br />sure setpoint. A duct static pressure sensor shall be factory mounted in the control panel. The field <br />shall furnish and install the pneumatic tubing for the duct static pressure sensor and the building <br />pressure sensor. The field shall furnish and install the outdoor air pressure sensor. <br />2.012 HEATING SECTION <br />A. The rooftop unit shall include an electrical resistance heating coil section. Staged electric heating <br />coil modules shall be factory installed downstream of the supply air fan in the heating section of the <br />rooftop unit. Heating coils shall be constructed of a low watt density, nickel - chromium alloy re- <br />sistance wire with intermediate supports that include ceramic bushings. The electrical contactors <br />shall be of the full line- breaking type with all the electrical power legs being disconnected when the <br />contactors are not energized. All electrical circuit wiring shall be designed with copper conductors, <br />aluminum wires are not acceptable. Heating element branch circuits shall be individually fused to a <br />maximum of 48 Amps per NEC requirements. The power supply for the electric heater shall be fac- <br />tory wired into the units main power block or disconnect switch. <br />B. The heating modules shall have an automatic reset, high temperature limit safety protection. A <br />secondary high limit protection shall also be provided that requires a manual reset. An airflow switch <br />shall be provided with the heating module to prevent the electric heater from operating in the event <br />of no airflow. <br />C. The electric heat elements shall be controlled by the factory installed DDC unit control system. <br />The heater shall have proportional SCR control. The unit controller shall modulate the electric heat- <br />er to maintain the discharge air temperature setpoint. <br />D. Field installed heating modules shall require a field ETL certification. Duct heaters mounted with- <br />in the rooftop unit in the field shall not be acceptable. The manufacturer's rooftop unit ETL certifica- <br />tion shall cover the complete unit including the electric heating modules <br />2.013 CONDENSING SECTION <br />A. Outdoor coils shall be cast aluminum, micro - channel coils. Plate fins shall be protected and <br />brazed between adjoining flat tubes such that they shall not extend outside the tubes. A sub - cooling <br />coil shall be an integral part of the main outdoor air coil. Each outdoor air coil shall be factory leak <br />tested with high - pressure air under water. <br />B. Outdoor air coils shall be protected from incidental contact to coil fins by a coil guard. Coil guard <br />shall be constructed of cross wire welded steel with PVC coating. <br />C. Fan motors shall be an ECM type motor for proportional control. The unit controller shall propor- <br />tionally control the speed of the condenser fan motors to maintain the head pressure of the refriger- <br />Reece, Noland & McElrath, Inc. 238147-5 Orange County Board of Elections <br />Orange Co., NC 05/04/18 <br />