الحمد لله والصلاة والسلام على رسول الله وعلى آله وصحبه أجمعين Choosing Single Packaged Direct Expansion Unit Selection Tips The units must be internationally reputed, have undisputed record of performance using(R407 or R410) and must be approved according to regulations Units must be factory assembled, precharged, prewired and tested.Each unit must consist of compressors, condenser evaporator, expansion valve, filter drier, sight glass interconnecting refrigerant, pipe work and all other standard accessories and controls. Casing The casing of A/C unit must be of weather proof construction suitable for outdoor installation, suitable for highly polluted areas made of heavy gauge galvanized steel, baked enamel and suitable coat finish. Top, bottom and sides of evaporator section must be insulated with 25-mm thick fiberglass or equivalent Evaporator of double skin construction is also acceptable All panels must be easily removable and all components must be easily accessible for servicing Evaporator section must have a suitable insulated drain pan and drain connections, the drain pan must be made of stainless steel Compressor Compressor must be of the reciprocating/scroll type direct coupled to an electric motor and complete with vibration isolators and crank case oil heater. Compressors 15 HP and below, can be of hermetic sealed type preferably with oil level sight glass Compressors above 15 HP must be of Semi-hermetic semi-sealed multi cylinder type with service valves and oil level sight glass All compressors above 15 HP must have suitable capacity control arrangement For multi-compressor units, each compressor must have its independent refrigerant circuit Condenser Condenser coil must be constructed of seamless copper tubes with copper or aluminium plate fins mechanically bonded to the tubes Spiral or round fins are not acceptable. Maximum number of fins not exceeds 14 per 25 mm. Condenser coil tubes and fins must be coated with special coating suitable to the working conditions The condenser fan must be propeller type with aluminium or steel blades direct driven upward or sideward discharge, provided with protective screens mounted to the casing Frame, blades, protective screens must be coated with special coat suitable for the environment Evaporator Evaporator coil must be of the same construction as of the condenser coil Evaporator coil face velocity must not exceed 2.8 m/sec Evaporator blower must be of the forward curved design and sized to suit the system pressure Fan motors must be installed on an adjustable base and secured by a locking device Fan drive must be through adjustable pitch pulley. Fan must be provided with sealed permanently lubricated ball or sleeve bearings and the entire fan assembly must be statically and dynamically balanced Motor Enclosure and Insulation The following are the minimum requirements of enclosure and insulation for motors Type of Motor Enclosure Insulation Compressor Motor Hermetic / Semi-Hermetic Special Condenser Fan Motor Drip-Proof Class "E" Evaporator Fan Motor Drip-Proof Class "A" Control Wiring & Protection Devices The thermostat control voltage must be 24 volts The A/C units being offered must feature the following minimum requirements of safety interlocking and protective devices Safety electrical and mechanical interlocking of evaporator fan motor with compressor / heater operation and interlocking through an airflow switch wherever duct heaters are provided Where multiple compressors are incorporated in the units their operation must be in sequence in accordance with return air temperature Manually reset and adjustable oil pressure differential cutout of 90-sec/120 sec. trip time rating for compressors of 25 HP and above Manually reset and adjustable high pressure and adjustable automatic low-pressure cutout for compressor protection Time guard circuit for preventing compressor rapid recycling preferably of adjustable range from 5 minutes to 15 minutes Time delay relay for sequence starting of multiple compressors system of minimum 15 sec. delay rating Separate single phasing preventor suitable for three phase 415 volts, 50 Hz. power supply to prevent the whole unit from single phasing phase shift and under voltage with a response time rating of maximum 2.0 sec Inherent thermal protection for compressor motor winding three phase evaporator fan and condenser fan, motor windings Inherent thermal protection or manually reset thermal or magnetic overloads for single-phase condenser and evaporator fan motors External thermal overload with manual reset or MCCB of suitable rating for all three phase motors in addition to (h) above The unit must be controlled by a duct thermostat housed in a suitable perforated transparent casing with a lock. Control voltage of the circuit to which thermostat is connected must be 24 V The control panel must be IP 54 at least في أمان الله
