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Choosing Single Packaged Direct Expansion Unit Selection Tips

Discussion in 'المنتدى التعليمي' started by mannan, 8/10/17.

  1. mannan

    mannan Administrator Staff Member



    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.


    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 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 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 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



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