1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.


Discussion in 'المنتدى التعليمي' started by mannan, 9/10/17.

  1. mannan

    mannan Administrator Staff Member



    الحمد لله والصلاة والسلام على رسول الله وعلى آله وصحبه أجمعين

    choosing D.X. UNITS Selection Tips



    All units must be rated in accordance with the ARI Standard and must be
    designed to conform to ASHRAE 15 latest revision and in accordance with UL 1995

    All units must be manufactured in a facility registered to ISO –9001

    All insulation and adhesive must meet NFPA 90A
    requirements for flame spread and smoke generation

    All units located outdoors near sea side or subjected to a
    highly polluted and corrosive atmosphere (see duty) must be
    coated with a special coat suitable for the following
    1) Highly concentrated acidic and alkaline atmosphere
    2) Solvent resistance
    3) Moisture
    4) Ultra violet rays
    5) Abrasion resistance
    6) Salt spray tests according to ASTM B117.85 2000 hours
    and must be repairable in the field

    Return Air Plenum

    Return air plenum must be an integral part of the unit, factory assembled
    Plenum must be suitable for accommodating 50 mm. thick metallic cleanable
    filters with a face velocity not exceeding 1.8 m/sec
    However, equal and approved field installed filter plenums are also acceptable

    Return Air and Fresh Air Filters

    1) Return air filters must be permanent and washable type made of aluminium
    or G.I. sheet designed for heavy duty and must be sized for maximum
    air velocity of 350 ft. per minute with initial pressure drop of not more
    than 0.25 inch W.G. at the rated air flow. Filter media must consist of
    cripped G.I. or aluminium arranged to give maximum area for airflow
    2) Return air filters must be enclosed in a separate filter frame
    integral unit.
    3) Fresh air filters must be of the same specification as return
    air filters.
    4) Thickness of return air filters must be 2" and that of fresh air filters must be of 4"

    Split System Direct Expansion Units

    The units must be of internationally reputed have undisputed
    record of performance.and must be approved

    The unit must consist of a remote air-cooled condensing unit and a matching air-handling unit
    Condensing units and air handling units must be of the same make

    The condensing unit must be of the air-cooled type, suitable for outdoor installation
    constructed of heavy gauge, galvanized sheet with baked enamel finish
    special coat to be applied. The unit must be complete with its own compressors
    air-cooled condenser, motors, fans, starters, relays and
    other standard accessories and controls


    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



Share This Page