Choosing Single Packaged Direct Expansion Unit Selection Tips

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