4.
CRITICAL ASSESSMENT
Critical assessments of components used in a centralized air conditioning system involve
thoroughly evaluating and analysing the performance, reliability, efficiency, and safety of each
component. The assessments aim to identify strengths, weaknesses, limitations, and potential
improvements related to the individual components within the system. By conducting these
assessments, it becomes possible to optimize system performance, enhance energy efficiency,
ensure safety compliance, and make informed decisions regarding component selection,
maintenance, and upgrades.
1) Compressors
The compressors shall be semi-hermetic, single rotor, rotary screw type with internal oil
separator. The compressor motor shall be refrigerant gas cooled, high torque, induction type, twopole, with inherent thermal protection on all three phases and shall be mounted on RIS vibration
isolator pads.
2) Electric Motors
Compressor motors shall be high torque, two pole, semi-hermetic, squirrel cage induction
type with inherent thermal protection on all three phases and cooled by suction gas. Full-load
power factor shall be, at minimum, 0.90. Motors not meeting this minimum power factor must be
capacitor-corrected to 0.90 or better.
3) Solid-State Motor Starters (each compressor)
Starter shall be designed using the current generation of reliable solid-state technology.
Each starter shall provide controlled motor acceleration and deceleration, and shall provide
protection for the following conditions: ground fault, phase rotation, electronic thermal overload,
over/under current, stalled motor, single phase, high load current and current unbalance. Acrossthe-line or wye-delta starters are not acceptable. Acceptable solid-state starter manufacturers are
GE, Cutler-Hammer, Benshaw or Reliance. The solid-state starters shall be capable of selfdiagnostics, metering, and have an LED display.
4) Evaporator
The evaporator shall be two circuit, direct expansion, shell-and-tube type with copper tubes
rolled into steel tube sheets. It shall be insulated with 3/4-inch (19 mm) closed cell polyurethane
insulation and designed for 150 psi (1034 kPa) water side working pressure and 354 psi (2441 kPa)
refrigerant side pressure. It shall be designed in accordance with ASME Pressure Vessel Code,
Section VIII, and have ANSI B9.1 pressure relief valves. Condensers:
Horizontal shell and finned tube type with steel shell and integral finned copper tubes rolled
into steel tube sheets. The condenser shall be equipped with intermediate tube supports and
construct in accordance with the requirements of ASME Unfired Pressure Vessel Code Section
VIII and ANSI B9.1 Safety Code. It shall be designed for 225 psi (1551 kPa) water side working
pressure and 350 psi (2413 kPa) refrigerant side pressure. It shall have ANSI B9.1 pressure relief
valves.
5) Refrigerant Circuit
The unit shall have two or more refrigerant circuits, completely independent of each other.
Each circuit shall be equipped with one compressor with integral oil separator, one microprocessor
controller, a factory-mounted control circuit transformer, electronic expansion valve, compressor
suction shutoff valve, discharge check valve, liquid line shutoff valves, replaceable core filterdryers, sight-glass with moisture indicator. Each circuit shall be capable of operating
independently, not being disabled in the event of fault(s) on the other circuit(s).
6) Electric Panel
The electric panel shall consist of two separated compartments, one for power components
and one for control components. The control section shall contain a microprocessor controller for
each circuit and one for the unit, providing operating and equipment protection controls. The
circuit controllers shall operate independently of each other.
7) Sound Reduction
The sound enclosure is constructed of prepainted steel formed into panels, gasketed and
lined with an aluminized polyester-faced sound absorbing insulation for maximum sound
attenuation. The combination of the metal panels and the specialized sound foam, provide an
excellent sound damping effect. The enclosure can be factory or field installed and is completely
removable. No field alterations are required to install the enclosure. Access to the compressors is
gained by removing any of the panels that are fastened with bolts not screws, facilitating service
access and proper reinstallation of panels. Heat is allowed to escape through strategically placed
air gaps in the enclosure to help prevent compressor overheating.
8) Water pump
Chilled water pumps transport cold water to the building or process loads, and then return
the heated water to the chiller for re-cooling. Condenser water pumps circulate the cooling water
between the chiller's water-cooled condenser and a cooling tower or another heat rejection device.
If the pumps do not sequence correctly or if there are low flow conditions, a chiller may
malfunction, and this problem may only become apparent once normal operation is restored.
Two common types of centrifugal impeller pumps are end-suction and split case
construction. When it comes to servicing these pumps, important considerations include
lubricating the pump and motor bearings, as well as cooling the water seal in larger pumps. Proper
alignment of the motor-pump shaft is crucial, and it should be periodically checked as the heavy
piping and supports may shift over time.
9) Cooling Tower
Cooling towers play a crucial role in the chiller system by transferring the unwanted heat
load, which is removed by the chiller and generated by the chiller's compressor work (known as
heat of compression), to the external environment. The heat is dissipated from the condenser water
through the operation of the cooling towers.
Cooling towers come in various types, including forced draft, induced draft, counterflow,
and crossflow designs. Each type has its own advantages and is chosen based on specific system
requirements. They are typically constructed using materials such as steel, fiber-glass reinforced
plastic (FRP), wood, or concrete, depending on factors such as durability, cost, and environmental
considerations.
Regardless of the type of cooling tower, there are certain service requirements that remain
consistent. Regular maintenance and inspection are essential for the various components of cooling
towers. This includes the fan motors, gear or belt drives, and water make-up float assemblies, all
of which need routine attention to ensure proper functioning and prevent any potential issues.
These components should be inspected, lubricated, and adjusted as necessary.
Another important aspect of cooling tower maintenance is the cleaning of tower basins,
fill, and distribution pans. Over time, these areas can accumulate debris, sediment, algae, and other
contaminants, which can negatively impact the tower's efficiency and performance. Regular
cleaning is necessary to maintain optimal water flow, prevent blockages, and ensure effective heat
dissipation.
Periodic inspections and cleanings of cooling towers are crucial for maintaining their
operational efficiency and prolonging their lifespan. It is recommended to follow manufacturer
guidelines and industry best practices to determine the frequency and extent of maintenance tasks
required for cooling towers. This proactive approach to maintenance helps prevent potential
problems, ensures efficient operation, and promotes the longevity of the cooling tower system.
10) Water Treatment
Water treatment is essential in maintaining the integrity and efficiency of water loops by
preventing and controlling various issues such as corrosion, scale presence, and biological growth.
Different treatment methods are employed based on the type of water loop.
Closed chilled water systems, although not exposed to the atmosphere, still require
corrosion inhibitors to control and prevent corrosion. Open cooling tower systems, on the other
hand, are more challenging to manage. Cooling towers function like large air washers and demand
regular maintenance to address corrosion-related problems. Various water treatment approaches,
including chemical, magnetic, and ozone methods, are successfully employed in different systems
today.
When water and pipes become fouled or scaled, heat transfer at the chiller and cooling coils
is hindered. This can lead to significant performance issues and damage the chiller's tubes if water
treatment is not properly executed. Therefore, it is advisable to regularly conduct eddy current
testing of tubes as a good practice, in addition to consistently implementing effective water
treatment measures.
Furthermore, controlling biological matter in cooling towers is crucial due to the significant
evaporation and potential drift of water into the atmosphere. This is not only a performance
concern but also an important aspect of maintaining a healthy environment. Several antimicrobial
growth products are available to minimize and prevent biological growth in the cooling tower
basin, aiding in the overall control of such issues.
11) Bus Control
Bus controls, which encompass various control systems in buildings, have evolved with
the advent of digital-based controls. These newer controls generally require minimal maintenance,
except for occasional software updates and calibration to ensure optimal performance. On the other
hand, older pneumatic systems rely on air compressor/driers, which necessitate specific routine
servicing. The presence of moisture in a pneumatic system can severely affect its proper operation,
leading to costly cleanup expenses. Therefore, it is essential to address moisture-related issues
promptly.
Additionally, it is important to regularly check and lubricate dampers and water control
valves to ensure their smooth operation. These components play a crucial role in maintaining the
desired control over the system and should be properly maintained. Bus controls have a significant
impact on the operation and performance of the chiller plant. Proper control of the chiller plant
pump sequence, air handler scheduling, and exhaust fan operation is crucial. These factors directly
affect the performance and efficiency of the chiller system.
It is also important to consider the temperature pull-down rates of the chilled water. Gradual
and steady temperature changes are recommended, as rapid temperature and/or flow adjustments
can lead to erratic and inefficient chiller operation. Such sudden changes can adversely affect the
overall system performance. In variable flow systems, it is crucial to confirm the minimum flow
requirements. Proper minimum flow rates need to be maintained to ensure the efficient operation
of the system.