COVER STORY
SKILLS WORKSHOP
HVAC&R SKILLS WORKSHOP
MODULE 40
PULLOUT
An introduction
to cooling towers
The evaporative cooling tower was originally developed as a water conservation device,
designed to reduce dependence on “once through” cooling systems and successfully replaced
many such systems. The development of the cooling tower also expanded the ability of designers
to provide efficient cooling in areas without large water supplies. Cooling towers are now applied
widely across a range of industries in Australia and New Zealand. Different cooling tower types
PULLOUT
and their characteristics are featured throughout this skills workshop.
Traditionally, in the building and HVAC&R industries, lowest first cost as
opposed to whole of life costs, has been the primary driver for equipment
and system selection, design and construction. Even designs that
incorporated energy or water efficiency features were often simplified or
rationalised during the competitive tendering and building construction
process. Designs whose energy efficient or sustainability features did
survive the construction process were then rarely commissioned,
controlled or operated so that the anticipated efficiencies were rarely
realised in practice. In the days of relatively cheap energy and water
abundance any industry concern over these issues was minimal.
Times have changed.
Natural draft hyper-bolic tower
Air
Out
Natural draft spray tower
Air
Flow
Hot water
in
Hot water
in
Air in
Cold water out
• Stack effect increases with height.
• Better and more consistent performance than atmospheric towers.
• Low maintenance costs.
• Not suitable for high dry bulb applications.
• High capital cost due to size.
Cold water
out
• Close water temperature control difficult.
• Air movement depends on induction forces.
• Used for high heat loads, large water flow rates and large
approach temperatures.
• Fluid movement is parallel or crossflow.
• Rarely used in HVAC&R systems.
• Low maintenance as no moving parts.
• Common in power generation and large industrial applications.
• No recirculation of air occurs.
• High pump head required as towers are high.
• Unobstructed location necessary.
• Performance depends on prevailing wind velocity and direction.
• Capital cost almost as high as for mechanical draft tower.
• Quiet operation and lower running costs.
• Rarely used in HVAC&R systems.
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Now designs have to be energy efficient to comply with building
regulations or voluntary building and system performance rating
schemes. Systems have to be commissioned, finetuned and periodically
recommissioned under most rating schemes. Owners want sustainable
building systems to provide safe, healthy, comfortable and productive
environments. Owners and operators also want optimised and integrated
systems that reduce operating and compliance costs.
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SKILLS WORKSHOP
New designs and innovations, new materials and methods of
manufacture have led to considerable improvements in performance and
environmental impacts. New methods of system control and new system
configurations are also achieving significant operational benefits.
Forced draft tower
Air out
Drift is reduced, fan efficiency is improved, fill and tower effectiveness
is increased, towers are smaller, lighter and more corrosion
resistant than ever before. Previously visible plumes can be made
to disappear, water consumption can be reduced, and system
energy consumption can be reduced. Advances in cooling tower
engineering and design means that towers can be constructed
on site faster and cheaper than was previously possible.
Hot water
in
Fill
Centrifugal
Fan
Induced draft tower
Air in
Air out
Cold water
out
• Fan situated at air intake.
Hot water
in
• High air entry and low exit velocities.
• May be susceptible to recirculation.
• Typically centrifugal fans with high resistance capability.
Fill
Air in
• Generally noisier than towers with axial flow fans.
• Generally incorporate fill.
• Discharge and intake air may be ducted.
• Fan, drive and motor located in dry air stream so less maintenance
and corrosion is likely.
Basin
Sump
Cold water
out
• Fans located at lower level so less vibration and better access.
Induced draft, single flow cross flow tower with fill
• Higher fan power requirement.
• Less physical space required for fan.
Air out
• Generally zero performance at zero fan speed.
• Common in HVAC&R applications.
Hot water
in
Ponds and fountains
Fill
Air in
Air in
Cold water
out
Induced draft counter flow tower with fill
• Fan situated at air outlet usually at high level.
• High air exit and low air entry velocities so less prone to recirculation.
• Large fans with low speed and noise possible.
Hot water
in
• Lower drift potential.
Cold water
out
• Can operate with or without fill.
• Fan, drive and motor located in moist air stream so electrical
protection is required and more maintenance and corrosion is likely.
• An attempt to combine the cooling tower heat rejection effect
with an architectural water feature.
• Fan more susceptible to vibration.
• Water quality issues.
• Less access for maintenance.
• Public exposure issues.
• Generally smaller footprint than forced draft.
• Lower performance levels expected.
• Generally up to 10–15% performance at zero fan speed.
• Performance depends on prevailing wind velocity and direction.
• Common in HVAC&R applications.
• Rarely used in HVAC&R systems.
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COVER STORY
HVAC&R SKILLS WORKSHOP
The relationship between cooling towers and public health
issues, particularly the potentially deadly Legionnaires’ disease is
also changing. Health authorities, building owners and system
maintainers realise that risks can be mitigated by appropriate
management strategies and that these strategies are generally well
understood by the industry. Risk assessment, water treatment and
management, system monitoring and maintenance, are all applied
to better manage the health risk. Compliance costs fall with a better
informed and educated industry properly managing the risk. n
MODULE 40
PULLOUT
Fluid cooler
Air out
Cooling
water in
Water
distribution
system
Hot
Fluid
in
Heat
exchanger
Cool
Fluid
out
Centrifugal
Fan
Counter-flow induced draft tower
Air out
Air in
Cooling
water out
Pump
Hot water
in
PULLOUT
• A closed circuit cooling tower in which the process fluid does not
contact the cooling air.
Air in
Air in
• Different types of fluids can be cooled including water, plating solutions,
quenching oils, chemical solutions, gases, refrigerants and air.
• The fluid cannot contact the atmosphere.
• Only a small quantity of water is required in the open evaporating
water circuit.
Cold water
out
• Vertical air movement is induced in opposition to the water flow
across the fill.
• Can be natural, forced or induced draft.
• Coldest water contacts the driest air for maximum performance.
• Reduced pumping requirement.
• Can be difficulties with access for maintenance.
• Can be heavier and larger than open circuit alternatives.
• Flexible air inlet location.
• Reduced water treatment and corrosion.
• Increased fan energy requirement.
• Air inlets on all sides reduce height and pump head.
• The tubes are sensitive to local dry areas caused by solid deposits.
• Better suited to sub zero operation.
• Good option with small packaged towers using centrifugal fans.
• Common in industrial applications.
• Common in HVAC&R applications.
Evaporative condenser
Crossflow induced draft tower
Air out
Air out
Hot water
in
Air in
Water
distribution
system
Hot water
in
Fill
Sump
Refrigerant
in
Heat
exchanger
Air in
Fill
Cooling
water in
Refrigerant
out
Basin
Cold water out
Air
in
• Horizontal air movement is induced over the fill and water flows
across the air stream.
Fan
Water basin
• Generally a larger plan area but lower height.
Cooling
water out
Pump
• Better access for maintenance.
• Lower pump head required due to reduced height.
• See Fluid coolers.
• Higher water loading reduces fouling potential.
• A closed circuit cooling tower in which the refrigerant is cooled
or condensed directly in a closed coil.
• Risk of recirculation higher due to reduced height.
• Less tolerant of obstructions to airflow.
• Can be air or water cooled or both.
• Air inlets on two sides desirable.
• Refrigerant management issues may restrict system location or size.
• Good option with small packaged towers using propeller fans.
• Sometimes referred to as DX systems.
• Common in HVAC&R applications.
• Common in industrial/refrigeration applications.
Next month’s workshop — Building management and control systems – Part 1
17