Process Design of Cooling Towers (Project Standards and Specifications)
Process Design of Cooling Towers (Project Standards and Specifications)
Process Design of Cooling Towers (Project Standards and Specifications)
KLM Technology
Group Rev: 01
Project Engineering
Standard
April 2011
www.klmtechgroup.com
TABLE OF CONTENT
SCOPE 3
REFERENCES 3
DEFINITIONS AND TERMINOLOGY 3
UNITS 8
GENERAL 9
TYPES OF COOLING TOWERS 10
Natural Draught Towers 10
Mechanical Draught Towers (see Figs. 1b, c and d) 11
DESIGN CONSIDERATIONS 16
Design Parameters 16
Ambient Air Temperatures 16
Approach 17
Cooling Range and Water Quantity 17
Effect of Altitude 17
Packings 17
Water Loadings 18
Drift Losses 18
Windage Losses 18
Recirculation 19
Hydrocarbon Detection System 19
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WATER QUALITY 19
Make-Up Water 19
Circulating Water 20
COLD CLIMATE DESIGN CONSIDERATIONS 20
General 20
Avoidance of Icing by Air Control 21
Avoidance of Icing by Control of Water Load 21
Effects of Anti-Icing Flow on Tower Capacity 22
SITING, SPACING AND ENVIRONMENTAL CONSIDERATIONS 23
General 23
Siting 23
Spacing 24
Environmental Considerations 24
GUARANTEES 29
APPENDIX A 30
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SCOPE
This Project Standards and Specifications covers the minimum process design
requirements, field of application, selection of types, design consideration and
thermal process design for cooling towers.
REFERENCES
Throughout this Standard the following dated and undated standards/codes are
referred to. These referenced documents shall, to the extent specified herein,
form a part of this standard. For dated references, the edition cited applies. The
applicability of changes in dated references that occur after the cited date shall
be mutually agreed upon by the Company and the Vendor. For undated
references, the latest edition of the referenced documents (including any
supplements and amendments) applies.
1. BSI (British Standards Institution)
BS 4485 "British Standard Specification for Water Cooling Towers"
- Part 1: 1969, "Glossary of Terms"
- Part 2: 1988, "Methods for Performance Testing"
- Part 3: 1988, "Thermal Design Principles"
- Part 4: 1975, "Structural Design of Cooling Towers"
2. CTI (Cooling Tower Institute, USA)
CTI Bulletin "Nomenclature for Industrial Water Cooling Tower"
NCL-109
- "Acceptance Test Code"
3. ASME (American Society of Mechanical Engineers)
- "ASME Test Code Section VIII
In the preparation of this glossary care has been taken to standardize only
suitable terms and definitions, dealing with the thermal design as mentioned by
the British Standard Glossary and CTI of USA.
Air Flow - Air flow is total quantity of air including associated water vapor flowing
through the tower
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Ambient Wet Bulb Temperature - Ambient wet bulb temperature is wet bulb
temperature of air measured windward of the tower and free from the influence of
the tower.
Basin Kerb - Basin kerb is top level of the retaining wall of the cold water basin;
usually the datum point from which tower elevation points are measured.
Cell Height - Cell height is the distance from basin kerb to top of fan deck but not
including fan stack.
Cell Length - Cell length is the dimension parallel to longitudinal axis and the
plane where louvres are usually placed.
Cell Width - Cell width is the dimension perpendicular to tower longitudinal axis
and usually at right angles to the louvre area.
Circulating Water Flow - Circulating water flow is the quantity of hot water
flowing into the tower.
Cold Water Basin (Basin Pond) - Cold water basin is a device underlying the
tower to receive the cold water from the tower, and direct its flow to the suction
line or sump.
Column Anchor - Column anchor is a device for attaching the tower structure to
the foundation; it does not include the foundation bolt.
Cooling Range (Range) - Cooling range is the difference between the hot water
temperature and the recooled water temperature.
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Discharge Stack - Discharge stack is that part of the shell or casing of a forced
draught tower, through which the outlet air is finally discharged. (See "fan stack"
for induced draught towers and "shell" for natural draught towers.)
Down Spout - Down spout is a short vertical pipe or nozzle used in an open
distribution system to discharge water from a flume or lateral on to a splash plate.
Drift Loss - Drift loss is water lost from the tower as liquid droplets entrained in
the outlet air.
Effective Volume - Effective volume is the volume within which space the
circulating water is in intimate contact with the air flowing through the tower.
Fan - Fan is a rotary machine which propels air continuously. This is used for
moving air in a mechanical draught tower and is usually of the axial-flow propeller
type. The fan may be of induced draught or forced draught application.
Fan Casing - Fan casing is those stationary parts of the fan which guide air to
and from the impeller. In the case of an induced draught fan, the casing may
form the whole or part of the fan stack.
Fan Deck - Fan deck is surface enclosing the top of an induced draught tower,
exclusive of any distribution system which may also form a part of the enclosure.
Fan Drive Assembly - Fan drive assembly is components for providing power to
the fan, normally comprising driver, drive shaft and transmission unit, and
primary supporting members.
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Fan Duty (Static) - Fan duty (Static) is the inlet volume dealt with by a fan at a
stated fan static pressure.
Fan Duty (Total) - Fan duty (total) is the inlet volume dealt with by a fan at a
stated fan total pressure.
Fan Power - Fan power is the power input to the fan assembly, excluding power
losses in the driver.
Fan Stack - Fan stack is cylindrical or modified cylindrical structure enclosing the
fan in induced draught towers.
Fan-Stack Height - Fan-stack height is the distance from the top of the fan deck
to top of fan stack.
Fan Static Pressure - Fan static pressure is the difference between the fan total
pressure and the fan velocity pressure.
Fan Total Pressure - Fan total pressure is the algebraic difference between the
mean total pressure at the fan outlet and the mean total pressure at the fan inlet.
Film Packing - Film packing is an arrangement of surfaces over which the water
flows in a continuous film throughout the depth of the packing.
Heat Load - Heat load is rate of heat removal from the circulating water within
the tower.
Inlet Air - Inlet air is air flowing into the tower; it may be a mixture of ambient air
and outlet air.
Inlet Air Wet Bulb Temperature - Inlet air wet bulb temperature is average wet
bulb temperature of the inlet air; including any recirculation effect. This is an
essential concept for purposes of design, but is difficult to measure.
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Motor Rated Power - Motor rated power is nameplate power rating of the motor
driving the fan.
Nominal Inlet Air Wet Bulb Temperature - Nominal inlet air wet bulb
temperature is the arithmetical average of the measurements taken within 1.5 m
of the air inlets and between 1.5 m and 2.0 m above the basin kerb elevation on
both sides of the cooling tower.
Outlet Air - Outlet air is the mixture of air and its associated water vapor leaving
the tower. (See Air flow.)
Outlet Air Wet Bulb Temperature - Outlet air wet bulb temperature is average
wet bulb temperature of the air discharged from the tower.
Packing (Filling) - Packing is material placed within the tower to increase heat
and mass transfer between the circulating water and the air flowing through the
tower.
Plenum - Plenum is the enclosed space between the eliminator and the fan
stack in induced draught towers, or the enclosed space between the fan and the
packing in forced draught towers.
Purge (Blow Down) - Purge is water discharged from the system to control
concentration of salts or other impurities in the circulating water.
Recirculation (Recycle) - Recirculation is that portion of the outlet air which re-
enters the tower.
Shell - Shell is that part of a natural draught tower which induces air flow.
Sump (Basin Sump or Pond Sump) - Sump is a lowered portion of the cold
water basin floor for draining down purposes.
Standard Air - Dry air having density of 0.0011 kg/L, at 21C and 0.7 atm (531
mm Hg).
Tower Pumping Head - Tower pumping head is the head of water required at
the inlet to the tower, measured above the basin kerb to deliver the circulating
water through the distribution system.
UNITS
GENERAL
1. A water cooling tower is a heat exchanger in which warm water falls
gravitationally through a cooler current of air.
Heat is transferred from the water to the air in two ways:
a. by evaporation as latent heat of water vapor;
b. by sensible heat in warming the air current in its passage through the
tower.
As a general measure, about 80% of the cooling occurs by evaporation and
about 20% by sensible heat transfer. The transfer of heat is affected from the
water through the boundary film of saturated air in contact with the water
surface. This air is saturated at the water temperature. From this saturated air
film, heat transfer occurs to the general mass of air flowing through the tower.
2. In the interests of efficiency, it is essential that both the area of water surface
in contact with the air and the time of contact be as great as possible. This
may be achieved either by forming a large number of water droplets as
repetitive splash effects in one basic kind of tower packing, or by leading the
water in a thin film over lengthy surfaces.
3. Air flow is achieved either by reliance on wind effects, by thermal draught or
by mechanical means. The direction of air travel may be opposed to the
direction of water flow giving counterflow conditions, or may be at right angles
to the flow of water giving crossflow conditions. Although the methods of
analysis may be different for counterflow and crossflow conditions, the
fundamental heat transfer process is the same in both cases. In some
designs mixed flow conditions exist.
4. The cooling range of the tower corresponds to the difference in temperature
of the air-water film between entry to and exit from the tower. Air enters the
tower having wet and dry bulb characteristics dependent on the ambient
conditions. It is generally in an unsaturated state and achieves near-
saturation in passing through the tower. It may be considered saturated at exit
in all but very dry climates.
5. Performance Characteristics
a. The performance characteristics of various types of towers will vary with
height, fill configuration and flow arrangement crossflow or counterflow.
When accurate characteristics of a specific tower are required the cooling
tower manufacturer should be consulted.
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There are many types of tower used in evaporating cooling, generally they tend
to be divided into two groups depending upon the method used for moving air
through the tower:
a. natural draught;
b. Mechanical draught.
influence air flow but are not normally taken into consideration in tower
design.
The choice of counterflow, mixed flow or crossflow arrangements is
dictated primarily by site and economic considerations.
b. Advantages
The advantages are as follows:
- It is suited to large water flow rates.
- High-level emission of plume virtually eliminates fogging at ground
level and recirculation.
- It occupies less ground space than multiple mechanical draught towers
for large thermal duty.
- It is independent of wind speed and direction when compared with
atmospheric towers.
- There is no fan noise.
- There is no mechanical or electrical maintenance.
c. Disadvantages
The disadvantages are as follows:
- The chimney effect of the shell diminishes as the humidity decreases
and this may be a disadvantage in hot dry climates.
- Close approach is not economical.
- The considerable height of shell frequently arranged in multiple
installations presents an amenity disadvantage.