CTI Sponsored Educational Programcti.org/downloads/AHRWinterMeetingCT10121-Jan-2014.pdfCTI Sponsored...

January 21, 2014

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CTI Sponsored Educational Program Cooling Tower 101 – Fundamentals and Design2014 AHR Expo – New York City

CTI Sponsored Educational Program

Presented By:Kent Martens – SPX Cooling Technologies, Inc.


January 21, 2014

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CTI Mission Statement

To advocate and promote the use of environmentally responsible Evaporative Heat Transfer Systems (EHTS) for the benefit of the

public by encouraging:� Education� Research� Standards Development and Verification� Government Relations� Technical Information Exchange


January 21, 2014

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CTI Objectives

� Maintain and expand a broad base membership of individuals and organizations interested in Evaporative Heat Transfer Systems (EHTS).� Owner/Operators� Manufacturers� Suppliers

� Identify and address emerging and evolving issues concerning EHTS.

� Encourage and support educational programs in various formats to enhance the capabilities and competence of the industry to realize the maximum benefit of EHTS.


January 21, 2014

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CTI Objectives

� Encourage and support cooperative research to improve EHTS technology and efficiency for the long-term benefit of the environment.

� Assure acceptable minimum quality levels and performance of EHTS and their components by establishing standard specifications, guidelines, and certification programs.

� Establish standard testing and performance analysis systems and procedures for EHTS.

� Communicate with and influence governmental entities regarding the environmentally responsible technologies, benefits, and issues associated with EHTS.

� Encourage and support forums and methods for exchanging technical information on EHTS.


January 21, 2014

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CTI Certification Program

� STD-201

� The standard sets forth a program whereby the Cooling Technology Institute will certify that all models of a line of evaporative heat rejection equipment offered for sale by a specific Manufacturer will perform thermally in accordance with the Manufacturer’s published ratings.

� Applies to Mechanical Draft Evaporative Heat Rejection Equipment such as Cooling Towers, Closed Circuit Coolers (and Evaporative Refrigerant Condensers).


January 21, 2014

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Please visit our website atwww.cti.org


January 21, 2014

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Publication and Presentation Disclaimer 2014

The information contained in the following publication, paper or presentation is intended for education by the author or presenter, however information given is in no way an

endorsement of the Cooling Technology Institute. The publication, paper or presentation has been reviewed by the

CTI staff and program committee for commercial content, however there may be differing opinions regarding the

content of information. The Cooling Technology Institute accepts no liability for its content.


January 21, 2014

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Cooling Tower Fundamentals and Design

� Principles of Operation

� Selection Parameters

� Types of Towers

� Components

� Heat Transfer Surfaces

� Capacity Control

� Water Losses


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Principles of Operation


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What is a Cooling Tower?

� A specialized heat exchanger in which two fluids (air and water) are brought into direct contact to effect the transfer of heat.

� Cools a re-circulating flow of water to which heat has been added by the system served.

� Heat is rejected primarily through evaporation of a small percentage of the circulating water (about 1% of the circulating flow rate for every 10°F of cooling range.)

� Saving the user on water costs.


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What is a Cooling Tower?� A specialized heat exchanger …

� Why is this important???

• A cooling tower can only reject the heat that is sent to it

• Cooling towers do not control the cooling range…the process controls the range


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How Does It Work?

� Hot water from process flows over the fill

� Fan draws ambient air across water on the fill

� Heat from the water is transferred to the air by latent and sensible cooling

� Cooled water is collected and recycled through the system being serviced

Cooling TowerProcess


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How Does It Work?�Creating Heat Transfer Surface and Airflow

�Merkel Theory

� The heat transfer from a water droplet to the surrounding air is a function of the differences between air enthalpy and saturated air enthalpy at the temperature of the water droplet

Air at TA

Film of air over water droplet at TB

Q.

TA < TB

Air enthalpy < Water enthalpy


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� Sensible heat transfer – from contact of relatively cold air with hot water, just as in air-cooled heat exchangers,…radiators

The “Cooling” in Cooling TowersSensible/Latent heat transfer

• Latent heat of vaporization – as with perspiration


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The “Cooling” in Cooling Towers

Sensible

� Sensible Cooling of 1 lb of water 1 deg F rejects 1 bt u.

� Dry Bulb temperature is the driving force

� Hard to cool 95º water with 95º air

� Very minor effect at low dry bulb temps

Latent

� Evaporating that same 1 lb of water rejects 1,000 btu !

� Wet Bulb temperature is the driving force

� Responsible for vast majority of the cooling effect


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HVAC Cooling Towers are Rated in “Tons”� So … What’s a “TON”?

• Unit of capacity for refrigeration equipment

� First refrigeration applications were ICE PLANTS –

• Capacity measured in “tons (of ice) per day”

� A “Ton” of refrigeration - definition:

• Convert a ton (2000 lb) of 32 deg F liquid water to a ton of 32 deg F ice in one day.

• Heat of fusion = 144 btu/lb

• 144 btu/lb x 2000 lb / 24 hr = 12000 btu/hr

• This is a “Refrigeration Ton”

� Chiller OUTPUT is rated in Refrigeration Tons


January 21, 2014

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HVAC Cooling Towers are Rated in “Tons”� Refrigeration Ton

(Heat rejection) = 12,000 btuh

� Cooling tower tons are BIGGER! The tower needs to remove not only the heat the chiller removes from the building, but also has to get rid of the heat produced BY the chiller in removing that heat from the building. That is called the “heat of compression”.

• Heat of rejection = 12,000 btuh

• Heat of compression = 3,000 btuh

• Gross heat rejection = 15,000 btuh

� Based on electric-drive compression chillers

� Standard CTI conditions for cooling towers

• 3 gpm/ton, 95oF – 85oF – 78oF WB


January 21, 2014

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Heat of Compression

Heat of Compression is a “Nominal” value in chiller ratings

� 3,000 BTU = 0.88 kW

Chiller efficiency is expressed in kW/Ton• Today’s chillers are much more efficient

• At 0.53 kW per ton, actual heat of compression = 1,809 btuh/ton for a total of 13,809 btuh/ton

• 13,809 / 3 / 500 = 9.21º range vs “nominal” 10º range

• = 8% less heat load on the tower!

• Some chillers are now down to 0.49 kW/ton or even less!!


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Cooling Towers are Rated in “Tons” for HVAC

…so what?

Tower ratings include the chiller’s heat of compression, so you don’t have to add it to the

tower sizing

…so for a 400 ton chiller, select a 400 ton tower (not 500 tons)


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More on “Tons”

� The output of all 200 ton chillers may be the same… 12,000 btuh/ton

But their gross heat rejection may NOT be the same!Towers see the gross heat rejection of the chiller

Consider:Electric drive centrifugal, recip or scroll compression

Water source heat pump

Gas-fired absorber

Steam-fired absorber or Steam-driven centrifugal

Gross Heat Rejection

15,000 btuh/ton

16-19,000 btuh/ton

22,500 btuh/ton

30,000 btuh/ton

Tower Duty3 gpm/ton, 95-85-78

2.92 gpm, 102-90-78

4.5 gpm/ton, 95-85-78 or 3 gpm/ton, 100-85-78

3.33 gpm, 103-85-78

Impact on Tower Size

1.0

(use fluid cooler)

1.5x1.25x

1.6x


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Selection Parameters


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Common Terms: Example:

� Flow rate (gpm) = 3,000 gpm

� HW (condenser LWT) = 95º F

� CW (condenser EWT) = 85º F

� WB (entering wet bulb) = 78º F

� Range = HW – CW = 10º F

� Approach = CW – WB = 7º F


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What determines which?Determined by amount of heat from process

Determined by the cooling tower

Nature


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Wet Bulb Temperature� The lowest temperature achievable through

evaporation at given ambient temperature and relative humidity

� The temperature at 100% relative humidity when no further evaporation is possible

� As measured by a wet bulb thermometer


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Selection ParametersDesign wet bulb temperature by geography

� See ASHRAE Fundamentals, 2013 Edition for CD ROM


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Design

WBT Value

Hours/year at or

above value

0.4% 35

1.0% 88

2.0% 175

Select WBT occurrence percentage

according to how critical CWT will

be to your process

Selection ParametersDesign wet bulb temperature by geography

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Other Possible Considerations

� Fan or pump power limitations

� Space limitations

� Noise limitations

� Plume limitations

� Water consumption limitations

� Elevation > 500’ above MSL


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Load CharacteristicsVarying Heat Load

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Tower Size as °F (approach)Cold water temperature minus wet bulb temperature


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Cold Water vs. Wet Bulb

20 30 40 50 60 70 80

40

50

60

70

80

90

100

10°F RANGE

TYPICAL COOLING TOWER PERFORMANCEFULL LOAD – FULL FAN SPEED – FULL WATER FLOW

CO

LD W

ATE

R T

EM

PE

RAT

UR

E (

oF

)

WET BULB TEMPERATURE ( oF)


January 21, 2014

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Types of Cooling Towers


January 21, 2014

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Induced Draft Crossflow – Factory Assembled

Characteristics:

� Air flows across the falling water (crossflow)

� Gravity water distribution –lower pump head, cleanable while operating

� Larger footprint

� Tall, accessible plenum –easy mechanical access

� High discharge velocity (resists recirculation)


January 21, 2014

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Induced Draft Crossflow Factory Assembled – Film Fill

Advantages:

� Maintenance access

� Cleaning during operation

� Low fan power

� Reversible fans

� Better ice control

� Low profile

� Low pump head

� Lower noise

Disadvantages:

� Larger footprint vs. counterflow film fill

� Not for dirty water process


January 21, 2014

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Induced Draft Counterflow – Factory Assembled

Characteristics:

� Air flows counter to the falling water (counterflow)

� Pressurized water distribution –higher pump head, must shut down to clean nozzles

� Stacked components – smaller footprint

� Short, crowded plenum –restricted mechanical access

� High discharge velocity (resists recirculation)


January 21, 2014

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Induced Draft Counterflow Factory Assembled – Film Fill

Advantages:

� Small footprint vs crossflow

� Low fan power vs forced draft

Disadvantages:

� Internal access

� Distribution system cleaning

� Sound (falling water)

• Higher pump head


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Forced Draft CounterflowCharacteristics:


� Pressurized water distribution – higher pump head, must shut down to clean nozzles

� Stacked components –smaller footprint

� Pressurized box – must be kept sealed to prevent leakage

� Low discharge velocity –(subject to recirculation if outdoors)


January 21, 2014

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Forced Draft Counterflow – Film Fill

Advantages:

� Ideal for indoor (ducted) installations

� External static applications

� Low noise

� Low profile available

Disadvantages:

� 2x propeller fan horsepower

� Can’t reverse fans

� Prone to recirculation (outdoors)

� Sensitive to icing (outdoors)


January 21, 2014

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Closed Loop – Fluid Cooler

Fluidcoolers prevent process fluid contamination fr om airborne matter

Characteristics:

� Process fluid in closed loop

� Redistribution pump circulates cooling water over cooling medium, coils

� Available in crossflow or counterflow (crossflow shown)

� Available in induced or forced draft (induced draft shown)

� Closed loop cooling can also be accomplished with open cooling tower and heat exchanger


January 21, 2014

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Induced Draft Counterflow – Field Assembled

Characteristics:


� Pressurized water distribution, must shut down to clean nozzles

� Used for larger applications (typically > 10,000 tons)

� Fiberglass or concrete structure

� Fewer individual cells (vsfactory-assembled towers)

� Fewer, but larger fan motors


January 21, 2014

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Induced Draft Counterflow Field Assembled – Film Fill

Advantages:

� Typically, reduced energy usage (vs factory-assembled)

� Fewer piping and electrical connections

� Architectural design flexibility

� Blend into building design

� Aesthetics

Disadvantages:

� Internal access

� Distribution system cleaning

� Sound (falling water)

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Components


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Tower Components

Mechanical equipment

Water distribution

Drift eliminators

Structure

Fill

Inlet Louvers

Water collection/storage

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Heat Transfer Surfaces


January 21, 2014

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Film Fill - Counterflow

� PVC sheets

� Stretches water into a thin film on surface of PVC sheet

� Heat transfer takes place on the surface of the water film

� Much more surface available for cooling than splash fills

� Allows for smaller towers

� Low tolerance to fouling or to uneven water distribution

� Variations in fill design are available for poor water quality applications

High Efficiency

Low Fouling


January 21, 2014

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Film Fill - Crossflow

� PVC sheets

� Stretches water into a thin film on surface of PVC sheet

� Heat transfer takes place on the surface of the water film

� Much more surface available for cooling than splash fills

� Low tolerance to fouling or to uneven water distribution

� Allows for smaller towers

Crossflow Film Fill

(w/integral louvers and eliminators)


January 21, 2014

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Splash Fill (Crossflow shown – also Counterflow)

� Forms droplets of water

� Heat transfer takes place on surface of droplets

� Tolerant of fouling

� Variety of materials and shapes

� Substantially reducedperformance vs. film fill

� Limited to dirty water or high-temp process today


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Capacity Control


January 21, 2014

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Capacity Control

� CAPACITY varies directly with the fan speed ratio� Half speed on the fan produces 50% of cooling capacity

FAN hp varies with the cube of the speed ratio� 0.53 = 0.125

1/2 fan speed delivers:

1/2 the cooling

At 1/8 of the hp


January 21, 2014

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� Save Energy:� Run all fans on VFDs

� Ramp all fans up and down together

� Control the tower to minimize SYSTEM ENERGY

� Chillers save 1-3% kW/ton per 1 deg F reduction in CW supply temp

� Chiller kW (hp) is typically 10x the cooling tower fan hp

� Spend a little at the tower to save a lot at the chiller

� Adjust CHILLER selections to take advantage of local design WB

� 78-80 WB � 85 CW

� 70 WB � 75-80 CW

� 65 WB � 70-75 CW

Capacity Control


January 21, 2014

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� Save Energy - Condenser water setpoint:

� Do NOT modulate the fan to:maintain the ‘design’ CW temp(typically 85 deg CW)

� DO modulate the fan to: maximize chiller efficiency(typically 75 deg, 70 deg or lower – depends on the chiller)

Capacity Control


January 21, 2014

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Capacity ControlVarying Fan Speed…� Tower cooling capacity varies directly with fan speed

� 100% speed provides 100% performance� 75% speed provides 75% performance� 50% speed provides 50% performance

� Tower fan energy varies with the cube of the speed ratio� 100% speed: 1.003 = 100.0% hp� 75% speed: 0.753 = 42.2% hp� 50% speed: 0.503 = 12.5% hp

Varying Flow Rate…� Pump affinity laws work the same way

� Tower design must be able to handle wide range of flow rates


January 21, 2014

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Traditional Example - 3000 Ton Chiller Plant4 @ 750 ton chillers and 4-cell tower

9000 GPM, 95-85-78 4 cells @ 40 hp/fan

75% Load – 3 chillers and 3 towers running

High speed

High speed

High speed

Total Fan HP

End Results: 75% of the energy @ 75% load with traditional approach


January 21, 2014

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Variable Flow Example – 75% Load4 @ 750 ton chillers and 4-cell tower

9000 GPM, 95-85-78 4 cells @ 40 hp/fan

Now @ 75% Load – 3 chillers and 4 towers running

75% speed

75% speed

75% speed

75% speed

Total Fan HP

End Results: 67.6/120 = 44% energy savings versus traditional


January 21, 2014

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What could go wrong?

1. Poor water distribution2. Poor air distribution3. Unpredictable thermal performance4. Inefficient/wasteful energy usage5. Scale formation on fill6. Ice formation in cold climates

Reducing the flow too much can result in…

Maintaining adequate water distribution minimizes these problems!!!!


January 21, 2014

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Variable Flow Issues that Must be Addressed!

January 24, 2014

Design for proper nozzle hydraulics• Define minimum and maximum operating flowrates• Assure tower manufacturer designs for these ranges

Maintain uniform air-side pressure drop• Generally, proper water distribution will result in

appropriate air distribution• Avoids problems with scaling, ice formation, poor

performance


January 21, 2014

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Water Losses

January 21, 2014

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Water Losses

Evaporation� Vapor, pure water

� Max. 1.0% flow rate per 10º F Range

� Varies with heat load and temperatures

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Water Losses

Blowdown (bleedoff)� Liquid circulating water - with

high solids content - that is purged & replaced with makeup water containing a lower level of solids, thus avoiding precipitation of solids.

� Quantity = f (desired cycles of concentration, based on make-up water chemistry). Number of cycles determined by water treatment provider.

January 21, 2014

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Water Losses

Drift� Liquid water drops entrained in discharge air, <0.005% of flow

� Water loss is statistically insignificant, but drift can still cause problems with:

� spotting of nearby windows, cars

� solid deposits on nearby infrastructure

� ice buildup

� Should not be a problem with modern drift eliminators maintained in good condition


January 21, 2014

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Water Losses - Sample

� Assume: 1000 ton system (3000 gpm, 10 F range, 77 WB,3 cycles of concentration, 0.005% drift)

� Cooling tower consumption:

• Evaporation: 3000 x 0.01 = 30.00 gpm (66.7%)

• Blowdown (@ 3 Cycles)

B = (E-[(C-1) x D])/(C-1) = 14.85 gpm (33.3%)

• Drift: 3000 x 0.00005 = 0.15 gpm

Total = 45.00 gpm

� In this example, water usage is 1.5% of circulating water flow


January 21, 2014

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Here’s What We Covered…….

� Principles of Operation

� Selection Parameters

� Types of Towers

� Components

� Heat Transfer Surfaces

� Capacity Control

� Water Losses

January 21, 2014

Slide No.: 62


Questions?

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