Presented By: Kent Martens SPX Cooling Technologies, Inc. Slide No.: 1
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 Slide No.: 2
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. Slide No.: 3
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. Slide No.: 4
STD-201 CTI Certification Program 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). Slide No.: 5
Please visit our website at www.cti.org Slide No.: 6
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. Slide No.: 7
Cooling Tower Fundamentals and Design Principles of Operation Selection Parameters Types of Towers Components Heat Transfer Surfaces Capacity Control Water Losses Slide No.: 8
Principles of Operation Slide No.: 9
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. Slide No.: 10
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 Slide No.: 11
How Does It Work? Cooling Tower Process 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 Slide No.: 12
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 T A Film of air over water droplet at T B Q. TA < TB Air enthalpy < Water enthalpy Slide No.: 13
The Cooling in Cooling Towers Sensible/Latent heat transfer Sensible heat transfer from contact of relatively cold air with hot water, just as in air-cooled heat exchangers, radiators Latent heat of vaporization as with perspiration Slide No.: 14
Sensible The Cooling in Cooling Towers Sensible Cooling of 1 lb of water 1 deg F rejects 1 btu. 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 Slide No.: 15
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 Slide No.: 16
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 Heat of compression Gross heat rejection = 12,000 btuh = 3,000 btuh = 15,000 btuh Based on electric-drive compression chillers Standard CTI conditions for cooling towers 3 gpm/ton, 95 o F 85 o F 78 o F WB Slide No.: 17
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!! Slide No.: 18
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) Slide No.: 19
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: Gross Heat Rejection Tower Duty Impact on Tower Size Electric drive centrifugal, recip or scroll compression 15,000 btuh/ton 3 gpm/ton, 95-85-78 1.0 Water source heat pump 16-19,000 btuh/ton 2.92 gpm, 102-90-78 (use fluid cooler) Gas-fired absorber 22,500 btuh/ton 4.5 gpm/ton, 95-85-78 or 3 gpm/ton, 100-85-78 1.5x 1.25x Steam-fired absorber or Steam-driven centrifugal 30,000 btuh/ton 3.33 gpm, 103-85-78 1.6x Slide No.: 20
Selection Parameters Slide No.: 21
Selection Parameters Common Terms: Flow rate (gpm) HW (condenser LWT) CW (condenser EWT) WB (entering wet bulb) Range = HW CW Approach = CW WB Example: = 3,000 gpm = 95º F = 85º F = 78º F = 10º F = 7º F Slide No.: 22
Selection Parameters What determines which? Determined by amount of heat from process Determined by the cooling tower Nature Slide No.: 23
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 Slide No.: 24
Selection Parameters Design wet bulb temperature by geography See ASHRAE Fundamentals, 2013 Edition for CD ROM Slide No.: 25
Selection Parameters Design wet bulb temperature by geography Select WBT occurrence percentage according to how critical CWT will be to your process Design WBT Value Hours/year at or above value 0.4% 35 1.0% 88 2.0% 175 Slide No.: 26
Selection Parameters Other Possible Considerations Fan or pump power limitations Space limitations Noise limitations Plume limitations Water consumption limitations Elevation > 500 above MSL Slide No.: 27
Load Characteristics Varying Heat Load Slide No.: 28
Tower Size as F (approach) Cold water temperature minus wet bulb temperature Slide No.: 29
Cold Water vs. Wet Bulb TYPICAL COOLING TOWER PERFORMANCE FULL LOAD FULL FAN SPEED FULL WATER FLOW 100 COLD WATER TEMPERATURE ( o F) 90 80 70 60 50 10 F RANGE 40 20 30 40 50 60 70 80 WET BULB TEMPERATURE ( o F) Slide No.: 30
Types of Cooling Towers Slide No.: 31
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) Slide No.: 32
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 Slide No.: 33
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) Slide No.: 34
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 Slide No.: 35
Characteristics: Forced Draft Counterflow Air flows counter to the falling water (counterflow) 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) Slide No.: 36
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) Slide No.: 37
Characteristics: Closed Loop Fluid Cooler 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 Fluidcoolers prevent process fluid contamination from airborne matter Slide No.: 38
Induced Draft Counterflow Field Assembled Characteristics: Air flows counter to the falling water (counterflow) Pressurized water distribution, must shut down to clean nozzles Used for larger applications (typically > 10,000 tons) Fiberglass or concrete structure Fewer individual cells (vs factory-assembled towers) Fewer, but larger fan motors Slide No.: 39
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) Slide No.: 40
Components Slide No.: 41
Tower Components Mechanical equipment Water distribution Drift eliminators Structure Fill Inlet Louvers Water collection/storage Slide No.: 42
Heat Transfer Surfaces Slide No.: 43
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 Slide No.: 44
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) Slide No.: 45
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 reduced performance vs. film fill Limited to dirty water or hightemp process today Slide No.: 46
Capacity Control Slide No.: 47
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 0.5 3 = 0.125 1/2 fan speed delivers: 1/2 the cooling At 1/8 of the hp Slide No.: 48
Capacity Control 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 Slide No.: 49
Capacity Control 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) Slide No.: 50
Capacity Control Varying 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: 1.00 3 = 100.0% hp 75% speed: 0.75 3 = 42.2% hp 50% speed: 0.50 3 = 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 Slide No.: 51
Traditional Example - 3000 Ton Chiller Plant 4 @ 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 Total Fan HP High speed High speed High speed End Results: 75% of the energy @ 75% load with traditional approach Slide No.: 52
Variable Flow Example 75% Load 4 @ 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 Total Fan HP 75% speed 75% speed 75% speed 75% speed End Results: 67.6/120 = 44% energy savings versus traditional Slide No.: 53
What could go wrong? Reducing the flow too much can result in 1. Poor water distribution 2. Poor air distribution 3. Unpredictable thermal performance 4. Inefficient/wasteful energy usage 5. Scale formation on fill 6. Ice formation in cold climates Maintaining adequate water distribution minimizes these problems!!!! Slide No.: 54
Variable Flow Issues that Must be Addressed! 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 24, 2014 Slide No.: 55
Water Losses Slide No.: 56
Water Losses Evaporation Vapor, pure water Max. 1.0% flow rate per 10º F Range Varies with heat load and temperatures Slide No.: 57
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. Slide No.: 58
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 Slide No.: 59
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 Slide No.: 60
Here s What We Covered. Principles of Operation Selection Parameters Types of Towers Components Heat Transfer Surfaces Capacity Control Water Losses Slide No.: 61
Questions? Slide No.: 62