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2006 01-16 evap techniques r seidl

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michaeljmack

This document discusses indirect evaporative cooling systems using cooling towers. It provides an overview of three strategies - a direct 1-stage model, indirect 1-stage model, and 2-stage model. At the design level, the direct 1-stage model has the lowest energy usage at 0.167 kW/ton, while the 2-stage model has the highest at 0.541 kW/ton. However, the document notes that design values do not show the full picture, and an annual simulation is needed to evaluate annual energy use and life cycle costs of the different models.

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1 Indirect Evaporative Cooling Systems Using Cooling Towers Rev.1 – 01/16/06 Reinhard Seidl, P.E. Taylor Engineering 1 Overview Goals for today Why use systems without compressors? 3 Strategies for multi-stage cooling using cooling towers Where are these applicable? How does it work? Energy and initial cost considerations Wrap up
2 2 Overview Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis 3 Overview Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
3 Why use systems without compressors? 5 Cooling Without Compressors? Energy consumption can be greatly reduced Air-cooled chiller consumes about 1.2 kW/ton Water-cooled chiller & tower consumes about 0.8 kW/ton. Cooling tower consumes about 0.1 kW/ton.
4 Where is this applicable? 7 Applications Low temperatures can’t be achieved Only useful for systems with relatively warm air supply (65°F) Won’t work in “wet” climates like Florida, but will work in dry climates like bay area, Arizona and the like Underfloor (UFAD) Systems that move a lot of air by design (Labs, Hospitals)
5 8 Air cooled chiller-design values Chiller: 1.2 kW/ton CHW pump: 0.05 kW/ton Total: 1.25 kW/ton 9 Water cooled chiller-design values Towers: 0.1 kW/ton CW pump: 0.05 kW/ton Chillers: 0.6 kW/ton CHW pump:0.05 kW/ton Total: 0.8 kW/ton
6 10 Cooling tower-design values Towers: 0.1 kW/ton Note: using an oversized tower dramatically reduces tower energy use. For example, using a tower with twice the surface area results in a reduction of 50% in airflow, which in turn reduces fan energy to (½)3 or about 1/8th of the original fan energy. In this sense, any discussion about tower energy is only meaningful when part-load energy consumption values and initial investment are considered along with energy use. 11 Cooling tower – basic operation Closed towers (indirect system) and open towers (direct system) Open tower: water falls down and air passing over water evaporates some of it, cooling both air and water. Imagine taking a shower in a strong wind -> evaporation provides cooling effect Open tower: cooling water is exposed to outside air
7 12 Cooling tower – basic operation The ambient wet-bulb is a measure of the humidity. The higher the wet-bulb, the more humid the air. Wet-bulb temperature can never exceed dry-bulb temperature. Dry-bulb temperature is what we commonly refer to as just temperature The leaving water temperature of the cooling tower can never be less than the wet-bulb temperature of the entering air. 13 Cooling tower – basic operation RANGE: entering water temperature – leaving water temperature. In this example: 90°F - 80°F = 10°F APPROACH: difference between leaving water temperature and ambient wet-bulb temperature. In this example: 80°F – 62°F = 18°F The closer the approach, the more fan energy the tower will require, and the larger its surface will have to be Tr (10°F Ta (18°F)
8 14 Cooling tower – basic operation Closed tower: a closed coil isolates the cooling water from the water, circulated through the tower. This means less problems with water treatment for coils served by cooling water Range and approach definitions remain the same A closed circuit tower will be less effective (greater fan energy per unit of cooling) than an open tower at the same size 15 Cooling tower – basic operation Cooling towers work better in dry climates like Arizona, because the ambient wet-bulb there is lower That means colder water leaving the tower, for the same fan energy It also means more water is evaporated
9 16 Psychrometric diagram More detail on psychrometrics 17 120 tons capacity, low airflow Δh Δh * tower airflow (lbs/h) = cooling tower capacity (Btu/h) Example: Flow = 160 gpm Range = 18°F Capacity =160*18*500/12000 = = 120 tons Airside flow has to be: h1=(62° wb) = 27.7 Btu/lb h2=(70° wb) = 34.1 Btu/lb Φ=120 tons/Δh = 3,756 lbs/min Density ρ @ Ta,in = 0.0728 lb/ft3 Airflow = Φ/ρ = 51,590 cfm Tw,in=84°FTw,out=66°F Range=18°F Approach=4°F Ta,in=83/62°F Ta,out=70/70°F
10 18 120 tons capacity, higher airflow Δh Δh * tower airflow (lbs/h) = cooling tower capacity (Btu/h) Example: Flow = 160 gpm Range = 18°F Capacity =160*18*500/12000 = = 120 tons Airside flow has to be: h1=(62° wb) = 27.7 Btu/lb h2=(68° wb) = 32.4 Btu/lb Φ=120 tons/Δh = 5,085 lbs/min Density ρ @ Ta,in = 0.0728 lb/ft3 Airflow = Φ/ρ = 69,845 cfm Tw,in=84°FTw,out=66°F Range=18°F Approach=4°F Ta,in=83/62°F Ta,out=68/68°F 19 Pre-cooling effect Δh Δh * tower airflow (lbs/h) = cooling tower capacity (Btu/h) Example: When pre-cooling is not applied, the tower operates at roughly 70,000 cfm and 120 tons capacity to bring water within 4°F of entering air wet- bulb temperature (4°F approach). Note that the physical size of the tower determines the approach. A smaller tower, operating with the same airflow, would produce a higher approach (a higher leaving water temperature, less range) and would require a higher water flow rate for the same capacity. Tw,in=84°FTw,out=66°F Range=18°F Approach=4°F Outside air Leaving air, no pre-cooling
11 20 Pre-cooling effect Δh Δh * tower airflow (lbs/h) = cooling tower capacity (Btu/h) With pre-cooling, the entering air wet-bulb is reduced. The same tower can now produce colder leaving water temperatures. This also means a larger range, and less water flow for the same capacity. Note that, as the pre-cooling effect pushes the condition of air entering the tower closer to the saturation line, the sensible/total heat ratio of air passing through the tower changes, to maintain the same Δh and tower capacity. Tw,in=84°FTw,out=61°F Range=23°F Approach=4°F Outside air Leaving air, no pre-cooling Pre-cooled air Δh 21 Overview Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
12 22 Overview 2-Stage model (Shlomo Rosenfeld) Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis Direct 1-Stage model (Loek Vaneveld) Indirect 1-Stage model (Mark Hydeman) 3 Strategies for multi-stage cooling using cooling towers
13 24 Direct 1-Stage Design Note this is an open tower design. This may not be acceptable in some cases where concerns over water treatment and fouling of coils exist. In such a case, the use of a plate heat exchanger (typically employed on open towers) will not work, since the temperature losses inherent in such an approach make the design impractical. ? 25 Pre-cooling effect-special case Δh Note this slide shows the principle. The actual values for the tower under consideration are different (next slide) Note that tower pre-cooling energy extracted from the air is re- introduced into the tower through the water, and has to be cooled within the tower. Leaving air temperature is the same with or without pre-cooling, for the same airflow through the tower. Δh = Building or Process Load Δhp = Pre-cooling Load Tw,in=84°FTw,out=61°F Range=23°F Approach=4°F Outside air Leaving air Δh Δhp Pre-cooled air Δhp Dashed line = tower without pre-cooling, for same process load with same tower airflow
14 26 Direct 1-Stage Design Actual Values Note this is an open tower design. This may not be acceptable in some cases where concerns over water treatment and fouling of coils exist. In such a case, the use of a plate heat exchanger (typically employed on open towers) will not work, since the temperature losses inherent in such an approach make the design impractical. ? 79.7 tons pre-cooling 200 tons tower capacity 27 Indirect 1-Stage Design CT1/2: 180 gpm 78°F - 62°F = 120 tons @ 50 Hp 0.311 kW/ton CT3/4: 172 gpm 81°F – 66°F at 15 Hp, relate to orig.120 tons 0.093 kW/ton requires 7.5 Hp and 3.0 Hp spray pumps 0.065 kW/ton Total 0.469 kW/ton Evaporation water usage 0.037 gpm/ton
15 28 Indirect 1-Stage Design CT1/2: 180 gpm 78°F - 62°F = 120 tons @ 50 Hp 0.311 kW/ton CT3/4: 172 gpm 81°F – 66°F at 15 Hp, relate to orig.120 tons 0.093 kW/ton requires 7.5 Hp and 3.0 Hp spray pumps 0.065 kW/ton Total 0.469 kW/ton Evaporation water usage 0.037 gpm/ton 29 2-Stage Design CCT1/2 each: 180 gpm 78°F - 67°F = 82.5 tons @ 25 Hp 0.155 kW/ton* CCT3/4 each: 180 gpm 67°F - 62°F = 37.5 tons @ 50 Hp 0.311 kW/ton requires 5 Hp, 5 Hp, 2 Hp spray pumps 0.075 kW/ton Total 0.541 kW/ton Evaporation water usage 0.027 gpm/ton
16 30 2-Stage Design CCT1/2 each: 180 gpm 78°F - 67°F = 82.5 tons @ 25 Hp 0.155 kW/ton* CCT3/4 each: 180 gpm 67°F - 62°F = 37.5 tons @ 50 Hp 0.311 kW/ton requires 5 Hp, 5 Hp, 2 Hp spray pumps 0.075 kW/ton Total 0.541 kW/ton Evaporation water usage 0.027 gpm/ton Note: all kW/ton calculations are based on the total output (120 tons) of the system, not on the individual capacity of each tower 31 2-Stage Design CCT1/2 each: 180 gpm 78°F - 67°F = 82.5 tons @ 25 Hp 0.155 kW/ton* CCT3/4 each: 180 gpm 67°F - 62°F = 37.5 tons @ 50 Hp 0.311 kW/ton requires 5 Hp, 5 Hp, 2 Hp spray pumps 0.075 kW/ton Total 0.541 kW/ton Evaporation water usage 0.027 gpm/ton 73.4 tons pre-cooling 111 tons tower capacity 73.4 tons heat re-gain
17 32 Energy and Water Usage at Design Conditions Direct 1-Stage 0.167 kW/ton 0.032 gpm/ton Indirect 1-Stage 0.469 kW/ton 0.037 gpm/ton 2-Stage 0.541 kW/ton 0.027 gpm/ton Note that these values are fairly easily derived, but don’t show the whole picture. 33 Coil Calculations Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
18 34 Annual Simulation Could use a spreadsheet, maybe DOE2. Spreadsheet offers more flexibility Look at bin weather data file to run a simulation Use approximation of coil performance to arrive at results for varying airflows and air entering temperatures. Use LMTD and ε-NTU methods. 35 Bin Data and Building Load Match each temperature bin with a building load. This building load doesn’t necessarily have to be exact – a rough approximation is sufficient since we’re trying to find a comparison between models rather than an absolute number. Use the dry-bulb and coincident wet-bulb to predict how coil performance at tower inlets will vary.
19 36 Sample Bin Data SAN FRANCISCO, CLIMATE ZONE 3 DEG N LAT 38 ELEV 52 FT MAY JUN JULY AUG SEP OCT Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B TEMP RANGE 115/119 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 115/119 110/114 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 110/114 105/109 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 105/109 100/104 0 0 0 0 0 0 0.5 0 0.5 70 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 100/104 95/99 0 0 0 0 0 0 0.2 0 0.2 71 0 0.1 0 0.1 67 0 0 0 0 0 0 0.2 0 0.2 66 0 0 0 0 0 95/99 90/94 0 0.3 0 0.3 67 0 1.9 0.1 2 71 0 1 0 1 65 0 0.3 0 0.3 65 0 2.6 0 2.6 65 0 0.7 0 0.7 66 90/94 85/89 0 0.7 0 0.7 65 0 4.1 0.6 4.7 66 0 2.8 0 2.8 65 0 1.9 0 1.9 65 0 6.8 0.3 7.1 60 0 2 0.2 2.2 65 85/89 80/84 0 2.8 0.1 2.9 66 0 8.7 1.3 10 62 0 4.6 0.6 5.2 63 0 6.5 0.5 7 60 0 12 1.2 13 62 0 6.9 0.5 7.4 62 80/84 75/79 0 3.5 0.5 4 60 0.4 16 2.4 18.7 61 0.1 14 1.8 16.1 61 0 19 1.2 19.9 61 0.2 20 4.9 25 60 0.1 12 1.9 13.7 60 75/79 70/74 0 18 1.2 19.1 58 2 32 5.1 39.1 60 0.6 50 4.8 55.8 60 0.3 58 6.7 64.8 61 2.3 54 11 67.4 59 0 29 5.1 34.1 58 70/74 65/69 1.9 42 5.7 49.6 58 5.4 70 17 91.7 59 3 100 19 122 59 4.9 98 24 127 59 11 85 30 125 59 5.5 76 19 101 57 65/69 60/64 12 101 26 140 56 31 83 51 164 56 31 61 63 155 57 39 55 76 170 57 52 48 79 179 57 32 87 66 184 56 60/64 55/59 56 71 88 215 52 83 24 103 209 53 123 13 120 255 54 137 9.3 114 261 55 109 11 87 207 55 100 32 110 242 54 55/59 50/54 134 7.9 116 258 50 109 0.9 60 170 51 91 0.7 40 131 52 63 0.1 25 88.3 52 62 0.6 26 88.5 52 94 2.8 44 140 51 50/54 45/49 43 0.2 11 54 46 8.8 0 0.8 9.6 47 0.2 0 0 0.2 48 3.5 0 0.3 3.8 48 4.2 0 0.2 4.4 47 17 0 1.8 18.3 46 45/49 40/44 0.9 0 0 0.9 59 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.8 0 0 0.8 41 40/44 35/39 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 35/39 30/34 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 30/34 25/29 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 25/29 20/24 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 20/24 15/19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 15/19 NOV DEC JAN FEB MAR APR ANNUAL TOTAL S U M M E R W I N T E R Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total Obsn Hours Total 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs 01 to 08 09 to 16 17 to 24 Obsn Hrs 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 115/119 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 110/114 0 0 0 0 73 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 105/109 0 0 0 0 71 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 100/104 0 0.5 0 0.5 70 0 0.5 0 0.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 95/99 0 0.5 0 0.5 68 0 0.5 0 0.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 90/94 0 6.8 0.1 6.9 66 0 6.8 0.1 6.9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.3 0 0.3 63 85/89 0 18.6 1.1 19.7 65 0 18.3 1.1 19.4 0 0.3 0 0.3 0 0.4 0 0.4 60 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2.1 0.1 2.2 65 80/84 0 43.8 4.3 48.1 63 0 41.3 4.2 45.5 0 2.5 0.1 2.6 0 3.1 0 3.1 60 0 0.1 0 0.1 55 0 0 0 0 0 0 0 0 0 0 0 1.4 0 1.4 56 0 5.8 0.6 6.4 60 75/79 0.8 94.3 13.3 108 61 0.8 83.9 12.7 97.4 0 10.4 0.6 11 0 8.5 0.5 9 56 0 1.9 0 1.9 56 0 0.2 0 0.2 59 0 0.3 0 0.3 58 0 5.3 0.7 6 56 0 12 2.1 14 59 70/74 5.2 269 37.4 312 58 5.2 241 34.1 280 0 28.1 3.3 31.4 0 28 2.7 30.9 56 0 5.3 0.8 6.1 53 0 2.3 0.3 2.6 55 0 8 0.5 8.5 52 0.3 12 2.9 15.3 53 0.5 28 4.7 33.5 57 65/69 32.1 555 126 714 56 31.3 471 114 617 0.8 84.2 11.9 96.9 2.5 58 23 82.8 55 4.4 24 6.7 34.6 56 2.4 13 3.5 19.3 56 1.6 36 7.6 45.1 55 0.8 38 6.6 45.7 52 4.3 65 15 83.6 54 60/64 214 668 421 1303 54 198 435 360 992 16 234 61.5 311 40 85 76 201 52 17 69 38 125 53 12 55 32 98.5 53 29 91 60 180 53 15 113 46 174 51 29 92 54 174 52 55/59 748 667 927 2341 51 607 161 621 1389 140 506 306 952 78 46 95 219 49 42 82 78 202 49 41 88 73 201 49 62 68 88 218 49 84 71 127 281 48 104 33 121 258 49 50/54 962 400 892 2255 48 552 13 310 875 410 387 582 1380 81 10 39 130 44 82 53 87 222 45 61 67 87 214 44 72 21 55 148 45 105 5.8 62 173 45 87 2 41 130 45 45/49 563 159 384 1107 44 76.1 0.2 14 90.3 487 159 370 1017 35 0.2 4.4 39.4 40 75 12 35 123 40 82 20 44 146 40 48 1.9 14 63.6 40 40 1 3.1 44.3 40 15 0.1 1.6 16.9 41 40/44 297 35.3 102 435 40 1.7 0 0 1.7 295 35.3 102 433 4.1 0 0.1 4.2 36 26 0.5 2.4 29 36 43 2.5 8 53.1 35 14 0.2 1 15.5 37 3.5 0 0 3.5 37 0.3 0 0 0.3 36 35/39 90.9 3.2 11.5 106 36 0 0 0 0 90.9 3.2 11.5 106 0 0 0 0 0 1.4 0 0 1.4 33 7.4 0.4 0.6 8.4 31 0.4 0 0 0.4 33 0 0 0 0 0 0 0 0 0 0 30/34 9.2 0.4 0.6 10.2 31 0 0 0 0 9.2 0.4 0.6 10.2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 25/29 0 0 0 0 26 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 20/24 0 0 0 0 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 15/19 0 0 0 0 0 0 0 0 0 0 0 0 0 37 Sample Bin Data Temperature values below the 65°F mark can be ignored for the simulation, since the system will be in economizer. ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0
20 38 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0 39 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 92° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0
21 40 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 87° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0 41 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 82° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0
22 42 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 77° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0 43 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 72° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0
23 44 Calculation for Off-Design Values Lower wb means: Tower fan can run at less than 100%. How will coil react to less airflow, and what will pre- cooling effect be? 67° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0 45 Coil Calculations Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
24 46 Coil Calculations Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis Skip coil calculations 47 Coil Calculations-Step 1 Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
25 48 Coil Calculation - LMTD LMTD method: ΔT1= Thot,in – Tcold,out and ΔT2= Thot,out – Tcold,in ΔT2 – ΔT1 ΔTlmtd = ----------------- ln (ΔT2/ ΔT1) Q = UA ΔTlmtd Use this method to determine UA, based on the temperatures we expect from design. In other words, we don’t care exactly how the U and A are derived (fin spacing, number of rows etc). We’ll just assume that for the given problem, a coil can be purchased with the right UA. We will then use this number to simulate how that coil will operate under different conditions. 49 Coil Calculation – LMTD Example LMTD method: ΔT1= 83° – 80° and ΔT2= 68.2° – 65° 3.2°-3.0° ΔTlmtd = ----------------- = 3.1° ln (3.2°/ 3.0°) Q = UA * 3.1° = 125 gpm * 15°= 937.5 MBH UA = 302,524 Btu/h°F This number UA, which represents the overall heat transfer coefficient of the coil, can now be used to calculate performance under different conditions.
26 50 Coil Calculations-Step 2 Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis 51 Coil Simulation – ε-NTU ε-NTU method: By taking the UA we calculated earlier, and using the mass flow rates for each medium and the specific heat, we can determine what the leaving temperatures will be, based on the calculated effectiveness ε ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− = max min minmax min max min min C C 1 C UA exp C C 1 C C 1 C UA exp1 ε Hot act inHotoutHot C Q TT −= ,, Cold act in,Coldout,Cold C Q TT += Maxact inColdinHotMinMax QQ TTCQ ε= −= )( ,,
27 52 Coil Simulation – ε-NTU ε-NTU method: Note that the formula shown below for ε only holds for a perfect counterflow heat exchanger. For other types (most real heat exchangers are somewhere between a counter flow and parallel-flow exchanger). For such a case, the NTU is calculated, and ε is read off a chart Perfect Counter-flow Parallel flow – Counter-flow ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− = max min minmax min max min min C C 1 C UA exp C C 1 C C 1 C UA exp1 ε minC UA NTU = ε 1 C C max min = 0 C C max min = 53 Coil simulation – ε-NTU ε-NTU method: Note that the formula shown below for ε only holds for a perfect counterflow heat exchanger. For other types (most real heat exchangers are somewhere between a counter flow and parallel-flow exchanger). For such a case, the NTU is calculated, and ε is read off a chart Perfect Counter-flow Parallel flow – Counter-flow ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− = max min minmax min max min min C C 1 C UA exp C C 1 C C 1 C UA exp1 ε minC UA NTU = ε 1 C C max min = 0 C C max min = We will use only this method
28 54 Coil Simulation – ε-NTU Example ε-NTU method: Design at 83° / 68.2°F for 57,700 cfm of air and 65° / 80°F for 125 gpm of water (78.3 tons or 937 MBH) Reduce fan speed, use 40,000 cfm and Ambient reduced to 67°F Performance now: Air at 67° / 65.1°F for 40,000 cfm and 65° / 66.3°F for 125 gpm of water (7 tons or 84 MBH) 55 Coil Simulation – ε-NTU Example ε-NTU method: Design at 83° / 68.2°F for 57,700 cfm of air ε =0.83 and 65° / 80°F for 125 gpm of water Qmax = 1,126 MBH (78.3 tons or 937 MBH) Reduce fan speed, use 40,000 cfm and Ambient reduced to 67°F Performance now: Air at 67° / 65.1°F for 40,000 cfm ε =0.96 and 65° / 66.3°F for 125 gpm of water Qmax = 88 MBH (7 tons or 84 MBH) Note that Qmax is the amount of heat that could be exchanged with an infinitely large (or perfect) heat exchanger. ε is a measure of how well the actual exchanger under consideration approximates this ideal exchanger, and varies with selected temperatures and flows.
29 56 Coil Calculations-Step 3 Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis 57 Coil Simulation – ε-NTU Example ε-NTU method: Design at 83° / 68.2°F for 57,700 cfm of air ε =0.83 and 65° / 80°F for 125 gpm of water Qmax = 1,126 MBH (78.3 tons or 937 MBH) Fh Btu 396,63 h min 60* cuft lb 0763.0*cfm700,57* Flb Btu 24.0cC 111 ° = ° =φ= Fh Btu 550,62 h min 60* gal lb 34.8*gpm125* Flb Btu 0.1cC 222 ° = ° =φ=
30 58 Coil Simulation – ε-NTU Example ε-NTU method: Design at 83° / 68.2°F for 57,700 cfm of air ε =0.83 and 65° / 80°F for 125 gpm of water Qmax = 1,126 MBH (78.3 tons or 937 MBH) ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −−− = 63,396 62,550 1 62,550 302,524 exp 63,396 62,550 1 63,396 62,550 1 62,550 302,524 exp1 ε 396,63 126,938 832.68 C Q TT Hot act in,Hotout,Hot −= −= 550,62 126,938 6580 C Q TT Cold act in,Coldout,Cold −= += MBH938126,1*83.0QQ MBH126,1)6583(550,62Q Maxact Max ==ε= =−= 59 Coil Calculations-Step 4 Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
31 60 Coil simulation – ε-NTU example ε-NTU method: Design at 67° / 65.1°F for 40,000 cfm of air ε =0.96 and 65° / 66.3°F for 125 gpm of water Qmax = 88 MBH (7.0 tons or 84 MBH) Fh Btu 949,43 h min 60* cuft lb 0763.0*cfm000,40* Flb Btu 24.0cC 111 ° = ° =φ= Fh Btu 550,62 h min 60* gal lb 34.8*gpm125* Flb Btu 0.1cC 222 ° = ° =φ= 61 Coil simulation – ε-NTU example ε-NTU method: Design at 67° / 65.1°F for 40,000 cfm of air ε =0.96 and 65° / 66.3°F for 125 gpm of water Qmax = 88 MBH (7.0 tons or 84 MBH) ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −−− = 62,550 43,949 1 43,949 302,524 exp 62,550 43,949 1 62,550 43,949 1 43,949 302,524 exp1 ε 949,43 186,84 671.65 C Q TT Hot act in,Hotout,Hot −= −= 550,62 186,84 653.66 C Q TT Cold act in,Coldout,Cold −= += MBH2.848.87*96.0QQ MBH8.87)6567(949,43Q Maxact Max ==ε= =−=
32 62 Annual Energy Use Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis Back to coil calculations 63 Energy Usage from Annual Simulation Direct 1-Stage 1.7 MWh per year Indirect 1-Stage 20.4 MWh per year 2-Stage 17.5 MWh per year
33 64 Life Cycle Cost Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis 65 Initial Cost Direct 1-Stage CT1/2 $ 36,000 Coils (59,320 cfm)x2 $ 72,000 Total $ 108,000 Indirect 1-Stage CT1/2 $ 40,000 CT3/4 $ 120,000 Coils (78,500 cfm)x2 $ 92,000 Total $ 252,000 2-Stage CCT1/2 $ 125,000 CCT3/4 $ 105,000 Coils (114,000 cfm)x2 $ 145,000 Total $ 375,000 Note: pricing is for towers, and estimated coil + custom coil installation. Pricing does not include piping, valves and associated controls.
34 66 Simplified Life-Cycle Cost Direct 1-Stage first cost $ 108,000 Energy cost, 20 years $ 4,100 (1.7 MWh/a) Total $ 112,100 Indirect 1-Stage first cost $ 252,000 Energy cost, 20 years $ 49,000 (20.4 MWh/a) Total $ 301,000 2-Stage CCT1/2 $ 375,000 Energy cost, 20 years $ 42,000 (17.5 MWh/a) Total $ 417,000 Note: pricing is for equipment only. This includes towers, and estimated coil + installation of coil on tower. Pricing does not include piping, valves, rigging, setting, startup or associated controls. Direct 1-Stage also has lower water usage and maintenance costs (not included in this simple analysis) 67 Comparison to Refrigerated Model – Aircooled chiller Chiller first cost, 240 tons $ 132,000 Energy cost, 20 years $ 193,000 (80.6 MWh/a) Total $ 325,000 Note: pricing is for equipment only. This includes towers, and estimated coil + installation of coil on tower. Pricing does not include piping, valves, rigging, setting, startup or associated controls. Note: Chiller energy cost derived from IPLV data, published in manufacturer’s literature for a 240 ton screw chiller. Use IPLV by taking EER at 100%, 75%, 50% and 25% load to estimate energy use at each temperature bin. EER Btu W W Btu EER out in in out =→=
35 68 20-year life cycle cost 20-year Life cycle cost $0 $100,000 $200,000 $300,000 $400,000 $500,000 $600,000 $700,000 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3 0.32 Cost $ / kWh Lifecyclecost Direct Indirect 2-Stage Chiller Break-even at around $ 0.10/kWh Break-even at around $ 0.19/kWh Note: For a more realistic calculation, piping materials & labor have to be added to the calculation. This makes the chiller model look even better, and break-even occurs at higher electricity prices. 69 15-year Life cycle cost $0 $100,000 $200,000 $300,000 $400,000 $500,000 $600,000 $700,000 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3 0.32 Cost $ / kWh Lifecyclecost Direct Indirect 2-Stage Chiller 15-year life cycle cost Break-even at around $ 0.13/kWh Break-even at around $ 0.25/kWh Note: For a more realistic calculation, piping materials & labor have to be added to the calculation. This makes the chiller model look even better, and break-even occurs at higher electricity prices.
36 70 Questions ? 71 Psychrometric diagram Back to cooling tower principles WET DRY
37 72 Psychrometric diagram Back to cooling tower principles COLD HOT 73 Psychrometric diagram Back to cooling tower principles San Francisco Florida Las Vegas
38 74 Psychrometric diagram Back to cooling tower principles AbsoluteHumidity Temperature 50° 70° 90° 75 Psychrometric diagram Back to cooling tower principles Temperature 0.45 lbs water/100 lb air 1.6 lb water/100 lb air 2.7 lb water/100 lb air AbsoluteHumidity
39 76 Psychrometric diagram Back to cooling tower principles 20% RH 50% RH AbsoluteHumidity Temperature FOG 90% RH 77 Psychrometric diagram Back to cooling tower principles AbsoluteHumidity Temperature FOG For the same moisture content, warmer air has a lower relative humidity or a lower saturation rate because hot air can absorb more moisture 20% RH50% RH90% RH 1.08 lbs water/100 lb air
40 78 Psychrometric diagram Back to cooling tower principles Temperature 90°/90% 90°/33% AbsoluteHumidity 68 wb 87 wbWet bulb temp.

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2006 01-16 evap techniques r seidl

  • 1. 1 Indirect Evaporative Cooling Systems Using Cooling Towers Rev.1 – 01/16/06 Reinhard Seidl, P.E. Taylor Engineering 1 Overview Goals for today Why use systems without compressors? 3 Strategies for multi-stage cooling using cooling towers Where are these applicable? How does it work? Energy and initial cost considerations Wrap up
  • 2. 2 2 Overview Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis 3 Overview Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
  • 3. 3 Why use systems without compressors? 5 Cooling Without Compressors? Energy consumption can be greatly reduced Air-cooled chiller consumes about 1.2 kW/ton Water-cooled chiller & tower consumes about 0.8 kW/ton. Cooling tower consumes about 0.1 kW/ton.
  • 4. 4 Where is this applicable? 7 Applications Low temperatures can’t be achieved Only useful for systems with relatively warm air supply (65°F) Won’t work in “wet” climates like Florida, but will work in dry climates like bay area, Arizona and the like Underfloor (UFAD) Systems that move a lot of air by design (Labs, Hospitals)
  • 5. 5 8 Air cooled chiller-design values Chiller: 1.2 kW/ton CHW pump: 0.05 kW/ton Total: 1.25 kW/ton 9 Water cooled chiller-design values Towers: 0.1 kW/ton CW pump: 0.05 kW/ton Chillers: 0.6 kW/ton CHW pump:0.05 kW/ton Total: 0.8 kW/ton
  • 6. 6 10 Cooling tower-design values Towers: 0.1 kW/ton Note: using an oversized tower dramatically reduces tower energy use. For example, using a tower with twice the surface area results in a reduction of 50% in airflow, which in turn reduces fan energy to (½)3 or about 1/8th of the original fan energy. In this sense, any discussion about tower energy is only meaningful when part-load energy consumption values and initial investment are considered along with energy use. 11 Cooling tower – basic operation Closed towers (indirect system) and open towers (direct system) Open tower: water falls down and air passing over water evaporates some of it, cooling both air and water. Imagine taking a shower in a strong wind -> evaporation provides cooling effect Open tower: cooling water is exposed to outside air
  • 7. 7 12 Cooling tower – basic operation The ambient wet-bulb is a measure of the humidity. The higher the wet-bulb, the more humid the air. Wet-bulb temperature can never exceed dry-bulb temperature. Dry-bulb temperature is what we commonly refer to as just temperature The leaving water temperature of the cooling tower can never be less than the wet-bulb temperature of the entering air. 13 Cooling tower – basic operation RANGE: entering water temperature – leaving water temperature. In this example: 90°F - 80°F = 10°F APPROACH: difference between leaving water temperature and ambient wet-bulb temperature. In this example: 80°F – 62°F = 18°F The closer the approach, the more fan energy the tower will require, and the larger its surface will have to be Tr (10°F Ta (18°F)
  • 8. 8 14 Cooling tower – basic operation Closed tower: a closed coil isolates the cooling water from the water, circulated through the tower. This means less problems with water treatment for coils served by cooling water Range and approach definitions remain the same A closed circuit tower will be less effective (greater fan energy per unit of cooling) than an open tower at the same size 15 Cooling tower – basic operation Cooling towers work better in dry climates like Arizona, because the ambient wet-bulb there is lower That means colder water leaving the tower, for the same fan energy It also means more water is evaporated
  • 9. 9 16 Psychrometric diagram More detail on psychrometrics 17 120 tons capacity, low airflow Δh Δh * tower airflow (lbs/h) = cooling tower capacity (Btu/h) Example: Flow = 160 gpm Range = 18°F Capacity =160*18*500/12000 = = 120 tons Airside flow has to be: h1=(62° wb) = 27.7 Btu/lb h2=(70° wb) = 34.1 Btu/lb Φ=120 tons/Δh = 3,756 lbs/min Density ρ @ Ta,in = 0.0728 lb/ft3 Airflow = Φ/ρ = 51,590 cfm Tw,in=84°FTw,out=66°F Range=18°F Approach=4°F Ta,in=83/62°F Ta,out=70/70°F
  • 10. 10 18 120 tons capacity, higher airflow Δh Δh * tower airflow (lbs/h) = cooling tower capacity (Btu/h) Example: Flow = 160 gpm Range = 18°F Capacity =160*18*500/12000 = = 120 tons Airside flow has to be: h1=(62° wb) = 27.7 Btu/lb h2=(68° wb) = 32.4 Btu/lb Φ=120 tons/Δh = 5,085 lbs/min Density ρ @ Ta,in = 0.0728 lb/ft3 Airflow = Φ/ρ = 69,845 cfm Tw,in=84°FTw,out=66°F Range=18°F Approach=4°F Ta,in=83/62°F Ta,out=68/68°F 19 Pre-cooling effect Δh Δh * tower airflow (lbs/h) = cooling tower capacity (Btu/h) Example: When pre-cooling is not applied, the tower operates at roughly 70,000 cfm and 120 tons capacity to bring water within 4°F of entering air wet- bulb temperature (4°F approach). Note that the physical size of the tower determines the approach. A smaller tower, operating with the same airflow, would produce a higher approach (a higher leaving water temperature, less range) and would require a higher water flow rate for the same capacity. Tw,in=84°FTw,out=66°F Range=18°F Approach=4°F Outside air Leaving air, no pre-cooling
  • 11. 11 20 Pre-cooling effect Δh Δh * tower airflow (lbs/h) = cooling tower capacity (Btu/h) With pre-cooling, the entering air wet-bulb is reduced. The same tower can now produce colder leaving water temperatures. This also means a larger range, and less water flow for the same capacity. Note that, as the pre-cooling effect pushes the condition of air entering the tower closer to the saturation line, the sensible/total heat ratio of air passing through the tower changes, to maintain the same Δh and tower capacity. Tw,in=84°FTw,out=61°F Range=23°F Approach=4°F Outside air Leaving air, no pre-cooling Pre-cooled air Δh 21 Overview Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
  • 12. 12 22 Overview 2-Stage model (Shlomo Rosenfeld) Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis Direct 1-Stage model (Loek Vaneveld) Indirect 1-Stage model (Mark Hydeman) 3 Strategies for multi-stage cooling using cooling towers
  • 13. 13 24 Direct 1-Stage Design Note this is an open tower design. This may not be acceptable in some cases where concerns over water treatment and fouling of coils exist. In such a case, the use of a plate heat exchanger (typically employed on open towers) will not work, since the temperature losses inherent in such an approach make the design impractical. ? 25 Pre-cooling effect-special case Δh Note this slide shows the principle. The actual values for the tower under consideration are different (next slide) Note that tower pre-cooling energy extracted from the air is re- introduced into the tower through the water, and has to be cooled within the tower. Leaving air temperature is the same with or without pre-cooling, for the same airflow through the tower. Δh = Building or Process Load Δhp = Pre-cooling Load Tw,in=84°FTw,out=61°F Range=23°F Approach=4°F Outside air Leaving air Δh Δhp Pre-cooled air Δhp Dashed line = tower without pre-cooling, for same process load with same tower airflow
  • 14. 14 26 Direct 1-Stage Design Actual Values Note this is an open tower design. This may not be acceptable in some cases where concerns over water treatment and fouling of coils exist. In such a case, the use of a plate heat exchanger (typically employed on open towers) will not work, since the temperature losses inherent in such an approach make the design impractical. ? 79.7 tons pre-cooling 200 tons tower capacity 27 Indirect 1-Stage Design CT1/2: 180 gpm 78°F - 62°F = 120 tons @ 50 Hp 0.311 kW/ton CT3/4: 172 gpm 81°F – 66°F at 15 Hp, relate to orig.120 tons 0.093 kW/ton requires 7.5 Hp and 3.0 Hp spray pumps 0.065 kW/ton Total 0.469 kW/ton Evaporation water usage 0.037 gpm/ton
  • 15. 15 28 Indirect 1-Stage Design CT1/2: 180 gpm 78°F - 62°F = 120 tons @ 50 Hp 0.311 kW/ton CT3/4: 172 gpm 81°F – 66°F at 15 Hp, relate to orig.120 tons 0.093 kW/ton requires 7.5 Hp and 3.0 Hp spray pumps 0.065 kW/ton Total 0.469 kW/ton Evaporation water usage 0.037 gpm/ton 29 2-Stage Design CCT1/2 each: 180 gpm 78°F - 67°F = 82.5 tons @ 25 Hp 0.155 kW/ton* CCT3/4 each: 180 gpm 67°F - 62°F = 37.5 tons @ 50 Hp 0.311 kW/ton requires 5 Hp, 5 Hp, 2 Hp spray pumps 0.075 kW/ton Total 0.541 kW/ton Evaporation water usage 0.027 gpm/ton
  • 16. 16 30 2-Stage Design CCT1/2 each: 180 gpm 78°F - 67°F = 82.5 tons @ 25 Hp 0.155 kW/ton* CCT3/4 each: 180 gpm 67°F - 62°F = 37.5 tons @ 50 Hp 0.311 kW/ton requires 5 Hp, 5 Hp, 2 Hp spray pumps 0.075 kW/ton Total 0.541 kW/ton Evaporation water usage 0.027 gpm/ton Note: all kW/ton calculations are based on the total output (120 tons) of the system, not on the individual capacity of each tower 31 2-Stage Design CCT1/2 each: 180 gpm 78°F - 67°F = 82.5 tons @ 25 Hp 0.155 kW/ton* CCT3/4 each: 180 gpm 67°F - 62°F = 37.5 tons @ 50 Hp 0.311 kW/ton requires 5 Hp, 5 Hp, 2 Hp spray pumps 0.075 kW/ton Total 0.541 kW/ton Evaporation water usage 0.027 gpm/ton 73.4 tons pre-cooling 111 tons tower capacity 73.4 tons heat re-gain
  • 17. 17 32 Energy and Water Usage at Design Conditions Direct 1-Stage 0.167 kW/ton 0.032 gpm/ton Indirect 1-Stage 0.469 kW/ton 0.037 gpm/ton 2-Stage 0.541 kW/ton 0.027 gpm/ton Note that these values are fairly easily derived, but don’t show the whole picture. 33 Coil Calculations Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
  • 18. 18 34 Annual Simulation Could use a spreadsheet, maybe DOE2. Spreadsheet offers more flexibility Look at bin weather data file to run a simulation Use approximation of coil performance to arrive at results for varying airflows and air entering temperatures. Use LMTD and ε-NTU methods. 35 Bin Data and Building Load Match each temperature bin with a building load. This building load doesn’t necessarily have to be exact – a rough approximation is sufficient since we’re trying to find a comparison between models rather than an absolute number. Use the dry-bulb and coincident wet-bulb to predict how coil performance at tower inlets will vary.
  • 19. 19 36 Sample Bin Data SAN FRANCISCO, CLIMATE ZONE 3 DEG N LAT 38 ELEV 52 FT MAY JUN JULY AUG SEP OCT Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B TEMP RANGE 115/119 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 115/119 110/114 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 110/114 105/109 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 105/109 100/104 0 0 0 0 0 0 0.5 0 0.5 70 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 100/104 95/99 0 0 0 0 0 0 0.2 0 0.2 71 0 0.1 0 0.1 67 0 0 0 0 0 0 0.2 0 0.2 66 0 0 0 0 0 95/99 90/94 0 0.3 0 0.3 67 0 1.9 0.1 2 71 0 1 0 1 65 0 0.3 0 0.3 65 0 2.6 0 2.6 65 0 0.7 0 0.7 66 90/94 85/89 0 0.7 0 0.7 65 0 4.1 0.6 4.7 66 0 2.8 0 2.8 65 0 1.9 0 1.9 65 0 6.8 0.3 7.1 60 0 2 0.2 2.2 65 85/89 80/84 0 2.8 0.1 2.9 66 0 8.7 1.3 10 62 0 4.6 0.6 5.2 63 0 6.5 0.5 7 60 0 12 1.2 13 62 0 6.9 0.5 7.4 62 80/84 75/79 0 3.5 0.5 4 60 0.4 16 2.4 18.7 61 0.1 14 1.8 16.1 61 0 19 1.2 19.9 61 0.2 20 4.9 25 60 0.1 12 1.9 13.7 60 75/79 70/74 0 18 1.2 19.1 58 2 32 5.1 39.1 60 0.6 50 4.8 55.8 60 0.3 58 6.7 64.8 61 2.3 54 11 67.4 59 0 29 5.1 34.1 58 70/74 65/69 1.9 42 5.7 49.6 58 5.4 70 17 91.7 59 3 100 19 122 59 4.9 98 24 127 59 11 85 30 125 59 5.5 76 19 101 57 65/69 60/64 12 101 26 140 56 31 83 51 164 56 31 61 63 155 57 39 55 76 170 57 52 48 79 179 57 32 87 66 184 56 60/64 55/59 56 71 88 215 52 83 24 103 209 53 123 13 120 255 54 137 9.3 114 261 55 109 11 87 207 55 100 32 110 242 54 55/59 50/54 134 7.9 116 258 50 109 0.9 60 170 51 91 0.7 40 131 52 63 0.1 25 88.3 52 62 0.6 26 88.5 52 94 2.8 44 140 51 50/54 45/49 43 0.2 11 54 46 8.8 0 0.8 9.6 47 0.2 0 0 0.2 48 3.5 0 0.3 3.8 48 4.2 0 0.2 4.4 47 17 0 1.8 18.3 46 45/49 40/44 0.9 0 0 0.9 59 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.8 0 0 0.8 41 40/44 35/39 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 35/39 30/34 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 30/34 25/29 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 25/29 20/24 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 20/24 15/19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 15/19 NOV DEC JAN FEB MAR APR ANNUAL TOTAL S U M M E R W I N T E R Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total M Obsn Hours Total Obsn Hours Total 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 01 to 08 09 to 16 17 to 24 Obsn Hrs 01 to 08 09 to 16 17 to 24 Obsn Hrs 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 115/119 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 110/114 0 0 0 0 73 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 105/109 0 0 0 0 71 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 100/104 0 0.5 0 0.5 70 0 0.5 0 0.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 95/99 0 0.5 0 0.5 68 0 0.5 0 0.5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 90/94 0 6.8 0.1 6.9 66 0 6.8 0.1 6.9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.3 0 0.3 63 85/89 0 18.6 1.1 19.7 65 0 18.3 1.1 19.4 0 0.3 0 0.3 0 0.4 0 0.4 60 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2.1 0.1 2.2 65 80/84 0 43.8 4.3 48.1 63 0 41.3 4.2 45.5 0 2.5 0.1 2.6 0 3.1 0 3.1 60 0 0.1 0 0.1 55 0 0 0 0 0 0 0 0 0 0 0 1.4 0 1.4 56 0 5.8 0.6 6.4 60 75/79 0.8 94.3 13.3 108 61 0.8 83.9 12.7 97.4 0 10.4 0.6 11 0 8.5 0.5 9 56 0 1.9 0 1.9 56 0 0.2 0 0.2 59 0 0.3 0 0.3 58 0 5.3 0.7 6 56 0 12 2.1 14 59 70/74 5.2 269 37.4 312 58 5.2 241 34.1 280 0 28.1 3.3 31.4 0 28 2.7 30.9 56 0 5.3 0.8 6.1 53 0 2.3 0.3 2.6 55 0 8 0.5 8.5 52 0.3 12 2.9 15.3 53 0.5 28 4.7 33.5 57 65/69 32.1 555 126 714 56 31.3 471 114 617 0.8 84.2 11.9 96.9 2.5 58 23 82.8 55 4.4 24 6.7 34.6 56 2.4 13 3.5 19.3 56 1.6 36 7.6 45.1 55 0.8 38 6.6 45.7 52 4.3 65 15 83.6 54 60/64 214 668 421 1303 54 198 435 360 992 16 234 61.5 311 40 85 76 201 52 17 69 38 125 53 12 55 32 98.5 53 29 91 60 180 53 15 113 46 174 51 29 92 54 174 52 55/59 748 667 927 2341 51 607 161 621 1389 140 506 306 952 78 46 95 219 49 42 82 78 202 49 41 88 73 201 49 62 68 88 218 49 84 71 127 281 48 104 33 121 258 49 50/54 962 400 892 2255 48 552 13 310 875 410 387 582 1380 81 10 39 130 44 82 53 87 222 45 61 67 87 214 44 72 21 55 148 45 105 5.8 62 173 45 87 2 41 130 45 45/49 563 159 384 1107 44 76.1 0.2 14 90.3 487 159 370 1017 35 0.2 4.4 39.4 40 75 12 35 123 40 82 20 44 146 40 48 1.9 14 63.6 40 40 1 3.1 44.3 40 15 0.1 1.6 16.9 41 40/44 297 35.3 102 435 40 1.7 0 0 1.7 295 35.3 102 433 4.1 0 0.1 4.2 36 26 0.5 2.4 29 36 43 2.5 8 53.1 35 14 0.2 1 15.5 37 3.5 0 0 3.5 37 0.3 0 0 0.3 36 35/39 90.9 3.2 11.5 106 36 0 0 0 0 90.9 3.2 11.5 106 0 0 0 0 0 1.4 0 0 1.4 33 7.4 0.4 0.6 8.4 31 0.4 0 0 0.4 33 0 0 0 0 0 0 0 0 0 0 30/34 9.2 0.4 0.6 10.2 31 0 0 0 0 9.2 0.4 0.6 10.2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 25/29 0 0 0 0 26 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 20/24 0 0 0 0 23 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 15/19 0 0 0 0 0 0 0 0 0 0 0 0 0 37 Sample Bin Data Temperature values below the 65°F mark can be ignored for the simulation, since the system will be in economizer. ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0
  • 20. 20 38 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0 39 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 92° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0
  • 21. 21 40 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 87° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0 41 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 82° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0
  • 22. 22 42 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 77° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0 43 Calculation for Off-Design Values Successively enter db/wb combinations into tower selection to simulate operation 72° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0
  • 23. 23 44 Calculation for Off-Design Values Lower wb means: Tower fan can run at less than 100%. How will coil react to less airflow, and what will pre- cooling effect be? 67° ? ? ? ? ANNUAL TOTAL Obsn Hours Total M TEMP RANGE 01 to 08 09 to 16 17 to 24 Obsn Hrs C W B 115/119 0 0 0 0 0 110/114 0 0 0 0 73 105/109 0 0 0 0 71 100/104 0 0.5 0 0.5 70 95/99 0 0.5 0 0.5 68 90/94 0 6.8 0.1 6.9 66 85/89 0 18.6 1.1 19.7 65 80/84 0 43.8 4.3 48.1 63 75/79 0.8 94.3 13.3 108 61 70/74 5.2 269 37.4 312 58 65/69 32.1 555 126 714 56 60/64 214 668 421 1303 54 55/59 748 667 927 2341 51 50/54 962 400 892 2255 48 45/49 563 159 384 1107 44 40/44 297 35.3 102 435 40 35/39 90.9 3.2 11.5 106 36 30/34 9.2 0.4 0.6 10.2 31 25/29 0 0 0 0 26 20/24 0 0 0 0 23 15/19 0 0 0 0 0 45 Coil Calculations Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
  • 24. 24 46 Coil Calculations Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis Skip coil calculations 47 Coil Calculations-Step 1 Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
  • 25. 25 48 Coil Calculation - LMTD LMTD method: ΔT1= Thot,in – Tcold,out and ΔT2= Thot,out – Tcold,in ΔT2 – ΔT1 ΔTlmtd = ----------------- ln (ΔT2/ ΔT1) Q = UA ΔTlmtd Use this method to determine UA, based on the temperatures we expect from design. In other words, we don’t care exactly how the U and A are derived (fin spacing, number of rows etc). We’ll just assume that for the given problem, a coil can be purchased with the right UA. We will then use this number to simulate how that coil will operate under different conditions. 49 Coil Calculation – LMTD Example LMTD method: ΔT1= 83° – 80° and ΔT2= 68.2° – 65° 3.2°-3.0° ΔTlmtd = ----------------- = 3.1° ln (3.2°/ 3.0°) Q = UA * 3.1° = 125 gpm * 15°= 937.5 MBH UA = 302,524 Btu/h°F This number UA, which represents the overall heat transfer coefficient of the coil, can now be used to calculate performance under different conditions.
  • 26. 26 50 Coil Calculations-Step 2 Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis 51 Coil Simulation – ε-NTU ε-NTU method: By taking the UA we calculated earlier, and using the mass flow rates for each medium and the specific heat, we can determine what the leaving temperatures will be, based on the calculated effectiveness ε ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− = max min minmax min max min min C C 1 C UA exp C C 1 C C 1 C UA exp1 ε Hot act inHotoutHot C Q TT −= ,, Cold act in,Coldout,Cold C Q TT += Maxact inColdinHotMinMax QQ TTCQ ε= −= )( ,,
  • 27. 27 52 Coil Simulation – ε-NTU ε-NTU method: Note that the formula shown below for ε only holds for a perfect counterflow heat exchanger. For other types (most real heat exchangers are somewhere between a counter flow and parallel-flow exchanger). For such a case, the NTU is calculated, and ε is read off a chart Perfect Counter-flow Parallel flow – Counter-flow ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− = max min minmax min max min min C C 1 C UA exp C C 1 C C 1 C UA exp1 ε minC UA NTU = ε 1 C C max min = 0 C C max min = 53 Coil simulation – ε-NTU ε-NTU method: Note that the formula shown below for ε only holds for a perfect counterflow heat exchanger. For other types (most real heat exchangers are somewhere between a counter flow and parallel-flow exchanger). For such a case, the NTU is calculated, and ε is read off a chart Perfect Counter-flow Parallel flow – Counter-flow ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ −−− = max min minmax min max min min C C 1 C UA exp C C 1 C C 1 C UA exp1 ε minC UA NTU = ε 1 C C max min = 0 C C max min = We will use only this method
  • 28. 28 54 Coil Simulation – ε-NTU Example ε-NTU method: Design at 83° / 68.2°F for 57,700 cfm of air and 65° / 80°F for 125 gpm of water (78.3 tons or 937 MBH) Reduce fan speed, use 40,000 cfm and Ambient reduced to 67°F Performance now: Air at 67° / 65.1°F for 40,000 cfm and 65° / 66.3°F for 125 gpm of water (7 tons or 84 MBH) 55 Coil Simulation – ε-NTU Example ε-NTU method: Design at 83° / 68.2°F for 57,700 cfm of air ε =0.83 and 65° / 80°F for 125 gpm of water Qmax = 1,126 MBH (78.3 tons or 937 MBH) Reduce fan speed, use 40,000 cfm and Ambient reduced to 67°F Performance now: Air at 67° / 65.1°F for 40,000 cfm ε =0.96 and 65° / 66.3°F for 125 gpm of water Qmax = 88 MBH (7 tons or 84 MBH) Note that Qmax is the amount of heat that could be exchanged with an infinitely large (or perfect) heat exchanger. ε is a measure of how well the actual exchanger under consideration approximates this ideal exchanger, and varies with selected temperatures and flows.
  • 29. 29 56 Coil Calculations-Step 3 Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis 57 Coil Simulation – ε-NTU Example ε-NTU method: Design at 83° / 68.2°F for 57,700 cfm of air ε =0.83 and 65° / 80°F for 125 gpm of water Qmax = 1,126 MBH (78.3 tons or 937 MBH) Fh Btu 396,63 h min 60* cuft lb 0763.0*cfm700,57* Flb Btu 24.0cC 111 ° = ° =φ= Fh Btu 550,62 h min 60* gal lb 34.8*gpm125* Flb Btu 0.1cC 222 ° = ° =φ=
  • 30. 30 58 Coil Simulation – ε-NTU Example ε-NTU method: Design at 83° / 68.2°F for 57,700 cfm of air ε =0.83 and 65° / 80°F for 125 gpm of water Qmax = 1,126 MBH (78.3 tons or 937 MBH) ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −−− = 63,396 62,550 1 62,550 302,524 exp 63,396 62,550 1 63,396 62,550 1 62,550 302,524 exp1 ε 396,63 126,938 832.68 C Q TT Hot act in,Hotout,Hot −= −= 550,62 126,938 6580 C Q TT Cold act in,Coldout,Cold −= += MBH938126,1*83.0QQ MBH126,1)6583(550,62Q Maxact Max ==ε= =−= 59 Coil Calculations-Step 4 Take desired design values and calculate what coil overall heat transfer needs to be LMTD method Verify design condition with calculated coil heat transfer ε-NTU method Verify other condition with calculated coil heat transfer ε-NTU method Explanation of ε-NTU method Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis
  • 31. 31 60 Coil simulation – ε-NTU example ε-NTU method: Design at 67° / 65.1°F for 40,000 cfm of air ε =0.96 and 65° / 66.3°F for 125 gpm of water Qmax = 88 MBH (7.0 tons or 84 MBH) Fh Btu 949,43 h min 60* cuft lb 0763.0*cfm000,40* Flb Btu 24.0cC 111 ° = ° =φ= Fh Btu 550,62 h min 60* gal lb 34.8*gpm125* Flb Btu 0.1cC 222 ° = ° =φ= 61 Coil simulation – ε-NTU example ε-NTU method: Design at 67° / 65.1°F for 40,000 cfm of air ε =0.96 and 65° / 66.3°F for 125 gpm of water Qmax = 88 MBH (7.0 tons or 84 MBH) ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −−− ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ −−− = 62,550 43,949 1 43,949 302,524 exp 62,550 43,949 1 62,550 43,949 1 43,949 302,524 exp1 ε 949,43 186,84 671.65 C Q TT Hot act in,Hotout,Hot −= −= 550,62 186,84 653.66 C Q TT Cold act in,Coldout,Cold −= += MBH2.848.87*96.0QQ MBH8.87)6567(949,43Q Maxact Max ==ε= =−=
  • 32. 32 62 Annual Energy Use Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis Back to coil calculations 63 Energy Usage from Annual Simulation Direct 1-Stage 1.7 MWh per year Indirect 1-Stage 20.4 MWh per year 2-Stage 17.5 MWh per year
  • 33. 33 64 Life Cycle Cost Why use cooling towers instead of refrigeration? Compare 3 models annual energy use Compare 3 models life cycle cost Compare 3 models of cooling tower use @ Design Annual Analysis 65 Initial Cost Direct 1-Stage CT1/2 $ 36,000 Coils (59,320 cfm)x2 $ 72,000 Total $ 108,000 Indirect 1-Stage CT1/2 $ 40,000 CT3/4 $ 120,000 Coils (78,500 cfm)x2 $ 92,000 Total $ 252,000 2-Stage CCT1/2 $ 125,000 CCT3/4 $ 105,000 Coils (114,000 cfm)x2 $ 145,000 Total $ 375,000 Note: pricing is for towers, and estimated coil + custom coil installation. Pricing does not include piping, valves and associated controls.
  • 34. 34 66 Simplified Life-Cycle Cost Direct 1-Stage first cost $ 108,000 Energy cost, 20 years $ 4,100 (1.7 MWh/a) Total $ 112,100 Indirect 1-Stage first cost $ 252,000 Energy cost, 20 years $ 49,000 (20.4 MWh/a) Total $ 301,000 2-Stage CCT1/2 $ 375,000 Energy cost, 20 years $ 42,000 (17.5 MWh/a) Total $ 417,000 Note: pricing is for equipment only. This includes towers, and estimated coil + installation of coil on tower. Pricing does not include piping, valves, rigging, setting, startup or associated controls. Direct 1-Stage also has lower water usage and maintenance costs (not included in this simple analysis) 67 Comparison to Refrigerated Model – Aircooled chiller Chiller first cost, 240 tons $ 132,000 Energy cost, 20 years $ 193,000 (80.6 MWh/a) Total $ 325,000 Note: pricing is for equipment only. This includes towers, and estimated coil + installation of coil on tower. Pricing does not include piping, valves, rigging, setting, startup or associated controls. Note: Chiller energy cost derived from IPLV data, published in manufacturer’s literature for a 240 ton screw chiller. Use IPLV by taking EER at 100%, 75%, 50% and 25% load to estimate energy use at each temperature bin. EER Btu W W Btu EER out in in out =→=
  • 35. 35 68 20-year life cycle cost 20-year Life cycle cost $0 $100,000 $200,000 $300,000 $400,000 $500,000 $600,000 $700,000 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3 0.32 Cost $ / kWh Lifecyclecost Direct Indirect 2-Stage Chiller Break-even at around $ 0.10/kWh Break-even at around $ 0.19/kWh Note: For a more realistic calculation, piping materials & labor have to be added to the calculation. This makes the chiller model look even better, and break-even occurs at higher electricity prices. 69 15-year Life cycle cost $0 $100,000 $200,000 $300,000 $400,000 $500,000 $600,000 $700,000 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3 0.32 Cost $ / kWh Lifecyclecost Direct Indirect 2-Stage Chiller 15-year life cycle cost Break-even at around $ 0.13/kWh Break-even at around $ 0.25/kWh Note: For a more realistic calculation, piping materials & labor have to be added to the calculation. This makes the chiller model look even better, and break-even occurs at higher electricity prices.
  • 36. 36 70 Questions ? 71 Psychrometric diagram Back to cooling tower principles WET DRY
  • 37. 37 72 Psychrometric diagram Back to cooling tower principles COLD HOT 73 Psychrometric diagram Back to cooling tower principles San Francisco Florida Las Vegas
  • 38. 38 74 Psychrometric diagram Back to cooling tower principles AbsoluteHumidity Temperature 50° 70° 90° 75 Psychrometric diagram Back to cooling tower principles Temperature 0.45 lbs water/100 lb air 1.6 lb water/100 lb air 2.7 lb water/100 lb air AbsoluteHumidity
  • 39. 39 76 Psychrometric diagram Back to cooling tower principles 20% RH 50% RH AbsoluteHumidity Temperature FOG 90% RH 77 Psychrometric diagram Back to cooling tower principles AbsoluteHumidity Temperature FOG For the same moisture content, warmer air has a lower relative humidity or a lower saturation rate because hot air can absorb more moisture 20% RH50% RH90% RH 1.08 lbs water/100 lb air
  • 40. 40 78 Psychrometric diagram Back to cooling tower principles Temperature 90°/90% 90°/33% AbsoluteHumidity 68 wb 87 wbWet bulb temp.