대일아쿠아

Let? look at a typical case of fan selection for a new cooling tower design. The following is a sample calculation for the thermal condition of 113.0 - 89.6 - 80.6?, 63,843 gpm of water flow, and induced draft counter flow type.

First and foremost, it is to decide the size of tower cell, air inlet height and depth of fill considering the heat load. The basic parameters for the optimum thermal design are as follows; the simplest way to design a new tower is to start with largest cell size and largest fan.

• ●  42' x 42' with a 28' fan, 175 hp (Square cell is best but rectangular tower with a ratio of length of width = 0.8 to 1.2, but all tower components are based on a 6' x 6' base increment.)
• ●  Air inlet on 2 sides only & air inlet height 1/3 of cell width (In general, 1/4 to 1/3 to the cell width)
• ●  Height of plenum chamber which is a distance between the top of eliminator and the bottom of fan deck is generally obtained from 0.25 x { (Tower Width² + Tower Length²)^1/2} - Fan Diameter } as minimum. Using a larger fan reduces fan power and reduces minimum plenum chamber height, but be careful: recirculation is likely to increase if fan discharge velocity is too low. If the less plenum is used to, the air velocity in the center part of the eliminators can become so high that the drift losses will occur. Same thing in the upper portions of the fill. All these result in the increased pressure drop, reduced airflow, and reduced performance.
• ●  For the fill, use 4' (In general, 2' to 5' for film type of fill or 10' - 15' for splash fill)
• ●  Water Loading: 2 to 14 US GPM/ft² for counter flow, 4 to 24 US GPM/ft² for cross flow
• ●  Air Velocity at each location: The below is a guide line for the best performance with the minimum fan power. Especially the air velocity at the fill must be ranged in the below table. If the air velocity is low in the fill, the air can not be smoothly moved upward due to the restriction of downward water and if the air velocity is too high in the fill, the heat of water can not be completely removed as desired.
  
  The problem when the air speed at the air inlet exceeds below ranges is resulting in poor air distribution in the fill. For more details refer to the subjects of Air Distribution and Exit Velocity in Chapter 9, energy saving.

(1) Thermal Design Result: Considering the water and heat loading, 6 cells of 42' width x 42' length of tower size and 14' (33% or more to the width of tower cell) air inlet height is reasonable. For velocity recovery, 7^o of incline & 6.0' (21.4% to fan diameter) of venturi height was applied to this sample job. 1.0% fill support obstruction, without air inlet louver (10.0% air inlet obstruction),
concrete structures.

(2) Airflow & Pressure Drop: The airflows and pressure drops at each location and each cell were produced as follows. Airflow is varying every location since the air volume is proportionally increasing as much as the heat of water is transferring into the air stream.

• ●  Pressure Drop at Air Inlet & Louver = K x (1/2) x (density x 0.1922) x (V²/115,820) = 2.0 x (1/2) x (0.0717 x 0.1922) x (943.01²/115,820) = 0.1058 in H₂O
  
• ●  Pressure Drop at Fill: This is given from the pressure drop curve of a specific model of fill. The fill area is a net area deducting the fill obstruction area from the total cross sectional area of 42' x 42' tower. That is, 10,478.16 ft² = 0.99 x 42' x 42 x 6 cells'. Note that heat transfer in this fill obstruction is not occurring since the water fraction and air stream can not be by-passed through the parts of fills to be just laid onto the fill supports. So, this
  area called "Dead Zone in Heat Transfer" must deducted as much as the fill obstruction
  percentage.
  
• ●  Pressure Drop at Eliminator = K x (1/2) x (density x 0.1922) x (V²/115,820) = 2.0 x (1/2) x (0.0688 x 0.1922) x (608.77²/115,820) = 0.0423 in H₂O
  
• ●  Pressure Drop at Fan Inlet = K x (1/2) x (density x 0.1922) x (V²/115,820)
  = 0.25 x (1/2) x (0.0688x 0.1922) x (1,853.71²/115,820) = 0.0491 in H₂O
  [Note that the net fan inlet area was obtained from pie/4 (Fan Diameter² - Seal Disc²).
  That is, 573.52 ft² = pie / 4 x (28² -7.3333²).]
  
• ●  Velocity Pressure at Fan = (1/2) x (density x 0.1922) x (V²/115,820)
  = (1/2) x (0.0688 x 0.1922) x (1,853.71²/115,820) = 0.1963 in H₂O
  or Press. Drop = (V/4,008.7)² x 1/density ratio
  = (1,853.71/4,008.7)² x 1/(0.0750/0.0688) = 0.1963 in H₂O
  
• ●  Velocity Pressure at Stack Exit = (1/2) x (density x 0.1922) x (V²/115,820)
  = (1/2) x (0.0688 x 0.1922) x (1,661.07²/115,820) = 0.1576 in H₂O
  or Press. Drop = (V/4,008.7)² x 1/density ratio
  = (1,661.07/4,008.7)² x 1/(0.0750/0.0688) = 0.1576 in H₂O
  [Note that the net fan stack exit area was calculated as per the formula of pie / 4 x {fan diameter + (2 x tan 7?x venturi height)}² - pie / 4 x (seal disc diameter)². That is, 640.03 = pie / 4{28 + (2 x 0.122785 x 6.0)}² - pie / 4 x 7.3333². The tip clearance was neglected for
  the venturi.]
  
• ●  Velocity Recovery (Pressure Gain) = Efficiency of Fan Stack x VP at Fan x (1 - (Fan Dia. / Stack Exit Dia)⁴) 
  = 0.7571 x 0.1963 x (1 - (28 - 29.47)⁴) = 0.0276 in H₂O
  
  Since Hudson is considering 7?of stack incline and is neglecting the no air flow zone at the top stack due to the seal disc. (Unless the height of fan stack is as mush as the fan diameter, the area of seal disc in the fan must be subtracted from the fan stack top area. Refer to Flow Pattern, Chapter 4), the net fan stack area must be corrected to pie/4 x {fan diameter + (2 x tan 7^o x venturi height)}². The area obtained from this correction is 682.26 ft² and consequently the air velocity must be corrected to 1,558.24 fpm. Then, velocity recovery is obtained from 0.7 x (0.1963 - 0.1386 ) = 0.0404 in H₂0.
  
  Consequently, the fan brake horsepower based on above calculation can differ from Hudson? fan rating, since there is a little deviation in the velocity recovery. BHP per Hudson? fan rating program in case of inputting the 6.0' of venturi height and the 0.5100" Aq. of static pressure (not considering the 0.0276" of velocity recovery obtained from the thermal design result.) is 136.7HP. This is less than BHP when inputting 0.4825 Aq. of static pressure and not inputting the 6.0' of venturi height by 3.1 HP. So, this is a reason why we recommend the customers to use their own calculation on the velocity recovery due to the fan stack.
  
• ●  Total Static Pressure: This is a value deducted by pressure gain, that is, velocity recovery from a summation of pressure drops at air inlet & louver, fill, eliminator and fan inlet.
  
  Total Static Pressure = (0.1058 + 0.3128 + 0.0423 + 0.0491) - 0.0276 = 0.4825 in H₂O
  
• ●  Total Pressure: This is a summation of Total Static Pressure and Velocity Pressure.
  
  Total Pressure = 0.4825 + 0.1963 = 0.6788 in H₂O

(3) Fan Diameter: As previously mentioned, the minimum fan coverage to the cross sectional area of each cell must not be less than 30% for the induced draft type of cooling tower or must not be less than 30% of the area of fill.

• ●  Fan coverage to 30% of cross sectional area of 42' x 42' tower: 529.2 ft². This is equal to 25.96 ft.
• ●  Fan coverage to 30% of net area of fill: 523.91 ft². This is equal to 25.83 ft.
  
  Judging from above fan coverage, 26' or 28' fan can be used. Considering fan shaft power, fan cost, fan stack cost, and allowable space in the fan deck, 28' fan is more applicable. As see the above airflow and pressure table, the air velocity at fan is less than 2,000 fpm. Accordingly, 28' of fan diameter is a best choice.

(4) Fan Speed: This must be fixed before deciding the number of fan blades, since the fan speed
is directly corresponding to the fan brake horsepower and resonant frequency margin due to the change in the fan speed. If possible, it would be fine to select the fan speed considering the standard gear reduction ratio of manufacturers. If non-standard gear reduction ratio is used, the additional cost and delivery is requiring as usual. Model AGC-1712-14 in Amarillo? products is satisfying the mechanical power rating and gear ratio. Fan Speed = (Motor Full Load Speed / Exact Gear Reduction Ratio) = 1,770 / 14.000 = 126.4 rpm.

(5) Number of Fan Blades: This depends on the limitation of vibration at the gear reducer (i.e. material constructions), BHP/blade, fan speed, and minimum resonant frequency, etc. For maintaining the 80 micron of vibration at the position of gear reducer, the minimum 6 each of fan blade is required. However, this is exceeding the allowable BHP/blade (24 = max. BHP/blade - 4 hp = 24 - 4) for 28' fan. So, 6 blades of fan can not be used. Also, the pressure margin with 7 blades are too close to the critical operating line. Finally, the number of blade satisfying the allowable BHP/blade and the minimum 10% of pressure margin is 8 each.

(6) Automatic Selection: To optimum design the fan diameter & number of blades in accordance with the given airflow conditions, Hudson's fan rating program gives all the available fan diameter and number of blades based on the cost /hour. You can select a model having most less cost/hour, but it depends on the tower application designs. The operating cost/hour calculation is based on several variables:

• ●  Fan Price Modifier: 1.0
• ●  Asset Years: based on 10 years
• ●  Work Hours/Year: based on 8,000
• ●  Electric Power Cost: $0.46($/kw-hr)

But, you can calculate to modify the cost/hour using the following equation:

       (List Price x Price Modifier)       (Fan HP x 0.746 kw/hr) x (Cost/kw-hr)
--------------------------------------------------- + ----------------------------------------------------------
(Asset Years x Workhours / Year)                     Total Efficiency

Fan program is outputting all the available solutions for fan diameter and number of blade of fan on your given airflow conditions. For previous example, the result of solutions without information on the fan diameter and number of blades and under the conditions of;

• ●  Fan use: included draft cooling tower
• ●  Type of Blade: H Type
• ●  Static pressure: 0.4825" Aq.
• ●  Airflow: 1,063,126acfm
• ●  Exit Air Density: 0.0688 lb/ft³
• ●  Resonance Frequency Margin: 5%

Hudson's fan rating program outputs all the fan solutions for given airflow conditions. These solutions are all of the fan sizes and number of blades that meet the same airflow specifications, and their operating cost/hour values. If you absolutely do not want one of those fans, try with a different tip speed.