The fan stacks are used for maximizing the fan efficiency, for preventing the reverse running of fan, and for minimizing the discharge air recirculation. The fan stack consists of three major parts as follows;

As the air is induced out of the plenum chamber beneath the fan, it tends to flow very predictable streamline into the fan stack. The inlet section of fan stack must be designed to induce the air smoothly and to minimize the air turbulence. In most cases, R/D = 0.15 or R/D = 0.10 is recommended.

The straight zone of fan stack is also very important to the fan performance. The fan blade are deflected downward during the operation due to the axial load onto the fan blade surface. Therefore, the movement of fan blade tips must be limited within the straight zone of fan stack. The minimum height of straight zone in the fan stack is a summation of the vertical dimension at the maximum blade pitch angle, the maximum deflection of fan blades tip, and some extra allowance.

Once the air properly directed into the fan stack, the close tip clearance must be kept. The greater the tip clearance the less efficient the fan. The space between the fan tip and fan stack allows the creation of air vortex at the blade tips which shorten the effective length of the blade, reducing the fan performance. (A vortex from upper section of the fan blades back to the low pressure area beneath the fan allows; this produces a lowered air flow rate and reduced fan efficiency.)

Close tip clearance minimizes the magnitude of the disturbances, maximizing the fan performance. However, the tip clearance must be designed to accommodate the wind-affected deformation of the fan stack, thermal expansion of the fan blades, and the possible build-up of ice inside the fan stack under the reverse fan operation. Fans are often installed in cooling tower with the tip clearance of up to 2 inches because of the manufacturing tolerances inherent in large fiberglass stack segments.

If the tip clearance is larger than the below maximum values, a pressure loss due to the increase of fan stack sectional area will occur. A rapid decline in the fan efficiency due to the decrease of total pressure and airflow will be resulted in and the brake horsepower under this situation will be slightly decreased.

The power consumption is generally decreased as much as the tip clearance is increased, since the volumetric air flow rate is significantly decreased. The efficiency at the larger tip clearance is decreased. The efficiency of fan at the larger tip clearance is decreased, because the input power is not reduced as much as the airflow is decreased.

At a slightly tapered exit cone the velocity pressure compared to the plane of fan is significantly reduced. The recovery of velocity pressure is converted into static regain which lowers the total pressure requirements of the fan.

A poorly designed and fabricated fan stack is a potential cause of poor air distribution, low fan stack efficiency, and significant vibration of fan stack due to the resonant frequency of fan. For high efficient fan stack design, the normal height of total fan stack is ranged in the 0.6 to 1.0 to the fan diameter. The taller height of fan stack than 1 x fan diameter does not useful for the velocity recovery and only makes the problems like the heavy fan deck load and higher wind load. The short height of fan stack is making a problem of the reverse running of fan due to the external wind under the situation of the fan is off.

Example 7-1. Estimate the height of inlet, straight, and velocity recovery zones of fan stack for the 28 feet of fan in the diameter and 10 feet of fan stack in the height.

(Solution)
1) Fan Inlet Zone

2) Straight Zone

• Vertical Dimension of Blade Tip @ Max. Pitch Angle: 5.73 inch
• Maximum Deflection of Blade Tip: 14 inch
• Extra Dimension from the trailing edge of blade: 6 inch
• Then, the height of straight zone is 25.73 inch (= 5.73 + 13 + 6)

3) Velocity Recovery Zone

Example 7-2. Calculate the velocity recovery at the above given design conditions.

(Solution)
There is no regulation in estimating the velocity recovery at the fan stack, which is generally accepted by every one, and the designers have to decide it with the experience. For the angle of taper, 7 degree is most efficient through the a lot of tests. The following formulas could be used for estimating the velocity recovery.

1) Formulated by Hudson Products Corp.

2) Formulated by MRL Corp.

In order to obtain a velocity pressure at the top of fan stack for a given fan stack, the area at the top of fan stack must be calculated first as follows;

Diameter of Fan Stack Top = Fan Diameter + 2 x Tan 7^o x Venturi Height
Area of Fan Stack Top = 0.7854 x (Diameter of Fan Stack Top² - Air Seal Disk²)
= 0.7854 x [28 + 2 x Tan 7^o x 43.87 / 12)² - (88 / 12)²] = 613.6 ft²

Air Velocity @Fan Stack Top = Air Volume @ Fan / Area of Fan Stack Top = 1019716.289 / 613.6 = 1,661.86 ft/min

Velocity Pressure @Fan Stack Top = (Air Velocity @ Fan Stack Top / 4008.7)² x (Air Density / 0.075) = (1661.86 / 4008.7)² x (0.0696 / 0.0750) = 0.1594 inch Aq.

Let's fan stack efficiency using the formula of MRL Corp.

Fan Stack Efficiency = [0.8 - 0.2 x (Venturi Height / Fan Diameter)] x 100(%) = {0.8 - 0.2 x [(43.87 / 12) / 28]} x 100 = 77.4%

Velocity Pressure @ Fan = (Air Velocity @Fan / 4008.7)² x (Air Density @Fan / 0.075) = (1778.0 / 4008.7)² x (0.0696 / 0.0750) = 0.1825 inch Aq.

Velocity Recovery = Fan Stack Efficiency x (Velocity Pressure @Fan - Velocity Pressure @Fan Stack Top) = 0.774 x (0.1825 - 0.1594) = 0.0178 inch Aq.

(Note: The reason why the area of air seal disk must be subtracted from the above equation in calculating the area of fan stack top is because the air streamline does not exist above the air seal disk.)