Cooling Tower Fan Cylinder Characteristics

Critical Fan Cylinder Characteristics which affect the delivery of air to and through the Fan Assembly. It typically depends on Air Inlet, Fan Deck Opening, Structure of Fan Stack, Tip Clearance & Velocity Recovery from the Fan Stack

1. COOLING TOWER FAN CYLINDER www.southerncooling.net Fan Stacks are provided for protecting the operating personnel from the rotating Fan. Really??

2. Fan Stacks are not just provided for protecting the operating personnel from the rotating fan but it has far more utility than just a safety device. It is a vital link in the chain of critical components that contribute to the overall efficiency of the Cooling Tower. When correctly designed, manufactured and applied, it assures that the Fan will be capable of moving the maximum amount of air through the tower at the minimum required horsepower. www.southerncooling.net FAN CYLINDER AIR INLET. FAN DECK OPENING. STRUCTURE. FAN TIP CLEARANCE. VELOCITY RECOVERY. FAN CYLINDER CHARECTERISTICS INFLUENCING THE AIR FLOW www.southerncooling.net

3. Let’s consider the air around the entrance to the fan. In a tube-like fan ring, air is collected from all directions and accelerated to the velocity at the plane of the fan. Some of the air is moving at 90° to the fan axis. Due to lack of smooth transition, the inability of the airflow vectors to make rapid changes in the direction thereby leading to “Vena Contracta ” at the blade tip, thereby starving the blade of air in this area. www.southerncooling.net FAN CYLINDER To be effective the vertical distance over which the transition occurs should be no less than 15% of the fan diameter. This assures full area, low turbulent airflow through the cylinder & maximizing air loading near the tips of the fan blades. 1. AIR INLET Since most of the work is done by the outer portion of the fan blade, this may result in efficiency loss as high as 15-20 %. To prevent this loss an Inlet bell is used at the bottom of the fan ring. So due to this Inlet bell the diameter of the fan cylinder below the fan gradually decreases (an elliptical curve) as it approaches the elevation of the fan leading to a smooth air inlet. www.southerncooling.net

4. The fan deck floor opening is the entry point for air flow in the Fan cylinder. In field erected wooden cooling towers, the opening is usually cut in the field by the erector. The natural tendency of the erector is to cut the opening too small. This may result into protrusion of the fan deck into the fan cylinder opening disrupting the smooth flow of air at a critical point of entry. The max. protrusion of the fan deck into the fan opening should be no more than 1”. www.southerncooling.net FAN CYLINDER Structurally strong and dimensionally stable fan cylinders that are capable of resisting wind loads must be securely attached to the tower structure. The design of the fan cylinder and its anchorage must be rigid enough to withstand high wind loads without deflecting and contacting the blade tips of a rotating fan. The fan cylinder and floor anchorage design should be such that it limit deflections to less than the fan tip clearance at 30 psf minimum wind loads. 2. FAN DECK OPENING 3. STRUCTURE www.southerncooling.net

5. Tip Clearance between the blade and the inside of the fan cylinder or fan ring is critical to Axial Flow fan performance. If close tip clearances are not maintained, a loss of performance will be noted, in pressure capability and air flow. The primary reason for close tip clearance is to minimize air loss or leakage around the tip, known as a Tip Vortex. www.southerncooling.net FAN CYLINDER 4. FAN TIP CLEARANCE Exit air will be at a higher pressure than the incoming air due to the work expended by the fan. If a leakage path or clearance exists, the air will seek a path of least resistance and bypass the tip causing a vortex or loss of performance. Moreover, the tip clearance must be uniform around the fan’s circumference. Uneven tip clearance not only disrupts the airflow but also induce fan vibration. While establishing the fan tip clearance the fan cylinder distortions that may occur due to wind load or other external forces should be considered. Fan cylinder and fan blade tip clearance should be around 0.5% of the fan diameter. www.southerncooling.net

6. Velocity Recovery particularly in large fan cylinders is vital to overall fan efficiency. It is accomplished by a gradual increase in the diameter of the fan cylinder above the fan. www.southerncooling.net FAN CYLINDER 5. VELOCITY RECOVERY This recovers the wasted velocity pressure energy at the top of the fan stack and converts it into useful work at the plane of the fan. Amount of recovery is based on the venturi section length and exit diameter. A maximum Flare angle of 12° for the recovery stacks provide a maximum assumed velocity recovery of 75% of the difference in average velocity pressure. www.southerncooling.net