Maximize cooling Tower Fan Performance

Cooling tower effectiveness is critical for any process industry. It is the single equipment which is never stops till annual shutdown with minimum hours. The role of the cooling tower is to remove process heat from the operation and reject heat in the cooling tower by evaporation. The 90% cooling comes from latent heat. Hence, the water and air contact need to be good for effective operation. The falling liquid from the top is distributed in droplet form and comes in contact with the air, where droplet transfer latent heat to the air. The air flow is critical to maintain certain liquid to gas ratio to function the equipment in desired manner.

For a fan, the maximum performance more than just maximum air flow for minimum horsepower. The maximum fan performance also means that the fan achieves its expected service life (15-20Years) with no problems. It could also mean that fan meets its noise limitation specification, normally no one is looks for it. As we are working over vibrations.

We are going to discuss, how a fan selection in cooling towers are dependent on various critical factors. The installation and operating environment are also have a significant impact, whether a maximum fan performance will be achieved in life time or not.

Cooling Tower Fan selection:

It was observed that many of times plant fans are changed based upon the power consumption with evaluating the performance of the fan. Normally, ENCON vendors are readily available to replace a fan based on flow and velocity delivered by the existing fan with the same velocity with another fan. Even it was not ensured that the replaced fan is providing similar performance or not in terms of pressure and velocity head across the fills.

There are many fan combinations that will meet for any given duty or operating requirements. The goal is to be find one of the best. This requires optimized cost, power consumption, and noise level. The fan power consumption is determined by required air flow (m3/hr or CFM), the resistance offered by the box, fills, and louvers to that of air flow. It is called static pressure, and the fan efficiency. We are well aware that there are two kind of fan efficiency in any fan system 1) static efficiency 2) Total efficiency. These both terms causing a lot of confusion also.

Let’s brush the fan efficiency basics.

Fan Efficiency:

In cooling tower, the fan must move the air flow by passing through the beams, fills, louvers, mist tarp etc. The fan must move sufficient air against the resistance present in the tower. It is similar like pump where we are adding the resistance requirment and presented as a total head. The pump is dealing with liquid and liquid is not compressible whereas the air is compressible. In case of fan, the resistance is added in terms of static pressure and measured in N/m2 or pascal.

There is other job fan is to collect air from 360° around its inlet and draw it through the fan. This work is also measured in same units and it is called velocity pressure. The velocity head is not useful work as it is called parasitic loss.

For fan the total work is sum of these two-element static pressure +velocity pressure. This work is also called as total pressure and how well the fan does it job in terms of energy is called “Fan Efficiency”.

If we consider useful work than we call it static fan efficiency. In either case, the horsepower required includes the parasitic work required to collect the air into the fan inlet. This brigs up a question: Which is most important to consider, Total Efficiency or Static Efficiency?

Total Fan Efficiency = Total Pressure (Pa)X m3/sec/1000XkW 

                        Static Efficiency = Static Pressure (Pa) X m3/sec/1000XkW 

Figure-1 Fan operating Point for 28 feet with 8 numbers of blades

Let’s consider their relationship by looking at per performance curve one of the most use cooling tower fan type. The diameter is 8.5 m and 8 number of blades.

First, we look, how the static efficiency and total efficiency varies along the curve at 10°.

This pitch line describes the flow and static pressure response of given fan at given speed and density. For ease, we have chosen standard density 1.2 kg/m3(0.075lb/ft3) and 60.95m/sec (12,000fpm) tip speed. From figure-1, we can consider the five different operating points. It is important to note that the static pressure is always zero at the “free flow” point of the curve. It should be because that point, total pressure equals to velocity pressure & static pressure is zero

The table -1 is shows, how static and total efficiency varies with increasing flow.

Form table 1, it is evident that the total efficiency is relatively constant but static efficiency is maximum at low flow and high static pressure region. This optimum efficiency occurs just before the fan stall point. Each axial fan operates on these principles. 

In this case, you can use the cooling tower fill data for L/G vs velocity vs Heat transfer (similar two figure-2), This graph is showing the velocity curve against the heat transfer (X-axis) and L/G (Y-axis), the height of the fills are fix to 1 meter. It is showing the impact of velocity in the performance of cooling tower. There are many fan combinations of blade and fan speed where the fan output, in terms of static pressure and flow, meet the exact requirment of your fan system.

For best duty point, we can refer the sheet and velocity requirment, this will help to identify the speed and number blades required for optimum or maximum fan performance. Normally, the cooling tower vendors are not supplying sufficient data apart from the GA drawing and technical specification. 

Which is one of big challenge to evaluate and utilize maximum cooling tower performance conditions. Many of times, it has seen the cooling tower Guarantee Performance test also not conducted as per CTI205. Which is major gap in the performance of cooling towers and fan efficiency.

Pressure Loss in cooling tower fans:

The main component of resistance in any cooling tower are

1.    Tower Packing or Fill (70-80%)

2.    Air inlet at induced draft

3.    Mist Eliminator at top

4.    Air directions change losses 

Power versus Tip Speed:

Table-2 shows a comparison of varying speed and number of blades from fan supplier to meet the cooling tower required duty. For above case, assumes our duty is 566m3/sec 1,200,000CFM) and 79.6 pa actual static pressure and standard density. The fan diameter is 8.5 m. The major important point to be learn is that varying tip speed for a specific duty typically does not have a major impact on power for the same number of blades, since the fan duty point is the same for all speeds. Blade counts typically increases as we slow down to a

lower speed and number of blades required. Normally cooling tower supplier used a control over for a cell design and the requirment of blades and fan size. As the cost of fan changes dramatically. 

Figure-3 shows a dramatic change in fan power at different fan diameters. This is because the operating point moves towards the maximum static fan efficiency as the diameter increases, even though the static pressure and flow required is the same for all cases (566m3/sec, 79.6 Pa), SE & TE on the curve represent. There will also be a minor difference in power as a function of pitch angle and speed in the same fan. This is because of some angles are more efficient more than other. 

Summary on Fan Efficiency:

We can make some conclusion about fan efficiency

·      Static efficiency is showing actual work done by the fan, not total efficiency.

·      Static efficiency peak at high pressure and low flow for each blade pitch line.

·      Efficiencies varies along with pitch lines as CFM as CFM and ASP changes.

·      The efficiency, we achieve is primarily function of the fan selection and operating point, which is function of tower resistance system.

·      About seeking lowest horsepower, varying fan speed does not have major effect for the same blade count for specific duty, to reduce power.

·      Use the largest diameter fan possible to maximize static efficiency and minimize velocity pressure for a fix duty. Further improvement can be made by optimizing fan speed & pitch angle for the diameter chosen.

Concern related to Cooling Tower Fan:

1.    Blade Loading: fewer blades lower fan cost, but for the same duty, also increases the horsepower per blade. It is not wise to choose fan operating at the manufacturer maximum limit of horse power. Increasing blade count lowers vibrations and insures longer, trouble-free life for fans by lowering power load per blade and decreasing tip pressure pulse into stack.

2.    Beware of column Plenum Towers: Towers with common plenum are toughest duty that exist because of extreme turbulence in the fan inlet. 

Static Pressure Margin:

Check the operating point proximity to stall condition. A stall point on a curve is the area where the slope of SP versus CFM or TP versus CFM line changes from negative to positive. Normally supplier prevent choice of an operating point in a stall condition by the way their curves are presented. A good rule of thumb is to make sure that at least an additional 2° of pitch is available before a stall is indicated on the curve. This provides a safety margin over static pressure.

Fan environments affect performance:

The following factors needs to take care, these factors can affect performance significantly.

Tip clearance: Excessive tip clearances allow air recirculation, due to high pressure differential, around the tips of the blades, where most of the work is being done. Figure-4 shows a test result from a 4.3 m (14 ft.) fan with varying clearance. The base clearance is assumed 0.3%D for the curve. This is still extremely close and would not be recommended for normal operation. The actual clearance for various diameters can be easily calculated using the equation shown.

Leading Edge Erosion: This wear is caused by poor mist elimination which allows water droplets impinge on the blades leading edge. Consider the bade is travelling about 61m/s(12000 ft/min) at the tip. The impact of water droplet hitting a blade moving about 135 miles /hr can wear holes in the blade, at worst doing serious structural damage.

Vibration: vibration can occur due to several reason, blade pitch not consistent, blade track is not consistent, coupling imbalance, water in hollow blade etc. In this scenario only vibration frequency is provides the clue to the source of vibration. The basic principle are

If vibration is at a 1XRPM frequency, fan unbalance is the problem

If fan balance is the problem, please check the pitch and track first. Next step, even if blades are moment balanced, would be to check blade mass weight and make sure the heaviest blade is not opposite to the lightest blade. We can shift the some blade position. If this also fails, a field dynamic balance is best solution.

What is excessive vibration? One have a maximum 6 mils (peak to peak) on the gear box. The motor vibration is not indicative of fan performance. If vibration is a blade pass frequency, the problem is environment and structure. This could be from loose bolting or stack structure or a stack structure resonance. Changing speed usually not a viable option to avoid resonance in the tower because of high changing cost of gear ratio or invest in speed drive. 

Shaft/Guard Blade clearance: 

A fan blade is continually vibrating up and down several inches at the tip, in turbulent weather can more than double with deflection. All it takes is one more revolution for a tip to down and strike to guard to cause a fan wreck. Shaft guard within 8 inches of a tip should be removed or lowered. Some companies are following 12 inches minimum clearance with guard of large fans. 

Velocity recovery Stack: The purpose of velocity recovery stack is to convert velocity pressure into useful static pressure work. They are most important if your fan operating point requires velocity pressure in excess of 50 Pa(0.2 in of H2O). They are more effective at higher velocity pressure. Typical heights are about 0.4 X diameter. Figure-4 shows power saving for 8.5 m dai fan with typical 3.3 m velocity recovery stack (VRS).

Fan Installation effect on Performance

Blade Pitch Variation: For smooth operation, all blade pitches should be set with 0.2° total variation. A digital protector is highly recommended. Check the angle again after retorqueing.

Blade Track Variation: This equally important for smooth operation. Long heavy blades must be supported at the tip to a common line on the fan ring before tightening. Figure -5 shows how one manufacturer recommends to installing blades. 

Fan Operation Impact in cooling Towers

The performance of cooling towers is designed as per rated load conditions. Suppose if a cooling tower design 6300 m3/hr with range 8°C and having approach 4°C. The cooling tower having 3 cell and induced draft cross flow.

The air flow requirement per cell would be 358 m3/sec per cell. The design liquid to gas ratio is 1.5. The cooling tower design NTU (Merkel number) is 1.38.

In actual, the cooling tower is operating at 76% flow loading. The circulation flow is 4800 m3/hr and the operating range is 5°C. The change in water flow is resulted in new L/G, which is 1.1 and tower heat transfer availability is reduced by 20% & the approach is increased by 2°C

Hence, it is always better to tune your cooling tower for maximum heat transfer availability, which is possible by blade angle change or with new fan as per the requirment to adjust capacity of cooling tower.

Conclusion

Optimizing fan performance is a process that requires close attention to detail, from fan design, to choose of fan operating point, consideration of the fan environment, and finally proper installation. The reward is this close attention will be a long trouble-free life of our fans. The impact of the fan is critical in overall cooling tower system to produce desire cooling. we can across the many instance where the fan performance is sole responsible for poor approach and impacting the cost of operation.

For better reliability, we need to also work upon below given aspect.

Fiberglass reinforced Plastic (FRP) construction has the highest fatigue strength compared to all aluminum construction. High fan efficiency is the result of careful design. Generally, the smaller the hub seal and longer the active airfoil will yield highest static efficiencies and lowest velocity pressure.

Air recirculation at the hub, another inherent property of axial fan is the problem of swirl. Swirl is the tangential deflection of the exit air direction caused by the effect of torque. The air vector at extreme inboard section of the blade actually reverse directions and subtract from the net airflow. This is very much measurable quantity. An inexpensive hub component, the hub seal disc prevents this and should be standard equipment on any axial fan.

It is very important that an analysis is made of the complete fan system so that fan system efficiencies can be computed. To do this complete information should be furnished from the supplier of the equipment for static and velocity pressure loss for each component of the system.

The blade should have built in protection to prevent leading edge erosion. Fiberglass surface should be protected against the sun (UV) by a proactive coating for long life. Aluminum blade should be protected against corrosion. 

LITERATURE CITED

1.    Manroe Robert “Minimizing Fan Energy Costs, Part 1 and Part 2, Chemical Engineering, June 1985

2.    CTI TP74-03