Automatic Hub - Air Cooler  Axial Flow Fan

 


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Air would seem the best choice for coolant, instead of water, because of the fact it is around us in unlimited quantities. The successful performance of the air-cooled units encouraged engineers to consider the use of air coolant, and several works have done to have higher performance in air-cooled units. An air cooled unit consists of there basic parts; tube bundle, the structure and the mechanical drive system. Mechanical drive system is made up of fans, transmission and drivers. The most common fan-blade materials is aluminum, but in presented work, fiberglass reinforced plastic is considered. V-belts or gear can be used as transmission parts with electrical motor as driver, but V-belts are most popular. Fan speed is set by using a proper combination of sprocket or sheave sizes with timing belts or V-belts, and by selecting a proper reduction ratio with gears. Fig (1) provided a breakdown of the mechanical equipment. Two different categories of hubs can be fitted to fan shafts, fixed and automatic pitch types; the second has several advantages, for example in temperature control. In fixed hubs as shown in Fig (2) changing fans pitch require reinstalling system. As we know, air temperature in comparison with water is subjected to greater and more rapid fluctuation, particularly as a result of sun and showers and variations in weather. Several methods can be used to control temperature in air-cooled heat exchangers. By pass, Louvers, motor control and variable-pitch fans are some of usual methods. The last is considered in this paper. In most common Automatic Hab fan models the pitch of each fan may be altered manually to suit the prevailing operating conditions. In new Automatic Hub, air pressure is used to change blade’s angle and operator only push a switch to have desired blade’s angle. Previous models  used diaphragm and piston cylinder, so hub lubrication with synthetic lubricants should be done at least every year; it needs reassembly of the apparatus and so cause damage.   In new model simple service procedure and avoiding possibility of errors during disassembly, reassembly and start up was considered in design of different parts.  
Fig (1)-Breakdown of Mechanical systemFig(2)-Fixed Hub
Selecting fan pitch angle
Positive pitch (thick edge of blade down) is usually determined from specifications of heat exchanger. That is related to hot air temperature on induced draft units or ambient air temperature for forced draft units. Usually draft pitch angle will be exceeded in order to use all available motor amperage. Negative pitch, is normally set to attain minimum ‘zero’ air flow. This will be about minus 13o for induced draft units or minus 8o for forced draft units. If full reverse flow is desired, the negative pitch setting will be usually -30o to -35o.


Automatic Hub modeling

As mentioned before new Automatic Hub changes blades pitch with changing air pressure in an air-spring. Air-spring is designed by assuming the required pressure to rotate blades.

This will force the top plate to move. Then the connecting rods transform the up-down plate’s motion to cause rotation in blades housing.

Automatic Pitch Hub consists of 6 main parts as shown in Fig (3).

Fig (3)-Automatic Pitch Hub

1. Steel structure

Hub structure is constructed from materials selected to fight corrosion to insure many years of service. Plates are made from ST37 and then epoxies.CK45 has been used for bushes and GGG45 for bearing’s housing.MO40 and hot dip galvanize steel is used and finishes carefully where needed.

Then hub is fitted to fan shafts using a standard tapered bushing and place into the clamps. This structure can be used for other types of blades only with changing clamps.



2. Taper bearing, housing and bushes

Taper bearings are designed after obtaining drag and lift forces in blades from their drag and lift curves. Then by considering induced centrifugal forces, appropriate SKF taper bearings with reliability 0.9994 and safety factor 2.31 selected.

3. An air-spring to pressurize the moving plate


Air-spring is the main part in rotating procedure. This instrument consists of a tube part and two caps as shown in Fig (4) and should maintain the required force for moving top plate and also it should be appropriate vibration isolator and maintain the required stroke. The instrument has been provided from a German company; it is from WBE 400 series and can handle internal pressure up to 10 bars.

Fig (4)-Air-Spring

4. Springs

The function of the spring is to overcome the pressure instrument which tries to feather the blade. The designed spring has 1 mm space between coils in compression mode and 8 effective coils. Because of the distinguish pressure lines available in different petroleum industries the instrument should be able to adjust with line pressure. This can be achieved in new model, since its spring permitted to be changed easily if needed. This spring is shown in Fig (5).

 Fig (5)-Spring

5. Connecting rods and joints.

As mentioned before one of the advantages of new AV model is that rods should maintain tension instead of compression force, so no buckling will occur in rods. Connecting rods and their joints designed manually and by Ansys software to ensure the required safety factor.



6. Central Tapered Bush

This part of instrument’s duty as shown in Fig (6) is to connect hub with transmission part. Because of taper shape of this bush its installation is easy and no heat processing is required
Fig (6)-Central tapered Bush

Results

The assembled model performance has been tested in working model 3D software. It is shown in Fig (8). Clamp rotation is plotted against time in Fig (7) and it shows that the apparatus can rotate the blades up to 44 degrees and the practical experiments on the instrument verify this results. 
A new Automatic Hub for variable pitch fans presented here. New model has several advantages in comparison with its similar models, like easy and clean mounting and lighter weight. In the design point of view using tension mechanism instead of compression ones used in other models, permitted higher safety. We believe that this mechanism is suitable for automatic temperature control approach, because it works with pressure, so fully automatic air-cooled heat exchanger may be achieved by using these Automativ Pitch Hubs.





Fig (7)-Rotation (Deg) vs. Time (s)