With the exception of axial or mixed flow fans and some light duty “in-duct” centrifugal fans directly incorporated into the ducting, a fan requires a solid foundation. This is especially im­portant where the plant is handling corrosive or erosive fumes or solid particles such as wood refuse or coal dust. Foundations may be of concrete or a steel structure.

Standard fan baseplates are usually designed to be grouted to a continuous concrete foundation block. If the fan is to be in­stalled on a steel structure, the manufacturer should be in­formed of the size and position of the steelwork. Baseplate modifications may be necessary.

It is the user’s responsibility to provide adequate support for the fan unit. The size of a foundation block will depend upon the na­ture of the sub-soil and the magnitude of the vibrations pro­duced by the fan. Increasing the mass of the foundation block will reduce the amplitude of radiated vibration. A block isolated by proprietary elastomer mats may be necessary. Structural steelwork will be much more flexible than a concrete foundation block. Consideration must be given to the natural frequency of the support structure.

Further, the foundation should be sufficiently high to facilitate the connection of ducts and to ensure adequate space for drainage. The foundation block should be longer and wider than the baseplate to provide extra physical protection, from such items as forklift trucks and barrows.

Every fan set should therefore be erected on firm foundations of adequate depth, taking particular care with the levelling and alignment. Reinforced concrete is recommended, the minimum weight being four times the combined weight of all the rotating parts, or twice the dead weight of the whole unit, whichever is the greater.

Special care is necessary where sets are mounted on steel sup­porting structures. These can be used but must be level and well braced in all directions to ensure adequate rigidity. Such foundations are vital for trouble-free quiet operation. The mini­mum natural frequency of any part of the structure must be 50% higher than the running speed of the equipment. Before erec­tion, foundations should always be checked against the fan arrangement drawings.

Concrete foundations

Check the height and ensure that there is the required grouting allowance between the foundation and the fan base. Suitable pockets should be provided for the bolts to be grouted in after levelling and aligning. For vee belt indirect drives, the founda­tion bolts should be of adequate length and well embedded in the concrete. The location of plinths should be checked in rela­tion to the fan layout drawing. Use steel packers to obtain the correct height of the fan; the packers should be approximately the same width as the base plate and placed each side of and close to the H. D. bolts.

Note: It is the upper surfaces of the machined pads on the baseplate which are important for levelling. The under­side of the baseplate is usually not machined and will not be exactly straight.

Horizontal fans and motors are usually supplied complete and aligned on a common baseplate. Whether the fan has been tested or not, the fan and motor will have been accurately aligned prior to dispatch from the manufacturer’s works. Check the coupling alignment, see Chapter 12, Section??. Adjust the shimming until the coupling alignment is close, and consult the manual for thermal growth correction. Protect the tops of the foundation bolts and fit shuttering around the block in prepara­tion for grouting. A good mixture for grout is one part of cement to two parts of sharp sand. The final consistency should be able to flow easily.

Pour the grout and ensure the top of the foundation is covered evenly. If full depth cross members are fitted, ensure complete support. If a cast iron baseplate is used, fill it completely. Re­move the shuttering after five days, but allow the grout to harden for at least 10 days. Do not allow the grout to dry out too quickly, and protect it from direct sunlight. In hot environments it may be necessary to cover the grout with damp sacks. Also, protect the grout from frost if the temperature is low. Sacks, cov­ered with polythene sheets will be adequate unless the temper­ature is very low. Tighten up the foundation bolts and smear with grease.

Check the coupling alignment, remember thermal growth cor­rections and adjust the shimming under the motor feet if neces­sary. Record all the alignment settings. Spacer couplings are not an excuse for poor alignment. Diaphragm coupling life will be short with poor alignment. Fans with Cardan shafts should have some radial misalignment, and the manufacturer’s instructions should be followed.

The suction and discharge ducting should be finalised after the grout has hardened and final alignment has been completed. The ductwork must line up naturally with the fan connections. Do not force ductwork into alignment. Remove the ducting sec­tions adjacent to the fan and clean out the ductwork, as best as possible. Recheck coupling alignment with recorded figures and modify the ductwork if necessary. Jog the driver to check for correct rotation direction before replacing coupling spacers or drive pins. It is important that coupling bolts are tightened with the correct torque and that the appropriate quality of bolt is used. Information on the subject should be obtained from the manufacturer’s instruction book. Fit any loose equipment which was removed for transportation and wire up the unit, locking-off all local isolators.

Very large fans driven by electric motors over 1 MW, or steam or gas turbines, will probably be delivered in at least three sec­tions, fan casing, rotating assembly and driver. Starting with the heaviest section, level and grout in as described above. After the grout has hardened, align, level and grout in the second section. When the second section grout has hardened, then proceed with final alignment and assembly.

Supporting steelwork

Steelwork levels should be checked (including holes in beams). The steelwork should be level and rigid. Make certain that all bolts are tight: welded supporting structures are preferable.

Erection of complete units

Unless too large for transit (size 1400 mm and above), fans are assembled in the manufacturer’s works. It is therefore only nec­essary to mount them on a level foundation and fix in position using all the foundation points provided. It is important however to ensure that after the foundation bolts have been initially tight­ened, the fan pedestal is not twisted, as this may affect the bearing alignment. In extreme cases it will strain the casing which in turn may cause the inlet cone/venturi to foul the impel­ler. A spirit level should be used for levelling the unit and if nec­essary the support points should be shimmed.

The following should also be noted:

A) Correct positioning of a fan to the drawing should be based on the discharge flange.

B) The foundation/H. D. bolts, when set in concrete, should be left after initial grouting for 5 days to allow them to fully harden. If the foundation bolts are into steel, they can be tightened down evenly immediately the fan is considered to be level.

C) Connecting ductwork must not be tightened to a fan unit until it is fully and securely bolted down.

D) Inferior concrete foundations or grouting can be a cause of fan vibration, and if this is considered to be so, the only satisfactory solution is to renew concrete or grouting with a stronger mixture of good quality material.

E) The plinth should be feathered into a concrete floor.

Where ductwork, pipe connections, or other ancillary equip­ment are connected to the inlet or outlet of the fan, it is essential that they should be supported entirely independently of the ma­chine. Conversely, apart from the small fans noted in Section 18.2.1, fans must be independently supported and not sus­pended from ductwork, etc. When handling air or gas at high temperatures suitable expansion joints should be provided be­tween the inlet or outlet and the connecting ductwork.

Erection of CKD (Complete Knock Down) units

As the title of this section implies, these fans will be despatched in a completely knocked down state. They will, however, have usually been trial-assembled at the manufacturer’s works and the parts identified with part numbers and match points. Never­theless, it is strongly recommended that assembly on site should be carried out by the manufacturer’s own erectors. Fail­ing this the manufacturer could supply a supervisor, with the contractor providing the unskilled labour. Incorrect methods of lifting and assembly could result in damage and/or the balance quality of the rotating assembly being affected.

Posted in Fans Ventilation A Practical Guide