1. Leakage is a waste of the energy. Airflow is lost. Fan efficiency is diminished. 
2. Leakage of a contaminated airstream may pose an environmental or safety hazard.
3. Leakage of hot or contaminated air may damage the fan shaft bearings.
4. Leakage can result in noise. Sealing may reduce noise.

These, as well as other site-specific factors should be considered when determining the degree to which the housing should be sealed using shaft seals, common caulks and gaskets.

1. Protection from corrosion by acids and alkalies.
2. Protection from weather (Rain and UV).
3. Protection from abrasion and physical wear.
4. Protection against process contamination.
5. Provides a cleanable surface for decontamination or removal of material build-up.

Axial fans come in many variations (Propeller Fans, Tubeaxial fans and Vaneaxial Fans), but they all have one thing in common; they all rotate about their axis and they all move a column of air parallel to that axis. Axial fans are commonly found in applications where the emphasis is on moving large volumes of air against relatively low system pressures. 

In a centrifugal fan the air enters the impeller axially and is accelerated by the blades and discharged radially. A centrifugal fan induces airflow by the centrifugal force generated in a rotating column of air producing  potential energy (Static pressure) and also by the rotational (Tangential) velocity imparted to the air as it leaves the tip of the blades producing kinetic energy (Velocity Pressure) giving a centrifugal fan a much higher system pressure capability. A typical application for a centrifugal fan is conveying material (EG. Plastic pellets) generating a relatively small volume of air with a high system pressure.   

1. Abrasive wear or corrosion.
2. Water (Condensation) trapped in hollow airfoil blades.
3. Material build-up on the fan blades.
4. Deformation of fan components due to impact.
5. Deformation of fan components due to exposure to temperature.
6. Bearing wear/deterioration.
7. V-belt drive misalignment.
8. Fatigue type failure in fan components due to continued exposure to abnormal loads or vibration.
9. Loose hardware on the fan or a weak base structure supporting the fan.

Direct drive fans are compact, have the lowest first cost and require the least amount of maintenance. Direct drive fans also have fewer sources of vibration and are often used where very low levels of vibration are required (EG. Clean rooms). However, because fan speeds are limited to the available motor speeds, the number of viable direct drive fan selections may be limited. Due to the limited motor speeds available (EG. 3600, 1800, 1200 & 900 RPM), it may require selecting a more expensive lower speed motor to meet a given performance requirement. However, a variable frequency drive (VFD) may be added to vary the motor speed. A VFD increases the first cost, however this cost may be offset by the ability to reduce the fan speed (Decrease power consumption) when 100% airflow is not required.