Improving Aerial Cooler Fan and Acoustic Performance
Tip Seals and Bell Mouth Entries Offer Inexpensive Upgrades to Cooler Fan Performance
Fin/fan aerial coolers are user in over 95% of gas processing plants in Alberta. Due to age, deterioration, fouling or unusual plant loads, these heat exchangers often become a performance problem in hot weather and are also a prime consideration where gas plants are to blame for environmental noise problems. Process Engineers often advise increasing fan speed to improve hot weather performance whereas acoustical engineers invariably advise reducing fan speed if environmental noise is the problem. In many cases, however, operators need to solve one problem without creating or exacerbating the other. A proven way to either improve aerial cooler fan performance by 10 - 20% or reduce fan noise by 3 - 6 dB is to use aluminum honey comb tip seals and bell mouth fan ring entries.
Tip Seals
API 661 controls the design of aerial coolers. The maximum tip clearance (i.e., the gap between the tip of fan blade and the fan ring) should not exceed 0.5% of fan diameter or 3/4", whichever is less. This type of tip clearance is extremely important because the outermost 10% of the fan blade typically does over 50% of the air moving work. Any tip clearance, and resulting re-circulation, causes significant deterioration of the fan performance. However, strict API clearances are extremely difficult to meet with standard fan and fan ring manufacturing tolerances due to fan ring ovality and nonconcentricity. The solution is to install a crushable honey comb aluminum seal which is setscrewed to the fan ring. This honey comb aluminum is then partially crushed by a roller temporarily attached to a fan blade to create a perfectly concentric circle in which the fan can run. The efficiency improvement which an operator might expect to realize with a retrofit such as this will depend on the amount of clearance prior to the retrofit; however, typical improvements of the order 5 - 15 % would be common.
Photographs showing closup views of honey comb tip seals.
Bell Mouth Entries
The standard axial fan uses power generating a static pressure and a dynamic (or kinetic) pressure. Due to the fact that industrial fans are installed in an orifice style ring with no upstream and downstream diffusers or ductwork, the dynamic head is completely wasted as the air decelerates through the tube bundle. This wasted dynamic head typically constitutes 20-25% of the power absorbed by the fan. By installing a bell mouth entry to smooth the inlet flow to the fan, a portion of this waste energy can be recovered. Depending on the face velocity through the fan, a bell mouth entry retrofit might realize an improvement in the order of 5 - 12% for the fan.
Photograph of a bell mouth entry.
