The most common V-belts used on smaller positive displacement blower drives have "B" or "3V" designations. B series belts have a top width of 21/32" and the narrow but deep "3V" belts have a 3/8" top width. The "X" indicates premium belt construction for superior grip and flexibility, especially on smaller diameter sheaves. Most industrial V-belt part numbers indicate belt cross-section and length and are used by virtually all belt manufacturers. However, there is a difference in how the length is measured. For example, a 3V450 belt has an OUTSIDE length of 45.0". A B45 has an INSIDE length of 45", whereas the outside length is 48" (3" difference between outside and inside length).
Premature belt failure on properly designed blower belt drives usually takes place because of running the belts with too little tension or on worn sheaves. Another reason is using the wrong belt. Fractional horsepower belts (usually designated as 3L, 4L, or 5L) found in hardware stores may look similar to B and 3V belts dimensionally, but will last only days or a few weeks vs. years for industrial belts.