Genuine Replacement Pre-Filters For Whirlpool Whispure Air Cleaners - Whirlpool Part #8171434. Ready-to-use right out of the box, each pre-filter will last for about 3 months depending upon the environment and are sold in a 4-Pack for a year"s supply. One way to tell it is time to replace them is by the color. They are almost black to begin with and turn a light gray color when they need to be changed. Fits Models AP350, AP450 and AP510.