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36 March 2017 Tablets & Capsules analyzes how well the filters clean, but also how the col- lector performs as a whole. That enables manufacturers to make a much more accurate comparison when deciding which filters truly meet process needs. The Standard applies to "bag, cartridge, or envelope industrial dust collectors that recondition the filter media by using a pulse of compressed air to discharge the dust cake from the filter media while the air cleaning device remains on line." It prescribes a method for testing the performance of these dust collectors from inlet to outlet and in so doing accurately portrays the dynamics of the equipment in multiple-filter arrangements, unlike the MERV method, which bases its comparison on the initial efficiency of a single filter. Multi-stage test sequence Under Standard 199, testing is a six-stage affair that challenges the dust collector just as a real-life application would (Figure 3). In Stage 1, dust is fed to the collector at a specified rate—without pulse cleaning—until a specified differen- tial pressure is reached. In Stage 2, on-demand pulse cleaning commences while the air flow and dust feed continue for 4 hours. On-demand cleaning conserves compressed air and maintains the filter pressure drop in a range that allows adequate air flow from the dust-gener- ating process. It's the most common cleaning method used in the pharmaceutical industry. Stage 3 uses continuous pulse cleaning while maintain- ing the air flow and dust feed. This stage lasts 24 hours or until the specified maximum differential pressure is reached, whichever occurs first. The filters are cleaned at specified intervals, generally every 10 to 15 seconds. Stage 4 involves final dust loading with on-demand cleaning, while the air flow and dust feed are maintained for another 24 hours. Cleaning is triggered by high and low differential pressure setpoints that the dust collector manufacturer provides. Data collected over the final 4 hours of this stage are used to assess performance. Stage 5 simulates how the dust collector would perform in an upset condition. It calls for maintaining the dust feed while suspending pulse cleaning. This continues until the Figure 2 Schematic of a typical pulse cleaning system Figure 3 Sequence of six test stages as set forth in ANSI/ASHRAE Standard 199* Automatic timer control Diaphram valve Header (compressed air reservoir) Solenoid valves Nozzle Filter cartridge Dust Dust Pulse-jet blowpipe Pulsing air Courtsey of ASHRAE Upper dP setpoint Lower dP setpoint Stage 1 Initial dust loading Stage 2 Initial dust load- ing with on- demand cleaning Stage 3 Dust loading with continuous cleaning Stage 6 Post upset condition Stage 4 Final dust loading with on-demand cleaning Stage 5 Upset condition Differential pressure Elapsed time (not to scale) *Adapted from ANSI/ASHRAE Standard 199-2016