Plenum Design。  Use perforated plate to cover as much of the ceiling (or wall opposite the exhaust hood(s)) as practical。 The diameter of the perforation should be between 6。3 mm and 9。5 mm (1/4 in and 3/8 in)。 Perforated plenums work best when ceiling height is less than 4。58 m (15 ft)。 Use either of the following two choices for replacement air plenum design:

a。 Design for 5。1 m/s (1,000 fpm) replacement air velocity through the open area of the perforated plate if perforated duct is used inside the plenum as shown in Figure 2-3。

b。 Design for 10。2 m/s (2,000 fpm) replacement air velocity through the open area of the perforated plate if the plenum is served with ducts using diffusers, grills or registers as shown in Figure 2-4。

Figure 2-3。  Plenum design with perforated duct。

Figure 2-4。  Plenum design without perforated duct。

2-4。5。3 Perforated Duct Design。  Use perforated duct to evenly distribute the flow of replacement air inside a plenum or use alone when ceiling height is greater than

4。58 m (15 ft)。 Manufacturers provide several different types and sizes of perforated duct。 Use recommendations from the manufacturer for duct design。 The manufacturer will not only recommend the size, shape, and type of the required perforated duct, but also the location of the orifices and reducers to distribute the air properly。

2-5 CONTROLS。  Provide industrial ventilation system controls and associated alarms to ensure contaminant control, space specific balance and conditioning, a safe and healthy work environment, and system malfunction notification。

2-5。1 Gauges and Sensors。  Specify gauges and sensors to provide continuous monitoring of system performance。  The minimum requirements are:

2-5。1。1 Differential pressure sensors, with gauge readouts, across each replacement air filter section。 Set points on the gauge to trigger an alarm when the pressure drops or gains across the filter exceed the manufacturer's recommended value。 A pressure drop occurs when there is a blow through a filter and a pressure gain occurs when the filter gets loaded。

2-5。1。2 Operating light on replacement air system fan motor。

2-5。1。3 Static pressure sensor at the outlet of the replacement air fan with a gauge readout。 Set the points on the gauge to trigger an alarm when the pressure is lower than the recommended range (as determined by baseline testing)。

2-5。1。4 Hood static pressure sensor, for critical processes or process where extremely toxic substances are used, with a gauge mounted in a conspicuous place near the hood。 Set the points on the gauge to trigger an alarm when the static pressure is lower or higher than the recommended range (as determined by baseline testing)。 Do not use the type of inline flow sensor, which measures the pressure drop across an orifice plate。 Use only a static pressure tap and differential pressure gauge。

2-5。1。5 Differential pressure sensor across each exhaust air-cleaning device with gauge readout。 Set points on the gauge to trigger an alarm when the pressure drop across the device exceeds the manufacturer's recommended value。

2-5。1。6 Static pressure sensor at the exhaust fan inlet with gauge readout。 Set the points on the gauge to trigger an alarm when the pressure is lower than the recommended range (as determined by baseline testing)。

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