Laboratory Pressurization Conflict with Fume Hood Exhaust
A university laboratory has four 6-ft-wide fume hoods and one general exhaust. Design exhaust is 1,000 cfm per hood at an 18-in. sash height plus 800 cfm general exhaust. Design supply is 4,200 cfm, giving a nominal 0.05 in. w.c. negative pressure. The room has a 3-ft by 7-ft door with a door closer. During balancing, the flow hood reads 900 cfm at each hood (3,600 total), 700 cfm at the general exhaust, and 4,500 cfm total at the supply diffusers. Room pressure is −0.12 in. w.c.; door opening force is 22 lbf. The client requires 100 fpm hood face velocity at 18 in. and a maximum door force of 5 lbf. Outdoor temperature is 35°F and indoor temperature is 72°F.
1. Perform a mass balance on the measured flows and explain why the result conflicts with the measured room pressure.
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Measured exhaust = 4 × 900 + 700 = 4,300 cfm. Measured supply = 4,500 cfm. By mass balance the room should be 200 cfm positive, yet it is −0.12 in. w.c. Both cannot be correct. Pressure is a qualitative indicator, not a flow measurement. Likely causes include a supply flow-hood reading biased by diffuser induction, an exhaust traverse error, supply duct leakage outside the room, or transfer air from adjacent spaces through door gaps. Re-measure supply at the air terminal and at the fan, and exhaust at the hood collar and at the stack. Check door undercut and transfer grilles. Do not adjust pressurization until the mass balance closes.
2. Calculate hood face velocity at the measured flow and evaluate the room pressure against the door-force limit.
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Each hood is 6 ft wide; an 18-in. sash opening is 1.5 ft, so face area = 6 × 1.5 = 9 ft². At 900 cfm, face velocity = 900/9 = 100 fpm, which just meets the criterion. At a higher sash position the velocity would fall below 100 fpm. The −0.12 in. w.c. room pressure is more than twice the 0.05-in. design and explains the 22-lbf door force. Door force should be measured at the handle; 22 lbf is unacceptable for a 3-ft door and exceeds the 5-lbf limit. The high negative pressure is inconsistent with the measured supply exceeding exhaust, which reinforces that one of the flow readings is wrong.
3. State the corrective actions and the final acceptance tests.
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Corrective actions: (1) establish a calibrated mass balance; (2) adjust supply and exhaust so the room is −0.05 in. w.c. at design; (3) verify each hood at 100 fpm at 18-in. sash and set sash stops; (4) adjust the door closer and provide transfer air if needed-do not rely on door force to hold pressure. If the room is truly at −0.12 in. w.c., increase supply or reduce exhaust. Final tests: re-measure all hood exhausts, general exhaust, and supply; verify room pressure with a calibrated transducer; measure door force at the handle; and re-test hood containment after balancing. Resolve the flow-versus-pressure conflict before sign-off.