Baffles are the movable partitions used to create slotted openings along the back of the hood. These slots and baffles direct the air that is being exhausted out of the hood. Visible from the working area, the baffles work in concert with the front sash to control airflow and optimize fume hood efficiency. Slotted Fume Hoods are ideal as a capture mechanism for fumes produced by welding, grinding, buffing, etc. Unlike a standard fume hood, a slotted fume hood is more energy efficient and cost effective by creating a more restrictive and faster airflow pattern from a smaller fan.

Hood Design

Problem:

Solution:

C_e = (VP_d / SP_h)^1/2

where C_e is coefficient of entry,

VP_d is duct velocity pressure,

SP_h is hood static pressure.

SP_h = h_e + VP_dwhere h_e is the hood entry loss.

For plain opening hood,

VP_d = SP_h * Ce² = 3” * 0.82² = 2.017” wg

h_e= 3” – 2.017” = 0.983” wg

Percent loss in velocity head = 0.983 / 2.017 = 48.74 %

For flanged opening hood,

VP_d = SP_h * Ce² = 3” * 0.76² = 1.733” wg

h_e= 3” – 1.733” = 1.267” wg

Percent loss in velocity head = 1.267 / 1.733 = 73.11 %

Car Hood Design

For cone hood,

VP_d = SP_h * Ce² = 3” * 0.93² = 2.595” wg

h_e= 3” – 2.595” = 0.405” wg

Percent loss in velocity head = 0.405 / 2.595 = 15.61 %

With an increase of C_e, hood entry loss and % loss in velocity decreases.

Problem:

Find the static pressure of a hood if the velocity pressure is 4.5 'H₂O and the coefficient of entry is 0.82.

Solution:

C_e = (VP / SP)^1/2

where C_e is coefficient of entry,

VP is velocity pressure,

SP is static pressure.

SP = VP / C_e² = 4.5 / 0.82² = 6.7” of water

Problem:

A 25' wide hood needs a slot velocity of 200 fpm with a 40 cfs volume. What size of slot opening is required?

Solution:

Required area of slot = Q / V = (40 cfs * 60) / (200 fpm) = 12 ft²

Size of slot opening = 12 ft² / (25” / 12) = 5.76 ft = 69.12”

Problem:

Assume the hood static pressure in a 8' duct is 3.0' H₂O and the hood manufacturer indicates that Ce is 0.89. What is the estimated flow rate?

Solution:

Hood entry coefficient, C_e = √ (VP_duct / SP_hood)

Fume Hood Design

So, VP_duct = C_e ² * SP_hood = 0.89² * 3 = 2.376 “wg

V_duct = 4005 * √VP_duct = 6173.42 fpm

Q = V_duct * A = 6173.42 * [π * (8/12)² / 4] = 2154.93 cfm

Problem:

Assume a hood is exhausting 3,000 cfm of air and the hood static pressure was measured at 2.17' w.g. Three months later the hood static pressure is 5.0 ' H₂O. Assume continued standard conditions (55^oF & 29.92' H₂O). Calculate, by how much the air flow through the duct has been reduced?

Solution:

For standard air, Q = 4005 A C_e √ SP_hood

For the same hood, Q is proportional to √ SP_hood

So, Q₂ = Q₁ (√ SP_{hood, 2} / √ SP_{hood, 1}) = 3000 (√ 5 / √ 2.17) = 4553.83 cfm

Increase in air flow rate = (4553.83 – 3000) = 1553.83 cfm

% Increase in air flow rate = (4553.83 – 3000) / 3000 = 51.8 %

Problem:

Compare the flow rate for a lateral hood located at the edge of an open surface tank (4ft*3ft) with the rate for a canopy hood located 3 ft above the tank. The tank contains a solution of ammonium phosphate in water at 250^oF. Ammonia gas is released from the tank. State your assumptions.

Solution:

Lateral Hood:

Q = V (10 X² + A)

where, Q = Air Flow (cfm), V = Centerline Velocity at X distance from hood (fpm)

X = Distance of source from edge along axis (ft), A = Area of hood opening (ft²)

Assumptions: Capture Velocity = 75 fpm, Hood Dimensions = 4 ft by 3 ft (same as tank)

V = 75 fpm, X = 0 (hood at edge of tank), A = 4 * 3 = 12 ft²

Q = 75 * 12 = 900cfm

Rectangular Canopy Hood:

Height of hood above tank = 3 ftLow Canopy Hood required.

Q = 6.2 * b^1.33 * Δt^0.42 * L

where, Q = Air Flow (cfm), b = Width of hood (ft),

Δt = Difference in source and ambient temperature (F), L = Length of hood (ft)

Assumptions: Hood Dimensions = 4 ft by 3 ft (same as tank)

b = 3 ft, L = 4 ft, Δt = (250 – 70) = 180 F

Q = 6.2 * 3^1.33 * 180^0.42 * 4 = 946.75 cfm

Slot Hood Design Studio

Slot Hood Design Pictures

ATMOS360’S Close Capture dust hoods with pneumatic cylinder actuated stripping baffles demonstrate superior performance in papermaking and converting applications. In area where close capture hoods are not feasible (an unsupported web or spatial constraints) we utilize general Area exhaust hoods. This combination of close capture and area hoods are used successfully on weak sheets such as facial and tissue lines. They provide optimum dust collection performance that result in improved process reliability.