### 1910.94 Ventilation

Good Practices For Ventilation of Industry Emissions
(Copied from from OSHA Technical Manual May 2013.  A link to the manual is provided below)

1. Hood placement must be close to the emission source to be effective. Maximum distance from the emission source should not exceed 1.5 duct diameters.

a. The approximate relationship of capture velocity (Vc) to duct velocity (Vd) for a simple plain or narrow flanged hood is illustrated in Figure III:3-4. For example, if an emission source is one duct diameter in front of the hood and the duct velocity (Vd)  =  3,000 feet per minute (fpm), then the expected capture velocity (Vc) is 300 fpm. At two duct diameters from the hood opening, capture velocity decreases by a factor of 10, to 30 fpm.

FIGURE III:3-4. RELATIONSHIP OF CAPTURE VELOCITY (Vc) TO DUCT VELOCITY (Vd)

b. Figure III:3-5 shows a rule of thumb that can be used with simple capture hoods. If the duct diameter (D) is 6 inches, then the maximum distance of the emission source from the hood should not exceed 9 in. Similarly, the minimum capture velocity should not be less than 50 fpm.

FIGURE III:3-5. RULE OF THUMB FOR SIMPLE CAPTURE HOODS: MAXIMUM CAPTURE DISTANCE
SHOULD NOT BE MORE THAN 1.5 TIMES THE DUCT DIAMETER

c. Figure III:3-6 provides a guide for determining an effective flange width.

FIGURE III:3-6. EFFECTIVE FLANGE WIDTH (W)

2. System effect loss, which occurs at the fan, can be avoided if the necessary ductwork is in place.

a. Use of the six-and-three rule ensures better design by providing for a minimum loss at six diameters of straight duct at the fan inlet and a minimum loss at three diameters of straight duct at the fan outlet (Figure II:3-7).

FIGURE III:3-7. AN ILLUSTRATION OF THE SIX-AND-THREE RULE

b. System effect loss is significant if any elbows are connected to the fan at inlet or outlet. For each 2½ diameters of straight duct between the fan inlet and any elbow, CFM loss will be 20%.

3. Stack height should be 10 ft higher than any roof line or air intake located within 50 ft of the stack (Figure III:3-8). For example, a stack placed 30 ft away from an air intake should be at least 10 ft higher than the center of the intake.

FIGURE III:3-8. MINIMUM STACK HEIGHT IN RELATION TO IMMEDIATE ROOF LINE
OR CENTER OF ANY AIR INTAKE ON THE SAME ROOF

4. Ventilation system drawings and specifications usually follow standard forms and symbols, e.g., as described in the Uniform Construction Index (UCI).

a. Plan sections include electrical, plumbing, structural, or mechanical drawings (UCI, Section 15). The drawings come in several views: plan (top), elevation (side and front), isometric, or section.

b. Elevations (side and front views) give the most detail. An isometric drawing is one that illustrates the system in three dimensions. A sectional drawing provides duct or component detail by showing a cross-section of the component.

c. Drawings are usually drawn to scale. (Check dimensions and lengths with a ruler or a scale to be sure that this is the case. For example, 1/8 inch on the sheet may represent one foot on the ground.) Good practices to follow when reviewing plans and specifications are listed in Table III:3-3.

TABLE III:3-3. GOOD PRACTICES FOR REVIEWING PLANS AND SPECIFICATIONS

 Investigate the background and objectives of the project. Understand the scope of the project. What is to be included and why? Look for conciseness and precision. Mark ambiguous phrases, "legalese," and repetition. Do the specifications spell out exactly what is wanted? What is expected? Do plans and specifications adhere to appropriate codes, standards, requirements,policies, and do they recommend good practice as established by the industry? Will the designer be able to design, or the contractor to build, the system from the plans and specifications? Will the project meet OSHA requirements if it is built as proposed?