Building Discharge and Wind Engineering

Building discharges shall be located and designed in accordance with Chapter 14 of the ASHRAE Handbook of Fundamentals. Fume hood and other contaminated exhaust shall not be recirculated into the building air supply. Interactions with adjacent buildings and their supply air intake requirements shall be carefully evaluated.


Considerations must be given to a wind-tunnel study to assure that reentrainment of exhaust will not occur, and that potentially hazardous exhaust will not impact nearby buildings.

 

UC Practice

ANSI Z9.5 5.3.4

ASHRAE 1999 Handbook — HVAC Applications, Chapter 43

ASHRAE Handbook of Fundamentals, Chapter 14

 

Fume hood exhaust should have vertical stacks that terminate at least 10 feet above the roof deck or two feet above the top of any parapet wall, whichever is greater. Higher stacks may be found to be necessary, using the guidance in the ASHRAE Handbook of Fundamentals, Chapter 14, and/or based on wind tunnel studies.

 

The basic challenge in locating the hood discharge is to avoid re-entrainment of effluent into any building air intake or opening and to minimize exposure of the public. The selection of stack height is dependent on the building geometry and airflow pattern around the building and is as variable as meteorological conditions.

 

The 10 ft (3 m) height above the adjacent roof line called for by this standard is primarily intended to protect maintenance workers from direct exposure from the top of the stack. However, this minimum 10 ft (3 m) height may be insufficient to guarantee that harmful contaminants won’t enter the outside air intake of the building or of nearby buildings.

 

See Chapter 43 of the ASHRAE 1999 Handbook — HVAC Applications. Among the factors to consider in establishing stack configuration, design, and height are: toxicity, corrosivity, and relative humidity of the exhaust, meteorological conditions, geometry of the building, type of stack head and cap design, adjacency of other discharged stacks and building intake, discharge velocity, and receptor population.

 

ANSI Z9.5 5.3.5

 

A minimum discharge velocity of 3,000 fpm is required unless it can be demonstrated that a specific design meets the dilution criteria necessary to reduce the concentration of hazardous materials in the exhaust to safe levels at all potential receptors.

 

A discharge velocity of 2,500 fpm (12.75 m/s) prevents downward flow of condensed moisture within the exhaust stack. It is good practice to make the terminal velocity at least 3,000 fpm (15.3 m/s) to encourage plume rise and dilution. These factors affect the dilution of the exhaust stream and the plume trajectory. High-discharge velocity and temperature increase plume rise, but high velocity is generally less effective than increased stack height. Augmented exhausters can be used, e.g., Strobic Air blowers, but the conditions of service need to be considered to avoid subjecting such equipment to conditions that could rapidly damage them.

 

ANSI Z9.5 5.3.4

8 CCR 5154.1(e)(4)

ASHRAE Handbook of Fundamentals, Chapter 14

 

Wind engineering evaluations shall be conducted for all wind directions striking all walls of a building. Actual height and placement shall be confirmed via 3-D modeling in a wind tunnel where building exhaust is likely to have significant ground level impact, or is likely to affect air intake for nearby buildings.

 

Modeling should also be performed when radioactive or carcinogenic materials will be exhausted by the ventilation system.

 

8 CCR 5154.1(e)(4)(D)

 

Emergency generator exhaust shall be considered in the wind-engineering study.

 

Combustion product odor from emergency power units (EPUs) is a significant nuisance. EPUs shall be located remotely or have tall-enough stacks to clear adjacent building air intakes and windows using the one in five rule of thumb mentioned in Chapter 14 of the ASHRAE Handbook of Fundamentals.

 

UC Practice

ASHRAE Handbook of Fundamentals, Chapter 14

 

Aesthetic conditions concerning external appearance shall not supersede the requirements of this Section.

 

ANSI Z9.5 5.3.5