Basic Documentation
Table Of Contents
- About this Application Guide
- Chapter 1–Introduction
- Chapter 2–Goals of the Laboratory Environment
- Chapter 3–Unique Ventilation Needs of a Laboratory Facility
- Chapter 4–Ventilation Systems Classification
- Chapter 5–Laboratory Facility Exhaust Systems
- Chapter 6–Laboratory Containment Units - Ventilation
- Chapter 7–Room Ventilation, Makeup Air, and Pressurization Control Systems
- Chapter 8–Laboratory Temperature and Humidity Control Systems
- Chapter 9–Laboratory Emergencies - Ventilation System Response
- Chapter 10–Laboratory Ventilation System - Validation
- Chapter 11–Laboratory Ventilation System - Commissioning
- Glossary
- Index
Exhaust System Discharge
Stack Diameters and Exit Velocity
When conventional stacks are used, the airflow within the stack should be limited to
about 2,400 feet per minute (fpm) in order to allow internal condensation to flow
downward to a roof drain provided at the base of the stacks. Internal air velocities
much higher than this will likely carry condensation droplets out of the stack where
their discharge will be subject to the wind speed and direction. Since such droplets
may consist of condensed acid vapors or other corrosive fumes, they can have a
very detrimental effect on the structure, nearby areas, and equipment.
Typically, the exhaust should exit the stack at about 3,000 fpm and this increase in
velocity may be attained by adding a conical shape to the stack. Do not incorporate
other elements at the discharge end of the exhaust stack. Weather hoods or rain
caps adversely affect the ability of the stack to achieve an effective vertical exhaust
discharge. Rain or snow cannot enter an exhaust stack if the exhaust air exit velocity
is at least 2,500 fpm. The following equations calculate the required internal diameter
of stack and point of discharge based upon volumetric airflow in cfm (l/s) and the
velocity in fpm (m/s).
Exhaust Stack Airflow Velocity Calculations
To find the required internal diameter of an exhaust stack or the diameter of a conical
stack top for a given amount of airflow, utilize the following formula:
IP: Stack I.D. (inches) = 13.54 × (cfm ÷ fpm)
1/2
SI: Stack I.D. (mm) = 35.68 × (l/s
÷
m/s)
1/2
For a 3000 fpm (15.3 m/s) exit velocity the cone top I.D. becomes:
IP: Cone I.D. (inches) = 0.247 × (cfm)
1/2
SI: Cone I.D. (mm) = 9.14 × (l/s)
1/2
Table 2 provides a handy reference of required stack or cone internal diameters for
airflow velocities for exhaust volumes.
Table 2. Internal Exhaust Stack and Discharge Cone Diameters Required for Airflow
Velocities of 2,400 and 3,000 fpm (15.24 and 12.19 m/s) for Various Airflows.
Volumetric
Airflow
Internal Diameter
Corresponding To Velocity
cfm l/s 2400 fpm
(12.19 m/s)
3000 fpm
(15.24 m/s)
500 236 6.2” 157mm 5.5” 140mm
750 354 7.6” 192mm 6.8” 172mm
1,000 472 8.7” 221mm 7.8” 199mm
1,250 590 9.8” 248mm 8.7” 222mm
1,500 708 10.7” 272mm 9.6” 243mm
1,750 826 11.6” 294mm 10.3” 263mm
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