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
Chapter 2–Goals of the Laboratory Environment
In the same way, one air change per hour will not remove all of the airborne contaminants in
a room but will produce a dilution effect. As the rate of air changes per hour is increased,
we’ll reduce the concentration of hazardous airborne substances faster; but the time to
remove every last trace of such substances could be quite long and just like in the tank
example cannot really be determined.
ACH Calculations
To determine the airflow rate that is necessary to achieve one air change per hour (ACH), for
a given room, the room volume in cubic feet must first be determined. Then, the room volume
is divided by 60 (minutes per hour). This simple calculation yields the airflow rate in cubic feet
per minute necessary to achieve one air change per hour (ACH). Thus, a room having
dimensions of 12 feet in width, 20 feet in length and 10 feet in height will have an internal
volume of 2,400 cubic feet. Next, the 2400 cubic feet is divided by 60 minutes per hour which
yields 40 cubic feet per minute (cfm). Therefore, one ACH for the room will require a
continuous air supply of 40 cfm. It follows also that 10 ACH would require 10 × 40 cfm or 400
cfm. It is equally important to remember that an amount of exhaust equal to the supply airflow
must simultaneously be removed from the room as part of the ventilation process.
Note that air change rates are based upon the total internal volume of a room. In actuality
rooms will have furniture, equipment and of course, people. Although the actual net air space
left within the room therefore is less than the volume calculated using the room dimensions,
the procedure to determine the airflow cfm for a specific ACH rate in order to comply with
regulatory requirements or conform to HVAC standards is to always base the ACH on the
overall room volume. Thus, it is not permissible to deduct the volume of furniture or
equipment in a room when calculating the airflow required to attain a specific ACH ventilation
rate.
The ASHRAE organization provides the most comprehensive guide to the recommended
amount of outside fresh air and the minimum room ventilation rates that should be supplied to
various types of rooms. Table 1 is adapted from information following the ASHRAE
recommended outdoor fresh air rates and the minimum ventilation (ACH) rates for the types
of rooms that would normally be associated with a laboratory facility.
In addition to ASHRAE, other organizations that address laboratory safety have established
recommended ventilation rates for various types of rooms. The 1990 OSHA requirement (29
CFR Part 1910) for laboratories recommends a ventilation rate of 4 to 10 ACH for
laboratories with fume hoods. Research facilities that have instituted their own minimum
standards for laboratory ventilation rates most often maintain a minimum of 10 ACH.
Experienced laboratory HVAC designers favor the use of minimum ACH rates between 10
and 12 ACH.
Table 1. Minimum ACH and Outside Air Rates (From ASHRAE HVAC Applications 1995 &
ANSI/ASHRAE Std. 62-1989).
TYPE OF
ROOM
General
Office
Classroom Corridor Auditorium Cafeteria
ACH RATE
4 to 10 6 to 20 — 4 to15 12 to 15
OUTSIDE AIR
20 cfm 15 cfm 0.10 cfm per sq ft 15 cfm 12 cfm
6 Siemens Building Technologies, Inc.