Basic Documentation
Table Of Contents
- Ventilation System Characteristics
- Constant Air Volume (CAV) 1-Position
- Constant Air Volume (CAV) 2-Position
- Low Air Flow (LAF) 1 & 2-Position
- Variable Air Volume (VAV)
- Ventilation System Functions
- 1. Small Testing and Analysis Facility
- 2. Small College
- 3. Large College or University
- 4. University Research Laboratories
- 5. Government Agencies
- 6. Government Research
- 7. Small/Mid-Sized Commercial and Industrial Firms
- 8. Large Commercial and Industrial Firms
- Life Cycle Cost Analysis
- Life Cycle Energy Cost
- Life Cycle Cost
Siemens Industry, Inc. Page 9 of 10
Document No. 149-980
Table 3. Laboratory Annual Cubic Feet per Minute.
System
Type
Average Fume Hood & General Exhaust Cubic Feet per Minute (cfm) Calculation Procedure
The average annual fume hood cfm is the normal constant fume hood exhaust cfm. CAV – 1
&
LAF - 1
The average annual general exhaust cfm is the normal constant general exhaust cfm.
Step 1: Determine the laboratory's weekly occupancy percentage by dividing the normal Occupied Hrs/Wk
6
by 168.
(For 8 A.M. to 5 P.M. weekdays this typically is about 33%.)
Step 2: Determine the laboratory weekly unoccupancy percentage by subtracting the resultant of Step 1 from
100%.
Step 3: Determine the unoccupied cfm percentage by multiplying the resultant from Step 2 by 20%, which
represents the allowable minimum fume hood exhaust airflow.
Step 4: Add the two percentages from Steps 1 and 3.
Step 5: Determine the average cfm by multiplying the fume hood's maximum exhaust cfm by the resultant
of Step 4.
CAV – 2
&
LAF - 2
The average annual general exhaust usage is determined by the following expression:
[Occ G.E. cfm × Occ Hrs/Wk + Unocc G.E. cfm × Unocc Hrs/Wk] 168
Step 1: Determine the normal laboratory fume hood usage time percentage by dividing the likely actual weekly
usage hours by 168. (For a VAV system this typically ranges from 10% to 20%.)
Step 2: Determine the laboratory fume hood non-usage time percentage by subtracting the resultant from Step 1
from 100%.
Step 3: Determine the unoccupied cfm percentage by multiplying the result from Step 2 by 20%, which represents
the minimum fume hood exhaust airflow.
Step 4: Add the two percentages from Steps 1 and 3.
Step 5: Determine the average cfm by multiplying the fume hood's maximum exhaust cfm by the resultant
of Step 4.
VAV
The average annual general exhaust cfm is determined from the following expression:
[Average Occ G.E. cfm × Occ Hrs/Wk + Average Unocc G.E. cfm × Unocc Hrs/Wk] 168
6
The weekly occupied hours are the total of each day's regular occupancy hours plus the additional daily occupied hours for transitioning
into and out of the unoccupied period (typically 2 hours per day).