Formulas:

N = Q/V x 60 min/hr

Where:

Q= volumetric flow rate in cfm,

V = volume in cubic feet

N = air changes per hour (ACH) Note: in your lab this was referred to as RACH.

V = Qt

Where:

Q = volumetric flow rate in cfm

t = time in minutes

V = volume

Q = KG/C x 10⁶

Where:

K = mixing factor, unit-less (also referred to a safety factor)

G = Generation rate of contaminant in cfm

C = Concentration at standard (PEL,TLV or REL) in ppm

Q = Required volumetric flow rate in cfm

You will also need molar conversions.

The ASHRAE standard 62-2010 on IAQ suggests an "air per person" and "air per square foot" approach for determining the minimum

amount of OA to be supplied to various occupancies for dilution purposes.The ASHRAE standard assumes excellent mixing, distribution and delivery of the outdoor air to occupants. It also assumes a certain occupant loading.

In its simplest form, the amount of air to be supplied is estimated by an equation similar to that shown below.

QOA = Qp + Qb = RpPd + RbAb (36-10)

Where:

QOA = minimum design OA ventilation rate

Qp = ventilation required to control "people" sources of air contaminants

Qp = ventilation required to control "building" sources of air contaminants

Rp = minimum rate of outdoor air in cfm per person

Pd = number of people in the occupied area

Rb = minimum rate of outdoor air in cfm per square foot of occupied area

Ab = floor area in occupied portion of the building, square feet

Sample of ASHRAE 62-2010 Standard Table

Occupancy Outdoor Air (OA) Required Ventilation Rates Application Persons per 1000 sf cfm/person cfm OA /person + cfm/ft2

Office Space 5 17 5 + 0.06

Conference Room 50 6 5 + 0.06

Science Lab (school) 25 17 10 + 0.18

What is the required flow rate for a 2500 square footScience Lab with an occupancy of 40 people?