What is the basis for using ACH as a design parameter?

So why is ACH used as a design parameter? The basis for this comes from the differential equations that describe concentration buildup and concentration decay. As a first step, let's examine the concentration decay due to purging. In its basic form, the concentration decay equation is of the following form:

where C(t) = Concentration at time t.

C₀ = Concentration at time 0

Q  = Ventilation rate

V  = Volume of the space

Dt  = Change in time

In the equation, the units need to be in some consistent format. For example, if the time is given in minutes, the ventilation rate needs to be given in terms of volumes per minute. At the same time, the volumetric units used in the ventilation rate must match the units used to define the space volume such as cubic feet or cubic meters. For example, if the volume of the room is given in cubic meters (m³) and the time is given in hours, the ventilation rate must be given in terms of cubic meters per hour (m³/hr). When the value of Q is given in terms of cubic feet per hour or cubic meters per hour, the value of Q/V in the exponent is the ACH.

So how does changing the ACH effect the concentration of a pollutant in a space? Figure 1 shows how the concentration of a substance declines based on different ACH. The abscissa of this graph is change in time while the ordinate is the ratio of the concentration at a given time to the initial concentration. While there were several simplifying assumptions made in the development of this graph, it shows how increasing the ACH decreases the time required to reduce a concentration of a pollutant. 

Based on this simple analysis, it can be stated that using ACH as a design parameter does have a sound basis. However, it must be recognized that it is a very simplified approach. In an upcoming post I will examine how the ventilation rate affects the concentration of a pollutant that is being emitted into a space.

Commonly Used Acronyms

It is customary to not use an acronym without first defining the acronym. To be proper the acronym should  be spelled out in each post. I am going to take a shortcut and define commonly used acronyms in this post. In subsequent posts I will use just the acronym to save space. My apologies to all the style manuals out there.

Acronyms

CFD = Computational Fluid Dynamics

FEA = Finite Element Analysis

STP = Standard Temperature and Pressure

Organizations

AIHA = American Industrial Hygiene Association

ANSI = American National Standards Institute

ASHRAE = American Society of Heating Refrigeration and Air-Conditioning Engineers

ASME = American Society of Mechanical Engineers

ASSE = American Society of Safety Engineers

NFPA = National Fire Protection Association

Units

acfm = Actual cubic feet per minute

ACH  = Air Changes per Hour

fpm = Feet per minute

gpm = Gallons per minute

HP = Horsepower

in w.g. = Inches of water gauge

psia = Pounds per square inch absolute

psig = Pounds per square inch gauge

rpm = revolutions per minute

scfm = Standard cubic feet per minute

More will be added as the need arises.

Understanding and Using ANSI/AIHA/ASSE Z9.2-2012

Keith D. Robinson, P.E. will be teaching a course entitled PDC 109:  Understanding and Using ANSI/AIHA/ASSE Z9.2-2012 at the upcoming AIHce conference. This course provides an in-depth look at the requirements for Local Exhaust Ventilation (LEV) systems that are set forth in this standard. It is intended for Environment Health & Safety (EH&S) personnel, facility managers, system operators, and engineers. 

Please visit my main website at www.kdrobinsonpe.com for more information.

Is Air Changes per Hour (ACH) a valid ventilation design parameter?

One question discussed at the recent ASHRAE Winter Meeting in Orlando was whether or not Air Changes per Hour, commonly referred to as ACH, represents a valid design criterion for ventilation systems. There are several design standards that prescribe minimum ACH rates to keep contaminant levels at acceptable levels. In general, it is considered that the best way to decrease exposure to chemicals and pollutants in the indoor environment is to increase the ACH. However some research now suggests that increasing the ACH actually increases exposure rather than decreases exposure. Increasing ACH also increases the energy usage of the system due to fan energy and the need to heat or cool the incoming makeup air. So is this a valid method to use when designing ventilation systems?

The answer is an unqualified "Maybe." Each system and situation is different. Some situations, such as labs with fume hoods, need relatively still air to prevent recirculation of pollutants form entering the operator's breathing zone. Other situations, such as machinery rooms, need large amounts of air to evacuate potentially hazardous gasses if a leak occurs. Suffice it to say that it takes sound engineering judgment in cooperation with the Environmental Health and Safety (EH&S) department during the design phase to ensure that the correct design criteria are used for each specific system.

Please visit my website at www.kdrobinsonpe.com to learn more about Keith D. Robinson, P.E.