Part 2: What is the basis for using ACH as a design parameter?

In a previous post I examined how the concentration of a pollutant decreased over time as a function of different ventilation rates. This examination was limited to the case where the pollutant was at some fixed level in a space and then ventilation was introduced into that space. An example of this situation would be a pollutant leaking from a pipe into a closed room and then a valve being closed which stops the flow of the pollutant and then a fan being turned on to provide ventilation to the space. While this scenario is possible, it is probably more useful to examine the case where a pollutant is being emitted at some rate and ventilation is being supplied to attempt control the level of that pollutant. For simplicity, it will be assumed that the initial concentration of the pollutant is zero. With this simplifying assumption, this case can be modeled using a relatively simple differential equation:

where   C(t) = Concentration at time t

G = Generation rate of pollutant

Q = Ventilation rate

V = Volume of the space

Dt = Change in time

This equation comes from ACGIH's book Industrial Ventilation A Manual of Recommended Practice. As with the previous case, the units for each of the parameters must be consistent. If G is given in CFM, then the time will be minutes and the volume will need to be given in cubic feet. So what impact does changing the ACH have upon the concentration of the pollutant in a space? For this example, it is assumed that the rate of pollutant generation is 1 CFM (0.5 L/s) in a room with a volume of 10,000 cubic feet (283 cubic meters), a space roughly 29’ wide x 29’ long x 12’ tall (8.8 m x 8.8 m x 3.7 m).

As in the previous case, the ACH has a dramatic impact upon the final concentration of the pollutant in the room. At small values for ACH the concentration of a pollutant increases for quite some time until a steady state concentration is reached. For example with an ACH = 0.5 the concentration continues to increase for about 10 hours until the final concentration of 12,000 ppm (1.2% by volume) is reached. Contrast this with an ACH = 4 where the final concentration of 1,500 ppm (0.15%) is reached after an hour. As ACH increases, the final steady state concentration decreases. This chart suggests that the ventilation rate can be used to control the final concentration of pollutants in a space.

It can be reasonably concluded that using the ACH as a design parameter for a ventilation system has merit. However, it is necessary to again mention that several simplifying assumptions were made in the previous analysis which can have a dramatic effect upon the performance graphs presented here. The limitations of this method will be examined in an upcoming post.

AIHce 2016: Understanding and Using ANSI/AIHA/ASSE Z9.2-2012

See the main conference and expo website here.

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 about Keith D. Robinson, P.E.

The System Curve, The Fan Curve, and the Operating Point Lunch and Learn

I am pleased to announce that I will be presenting a complimentary Lunch and Learn session on April 21, 2016 entitled "The System Curve, The Fan Curve, and the Operating Point". During this session, the theoretical background for how a fan and duct system interact will be presented. Following this brief theoretical introduction participants will take flow and pressure measurements on a small duct and fan system to develop the system curve, the fan curve, and the operating point for the system. The skills developed during this session can be used by the participants to determine the performance of the industrial ventilation systems at their individual facilities. A boxed lunch will be provided.

The Lunch and Learn will be held at 12303 Airport Way, Suite 200 in Broomfield, CO. Space is limited to 10 participants. Please contact Keith Robinson at kdr@kdrobinsonpe.com or at 303-746-8904 to reserve your spot.

Please visit my main website at www.kdrobinsonpe.com for more information about this Lunch and Learn or Keith D. Robinson, P.E.

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.

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.

Industrial Ventilation

Industrial Ventilation Systems are a specialized subset of the typical Heating Ventilation and Air Conditioning (HVAC) systems. Unlike building HVAC systems, comfort is not the primary consideration with Industrial Ventilation systems. Instead they are primarily concerned with controlling pollutants within industrial spaces. Of course the environmental conditions within an industrial facility are important. However, in the absence of extreme conditions, i.e. extreme heat or humidity or extreme cold, the exposure to a hazardous substance is more important than comfort. An analogy to this is found in a survival situation: We can live three days without water but several weeks without food. As a result finding water is more important than finding food when faced with the choice.

Perhaps the best reference to have when working with Industrial Ventilation systems is the book "Industrial Ventilation A Manual of Recommended Practice" published by the American Conference of Governmental Hygienists (ACGIH). This handbook provides a wealth of information and is the first reference to consult when faced with a questions about industrial ventilation systems.

Keith D. Robinson, P.E.

Welcome to the blog of Keith D. Robinson, P.E., a engineering consultant in Colorado Springs, Colorado. The goal of this blog is to discuss issues that arise in the engineering world and to hopefully provide some useful information to the readers. In general the information will come from the main areas in which I practice engineering but occasionally I will bring in information from outside my usual areas. These main areas will be Computational Fluid Dynamics (CFD), Finite Element Analysis (FEA), Industrial Ventilation, and Process/System simulation.

For more information about me and what I do, please check out my website at www.kdrobinsonpe.com