Hvac Ventilation

Page 1 of 4 1-31-07 Bill Greco Vapor Dilution with respect to Outside Air over a period of Time The following concerns Pharmaceutical production areas, research areas and other areas where annoying or dangerous vapors can be released. Many times vapors are not controlled by special ventilation apparatus such as hoods or carbon filtration. Vapors can be caused by laboratory accidents, use of cleaning solvents applied outside of the influence of the exhaust system or by special adhoc procedures conjured up by jury rigged processes. The primary general HVAC system often becomes the last line of defense to confine and dilute these vapors which have escaped into a conditioned space. A primary general HVAC system is meant to serve as supply and exhaust of air to control comfort or specifically produce a controlled environment within regulated room classification requirements. If vapors are re-circulated and confined within a primary general HVAC system, only outside air will dilute the unwanted vapor. Unfortunately opening a window in most cases is not a possibility. A prime function of many substances especially solvents is to evaporate into the local atmosphere leaving behind a physically or chemically transformed material formerly associated with it. Evaporation of many substances cause fumes, gases, vapors and / or mists. It is nearly always possible to estimate the rate or use of total liquid composition, to determine the rate of evaporation. This makes it especially simple, to handle vapor ventilation problems on a quantitative basis with relation to time. The following could be used to calculate any concentration of vapor or could be representative of the differential of a mixture of some substance being added over a time period in a tank of fluid through which fresh fluid is being added and continuously removed. The procedure can be applied to any case where air is initially charged with contaminant and it is desired to estimate the conditions in the space in relation to time.

Figure-1 shows a diagram of a primary general HVAC system that is conditioning re-circulating air, exhausting air and introducing outside air. Estimating the rate of generation of vapors : A weight of any liquid equal to it’s molecular weight produces on evaporation, the same volume of vapor as all other liquids. If the weight is given in pounds, the volume of vapor formed by one pound molecular weight is 359 cuft at 320 F., at room temperature of 700 F:

Assume evaporation of one pound of turpentine C10 H16 which has a molecular weight of 136.23 that has evaporated over a period of one hour. The total volume of vapor per hour is :

2.839 cuft/ hour or 0.047 cuft/min or a generation rate (G) of 0.047 cfm

Page 2 of 4 1-31-07 Bill Greco Vapor Dilution with respect to Outside Air over a period of Time

An increase in the quantity of vapor in the space during the time interval dt, is equal to the rate of vapor being added to the space minus the removal rate of vapor leaving the space. The relation between the ratio of concentration of vapor c and time t can be developed from the equation:

where: Q = Ventilation Rate, cfm (air exhausted) G = Vapor generation rate, cfm c = Ratio of substance vapor to air in parts per million expressed as a fraction at time t c_initial = Initial ratio of substance vapor parts per million at time t=0 P = Cubic Displacement of the space c x co

t dc/G – Qc = 1/P xdt

equation-2

0

which becomes:

and for cases where the initial concentration is set at zero :

solving for t =

and:

Simplifying:

Changing signs:

Page 3 of 4 1-31-07 Bill Greco Vapor Dilution with respect to Outside Air over a period of Time :

Multiply by P =

Divide by 0.435Q =

Rearranging =

A note concerning logs when addressing this problem: The terms log and log10 are what is known as common logs or Briggs* logarithms, logs to the base 10. The terms loge and ln are known as napierian or natural logarithms, logs to the base e or the infinite series 2.71828..... This is mentioned to avoid confusion when solving for (t) using the equations in this report. *= Briggs or Briggsian logs are a less used name for common logs, named for the English mathematician and Oxford professor Henry Briggs (1561-1630), who proposed that logarithms to base 10 would be more useful than Naperian logarithms. Other equation forms that solve for (t) are:

Page 4 of 4 1-31-07 Bill Greco Vapor Dilution with respect to Outside Air over a period of Time Example-1: Assume a research lab 30 feets long x 15 feet wide with a 10 foot high ceiling. A repetitive procedure is being performed in which a lab technician is using 1 pound of acetone on a lab bench for 30 minutes during each process, the procedure must be conducted outside of the labs hood area. The primary general HVAC system as shown in Figure-1 supplies conditioned air to the space, 170 cfm is being exhausted from the room. The room temperature is controlled to 700 F. The procedure is started with 0 parts per million (ppm) in the room. How long will the procedure be allowed to continue prior to reaching acetone’s threshold level of 1,000 parts per million ? acetone (CH3 COCH3 ) has a molecular weight of 58.08:

6.658 divided by 30 minutes = 0.188 cfm G= 0.188 cfm P= (30)(15)(10) = 4,500 cuft c= 1,000 / 1,000,000 or 0.001 Q= 170 cfm

If the process is stopped at 1,000 ppm, how long will it take the room air in example-1 to dilute and reach 500 ppm ? c_final = 500/1,000,000 = 0.0005

Refernce: Hemeon, W.C.L.

Plant and Process Ventilation Second Edition

pp 228-232