GENERAL DILUTION VENTILATION

General Dilution Ventilation

General Dilution Ventilation 2

General Dilution Ventilation

The supply and exhaust of air in a buildingTypes of general dilution ventilation:

Type1: dilution ventilation (D.V.) D.V. Is the dilution of contaminated air with uncontaminated

air for controlling potential airborne health hazards, fire and explosive conditions, odors and nuisance type contaminants

D.V. Also includes the control of airborne contaminants such as vapors, gases and particulates generated within tight buildings

D.V. Is not as satisfactory for health hazard control as is local exhaust ventilation

Type 2 : heat control ventilation It is the control of indoor atmospheric conditions found in hot

industrial environments. The purpose is to prevent discomfort or injury to workers


Dilution Ventilation for Health

Dilution ventilation is generally used to control the vapors from organic liquids with a TLV of 100 ppm or higher.The limiting factors for D.V. For health are:

The quantity of contaminant generated must not be too great or the air flow rate necessary for dilution will be impractical.

Workers must be at an appropriate distance from the contaminant source or the exposed contaminant must be in sufficiently low concentrations so that workers will not have an exposure in excess of the established TLV.

The toxicity of the contaminant must be low. The emission rate of contaminants must be reasonably

uniform.


Parameters Required for Determination of Dilution Ventilation Rates Solvent vapor per minute (i.E. Evaporation rate )Specific gravity of liquidMolecular weightAcceptable health standard (threshold limit value

i.E. TLV) K factor for incomplete mixing


General Dilution Ventilation Equation

Rate of accumulation = Rate of generation – Rate of removalVdc = Gdt – Q’CdtWhereV = Volume of roomG = Rate of generationQ’ = Effective volumetric flow rateC = Concentration of gas or vapor in ppm t = timeFor steady state condition, change in concentration, dC = 0Gdt = Q’CdtFor constant concentration C and uniform generation rate G, the above equation may be integrated as Q’ = G/CNow, G = (403 * SG * ER)/MW


General Dilution Ventilation Equation

Q’ = (403 * 106 * SG * ER)/MW * CWhereSG = Specific gravityER = Emission rate in pints/minuteMW = molecular weightG = Rate of generation in cfmC = Concentration of gas or vapor in ppm

Actual Ventilation Rate Q = Q’ * KWhereK = factor for incomplete mixing and lies between 1 and 10 and depends on:

Efficiency of mixing Toxicity of chemicals Duration of the process


Contaminant Concentration Build UpVdc = Gdt - Q’CdtRearranging the terms and integrating from time

t1to t2 and concentration C1to C2 , we getln[(G-Q’C2 )/ [(G-Q’C1 ) = -Q’/V(t2-t1)Δt = t2 - t1

Δt = -V/Q’ * ln [(G-Q’C2 )/ [(G-Q’C1 ) If initial concentration C1 = 0 and Q’ = Q/K thenΔt = K(V/Q)ln [G )/ [G-((Q/K)C2 )]Note: C is in parts /106


Rate of PurgingFor this case, rate of contaminant generation G = 0VdC = -Q’CdtdC/ C = (-Q’/V)dtIntegrating from time t1to t2 and concentration C1to C2 , we getln(C2 / C1) = -Q’/V(t2-t1)

t2 - t1 = -(V/Q’) ln(C2 / C1)If initial time t1=0 thent2 = -(V/Q’) ln(C2 / C1) = -(V/Q’) ln(C1 / C2)Q’= Q/K t2 = K(V/Q) ln(C1 / C2)Wheret2=time, minutes


Mixtures-dilution Ventilation for Health

When two or more hazardous substances are present, then their combined effect known as the additive effect should be given primary consideration

If(C1/TLV1) + (C2/TLV2) +……… (Cn/TLVn) > 1then the threshold limit of the mixture is

considered to be exceededWhereC = observed atmospheric concentrationTLV = corresponding threshold limit


Ventilation for Heat ControlHistory of heat stressSteel industryGlass industryMining industryPaper industry

Heat load on a personMetabolismConductionConvectionRadiationEvaporation


Heat Balance and Exchangedelta s = (M – W) + C + R - Edelta s = change in body heat content(M-W) = total metabolismC = convection heat exchangeR = radiative heat exchangeE =evaporative heat lossC and R are positive if delta s increases in heat

Data required:Measurement of metabolic heat productionAir temperatureAir water vapor pressureWind velocityMean radiant temperature


Methods of Heat Exchange Convection Radiation Evaporation

Convection

C = 0.65Va0.6 (ta-tsk)

WhereC = convective heat exchange, Btu/hVa = air velocity, fpmta = air temperature, Ftsk = mean weighted skin temperature, usually assumed to be

95 F


Methods of Heat ExchangeRadiationR = 15.0 (tw - tsk)Where:R = radiant heat exchange , Btu/hrtw = mean radiant temperature,Ftsk = mean weighted skin temperature(usually

95 F)


Methods of Heat ExchangeEvaporationE = 2.4Va

0.6(ρsk - ρa)Where:E = evaporative heat loss, Btu/hVa = air velocity, fpmρa = water vapor pressure of ambient air,mm Hgρsk= water vapor pressure on the skin, (assumed to be 42 mm Hg at a 95 F skin temperature)


Acute Heat Disorders1. Heat stroke A major disruption of central nervous function Lack of sweating Rectal temperature > 410C

Treatment Placing the patient in a shady area Removing the outer clothing Wetting the skin Increasing air movement Professional help


Acute Heat Disorders2. Heat exhaustionCauses Lack of acclimatization Failure to consume sufficient waterSymptoms Clammy, moist skin Weakness or extreme fatigue Giddiness Nausea Head ache Low weak pulseTreatment Resting in a cool environment where there is free flowing,

dry air usually remediates the symptoms quickly.


Acute Heat Disorders3. Heat cramps Characterized by spasms in skeletal muscles Occurs in people when body water and electrolyte levels

have not been restored after extended periods of heavy sweating during exercise and/or heat stress

Old Treatment Use more salt at meal times and provide 0.1% salt in

drinking water Salt tablets can also be usedRecommended Treatment Use of electrolytes (Mg, Ca, K)


Acute Heat Disorders4. Heat rash It is an acute inflammatory skin diseaseOccurs commonly in areas of skin folds or

where there is abrasive clothing

TreatmentThe infected areas should be kept dry,

unabated and open to free flowing, dry air


Heat Strain MeasurementThe severity of heat strain will vary greatly

among people exposed to the same level of heat stress. Acute heat strain is indicated by:

Visible sweatingDiscontinued sweatingElevated heart rateElevated deep body temperatureDecreased systemic arterial blood pressurePersonal discomfort Infrequent urination


Heat Stress MeasurementDry-Bulb temperatureTemperature as registered by a thermal sensor

Natural Wet-Bulb temperatureTemperature measured by a thermometer whose bulb is

covered by a wetted wick and exposed to natural air movement unshielded from radiation

Psychrometric Wet-Bulb temperatureTemperature as registered by psychrometer

Globe temperatureIt is the infrared radiant heat transfer measured by a

temperature sensor at the center of a 6 inch hollow copper sphere which is painted both on the inside and outside


WBGT IndexIndoorsWhen radiant heat transfer is negligibleWBGT = 0.7(Wet Natural Bulb Temperature) +

0.3(Globe Temperature)

OutdoorsWhen there is no source of radiant heat transferWBGT = 0.7(Wet Natural Bulb Temperature) +

0.2(Globe Temperature) +0.1 (Dry Bulb Temperature)


Dilution Ventilation for Fire and ExplosionIt is necessary that the concentration of vapor in the work

area should be below the lower explosive limit (LEL) (preferably<25% LEL). This is for fire and explosion only and not for health hazard

Q = 403 * SG *100 *ER * SF/MW * LEL *BWhere

LEL = lower explosive limit, parts per 100SG = specific gravityER = emission rate, pints/minSF = safety factorMW = molecular weightB = constant ( 1 for temperatures up to 2500 F 0.7 for temperatures > 2500 F)

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GENERAL DILUTION VENTILATION