General Dilution Ventilation
General Dilution Ventilation
The supply and exhaust of air in a building
Types of general dilution ventilation:
 Type1: dilution ventilation (D.V.)
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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
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It is the control of indoor atmospheric conditions found in hot
industrial environments. The purpose is to prevent discomfort or
injury to workers
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Dilution Ventilation for Health
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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.
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Parameters Required for Determination
of Dilution Ventilation Rates
 Solvent vapor per minute (i.E. Evaporation rate )
 Specific gravity of liquid
 Molecular weight
 Acceptable health standard (threshold limit value i.E.
TLV)
 K factor for incomplete mixing
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General Dilution Ventilation Equation
Rate of accumulation = Rate of generation – Rate of removal
Vdc = Gdt – Q’Cdt
Where
V = Volume of room
G = Rate of generation
Q’ = Effective volumetric flow rate
C = Concentration of gas or vapor in ppm
t = time
For steady state condition, change in concentration, dC = 0
Gdt = Q’Cdt
For constant concentration C and uniform generation rate G, the
above equation may be integrated as
Q’ = G/C
Now, G = (403 * SG * ER)/MW
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General Dilution Ventilation Equation
Q’ = (403 * 106 * SG * ER)/MW * C
Where
SG = Specific gravity
ER = Emission rate in pints/minute
MW = molecular weight
G
= Rate of generation in cfm
C
= Concentration of gas or vapor in ppm
Actual Ventilation Rate Q = Q’ * K
Where
K = factor for incomplete mixing and lies between 1 and 10 and
depends on:
 Efficiency of mixing
 Toxicity of chemicals
 Duration of the process
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Contaminant Concentration Build Up
Vdc = Gdt - Q’Cdt
Rearranging the terms and integrating from time t1to t2
and concentration C1to C2 , we get
ln[(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 then
Δt = K(V/Q)ln [G )/ [G-((Q/K)C2 )]
Note: C is in parts /106
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Rate of Purging
For this case, rate of contaminant generation G = 0
VdC = -Q’Cdt
dC/ C = (-Q’/V)dt
Integrating from time t1to t2 and concentration C1to C2 , we get
ln(C2 / C1) = -Q’/V(t2-t1)
t2 - t1 = -(V/Q’) ln(C2 / C1)
If initial time t1=0 then
t2 = -(V/Q’) ln(C2 / C1) = -(V/Q’) ln(C1 / C2)
Q’= Q/K
t2 = K(V/Q) ln(C1 / C2)
Where
t2=time, minutes
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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) > 1
then the threshold limit of the mixture is considered to
be exceeded
Where
C
= observed atmospheric concentration
TLV = corresponding threshold limit
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Ventilation for Heat Control
History of heat stress
 Steel industry
 Glass industry
 Mining industry
 Paper industry
Heat load on a person
 Metabolism
 Conduction
 Convection
 Radiation
 Evaporation
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Heat Balance and Exchange
delta s = (M – W) + C + R - E
delta s = change in body heat content
(M-W) = total metabolism
C
= convection heat exchange
R
= radiative heat exchange
E
=evaporative heat loss
C and R are positive if delta s increases in heat
Data required:
Measurement of metabolic heat production
Air temperature
Air water vapor pressure
Wind velocity
Mean radiant temperature
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Methods of Heat Exchange
 Convection
 Radiation
 Evaporation
Convection
C = 0.65Va0.6 (ta-tsk)
Where
C = convective heat exchange, Btu/h
Va = air velocity, fpm
ta = air temperature, F
tsk = mean weighted skin temperature, usually assumed to be 95 F
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Methods of Heat Exchange
Radiation
R = 15.0 (tw - tsk)
Where:
R = radiant heat exchange , Btu/hr
tw = mean radiant temperature,F
tsk = mean weighted skin temperature(usually 95 F)
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Methods of Heat Exchange
Evaporation
E = 2.4Va0.6(ρsk - ρa)
Where:
E = evaporative heat loss, Btu/h
Va = air velocity, fpm
ρa = water vapor pressure of ambient air,mm Hg
ρsk= water vapor pressure on the skin, (assumed to be
42
mm Hg at a 95 F skin temperature)
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Acute Heat Disorders
1. 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
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Acute Heat Disorders
2. Heat exhaustion
Causes
 Lack of acclimatization
 Failure to consume sufficient water
Symptoms
 Clammy, moist skin
 Weakness or extreme fatigue
 Giddiness
 Nausea
 Head ache
 Low weak pulse
Treatment
 Resting in a cool environment where there is free flowing, dry
air usually remediates the symptoms quickly.
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Acute Heat Disorders
3. 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 used
Recommended Treatment
 Use of electrolytes (Mg, Ca, K)
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Acute Heat Disorders
4. Heat rash
 It is an acute inflammatory skin disease
 Occurs commonly in areas of skin folds or where
there is abrasive clothing
Treatment
 The infected areas should be kept dry, unabated and
open to free flowing, dry air
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Heat Strain Measurement
The severity of heat strain will vary greatly among
people exposed to the same level of heat stress.
Acute heat strain is indicated by:
 Visible sweating
 Discontinued sweating
 Elevated heart rate
 Elevated deep body temperature
 Decreased systemic arterial blood pressure
 Personal discomfort
 Infrequent urination
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Heat Stress Measurement
Dry-Bulb temperature
Temperature as registered by a thermal sensor
Natural Wet-Bulb temperature
Temperature measured by a thermometer whose bulb is covered
by a wetted wick and exposed to natural air movement
unshielded from radiation
Psychrometric Wet-Bulb temperature
Temperature as registered by psychrometer
Globe temperature
It 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
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WBGT Index
Indoors
When radiant heat transfer is negligible
WBGT = 0.7(Wet Natural Bulb Temperature) +
0.3(Globe Temperature)
Outdoors
When there is no source of radiant heat transfer
WBGT = 0.7(Wet Natural Bulb Temperature) +
0.2(Globe Temperature)
+0.1 (Dry Bulb Temperature)
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Dilution Ventilation for Fire and
Explosion
It 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 *B
Where
LEL
SG
ER
SF
MW
B
= lower explosive limit, parts per 100
= specific gravity
= emission rate, pints/min
= safety factor
= molecular weight
= constant ( 1 for temperatures up to 2500 F
0.7 for temperatures > 2500 F)
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