Transcript Natural Ventilation Presentation

Natural Ventilation
Natural Ventilation
Calculation of rate of ventilation air flow
Q = H/(60 * CP * ρ * Δt) = H/1.08 * Δt
Where
H = Heat removed in Btu/hr
Δt = indoor outdoor temperature difference(oF)
CP = 0.245 Btu/lb/ oF
ρ = 0.075 lb/ft3
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Flow Due to Thermal Forces (Stack Effect)
Q = C * K* A * √ ( h * [ ( ti – to ) / ti ] )
Q = air flow in cfm
A = free area of inlets or outlets (assumed equal) in ft2
h = height from inlets to outlets, ft
ti = indoor air temperature, oF
to = outdoor air temperature, oF
C = Constant of proportionality =14.46
K = 65% or 0.65 for effective openings
= 50% or 0.50 for unfavorable conditions
Substituting the values for C and K the equation reduces to
Q = 9.4* A * √ ( h * [ ( ti – to ) / ti ] )
Q = 7.2 * A * √ ( h * [ ( ti – to ) / ti ] )
(for effective openings)
(for unfavorable conditions)
When to> ti replace denominator in equation with to.
Assumptions:1. No significant building internal resistance
2. Equation is valid for temperatures ti and to close to 80oF
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Factors affecting flow due to wind
 Average velocity
 Prevailing direction
 Seasonal and daily variation of wind speed & direction
 Terrain features (local)
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Calculation of Air Flow(due to Wind)
Q = EAV
Q = air flow in ft3/min
A = free area of inlet openings in ft2
V = wind velocity in ft/min
E = effectiveness of openings
= 0.5-0.6
perpendicular winds
= 0.25-0.35 diagonal winds
V for design practice = 1/2*seasonal average
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Flow Due to Combined Wind and
Stack Effect
 When both forces are together, even without interference, resulting
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air flow is not equal to the two flows estimated separately.
Flow through any opening is proportional to the square root of the
sum of heads acting on that opening.
Wind velocity and direction, outdoor temperature, and indoor
distribution cannot be predicted with certainty, and refinement
calculations is not justified.
A simple method is to calculate the sum of the flows produced by
each force separately.
Then using the ratio of flow produced by the thermal forces to the
aforementioned sum, the actual flow due to the combined forces can
be approximated.
When the two flows are equal, actual flow is about 30% greater than
the flow caused by either force.
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Types of Natural Ventilation Openings
 Windows :
There are many types of windows.
Windows sliding vertically, sliding horizontally, tilting,
swinging.
 Doors, monitor openings and skylights.
 Roof Ventilators (weather proof air outlet).
 Stacks connecting to registers.
 Specially designed inlet or outlet openings.
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Natural Ventilation Rules
1.
Buildings and ventilating equipment should not usually be oriented
for a particular wind direction.
2.
Inlet openings should not be obstructed by buildings , trees,
signboards, or indoor partitions.
3.
Greatest flow per unit area of total opening is equal to inlet and
outlet openings of nearly equal areas.
4.
For temperature difference to produce a motive force, there must be
vertical distance between openings; vertical distance should be as
great as possible.
5.
Openings in the vicinity of the neutral pressure level are least
effective for ventilation.
6.
Openings with areas much larger than calculated are sometimes
desirable(e.g.hot weather,increased occupancy). The openings should
be accessible to and operable by occupants.
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Infiltration
Infiltration is air leakage through cracks and interstices, around
windows and doors, and through floors and walls into a building
Leakage rate (houses)0.2 to 1.5 air changes /hr in winter
Infiltration through a wall
Q = C*(ΔP)n
Q = Volume flow rate of air ft3/min
C = Flow coefficient(Volume flow rate per unit length of crack or
unit area at a unit pressure difference)
ΔP = Pressure difference
n = Flow exponent 0.5 –1.0 normally 0.65
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Pressure Difference Due to Thermal
Forces
Pc = 0.52*P*(1/To-1/Ti).
Pc = theoretical PC = pressure difference across enclosure due to
chimney effect(inches of water).
P = atmospheric pressure lb/sq.inches.
h = distance from neutral pressure level or effective chimney
height.
To = absolute temperature outside 0R.
Ti = absolute temperature outside 0R.
Apply for character of interior separations correction.
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Infiltration
 Air moves in and out of buildings at varying rates depending
upon a number of factors relating to both the structure and the
local meteorological conditions. Two terms are: infiltration and
ventilation. Both are measured as air exchange rate, or air
changes per hour (ACH).
 The ASHRAE defines infiltration as “uncontrolled airflow through
cracks and interstices, and other unintentional openings.”
 Infiltration occurs because no building is completely airtight;
wind pressures and temperature create driving forces which
push or draw the outdoor air through openings into the
building.
 Infiltration is the rate of exchange of outdoor air with the entire
volume of indoor air, quantitated as ACH.
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Factors Affecting Air Infiltration
Type of structure and construction
 Meteorology
 Heating & cooling systems
 Occupant activity
Structural parameters
 Quality of construction
 Materials of construction
 Condition of the structure
Meteorological parameters
 The airflow rate due to infiltration depends upon
pressure differences between the inside and outside of the
structure and the resistance to flow through building openings
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Wind Effects
 Shell and exterior air barriers.
 Interior barriers to flow that cause internal pressure buildup and
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thus reduce infiltration.
Lack of precise knowledge of the detailed wind pressure profiles
on building surfaces.
Influence of complex terrain, presence of trees and other
obstacles that create channeling and may increase the
magnitude of wind force and alter its direction close to the
structure.
Sheltering, urban canyon and building wake phenomena due to
surrounding buildings and other neighborhood factors.
Fluctuating winds, rather than linear wind forces, that may
effect infiltration rates through window cracks.
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Temperature Effects
 Temperature inside a structure is often different from the
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outside ambient temperature.
Maximum temperature differences occur when the indoor
environment is heated.
Temperature differences cause differences in air density inside
and outside, which in turn produce pressure differences.
In the winter when indoor air temperatures are high relative to
those outdoors, the warmer and less dense air inside rises and
flows out of the building at its top.
This air is replaced by cold outdoor air that enters near the
bottom of the building or from the ground.This phenomenon is
called the building “Stack Effect”.
During hot weather when air conditioning produces lower
temperatures inside than outside, the reverse process occurs.
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Humidity Effects
 Stricker in 1975 reported that homes with low infiltration
rates had high humidity.
 In a study by Yarmac et al. in 1987 in 25 houses in the
southern U.S., no apparent relationship was found between
relative humidity and air exchange rate. One explanation
for this lack of association is that absolute humidity, rather
than relative humidity, may be a better measure of any
effect the water content of the air has on infiltration.
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Pressure Difference Across the
Building Envelope
ΔP = Po+Pw-Pi
Where
ΔP = pressure difference between outdoors and indoors at the
location
Po = static pressure at reference height in the undisturbed flow
Pw = wind pressure at the location
Pi = interior pressure at the height of the location
1. The more usual case is when both wind and indoor outdoor
temperature differences contribute to the ΔP across the
building envelope
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Pressure Difference Across the
Building Envelope
2.Temperature differences impose a gradient in the pressure
differences which is a function of height and the temperature
difference
This effect is additive to the wind pressure expression and is
expressed by ASHRAE, 1989 as
ΔP = Po+Pw-Pi,r+ ΔPs
Where
ΔPs= the pressure caused by the indoor-outdoor temperature
difference (stack effect)
Pi,r = the interior static pressure at a reference height (it assumes a
value such that inflow equals outflow)
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Bernoulli’s Equation
PV = (Cp*ρ*V2)/2
Where
PV = surface pressure relative to static pressure in undisturbed
flow,Pa
Cp = surface pressure coefficient
ρ = density of air,kg/m3
V = wind speed in m/s
Under standard conditions (100.3 Pa or 14.7 psi) and 200 C, this
equation reduces to:
PV = (Cp*0.601*V2)
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Bernoulli’s Equation
Cp varies with location around the building envelope and wind
direction
The differences in air density due to temperature differences
between the interior and exterior of a building create the
pressure difference which drives infiltration
To estimate this pressure difference, ΔPs, it is necessary to know
the NPL
This pressure difference can be expressed as:
ΔPs = ρi*g*h*(Ti-To)/ To
Where:
ΔPs = pressure difference, Pa
ρi
= density of air, kg/m3
g = gravitational constant, 9.8m/sec2
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Bernoulli’s Equation
h=distance to NPL(+ve if above, -ve if below from the location of
the measurement
Subscripts:
i=inside
o=outside
It is difficult to know the location of the NPL at any one moment,
but there are some general guidelines
According to ASHRAE,1989, the NPL in tall buildings can vary from
0.3 to 0.7 of total building height
In houses with chimneys, it is usually above mid-height, and
vented combustion sources for space heating can move the NPL
above the ceiling
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Measurement Techniques
 Tracer gas
 Fan pressurization
 Effective Leakage Area(ELA)
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Tracer Gas
 It is a different measure of air exchange rate.
 The gas concentration will decrease as dilution air flow
into the building.
 The rate of decrease is proportional to the infiltration
rate.
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Assumptions
 The tracer gas mixes perfectly and instantaneously
 The effective volume of the enclosure is known
 The factors that influence air infiltration remain
unchanged throughout the measurement period
 Imperfect mixing occurs when air movement is
impeded by flow resistances or when air is trapped
by the effects of stratification
 This causes spatial variation in the concentration of
the tracer gas within the structure, this may cause
bias in sampling locations
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Assumptions (contd…)
 Fans are often used to mix the tracer gas with the building
air.
 Effective volume is assumed to be the physical volume of
the occupied space.
 Areas which contain dead spaces that do not communicate
with the rest of the living space will reduce the effective
volume.
 Variations in conditions during the measurement
period,such as door openings or meteorological changes,
will cause a departure from the logarithmic decay curve
and the equation on which infiltration is calculated will no
longer hold.
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Types of Gases of Used As Tracers:
 Helium,Nitrous oxide, Carbon dioxide,Carbon monoxide,
Sulfur hexaflouride, and perfluorocarbons
 Non-toxic at concentrations normally used in such studies,
non-allergenic, inert, non-polar, and can be detected easily
and at low concentrations
 Most frequently used are SF6 and Perfluorocarbons
 Carbon dioxide or carbon monoxide can be used if initial
concentrations are substantially above background but well
below concentrations of health concern
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Tracer Gas Dilution: SF6
 Specific instructions for this method can be found in
the American Society of Testing Materials
(ASTM)Standard Method for Determining Air Leakage
Rate by Tracer Dilution (E741).
 The basic apparatus for this method includes: tracer
gas monitor, cylinder of tracer gas, sample collection
containers and pump, syringes, circulating fans, and
a stopwatch.
 Meterological parameters which are recorded include:
wind speed and direction, temperature (indoors and
outdoors), relative humidity barometric pressure.
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Tracer Gas Dilution: SF6
 For SF6 concentrations in the range of 1-500 ppm, a portable
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infrared gas analyzer is used.
For SF6 concentrations in the ppb range/a gas
chromatograph(GC)with an electron capture detector is used.
A field GC is preferable so that the concentration of SF6 can
be immediately verified and optimum sample integrity
maintained.
If it is injected in undiluted form, SF6 may tend to sink and
accumulate in low areas.
Documenting various structural parameters and occupant
activities which may be occurring during the sampling time as
well as the meterological parameters.
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Tracer Gas Dilution: SF6
 Structural parameters include: windows (number, location,
type), noticeable leakage paths, wall construction, location
of chimneys, vents and other direct indoor-outdoor
communication points, and type and capacity of the
heating and/or air conditioning systems.
 Occupant activity such as opening and closing of doors
(interior or exterior) or vents will affect the infiltration rate
as well as the distribution of the tracer gas within the
structure.
 Operational status of the heating or cooling system should
also be recorded.
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Calculation of Air Exchange Rate
 C=Co-It
Where:
 C = tracer gas concentration at time t
 Co= tracer gas concentration at time =0
 I = air exchange rate
 T = time
This relationship assumes that the loss rate of the initial
concentration of tracer gas is proportional to its concentration
If the ventilation system recirculates a fraction of the indoor air,
then the above assumption may not hold
Above equation then can be rearranged to yield the expression
 I = (1/t)*Ln(Co/C)
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Fan Pressurization
 It is sometimes also called depressurization.
 It is not a direct measure of infiltration.
 It characterizes the building leakage rate
independent of weather conditions.
 Measurements are made by using a large fan to
create an incremental static pressure difference
between the interior and the exterior of the building.
 The air leakage rate is determined by the relationship
between the airflow rates and pressure differences.
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Fan Pressurization (Contd…)
 The fan is usually placed in the door, and all direct
openings in the building envelope, e.g.,windows,
doors, vents, and flues, are sealed off.
 The airflow rate through the fan is determined by
measuring the pressure drop across a calibrated
orifice plate.
 The resulting leakage occurs through the cracks in
the building envelope, and the effective leakage area
can be calculated from the flow profile.
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Advantages and Disadvantages of Fan
Pressurization
Advantages:
 It does not require sophisticated analytical equipment
as do the tracer techniques
 It allows for a comparison of homes based on their
relative leakiness irrespective of the prevailing
weather conditions at the time of measurement
 It can be used to measure the effectiveness of
retrofit measures
Disadvantages:
 This is an indirect measure of infiltration and hence
approximates the actual process through an
inherently artificial process, pressurization or
depressurization
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General Steps
 Note the physical characteristics of the building.
 Close all normal openings (e.g.,windows, doors,
vents, and flues).
 Record meteorological conditions and indoor
temperature and relative humidity, and install the
blower assembly.
 The blower should run at such speeds as to induce
pressure differences of 0.05 to 0.3 in. water (12.5 to
75 Pa).
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Effective Leakage Area(ELA)
 Another indirect method to estimate air infiltration.
 It can be interpreted physically as an approximation
of the total area of physical openings in the building
envelope through which infiltration occurs.
 The empirical model used to estimate air exchange is
based on pressure differences.
 The method involves measuring the dimensions of
each opening and converting this value to a leakage
area equivalent value.
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Calculation of ELA
ELA = Q4/(2*ΔP/ ρ)0.5
Where
ELA = effective leakage area,m2
Q4
= airflow at 4 Pa(m3/sec)
ΔP
= the pressure drop causing this flow,I.e.,4 Pa
ρ
= density of air,1.2 kg/m3
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