Natural Ventilation. 2 Calculation of rate of ventilation air flow Q = H/(60 * C P * ρ * Δt) =...

Natural Ventilation

Natural Ventilation 2

Natural VentilationCalculation 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


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 ] ) (for effective openings)

Q = 7.2 * A * √ ( h * [ ( ti – to ) / ti ] ) (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 t i and to close to 80oF


Factors affecting flow due to wind

Average velocityPrevailing directionSeasonal and daily variation of wind speed & directionTerrain features (local)


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


Flow Due to Combined Wind and Stack Effect When both forces are together, even without interference, resulting

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.


Types of Natural Ventilation OpeningsWindows :

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.


Natural Ventilation Rules1. 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.


InfiltrationInfiltration 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


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.


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.


Factors Affecting Air InfiltrationType of structure and construction Meteorology Heating & cooling systems Occupant activityStructural parameters Quality of construction Materials of construction Condition of the structureMeteorological 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


Wind Effects Shell and exterior air barriers. Interior barriers to flow that cause internal pressure

buildup and 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.


Temperature Effects Temperature inside a structure is often different from the

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.


Humidity EffectsStricker 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.


Pressure Difference Across the Building EnvelopeΔP = 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


Pressure Difference Across the Building Envelope2.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)


Bernoulli’s EquationPV = (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)


Bernoulli’s EquationCp 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


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


Measurement Techniques

Tracer gas

Fan pressurization

Effective Leakage Area(ELA)


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.


AssumptionsThe tracer gas mixes perfectly and

instantaneouslyThe effective volume of the enclosure is knownThe 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


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.


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


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.


Tracer Gas Dilution: SF6 For SF6 concentrations in the range of 1-500 ppm, a

portable 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.


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.


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 = timeThis 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)


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.


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.


Advantages and Disadvantages of Fan PressurizationAdvantages:

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 measuresDisadvantages:

This is an indirect measure of infiltration and hence approximates the actual process through an inherently artificial process, pressurization or depressurization


General StepsNote 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).


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.


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

Natural Ventilation. 2 Calculation of rate of ventilation air flow Q = H/(60 * C P * ρ * Δt) =...

Documents

Transcript of Natural Ventilation. 2 Calculation of rate of ventilation air flow Q = H/(60 * C P * ρ * Δt) =...