International MSc Programme Sustainable Energy EngineeringInternational MSc Programme Sustainable Energy Engineering

SUSTAINABLE ENERGY UTILIZATION

Lecture:- VENTILATION DEMAND

Assist. Prof. Igor BALEN

Air Handling System

Room WithDefined

Requirements

SupplyAir

OutletAir

Purpose of an air-handling system

Basic concepts and terminology

VENTILATION includes the intentional introduction of air from the outside into a building.

Basic concepts and terminology

VENTILATION AIRis air used to provide acceptable indoor air quality (IAQ) and thermal comfort (TC).

VENTILATION DEMANDS- provide outside air (oxygen) for breathing of humans (or/and animals)- control of indoor air contaminants- covering of the building’s thermal loads (temperature and humidity control)- setting of uniform conditions in the occupied zone

Basic concepts and terminology

AIR-HANDLING UNIT – conditions air for a building100% OUTSIDE AIR UNIT- no recirculation of return air throughthe air-handling system. - all the supply air is treated outside air(called makeup air)- all return air is dischar-ged directly to the out-side as relief air- air-handling unit that provides 100% outside air to offset air that is exhausted is called a makeup air unit (MAU).

- the pressure in ventilated spaces is, in most cases, equal to the atmospheric pressure of the outside; the air-flow delivered to a space (SA) equals the airflow brought back from a space (RA); such spaces are described as neutral or balanced.

- in mechanical ventilation systems, SAF and RAF might be sized differently, when necessary.- in these cases, a building can be:

(1) pressurized (positive) relative to outdoors - VSA > VRA- certain amount of air is exfiltrated from the space through openings and cracks(2) depressurized (negative) relative to outdoors - VSA < VRA- certain amount of air is infiltrated to the space through openings and cracks

- examples of pressurized spaces: clean rooms, operation theaters- examples of depressurized spaces: kitchens, toilets, laboratories working

with toxic substances

Pressure in ventilated spaces

Pressure in ventilated spaces

- normally, slightly positive space pressure should be maintained if therequired level of IAQ and environmental control of the conditioned space is higher than that of the surrounding area.

- controlled pressure difference between space air and the air in the surrounding area (positive or negative) depends on the space requirenments and usually is below 13 Pa - an excessive pressure differential may make opening and closing doors difficult.

The space pressure difference between the conditioned space and the surrounding area is affected by:(a) stack effect,(b) wind effect,(c) operation of the air systems and exhaust systems, and(d) the air tightness (amount of leakage area) of the building shell

Pressure in ventilated spaces

STACK EFFECT- the phenomenon where the temperature difference between the cold outdoor and warm indoor air columns causes a density difference between these air columns that creates a pressure difference between the cold and the warm air columns.

Pressure in ventilated spaces

STACK EFFECT- neglecting vertical density gradients, the stack pressure difference for a horizontal leak at any vertical location is given by:

)()( HHgp NPLiost −−=Δ ρρ [Pa]

- for a high-rise building with two openings on the external wall, measured from lower opening, can be calculated from:

)/()/(1 221

0

oiNPL TTAA

HH+

= [m]

ρo – outdoor air density [kg/m3] To – outdoor air temperature [K]ρi – indoor air density [kg/m3] Ti – indoor air temperature [K]HNPL – height of neutral pressure level Ho – vertical distance between

above reference plane [m] openings [m]H – height above reference plane [m] A1 – area of lower openings [m2]

A2 – area of higher openings [m2]

Pressure in ventilated spaces

STACK EFFECT – AIRFLOW- airflow caused by stack effect when inlet and outlet areas are equal:

ioiNPLDst TTTHgACV /)(2 −Δ=& [m3/s]

CD – discharge coefficient for opening (usually 0.65)A – free area of inlet openings [m2]ΔHNPL – height from midpoint of lower opening to NPL [m]

- increasing the outlet area over inlet area (or vice versa) increases airflow but not inproportion to the added area.- when openings are unequal, use the smaller area in above equation and add the increase as determined from the figure.

Pressure in ventilated spaces

WIND EFFECT- pressure changes for airflow around obstacle:

Pressure in ventilated spaces

WIND EFFECT- airflow around building:

Pressure in ventilated spaces

WIND EFFECT- wind pressure relative to outdoor static pressure on the building surface:

2

2wCp opw ρ=Δ [Pa]

- approaching wind speed at the observed height is calculated from the meteorological wind speed, corrected for height and terrain roughness:

n

met

wmeto H

Hwaw ⎟⎟⎠

⎞⎜⎜⎝

⎛= [m/s]

Cp – wind surface pressure coefficientw – approaching wind speed at winward side [m/s]ao – correction factor for terrain roughnesswmet – wind speed from a meteorological station [m/s]

velocity profile exponent

- for urban areas: ao=0.35, n=0.4- for suburban areas: ao=0.6, n=0.28- for airports: ao=1.0, n=0.15

Pressure in ventilated spacesWIND EFFECT- local wind surface pressure coefficient (Cp×100) for tall building wall:

Pressure in ventilated spacesWIND EFFECT- surface averaged wind surface pressure coefficient Cs:

For tall building wall For low-rise building wall

If the wind direction is normal to the windward surface of a high-rise building with a depth-width (L/W) ratio of 4, the average Cp value is about +0.60 on the windward side, about -0.5 on the leeward side and on the flat roof, and about -0.25 on the other two sides.

Pressure in ventilated spaces

WIND EFFECTThe influence of the wind effect on space and building pressure characteri-stics is as follows:

1. Rooms on the windward side of the building are usually at a positive pressure and on the leeward side at a negative pressure relative to the corridor pressure. It is best to build clean spaces, such as conference rooms, on the windward side of the prevailing wind, and laboratories with toxic gas exhaust systems on the leeward side of the building.

2. Outdoor air intake should be located on the side with a positive surface pressure coefficient Cp in the prevailing wind. Exhaust outlets should be located where Cp is negative, preferably on the rooftop.

3. Sufficient total pressure must be provided by the supply fan to overcome the negative pressure at the outdoor intake and by the exhaust fan to overcome the positive pressure at the exhaust outlet. Alternatives should be provided to allow outdoor air intake and outlet when the wind direction isdifferent from that of the prevailing wind.

Pressure in ventilated spaces

WIND EFFECT – AIRFLOW- airflow caused by wind only:

AwCV ww =& [m3/s]

Cw – effectiveness of openings (Cw is assumed to be 0.5 to 0.6 forperpendicular winds and 0.25 to 0.35 for diagonal winds)

A – free area of inlet openings [m2]w – wind speed [m/s]

COMBINED DRIVING FORCES- wind pressure, stack pressure, and mechanical system acting together:

Iwst pppp Δ+Δ+Δ=Δ [Pa] ρ/2 pACV D Δ=& [m3/s]

pressure that acts to balance inflows and outflows(including mechanical system airflows)

Ventilation requirements

OUTDOOR AIR REQUIREMENT PER PERSONASHRAE Standard 62 - the recommended minimum outside air ventilation per person for breathing, for any type of space (non-smoking) is 8 L/s(Vo,p=30 m3/h) - satisfies the odor perceptions of 80% or more of visitors.- outside air supply per person can be higher, depending on the type of space occupancy (see recommendations in literature); it can be up to 100 m3/h for offices in modern high multi-floor buildings.- for usual residential and commercial applications, outside air supply per person is in range Vo,p=30-60 m3/h (>50 m3/h satisfies 90% or more of persons)- for smoking sapaces, outside air supply should be increased for at least+20 m3/h

- for N persons in a space, total minimum outdoor airflow rate is:

poo VNV ,&& = [m3/h]

Ventilation requirements

OUTDOOR AIR REQUIREMENT ACCORDING TO THE ALLOWABLE CONCENTRATIONS OF AIR CONTAMINANTS- pollutants effect health of the occupants- pollutants include non-biological particles (synthetic vitreous fibers, combustion products, nuisance dust, and others); bioaerosols; gases and vapors that may be generated due to industrial processes (usually known from the type of process), by building materials, furnishings, and equipment, by occupants and their activities in a space, or brought in from the outdoors.- different standards for industrial and nonindustrial indoor environments- outside air supply rate, used to dilute the concentration of a specific indoor air contaminant, can be calculated as:

oi

cono CC

mV−

=&& [m3/h]

mcon – total contaminant source strength [μg/h]Ci – steady-state indoor concentration [μg/m3]Co – outdoor concentration [μg/m3]

Ventilation requirements

OUTDOOR AIR REQUIREMENT ACCORDING TO THE ALLOWABLE CONCENTRATIONS OF AIR CONTAMINANTS- indoor concentration of contaminants Ci should meet the specified value stated in standards (i.e. CO concetration in garages and tunnels)- concentration of contaminants is usually expressed in the following units:ppm – parts of contaminant by volume per million parts of air by volumeμg/m3 – micrograms of contaminant per cubic meter of air

ppm=(24.45/M)(1000 μg/m3); M – relative molecular mass of contaminant

- example – CO2 concetration in indoor environmentssteady-state indoor concentration 1000 ppm, normal outdoor concentration 350 ppm, volume production of CO2 by one person 18 L/h

7.700035.0001.0

3600/18, =

−=

−=

oi

conpo CC

VV&

& L/s per person

Ventilation requirements

HUMAN RESPONSE TO CARBON-MONOXIDE (CO)

Ventilation requirements

OUTDOOR AIR REQUIREMENT ACCORDING TO THE ALLOWABLE CONCENTRATIONS OF AIR CONTAMINANTS- with balanced ventilation system, for a single space, change of the contaminant concentration in time can be calculated from:

θθ

θθθθ

θ ,,0,,

, )()1( sACH

siACHcon

i CeCCeVACH

VC +−+−

⋅= ⋅−

=⋅−

&

Ci,θ – contaminant concentation in space at perfect mix [m3/ m3]Vcon,θ – total contaminant source strength [m3/h] ACH – air changes per hour [1/h]V – space volume [m3] θ – time [h]Ci,θ=0 – start contaminant concentation in space [m3/ m3]Cs,θ – contaminant concentation in supply air [m3/ m3]

Ventilation requirements

OUTDOOR AIR REQUIREMENT ACCORDING TO ACH- number of air changes per hour (ACH) represents a ratio of the outside airflow entering a space in one hour to the space volume (internal)- therefore, the outdoor airflow rate is :

VACHVo ⋅=& [m3/h]

- ACH depends on the space volume, shape, type of occupancy...- ACH criteria for ventilation requirement is used when contamination sources are not specified; it is also used as a control for the supply airflow calculations by other methods.- for usual residential and commercial spaces, ACH is in range 4-8 h-1.- for different types of spaces, recommended ACH can be found in thetables given in literature.

Ventilation requirements

SUPPLY AIR REQUIREMENT ACCORDING TO CALCULATED COOLING/HEATING LOAD- volume airflow rate of ventilation system, to maintain a required temperature, can be calculated from cooling and/or heating load results

supply airflow from sensible cooling load: supply airflow from sensible heating load:

ACp

COOLsAC tc

qV

Δ=ρ

,& [m3/s]Hp

HEATsH tc

qV

Δ=ρ

,& [m3/s]

- if the ventilation system operates with constant airflow during the whole year, calculation using cooling load usually gives higher airflow because the temperature difference between supply air and room air is much lower:COOLING SEASON: ΔtAC=3-8(10)ºCHEATING SEASON: ΔtH=10-25ºC

Ventilation requirements

SUPPLY AIR REQUIREMENT ACCORDING TO DEHUMIDIFICATION DEMAND- volume airflow rate of ventilation system, to maintain a required humidity by decreasing humidity ratio x, can be calculated from latent load results:

AC

lAC xr

qVΔ

=0ρ

& [m3/s] or

- used in cases of high latent loads in a space (i.e. ventilation of swimming pools)

AC

wAC x

mVΔ

=ρ&& [m3/s]

Ventilation demand

ROOM AIR MOVEMENT – ENTRAINMENT FLOW- also known as conventional mixing flow- conditioned air is normally discharged from air outlets at velocities much greater than those acceptable in the occupied zone; diffuser jets mix with the ambient room air by entrainment, which reduces the air velocity and equalizes the air temperature- creates relatively uniform air velocity, temperature, humidity, and air quality conditions in the occupied zone.

Ventilation demand

ROOM AIR MOVEMENT – DISPLACEMENT FLOW- the movement of air within a space in a piston-type motion- air is supplied from air outlets at low air velocities- outlets are located at or near the floor level, and the supply air is introduced directly to the occupied zone- to function properly, stable vertically stratified temperature field is essential- desirable for removing pollutants generated within a space

Ventilation demand

OCCUPIED (COMFORT) ZONE

Distance from internal walls

Distance from external walls

Window

Outside of this zone it is not necessary to maintain thermal comfort parameters