01 VEN Introduction

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Ventilation

1 Introduction - Basic principles

Vladimír Zmrhal (room no. 814)

http://users.fs.cvut.cz/~zmrhavla/index.htm Dpt. OfEnvironmental

Engineering

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Principle of ventilation

Ventilation

is intentional supply of outdoor air into the buildings (cars,

trains …)

to dilute and remove indoor air contaminants

affect the indoor air quality

healthy buildings

Good indoor air quality (IAQ) is necessary for maintaining

health and high productivity !!!


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Principle of ventilation

Natural ventilation

is driven by pressure differences across the building envelope,

caused by wind and air density differences because of

temperature differences between indoor and outdoor air

the flow of air through open windows, doors, grilles and

other planned building envelope penetrations

Mechanical (forced) ventilation

intentional movement of air into and out of building using

mechanical force

is driven by fans

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Terminology

Nominal air exchange rate (ventilation rate)

[1/h]

Space air exchange rate

[1/h]

[m3/h]

sS

r

V I

V

⋅

=

.

e

r

V I

V =

s e c V V V ⋅ ⋅ ⋅

= +

V e … outdoor air volume air flow

rate [m3/h]

V c … recirculation air flow rate

[m3/h]

V r …interior volume of space [m3]


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Terminology

Outside air fraction

[-]

Percent outside air

[%]

X oa= 100 % …means no recirculation of return air

. .

e eoa

s e c

V V X

V V V ⋅ ⋅ ⋅

= =

+

.

100eoa

e c

V X

V V

⋅ ⋅= ⋅

+

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Terminology

Airflows

1 Outside air

2 Supply air

3 Return air

4 Exhaustt air

5 Recirculated air 6 Indoor air

7 Transfer air

8 Mixed air

9 By-pass air


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Terminology

Air-Handling Unit

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Heat and Pollutants

Sources of heat and pollutants

internal

- persons

- electronic appliances, lighting, technology, electric motors

- animals, biological processes in agriculture, … external

- outdoor air

- external climatic conditions


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Indoor Contaminants

CO2

water vapour

combustion products

tobacco smoke

VOC

aldehydes

pesticides

asbestos

radon

…

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Ventilation Requirements

hygienic – for the people

technological – for technology processes

biological – agricultural processes

micro-biological

safety

fire


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Parameters of the pollutants

Concentration of pollutants

mass [mg/m3]

volumetric [cm3/m3]

ppm (parts per million) 1 ppm = 10-4 %

number of particles in m3 – clean rooms

Dimension of the particles

a = 0 – 1 0 0 µm

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Conversion X [mg/m3] Y [ppm]

M …molecular weight of the gass [g/mol]

Note:

1000 ppm = 0,1 % vol.

= 324,44[ppm] [mg/m ]Y X

M


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Mass Balance of the Ventilated Room

τ τ τ ⋅ ⋅ ⋅

+ = +s r source exhaust supply storage

G d V c d V c d V dc


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τ → ∞; steady state

solution

τ ⋅

G d τ ⋅

+ sV c d τ ⋅

= V c d + r V dc

τ

τ

τ

⋅ ⋅

⋅

⋅ ⋅ ⋅

⋅

⋅

⋅

= =− −

+ − =

=

+ −

∫ ∫0 0

s s

s r

c

c r

s

G GV

c c PEL c

G V c V c d V dc

dc V d

V Gc c

V


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τ τ

+ − + −

= ⇒ =+ − + −

0 0

ln lns s

r

r s s

G Gc c c c

V V V V G GV V c c c c V V

τ τ − − = + + −

0 1r r

V V

V V

s

Gc c e c e

V

( ) ( )τ τ τ

= − + + − − − −

2

0 0 02 2r r r G V G V G V V

c c c c c c



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Concentration of the pollutant

Time τ ττ τ [h]

C o n c e n t r a t i o n c

[ g / m 3 ]

V→→→→ 0 m3/h

V1

V2 = 2V1

V

G = const.

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s i r

source exhaust storagesupply

Q d V c t d V c t d V c dt τ ρ τ ρ τ ρ ⋅ ⋅ ⋅

⋅ + ⋅ ⋅ ⋅ ⋅ = ⋅ ⋅ ⋅ ⋅ + ⋅ ⋅ ⋅

Thermal Balance of the Ventilated Room


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c …specific heat of the air = 1010 [kJ/kgK]

ρ …density of the air = 1.2 [kg/m3]

t →∞; steady state

Q d τ ⋅

sV ct d ρ τ ⋅

+ i V ct d ρ τ ⋅

= r V cdt ρ +

( )i s

QV

c t t ρ

⋅

⋅

=−

Thermal Balance of the Ventilated Room

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Limit Concentrations

Maximum permission concentration (MPC)

… in any time !!

for τ →∞ and steady state

≤max c MPC

⋅

⋅= +

max s

Gc c

V


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Permission exposure limit PEL

for τ →∞ and steady state

WHY ?

⋅

⋅

=− p

GV

PEL c

Návrh větrání

?!=c PEL

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Permission exposure limit PEL

…where τ = 8 hours

0

1avg c c d

τ

τ τ

τ = ∫

( )τ τ

⋅ ⋅

− ⋅

⋅ ⋅

= − − − + + ⋅

0

11 I avg s s

G Gc c c e c

I V V

≤,8avg c PEL


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Average concentration for periodical pollutant generation

( )τ τ τ

⋅ ⋅

−

⋅ ⋅

=

= − − − + +

∑ 0,1

1 11 i i

nI i i

avg i s s i i tot i

i i

G Gc c c e c

I V V

1

8 hoursn

tot i i

τ τ =

= =∑


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<

≤

max

prům

c MPC

c PEL

!


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Example 1

Permanent pollutant generation Ventilation strategy:

1) c max,1 > MPC

c avg,1> PEL

!!! V e ↑↑↑↑

2) c max,2 < MPC

c avg,2

< PEL

OK

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Example 2

Variable pollutant generation Ventilation strategy:

1) c max,1 < MPC

c avg,1> PEL

!!! V e ↑↑↑↑, τ ττ τ ↓↓↓↓

2) c max,2 > MPC

c avg,2 < PEL

!!! V e ↑↑↑↑ , τ ττ τ ↓↓↓↓


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Example 3

Periodical pollutant generation Ventilation strategy:

c max < MPC …OK

c avg > PEL …!!!

!!! V e ↑↑↑↑, τ ττ τ ↓↓↓↓

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Hygienic aspects

Adition of the pollutants

for two and more chemical substance

+ + + ≤1 2 1

1 2

... 1n

n

c c c

PEL PEL PEL

+ + + ≤1 2

1 2

... 1n

n

c c c

MPC MPC MPC

= + + +1 2

1 2

; ... n

n

G G GV

PEL PEL PEL


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The pollutants with independent effect

= = =1 2

,1 ,2 ,

1 2

; ; ... ne e e nn

G G GV V V

PEL PEL PEL

( ),1 ,2 ,max ; ; ...e e e e nV V V V =

Hygienic aspects

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Mauna Loa

(Havaj)> 380 ppm

Carbon dioxide CO2


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Example1

Example 1: Pettenkoffer‘s criterion

1 people exhale 0.42 m3/h of air

Concentration of CO2 in exhaled air is 4 % vol.

Concentration of CO2 in outdoor air CCO2 = 0.035 % vol. = 350 ppm

Max. concentration of CO2 in the room Cmax = 0.1 % vol. = 1000 ppm

Calculate volume air flow rate of outdoor air Ve = ? [m3/h]

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Pettenkofer‘s criterion

Max von Pettenkofer (1818 - 1901)

main metabolite: CO2 a water vapour

CO2 production depends on activity level - 16 dm3/h CO2 for

unsleeping person (10 dm3/h when sleeping)

CO2 concentration in indoor environmentsinforms about quality of ventilation

CL = 0.1 vol. % = 1000 ppm

→ Pettenkofer criterion

25 m3/h per person


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• 360 - 400 ppm: concentration in outdoor air

• 800 - 1 000 ppm: recomended indoor air concentration

• 1 200 až 1 500 ppm: maximum (real) indoor air concentration

• > 1 500 ppm: tideness, sleepiness, lethargy, headache…

• < 5 000 ppm: maximum safety concentration without health risk

• > 5 000 ppm: sickness, higher pulsation (sick building syndromme -

SBS)

• > 10 000 ppm: health hazards

• > 40 000 ppm: dangerous to live

1 000 ppm= 0,1% obj .

Carbon dioxide CO2

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Space Occupancy

[m2/person]

Required air flow rate

[m2/h.person]

Office spaces 10 36

Land-scape office 15 36

Classroom 2 29

Kidergarten 2 30

Conference room 2 36

Department store 7 23

EN 15251 - Indoor environmental input parameters for design and

assessment of energy performance of buildings- addressing indoor

air quality, thermal environment, lighting and acoustics

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ASHRAE

10 cfm = 17 m3/h


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Health Risk

Group 1 Group 2 Group 3

Risk of death or

sickness

Risk of respiratory

sickness

Risk of less important

sickness or discomfort

Radon Fungi Temperature

Carcinogenic VOC Allergens Noise

Asbestos Dust particles Lighting

Passive smoking (ETS) NOx Non-carcinogenic VOC

CO (high concentrations) Ozone CO (low concentrations)

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Example 2

Example 2: Calculation of volume air flow rate

Mass flow of pollutant G = 0.2 kg/h of FexOy

Permission exposure limit PEL = 0,015 g/m3

Concentration in outdor air c s = 0

Calculate volume air flow rate V = ?


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Example 3

Example 3: Calculate time τ ττ τ of ventilation after accident of NH3 in

machine room.

Volume of the machine room V r = 500 m3

Amount of escaped NH3 m = 1.2 kg

Max. permission concentration MPC = 0.036 g/m3

Ventilation rate I = 5 h-1

How long it takes achieving of MPC …

τ ττ τ = ?

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Thank you for your

attention

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