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Ventilation

1 Introduction - Basic principles

Vladimír Zmrhal (room no. 814)

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

Environmental

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

VI

V

⋅

=

.

e

r

VI

V=

s e cV V V⋅ ⋅ ⋅

= +

Ve … outdoor air volume air flow

rate [m3/h]

Vc … recirculation air flow rate

[m3/h]

Vr…interior volume of space [m3]

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Terminology

Outside air fraction

[-]

Percent outside air

[%]

Xoa = 100 % …means no recirculation of return air

. .

e eoa

s e c

V VX

V V V⋅ ⋅ ⋅= =

+

.

100eoa

e c

VX

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 – 100 µm

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Parameters of the pollutants

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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

Parameters of the pollutants

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Mass Balance of the Ventilated Room

�� � �

τ τ τ⋅ ⋅ ⋅

+ = +s r

source exhaustsupply storage

Gd 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 + rV 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 Vd

VGc c

V

Mass Balance of the Ventilated Room

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τ τ+ − + −

= ⇒ =+ − + −

0 0

ln lns s

r

rs s

G Gc c c c

V VV VG GV Vc c c cV 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 rG V G V G V

Vc c c c c c

Mass Balance of the Ventilated Room

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Concentration of the pollutant

Time ττττ [h]

Concentration 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 dtτ 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 dτ⋅

sV ct dρ τ⋅

+ iV ct dρ τ⋅

= rV cdtρ+

( )i s

QV

c t tρ

⋅⋅

=−

Thermal Balance of the Ventilated Room

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Limit Concentrations

Maximum permission concentration (MPC)

… in any time !!

for τ → ∞ and steady state

≤maxc 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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Limit Concentrations

Permission exposure limit PEL

…where τ = 8 hours

0

1avgc c d

τ

τ ττ

= ∫

( )τ

τ

⋅ ⋅

− ⋅⋅ ⋅

= − − − + + ⋅

0

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avg s s

G Gc c c e c

I V V

≤,8avgc PEL

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Average concentration for periodical pollutant generation

( )τ ττ

⋅ ⋅

−⋅ ⋅

=

= − − − + +

∑ 0,1

1 11 i i

nIi i

avg i s s iitot i

i i

G Gc c c e c

I V V

1

8 hoursn

tot ii

τ τ=

= =∑

Limit Concentrations

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Limit Concentrations

<≤

max

prům

c MPC

c PEL

!

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Example 1

Permanent pollutant generation Ventilation strategy:

1) cmax,1 > MPC

cavg,1> PEL

!!! Ve ↑↑↑↑

2) cmax,2 < MPC

cavg,2 < PEL

OK

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Example 2

Variable pollutant generation Ventilation strategy:

1) cmax,1 < MPC

cavg,1> PEL

!!! Ve ↑↑↑↑, ττττ ↓↓↓↓

2) cmax,2 > MPC

cavg,2 < PEL

!!! Ve ↑↑↑↑ , ττττ ↓↓↓↓

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Example 3

Periodical pollutant generation Ventilation strategy:

cmax < MPC …OK

cavg> PEL …!!!

!!! Ve ↑↑↑↑, ττττ ↓↓↓↓

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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 n

n

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 environments

informs 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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Ventilation Requirements

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Ventilation Requirements

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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Ventilation Requirements

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 cs = 0

Calculate volume air flow rate V = ?

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Example 3

Example 3: Calculate time ττττ of ventilation after accident of NH3 in machine room.

Volume of the machine room Vr = 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 …

ττττ = ?

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Thank you for your

attention