!"#$%#

& #'

()#) ( #

*+#,-./ ,(

/ (/ 0+

/ 12+

1((3

4 5#

'(236(1(7#'(

#')#

'(3(#

*18&!9(:'#

!""#$% % %%#%#!&'(

514'4

;9:51

'+(

!!!!!#,(3(# ' '4

%+,#

''4

. " 3

3

" (3#

'0+(#

)$

!)*+ !, ************************************************************************************************************* -)*) % ********************************************************************************************************************************** .)*/ & ************************************************************************************************************************************* 0

" #$% &/*+ $, ************************************************************************************ 1/*) 2,# ********************************************************************************************************************* +3/*/ ! ******************************************************************************************************* +3

/*/*+ !,

********************************************************************************************* +3/*/*) 4 ******************************************************************************** ++/*/*/ !,, ***************************************************************************************** +)

/*5 $, ******************************************************** +//*- !2 ******************************************************************************************************** +//*. ,#6, ************************************************ +/

/*.*+ $ ************************************************************************************************ +5/*.*) ,# , ********************************************* +-/*.*/ 7 ****************************************************************************************************** +-/*.*5 $ ******************************************************************************************************* +./*.*- $ **************************************************************************** +0/*.*. ,#,8 **************************************************************************** +1

/*0 **************************************************************************************************** )3/*9 4,, ******************************************************************************************************* )3

/*9*+ : ,, ************************************************************************* )3/*9*) $77 ******************************************************************************************** )+

/*1 !, ******************************************************************************************************************* ))

' ( "5*+ % ****************************************************************************************************************************** )/5*) 2,#,#6 ,, ************************************************************************************** )/5*/ !;28 ************************************************************************************************************ /+5*9*) ,8 **************************************** /+5*9*/ ,8,, *************************************** /)

5*1 ,,, ************************************************************************************** /)

! ""-*+ % ****************************************************************************************************************************** //-*) %,

*************************************************************** //-*/

******************************************** /5-*5 ,, ******************************** /--*- 4$ ************************************************************************ /.

-*-*+ ,, ************************************************************** /.-*-*) ,, ***************************************************************************** /.-*-*/ $,# ************************************************************************************** /.

) * "&.*+ % ****************************************************************************************************************************** /1.*) #,#,> ************************************************************************* /1.*/ ,

********************************************************************************* /1.*5 #,, ****************************************************************************** 53

+ ( '0*+ % ****************************************************************************************************************************** 5+0*) 4 ************************************************************************************************ 5+0*/ ! ************************************************************************************************************* 5+0*5 ,> ******************************************************************************** 5+0*- ************************************************************************************************************************* 5)0*. ,

********************************************** 5/

, - '!9*+ **************************************************************************************************************************** 5-

9*+*+ ***************************************************************************************************************** 5-9*+*) ************************************************************************************************************ 5.9*+*/ 4 ***************************************************************************************************** 5.9*+*5 ,> ************************************************************************************************** 509*+*- ,, ***************************************************************************************** 509*+*. ******************************************************************************************* 599*+*0 2 ************************************************************************************************************* 519*+*9 ' ************************************************************************************************************** -)

9*) %,, ******************************************************************************************************************* -/9*)*+ ***************************************************************************************************************** -/9*)*) ************************************************************************************************************ -/9*)*/ 4 ***************************************************************************************************** -59*)*5 ,> ************************************************************************************************** -59*)*- ,, ***************************************************************************************** --9*)*. ******************************************************************************************* --9*)*0 2 ************************************************************************************************************* -.9*)*9 ' ************************************************************************************************************** -0

9*/ 2 **************************************************************************************************************************** -99*/*+* ***************************************************************************************************************** -99*/*) ************************************************************************************************************ -99*/*/ 4 ***************************************************************************************************** -19*/*5 ,, ***************************************************************************************** .39*/*- ,> ************************************************************************************************** .)9*/*. ************************************************************************************************************** ./9*/*0 ****************************************************************************************************************** .59*/*9 2 ************************************************************************************************************* .59*/*1 ?' ****************************************************************** .5

9*5 @

*********************************************************************************************************************** .-9*5*+ *********************************************************************************************************************9*5*) ****************************************************************************************************************9*5*/ 4 *********************************************************************************************************9*5*5 ,> ******************************************************************************************************

9*- ********************************************************************************************************************************** .09*-*+ *********************************************************************************************************************9*-*) ****************************************************************************************************************9*-*/ 4 *********************************************************************************************************9*-*5 ,> ******************************************************************************************************

)&

'(*%%$(!.

/ 5199>:,/ *1899>(:';/ 0()4##;#4#,#)#1##4,)1!4,)1

5:A/ 9#79:0(/ B9#0#A'!,'1(1/ ADB#1#*E;/ FB#1#;BG;B

2(;/ 6(1(B#1#1,7';'7

'(.

! "

#

$

!

%$

#

! & ' '"'(

)

*

+ '

, , ,

&-.$

& '

, ".'%/"$$01/"

!

Temperature [C]

Hei

ght a

bove

floo

r

1 - 2C

Displacement Mixing

" #$%

"Plane, wall-mounted

Semi-circular,corner-mounted

Circular, free-standing

Floor-mounted

& '(

,,2'3.4

!"

#

) '$(

,

!

"##$#!

%##&'%#!(##$#)&$!

*%$#"!

&$ )# &!

*%%$#%%#!

*%'%#&!

*+#'$#&'%

#

*%&)$%#!

*, $ &% #

+%$$%!-%&

! *.#&/#%&

"#*+#'$#&!

$*0#)&" % ! % ##& !&

#"$%!%#-$#!!1$$#)#%#!-'2/#2%#!% # $ & $&&'

3

#)4%##&!

!" #"$" "#! #"%#" #"&' #"##%#" #"##(((&' "#($"#(( ")

*& +,,

-

+567

+#567

+#

$567 1#57 3# 57 8$57

+#)9$

+#)%&$)

+#)%&$)

#%#5:7

+##5;7 3#57

-9##

5:7

8:9"57

5$

& .,$,

F$:57 F$:57 8$:57

>'#$###$57

8$%###57

8$%##$57

/##$###$57

! $###$

851)1##"5C7

?#C7

?##5C7

3#

+&@)I2$

)

93 Basic knowledge about displacement ventilation

SummaryThis chapter presents the basics for calculating thetemperature gradient and estimating the contaminantconcentration in a displacement ventilated room. Mainitems in this chapter are: Air flow patterns Temperature distribution Convection flows Contaminant distribution Thermal comfort

Conclusions The contaminant concentration is always better in

the occupied zone in a displacement-ventilatedroom than in a room ventilated by mixingventilation.

Theoretically we need a supply air volume flow of20 l/s per person to keep the occupied zone freefrom contaminants. However due to the freeconvection around a person also a smaller supplyair volume flow gives a much better air quality inthe breathing zone. A supply air volume flow of10 l/s per person gives e.g. a concentration that isonly 20% of the concentration in the ambient atthe same level.

The vertical temperature distribution has to be gi-ven attention. Make sure that a suitable diffuser isutilised in order to avoid cold air along the floor.

3.1 Principles of DisplacementVentilation

The air-flow pattern in a ventilated room is mainlydivided into two different types, mixing (dilution)ventilation and displacement ventilation. In mixingventilation the air is supplied in such a way that theroom air is fully mixed and the contaminantconcentration is the same in the whole room. Indisplacement ventilation, which is the subject of thisbook, a stratified flow is created using the buoyancyforces in the room. The air quality in the occupiedzone is then generally better than with mixingventilation. The ventilation system supplying the airto the room is not considered in this book, only the airflow within the room.

Figure 3.1 Schematic illustration of the air flow that might be found in a room ventilated by displacement ventilation

Displacement ventilation has for many years beenused in industrial premises with high thermal loads.Since mid 80s it has also been used in non-industrialpremises to a large extent, especially in the Scandina-vian countries. In recent years the interest indisplacement ventilation has increased all over theworld. Displacement ventilation presents theopportunity to improve both the temperatureeffectiveness and the ventilation effectiveness. Theprinciple is based on air density differences wherethe room air separates into two layers, an upperpolluted zone and a lower clean zone, see Figure 3.1.This is achieved by supplying cool air with a lowvelocity in the lower zone and extracting the air in theupper zone. Free convection from heat sources createsa vertical air movement in the room. When theconvection heat sources in the room are also thecontamination sources, the convection flows trans-port the warm polluted air up to the upper zone. Theconvection flow rates relative to the ventilation flowrate determine the height of the boundary betweenthe two zones. The sum of the warm convection flowrates to the upper zone minus the downward directedflows from cold surfaces to the lower zone is equal tothe ventilation flow rate in the room. An increasedventilation flow rate thus moves the boundaryupwards and a decreased flow rate moves theboundary downwards at fixed convection flow rates.

10

The supply air temperature must be lower than theroom air temperature. If the supply air temperature iswarmer there will be a short-circuit, see Figure 3.4.However the vertical air flow has a certain amount ofentrainment which causes some circulation in the restof the room, this is sometimes used for heating anempty room before occupational time.

Figure 3.4 Short-circuit of airflow in a roomwhen the supply air temperature iswarmer than the room airtemperature.

3.3 Temperature distributionSince displacement ventilation supplies cold fresh airdirectly to the occupied zone, a potential draught riskexists at floor level. In addition, the temperaturestratification may cause discomfort. See Figure 3.5.The temperature will, however, not vary much in thehorizontal direction, except close to the diffuser.

Figure 3.5 Temperature stratification in adisplacement ventilated room.

3.3.1 Temperature at the floorThe temperature of the supply air in the floor arearises due to induction and convection, as radiationfrom the other warmer surfaces in the room in turn

3.2 Air flow patternIn a displacement ventilated room the air flow patternis governed by the convection flows from heatsources and sinks present in the room. This meansthat a distinctive feature of displacement ventilationis the formation of horizontal air layers. The warmestair layers are at the top and the coolest air layers are atthe bottom. The air moves easily within a horizontallayer but the transportation between the layers needsa stronger force. See Figure 3.2. This means that theextract should be positioned at the layer in which thepollutants are. In most cases this means that theextract should be at the highest point in the room.

Figure 3.2 Horizontal air movement.

The vertical air movement is caused by convectionflows from warm or cold sources. Warm objects suchas people, computers, lamps etc. create risingconvection flows. Depending on the power andgeometry of the heat source the convection flowswill rise all the way to the ceiling or settle at a lowerheight see Figure 3.3.

Figure 3.3 Vertical air movement.

0 0,2 0,4 0,6 0,8 1 1,2( - s) /(e - s)

0,0

0,5

1,0

1,5

2,0

2,5

Hei

ght a

bove

floo

r [m

]

Floor

Ceiling

11

heats the floor. A dimensionless temperature of the airnear the floor is often presented as

(3.1)

where: f is the air temperature near the floor s is the supply air temperature e is the exhaust air temperature

The total temperature difference gives together withthe air volume flow rate the amount of heat removedfrom the space:

qv cp (e - s ) / 1000 = tot (3.2)

whereq v is the volume air flow rate [l/s] is the air density = 1,2 kg/mc p is the specific heat of the air = 1004 J/kgC tot is the heat removed from the space [W]

Based on a literature review (Mundt, 1990) thefollowing equation can be used to estimate the dim-ensionless temperature of the air near the floor.

(3.3)

whereA is the floor area [m]r is the heat transfer coefficient due to

radiation ( 5 W/m K ) cf is the heat transfer coefficient at the floor

due to convection ( 4 W/m K )

In Figure 3.6 the dimensionless temperature of the airnear the floor is shown as a function of the ventilationflow rate per m2 floor area. The points shown in thefigure are from measurements with distributed heatsources presented in eleven different references(Mundt, 1996).

Figure 3.6 Dimensionless temperature of theair near the floor as a function ofthe ventilation flow rate per m2 floorarea with different heat transfercoefficients due to convection.

3.3.2 Vertical temperature distributionThe vertical temperature distribution in the roomdepends on the location of the heat sources. Whenthe heat sources are in the lower part of the room thetemperature gradient is larger in the lower part andthe temperature more constant in the upper part. Onthe other hand, when the heat sources are locatedmostly in the upper zone, the temperature gradient issmaller in the lower part and increases in the upperpart, see Figure 3.7. For a given arrangement of heatsources, the relative temperature distribution isrelatively independent of the heat load.

Figure 3.7 Temperature gradient in adisplacement ventilated room withthe heat sources at differentlevels.

=f - se - s

Heat sources in the

lower part of the room

Heat sources in the

upper part of the room

0 0,2 0,4 0,6 0,8 1 1,2Temperature ratio ( - s) /(e - s)

0,0

0,5

1,0

1,5

2,0

2,5

Hei

ght a

bove

floo

r [m

]

Floor

Ceiling

( ) = qv 10-3 cp

A +1r

1cf + 1

1

Ventilation flow rate per m floor area, qv / A [l/sm]

0

0,2

0,4

0,6

0,8

1,0

0 1 2 3 4 5 6 7 8

cf = 5 W/mK cf = 3 W/mK

= (

f-

s ) / (

e-

s )

12

The temperature gradient is strongly influenced bythe elevation of the heat sources. In rooms where theheat sources are located at a high level, displacementventilation is efficient for keeping the occupied spacescool. See Figure 3.8.

However, the air temperatures near the floor f andthe vertical temperature gradient are not only afunction of flow rate and load, they are also a functionof the type of heat source in the room.

According to Nielsen (1996) and Brohus and Ryberg(1999) the relative air temperature near the floor, (see Eqn. 3.1) varies between 0,3 and 0,65 for differenttypes of heat sources. See Figure 3.9.

Figure 3.9 Vertical temperature distribution fordifferent types of heat loads.

A concentrated heat load as e.g. a small furnace in anindustrial environment can give a -value of 0,3.Ceiling light will give a vertical temperature gradientwith a floor temperature of = 0,5, which is generatedby radiation from the light source. When people arethe primary heat source, will have a value of 0,58,and evenly distributed heat sources will give a valueof 0,65. It is obvious that this variation can be of thesame magnitude as the one found at different flowrates.

The different temperature gradients are shown inFigure 3.9 where it is assumed that the verticaltemperature distribution is a linear function of theheight. If many different heat sources are present inthe room it is advised to use the 50% rule (Chapter3.4).

3.3.3 Temperature effectiveness

As the exhaust temperature is higher than the airtemperature in the occupied zone, a temperatureeffectiveness can be defined

(3.4)

where oz is the mean temperature in the occupied zone

=e - soz - s

Distributed heat sources

Sedentary persons

Ceiling light

Point heat source

Temperature ratio ( s) /( e s)

Hei

ght a

bove

floo

r [m

]

Floor

Ceiling

0 0,3 0,5 0,58 0,65 1

Figure 3.8 Roof heated by sun - an example where displacement ventilation is efficient.

Hei

ght a

bove

floo

r

Temperature

13

3.4 Practical assumptions forthe temperaturedistribution

As shown in Figure 3.5 and Figure 3.7, the temperatureincreases with height, and the temperature profiledepends on the location of the heat sources and theflow rate. For most practical purposes, we may assumea temperature profile as shown in Figure 3.10.

Figure 3.10 The 50%-rule for verticaltempera ture distribution.

The 50%-rule for the vertical temperature distri-bution says that the air temperature at floor level ishalf-way between the supply air temperature and theextract air temperature. This is a general experiencethat may be used as a first approximation for mostnormal rooms and normal air diffusers.

Example:If the heat balance and air flow rate in the room yieldsa temperature increase of e - s = 10C , then thetemperature at floor level will become approximately5C higher than the supply air temperature.

3.5 The Archimedes numberSeveral phenomena in a ventilated room, like thevertical temperature gradient, velocity levels instratification flow, stratification level and ventilationeffectiveness can all be described by the Archimedesnumber. The Archimedes number is simply a ratiobetween the buoyancy forces and the inertia forces.In its original form it is defined as:

where: density difference between the colder and the

warmer air [kg/m]g acceleration of gravity = 9,81 m/sL a characteristic length [m] density of the air [kg/m] air velocity [m/s]

The Archimedes number can be expressed in a num-ber of ways, using temperature differences to expressdensity differences etc. But the basic fact is alwaysthe same:- Larger numbers means that the buoyancy forces

are dominant- Smaller numbers means that inertia forces

(velocities) are dominant

3.6 Convection flows the engines ofdisplacement ventilation

Natural convection flows are the engines ofdisplacement ventilation. A natural convection flowis the air current that rises above warm objects likepeople or computers, rises along a warm wall, ordescends from cold objects like windows or outerwalls, due to buoyancy. See Figures 3.11 - 3.13. Tounderstand displacement ventilation, one has tounderstand the nature of the natural convection flows,and to know the magnitude of these flows. Theconvection flow rising above a hot object is called athermal plume, or simply a plume. Empirical, analyticaland computational fluid dynamics are the commonlyused approaches to evaluate air temperatures,velocities and airflow rates in thermal plumes abovedifferent heat sources and convection flows at verticalsurfaces.

Ar =g L

2Extract air temperature, e

Supply air temperature,

s

Air temperature

at floor, f

50% 50%

Temperature

Hei

ght a

bove

floo

r [m

]

Floor

Ceiling

14

Figure 3.11 Convection flows - the engine ofdisplacement ventilation.

All plumes encountered in practical ventilation areturbulent flows, and follow the similarity laws for fullyturbulent flows.

Figure 3.12 Convection flows at verticalsurfaces.

Figure 3.13 Thermal plume above a horizontalsource.

The amount of air in the convection flows increaseswith height due to entrainment of the surrounding air.The amount of air transported in a natural convectionflow depends on the temperature and the geometryof the source and the temperature of the surroundingair. As the driving force in convection flows is thebuoyancy force caused by the density difference (i.e.the temperature difference) a temperature gradient inthe room influences the plume rise height.

3.6.1 Point and line sourcesThermal plumes above point and line sources (Figure3.14) have been studied for many years. Among theearliest publications are those from Zeldovich (1937)and Schmidt (1941). Turner (1973) gives acomprehensive record of most of the phenomenaencountered in connection with buoyancy effects influids. Analytical equations to calculate velocities,temperatures and air flow rates in thermal plumes overpoint and line heat sources with given heat loadswere derived based on the momentum and energyconservation equations and assuming Gaussianvelocity and excessive temperature distribution inthermal plume cross-sections (Mundt, 1996). Theseequations correspond with those receivedexperimentally by other researchers (Mierzwinski,1981, Popiolek, 1981) and are listed in Table 3.1. Theequations in Table 3.1 were derived with theassumption that the heat source size was very smalland did not account for the actual source dimensions.

Figure 3.14 Plumes from a point source andfrom a line source.

The coefficients in the equations differ slightly in dif-ferent references depending on the entrainmentcoefficients used. is the convective heat flux in W

z

Flow, qv

Point source Line source

su

Flowqv

Hot wallsu >

Flowqv

Cold wallsu <

su

15

or W/m from the heat source and z is the height abovethe level of the heat source. The convective heat flux can be estimated from the energy consumption ofthe heat source tot by

= k tot (3.5)

The value of the coefficient k is 0,7-0,9 for pipes andducts, 0,4-0,6 for smaller components and 0,3-0,5 forlarger machines and components (Nielsen, 1993 B).

3.6.2 Convection flow along verticaland horizontal surfaces

Convection flow along vertical surfaces is also of majorinterest. When the vertical extension of the surface issmall the convection flow is mainly laminar and atlarger extensions the flow is turbulent. The basicequations for a surface with a constant temperatureare given in Table 3.2 (Jaluria, 1980, Etheridge andSandberg, 1996).

is the temperature difference between the surfaceand the surrounding air and z is the height from thebottom of the surface. The flow changes from laminarto turbulent at GrPr=7108, which for air and mode-rate temperature differences means around z = 1mand for air at higher temperatures around z = 0,5m.

Convection flows from horizontal surfaces are verydifficult to determine in the same basic way as forpoint, line or vertical sources. The reason is that theflows behave in a very unstable way and leaves theflat surface from different positions at different times,

partly depending on the total air movement in theroom. These surfaces are mostly treated as plumesfrom extended sources see chapter 3.6.3.

3.6.3 Extended sourcesIn reality heat sources are seldom a point, a line or aplane vertical surface. The most common approachto account for the real source dimensions is to use avirtual source from which the airflow rates arecalculated (Elterman 1980, Mundt 1992, Skistad 1994),see Figure 3.15. The virtual origin is located along theplume axis at a distance z0 on the other side of the realsource surface.

Figure 3.15 Illustration of the position of thevirtual sourcesource surface.

Table 3.2 Characteristics of convection flows along vertical surfaces

Parameter Laminar region Turbulent regionMaximum velocity, vz [m/s] vz = 0,1 z vz = 0,1 zThickness of boundary layer [m] = 0,05 (z/ ) 0,25 = 0,11 - - 0,1 z 0,7Airflow rate, qv,z [l/sm width] qv,z = 2,87 0,25 z0,75 qv,z = 2,75 0,4 z1,2

Table 3.1 Characteristics of thermal plumes above point and line sources.

Parameter Point source Line sourceCentreline velocity, vz [m/s] vz = 0,128 1/3 z 1/3 vz = 0,067 1/3Centreline excessive temperature, z [oC] z = 0,329 2/3 z 5/3 z = 0,094 2/3 z 1Airflow rate, qv,z[l/s for point source, l/sm for line source] qv,z = 5 1/3 z 5/3 qv,z = 13 1/3 z

b) Extended source

Virtual source

z

Flow, qv

a) Point source

z0

16

The adjustment of the point source model to therealistic sources using the virtual source method givesa reasonable estimate of the air flow rate in thermalplumes.

The weak part of this method is how to estimate thelocation of the virtual located point source. Themethod of a maximum case and a minimum caseprovides a tool for such estimation. See Figure 3.16(Skistad 1994). According to the maximum case, thereal source is replaced by the point source such thatthe border of the plume above the point source passesthrough the top edge of the real source (e.g., cylin-der). The minimum case is when the diameter ofvena contracta of the plume is about 80% of the uppersurface diameter and is located approximately 1/3diameter above the source. The spreading angle ofthe plume is set to 25. For the low-temperaturesources, Skistad (1994) recommends the maximumcase, whereas the minimum case best fits themeasurements for larger, high temperature sources.The maximum case gives z0 = 2,3D and the mini-mum case z0 = 1,8D with z0 defined in Figure 3.16.

For a flat heat source Morton (1956) suggests theposition of the virtual source to be located at z0 =1,7-2,1D below the real source.

Mundt (1996) calculates the thickness of the boundarylayer (see Table 3.2) at the top of a vertical extendedheat source and adds this to the source radii and thencalculates the position of the virtual source as z0 =2,1(D+2) before using the point source equation.According to Bach et al (1993) the volume flow fromthe vertical surfaces should be added to the volumeflow calculated by the equations for point or linesources.

ExampleCalculate the convection flow rate 0,5 m above a cy-linder with height 1 m and diameter 0,4 m. Theconvective heat flux is 50 W.

In the maximum case we getz0 = D/(2 tan12,5O) = 2,255 D = 0,9 m

andz = z0 + h = 0,9 + 0,5 = 1,4 m

from Table 3.1 we useq,z = 5 1/3z5/3

which givesq,z = 5 501/3 1,45/3 = 32 l/s

In the minimum case we getz0 = 0,8D / (2 tan 12,5O) = 1,804 D = 0,72 m

andz = z0 - D/3 + h = 0,72 - 0,3 + 0,5 = 1,09 m

which givesq,z = 5 501/3 1,095/3 = 21 l/s

(the position of the virtual source is in this case(1,804 - 1/3) D = 1,47 D below the upper edge ofthe source)

Figure 3.16 Convection flow above a verticalcylinder

3.6.4 Plume interactionWhen a heat source is located close to a wall theplume may be attached to the wall, Figure 3.17. In thiscase the entrainment will be reduced compared to theentrainment in a free plume. The airflow rate from aheat source can then be calculated as half of the flowfrom a source with a heat emission of 2 (Nielsen,1993 B).

(3.6)

If the heat source is located in a corner the airflow rateis equal to 25% of the airflow from a heat source witha heat emission of 4 (Kofoed, 1991):

(3.7)

q,z =5 (2 )1/3 z5/3

2 = 3,2 1/3z 5/3

Minimum case

Maximum case

d0

D

z0

z

H

h d0

D

z0

z

H

h

D/3

q,z = 2 1/3 z5/3

17

When several heat sources are positioned close toeach other the plumes merge into a single plume, seeFigure 3.17. The total flow from N identical sources isthen given by, (Nielsen, 1993 B)

(3.8)

whereq v, z is the flow in the plume from one of the sources

When the heat sources are more separated the totalflow is equal to the sum of the flows from each heatsource.

a) Plume attached to a wall

b) Interaction between two plumes

Figure 3.17 Thermal plumes

3.6.5 Plumes and temperaturegradients

When there is temperature stratification in a room, asin a room ventilated by displacement ventilation, theplumes are influenced by the temperature stratifi-cation. The driving force for the plume is the tempera-ture difference between the plume and the surround-ings and when this difference diminishes the plumeswill disintegrate and spread horizontally in the room,see Figure 3.18.

Batchelor (1954) noticed the influence of a temperaturegradient in the surroundings and Morton et al (1956)gave a solution for calculating the maximum plumerise from a point source in surroundings with atemperature gradient. The volume flow rates in theplumes in a room with temperature stratification is

slightly decreased compared to the volume flow ratescalculated with the equations presented for a nonstratified medium, Mundt (1992). Jin, (1993) studiedthe maximum plume rise height for plumes abovewelding arcs.

Figure 3.18 Schematic illustration of the air flowpattern in a room ventilated by displacement.

In the presence of a temperature gradient, theconvective plume reaches the equilibrium height (zt)where the temperature difference between the plumeand the ambient air disappears, see Figure 3.19. Alsothere is another level in the plume, where the airvelocity equals to zero. This is referred to as themaximum height of the plume (zmax ).

ztzmax

z*

2,1

2,8

z**

2,0

2,95

Point source

Line source

s = > 0ddz

Figure 3.19 Vertical plume in a room withtemperature gradients andstratification

q,z,N = N1/3 q,z

Plume 1

Plume 2

Plume 3

room

Plume1

Plume2

18

The plume spreads horizontally between these twoheights. The convective flow below zt can becalculated from the following model (Mundt, 1996).

Point sourceThe position of the virtual source is calculated. Adimensionless height z* above the virtual source iscalculated

z* = 2,86 z s3/8 cf-1/4 (3.9)where:s vertical temperature gradient ( /z) in the

room [C/m] cf convective heat from the source [W]

As can be seen from Figure 3.19 only z* values lessthan 2,1 are relevant to further calculations. The volumeflow rate at the height z* is then given by

qv = 2,38 cf3/4 s-5/8 (0,004 + 0,039 z*+ 0,380 z*2 - 0,062 z*3) (3.10)

whereqv is the volume flow rate in l/s

The maximum height zmax is given by Equation (3.9)for z* = 2,8

zmax = 0,98 cf1/4s-3/8 (3.11)

and the height zt by Equation (3.9) for z* = 2,1

zt = 0,74 cf 1/4 s -3/8 (3.12)

Line sourceThe position of the virtual source is calculated. Adimensionless height z** above the virtual source iscalculated

z** = 5,78 z s1/2 cf-1/3 (3.13)where:s vertical temperature gradient ( /z ) in the

room [C/m] cf convective heat from the source [W]

As can be seen from Figure 3.19 only z** values lessthan 2,0 are relevant to further calculations. The volumeflow rate at the height z** is then given by

qv,l = 4,82 cf 2/3 s-1/2 (0,004 + 0,477 z**+ 0,029 z**2 - 0,018 z**3) (3.14)

whereqv, 1 is the volume flow rate in l/(s m)

The maximum height zmax is given by Equation (3.13)for z**=2,95

zmax = 0,51 cf 1/3 s-1/2 (3.15)

and the height zt by Equation (3.13) for z**=2,0

zt = 0,35 cf 1/3s-1/2 (3.16)

Personalcomputer75W

Fluorecentlamp 36W

Desk lamp60 W

0,3 0,5 1,0 1,2 1,4

Height above object, z [m]

3

5

10

30

50

80

Con

vect

ion

flow

rate

, qvz

[l/s]

Figure 3.20 Convection volume flow above a sedentary person and above some objects.From Mundt, 1992/Nielsen, 1993 B.

Height above floor, z [m]

Con

vect

ion

flow

rate

, qvz

[l/s]

10

30

50

80100

20

200

1,0 2,0 3,0 4,0 5,0

Vertical temp. gradient:

s = 0,3 C/m

s = 0,09 C/m

Equation, Table 3.1

19

3.6.6 Convection flows from realobjects

From the theories above, and practical experiments,Nielsen (1993 B) has summarised the convection flowsabove some common objects found in non-industrialenvironments, see Figure 3.20. The line drawn in thefigure to the left is calculated by the equation for the

Hei

ght a

bove

floo

r [m

]

0

0,5

1,0

1,5

2,0

2,5

s = d/dz = 1,5 C/m

qv,z = 20 l/s

Figure 3.23 Schematic illustration of the contamination distribution in a room ventilated bydisplacement ventilation, when the contaminant source (the person) is not thewarmest source.

Figure 3.22 Schematic illustration of the contamination distribution in a room ventilated bydisplacement ventilation and with warm contaminant sources.

Figure 3.21 Convection flow in plume above asedentary person in a normalenvironment.

0 0,2 0,4 0,6 0,8 1,0

Hei

ght a

bove

floo

r, z

[m]

0

0,5

1,0

1,5

2,0

2,5

Contamination ratio, croom/ce

Hei

ght a

bove

floo

r, z

[m]

Contamination, croomTemperature,

0

0,5

1,0

1,5

2,0

2,5

roomplume1

plume2

croom

20

air flow rate in Table 3.1. The convection flow abovea seated person is thus approximately 20 l/s, see Figure3.21. In order to keep the inhaled air at a lowerconcentration than the ambient a lower airflow mayhowever be used in calculations, see Chapter 3.8.

3.7 Contamination distributionThe contamination distribution in a displacement-ventilated room depends on the position of thecontamination sources and if the heat sources arealso the contamination sources. In the ideal case withwarm sources all contaminants are transporteddirectly into the upper zone by the convection flows,see Figure 3.22.

However if the source is too weak, the plume mightdisintegrate at a lower level and the contaminants willthen be trapped at this level, see Figure 3.23, and onlyslowly transported indirectly by the strongerconvection flows to the upper zone.

Figure 3.24 Poor building air tightness andinsulation may reduce the benefitof displacement ventilation, andmake it more like mixingventilation.

The contaminant concentration is of course alsoinfluenced by the downward directed convectionflows that might occur at the outer walls in coldseasons, especially when the walls are poorlyinsulated. These downward flows will then transportthe contaminants from the upper zone back to thelower zone. However as long as there is a positiveconcentration gradient in the room, the contaminantconcentration in the occupied zone will always belower than by mixing ventilation.

The influence of a poorly insulated roof will, in thecold season decrease the concentration gradient, dueto the downdraught of cold air, just like with the coldwalls. However if the roof is heated by the sun thiswill help stabilise the displacement ventilation as itheats the air in the upper zone. (See Figure 3.8).

3.8 Ventilation effectivenessDifferent definitions of ventilation effectiveness havebeen introduced. In defining ventilation efficiency, adistinction must be made between two terms: the contaminant removal effectiveness, c, which

is a measure of how quickly an airbornecontaminant is removed from the room (Brounsand Waters, 1991) and

the air change efficiency, a, which is a measure ofhow quickly the air in the room is replaced(Sutcliffe, 1990).

In a displacement ventilated room the air changeefficiency is mostly higher (a 60-70 %) than in aroom ventilated by mixing ventilation (a 50 %),(Mundt, 1994). A good survey of the relation betweenthe different versions of ventilation effectiveness isgiven by Nielsen (1993), pp. 17 19. The most rele-vant versions of ventilation effectiveness fordisplacement ventilation in non-commercial premisesare treated below.

3.8.1 Contaminant removaleffectiveness

The contaminant removal effectiveness is defined by

(3.17)

wherec e is the contaminant concentration in the exhaustc s is the contaminant concentration in the supplyc mean is the mean contaminant concentration in the

room

or for the occupied zone

(3.18)

wherec oz is the mean contaminant concentration in the

occupied zone

c =ce - cs

cmean - cs

c =ce - cscoz - cs

21

Figure 3.26 Iso-concentration map showingthe dispersion pattern of a tracergas emitted directly above a 4 Wheat source in the lower zone.(Stymne et al, 1991)

As pointed out above, the ventilation flow rate mustnot always be set to cover the convection flows abovethe occupants present in a room. Figure 3.27 showsthe improvement in inhaled air quality relative to theair quality in the ambient as a function of theventilation flow rate per person.

Figure 3.27 The ratio between theconcentration in the breathingzone and in the ambient air at thesame height (Etheridge andSandberg, 1996)

3.8.2 Personal exposure index.The thermal flow around a person, and also the airflowcreated by the movement of the person, may give ahead high contaminant concentration that is differentfrom that measured without any occupants beingpresent.

Figure 3.25 Thermal flow around a personmay give cleaner breathing air.

This can be expressed by the following personalexposure index, Brohus and Nielsen (1996A):

(3.19)

wherec exp is the inhaled concentration.

It is possible to work with a stratification height thatis lower than the height of the breathing zone. Thepersonal exposure index will often be larger than thelocal ventilation index because clean air is moved fromthe lower part of the room up to the breathing zone bythe free-convection boundary layer around theperson, see Figure 3.25 and Figure 3.26.

Measurements of the personal exposure index madein situations with air movement in the occupied zoneand contaminant sources close to a person can giverise to a very small exposure index, see Brohus andNielsen (1996 B).

Although the personal exposure index shows theability of improved air quality in the inhaled airdisplacement ventilation should not be used whenthe contamination sources are mostly cold.

exp =ce - cs

cexp - cs

22

With a ventilation flow rate of 20 l/(s,person) theboundary is above the person. A ventilation flow rateof 10 l/(s,person) gives however a concentration whichis only 20% of the concentration in the ambient at thesame level.

Measurements by Mundt (1994) also showed the rapidalmost instantaneous recreation of the thermal flowaround a person when the person moves from oneplace to another in a room.

Particle transportation in a displacement-ventilatedroom was studied by Mundt (2000), the results indicatethat there seem to be little risk of re-suspension ofparticles from the floor into the supply airflow. Thesizes studied were however only particles larger than0,5 m and more research is needed for smallerparticles.

3.9 Thermal comfortOne of the limiting factors for the thermal comfort indisplacement ventilation is the air velocity at floorlevel. There is a zone close to the air supply where theair velocity is greater than that recommended, 0,15 m/s in winter time and 0,25 m/s in summer time (ISO7730). The extent of this zone, which depends on theair supply device, should be documented in themanufacturer 's catalogue .

The other limiting factor is the temperature gradient,which should be less then 3 C/m between 0,1 m and1,1 m above the floor (ISO 7730). In some countriesthe limit is set to 2C/m.

!"

"

#

$%&%'

$()&

*

+

()$

# +(),

(),!

Typical depth 20 cm

~ 2 - 5 cm

* !

+-+./+

/+

+0

%$12

/3

4+!+())

!

+()5 +

!"#$%&%'()*%'(+%'(,-#%'+./

0 1"

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Dist. from centre line of diffuser, [m]

Diff

user

Adjacent zone

0.15 m/s

0.20 m/s0.5

1.0

1.5

- 0.5

- 1.0

- 1.50.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Dist. from centre line of diffuser, [m]

Dist from diffuser front, [m]

Diff

user

Adjacent zone

0.15 m/s

0.20 m/s

Forwards discharge

Sideways discharge

Dist from diffuser front, [m]

0.5

1.0

1.5

- 0.5

- 1.0

- 1.5

0

+!

0)&2

0)$2

6&1

678&19

!&&:

6

02

0

2 ;0&77$20$%%%2()<

!

+ 23443'5,

%

0),2

/+

!

"%%?51

)&5

$5

#"0),2

0

()

+

0!2

()?

: ;#4

#48

:

#"-C/+D./ + ! /++()7

) 1#3'(0'(0

!/

++

0)?2

!

("0)?20)52

1 ?!"'

/

6,7 67 67( 5+ (5+( 5

( :): 0)+( :50 0

/+

#/++ /

!

/+

" !

( #

()&%

"

'(

;0&77)A2

0!2 02"!;>0&77%2

'(50 (@::55 67

= 0,54 m

= 0,45 m

5

# !

!

#

()&&

#44

!

>!

!!44

0$=)12*

!

4

!"/

0)72

+

!()&$#//

!

/02

)

A

()&, 02%

!"

0 ##

0()&52%$& 0()&52%

(

+ !"-#'./

/+()&

& '

"%&% '( > / +

(02

!02,EF$

(

$

"%&% )*+*,

!

()$%

>()&))&

"%&%! )*+

!

/ +

( %

)

!

-* "()$$

C

.*+ >

/ ()$,>! "+

1!

33

5 Design procedures

5.1 SummaryThe design of a ventilation system should alwaysfollow a systematic procedure as follows: First, choose a suitable ventilation principle.

(Displacement ventilation is not always the bestfor all purposes!)

If displacement ventilation is chosen, calculate therequired ventilation airflow rate with regard to airquality and temperature conditions.

Select suitable diffusers with regard to verticaltemperature distribution and adjacent zones.

5.2 Strategic design of the roomair conditioning process

a) Target levelsThe aim of the room air conditioning is to maintaindesired conditions, i.e. target levels, in the room du-ring different operating conditions in the mosteconomical way (energy usage, cost efficiency).Depending on the design criteria the designer has dif-ferent strategies to choose from in order to achievespecified targets. The room air conditioning designand evaluation process is illustrated in Figure 5.1.

b) StrategyThe room air conditioning strategy is a fundamentalscheme that describes the targeted temperature,humidity and contaminant distributions as well as airflow patterns within the air-conditioned room. Theroom air conditioning system consists of differentmethods and their controls that all together create thesystem performance. The system performance isevaluated by comparing the achieved conditions tothe chosen strategy. Both the methods (room airdistribution, exhaust, room heating and cooling, etc.)and processes and disturbances inside the roominfluence the resulting conditions.

c) SystemThe room air distribution method is often consideredas a principal parameter to apply a certain room airconditioning strategy and heating and cooling asassisting methods. However, it must be noted that insome cases a strategy can be fulfilled also withoutany mechanical air distribution installations usingbuoyancy forces. The classification of ideal room airconditioning strategies is summarized in Figure 5.2

a)TARGET OF THE

INDOOR AIR CONDITIONS

a)TARGET OF THE

INDOOR AIR CONDITIONS

c)AIR CONDITIONING

SYSTEM & CONTROL

c)AIR CONDITIONING

SYSTEM & CONTROL

AIR CONDITIONING SYSTEM PERFORMANCE

VALUATION

AIR CONDITIONING SYSTEM PERFORMANCE

VALUATION

ROOMINDOOR / OUTDOOR

LOADSDISTURBANCES

ROOMINDOOR / OUTDOOR

LOADSDISTURBANCES

Temperature [C]Humidity [%RH]Air quality [ppm]

Investment cost [ ]Running Cost [ ]

Temperature [C]Humidity [%RH]Air quality [ppm]

Investment cost [ ]Running Cost [ ]

oC oC

b)ROOM AIR CONDITIONING

STRATEGY

b)ROOM AIR CONDITIONING

STRATEGY

Figure 5.1 The Room Air conditioning and Evaluation Process. (Hagstrm 2000)

34

Figure 5.2 The summary of the ideal room air conditioning strategies.(Hagstrm 2000)

5.3 Displacement ventilationand room air conditioningstrategies

Displacement is an efficient air distribution methodwhen: the aim is air quality in rooms where the

contaminants are warm, large heat surpluses are required be removed by

large quantities of air (more than about 60 - 70W/m2 or more than around 10 l/sm2 (36 m3/hm2)).

The design criteria for these cases differ and they arediscussed later in this chapter.

It is necessary to emphasize the difference betweenthe room air conditioning system and the airdistribution method. Choosing displacementventilation as an air distribution method does not byitself result in a stratification strategy, if the wholeroom air conditioning system is not designed for thatpurpose. As an example overheated supply air throughdisplacement units results in close to mixed conditions(Halton Oy, (2000)). Thus, it is possible to usedisplacement ventilation for example pre-heating ofthe space in the morning. However, due to the short-circuiting effect, constant heating of a room by hotventilation air should not be used in connection withdisplacement ventilation.

PISTON STRATIFICATION-DISPLACEMENT

ZONING MIXINGStrategy

Description Unidirectional flow through the room

Utilise density differences

Air flow from clean zones to contaminated zones

Uniform conditions in all parts of the room

Air quality;temp, humidity RFcontaminants, c

, RF, c

Room dimension

, RF, c

Room dimension

, RF, c

Room dimension

, RF, c

Room dimension

s = supplye = exhaust

s

e

s

e

s

e

s

e

Maincharacteristics

Flow pattern controlled by low momentum supply air, strong enough to overcome disturbances

Flow pattern controlled by buoyancy

Flow pattern controlled by high momentum supply air

Flow pattern controlled partly by buoyancy and partly by supply air momentum

Ventilationeffectiveness

1 = - - e

oz

s

s c = coz - cs

ce - cs

35

Figure 5.3 Vertical air temperature distributionin a room with cooled ceiling.Temperatures relative to temp. 0,1metre above the floor. Tan (1998)

Another example is a system consisting ofdisplacement ventilation air supply and cooledceilings. Low velocity air supply and cooled ceilingsystems behave like mixing systems when the cooledceiling provides a substantial part of the cooling. SeeFigure 5.3 (Tan (1998) et al.)

5.4 Factors influencing thethermal stratification andthe design methods

While the contaminant stratification level is mainlyaffected by the relation of supply airflow rate andconvective airflow rate, thermal stratification is alsoaffected by thermal radiation exchange between dif-ferent room zones. The thermal radiation from upperzone warms up the air temperature at floor level.From this fact, it follows that if the supply air flowrate in the room is decreased => the temperaturestratification and ceiling temperature will increase =>the thermal radiation from upper zone to lower zonewill also increase and thus increase the air temperatureat the floor level => the temperature stratification willthen be decreased. This process has been presentedby Mundt (1996) in her doctoral thesis. When thevertical temperature gradient has reached itsmaximum, the temperature in the whole room willstart to rise. This is demonstrated in Figure 5.4.

Figure 5.4 Temperature profiles measured atvarious times during a meeting in aroom with constant supply airtemperature. (Skistad 1994).

The first displacement ventilation design methodsapplicable for manual calculations are based on theempirical coefficients, in which the influence of thethermal radiation exchange between upper and lowerpart of the room is built in. Such methods are presentedas an example by Halton (2000) and Skistad (1994).The value of these methods is their ease of use andalso the accuracy of the estimation which in manycases is still reasonable.

More detailed methods allowing computationaltreatment of radiation exchange and situations beyondthe traditional cases have been presented by Livtchak(2001) and Mundt (1996). However, these methodsare iterative and too complex to be used manually,and need to be coded into software.

It is also possible to use computational fluid dynamics(CFD) software to simulate large, complex spaces.However, one needs to pay special attention todescription of radiation exchange and the right inter-pretation of boundary conditions in heat andcontaminant sources and also in supply air units.

0,00

0,50

1,00

1,50

2,00

2,50H

eigh

t abo

ve fl

oor l

evel

, z [m

]

0,8 1,0 1,2 1,4Relative air temperature

(relative to temp. at 0,1 m above the floor)

= 0

= 0,4

= 0,5

= 0,6

= ratio of the cooled ceiling cooling output to the total cooling output (Tan 1998)

Cooled ceiling

17 19 21 23 250

0,5

1,0

1,5

2,0

2,5

Temperature, [C]

Hei

ght a

bove

floo

r, z

[m]

16 18 20 22 24 26

9AM 10AM 12AM 1 PM

Supply airtemperature,p = 17.8 C

Rule-of-thumb curve

36

5.5 Displacement VentilationDesign Procedure

5.5.1 Air quality: Design criteria forcontaminant stratification

The design criterion for the air quality based designis that the supply airflow rate is equal to the sum ofconvective flows at the stratification height (shiftzone). Moreover, it should be ensured that anycontaminants that are carried upwards by theconvection do not re-circulate into the occupied zone.Once the required supply airflow rate is defined it isnecessary to check that both the contaminantconcentration and the thermal conditions requirementsare satisfied within the occupied zone.

It must be noted that the vertical stratification of thecontaminants occurs only when the contaminantsource is inside the warm convective current or thecontaminant is lighter than air. If the heat andcontaminant sources are separate there is a risk thatcontaminants are not carried out from the occupiedzone.

5.5.2 Temperatures: Design criteria forthermal comfort

The design criteria for temperature-based design arethe removal of excess heat from the occupied zoneand thermal comfort. Thus, the supply airflow rate isnot chosen based on the convective flows but on: occupied zone temperature requirement (Minimum

temperature at floor level and maximumtemperature at the edge of the occupied zone)

maximum vertical temperature gradient within theroom.

5.5.3 Design Procedure flow chartThe displacement ventilation design procedure tak-ing into account both contaminant and temperaturestratification is presented as a flow chart in Figure5.5. The application of the design procedure isdemonstrated with practical examples in chapter 8.The following notes apply to the chart:

S1: Typically stratification layer is selectedslightly above the breathing zone.

S2: Take into account both ascending anddescending air currents.

S5: According to (Nielsen 1993) the lower(occupied) zone concentration is 0.1-0,3times the exhaust air concentration. Usinga conservative estimate of 0,3 it can bechecked whether the occupied zoneconcentration is below acceptable level. Ifthe occupied zone concentration is higherthan required, then increase the supplyairflow rate accordingly.

T1: -Occupied zone temperature requirement(qmin at floor level and qmax at the edge ofthe occupied zone)-Maximum temperature gradient

T3: Use the comfort criteria:a) The vertical stratification is calculatedby multiplying the maximum temperaturegradient with the room height.b) Estimate the air temperature at the floorlevel using 50% rule.

T5: Use equation 3.2

T6: This can be done for example usingdimensionless temperature method thatwas introduced in chapter 3.3.

Figure 5.5 Displacement Ventilation DesignProcedure.

37

Air Quality

Select stratification height

Determine the convective flow rates through the stratification height

Calculate the exhaust contaminant concentration, ce

Evaluate the concentration in the occupied zone, coz

Choose supply air flow rate,qp = sum of convective flows

Temperature

Select thermal comfort criteria

Calculate the heat surplus to be removed by the ventilating air

Calculate the supply air temperature, p

Calculate the supply air volume flow rate, qp

Calculate the maximum temperature increase from supply

to exhaust air,e - p

Re-evaluate the air temperature increase at floor level, f

S1

S2

S3

S4

S5

T1

T2

T3

T4

T5

Result

Check that the air flow rate is sufficient according to codes and standards.

Choose the air volume flow qp with regard to temperatures, air quality and regulations.

Re-calculate the vertical temperature distribution in the room,and estimate the pollutant stratification height.

Select diffusers and ensure that the adjacent zones are acceptable.

T6

R1

R2

R3

R4

38

!"

!

!## $" # # % '(() &*+,"

-,.%'(() # /01 ,01 '0."2

##

### # # *01

"# # #"#"3##4

5 "

&)+5

+, ' 6 +78

#9:'078

265#2+78:/78

$

= 10 l/s6 - 8 l/s

!!

5#

5#6

"#6

8;<

=6"6

###25#

Supply air temperature

=16,5C

Extract air temperature

= 25,5C

Displacementventilation

Supply air temperature

=18,5C

Extract air temperature

= 28,5C

Displacementventilation

Supply air temperature

=14C

Extract air temperature

= 23CMixing

ventilation

Supply air temperature

=14C

Extract air temperature

= 23C

Mixingventilation

[C]

2,5C

9C

9C

Average temperature in the occupied space

= 23C

[C]18 20 22 24 261614 18 20 22 24 261614

Average temperature in the occupied space

= 23C

4,5C

9C

10C

!"#

$!%!

$!

! &'(!

(!()*! (( !

$ !

!++!,

$!%

!

!

Reduced air volume flow = Reduced adjacent zone

Temperature sensor

Air quality sensor

Occupied zone

!

#-# !+!

!.!!

!

#!!

/#!

.!#%0

$ ! . 12! #03 !

45,6

3 !45+6

3 7(4+888

!

!%

7(

988 7(

! ! 7

(

+888.1:

"#$

% &

!!%

!+;6!!

!526

526!

' ( ) % % )&)

=

25C

=

1000 ppm

=

21C

CO2-concentration

600 ppm 1000 ppm

Max

Min

Temperature in lower zone,

NB:Supply temp > room temp 1 !

22C

18C

23C18C 21C

20C

* + !

*

+

%

586&5+6) #5+6!

586 !

45

8 Case studies

8.1 RestaurantThe first case study is a restaurant with dense seating and both smoking and non-smoking areas. This is atypical case where air quality is the major issue.

8.1.1 Description

Figure 8.1 Therestaurantthat shall beventilated.

The restaurant to be ventilated, shown in Figure 8.1, has a floor area of 132 m. It will accommodate amaximum of 96 customers, 48 in the smoking area and 48 in the non-smoking area.

Table 8.1 Data for the restaurant.

Room dimensions Height 3,0 m Floor area 132,0 m Room volume 396,0 m

Max. number of people in the restaurant Smokers 48 pers. Non-smokers 48 pers. Employees 6 pers. Max occupancy 102 persons

Floor area per customer 1,38 m/customer

46

8.1.2 Design criteriaThe thermal comfort criteria are as follows:

Table 8.2 Thermal comfort requirements.

Temperatures in the occupied spaceMax. temp. 26 CMin. temp. 20 CTarget temperature 23 CMaximum vertical temperaturegradient 2,0 C/m

The air quality demands of the restaurant owner are: it complies with the governmental regulations (*) the air quality is good the non-smokers are affected by tobacco smoke as

little as possible

(*) National requirements for the ventilation air flowrate per customer must be complied with .

8.1.3 Ventilation strategyIn the smoking area, air quality is a major concern.Although displacement ventilation is the chosenmethod, by itself it may not be adequate to ensuregood air quality for the non-smokers. The ventilationshould be designed so that little or no air from thesmoking area creeps into the non-smoking area. Thiscan be achieved by supplying as much air as possibleinto the non-smoking area, and extracting air fromthe smoking area. The smoking zone and the buildingelements should be arranged so that smoke-contaminated air does not infiltrate the non-smokingzone. This is illustrated in Figure 8.2.

Figure 8.2 Seating arrangement and ventilation strategy for the restaurant.

Wardrobe

Locationof supply air

Locationof extract air

Main air flowdirection

Smok

ing ar

ea

Non-smoking area

Kitchen

Doors for waiters

Column (unmovable)

(1)

(2)

(3)

Entrance doors

(4)

(5)

47

8.1.4 Design for air qualityThe maximum number of people in the room is 102.Using a ventilation rate of 20 l/s per person, theventilation flow required for contaminant stratificationabove the sedentary peoples heads is calculated (seechapter 3):

qs = 102 20 l/s = 2 040 l/s (= 7 344 m/h)

According to the considerations in Chapter 3, we mayalso consider ventilation rates down to 10 l/s perperson. This has to be considered together with theventilation rates with respect to other criteria.

8.1.5 Design for thermal comfortThe heat gain of the room is given in Table 8.3 andshows that almost 90% of the heat surplus comes fromthe customers.On a typical day, the restaurant is open for a coupleof hours at lunchtime and a much longer period in theevening. The necessary airflow rate to remove theheat surplus is determined by taking into considerationthe heat accumulation in the building fabric. For thisexample it is assumed that the heat accumulation inthe building fabric reduces the need for air-coolingby 40%. Thus, the net requirement for cooling by theventilation air becomes:

net = 0,6 9,87 kW 6 kW,

This corresponds to a specific heat load of45,5 W/m.

The room is 3 metres high. A vertical temperaturegradient of 2C/m corresponds to a temperaturedifference between floor and ceiling of 6C. By the50% rule there is a temperature difference of 12Cbetween extract air and supply air. However, this ismore than most air diffusers can handle withoutcausing draughts along the floor, so a maximumtemperature difference between the extract and supplyair is calculated as:

Figure 8.3 Temperature diagram at maximumtemperature difference betweenextract air and supply air.

This gives a ventilation rate of:

qs = 478 l/s ( = 1 720 m/h)

Comment:The maximum temperature difference of 10Cbetween extract and supply air is similar to thatnormally used for mixing ventilation. Thus, the airvolume flow for removal of heat surplus will be thesame for both displacement and mixing ventilation.

Table 8.3 Heat gain without regard to heat accumulation in building elements.

People: 102 persons 85 W/person 8 670 W 65,7 W/mLighting: 12 lamps 100 W/lamp 1 200 W 9,1 W/mSum: 9 870 W 74,8 W/m

= e s = 10C

A temperature-diagram for this case is shown in Figure8.3.

Hei

ght a

bove

floo

r [m

]

Supply air temperature

s =16C

Extract air temperature

e =26C

Air temperature at floor level, f = 21C

Temperature in the occupied space r ~ 23C

Temperature [C]

50% 50%

18 19 20 21 22 23 24 25 26 27171615140

0,51,01,52,02,53,0

48

8.1.6 Resulting ventilation data

Ventilation air flow rateWhen comparing the ventilation rate from air qualityconsiderations and from thermal comfortconsiderations, the ideal flow rate of 20 l/s per per-son gives a very high ventilation rate, while theventilation rate for thermal comfort is very low.

A compromise of qs = 9,5 l/s per person is chosen,giving a temperature difference of 5C between supplyand extract air. This provides a reasonable air qualitywhen the restaurant is full, and a reasonabletemperature difference between the supply air and theextract air. However, most of the time, the restaurantis only half full, in which case, the air flow rate willbe up to 20 l/s per person. This will give excellent airquality.

Hei

ght a

bove

floo

r [m

]Supply air

temperature s =19,5C

Extract air temperature e =24,5C

Air temperature at floor level, f = 22C

Temperature in the occupied space r ~ 23C

Temperature [C]

50% 50%

18 19 20 21 22 23 24 25 26 27171615140

0,51,01,52,02,53,0

Table 8.4 Ventilation rates.

Ventilation rates total per person per floor areaqs qs/n qs/Afl/s l/s pers l/s m (m/h m)

With respect to air quality 2040 20,0 15,45 (55,6)With respect to thermal comfort 478 4,7 3,62 (13,0)National regulations - - - -

Choice 1020 10 7,73 (27,8) Air change rate: 9,3

Table 8.5 Design air quality for the ventilation of the restaurant.

Min. vent. Max. ventAir volume flow per person: qs/n 10 l/s 20 l/sCO2-concentration increase ce - cs 556 ppm 278 ppmCO2-concentration in extract air ce 906 ppm 556 ppm

Table 8.6 Design temperatures for the ventilation of the restaurant.

Air flow rate qs 1000 l/s = 3600 m/hTemp. difference extract - supply qe - qs 5,0 CAir temperature at floor level: qf 22,1 CSupply air temperature: qs 20,1 CExtract air temperature: qe 25,1 CAverage vertical temp. gradient s 0,8 C/mTemperature 1,1 m above floor q1,1m 23 CTemp-diff. 1m - supply q1m- qs 2,9 C

Figure 8.4 Temperature diagram at the chosenventilation rate and maximum heatgain.

49

Design air qualityThe CO2-emission of a seated person is about 20 l/h= 0,006 l/s (Recknagel et.al. 2001). Assuming thatthe CO2-concentration in the supply air is cs = 350ppm (the outdoor concentration), we can calculatethe concentration in the extract air, ce.

The air quality in the breathing zone will be betterthan in the extract air.

Design temperaturesWith this choice of ventilation rate, the design data isshown in Table 8.6, and the temperature diagram inFigure 8.4.

8.1.7 Arrangements

Location of air diffusersWhen locating the air diffuser, remember that manyrestaurants are refurbished and modified several ti-mes during the lifetime of a ventilation system. Thus,air terminal devices and ducts should be located insuch a way that only minor changes are required whenthe room is refurbished. Moreover, the ventilation sys-tem should not require that the furniture should be

kept away from those areas where a restaurant opera-tor would find it natural to place furniture.

The air diffusers are located beside two columns (pos1 and pos 2 in Figure 8.2) and in the passage outsidethe door between the kitchen and the restaurant (pos3 in Figure 8.2). There will probably be no seating inthese areas, so that the adjacent zone can be large, ifnecessary. Diffusers have not been placed along thewalls, because they would be too close to some of theseats.

Units 1 and 2Units (1) and (2) are located as shown in Figure 8.5.Two semi-circular wall units with the same diameterand width of the columns i.e. 0,7 m are installed inthese locations. The unit is shown in Figure 8.6.

The adjacent zone diagram for this unit is shownbelow. Looking at the seating plan, it can be seen thatthe nearest ankles are about 1,5 metres from thediffusers. From Figure 8.7 it can be seen that morethan 350 l/s can be supplied from each unit. To allowfor some margin of safety, choose

qs = 320 l/s from each of unit1 and unit 2

(1)

(2)

Figure 8.5 Air diffuser (1) and (2).

50

Some seats are closer than 1,5 metres from diffuserno.2. To protect these seats from draughts, partitionsare placed between them and diffuser 2. This is shownin Figure 8.8.

Figure 8.7 Adjacent zone length for units1 and 2.

Unit 3For unit 3, the available space is shown in Figure 8.9.The front surface area is 2,5 metres wide and 1,25metres high. The distance from the diffuser to theankles of the nearest customers is about 1,5 metres.

Figure 8.6 Semicircular wall unit for places1 and 2.

Figure 8.8 Partitions between the diffuser and the closest seats protecting the guests from draught.

Protecting wall

Diffuser

0 100 200 300 400 500 600 700 800 900Supply air flow, qs [l/s]

0

1

2

3

4

5

Adj

acen

t zon

e le

ngth

, ln

[m]

t room - tsupply = 3C

Supply unit 1 and 2B=0,7m, H=1,8m W=0,65m

51

Figure 8.9 Diffuser no. 3.

In this space two plane units were installed as shownin Figure 8.10.

Figure 8.10 Diffuser chosen for place 3.

The dimensions of this unit are: height = 1,2 m, width= 1,1 m and depth 0,3 m.The length of the adjacent zone for an under-temperature of 3C is shown in Figure 8.11 as afunction of the supply volume flow. For an adjacentzone of 1,5 metres, each unit supplies approximately180 l/s. Two units are placed at this location, givingtotal supply airflow of 360 l/s.

Figure 8.11 Adjacent zone length for the unitsat place 3.

Warning: When putting two or more units besideeach other, the airflow from the diffusers merge, andthe adjacent zone becomes larger than that from onesingle unit. To allow for this effect, the supply rate isreduced from 2 x 180 l/s = 360 l/s to

2 x 160 l/s = 320 l/s from unit 3.

Location of extract units

Figure 8.12 Location of extract units.

A total of 1000 l/s is extracted from the room. Moreair is extracted from the smoking zone than from thenon-smoking zone, to prevent smoke drifting into thenon-smoking area. In addition, a threshold is put belowthe ceiling between the two zones. Its location is shownin Figure 8.12. Also, see Figure 8.13.

2,5m

1,25 m

1,5 m

0 100 200 300 400 500Supply air flow, qs [l/s]

0

1

2

3

4

5

Adj

acen

t zon

e le

ngth

, ln

[m]

t room - tsupply = 3C

Supply unit 3B=1m, H=1,2m W=0,3m

(1)

(3)

(4)

(5)

320 l/s

320 l/s

320 l/s320 l/s

640 l/s

Treshold below ceiling

(2)

52

Figure 8.13 Threshold below the ceiling between the smoking and non-smoking zones.

8.1.8 Key numbers

Table 8.7 Key numbers for the restaurant..

Gross heat surplus 74,8 W/mHeat surplus removed by ventilation 45,5 W/mVentilation rate 7,7 l/s m = 27,8 m/h m

10 l/s pers = 36 m/h persAir changes: 9,3 per hour

Treshold Extract, 670 l/s

Extract, 330 l/sNon-smoking area

Smoking area

53

Figure 8.14 A single cell office withdisplacement ventilation.

8.2.1 DescriptionThe office is shown in Figure 8.14. The diagram alsoshows the locations of the diffuser and the extractunit.

Table 8.8 Room dimensions.

Length 4,2 m Width 2,4 m Height 2,5 m Floor area 10,1 m Room volume 25,2 m

Normally, there is one person in the room. For shorteror longer periods, there may be two people.

8.2.2 Design criteriaThe design criteria for thermal comfort are as follows:

Table 8.9 Thermal comfort criteria.

Max. temp. in the occupied space 24 CMin. temp. in the occupied space 20 CMax. vertical temp. gradient 2,0 C/m

The air quality demands are: It complies with the governmental regulations The air quality is generally good

Diffuser Air extract

8.2 Case: Single cell office

The second case study is a single cell office. Unlike the restaurant, the main task of the ventilation is to removesurplus heat. Air quality is usually taken care of by the large amount of air used for removing the heat surplus.

54

8.2.3 Ventilation strategyAs pointed out earlier, air quality is not the main issuein cell offices. In most cases ventilation is use for heatremoval. However, displacement ventilation has beenused in many cell offices, so we illustrate the designprocess by the following example.

8.2.4 Design for air quality

Ventilation air flow rateTo achieve the very best air quality, a stratificationheight of 1,3 metres above the floor is preferred,keeping the sedentary person well within the lowerzone. See Figure 8.15.

Figure 8.15 Designing for contaminantstratification 0,3 m above the head

There are two convection sources in the room shownin Figure 8.15, the person itself and the table lamplight bulb.

The convection flow from a seated person 1,3 metresabove the floor (qv) is found in Figure 3.20. With avertical temperature gradient of 0,3 C/m in the room,qv is approximately 25 l/s. When the thermalstratification is approximately 1,5 C/m, theconvection flow rate is approximately

qv,person = 20 l/s

The light bulb can be regarded as a point source. A60W light bulb emits about 20W as radiated heat and40W as convected heat. Thus, the strength of the heatsource, used in the formula, is 40 W with the followingresult:

qv = 5 (40)1/3 0,35/3 = 2,3 l/s

Figure 3.20 gives a convection flow rate of 3,5 l/s.Thus, measurements show a higher flow rate. Thismay be due to the fact that the light bulb is no pointsource, but a real source with a finite size. For thispurpose we use

qv, lamp = 3 l/s

The ventilation demand with respect to stratificationis the sum of the convection flow rates through thechosen level of stratification:

qv = 20 l/s + 3 l/s = 23 l/s ( = 83 m/h)

In this case, we ignore the cold draught from thewindow and other disturbing air flows because it isassumed they are small in comparison to the air flowsalready taken into account.

Air qualityThe exhaust concentration is calculated as follows:Given a CO2 emission of 20 l/h = 0,006 l/s for oneperson (Recknagel et.al.2001) we get an increase inthe CO2 concentration from the supply air to the ex-haust of:

ce = 0,006 l/s / 31 l/s = 194 ppm

According to studies by Nielsen (1993, 2) the lower(occupied) zone concentration is 0.1-0,3 times theexhaust air concentration. Using the conservativeestimate, the concentration increase in the occupiedzone is 0,3 times the increase in the extract. The CO2concentration increase from the supply air to thebreathing zone is therefore:

cexp = 0,3 ce = 0,3 194 ppm = 58 ppm

With an outdoor CO2 concentration of 350 ppm, theCO2 concentration in the breathing zone becomes:

cexp = 350 ppm + 58 ppm = 410 ppm

Compared with a typical limit of 1000 ppm, this airquality is very good. If this is applied to perfect mixingventilation, the concentration in the breathing zonewould be the same as in the extract, i.e. 350 ppm +194 ppm = 544 ppm. This is also very good.

1,3 m

0,3 m

2 l/s 20 l/s

22 l/s

22 l/s

55

8.2.5 Design for thermal comfortThe heat balance of the room is similar to that in Table8.10. For simplicity, heat loss or gain to thesurroundings is ignored, but the heat accumulationinto the building fabric and the solar heat gains du-ring the day are taken into account by using typicalnet values.

Table 8.10 Heat balance of the office.

Lighting 100WPeople 85WSolar load 200WSum: 385W

The maximum temperature difference allowed inpractice is 8 - 10 (provided the air diffuser is capableof handling this under-temperature without anexcessive adjacent zone). If a 9C temperatureincrease from supply to extract is taken, then the res-ulting airflow rate is:

8.2.6 Resulting ventilation data

Ventilation airflow rateThe ventilation rate required for the removal of surplusheat is much greater than that required for good airquality, so selecting the greater of the two:

qs = 32 l/s (= 115 m/h)

Design air qualityUsing the same calculations as in paragraph 8.4.2,the following results are achieved:

Table 8.11 Design CO2-concentrations.

Supply air 350 ppmNumber of persons 1 2 Increase due to persons 188 375 ppmOccupied zone 406 463 ppmExtract 538 725 ppm

= ) - ( c

= qsep

vC 9 CJ/kg 004 1 1,20kg/m

W385

l/s32 = m/s 0,032 =

Design temperaturesUsing the 50% rule with a 9C temperature increasefrom supply to extract air, and a supply air temperatureof 16C, the resultant temperature distribution in theroom is:

Table 8.12 Design temperatures.

Air temperature at floor level: qf 20,5 CSupply air temperature: qs 16,0 CExtract air temperature: qe 25,0 CAverage vertical temp. gradient

s = Dq/Dz 1,8 C/mTemperature 1,1 m above floor

qoz = q1,1= 22,5 CTemperature 1,8 m above floor q1,8 23,7 C

The resulting temperature distribution is shown in thefigure below:

Figure 8.16 Temperature diagram for the singlecell office.

00.51.01.52.02.5

Hei

ght a

bove

floo

r [m

]

Temperature [C]

18 20 22 24 26

50% 50%

16appr. 6C

Supply air, p

Extract air, eRoom air,

Floor air, f

56

Figure 8.17 Low induction diffusers may givecold draught along the floor.

To avoid this problem, a diffuser that mixes room airinto the supply air discharged into the room is required.

Figure 8.18 High induction diffusers may levelout temperatures.

Temperature [C]

0

0.5

1.0

1.5

2.0

2.5

Hei

ght a

bove

floo

r [m

]

18 19 20 21 22 23 24 251716

Low induction -cold draught along the floor

Supply air, p

Extract air, e

Room air, Floor air, f

Temperature [C]

0

0.5

1.0

1.5

2.0

2.5

Hei

ght a

bove

floo

r [m

]

18 19 20 21 22 23 24 251716

Supply air, p

Extract air, e

Room air,

Floor air, f

High induction -evens out temperatures

Warning!This case study is an example where the air is suppliedat a very low temperature. It is therefore essential thatthe designer selects an air diffuser that can handlethis under-temperature without causing draughts alongthe floor.

8.2.7 Arrangements

Location of air diffuser and extractopeningA frequently used arrangement supplies the air fromthe inner wall behind the door. Consequently, thedraught zone is far away from peoples ankles.Furthermore, in this position, the diffuser will not becovered by furniture. The extract unit is also locatedon the inner wall. Moreover, the amount of ductworkis minimised. See Figure 8.19.Figure 8.19 Supply air diffuser, extract unit and

ducts

A suitable air diffuser is shown in Figure 8.20.The unit fits into the wall, and the front sheet is 600mm wide and 500 mm high.

Warning!The documentation is valid for a temperaturedifference of 5C between the occupied zone and thesupply air. Figure 8.16 demonstrates that the expectedtemperature difference between the supply air and theoccupied zone is slightly above 6C. Thus, a largeradjacent zone is necessary than that specified in themanufacturers diagram (Figure 8.20).

57

Moreover, the adjacent zone is larger when the unit islocated on a plane wall rather than in a corner.However, it is not a problem if the adjacent zone isone metre or slightly more. See Figure 8.21. Theref-ore, the selected diffuser is suitable..

Figure 8.20 The air diffuser chosen for the celloffice.

Adjacent zone ln < 1 metre

pmax = 54 Pa

50 100 150 200 250 300 m3/h

7550 l/s

0,8 1,2 l02

20

q = 31 l/s 1 < 0,8 m

10

20

50

100

200Pa

LA 3530

25

s

0

Figure 8.21 The adjacent zone in front of thediffuser.

Caution:Before finally selecting an air diffuser, the heightmeasurement and the air temperature increase at theedge of the adjacent zone should be checked. If it isnot stated in the documentation, then it should beobtained from the manufacturer!

8.2.8 Key numbers

Table 8.13 Key numbers for the cell office.

Gross heat surplus 38,2 W/mHeat surplus removed by ventilation 38,2 W/mVentilation rate 3,2 l/s m = 11,4 m/h m

32 l/s pers =115 m/h persAir changes: 4,6 per hour

58

Figure 8.22 The auditorium.

8.3.1 DescriptionThe auditorium is shown in Figure 8.22. Smoking isnot allowed. A maximum of 280 people can be seatedin the room, but normally the auditorium is only partlyfull. A typical pattern of usage is that the room isoccupied for 45 minutes followed by a 15 minutesbrake. This goes on from 08:15 in the morning toabout 15:00 in the afternoon.

Table 8.14 Data for the auditorium.

Room dimensionsHeight 6,0 mAverage width 15,5 mLength 22,0 mFloor area 341,0 mRoom volume 1600 m

Max. no. of people in the room 280 personsFloor area per person 1,14 m/person

8.3 Case: Auditorium

This case study is an auditorium. This is an example of people being seated atdifferent heights.

6 m

15 m 22 m

3,5 m

8.3.2 Design criteria

Table 8.15 Thermal comfort requirements.

Temperatures in the occupied spaceMax. temp. 26 CMin. temp. 20 CTarget temperature 23 CMax. vertical temp. gradient 2,0 C/m

The air quality demand is that the CO2-concentrationin the breathing zone shall not exceed 1000 ppm.

59

8.3.3 Ventilation strategyThis is a case where the room is tall, and the emissionfrom the persons (not the heat surplus) is the mainproblem. Thus, displacement ventilation is a naturalchoice.

Figure 8.23 Ventilation principles for theauditorium.

Both of the arrangements shown in Figure 8.23 hasbeen used with success. In Figure 8.23a), the air issupplied in front of the audience, and in Figure 8.23b)the air is supplied beneath the seats. In both cases theair is extracted at ceiling level.

When people are seated at different heights, the feedair to the convection currents come from differentlevels. This is illustrated in Figure 8.24. This meansthat the air quality for the persons seated in the upperpart of the room will not be as good as for those seatedlower down.

a)

b)

Figure 8.24 Convection currents around the people get feed-air from different heights.

60

Figure 8.25 Supply air under the seats.

Locating the air supply under the seats may be a bet-ter solution if one can trap the fresh air between therows, as shown in Figure 8.25 a). However, whenthere are openings between the rows, as in thestairways, the fresh air will float like water down thestairs, and not be captured by the convection currentsaround the people.

For this case, we choose air supply under the seatsand extract at the ceiling, as shown in Figure 8.23 b).

8.3.4 Design for thermal comfortThe heat gain of the room is given in Table 8.16. Mostof the heat surplus comes from the audience. Themaximum load in the auditorium lasts 45 minutes,and then the auditorium is empty for 15 minutes beforethe next lecture. The effective heat gain is subject tocomputation of usage and heat accumulation in thebuilding elements. For this purpose we assume thatcalculations has given that the ventilating air mustremove 70% of the maximum heat load.

Table 8.16 Heat gain in the auditorium at maximum audience.

People in the room: 280 persons 85 W/person = 23 800 W 69,8 W/mLighting 341 m 10 W/m = 3 410 W 10,0 W/m Sum: 27 210 W 79,8 W/m

Heat to be removed by ventilating air 19 047 W 55,9 W/m(according to calculations of heat accumulation in the building materials and non-stationary effects)

a) Supply air is contained between the rows. b) Supply air is foating down the stairways.

Before we calculate the ventilation airflow rate, wetake a look at the temperature distribution in the room.First, if we assume that 1/3 of the total temperatureincrease in the room is from the supply air to the floorair temperature, the temperature distribution will beapproximately as in Figure 8.26, when the temperaturerequirements shall be satisfied. This is a case wherethe temperatures are just within the design limits. Oneshould aim at having a more even temperaturedistribution, so that the temperature in the upper partof the room is lower, and the floor level temperatureis higher.

To achieve this, we can utilise a supply unit with acertain degree of room air entrainment, or locate thesupply units under the seats. In this case, the 50%rule is a more probable result with respect to thetemperature distribution. See Figure 8.27.

61

Figure 8.26 Thumb-rule temperature distribution, 1/3 temperature increase to floor temp.

Figure 8.27 Temperature distribution according to 50% rule.

Temperature [C]18 19 20 21 22 23 24 25 26 2717

33% 66%

16

appr. 4,5C

Hei

ght a

bove

floo

r [m

]

0

1

2

3

4

5

6

Extract air temperature e =27C

Supply air temperature s =17C

Air temperature at floor level, f = 20C

Temperature [C]

50% 50%

18 19 20 21 22 23 24 25 26 271716

appr. 6 C

Hei

ght a

bove

floo

r [m

]

0

1

2

3

4

5

6

Extract air temperature e =26C

Supply air temperature s =16C

Air temperature at floor level, f = 21C

62

The temperature increase from supply to extract airis 10C. When 19 000 W shall be removed with thistemperature difference, the ventilation air flow ratebecomes:

qs = 1 517 l/s (= 5 464 m/h)

8.3.5 Design for air qualityIn this case we