Zonal Air Distribution

8.7.S. I Design Requirements for Achieving the Zoning Strategy

The aim of the zoning strategy is to have control of temperature, con­centration, or humidity over a certain volume of the room, while the rest of the room is left with less attention. In most cases the accumulation of heat.

Zonal Air Distribution

The key flow elements in the zoning strategy are the supply air jets, plumes of the buoyancy sources, buoyant airflows along the surfaces, and tur­bulent mixing between the controlled and the uncontrolled zones, as in Fig. 8.32. These flow elements have significant influence on the effectiveness of the system.

There are four principal ideas in achieving uniform conditions in the con­trolled zone and a high hear and contaminant removal effectiveness:

1. Supply air is evenly distributed into the controlled zone. I’he momentum of the jets is high enough to ensure the uniform conditions but also low enough to avoid mixing in the whole room—i. e., the turbulent mixing between the zones is low. This means that usually the number of inlet devices is high.

2. The momentum flux of the plumes is high enough to penetrate the supply airflow patterns. The penetration depends on the location of the plumes in relation to the supply airflow patterns.

3. The plume airflow rate in relation to the extract airflow rate from the uncontrolled zone is low enough to avoid undesired return flow from the uncontrolled zone to the controlled zone.

4. Disturbance flows on the zone boundary should be avoided because of the undesired return flow from the uncontrolled zone to the controlled zone.

The accumulation of heat, contaminants, and humidity is usually vertical in the room, but horizontal zoning is also possible. The same principal ideas should be followed in those cases.

8.7.5.2 Two-Zone Model for Zoning Strategy

When the zoning strategy is applied, the two-zone model is a useful and simple tool for the determination of the thermal, contaminant, and humidity accumulations. Principles of two-zone modeling are presented in Section 8.4.

V

/

Room dimension

A

EX

X T, C, x

Zonal Air Distribution

Su

подпись: su

Mr

* rv

A

Rrui w

*bt 1 T <lcb <lcbm y

Q

‘Uz

^db

T,,|,

A ^

Q Urx

^Izcv

A/ex

*,t,4

^ . TrllZ

/ ‘/il I

9b,

X %

V>U’U7.

F

K

<?«

0,

J;

11.

Bl.

Vc,mt

Tcbmj

JKm

H

*

T

ED. <?,, i b’lhl

…… …. *t“iw

N. T. y

1 ^ rnoi

 

<hut

<W

^UZ/

 

Zonal Air Distribution

Hi

 

TW

‘/i//

 

Orl,

 

FIGURE 8.33 Two-zone temperature model of the zoning strategy.

Relatively uniform conditions in the controlled zone are characteristic tor the zoning strategy. By assuming uniform conditions also in the uncontrolled zone, the following two-zone model can be developed. The controlled zone boundary should be defined high enough to get nearly all the induction air of the supply air devices from the controlled zone. This depends on the air distri­bution method used and the dimensioning of the devices.

Figures 8.33 and 8.34 describe a two-zone model application of the zon­ing strategy where all the main variable parameters are presented. Figure 8.33 (temperature model) describes the accumulation of heat and Fig. 8.34 {con­centration model) the accumulation of contaminants. After solving for the temperatures, heat flows, and airflows, contaminant concentrations can lie calculated. The models are here determined for stationary loads, airflow rates, and indoor/outdoor conditions, but they can be developed also for dynamic simulations.

R

U/

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Hi,

(jub lluh

Gwii

<

->C,„

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i……………………. T fob /f

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A

LJaih

‘Gab

Ge. b G(bm{

Peb ^*cbm l

L1cm

A

Tfuzf ‘* I*l] If/ ^ </uIf

Jnf ™u if

* b

(*h Gbf QV c„,

H,.

 

Zonal Air Distribution

‘fex

 

Zonal Air Distribution

I

 

‘Till

 

Zonal Air Distribution

Q

 

Zonal Air Distribution

G|w*

^liTCX

The temperature model is based on the air mass flow rate and heat flow rate balances in the lower (controlled) zone (lz) and the upper (uncontrolled) zone (uz).

The plume airflows (qc) are determined as described in Section 7.5. The turbulent mixing (iqfjt) between zones and the penetration of the plume air­flows iqcbm) through the supply airflow patterns must be determined specially for the air distribution method and devices used as well as the locations of plumes and supply air devices.

The radiation heat transfer (cf>r) from the heat loads such as machines, lamps, persons, and sun has to be determined separately for the lower zone ((/>r/.) and upper zone ($ruz). The radiation between zone wall surfaces (UJ has to be determined as well.

Local exhaust airflows from the hoods in the lower zone (qjzcx) reduce the heat removal effectiveness, because the return airflow rate (qb) from the upper zone to the lower zone is increased.

The infiltration airflow into the lower zone (q^z) is assumed to be higher than the exfiltration. The difference {bq^z) describes the airflow rate into the upper zone caused by filtration.

The wall surface temperatures (Twul) and (TuAz) are calculated separately for each wall and window surface by means of heat transfer through the wall: Twuz = Tuz — (Uuz/hu?)(Tuz -T0) and Twh = Ti2 — (Uh/bh)(Th — Ta), where b is the heat transfer coefficient on the surface and U is the overall heat trans­fer coefficient. The airflows along the wall surfaces through the zone bound­ary (qwb) are calculated separately for each wall and window surface.

The concentration model is based on the air mass flow rate and contami­nant flow rate balances in the lower (controlled) zone (lz) and the upper (un­controlled) zone (uz).

The airflows on the wall surfaces and the air filtration through the walls significantly influence contaminant accumulation, and therefore it is essential to carry out the calculation also for the cold season.

Nomenclature for the figures

A = area [m2]

B = coefficient

C = concentration [mg m-3]

H = height [m]

G = contaminant flow rate [mg s_1 ] q = airflow rate [dm3 s‘11 <jk = heat flow rate [W]

T = temperature [K, °C]

Subscripts

A — air

B = boundary, through the boundary c = convection cd = conduction ex = exhaust, extract

/ = filtration lz = lower zone m — mixing, mixed

O = outside r = radiation s = supply

T = turbulent mixing, total uz = upper zone tv = wall

8.7.S.3 Characteristics Of the Zoning Strategy

The air distribution method and dimensioning of the air supply devices are important factors in determining the accumulation of hear and contami­nants. Examples of this are presented in Section 8.4. After the behavior of the air distribution method and devices are known, the characteristic effects of the other airflow elements can be calculated.

The simplified examples in Figs. 8.35-8.39 are calculated for the case of an industrial hall with length 40 m, width 25 m, and height 8 m. The zone level is determined at the height of 4 m. The dimensions of the heat (50% convection) and contaminant sources located at floor level are 1 m, 1 m, and 1 m. The heat load of the lights located in the upper zone at the level of 6 m is 15 W m-2. For the walls the U-value is 0.5 W nr2 K-1. The turbulent mix­ing between the zones is estimated as qbl = 0.5^ or qbl ~ 1.0<js. The penetra­tion of the plumes through the zone boundary is estimated to be 100%, i. e., V q(bm = Јqc. Contaminant concentration outside is Ca =0. Other values are listed in the tables of the figures.

The effect of the plume airflow rate and the turbulent mixing airflow rate through the zone boundary is presenred in Fig. 8.35. The heat removal effec­tiveness 6r and contaminant removal effectiveness ec are presented as func­tions of the relative airflow rate.

Zonal Air Distribution

— • o — ■ ■ 1. Temp. 0.5 —■—1. Com. 0.5

— ■ -2. Temp. 1.0 —*—2. C’ont. 1.0

подпись: - • o- ■ ■ 1. temp. 0.5 —■—1. com. 0.5
- ■ -2. temp. 1.0 —*—2. c'ont. 1.0

0.0 1.0 2.0 3.0

‘7ihftl /<7s

подпись: 0.0 1.0 2.0 3.0
‘7ihftl /<7s
The effect of the supply airflow rate and the heat load is presented in Fig. 8.36. The heat removal effectiveness e, and contaminant removal effectiveness

Ta = 25 °C TUI = 26 "C = 0 = = 0 = 0

Tflztd = ^uzcd = 0

Case 1: qbJqi — 0.5 Case 2: qbi! qs = 1.0

FIGURE 8.35 Effectiveness ЂT(temp.) and e, (cont.) as functions of the ratio qcbm /with the ratio <?(*/<), as a parameter.

3 A A

2.60 « 2.40

— • o — • -1. Temp. 60 —■— 1. Cont. 60

— — a • — 2. Temp. 100

> 1 00

S I. oU i

—*—2. (.ont. 100

2 160

————- -1

2

. . . i

K * ’ ~

> 7 8 91 qs(Us m2

0 11 1

T0 = 25 °C Tm = 26 °C

Tfu’b ~ 0 <?/lz = Ifuz = 0

<J7*X = 0

Ј/l/i’d = fitted ~ 0

Case l:4>, = 60W m’2 Case 2: <I>, = 100 W in“2 O, = 500 W apiece

‘lb Ah =1-0

FIGURE 8.36 Effectiveness <r(temp.) and tr (cont) as functions of the supply airflow rate q. (L s~’nr2) with the total heat load <b. (W m*!) as a parameter.

Ec are presented as functions of the supply airflow rate. The power of one heat source is 500 W.

The effect of the local exhaust airflow rate in the lower zone is presented in Fig, 8.37. The heat removal effectiveness e7 and contaminant removal effec­tiveness ec (determined by extract air) are presented as functions of the local exhaust airflow rate. The total heat load is 60 W m~- and the power of one heat source is 500 W. The supply airflow rate is 8 L s_1 m~2.

The effect of the downward airflow along the wall surfaces is presented in Fig. 8.38. The heat removal effectiveness er and contaminant removal effec­tiveness Ђc are presented as functions of the outdoor temperature T0. The total heat load is 60 W m-2 and the power of one heat source is 500 W. The supply airflow rate is 8 L s-1 nr2, [n winter seasons heat losses through the walls and the airflow along the walls increase the relative temperature difference and de­crease the concentration difference.

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2.40

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G 220

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Ss,

1.20 1.00

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—(

1 — — —

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— <

0 12 3 4 5 6 7

Ta = 25 "C TM = 26 °C

<twb = 0

<7/1/. = tffui — 0 q, = 8 L s_1irr2

*7bred ~ *"/ uzed = 0

<1), = 60 W nr2 <f>, = 500 W apicce = i

подпись: ta = 25 "c tm = 26 °c
<twb = 0
<7/1/. = tffui - 0 q, = 8 l s_1irr2
*7bred ~ *"/ uzed = 0
<1), = 60 w nr2 <f>, = 500 w apicce = i

Temp. 60 Cont. 60

подпись: temp. 60 cont. 60

FIGURE 8.37 Effectiveness Ђr (temp.) and Ђf (cont) as functions of the local exhaust airflow rate from the lower zone

A

<U

•c 2

U jC

L5

.20

1.00

* * 1

K> W.

3.00

2.80

2.60

2.40 2.20

00

1.80

1.60

1.40 i.

-25 -15 -5

15

25

Zonal Air Distribution

Th = 20-24 °C Tu, = 21-26 °C

*7/1? = fuz ~ 0 t/* = 8 L s’1 m-2 Ocd = 42.0 W m-2 <t>i = 60 W nr2 0r = 500 W apiece

‘litres ~ 1

 

O •• Temp. 60 — Cunt. 60

 

FIGURE 8.38 Effectiveness t r (temp.) and tr: (cont) as functions of the outdoor temperature T„.

The effect of the disturbance of the supply airflow on the plumes is pre­sented in Fig. 8.39. The heat removal effectiveness eT and contaminant re­moval effectiveness ec are presented as functions of the penetration factor ‘F of the plume, which is the ratio of penetrated plume airflow rate to the whole plume airflow rate. The total heat load is 60 W m-2 and the power of one heat source is 500 W. The supply airflow rate is 8 L s_1 m-2.

These case examples illustrate the dependence of the stratification of tem­perature and contaminants on several parameters, which in some cases in­crease and in other cases decrease the effectiveness. All the parameters should be included in calculations when designing the system combination of the room air conditioning methods.

-1 m-2

подпись: -1 m-2

Temp. 60 Cont. 60

подпись: temp. 60 cont. 60

3.0 2.80 2.60

2.40 2.20

2.0 1.80 1.60

1.40 1.20 1.00

подпись: 3.0 2.80 2.60
2.40 2.20
2.0 1.80 1.60
1.40 1.20 1.00

J

. 1

F

— -1

* T

U> * *

0.2 0.4 0.6 0.8

Penetration factor 4*

T0 = 25 °C Tm = 26 °C <7 wh = 0 ‘1 — <7/uz ~ 0

^s=SL s

*7l7cd ~ ‘7 ll/Cll = 0

4>, = 60 W m-2 <t>, = 500 W apiece ctbt/lh = 1

FIGURE 8.39 Effectiveness eT (temp.) and ef. (cont) as functions of the penetration factor of the plume airflow rate.

Measured results of effectiveness and turbulent mixing are presented in literature by Bach (several air distribution methods)3 and Hagstrom et al. (mixing air distribution methods in zoning strategy).4’5 A typical exam­ple of an air distribution method and device in the zoning strategy is the so-called active displacement method, which is based on a nozzle duct de­vice.6

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