Reference

NBS

PUBLICATIONS

NBSIR 85-3208

NAT'!, INST. OF STAND & TECH

AlllOfc. 034057

Ventilation Effectiveness in

Mechanically Ventilated Office

Buildings

Andrew K. Persily

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

August 1 985

Prepared for: U.S. Department of Energy —

Q

C^

7 ' —u»f Building and Community Systems

2 qq

jton, DC 20585

.1156

85-3208

1985

NATIONAL BUREAU OF STANDARDS

LIBRARY

NBSIR 85-3208

VENTILATION EFFECTIVENESS IN

MECHANICALLY VENTILATED OFFICE

BUILDINGS

Andrew K. Persily

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Building Technology

Gaithersburg, MD 20899

August 1 985

Prepared for: U.S. Department of Energy

Office of Building and Community Systems

Washington, DC 20585

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Abstract

Mechanical ventilation systems in large office buildings are designed to meet

space conditioning loads and to maintain acceptable indoor air quality. In

order to achieve acceptable air quality, the ventilation systems are designed to

bring in a minimum amount of outside air whenever the building is occupied. In

addition to minimum outside air intake levels, there must also be adequate

distribution of this air within the building. Although the net ventilation rate

of a building or room may be sufficient, poor air distribution may lead to some areas within the space being inadequately ventilated. The concept of

ventilation effectiveness has been developed to quantify the air distribution

characteristics of a ventilated space. This paper examines several definitions

of ventilation effectiveness and associated tracer gas measurement techniques.

Techniques for making ventilation effectiveness measurements in mechanically

ventilated office buildings are discussed with reference to building and

mechanical equipment design and tracer gas instrumentation. Specific strategies

are proposed for measuring ventilation effectiveness on different scales ranging

from individual rooms to whole buildings.

Key words: air distribution; building performance; measurement; mechanical

ventilation; tracer gas measurement; ventilation; ventilation

effectiveness; ventilation system performance.
TABLE OF CONTENTS

NOMENCLATURE iii

1 « INTROEU CTION......................9.e....e.oe...eeeo... ............. ........1

2. BUILDING DESIGN AND EQUIPMENT............. .2

3. TRACER GAS EQUIPMENT 3

4. THEORETICAL BACKGROUND 4

4.1 Concentration Efficiencies 4

4.1.1 Definitions 4

4.1.2 Two-Zone Model 6

4.2 Age Distributions 7

4.3 Multi-Zone Approach 9

5. EXPERIMENTAL TECHNIQUES 10

5.1 Concentration Efficiencies 10

5.2 Age Distributions..... 11

5.3 Multi-Zone Approach 13

6. CONCLUSIONS 14

7. ACKNOWLEDGMENTS 14

8. REFERENCES 15

9. APPENDIX: EXPERIMENTAL DETAILS 17

ii
NOMENCLATURE

A^Aj constants in two-zone ventilation model

C tracer gas or pollutant concentration

Ce exhaust air concentration

Cj concentration at point J

C

s supply air concentration

C average concentration in a space

f(t) frequency distribution of fluid elements leaving a space

F(t) age distribution of fluid elements leaving a space

k number of zones in a multi-zone model

m volume of tracer gas injection

m tracer gas injection rate gas (vol/t)

N ventilation rate (t“^)

Q airflow rate (vol/t)

Qjj airflow rate from zone i to zone j (vol/t)

Qs supply airflow rate (vol/t)

r fraction of exhaust air which is recirculated

s fraction of supply air which bypasses occupied zone

t time

V room or building volume

<e a> room average relative air-diffusion ventilation efficiency

e a> j

relative air-diffusion ventilation efficiency at point J

e a local air exchange indicator

£

j steady-state relative ventilation efficiency

e room average steady-state relative ventilation efficiency

e r pollutant removal efficiency

iii
£ t

H

i £ t

n

<J> Ct)

$(t)

T t

pP>

instantaneous transient relative ventilation efficiency

room average instantaneous transient relative ventilation efficiency

integral transient relative ventilation efficiency

ventilation efficiency from Janssen model

frequency distribution of fluid elements within a space

age distribution of fluid elements within a space

constant in two-zone ventilation model, or mixing factor

inverse of air exchange rate (t)

turnover time (t)

mean residence time of contaminant leaving the room (t)

room average of the internal age (t)

mean age of the internal air at point J (t)

iv
1. INTRODUCTION

The establishment of minimum ventilation rates in buildings to maintain the

health and comfort of occupants has been a subject of interest for many years

and continues to be an important area of concern. Efforts to conserve energy

for space conditioning have led to reduced building ventilation rates, and

increased concern about minimum outside air intake levels. Standards and

recommendations for minimum ventilation rates are generally in units of

volumetric airflow rate per person or per unit floor area, and depend on room

type (kitchen, conference room) and activity (smoking) (ASHRAE 1981). Building

ventilation systems are designed to satisfy these minimum outside air intake

levels whenever the building is occupied. Even if the ventilation system is

bringing in a sufficient amount of outside air, the air may not be well

distributed within the interior space. Airflow rate, supply air temperature,

and the location, size and type of supply outlets and return grilles affect the

distribution of the ventilation air. The concepts of ventilation effectiveness

(Sandberg 1981, Skaret and Mathisen 1982) are used to quantify the effectiveness

of ventilation air in removing the existing air within a room and replacing it

with freshly conditioned air.

When evaluating a ventilation system's ability to remove an internally generated

pollutant, the pollutant emission characteristics are important. If the

pollutant is generated continuously, or for a period of time which is long

compared to the inverse of the building air exchange rate, the ventilation

system's air distribution performance is characterized by the equilibrium

concentration of the pollutant at various locations. Brief pollutant discharges

are characterized by time-integrated exposures to the pollutant and peak

concentrations. The spatial characteristics of pollutant emission is another

important factor, i.e. whether the pollutant is emitted from specific locations

or uniformly over a large area.

The ventilation effectiveness for a space and its ventilation system is

determined by several variables, including differences between the supply and room air tempertures. The temperatures of room surfaces in relation to the room

air temperature also affect the airflow within the room, as do the type and

position of supply and exhaust air openings and the flow rates through them.

The direction and extent of spread of the airflow from supply terminals has been

studied as a function of the variables discussed above (Straub et al 1956 and

1957, Nevins 1976), and design guidelines exist for the selection of supply

outlets (ASHRAE 1985). These terminals are generally selected to provide good

mixing of the supply air with the room air in order to avoid subjecting the

occupants directly to the supply air temperature, or to excessive temperature

variations and drafts. The concepts of "effective draft temperature" and the

Air Diffusion Performance Index (ADPI) have been developed to quantify air

diffusion performance in terms of thermal comfort (ASHRAE 1985).

While ventilation effectiveness has become an active field of study in recent

years, ventilation system performance has been studied for many years (Lidwell

I960, Jennings and Armstrong 1971, Kusuda 1976). Most of the recent ventilation

effectiveness research has involved experiments in test rooms with

reconfigurable intake and exhaust openings and controllable supply air

temperatures and ventilation rates (Sandberg 1981 and 1983, Sandberg, Blomqvist

and Sjoberg 1982, Malmstran and Ahlgren 1982, Skaret and Mathisen 1982 and

1983). In these tests, ventilation effectiveness is measured as a function of

opening position, airflow rate and temperature difference between the supply and

1
room air. While these experiments have been extremely valuable, the test

conditions are quite different from the situation encountered in offices in real

buildings. In real offices, air enters and leaves from locations other than the

supply and exhaust air openings. There are leaky windows, doors to other

offices, and additional openings in the room boundary. The extent to which

these other airflows affect ventilation effectiveness is not clear. The

ventilation effectiveness can still be measured in real offices, but the results

may not lend themselves to straightforward interpretation. Additional

ventilation effectiveness measurements have been conducted in naturally

(Maldonado and Woods 1983) and mechanically (Offermann et al 1983) ventilated

residential buildings. There have also been a limited number of measurements in

large mechanically ventilated buildings (Skaret and Mathisen 1985).

This report is concerned with the measurement of ventilation effectiveness in

office buildings. Office buildings and their mechanical ventilation equipment

are discussed, along with tracer gas equipment for ventilation effectiveness

measurements. Several definitions of ventilation effectiveness are presented

and related to the situation encountered in office buildings. Finally,

experimental techniques are proposed for use in office buildings to measure

ventilation effectiveness in individual offices, zones within buildings, and

whole buildings. An appendix gives examples of techniques for measuring

ventilation effectiveness in office spaces, including tracer gas injection and

sampling locations.

2. BUILDIfC DESIGN AND EQUIPMENT

Office buildings vary in size, zoning, air handling equipment, and air

distribution system design. All of these factors must be considered when

designing experiments to measure ventilation effectiveness. Tests may be

designed to study a single room, a zone containing several rooms, or a whole

building. The specific area of interest will affect which techniques can be

used and how they are applied.

The design of air distribution systems in office buildings is quite varied and

cannot be completely covered here, but additional information is available

elsewhere (ASHRAE 1984). Office buildings air handlers, located in mechanical

equipment rooms, serve particular sections or zones of a building. The nunber

of air handlers and the manner in which the occupied space is divided into zones

varies greatly among buildings. The air handlers contain supply fans that force

conditioned air through a system of ductwork to supply air terminals in the

occupied space. The air leaves the space predominantly through exhaust or return air grilles, and enters a network of return ducts. The return air is

generally drawn by a return air fan back to the mechanical equipment room. Some

of the return air is exhausted to the outside and the rest is recirculated back

into the supply airstream, where it mixes with new outside air. The amount of

air that is recirculated and the amount of outside air intake is determined by

the HVAC control system, based on the outdoor conditions and the space

conditioning load. There is also a significant amount of airflow into and out

of the building due to envelope leakage, often similar in magnitude to the

intentional ventilation rate. The ventilation requirements mentioned earlier

translate into a design minimim rate of outside air intake, intended to be

brought into the supply airstream independent of the amount of recirculation or total airflow. This design minimum is supposed to be achieved either by

employing a minimum position of the outside air intake dampers or a subset of

2
the intake dampers that never close. Some building operators rely on uncontrolled air leakage through the building envelope to provide a sufficient

amount of outside air and close the outside air intake dampers completely. This

strategy provides little or no control over air distribution and indoor air

quality.

Within a conditioned zone, even a single room, there are generally several

supply air outlets served by a common supply duct. Similarly, there are several

exhaust air openings that feed into the return air system. In this report we

are discussing systems which employ supply air outlets which are designed to

entrain room air into the primary supply airstream. Figure 1 shows a schematic

of an air distribution network for an office building zone. A supply air shaft,

which also serves other areas, passes through this zone, and a supply air duct

branches off of the shaft to serve this zone. The supply duct branches off

further to several supply air outlets throughout the space. In most modern

office buildings, the ductwork shown in figure 1 is located above a dropped

ceiling, while the space below may be divided by internal partitions. The space

above the dropped ceiling is generally used as a return air plenum, i.e. the

exhaust or return air grilles are simply openings in the dropped ceiling through

which air flows from the occupied space. The return air plenum is generally

open over an entire floor. This return air flows through the plenum and enters

the return air shaft through an opening in that shaft. In general, there are

only about two or three such openings into the return air shaft on each floor.

3. TRACER GAS EQUIPMENT

The ventilation effectiveness measurement techniques discussed below employ

tracer gases to either simulate a pollutant or to tag the supply air. In

designing these tests, one must consider the characteristics of the tracer gas

analysis equipment including the range of measurable concentration and the

sampling frequency. Since one often needs to measure the concentration at

several locations, the sampling frequency of the equipment determines how often

the concentration at each location can be measured. For some ventilation

effectiveness measurement techniques, an average tracer gas concentration is

useful and can be obtained by filling an air sample container over an appropriate interval of time.

Ventilation effectiveness measurements also require tracer gas injection

equipment. The injection flow rates must be matched to the measurable

concentration range, the ventilation airflow rates, and the volumes of the zones

of interest. It is generally desirable to inject a tracer gas, which simulates

a pollutant, in an unobtrusive manner, i.e. at low enough flow rates such that

the airflow patterns within the room are not disturbed.

There are many tracer gases and concentration measuring devices which can be

used in ventilation effectiveness measurements. Many of these devices have been

previously employed to study infiltration and ventilation in buildings (Hunt

1980), and include automated systems that can operate unattended for long

periods of time (Grot, Hunt and Harrje 1980, Grot 1982). Equipment capable of

measuring the concentrations of several different tracer gases has been used to

study airflows between different zones in a building and can be useful in

studies of ventilation effectiveness (I 1 Anson et al 1982, Dietz et al 1984).

3
4. THEORETICAL BACKGROUND

Several definitions and theoretical frameworks have been used to discuss

ventilation effectiveness. Three approaches to ventilation effectiveness are discussed below. The first type of ventilation effectiveness measures are referred to as "concentration efficiencies" and are based on relations between

gas concentrations in the supply air, the exhaust air, and at various locations

in the room. Efficiencies based on age distributions and residence times, using

approaches of chemical reactor engineering, are also presented. Finally, multitracer techniques to measure interzone airflow rates are discussed.

Figure 2 depicts three important cases of ventilation air movement which are referred to in discussions of ventilation effectiveness. Maximum ventilation

effectiveness is associated with the idealized case in which the supply air

displaces the rocm air without mixing, so-called "piston flow." While piston

flow corresponds to maximum ventilation effectiveness, it may lead to

undesirable thermal comfort conditions. In some discussions of ventilation

effectiveness, the idealized case of perfect mixing serves as a reference.

Perfect mixing is defined to occur when the supply air or an internally

generated pollutant is uniformly and instantaneously mixed with all the room

air, thus the pollutant concentration is uniform throughout the room. Ventilation effectiveness is generally minimized when a significant portion of

the supply air "short-circuits" the occupied zone and flews out of the space

without reaching the occupants.

4.1 Concentration Efficiencies

4.1.1 Definitions

Much work has been done using ventilation efficiencies based on relations

between pollutant, or tracer gas, concentrations in a rocm or zone. The first

group of these definitions are based on a ventilation system's steady -state

performance. The steady-state relative ventilation efficiency e j, denoted as

by Sandberg (1981) and ejj by others (Skaret 1982, Malmstrom ana Ahlgren 1982),

expresses how the system's ventilation effectiveness varies throughout a room.

It is defined as

Sj = <ce - c

s

) / (Cj - c

s

) (1)

where

C

e

= concentration in the exhaust air

Cj = concentration at point J in the room

C

s = concentration in the supply air.

These are concentrations of a general pollutant, or a tracer gas, which may be

carried in the supply stream and/or generated within the room. For many

pollutants, the supply air concentration C

s is practically zero when there is no recirculation of the return air. The concentration at point J can be replaced

by the average room concentration C to define a room average relative

ventilation efficiency F.

For a pollutant generated within a rocm, one can examine the three cases shown

4
in figure 2. In the case of perfect mixing, the tracer concentration is uniform

throughout the rocm and equal to the exhaust concentration, thus the relative

ventilation efficiency equals one. The case of perfect mixing then serves as a reference. When there is less than perfect mixing, the exhaust air

concentration may be higher than the concentration at a point J within the roan,

depending on the location of the pollutant source and the point J, leading to

values of £j which are greater than one. This is generally a favorable

situation.

J In the extreme case of piston flow, the value of £j depends on the

position of point J relative to the location of the pollutant source. If the

pollutant is generated downstream of point J, then C,- will equal C

s and the

relative ventilation efficiency is infinite. When there is short circuiting of

the supply air to the exhaust air outlet, and the pollutant is generated in the

space which is being bypassed by the ventilation air, will be greater than

Cg. The relative efficiency is then less than one, a generally undesirable

situation.

Sandberg also defines "transient" ventilation efficiencies for the situation in

which a rocm starts with a spatially homogenous pollutant concentration and

there are no pollutant sources. If the air within the room is perfectly mixed,

the concentration C decreases with time t according to

C = C( 0)e“Nt (2)

where N is the ventilation rate of the room in exchanges per hour and C ( 0) is

the initial pollutant concentration. In the more typical case of imperfect

mixing, the concentration decay is more complex and can be analyzed by dividing

the room into a number of perfectly mixed zones which exchange air with each

other. The mathematics for multi-zone decay is discussed by Sinden (1978). In

multi-zone models, the concentration in each zone is the sum of k decay terms

similar in form to eq (2), where k is the number of zones in the model. After

some time t0 , the concentrations in all the zones decay at the same rate and the

ratio of the concentrations in any two zones is a constant. In a specific two

zone application, one zone represents the occupied space, characterized by C^,

and the other zone is characterized by the exhaust air concentration Ce

. Thus,

after a sufficiently long time (t»tQ ), these concentrations are given by

c

e

(t) = A

ee-Nat (3a)

Cj(t) = Aje-Not (3b)

where Ag, Aj and a are constants. As noted above, after a sufficiently long

time the ratio of Ce

(t) to Cj(t) becomes constant. Sandberg defines this ratio

as the "transient relative ventilation efficiency,"

e t

= C

e

(t) / Cj(t). (4)

As in the case of the steady -state relative ventilation efficiency, the average roan concentration C(t) can be substituted for C^(t) to define a roan average

transient efficiency eV. The constant a in eq (3) has been referred to as a mixing factor (Kusuda i 976 ) and proposed as a measure of ventilation

effectiveness (Skaret and Mathisen 1983). Sandberg cautions against considering

Na to be a local ventilation rate because spatial differences in ventilation

capability will be underestimated and the value depends on when the measurements

are made (Sandberg 1981).

5
Sandberg defines an integral transient ventilation efficiency based on the areas

under the dilution curves,

£ t

C

e (t)dt / /

Cj (t)dt. (5)

Hie value of corresponds to the steady-state relative ventilation efficiency

£j for a homogeneous, unobtrusive pollution source in a room. This equivalency

can be generalized to an arbitrary pollution source distribution. f When the

initial concentration distribution used in the determination of e t is

"geometrically similar" to the steady-state pollution source distribution

corresponding to ej, then the transient ventilation efficiency e t equals the

steady-state ventilation efficiency £j.

All of these concentration definitions are based on a model of the office space

in which all the air flows in through a supply vent and out through an exhaust

vent. In real buildings, air also enters and leaves through windows, doors to

the outside and adjacent offices, and other air leakage locations. Some of this

air is from outside and some is "stale" air frcm other occupied spaces. These

additional air flows are not accounted for by the above definitions.

4.1.2 Two-Zone Model

A two-zone model, in which the space under consideration is divided horizontally

into two perfectly mixed zones, is often used to study office ventilation

effectiveness. Figure 3 shows four different cases of supply and exhaust air

vent location for a two-zone ventilation model. The diagrams are only meant to

depict in which zone the supply and exhaust vents are located, not the number of

vents or their specific locations within the zone. Q is the airflow into and

out of the total space volume, and 3Q is the flow between the zones where 6 is a fraction which ranges from zero to infinity. 6=0 corresponds to no mixing,

and 6 = 00 corresponds to complete mixing between the zones. Case A is

representative of the vent arrangement found in most modern U.S. office

buildings, and some have suggested that it may lead to situations where a significant fraction of the supply air short-circuits the occupied zone.

Sandberg (1981) has applied the two-zone model, for cases A and D, and for the

case of perfect mixing, to calculate the steady-state relative ventilation

efficiency £j. He assumes the two zones are of equal volume, and characterizes

the upper zone by the exhaust air concentration C

?

. He solves for the steadystate pollutant concentration in the lower, occupied zone and the ventilation

efficiency ej, depending on whether the pollutant is released in the lower zone,

upper zone or uniformly in both zones. One can determine both the ventilation

efficiency and the mixing coefficient 6 from Cj, C

e

, the room volume V, and the

tracer gas injection rate m.

Others (Sklret and Mathisen 1982, Malmstrbm and Ahlgren 1982) have also used a two-zone model to calculate ventilation efficiencies as a function of the mixing

parameter 6. They consider all four of the cases shown in figure 3, and use this two-zone approach in test rooms to make actual measurements of ventilation

efficiency as a function of vent position, ventilation rate, and the temperature

difference between the supply and the roan air. When the supply air terminal is

located in the upper zone, the ventilation efficiency decreases as the supply

air overtemperature increases, due to thermal stratification. The highest

ventilation efficiencies are measured when the supply and exhaust air vents are located in different zones. For these cases, C and D, the ventilation

6
efficiency decreases with increasing values of 3, while for case A, an increase

in 3 leads to increased ventilation efficiency.

Figure 4 shows calculated values of ventilation efficiency for two cases of a two-zone model as a function of the mixing parameter 3 (Sandberg 1981). As

mentioned above, when the supply and exhaust vents are in different zones, the

ventilation efficiency is greater than 1.0 and an increase in mixing between the

two zones decreases the ventilation efficiency. When both vents are in the

upper zone, an increase in mixing increases the ventilation efficiency towards

one. Figure

f 5 shows measured values of the integral transient ventilation

efficiency e I at various heights in a small test room (41 m-^) with an exhaust

inlet located on a sidewall, 0.2 m below the ceiling, and a supply outlet

mounted in the center of the ceiling (Sandberg 1981). The ventilation

efficiency is plotted against air exchange rate for various temperature

differences between the supply and room air. In general, the larger the

temperature difference, the lower the ventilation efficiency, presumably due to

increased thermal stratification. Also, except for the largest temperature

difference, the ventilation efficiency increases with increasing air exchange

rate.

As mentioned earlier, these two-zone approaches assume air flows in and out of

the roan only through the vents. In real offices, there are many other

airflows. If one includes the additional airflows, the relations between

equilibrium concentrations, ventilation rate, and ventilation efficiency no longer apply. In addition, mechanical ventilation systems often employ

recirculation of the exhaust air and this is not dealt with in these models.

Janssen has developed a two-zone approach to ventilation effectiveness which

includes recirculation and is based on the schematic of office ventilation shown

in figure 6 (Harrje and Janssen 1984). In this model s is the fraction of the

supply airflow Qs which bypasses the occupied part of the room, and r is the

fraction of the exhaust air which is recirculated and mixed with the new outside

air. Janssen defines a ventilation efficiency as

n = (Q01 - q 1q_s

) / qq1 = (1-s) / ( 1-sr) (6)

where Qqi is the flow rate of outside air into the building and Qjq-s is the

amount of this outside air which bypasses the occupied space and is exhausted.

This ventilation efficiency has a maximum value of 100%, corresponding to no bypassing (s=0). One can measure its value by injecting a tracer gas into the

supply air such that the exhaust air concentration builds up before the

concentration in the occupied zone. While this may be difficult in some cases,

if it is possible then one can use the decay in exhaust air concentration to

determine n. While this model accounts for recirculation, it neglects flow

between the occupied zone and the rest of the building and its value will be

influenced by air leakage through the building envelope.

4.2 Age Distributions

Age distributions and residence times have been used in the analysis of fluid

flow through chemical reactors to quantify departures frcm perfect mixing and

piston flow (Danckwerts 1 953 ) • Sandberg (1983) and others (Skaret and Mathisen

1985) have employed these concepts in the study of ventilation airflow. The age

of a fluid element is defined to be the time that has passed since the element

entered the space. Three populations of fluid elements of interest are defined:

the total internal population of all elements within the space, the local

7
internal population at some point within the space, and the external population

of elements leaving the space. Each population is characterized by a cumulative

age distribution F(t), which is the fraction of the population with an age less

than or equal to t. A frequency distribution f(t) is defined as the derivative

of F(t). Sandberg describes how to measure some of the age and frequency

distribution functions of the three populations described above using tracer gas

methods. These measurement procedures are reproduced in table 1 (Sandberg

1983). F anf f are the age distribution and frequency distribution,

respectively, of the fluid elements leaving the room, while $ and <P are the age

and frequency distributions of the population within the room.

One defines moments of these frequency distributions for use in the analysis of

air quality and ventilation effectiveness. The nth moment of the frequency

distribution is

i[n) = /t°f (t) dt jq r 00 = n /

t^ n”^ [l-F(t)] dt (n>0).

(7a)

(7b)

The first moment y£^ is equal to the mean age for the population described by

the frequency distribution f(t).

Sandberg (1983) defines a "relative air-diffusion efficiency" <e > according to

<e a>

= T n

/v>

( 1 )

( 8 ) y£^ is the room average of the internal air age, and x n is the inverse of the

air exchange rate of the space. One may also define an air-diffusion efficiency

at a point J £- j by substituting the average internal air age at point J y,H'

into eq (8). For the idealized case of perfect mixing, yi equals x n , and the

air diffusion efficiency is one. For pure piston flow, <e

g> equals 2.0. Skaret

and Mathisen (1985) define an air exchange efficiency which is equal to <e g>

divided by 2.0. They characterize local conditions with a local air exchange

indicator

/ y

( 1 ) (9)

is less than the room average age

3tJ that location, then has a value g

( 1 ) If the local interior air age y ( generally favorable situation atJ that location, then e g

has a value greater than

one. Another quantity of interest is the mean residence time of an internally

generated contaminant leaving the room y^*'. This quantity can be determined in

several ways, and compared to x R to evaluate ventilation system performance.

As in the case of concentration efficiencies, age distribution analysis is based

on the assumption that air only enters through supply inlets and only leaves

through exhaust outlets. Additional inflows and outflows complicate the

situation, and it is not clear hew the interpretation of the measured results

should be modified. Also, existing tracer gas measurement techniques assume

that the ventilation system operates under conditions of 100% outside air intake

(no recirculation of return air). The various ages are still useful concepts,

but their measurement under recirculation is more difficult.

8
4.3 Multi-Zone Approach

We have reviewed ventilation efficiencies based on measured concentrations and

on residence times. These concepts are straightforward to apply and interpret

for spaces in which air flows only through the supply and exhaust air vents. In

real office buildings, however, there are many more airflows than the four shown

in figure 3. In the airflow schematic shown in figure 7, zone #1 can be

considered to be the occupied portion of a room and #2 the ventilation zone,

corresponding to case A in figure 3. Zone #3 is the rest of the building, and

the background is the outside. There are three zones exchanging air with each

other and the outside, for a total of twelve airflows.

Multi-zone airflow analysis techniques have been discussed previously, along

with the use of multiple tracer gases to determine all the various airflow rates

(Sinden 1978, I'Anson et al 1982, Dietz et al 1984 ). In the multi-zone

techniques which we consider, the building is divided into three zones, as shown

in figure 7, and a different tracer gas is released in each zone. If tracer gas

A is released into zone 1, tracer B into zone 2 and C into 3, then the

concentration of tracer gas A in each zone is described by the following

equations

Qji is the air flow rate from zone i to zone j, mx is the injection rate of

tracer gas x, and C

ix is the concentration of tracer x in zone i. There are similar sets of equations for tracer gases B and C.

In a steady-state application, there is constant injection of a different tracer

gas into each zone. After a sufficiently long period of time, one measures the

equilibrium concentration of each of the three tracer gases in each of the three

zones. From these measured concentrations and the injection rates, one determines the airflow rates shown in figure 7. In another technique, a pulse

of a different tracer gas is released into each zone and the subsequent tracer

concentrations are monitored. From the measured concentration and eq (10), one can solve for all the airflows in figure 7. These techniques involve more

complex tracer gas concentration measurement and injection procedures, but they

provide a complete description of the airflows involved.

dC 1A/dt = mA

- C 1A (Q10 + Q 12 + $13) + C2AQ21 + C3AQ31

dC^/dt = - C2a (Q2o + q21 + Q23^ + C 1AQ 12 + C3AQ32

dC 3A/dt = “ C3A (q30 + q31 + q32^ + C 1AQ 13 + C2AQ23* (10c)

(10b)

(10a)

9
5. EXPERIMENTAL TECHNIQUES

The following section describes the experimental application of the ventilation

effectiveness definitions described above. The various techniques are discussed

with reference to isolated zones, large zones, and whole buildings. An isolated

zone is a room or space with its own supply and exhaust vents, and is enclosed

by walls but not further divided by walls. Such a zone may exchange air with

the outside through windows and other leakage sites, and with the rest of the

building through doorways and other openings. Isolated zones without air

leakage are used in most of the previous experimental work done in test rooms. Large zones consist of several offices or isolated zones, and are generally

subdivided by permanent partitions. They may be served by one or more air

handlers, and they may share these air handlers with other zones. An example of

a large zone is an entire floor of a building. All of these techniques are based on the assumption that there is no recirculation of return air.

5.1 Concentration Efficiencies

The following experimental techniques are designed to measure the concentration

efficiencies discussed in section 4.1.1. The first technique involves the

steady-state relative ventilation efficiency ej employing a two-zone mc>del (case

A in figure 3) and a constant injection of tracer gas at a known rate m into the

working space. To simulate a diffuse pollutant source, one should employ a diffuse tracer gas release, such as a well-distributed, multi-point injection.

If one is simulating the release of carbon dioxide or odors from people, then

the use of single injection points is appropriate. The number of injection

points should be based on reasonable levels of occupant density. The simulation

of a diffuse pollutant release, for example from carpets, requires a more

homogeneous tracer gas source.

The lower, occupied portion of the room is characterized by the tracer

concentration Cj, and the upper zone by the exhaust concentration C

e

. This

situation has been analyzed by Sandberg (1983). The steady-state concentration

in the occupied zone is

Cj = ((0+1) / 0) mV/Q, (11)

the steady-state concentration in the exhaust is

Ce

= mV/Q, (12)

and the steady-state relative ventilation efficiency is

ej = C

e

/ Cj = fV( 3+1). (13)

0 is the mixing parameter discussed in section 4.1.2. From measurements of

these two steady-state concentrations, the injection rate m, and the room volume

V, one can estimate the ventilation efficiency the mixing parameter 3, and

the airflow rate Q. These measurements are conducted with the HVAC system

running at 100% outside air so that the supply air concentration is zero. In

general, there will be several exhaust air vents serving the test space and they

should all be monitored. To avoid errors due to fluctuations in the

concentrations, average concentrations can be obtained with air sample

containers. To obtain tracer concentrations within the range of one’s analysis

equipment, for the injection flow rates which can be accurately measured, a

10
dilute mixture of tracer gas may be required. The amount of dilution can be

estimated from eq (12) assuming reasonable values for the air exchange rate and

the tracer injection rate and concentration. Since the room being tested will

exchange air through locations other than the HVAC air terminals, the estimates

of Q, 8 and e-j will not be exact. These additional airflows will decrease both

C.: and C

e by unknown amounts and may increase or decrease the measured value of

e'L Some of the additional air will flow in from outside while the rest will be

from other locations within the building. To keep such concerns to a minimum,

this measurement procedure is best applied to zones which are as isolated as possible from the rest of the building and the outdoors. To measure in a large zone or whole building, one must release the tracer uniformly throughout

the entire occupied space. Such an injection strategy may be difficult.

The measurement of the transient ventilation efficiencies defined by eq (4) and

(5) requires a uniform tracer gas concentration throughout the space in

question. This initial condition can be achieved by injecting tracer into the

supply duct at a constant rate and waiting until all locations in the space are

at the same concentration. The test begins at t=0 with an essentially uniform

tracer distribution within the space and the HVAC system running at 100% outside

air. One then monitors the decay in tracer concentration within the space Cj

and in the associated exhaust C

e

.

After a sufficient period of time, the concentrations at all locations will

decay at the same rate. The transient relative ventilation efficiency is then

given by eq (4)

et

= Ce

(t) / Cj(t). (4)

In this case C^(t) should be the average concentration for all exhausts,

although the differences among the separate exhaust vents may be of interest.

Cj can be measured at several locations within the space to obtain information

on local variations in ventilation effectiveness. After a sufficient period of

time, the efficiency in eq (4) will have a constant value except for random

variations. To avoid errors due to such fluctuations, average concentrations

can be measured at each location using air sample containers which are filled

over an appropriate length of time. If this averaging period starts at t=0 and

ends when all concentrations have decreased to zero, the ratio of the average

concentrations in the exhaust and at point J is equal to e^. in eq (5). This

technique can be applied simultaneously to isolated zones and to larger zones.

In determining the ventilation efficiency for a given zone, one uses the exhaust

air concentration for that zone. Since the injection strategy is sirppler for

measurements of the transient efficiencies, and since the value of is related

to the steady state efficiency ej, it may be more appropriate in large zones to

measure the transient effj.ciency

J than the steady-state efficiency. The fact

that the measurement of e^. begins with a uniform tracer gas concentration

throughout the space means that it is similar in value to the steady-state

ventilation efficiency e* for a pollutant source which is uniform throughout the

space.

J

5.2 Age Distributions

There are several ways to characterize ventilation effectiveness using age

distribution techniques. The techniques vary in the tracer gas injection

procedures and in the concentration measurement locations. Four different

experimental procedures are outlined in table 2 and discussed below.

11
The first experiment involves the injection of tracer into the room at a constant rate m to simulate a pollutant source. The mean residence time of the

contaminant leaving the room can the be computed according to (see table 1)

*”» T t

= F(t) ] dt r °° 0

/

t 1-Ce (t)/(m/Q) 3dt,

•'n (14)

is referred to as the "turnover” time of the contaminant. After a sufficiently long period of time T, C

e (t)/(m/Q) will approach one andyy^

be determined by

T

yp } = T - [ fQ

Ce

( t)dt ]/Ce

(~).

can

(15)

CA°°) is the equilibrium concentration of tracer in the exhaust duct and equals

m/Q. Thus, from the values of C

e

(°°) and m, one can determine the ventilation

airflow rate Q. The integral in eq (15) is simply the average exhaust air

concentration from 0 to T, and can be determined with an air sample container.

By relating the turnover time to the turnover time under conditions of

perfect mixing, T n

= V/Q, one obtains a pollutant removal efficiency

£ r=VV (16)

Values of c r greater than one correspond to more effective pollutant removal

than would occur under conditions of perfect mixing. Because of additional

airflow through the room, rti/Ce

(°°) will equal the ventilation airflow Q plus

other incoming air leakage. There is no way to distinguish between the supply

airflow and air leakage from outside or from other rooms. Application of this

experiment to a large zone or whole building would require one to inject the

tracer gas uniformly throughout the entire space, which may be difficult.

In the second experiment a pulse of tracer gas m is released into the room to

again determine yy'. In this case,

41>= jftce (t)/(m/Q) dt. (17)

This integral must be determined numerically from measurements of exhaust

concentrations. Exhaust air concentrations made roughly every minute should be

sufficient to evaluate the integral in eq (17), however one still needs to know

the value of Q to determine y|^ and t Q may be determined from the equality

of (m/Q) and the integral of C

e over time. Again, the value of Q determined in

this way includes airflows other than that through the ventilation system.

Application of this experiment to a large zone would be extremely difficult

because it would require the synchronization of the tracer gas pulses in each of

the constituent zones. Thus, this second procedure is probably most appropriate

for a smaller zone which is as isolated as possible from the rest of the

building and the outside.

The third experiment is designed to measure the room average of the internal air

age yy' and the average internal air age at a point J yy. by injecting tracer

at a constant rate m into the supply air duct, yy' is determined by measuring

the exhaust air concentration according to

/

oo t[1- C

e (t)/(m/Q)] dt. (18)

12
The integral must be determined numerically from measurements of C0

(t).

equilibrium exhaust

to determine Q.

^ — — —- 1—

<e > according to eq (8). The local mean internal air age can be

determined by measuring the concentration at point J Cj (t ),

The

air concentration C be used

e

(») is equal to (m/Q), and can " can then be used to determine the air-diffusion efficiency

( 1 ) /

'oo C1- Cj (t)/(m/Q) ] dt. (19)

3 -o This integral can be evaluated by using an air sample container which is filled

until equilibrium is established. Thus, one can determine by monitoring

the exhaust concentration during the test, and determine yiy at several

locations from air sample containers filled during the test. This technique is

applied to a particular zone by injecting the tracer into the supply duct for

that zone.

The fourth experiment yields the same information as the third by turning off

the tracer gas injection after equilibrium has been reached during the previous

experiment. During this "step-down" or decay test,

co y^ = (1/t ) ft Ce (t)/C

e

(0) dt. (20)

J Q

and

oo 4

1) = ( 1/Cj ( 0)

) /

C

J (t) dt. ( 21 )

The value of x n in eq (20) can be determined from the values of C

e

(o°) and m

before the decay begins. Equation (20) requires continuous monitoring of C

Q to

be evaluated, while eq (21) can be evaluated through the use of air sample

containers. Skaret and Mathisen (1985) point out that the results obtained in

the decay regime are probably more relaible than those obtained during the

concentration buildup when building envelope air leakage occurs. This is

because when there is leakage not all of the incoming air can be tagged with the

tracer gas.

All of the above age distribution measurement techniques are complicated by the

existence of airflows other than those through the ventilation system. Because

of these other airflows, the equations used to determine Q are no longer

appropriate.

5.3 Multi-Zone Approach

The single tracer measurement techniques are complicated by the fact that the

spaces are subject to airflows other than those through the supply and exhaust

air vents. These additional airflows prevent straightforward relations between

the tracer gas measurements and the airflows of interest. Multiple tracer gas

techniques can be used to overcome the limitations of the single tracer

techniques, although they involve more complex tracer gas measuring equipment.

Figure 7 is a schematic of a three-zone model of the ventilation of a single

office. The office itself is divided into two zones. The lower, occupied zone

is #1 and the upper zone is #2. The rest of the building interior is referred

to as zone #3. Each zone is assumed to be uniformly mixed. There are a total

of twelve different airflows between each of the zones and the outside, and

these flows characterize the ventilation of the office in question. The

measurement of these different airflows with several tracer gases is discussec

13
earlier. The details of injection and sampling for each zone depend on the

building's floor plan and air distribution system. Regardless of which tracers

are used, for each gas and each zone there is a tracer gas mass balance, for a total of nine equations. There are also three independent mass balances of the

airflows Q-y, providing twelve equations to determine the twelve airflow rates

shown in figure 7. The values of Qjj are not used in existing definitions of

ventilation effectiveness, but they provide a complete characterization of the

ventilation of the office in question.

6. CONCLUSIONS

We have reviewed several approaches to evaluating ventilation effectiveness in

mechanically ventilated offices and office buildings. Two basic approaches

which employ a single tracer gas, concentration efficiencies and age

distribution measurements, were examined in detail. The steady-state relative

ventilation efficiency is in general most appropriate to isolated zones,

while the transient ventilation efficiencies, e ^ and e^, can also be applied in

larger zones and whole buildings. Four different age distribution measurement

procedures are described. The first two yield values of the pollutant removal

efficiency e r and are most appropriate to isolated zones. The third and fourth

procedures provide estimates of the air diffusion efficiency ^^and the local

air exchange indicator e 3and can be applied to large zones and whole

buildings. These single tracer approaches have been applied to test rooms in

which air enters and leaves only through well defined locations.

Real offices have additional uncontrolled air leakage through windows and doors

to the outside and adjoining offices. These additional airflows complicate the

application of concentration and age distribution efficiencies such that

airflows and other quantities of interest cannot be measured. Ventilation

effectiveness can still be measured, but the results no longer the ventilation

system performance. Regardless, single tracer measurements of ventilation

effectiveness are useful because of their simplicity, and in some cases may

provide an indication of what is occuring in the ventilated space. To

completely characterize the airflows in and out of an office space, multiple

tracer gas techniques must be used. These techniques are more complex, but they

provide all the information required to evaluate the ventilation of an office

space.

7. ACKNOWLEDGMENTS

This research was supported by the U.S. Department of Energy through an interagency agreement with the National Bureau of Standards.

14
8. REFERENCES

American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.,

ASHRAE Handbook o£ Fundamentals. , 1985.

American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

,

ASHRAE Handbook. Systems . 1984.

American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

,

"Ventilation for Acceptable Indoor Air Quality, ASHRAE Standard 62-1981,

1981.

Danckwerts, P.V. , "Continuous Flow Systems. Distribution of Residence Times,"

Chemical Engineering Sgiensfiy Vol.2, 1953.

Dietz, R.N. , Goodrich, R.W. , Cote, E.A. , "Performance of a Passive

Perfluorocarbon Tracer System for Building Ventilation and Air Exchange

Measurements," ASTM Symposium on Measured Air Leakage Performance of

Buildings, Philadelphia, 1984.

Grot, R.A. , "The Air Infiltration and Ventilation Rates in Two Large Commercial

Buildings," Proceedings of ASHRAE/DOE Conference on Thermal Performance of

the Exterior Envelopes of Buildings II, Las Vegas, 1982.

Grot, R.A. , Hunt, C.M. , Harrje, D.T., "Automated Air Infiltration Measurements

in Large Buildings," in Ain Infiltration Instrumentation and. Measuring

Techniques proceedings of the First Air Infiltration Centre Conference,

Bracknell, UK, 1980.

Grot, R.A. , Persily, A.K. , "Air Infiltration and Airtightness Tests in Eight

U.S. Office Buildings," in Ain Infiltration Reduction in Existing Bu ild ings

proceedings of the Fourth Air Infiltration Centre Conference, Elm,

Switzerland, 1983.

Harrje, D.T. , Janssen, J.E. , "A Standard for Minimun Ventilation," in The

Implementation Ain Infiltration Standards in Buildings proceedings of

the Fifth Air Infiltration Centre Conference, Reno, Nevada, 1984.

Hunt, C.M. , "Air Infiltration: A Review of Some Existing Measurement Techniques

and Data," in Hunt, C.M. , King, J.C. , and Trechsel, H.R. , eds. , Building

Air Change Rate and Infiltration Measurements . STP 719, American Society

for Testing and Materials, 1980.

I’Anson, S.J. , Irwin, C. , Howarth, A.T. , "A Mulitple Tracer Gas Technique for

Measuring Air-Flows in Houses," Building and Environment . Vol.17, 1982.

Jennings, B.H. , Armstrong, J.A. , "Ventilation Theory and Practice," ASHRAE

Transactions , Vol.77(I), 1971.

Kusuda, T., "Control of Ventilation to Conserve Energy While Maintaining

Acceptable Indoor Air Quality," ASHRAE Transactions . Vol.82(I), 1976.

Lidwell, O.M. , "The Evaluation of Ventilation," Journal of Hygiene . Vol.58,

1960.

15
Maldonado, E.A.B., Woods, J.E. , "Ventilation Efficiency as a Means of

Characterizing Air Distribution in a Building for Indoor Air Quality

Evaluation," ASHRAE Transactions. Vol. 89(H), 1983.

Malmstran, T. , Ahlgren, A., "Efficient Ventilation in Office Rooms," Environment

International . Vol. 8, 1982.

Nevins, R.G. , Air Diffusion Dynamics . Business News Publishing Company,

Birmingham, Michigan, 1976.

Offermann, F.J. , Fisk, W.J. , Grimsrud, D.T. , Pedersen, B., Revzan, K.L.

,

"Ventilation Efficiencies of Wall-or-Window Mounted Residential Air-to-Air

Heat Exchangers," ASHRAE Transactions , Vol. 89(11), 1983.

Sandberg, M. , "What is Ventilation Efficiency?," Building and Environment , Vol. 16, No. 2, 1981.

Sandberg, M. , Blomqvist, C. , Sjoberg, M. , "Warm Air Systems. Part 2. Tracer Gas

Measurements and Ventilation Efficiencies," The National Swedish Institute

for Building Research, Bulletin M82:23, 1982.

Sandberg, M. , "Ventilation Efficiency as a Guide to Design," ASHRAE

Transactions . Vol. 89(11), 1983.

Sinden, F.W. , "Multi-Chamber Theory of Air Infiltration, " Building and

Environment . Vol. 13, 1978.

Skaret, E. , Mathisen, H.M. , "Ventilation Efficiency." Environment International . Vol. 8, 1982.

Skaret, E. , Mathisen, H.M. , "Ventilation Efficiency - A Guide to Efficient

Ventilation," ASHRAE Transactions. Vol. 89(11), 1983.

Skaret, E. , Mathisen, H.M. , "Test Procedures for Ventilation Effectiveness Field

Measurements," proceedings of the International Symposium on Recent

Advances in the Control and Operation of Building HVAC Systems, Trondheim,

Norway, 1985.

Straub, H.E. , Gilman, S.F. , Konzo, S. , "Distribution of Air Within a Room for

Year Round Air Conditioning. Part I," University of Illinois Engineering

Experiment Station Bulletin, No. 435, 1956.

Straub, H.E., Chen, M.M. , "Distribution of Air Within a Room for Year Round Air

Conditioning. Part II," University of Illinois Engineering Experiment

Station Bulletin, No. 422, 1957.

16
9. APPENDIX: EXPERIMENTAL DETAILS

In this section, experimental details are presented for ventilation

effectiveness measurements. These details include tracer gas injection rates

and strategies, sampling locations and other information, and are presented for

isolated zones, large zones, and whole buildings. A multi-tracer measurement

procedure ia also discussed.

A1. Isolated Zone

The details of ventilation effectiveness measurements in an isolated zone are presented for the rocm shown in figure A1. This is a small office (53.1 nP)

with a supply vent located under the window and an exhaust vent located above

the door. The exhaust vent is simply a grill that opens into the hallway, which

is exhausted through a return duct. In the experiments described below, the

door is assumed to be closed. Without making the measurements, we do not know

the ventilation flow rate through the room. This information is sometimes

available from mechanical equipment specifications, but these can be different

from the actual airflow rates. We assume the room is ventilated at 3 air

changes per hour, which equals 0.044 nP/s.

Concentration Efficiencies

To measure the steady-state relative ventilation efficiency, we use the two-zone

approach outlined earlier. The tracer gas, sulfur hexafluoride (SFg), is

injected at a constant rate m into the working space. The injection rate can be

estimated from eq (12) assuming a steady-state exhaust concentration C

e equal to

100 ppb. As stated above Q = 0.044 nP/s, and therefore m = 0.26 cc/min of SFg.

This is a very low flow rate to measure and control. If one uses a 0.1% mixture

of SFg in air, then the injection flow rate is 260 cc/min, which is much more

manageable. If the actual ventilation flow rate is very different from the

assumed value, one can adjust m as needed. If one wishes to inject the gas

diffusely within the occupied zone, this can be done with a perforated tube

running across the floor of the room, as depicted by the dotted line in figure

A1. If one is simulating a pollutant emitted by people, than a single point

source for each person is appropriate. After the injection starts, one monitors

the SFg concentration at the exhaust vent to determine when steady-state

conditions have been achieved. Once equilibrium exists, one fills several air

sample containers at table-top or breathing zone height to obtain values of the

steady -state concentration at several locations in the occupied zone C • . These

containers can be filled from outside the room through tubes which run

J under the

door to avoid disturbing the airflow patterns within the room. The containers

can be filled over a period of time on the order of 10 minutes to obtain average

values of Cj. During this same period, one monitors C

Q to determine its average

value. From C

e , m and the room volume V, one determines the ventilation flow

rate Q from eq (12). For each value of Cj, one can determine a mixing parameter

3 and a relative ventilation efficiency ej from eq (11) and (13). Because there

will be airflow in and out of the room through windows, doors and other leakage

locations, the estimate of Q will not be exact, nor will the estimates of 6 and

e r

To measure either of the transient ventilation efficiencies e t or one must

start with a uniform tracer concentration within the room. This can be achieved

by injecting SFg into the supply vent at a constant rate. To begin with 200 ppb

throughout the roan, one can determine the injection flow rate m by setting rn/0

17
equal to 200 ppb. For Q equal to 3 air changes per hour, the injection rate is

0.53 cc/min. Again, with a 0. 1% mixture of SFg in air, the injection rate is

530 cc/min. After the injection is started, C

e should be monitored to determine

when steady -state has been achieved. At this point, the injection is stopped

and the measurement begins. To determine the integral ventilation efficiency

E£, one needs the average value of Ce from t=0 until C

e has decreased to zero. The average values of C and Cj over this period can be obtained using air

sample containers. To determine the instantaneous ventilation efficiency e j., one needs average values of Cg and Cj for shorter time periods, after the

concentrations have begun to decay at the same rate.

Age Distribution

No. 1

The first experiment involves the injection of tracer into the room at a constant rate m to simulate a pollutant and to determine the mean residence time

of tine contaminant leaving the room yV'. In this experiment, one can use the same injection arrangement for measuring the steady-state relative ventilation

efficiency and the same dilute mixture of SFg. One monitors the exhaust

concentration C

e starting when the injection starts and ending when steady-state

is achieved. The value of y£'' is calculated over the monitoring period from eq

(15). A sample container is used to obtain an average value of C

0

, but one must

still monitor C

Q during the test to assure that equilibrium has been

established. To determine the pollutant removal efficiency, one also needs the

value of x n

= V/Q. can be determined frcm the equality of C

e

(°°) and m/Q.

No. 2

This experiment involves the release of a pulse of tracer gas into the room to

simluate a pollutant and again yields a value of y> . One monitors C

e throughout the test and determines y£" from eq (17). To obtain a value of Q

for determination of one employs the equality of m/Q to the area under the curve of C

e vs time. One can obtain an approximate value for the tracr gas

pulse volume by assuming C

e

= 25 ppb and that it takes two hours for C

g

to

decrease to zero. In this case, m = 8 cc of SFg. This gas should be released

over a time period which is very short compared to = 20 min. Choosing a 30

sec injection time, the injection rate must be 16 cc/min. One must be certain

to get all of the tracer into the room.

No. 3

In the third experiment, one injects tracer into the supply duct at a constant

rate m to obtain estimates of yv', the room average interior air age, and y^y,

the average internal air age at a point J. y£" is determined yith eq (18) and

requires continuous monitoring of the exhaust concentration. y£y can be

determined at several locations in the occupied zone by filling Sample

containers until C

e attains its equilibrium value (eq (19)). In this experiment

one can use the same injection rate, 260 cc/min, and the same dilution mixture

of SFg, 0.1%, as in the determination of ej. To determine T n one requires Q,

which can be obtained frcm the equality ofJ C

e

(°°) and m/Q.

18
No. 4

This experiment takes place during the decay in concentration after the

injection in No. 3 is stopped. is determined from continuous monitoring of

the exhaust Ce and eq (20). The values of are obtained by filling

containers in the occupied zone and using eq \21).

A2. Large Zone

The large zone for which ventilation effectiveness measurements are described

below is shown in figure A2. It is a large office area with permanent walls

enclosing the area shown in the figure. The left and bottom walls are on the

outside of the building and have windows where noted. The volume is 327 nr. There is a 1 m high return air plenum above the ceiling, which contains the

supply ductwork. The room is ventilated by six 1.5 m slots, 0.6 m of which

provide supply air. The other 0.9 m is a return air vent which simply opens

into the return air plenun above the dropped ceiling. Assuming that the zone is

ventilated at 3 air changes per hour, then Q equals 0.27 nr/s and the value of

t n for the zone is 20 min.

Concentration Efficiency

The steady-state ventilation efficiency measurement is very similar to the one described above for a single room, except this large zone has a more complex

exhaust system. The tracer gas injection rate rh required to yield a steadystate exhaust concentration of 100 ppb is 0.26 cc/min. Therefore, a 0,tf>

mixture of SFg in air will require an injection rate of 260 cc/min. One

possible injection strategy is indicated in figure A2, where perforated tubes

(denoted by dotted lines) are distributed around the room. Other injection

schemes may be used, depending on the nature of the pollutant one is trying to

simulate. Six workplace sampling points are shown for the measurement of Cwith sample containers. The six exhaust vents also need to be sampled

individually with containers and the resultant concentrations averaged to

determine C

e for the zone. One cannot simply measure C

e in the plenum space,

because such a measurement will in general not provide a representative value of

C

e for the space. One should monitor some of the exhaust concentrations during

the test to determine when steady-state exists, and when to begin filling the

sample containers.

To measure the transient ventilation efficiency one needs to begin with a uniform concentration of tracer throughout the zone. An initial concentration

of 200 ppb can be obtained by injecting SF^ at a constant rate into the main

supply duct and waiting until the tracer gas concentration is uniform within the

space. In this case, the SFg injection rate is based on the airflow rate

through the supply duct. By monitoring the concentration at the various

workplace locations, one can determine when a uniform tracer gas concentration

has been achieved. At this point the supply duct injection is stopped, and the

measurement begins. During the experiment one samples the concentrations at all

six exhaust vents and at workplace locations of interest. To measure the

integral transient ventilation efficency e^., one fills air sample containers at

the different locations, starting when the injection stops and ending when all

the concentrations decrease to zero. The instantaneous transient ventilation

efficiency is determined from short term average concentrations at these same locations after the tracer gas decay rate is uniform at all the locations.

19
Age Distribution

No. 1

In this test one injects tracer at a constant rate to simulate a pollutant

source in the occupied space using the same flow rate and dilute SF5 mixture

that was suggested for measuring the steady -state ventilation efficiency. One

monitors several of the exhaust concentrations during the test to determine when

steady -state has been achieved. During this period one fills air sample

containers from the exhaust, starting when the injection begins and ending after

steady-state exists, to determine the average exhaust concentration during the

period. As in the case of ej, it may be difficult to inject the tracer

uniformly throughout larger zones with complex layouts.

No. 2

In this test one releases a pulse of tracer gas into the occupied zone, and can

use the injection scheme already mentioned. Assuning an average SF5

concentration of 25 ppb over a 2 hour period and an airflow rate through the room of 0.27 nw/s, one obtains a tracer pulse volume of 50 cc. This experiment

requires continuous monitoring of the exhaust concentration, and since this zone has six exhaust vents, this may present a problem. It takes one minute to

measure the SFg concentration with electron capture detectors, therefore each

exhaust location can be sampled only once every six minutes. If this

measurement frequency is not sufficient to evaluate the integral in eq (17), one may sample a mixture of the air from all six exhaust locations. The value of Q

is determined from the equality of m/Q and the integral of C

e over time.

No. 3

This experiment requires a constant injection of SFg into the supply duct at the same rate and dilution as in the measurement of the steady-state relative

ventilation efficiency. The injection must be made into the supply duct before

it branches off to the six supply vents for this zone. However, if this supply

duct also serves other areas then this injection location is inappropriate. In

addition, the tracer gas must be well-mixed with the supply air before it

reaches the supply vents. If a well-mixed injection can be made into the supply

duct of this room, then one continuously monitors the exhaust air concentrations

to determine the room average internal air age from eq (18). One uses air

sample containers to determine local mean internal ages at locations within the

workplace using eq (19). The exhaust concentrations are monitored and the

containers are filled until equilibrium is attained in the zone. The value of Q

is determined from the fact that C

e

(°°) = m/Q.

No. 4

This experiment is the decay in concentration after the injection in experiment

No. 3 is stopped. Continuous exhaust concentrations and average workplace

concentrations are measured as in experiment No. 3. The measurement continues

until all the concentrations decrease to zero.

20
A3. Whole Building

To demonstrate the techniques which can be used for whole building ventilation

effectiveness measurements, we consider an office building with a volume of

about 1.25 x 1(F m’. The whole building is served by a single supply fan that

feeds into a large supply duct that handles all the floors. The return air from

each floor flows into a large return duct that flows up to a return air fan.

The building employs ceiling return air plenums.

Concentration Efficiency

The measurement of the steady-state ventilation efficiency on a whole building

scale is not discussed due to the potential difficulty of injecting the tracer

gas uniformly throughout all the workplace locations throughout the building.

This measurement is generally practical only for individual offices or large

zones of manageable size.

To achieve the desired initial conditions of uniform tracer gas concentration

throughout the building for measuring the transient relative ventilation

efficiency, one must inject tracer at a constant rate into the supply air duct

for the whole building. At 3 air changes per hour, the injection rate should be

about 1.25 1/min to obtain a concentration of 200 ppb. By sampling the air at

each floor's opening into the return air duct, one can determine when steadystate exists. At this point the tracer gas injection is stopped and the

measurement begins. One samples workplace and exhaust concentrations for any

office or zone as described earlier using sample containers. If one has

sufficient containers, and can control all of them, one can determine an average

floor concentration and an average transient ventilation efficiency for the

floor. Similarly, a total building average transient ventilation efficiency can

be determined from an average concentration determined over the total building

workplace. Some questions exist concerning timing since after the tracer

injection is stopped in the penthouse, it takes different amounts of time for

the 0 ppb supply air to reach different floors. One could account for this

delay when filling the sample containers on different floors, but the delay time

is short compared to the characteristic time of the system, and therefore this

is probably an insignificant effect.

Age Distributions

No. 1

This experiment requires a constant, uniform injection of tracer gas throughout

the building workspace which may be difficult to accomplish, as mentioned for

the steady-state ventilation efficiency. Therefore, this experiment is

generally inappropriate for a whole building.

No. 2

As in experiment No. 1, it would be very difficult to inject a pulse of tracer

uniformly throughout a building at one time. Again, this experiment is probably

inappropriate for a whole building.

No. 3

This experiment requires a constant injection of SF^ into the main supply duct

21
at about 0.6 1/min to obtain an equilibrium concentration of about 100 ppb. One

can determine the average internal air age for the building by monitoring the

building return air concentration using eq (18), and the same age for a floor

from the floor return air concentration. Again, these exhaust concentrations

must be monitored continuously, and the sampling frequency required to evaluate

the integral in eq (18) determines how many floors can be studied in one experiment. Also, the value of jA'' for each floor can only be compared to the

whole building tn because one does not know the value of m for individual

floors. Local mean internal ages can be determined with eq (19) using air

sample containers filled at workspace locations, starting at the injection time

and ending when the equilibrium concentration has been obtained.

No. 4

This test is simply the decay in concentration after the injection of No. 3 has

stopped. One must measure the floor and whole building exhaust (return)

continuously to determine lA*' using eq (20). Again, one must use xn for the

whole building from No. 3 since one cannot determine xn for individual floors.

The values of for workspace locations are determined from containers filled

starting when the injection stops until the concentration decreases to zero.

A4. Multi-Tracer

In this section an example is outlined of a multi-tracer experiment involving

the zone in figure A2 as part of the whole building discussed in section A3.

This experiment involves three zones, as shewn in figure 7, in which zone #1 is

the workspace of the test area and zone #2, referred to as the ventilation zone,

corresponds to the upper zone of a two-chamber model of an office area. Zone #3

refers to the rest of the building. In this experiment a different tracer gas

is injected at a constant rate into each of the three zones and the equilibrium

concentrations of each gas in each zone are measured. Due to the large building

volumes and the cost of the tracers, measurements in the ppb range are desirable. Various refrigerants can be used as tracers with analysis times on the order of five minutes per sample. A wide range of measurable concentrations

are required, particularly for tracers A (zone #1) and B (zone #2). Because the

volume of zone #3 is in general much larger than #1 and #2, the concentrations

of tracers A and B will be much lower in zone #3 than in the other two zones.

The injection and sampling arrangements for this test will depend on the zone

volumes, the ventilation system design, and the building zoning arrangement.

The injection of tracer gas A into zone #1 should be a diffuse arrangement as discussed earlier. It is difficult to estimate the injection rate without

knowing the airflow rates which are being measured. An estimation can be made

based on the total room volume (327 nw) and an assumed ventilation rate of 3 air

changes per hour. If the maximum measurable concentration of tracer A is 500

ppb, then the injection rate should be 1.3 cc/min. The actual concentration in zone #1 will be different than 500 ppb due to two opposing influences. Mixing

between the room and the rest of the building will tend to decrease the

concentration, while a lack of mixing between zones #1 and #2 will tend to

increase the zone #1 concentration. The air samples in zone #1 should be taken

at occupied locations of interest which are not too close to the tracer gas

injection points.

Tracer gas B is injected in zone #2 and should be released into the supply air

stream. The injection rate should be estimated just like the injection rate of

22
tracer gas A, but using the maximum measurable concentration of tracer B. This

injection can take place in the supply duct before it splits off into the six

outlets which serve the room, but only if this duct serves no other rooms. Otherwise, the injection must be split proportionately among the six outlets.

The air for zone #2 should be sampled at the six exhaust vents, either

separately at each vent or from a mixture of air from all the vents.

The injection on tracer gas C into the rest of the building is the most

complicated. This injection should be uniform, in terms of tracer flow rate per

unit volume, throughout the building except for the room being tested. On all

the floors except for the floor containing the test room, the tracer can be

injected into the supply duct for each floor. The injection rate should be

based on the floor volume, an estimate of the air .exchange rate, and a target

concentration of about 75% of the maximum measureable concentration of tracer

gas C. On the floor containing the room, the tracer should be injected at

branches in the supply duct such that the entire floor receives the gas except

the test room. This may require several injection points, and each injection

rate should be sized according to the same target concentration and based on the

volume of the floor section served by the duct branch which is receiving the

injection. The tracers in this test can be injected using passive emitters

similar to those employed in the PFT infiltration measurement technique (Dietz

et al 1984).

After the injections are begun, one samples the air at each location to

determine when equilibrium has been established. Once equilibrium has been

achieved, an average value of each of the three tracer concentrations in each of

the three zones is measured. Using eqs (10) and their versions for the other

two tracers, one solves for the twelve flows shown in figure 7. The solution

also requires the mass balances of the airflows for each zone.

23
Table 1 Age Distributions Obtained from Different Injection Procedures

(Sandberg 1983)

INJECTION POINT SUPPLY DUCT IN THE ROOM "\LOCATI0N OF IN-ADMEASURING

JECTIOrr\. POINT

PROCEDURE |

EXTRACT DUCT IN THE ROOM EXTRACT DUCT

PULSE D [m3

] c ( T ) C (t) Cp

(T) Cp

(T) t (t) - P - p

C (T) C (t) ffrl T e

r e f( T ) * —— * —

J

C

e

( T )dT ( "/Q) oi^Cp (T)dT (m/Q)

J

Ce

( T )dT (n/Q)

STEP-UP a [m3 /s]

c (t) c ( t) f(t) * —— * —

C

e

(-) (m/Q)

C (t) Cp (t) *PF

(T) * /C

= 4

P (-) (m/Q)

C (t) C (t) F(t) * — - -2—

C

e

(«») (m/Q)

STEP-DOWN C (t)

1 - F(x) * -£

c

e

(o)

Cp

(T)

1 - *p (T) 3 c~ToT

C (t)

1 - F(t) = —

c

e

(o)

POPULATION AIR LEAVING THE ROOM LOCAL POPULATION OF AIR AT POINT P CONTAMINANT LEAVING THE ROOM

Table 2 Sunniary of Age Distribution Measurement Techniques

No. Injection Concentration Calculation Age

1 Constant rate

into room

C

e

(t) Time average up>

2 Pulse into room

Ce

(t) Integration up)

3 "Step-up"

Constant rate

into supply duct

C

e

(t)

Cj (t)

Integration

Time average

up)

* 1 ?

4 "Step-down"

(No injection)

C

e

(t)

Cj(t)

Integration

Time average

uP>

*

1 ?

= mean residence time of contaminant leaving the roan,

up) = roan average of the internal air age.

up) = mean age of the internal air at point J.

24
Exhaust air terminal

Connection between return

air shaft and ceiling plenum

Figure 1 Example of an Air Distribution Network

25
PISTON FLOW

i 1

SHORT CIRCUITING

Figure 2 Representative Examples of Ventilation Air Movement

26
Figure 3 Flew Diagrams of Two-Zone Models

27
Case D in Figure 3

Case A in Figure 3

instantaneous transient relative

ventilation efficiency

steady-state relative ventilation

efficiency with a pollution source

in the upper and lower zones

steady-state relative ventilation

efficiency with a pollution source

in the lower zone

steady -state relative ventilation

efficiency with a pollution source

in the upper zone

Figure 4 Calculated Ventilation Efficiencies for Different Pollution

Source Locations and Different Ventilation Systems

(Sandberg 1981)

28
100

e t

%

i

123 ^ n(h"J

£ t at 2.1 m

/

/

7060

50

3 U n( h ’)

£

t

at = o °c

A T = 2 °C

at = n °c

AT = 8 °C

Figure 5 Measured Values of Integral Transient Relative Ventilation

Efficiency at Different Room Heights under Various Conditions

of Supply Air Overtemperature and Ventilation Rate

(Sandberg 1981)

29
Figure 6 Ventilation System Schematic (Harrje and Janssen 1984)

OUTSIDE

30
Window

Door

TOP VIEW

Exhaust

vent

Door

3.35 m

Supply vent

SIDE VIEW

Figure A1 Example of an Isolated Zone

Window

31
J3

E2) (S2 @

El (SI

E4 (S4 E3) S3 Door

J5

S6 E5 (S5

J6

Injection

Return

Supply

Window

Supply

(E#) Return

(j#) Workplace

Figure A2 Example of a Large Zone

32
«0«M NIS-4 14A |HVII«I

U.». DIPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 85-3208 AUGUST 1985

4. TITLE AND SUBTITLE

Ventilation Effectiveness in Mechanically

Ventilated Office Buildings

5. AUTHOR(S)

Andrew K. Persily

(. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. Contract/Grant No.

NATIONAL BUREAU OP STANDARDS

U.S. DEPARTMENT OP COMMBtCE S. Type of Report & Period Covered

GAITHERSBURG, MD 20899

J. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Department of Energy

Office of Building and Community Systems

Washington, D.C. 20585

10.

SUPPLEMENTARY NOTES

| 1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Mechanical ventilation systems in large office buildings are designed to meet space

conditioning loads and to maintain acceptable indoor air quality. In order to

achieve acceptable air quality, the ventilation systems are designed to bring in a minimum amount of outside air whenever the building is occupied. In addition to

minimum outside air intake levels, there must also be adequate distribution of this

air within the building. Although the net ventilation rate of a building or room

may be sufficient, poor air distribution may lead to some areas within the space

being inadequately ventilated. The concept of ventilation effectiveness has been

developed to quantify the air distribution characteristics of a ventilated space.

This paper examines several definitions of ventilation effectiveness and associated

tracer gas measurement techniques. Techniques for making ventilation effectiveness

measurements in mechanically ventilated office buildings are discussed with reference

to building and mechanical equipment design and tracer gas instrumentation. Specific

strategies are proposed for measuring ventilation effectiveness on different scales

ranging from individual rooms to whole buildings.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s

air infiltration; building ventilation; mechanical ventilation; office building

ventilation; ventilation; ventilation efficiency, tracer gas measurement

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