Mixing Ventilation. Guide on mixing air distribution design. rehva Federation of European Heating, Ventilation and Air Conditioning Associations

Mixing

Ventilation

Guide on mixing air

distribution design

Dirk Müller (Ed.)

Claudia Kandzia

Risto Kosonen

Arsen Krikor Melikov

Peter Vilhelm Nielsen

rehva

Federation of European Heating, Ventilation and Air Conditioning Associations

Mixing Ventilation

Guide on mixing air distribution design

Dirk Müller (Ed.)

Claudia Kandzia

Risto Kosonen

Arsen Krikor Melikov

Peter Vilhelm Nielsen

This Guidebook is the result of the efforts of REHVA volunteers. It has been written with care, using the best available information and the soundest judgment possible. REHVA and its volunteers, who contributed to this Guidebook, make no representation or warran-ty, express or implied, concerning the completeness, accuracy, or applicability of the information contained in the Guidebook. No liability of any kind shall be assumed by REHVA or the authors of this Guidebook as a result of reliance on any information con-tained in this document. The user shall assume the entire risk of the use of any and all information in this Guidebook.

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Copyright © 2013 by REHVA

Federation of European Heating, Ventilation and Air Conditioning Associations www.rehva.eu

All rights reserved

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopies or any other information stor-age and retrieval system, without permission in writing from the publisher.

Requests for permission to make copies of any part of the work should be addressed to: REHVA Office, Washington Street 40, 1050 Brussels – Belgium

e-mail: info@rehva.eu

ISBN 978-2-930521-11-4 Printed in Finland by Forssa Print

TERMINOLOGY ... 1

1 INTRODUCTION TO AIR DISTRIBUTION IN SPACES ... 2

Governing equations for the description of air motion in spaces ... 2

Ventilation effectiveness and air change effectiveness ... 4

Room air distribution ... 5

Mixing ventilation ... 6

Displacement ventilation ... 9

Unidirectional (piston) ventilation ... 10

An overview of air distribution principles with a full surface supply area ... 10

The family tree of air distribution patterns based on supply opening location ... 11

Mixing and displacement air distribution patterns based on driving force ... 11

Nature of airflow in rooms ... 13

2 HUMAN RESPONSE TO AIR MOVEMENT ... 14

Air distribution and thermal comfort ... 14

Air distribution and indoor air quality ... 17

Air distribution and acoustic ... 18

Sound pressure level weightings ... 19

Air distribution and other personal related factors ... 20

3 FLOW ELEMENTS ... 22

Jet flow models ... 22

The universal velocity profiles ... 28

Velocity decay shown in log-log graph ... 29

Entrainment in a circular free jet ... 29

Transformation of jet flow in a room ... 29

Non-Isothermal Free Jets and Wall Jets ... 31

Vertical circular free jet ... 32

Horizontal three-dimensional free jet ... 32

Horizontal three-dimensional wall jet ... 33

Horizontal two-dimensional wall jet ... 33

Buoyant flow elements ... 34

Modelling of thermal plumes ... 36

Interaction of plumes ... 38

4 DIFFUSERS FOR MIXING AIR DISTRIBUTION ... 39

Air diffusers with high momentum ... 39

Air diffusers with low momentum ... 42

The flow from different air terminal devices ... 43

Guaranteeing functional requirements of diffuser ... 44

Identification of diffuser characteristics ... 45

Throw pattern of variable air flow rate ... 46

5 DIFFUSER MODELING AND BENCHMARKS FOR CFD PREDICTIONS ... 50

Boundary conditions based on flow elements ... 50

Simplified boundary conditions ... 50

The prescribed velocity method ... 51

The momentum method ... 51

Fully resolved diffusers ... 51

CFD Modeling of Complex Air Diffusers ... 52

Mixing ventilation benchmark tests ... 54

Two-dimensional flow in a room with slot inlet and mixing ventilation ... 55

Three-dimensional flow around a person in mixing ventilation ... 55

Computer simulated person - Thermal comfort ... 56

6 MIXING AIR DISTRIBUTION DESIGN ... 57

Selection of air distribution method ... 57

Selection of air diffusers ... 63

Exhaust openings ... 65

7 EVALUATION OF MIXING AIR DISTRIBUTION ... 68

Airflow characteristics ... 68

Physical measurements ... 70

Full-scale room assessment ... 70

Heat sources simulation ... 71

Pollution simulation ... 73

Air distribution measurement and assessment criteria ... 75

Scale-model evaluation of air distribution ... 78

Field measurements ... 78

8 CASE STUDIES ... 81

8.1 Dimensioning of diffusers and design of different mixing ventilation systems 81 8.2 Airflow interaction in spaces ... 84

8.3 Impact of the thermal load on the room airflow pattern ... 93

8.4 Air distribution in classroom ... 94

8.5 Airborne Cross Infection Risk between People in a Mixing Ventilated Room . 98 8.6 Examples of mixing air distribution ... 99

8.7 An experiment for transient room airflow ... 102

8.8 Experimental study on the effect of turbulence intensities on the maximum velocity decay of attached plane jet ... 106

This book has been developed by a group of experts representing wide area of expertise from HVAC industry and building management assisted by experts from REHVA office. Each person gave a personal and valuable contribution to improve the quality of the guidebook. The following experts were involved:

Authors: Dirk Müller (Ed.) Claudia Kandzia Risto Kosonen Arsen Krikor Melikov Peter Vilhelm Nielsen

Task force members: Alfred Moser Elisabeth Mundt Hazim Awbi Marcel Loomans Mats Sandberg Reviewers: Marcel Loomans Mats Sandberg Alfred Moser Elisabeth Mundt

About this guideline

Ventilation is a process and practice of keeping an enclosed space supplied with proper air for breathing. The word “ventilation” in Latin “ventilare”, originates from the word “ventus” which means wind. Apart of improving indoor air quality air supplied to spaces is used for removing or supplying heat in order to provide thermally comfortable environment for occupants. Air can be transported to spaces by use of mechanical systems or by use of natural forces. The performance efficacy of any ventilation methods depends on the air distribution in spaces. The air distribution in spaces is a major important factor for occupants’ health, comfort and performance as well as for efficient energy use. In many buildings mechanical systems are used to transport and supply clean and cooled/heated air to occupied spaces. Air distribution in such spaces and in ventilated spaces in general depends on inertia and buoyancy forces, initial conditions of the supplied flow and its turbulence structure, boundary conditions in the space, etc. Different air distribution strategies based on mechanical systems for air transportation are used today to achieve the goal of ventilation. One of the air distribution strategies is to mix the supplied clean and cool/warm air with the polluted and warm/cold air in the space. Dilution of the supplied air with the room air is

the aim of this air distribution strategy known as mixing ventilation.

At present mixing ventilation is mostly used in practice. Although it has been applied for many years the condi-tions for its optimal performance in practice are not widely understood. The existing knowledge is not struc-tured in a simple, short and understandable way for the majority of the HVAC community members. This defines the motivation in developing this book of guidelines. The objectives are to introduce the main factors important for achieving different air distribution patterns in rooms, the airflows generated by artificial and natural way in spaces and their interaction is presented. Human re-sponse to air movement and its importance for occu-pants’ comfort is summarised. The main principles and design considerations used today for design of comfort-able and efficient mixing air distribution are systemised and the methods for prediction and assessment of the environment obtained with mixing ventilation are out-lined. Case studies on mixing air distribution in rooms and vehicle compartments are presented and dis-cussed.

The authors

REHVA –

Member countries of REHVA: Belgium | Croatia | Czech Republic | Denmark | Estonia | Finland | France | Germany | Hungary | Italy | Latvia | Lithuania | The Netherlands | Norway | Poland | Portugal | Romania | Russia | Serbia | Slovakia | Slovenia | Spain | Sweden | Switzerland | Turkey | United Kingdom.

In this guidebook most of the known and used in practice methods for achieving mix-ing air distribution are discussed. Mixmix-ing ventilation has been applied to many differ-ent spaces providing fresh air and thermal comfort to the occupants. Today, a design engineer can choose from large selection of air diffusers and exhaust openings.

Energy efficiency and health has become more and more important during the last years. Buildings consume a large share of primary energy in the European Union and thus, all new and retrofitted buildings have to follow strict codes and standards con-cerning their acceptable energy demands. On the other hand research documents that indoor environmental quality has a major influence on human health and occupants’ productivity and it is not reasonable to save energy by delivering less fresh air to spac-es. Thus, an important future goal will be to ventilate occupied spaces with the lowest possible energy consumption.

Therefore air distribution methods that provide indoor environment of high quality at decreased energy use need to be devel-oped. Traditional mixing ventilation is based on the supply of momentum flow from the diffusers. New layouts as ceiling mounted large area diffusers and diffuse ceiling inlet do also create mixing flow due to buoyancy effect and can also be consid-ered as solutions for mixing ventilation (see chapter 1 in this Guidebook).

Other methods for air distribution such as impinging jet ventilation (Karimipanah and Awbi 2002), air distribution by wall con-fluent jets (Cho et al. 2004), stratum venti-lation (Lin et al. 2009), etc. are under de-velopment and may be implemented in practice. The possibility of using transient

air distribution for better control over air-flow pattern in rooms and for providing occupants with improved comfort have been studied. In Chapter 8 (case study 8.7) of this guidebook an example of transient room airflow is discussed.

Modern mixing ventilation systems shall provide exactly the right amount of fresh air to every occupant in a room. This means that volume flow rates are not constant over time and that room air distribution systems may be designed to allocate and ventilate zones of room where occupants are present. Even systems that sense and follow occu-pants moving in room may be developed. In this way air distribution system fully adapted to the strength and location of heat and pollution load may be developed.

Large differences exist between occupants with regard to preferred environment (thermal and olfactory), clothing thermal insulation and activity. Therefore the uni-form environment aimed at in rooms with total volume air distribution, such as mix-ing air distribution, may not be the optimal one. New systems based on advanced air distribution such as personalized ventilation need to be further developed, studied and implemented in buildings (Melikov 2011).

User interactive systems that allow for in-dividual control of micro-environment at each workstation are expected to be devel-oped. Air distribution systems and air sup-ply diffusers that provide each occupant with possibility for controlling airflow characteristics (direction, velocity, dynam-ics of velocity fluctuations, temperature, etc.) may be developed.

There is still a lot to be achieved in the field of HVAC!

Air change rate: The ratio of the volumetric air supply to a space to the volume of that space. It is usually measured in air changes per hour, and normally relates to the fresh air change rate. Air change efficiency c_{: Information, how}

efficient room air is replaced by supply air. Air Diffusion: The process of delivering air into the room.

Air throw: The distance an air stream travels after leaving a diffuser before its velocity is reduced to a specific value. The throw is defined for isothermal flow conditions.

Ar-number: Ratio between the buoyancy force and the momentum force.

Buoyancy: The gravity force exerted on a vol-ume of air that has a density difference to ambi-ent air.

Contaminant removal effectiveness: A meas-ure of how quickly an airborne contaminant is removed from the room. The effectiveness is determined by comparing the concentration in the exhaust with the mean concentration in the room. Draught: Unwanted local cooling of the body caused by air movements

Displacement air diffusion: Air diffusion where the mixing of supply air and room air external to the air terminal device is at a minimum.

Mixing air diffusion: Air diffusion where the mixing of the supply air with the room air is intended. The air is supplied to the room with a high inlet velocity. The result is a high turbu-lence intensity aiming to generate good mixing and uniform temperature and pollution distribu-tion in the occupied zone.

Occupied zone: Volume of the room with height of 1.8 m above the floor and distance 0.6 m from

the walls (with and without windows) and the ceiling expected to be occupied by people. Operative temperature: Calculated as average of the air temperature and the mean radiant temperature of the room. This temperature is a measure for the perceived temperature.

Piston flow: A theoretical airflow pattern where the air from the supply passes like a piston across the room and pushes the old air our through the exhaust.

Reynolds number: Ratio between the inertia force and the viscous force

Speed: The magnitude of the air velocity vector at the measuring point.

Speed, mean: The magnitude of the air velocity vector at the measuring point averaged over a period of time.

Speed, standard deviation of speed: A statisti-cal measure of the scatter of the instantaneous speed measurements around the mean speed in a frequency domain.

Supply air temperature: The temperature of the supply air measured at the plane of entry to the room.

Thermal comfort: Condition of mind which express satisfaction with the thermal environ-ment.

Thermal plume: The air curtain rising from a hot body (or descending from a cold body). Turbulence intensity: The ratio of the standard deviation of air speed to the mean speed. The value can be expressed in percent.

Velocity: a measure of the distance travelled per unit of time. It is defined by its magnitude and direction at any point of the flow.

Ventilation aims to provide building occu-pants with clean air for breathing. In this sense ventilation aims to exchange polluted indoor air with less or non-polluted outdoor air, thus insuring healthy and comfortable environment in the occupied zone of rooms. This guidebook is focused on mechanical ventilation systems.

Mechanical ventilation systems supply into rooms either filtered outdoor air or filtered outdoor air mixed with re-circulated filtered room air. The air is supplied cooler than the room air when heat generated in spaces has to be removed. It is supplied with tempera-ture equal to the room temperatempera-ture in order to provide occupants with fresh air or warmer than the room air temperature in order to provide heating for occupants. Furthermore the supplied air can be humid-ified or dehumidhumid-ified.

The guidebook aims to present the basic principles of air distribution in spaces. The guidebook does not address different kinds of air handling units or any other parts of an air distribution system except of air supply openings. Location and type of air supply openings or air supply diffusers play a ma-jor role for air distribution in spaces. There-fore practical information about different kinds of air diffusers is provided.

The air distribution strategies in spaces can be divided into two groups, namely total volume air distribution and localized air distribution. The total volume air distribu-tion aims to provide good air quality and acceptable velocities and temperatures in the occupied zone, i.e. occupants are ex-posed more or less to a uniform environ-ment. The main total volume air distribu-tion principles

applied in practice are: mixing air distribu-tion and displacement air distribudistribu-tion. A hybrid air distribution of mixing and dis-placement is also use.

Other air distribution strategy, known as personalized ventilation, aims at supplying entire or part of the ventilation air close to each occupant before it is mixed with the polluted and usually warmer room air. Usu-ally direction and flow rate and in some cases the temperature of the supplied per-sonalized air can be controlled individually. These measures may achieve a preferred environment for each occupant. This type of localized air distribution ventilation is typi-cally implemented in conjunction with background total volume ventilation.

Governing equations for the description of air motion in spaces

The governing equations for mass flow and energy flow in a room are the continuity equation, the Navier Stokes equations and the energy equation. These equations are presented in dimensionless form in order obtain normalized variables. These varia-bles are the base for the following overview of air distribution systems as well as for many of the equations in this guide book.

All distances are normalized by a istic length of interest. A typical character-istic length in ventilation problems is the height of the supply opening, ho, or the square root of the supply area, ao. The height of a hot or cold vertical surface, l, can also be used as a characteristic length in situations where free convection is the most important as for example in case of cold downdraught in an atrium at low outdoor temperatures.

Velocities are typically normalized by the supply velocity at the diffuser, uo. The sup-ply velocity is defined as supsup-ply flow rate divided by the supply area. Temperatures, T, are normalized by the following expres-sion: o e o T T T T T    * (1.1)

Where To and Te are supply air temperature and return air temperature, respectively.

The dimensionless governing equations can be expressed as: 0 * * * * * *          z w y v x u (1.2)





x, y, and z are the three co-ordinates in an orthogonal coordinate system, u, v, and w are velocities in the co-ordinate directions, t is time and p is pressure. β, g, cp, ρo, μo and

uo are respectively thermal expansion coef-ficient, gravitational acceleration, specific heat,density, viscosity and supply velocity respectively.

The following dimensionless numbers are present in the equations:

o o o ohu    Re (1.5)





qo is the flow rate equal to the product of the supply velocity and supply area uo∙ao. The dimensionless numbers Re, Ar and Pr are called Reynolds number, Archimedes number and Prandtl number, respectively. The Reynolds number can be considered as the ratio of turbulent diffusion to laminar diffusion, and the Archimedes number as the ratio between thermal buoyancy force and momentum force. The Prandtl number is the ratio of momentum diffusivity to mass diffu-sivity, and it is equal to ~ 0.7 for the air.

Ar and 1/Re appears in the Navier Stokes equations, and 1/(RePr) in the energy equation. By analysing Ar it can be seen that an additional number Arratio can be

defined, see Equation (1.7). This ratio is used to define the different flow situations in the following discussion on different air distribution systems. It can be understood as a simplified version of Ar.

Practical cases of room air flows cannot be predicted by an analytical solution of the governing equations. The flow field can be calculated using numerical methods and turbulence models for the highly fluctuating

and vortex based flow fields. Turbulence models are still under development for room airflows and calculations results should be validated carefully with experimental data.

The geometry of a room, geometry and location of air supply diffusers and return air openings, position of heating or cooling load, orientation in relation to gravity effect and the dimensionless numbers (Re, Ar, Pr) will, together with the governing equations, express a total description of the air distri-bution in a room. Studies show that flow in rooms often will have a large Reynolds number and therefore can be considered as a fully developed turbulent flow.

This means that the level of the Reynolds number is irrelevant for the dimensionless room flow pattern. However in some areas of rooms, including the occupied zone, a low Reynolds number flow can exist when the air supply velocity is low. Figure 1.1 shows measurements of the maximum ve-locity, urm, in the occupied zone of a room with mixing ventilation from a wall mount-ed diffuser versus the air change rate, n.

Figure 1.1. Maximum velocity, urm, measured

in the occupied zone of a room versus air change rate in the case of isothermal mixing air distribution.

Air change per hour is a measure of how many times the air within a defined space is replaced. The flow is isothermal. It is known from the similarity principles that any velocity as e.g. the maximum velocity in the occupied zone is a linear function of the air change rate (or of the supply

veloci-ty) in case of a fully developed isothermal and turbulent flow. In Figure 1.1, this is the case for velocities larger than 0.25 m/s. But the figure also indicates that the flow in the occupied zone is a low Reynolds number flow for velocities below 0.25 m/s (differ-ences between the red lines).

Ventilation effectiveness and air change effectiveness

In order to assess and compare different air distribution patterns several indexes have been defined and used widely. These are introduced and discussed in detail in the REHVA’s Guidebook No. 2 on Ventilation Effectiveness (Mundt et al. 2004).

Two of the most important indices are the air change efficiency, εa, and the contami-nant removal effectiveness, εc_{. The air} change efficiency, εa, evaluates how effec-tive the room air is replaced by the supplied clean air. The most effective flow is the piston flow without any kind of mixing between the supplied clean air and the room air. This flow type has air change efficiency equal to 100%. Perfect mixing leads to an air change efficiency equal to 50%.

The contaminant removal effectiveness, εc_, is focused on the transport of pollution. It is a measure how quickly an airborne contam-inant is removed from the room and is de-fined as: m ean e c c c   (1.9)

where ce is the contaminant concentration in the exhaust and cmean is the mean con-centration of contaminant in the room. This expression assumes that the supplied air is not contaminated. Here, perfect mix-ing results in a contaminant removal effec-tiveness equal to 1.

Flows in rooms can be characterizes based on the air change efficiency and the con-taminant removal effectiveness. This is schematically shown in Figure 1.2. Perfect mixing ventilation is defined by an air change efficiency equal to 50% and a con-taminant removal effectiveness equal to 1. Short circuit flows lead to values smaller than 50% for the air change efficiency. The quality of displacement air distribution depends on the contaminant source. Only contaminant sources with heat production can be treated effectively by displacement ventilation (Wildeboer and Müller 2006).

Figure 1.2. Definition of different flow types based on air change efficiency and

contaminant removal effectiveness.

Room air distribution

In this section all total volume air distribu-tion principles are addressed based on discussion of air flow pattern in a box-like geometry with heat sources or cooling loads. The supply and exhaust air openings are considered to have different locations in the geometry.

The air distribution in a room with a fully developed turbulent flow can be described by the Archimedes number, which has been

already defined. It can be considered as the ratio between the buoyancy force and the momentum force. In a given geometry the Archimedes number can be expressed as Ar ~ ∆To/uo2 or as ∆To/qo2 (Arratio) because

the supply area ao is constant.

Apart from the Archimedes number, sev-eral boundary conditions including geome-try, supply and return air openings, differ-ent sources and sinks of heat load includ-ing their strength and location, enclosure surface temperature, etc. have influence on the air distribution in spaces. It can be very complicated to describe all details of the boundary conditions because they are in-dividual for different rooms. But a few primary and common parameters, which are the important ones, shall be consid-ered. These primary variables are:

 Cooling or heating mode.

 Archimedes ratio ∆To/qo2 - flow rate of air supplied to the room, qo and tem-perature difference between return and supply air, ∆To.

 The ratio between the total area of the supply openings and the

wall/ceiling/floor area, ao/A.

 Location of the air supply opening(s): close to the ceiling (high) or close to floor level (low)

The ratio between the total area, ao, of the supply openings and the surface area, A, of wall/ceiling/floor at which the supply open-ings are located, ao/A, is an important parame-ter for the room air distribution. At a given supply flow rate the momentum flux of the flow from the openings controls the air movement if ao/A is small, while the thermal plumes from heat sources control the air movement in the occupied zone if ao/A is large and the air change rate is moderate.

It is possible to obtain a unidirectional flow in the room if ao/A is equal to 1.0. The ratio,

ao/A, is considered to be small for values smaller than 10-3_{, medium for values}

be-tween 10-3 to 0.3, and large for values larg-er than 0.3. The values smalllarg-er than 10-3 _are

typical for diffusers designed for mixing ventilation, and the value 6·10-3 _{is typical}

for displacement ventilation diffusers. Ceil-ing mounted diffusers for vertical ventila-tion have an area ratio around 3·10-2. The ratio 1.0 corresponds to supply through the whole ceiling, wall or floor, providing the possibility of a piston flow dependent on the level of the momentum flow and direc-tion and level of the buoyancy force.

The location of the air supply and air ex-haust openings in relation to the direction of gravity is also important. A high loca-tion of the air supply opening will in some cases give a different air distribution pat-tern than a low location of the air supply opening. Furthermore, the location of the air exhaust opening can also be important for the ventilation effectiveness as de-scribed in the REHVA Guidebook No.2 (Mundt et al. 2004).

In a cooling mode the air distribution pat-tern in a room can be represented in a three dimensional system defined by the flow rate supplied to the room, qo, the difference between exhaust and supply air tempera-ture, ∆To, and the ratio between the total area of the supply openings and the wall area, ao/A. This is shown in Figure 1.3 (see Nielsen 2011).

The variables in Figure 1.3 can address a design graph for the room air distribution. For example Figure 1.4 shows design graph (qo - ∆To graph) for a constant value of ao/A. The right side of the curve defines

momentum driven flow while the left side defines a flow driven by buoyancy forces.

Figure 1.3. Three-dimensional representation of the room air distribution for the case of cooling.

Figure 1.4. Principle determination of airflow in a room with a given ao/A ratio based on the

critical Archimedes ratio. For the conditions in the left side of the curve the air distribution is dominated by convective flow while inlet momentum flow is dominating for the conditions at the right side of the curve.

The curve indicates the position of the critical Archimedes number where the air movement changes between the two dif-ferent flow types. The graphs in Figure 1.5 to 1.9 also indicate those left side and right side locations for convection and momen-tum controlled flow.

Mixing ventilation

Mixing room air distribution aims for dilut-ing of polluted and warm/cool room air with cleaner and cooler/warmer supply air. The air is supplied to the room with high initial mean velocity and the established

To qo a / Ao ∆T º /qº² ∆T º /qº² small and inlet momentum is dominating ∆T º q º large and convective flow is dominating

velocity gradients generate high turbulence intensity aiming to promote good mixing and uniform temperature and pollution distribution in the occupied zone.

Mixing ventilation is an expression for an air distribution pattern, and not for a venti-lation system. It can also be called an air distribution pattern with mixing effect or mixing air distribution. Mixing ventilation is traditional considered to be the air distri-bution which is obtained by the use of dif-fusers with a high momentum supply flow as discussed in Chapters 3, 4 and 6.

The chart in Figure 1.5 indicates how the air distribution pattern with mixing effect is generated by diffusers supplying a high momentum flow to the room. To obtain this high momentum flow at air change rates of 1 to 5 h-1 it is necessary to have diffusers with small openings (small ao/A). The supply air velocity is high and

diffus-ers are therefore located outside the occu-pied zone of the room as shown in the photo. Entrainment of the room air by the supplied jets creates a high mixing in the occupied zone and a contaminant removal effectiveness εc _{of ~ 1.0 is achieved.}

The air supply openings can in principle have any location outside the occupied zone. As already defined two types of air distribution, namely air distribution based on a buoyancy effect (large ∆To and rela-tively small qo) and air distribution driven by momentum of the supplied flow (large qo) can be achieved as illustrated in the graph in Figure 1.5 although the momen-tum driven flow is the general situation for small ao/A.

The ∆To-qo chart in Figure 1.5 also identifies an area representing a clean room with a very high supply flow rate and the photo in the figure shows an example on such a room.

Figure 1.5. Mixing ventilation with a high location of the air supply openings and clean room with a high turbulent flow is defined in the qo-∆To chart. The two photos show a meeting room with a

The air distribution in rooms with vertical ventilation from openings in the ceiling with a medium area, ao/A, will be con-trolled by the thermal forces, i.e. large ∆T/qo2 (Figure 1.6). In this case the cool-ing mode is the common situation in order to avoid stratification. The flow in the room will show a high level of mixing effect when the heat sources are located below the diffusers but there will be some displacement effect when the heat source are located outside the diffuser (Nielsen et al 2007). This air distribution system can in most cases be considered as a mixing ventilation system.

High momentum mixing flow with medium opening areas in the ceiling is considered as clean room mixing ventilation based on a highly turbulent flow (Figure 1.6, right side of the curve).

Air distribution pattern in rooms with diffuse ceiling inlet, i.e. where the whole ceiling is the supply opening (ao/A = 1.0) might be controlled either by buoyancy flows from heat sources (large ∆T/qo2) or by momentum flow (Figure 1.7). In the case of air distribu-tion pattern controlled by buoyancy flows the contaminant removal effectiveness εc is around 1.0, (air change rate equal to 1 – 5 h-1).

Figure 1.6. Vertical ventilation with air supply from openings in the ceiling. ao/A ~ medium.

The photo shows this type of flow from ceiling mounted textile terminals.

Figure 1.7. Diffuse ceiling inlet and unidirectional clean room flow. ao/A = 1.0. The two photos show

A high load can be handled without signifi-cant draught because the air velocity is low. This air distribution system is also defined as a mixing ventilation system in this guide book and the type of air distribution pattern is described by Nielsen and Jakubowska (2009). In this case the cooling mode is the common situation in order to avoid stratifi-cation. A system with preheating after night set back is discussed by Nielsen et al. (2010).

At high air change rates, 50 – 100 h-1, pis-ton flow takes place, and the system is a clean room air distribution system with very high ventilation effectiveness, see the later section “Unidirectional (piston) venti-lation”. The air change efficiency εa

is equal to 100%.

Displacement ventilation

Displacement ventilation is an expression for an air distribution pattern, and not for a ventilation system. It can also be called an air distribution pattern with displacement effect or displacement air distribution.

Displacement ventilation takes only place when thermal plumes from heat sources control the air movement in the room and the air flow pattern is not disturbed by the momentum provided by the supply open-ings. The supply openings are normally located at a low level (at a wall or at the floor) and the return openings are located at a wall just below the ceiling or at the ceil-ing. Displacement air distribution aims to replace but not mix the polluted room air with clean air.

The clean and cool air is supplied close to the floor at low velocity. Therefore in

rooms with displacement air distribution the highest velocity and the lowest tempera-ture occur near the floor close to the supply device (approx. 0.05 m above the floor). Unlike in rooms with mixing ventilation vertical temperature gradient exist in rooms with displacement ventilation.

The opening supply areas are large enough (medium ao/A) to provide air with very low momentum at normal air change rate of 1 to 5 h-1. The location of the supply openings is less significant for the room air distribution because the flow is mainly driven by buoy-ancy effects (Figure 1.8). The flow may be thermally stratified in the occupied zone and it is possible to work with a contami-nant removal effectiveness larger than 1.0. Displacement ventilation is described in the REHVA Guidebook Nr. 1 (Skistad et al. 2002). Displacement ventilation achieved with under-floor air distribution is de-scribed by Bauman (2003). High flow rate and medium supply area as shown in the right side of the chart in Figure 1.8 is nor-mally not a practical solution for an air distribution system because high velocities will be generated in the occupied zone.

Diffuse floor inlet or supply through the carpet is a special type of displacement ventilation where ao/A is equal to 1.0 (Figure 1.9). In this case the air distribution pattern with displacement effect is only driven by buoyancy forces (Figure 1.9, left side in the chart). This principle has high contaminant removal effectiveness εc_{. High} momentum unidirectional flow from the floor, as shown on the right side of the graph in Figure 1.9, is not used in practice because it will not ensure clean air in the working zone in the room.

Unidirectional (piston) ventilation

Piston flow will take place when ao/A = 1.0 and when the air distribution is controlled by the momentum flow (right side of the critical Archimedes curve in the graph Figure 1.7). A very high flow rate is required to obtain the momentum flow which can overcome and destroy the plume from, for example, a person. The air change rate can typical be 50 to 100 h-1. Figure 1.7 show a common layout for this type of air distribution when the whole ceiling is a supply opening; unidirectional flow from one of the walls can also be used.

An overview of air distribution principles with a full surface supply area

The interconnections between the different air distribution modes with full surface supply areas (ao/A = 1.0) have been studied by Linke (1962). Air is supplied, either from below, or from above, with an opposite location of the return opening. Heat is evenly supplied in a horizontal position low in the occupied zone. The different flow patterns are shown in

Fig-ure 1.10. They correspond to the flows

shown in Figure 1.7 and Figure 1.9.

Figure 1.8. Displacement air distribution (Displacement ventilation) is defined in the chart. The supply openings are located at a low level, and the return openings are at a high level. The supply area of the opening is relatively large. The photo shows displacement ventilation diffusers in a restaurant.

Figure 1.9. Displacement flow with diffuse floor inlet. ao/A = 1.0.

Figure 1.10. Air distribution modes with a full surface supply area (Linke 1962). ao/A = 1.0.

The family tree of air distribution patterns based on supply opening location

The whole “family” of air distribution pat-terns can in the case of cooling be described in two three-dimensional charts, “family trees”, one for a high location of the air supply opening and one for a low location of the air supply opening. The charts are shown in Figure 1.11 and Figure 1.12. (Nielsen 2011).

If the air supply openings are located at the upper room regions, it is difficult to work with stratification effects. Thus it is almost impossible to obtain better values for the air change efficiency εa

and the contaminant removal effectiveness εc _{than in the case of}

perfect mixing ventilation. Most of the flow is characterized by a strong mixing, either due to the high momentum of the supply airflow or due to the interaction of the cold supply air moving downwards with the up-ward flow of thermal plumes generated by heat sources. Thus, a contaminant removal effectiveness εc _{of approx. 1.0 is typical for} this air distribution patterns. Only in the case of downward flow from diffuse ceiling inlet (ao/A = 1.0) and high flow rates the

estab-lished piston flow makes it possible to obtain a very high ventilation effectiveness and this is convenient way to supply clean air to the working zone in a clean room.

Air distribution with mixing effect can be achieved with supply openings located at the low room height. It is important that the region of the generated flow with high ve-locity is outside the occupied zone. A low location of the supply openings allows achieving a high contaminant removal ef-fectiveness, εc, because the stratification effect can be used and displacement air distribution is achieved. This is especially the case when the openings are large and therefore have low momentum flow.

Mixing and displacement air distribution patterns based on driving force

Both, the air distribution with mixing and with displacement can be driven by either buoyancy or momentum effect as discussed earlier (Figure 1.11 and Figure 1.12). In

Figure 1.13, examples of mixing and

dis-placement air distribution patterns with buoyancy and momentum effects are shown.

∆Tº

q º

Displacementventilation with diffuse floor inlet

Clean room with piston flow Diffuse

ceiling inlet

Figure 1.11. Different air distribution modes for cooling with high location of supply openings. The Figure is a three-dimensional expression of the Figures 1.5, 1.6 and 1.7.

Figure 1.12. Different air distribution modes for cooling with low location of supply openings. The Figure is a three-dimensional expression of the Figures 1.8 and 1.9.

Figure 1.13. Total volume air distribution modes with mixing and displacement patters based on buoyancy or momentum driving forces.

a /A0

q0

Vertical ventilation

Mixing ventilation

Clean room with turbulent mixing Diffuse

ceiling inlet

Clean room with piston flow T0 a /A0 q0 Mixing ventilation Displacement ventilation Diffuse inlet floor T0

Diffuse ceiling/floor inlet, i.e. when the air is supplied over the whole area of the ceil-ing/floor (ao/A = 1.0), have been rarely used for comfort ventilation in occupied public buildings, such as office rooms, etc. For typical practical applications ventilation air is supplied from air terminal devices in-stalled either on the ceiling or the wall or the floor (ao/A < 1.0), In this case the gen-eral rule is that air distribution with mixing effect is used for cooling and heating while the air distribution with displacement effect is used mainly for cooling.

Nature of airflow in rooms

In this guidebook as well as in most of the literature on air distribution in spaces only the steady state flow situation is consid-ered and described. However, in many practical air distribution applications this is not the case. The air distribution is result of complex interaction of different flows as discussed in detail in Chapter 6 of this guidebook.

Large spatial gradients in velocity, tempera-ture and pollution concentration near air supply openings and inner surfaces exist. Spatial gradients may exist in the occupied zone as well. Often the mixing in rooms is not complete. It changes with time due to changes in heat loads (solar heat load, number of occupants, lighting, etc.), dis-turbances from walking occupants (Hal-vonova and Melikov 2010), local supply or exhaust flows, etc.

Therefore room air distribution in space and in time domains should be carefully considered in order to avoid zones of high velocity and low temperature as well as zones of polluted air which may affect occupants’ comfort and well- being.

Courtesy

Photos of room air distribution systems have been supplied by Lindab A/S, Institute for Science and International Security, KE Fibertec A/S, Wessberg and Shelco Inc.

2 Human response to air movement

spaces aims to generate healthy, comforta-ble and work stimulating environment in the occupied zone. Airflow with different characteristics, including air temperature, mean velocity, turbulence intensity, fre-quency of velocity fluctuations, flow direc-tion, etc., can be generated. It affects occu-pants’ thermal comfort and inhaled air qual-ity. In this chapter human response to air movement is discussed shortly.

Air distribution and thermal comfort

Thermal comfort is that condition of mind which expresses satisfaction with the thermal environment (ISO Standard 7730 2005, ASHRAE Standard 55 2010). Peo-ples’ thermal sensation is related to the thermal balance of their body as a whole. This balance is influenced by physical activity and clothing, as well as several environmental parameters: air tempera-ture, mean radiant temperatempera-ture, air veloci-ty and air humidiveloci-ty. The ranges of envi-ronmental parameters for whole body thermal comfort are prescribed in hand-books and standards (ASHRAE Handbook Fundamentals 2009, ISO Standard 55 2005, ASHRAE Standard 55 2010, EN15251 2007, etc.). During design and assessment of indoor environment the thermal sensation of an “average person” can be predicted by the PMV (Predicted Mean Vote) index on seven point scale - cold, cool, slightly cool, neutral, slightly warm, warm, hot (Fanger 1972, ISO Standard 7730 2005, ASHRAE Handbook Fundamentals 2009) when the above de-fined environmental parameters have been estimated or measured. The predicted per-centage dissatisfied (PPD) index provides information on thermal discomfort or

thermal dissatisfaction by predicting the percentage of people likely to feel too warm or too cool in a given environment. The PPD can be obtained from the PMV index (Fanger 1972, ISO Standard 77 2005). For majority of people thermal comfort is achieved in a relatively narrow range of operative temperature (operative temperature is based on the air tempera-ture and the mean radiant temperatempera-ture, ISO standard 7730 2005). In warm or cold environments, there can often be influence of adaptation. Apart from clothing, other forms of adaptation, such as body posture and decreased activity, which are difficult to quantify, can result in the acceptance of higher indoor temperatures. People used to work and live in warm climates can more easily accept and maintain a higher work performance in hot environments than those living in colder climates (ISO Stand-ard 7730 2005).

The heat exchange between human body and the environment is influenced by the airflow characteristics at the vicinity of the body. Air distribution in rooms, especially rooms with displacement air distribution, may lead to non-uniformity in temperature field which may cause local discomfort of “warm head” and “cool feet” when the difference in air temperature at the head level (1.1 m above the floor) and at the ankle level (0.1 m above the floor) is large (Wyon and Sandberg 1996). In the com-fortable range of operative temperature (20–26°C) the air velocity required for whole body thermal comfort is below 0.2 m/s, though some people may prefer higher velocity (discussed later in this sec-tion). There is no minimum air velocity that is necessary for thermal comfort.

As it is discussed in the following chapters the characteristics of the room airflow differ spatially and depend on the boundary condi-tion, the positioning of the air supply termi-nal device, the discharge parameters of the ventilation air, the interaction of the supplied ventilation flow with the thermal flows gen-erated by warm/cold surfaces (e.g. down-ward/upward flow from cold/warm win-dows, thermal plume from human body, spot lighting, office equipment), etc. Thus loca-tions with air movement of relatively high velocity and sometimes low temperature may occur in the occupied zone. This en-hances the heat loss from the body parts exposed to the air movement.

When a person feels thermally neutral or cooler the increased local heat loss due to high velocity may cause local discomfort due to draught. The risk of draught increas-es when the airflow temperature decreasincreas-es and the mean velocity and turbulence inten-sity increase. The percentage of occupants dissatisfied due to draught, DR (%), can be predicted by the following equation:

) 14 . 3 37 . 0 ( ) 05 . 0 )( 34 ( DR T u 0.62 uTu (2.1)

In this equation ta [C] is the air tempera-ture, u [m/s] is the mean velocity, and Tu [%] is the turbulence intensity of the flow. The equation is valid when u is higher than 0.05 m/s; for u smaller than 0.05 m/s, u = 0.05 m/s should be used; for DR > 100% DR = 100% should be used. The equation is based on results from human subject experiments (Fanger et al. 1988) and is included in the present international and national standards (ISO Standard 7730 2005, etc.). Equation (2.1) can be used when air temperature, mean velocity and turbulence intensity at the location of the

occupants are known (obtained by airflow predictions or measurements at four heights 0.1, 0.6, 1.1 and 1.7 m as recommended in ISO Standard 7726 1998). The air tempera-ture, mean velocity and turbulence intensity can be easily measured in practice (dis-cussed in Chapter 8). Nevertheless the above equation (2.1) can be simplified by assuming that for the range of velocities that may cause an increase of the local cooling of the body (above 0.1–0.15 m/s) the turbulence intensity is approx. 40% in the case of mixing air distribution and ap-prox. 20% in the case of displacement air distribution. In Figure 2.1 the combinations of air temperature, mean velocity and tur-bulence intensity that may cause 20% dis-satisfied occupants due to draught are shown. As illustrated in the figure at air temperature of 23°C, 20% dissatisfied may be expected when the mean velocity of the flow is 0.17 m/s and turbulence intensity is 40%. The same 20% dissatisfied can be obtained when the airflow has a much higher mean velocity of 0.23 m/s, but lower turbulence intensity of only 20%. In prac-tice, an increase of the airflow rate is often required for the removal of heat or pollu-tion generated in rooms. The increased ventilation rate however will lead to an increase of velocity in the occupied zone which may result in increased risk of draught discomfort. In this case, the use of air distribution which ensures low turbulent flow may be considered.

The airflow direction is another parameter important for occupants’ thermal sensation. For the same conditions, i.e. mean velocity, turbulence intensity and temperature, air-flow from the rear feels more uncomforta-ble by people than an airflow from the front, and airflow from below feels more uncomfortable than airflow from above

(Mayer and Schwab 1988, Toftum et al. 1997, Zhou 1999, etc.). The results ob-tained from human subject experiments shown in Figure 2.2 and Figure 2.3 clearly demonstrate the importance of airflow di-rection for peoples’ draught sensation.

Figure 2.1. Combinations of air temperature, mean velocity and turbulence intensity that will cause 20% dissatisfied occupants according to the DR model, eq. 2.1 (ISO Standard 7730 2005). The impact of airflow direction on people’s thermal sensation can be due to several reasons. Non-uniform distribution of ther-mal receptors (sensitive to changes in skin temperature due to convective heat

ex-change caused by the airflow) over the body is one of the reasons. Also, people are used to airflow from front since it exists when they walk. Another important reason is the interaction of the ventilation flow with the free convection flow around hu-man body and with the thermal plume gen-erated by the body as shown in Figure 2.4. For a seated person the free convection boundary layer at the back is thin due to the body posture and the blocking effect of the chair backrest and thus it can be easily pen-etrated by the ventilation flow while the free convection flow at front generated by lower and upper chest, thighs and legs is thick and more difficult for penetration. Ventilation flow from bellow will assist the free convection flow and will increase the body heat loss, especially at the feet. In contrary, ventilation flow from above with much higher velocity will be needed to destroy the opposing upward flow of the thermal plume generated by the body (with mean velocity up to 0.25 m/s) and to pro-vide cooling of the upper body part. This airflow INTERACTION is discussed in detail in Chapter 7 of this guidebook.

u[m/s] T[ ]°C Tu = 40% Tu = 0%1 Tu = 0%6 Tu = 0%2 0.4 0.3 0.2 0.1 0 18 20 22 24 26 Category B: DR = 20%

Figure 2.2. Percentage of subjects dissatisfied due to draught at the face when exposed to airflow from rear and from front; turbulence intensity of the flow – 5% (Mayer and Schwab 1988).

Figure 2.3. Percentage of subjects dissatisfied due to draught at the face when exposed to airflow from bellow and from above; turbulence intensity of the flows – 5% (Mayer and Schwab 1988).

100 100 80 80 60 60 40 40 20 20 0.2 0.2 0.1 0.1 0 0.3 0.4 0.5 0 0.3 0.4 0.5 0 0 pe rc en ta ge o f d iss at is fie d be ca us e of [ ] dra ug ht a t n ec k / f ac e % pe rc en ta ge o f d iss at is fie d be ca us e of [ ] dra ug ht a t f ac e %

mean air velocity m/s[ ]

(turbulence intensity 5%) mean air velocity m/s(turbulence intensity 5%)[ ] neck

Figure 2.4. Interaction of ventilation flow with free convection flow around human body. The frequency of velocity fluctuations has an impact on the thermal sensation of people (Fanger and Pedersen, 1977; Zhao and Xia, 1998). Sedentary subjects experienced a maximum discomfort at frequencies around 0.3-0.5 Hz at a mean air velocity of 0.3 m/s when exposed to a periodically fluctuating airflow at an air temperature preferred by the subject (Fanger and Pedersen 1977). How-ever, in real spaces occupants are exposed to airflow with a randomly fluctuating velocity with frequency from very low up to 2 Hz (Finkelstein, et al. 1996). Experiments with people exposed to airflows with different velocity and equivalent frequency of veloci-ty fluctuations as they occur in rooms in practice, revealed that people were signifi-cantly more sensitive to airflow with equiva-lent frequency between 0.2 and 0.6 Hz (Zhou and Melikov 2002, Zhou et al. 2002). The equivalent frequency is an integral measure for the frequency of the random velocity (speed) fluctuations in rooms. In practice in rooms with total volume ventila-tion it is difficult to control accurately the airflow characteristics. However in the case of some types of air distribution, such as personalized ventilation, it is possible to control the characteristics of locally applied airflow and to explore the impact of equiva-lent frequency together with the impact of air temperature, mean velocity, turbulence intensity and airflow direction on occupants’ thermal comfort.

The increased heat exchange between hu-man body and room environment due to air movement may have adverse effect on peo-ples’ thermal comfort. As already dis-cussed, for occupants who feel thermally neutral or cooler increased local heat loss from the body due to high velocity may cause draught discomfort. On the contrary, increased heat loss from the body due to air movement improves the thermal comfort of people who feel warm or hot. Air move-ment with elevated velocity under individu-al control is recommended in the present thermal comfort standards (EN15251 2007, ASHRAE Standard 55 2010, ISO Standard 7730 2005) in order to offset indoor tem-perature rise and provide thermal comfort for the occupants. In practice, air movement with elevated velocity can easily be gener-ated by a fan (desk fan, standing fan, ceil-ing fan, etc.). However the strategy of in-creasing room air temperature and improv-ing occupants’ thermal comfort by air movement at elevated velocity should be considered with caution because it may lead to increase of energy consumption (Schiavon and Melikov 2008).

Air distribution and indoor air quality

The main purpose of ventilation is to provide occupants with clean air for breathing. Sig-nificant knowledge exists today on the im-pact of room air distribution on dispersion and removal of pollution generated by dif-ferent sources in spaces, such as occupants, building materials, office machines, etc. The mixing air distribution aims to dilute the pollution generated in the ventilated space. Thus uniform pollution concentration in the occupied zone is obtained, though near pol-lution sources the concentration is high. Because of its principle (not to mix with but to replace the polluted room air), displace-ment air distribution has potential to remove

more efficiently pollution than mixing air distribution. However the pollution removal efficiency of displacement air distribution is dependent on the type of pollution source (passive source generating only pollution or active source generating pollution and heat), location of the pollution source, size of the generated pollution particles, etc. (Brohus and Nielsen 1996, Bjørn and Nielsen 2002). Furthermore, the performance of displace-ment air distribution with regard to removal of pollution decreases with the presence of walking people, due to the interaction with room airflows, etc. (Halvonova and Me-likov 2010, Cermak and MeMe-likov 2006, etc.). Therefore, in practice depending on occupants’ activity, displacement air distri-bution may not be advantageous to the mixing air distribution with regard to in-haled air quality.

Perceived air quality (PAQ) depends on the level of pollution in the inhaled air. Accept-ability of PAQ decreases when the pollu-tion level in the inhaled air increases. As-suming other conditions in the room are unchanged, the percentage of dissatisfied occupants with perceived air quality de-creases with the increase in the ventilation effectiveness. PAQ also depends on the temperature and relative humidity of the inhaled air. PAQ will decrease when the temperature and relative humidity of the air inhaled increase, i.e. when its enthalpy increases (Fang et al. 1998, Melikov and Kaczmarcyk 2012, Böttcher 2003). The effect of the enthalpy on PAQ decreases with the increase of the pollution level in the air. Apart from the temperature, relative humidity and pollution level the air move-ment also affects the perceived air quality. Increased air movement at the facial region makes people to feel the air more fresh and with better air quality. Increased air

move-ment diminishes the negative impact of high air temperature, relative humidity and pollution level on the perceived air quality (Melikov and Kaczmarczyk 2012, Zhang et al. 2010). The positive impact of air move-ment on the perceived air quality increases at high air temperature, relative humidity and pollution level. In rooms without pollu-tion sources at low heights and under steady state conditions (e.g. without walk-ing occupants), inhaled air with displace-ment air distribution will be cleaner than with mixing ventilation, because free con-vection flow around human body will move the supplied clean air upward to the breath-ing zone. Yet, the PAQ may not be better than in rooms with mixing air distribution if the temperature of the inhaled air is high (Melikov et al. 2005). The potential of ele-vated air movement for diminishing the negative impact of increased temperature and relative humidity on PAQ can be effi-ciently explored when room environment is kept warmer than neutral. In this case the air movement with elevated velocity will improve occupants’ thermal comfort and perceived air quality but it will not decrease the SBS (Sick Building Syndrome) symp-toms, such as headache, difficulty to con-centrate, etc., if the air is polluted (Melikov and Kaczmarczyk 2012).

Air distribution and acoustic

Beside thermal comfort the aspect of acous-tic comfort is also a relevant criterion in HVAC applications, since whenever me-chanical power is generated or transmitted, a fraction of the power is converted into sound power. In particular, small supply openings leading to small ratio of the sup-ply opening areas ao to the surface area A of wall/ceiling/floor on which the supply openings are located, ao/A, may lead to acoustic discomfort.

The relevant acoustic characteristics in the field of HVAC components are the sound power and the sound intensity level. The sound power of a source is its rate of emis-sion of acoustical energy and is expressed in watts. For the corresponding sound power level Lw in dB, the power reference is 10-12 W and w is the acoustic power emitted by the source in watts.

) 10 / log( 10 12  w Lw (2.2)

The sound power level in dB of an acous-tic source is independent of location and space. It is the same at any distance. A typical conversation level for example is estimated to be at 70 dB. In contrast, the sound intensity level is always depended on the distance from the acoustic source. The sound intensity I at a point in a speci-fied direction is the rate of sound energy through unit area at that point and is ex-pressed in W/m². The corresponding sound intensity level LI is expressed in dB with a reference quantity of 10-12 W/m².

L_I 10log(I/1012) (2.3) The sound pressure is the local pressure deviation from the atmospheric pressure caused by a sound wave. Sound pressure in air can be directly measured using a microphone and the unit for sound pres-sure is pascal. Sound prespres-sure level (SPL) or sound level is a logarithmic measure of the effective sound pressure of a sound relative to a reference value. It is measured also in the dimensionless quantity decibels (dB) above a standard reference level. The commonly used "zero" reference sound pressure in air is 20 µPa, which is usually considered the threshold of human hearing (at 1 kHz).

The human perception of noise is related to the sound pressure level. Using the same supply opening with a given sound power level in two rooms can lead to different sound pressure levels and acoustic comfort. The distance to the supply opening, the reflection and absorption effects at all room surfaces including the furniture will control the sound pressure level in the occupied space. It is noted that an increase of the perception of noise by a factor of two is related to a sound pressure level increase by 6 dB. Adding two equal sound sources leads to a sound pressure level increase of 3 dB based on the value of a one source.

Sound pressure level weightings

The human perception of sound is not relat-ed to frequency independent response to the sound pressure level. The human ear does not perceive very low- and very high-frequency sounds as well as sounds in the range between 500 and 5,000 Hz. This frequency range is used for our oral com-munication. Because of the frequency de-pendent human sound perception, three weightings have been established for scal-ing the sound pressure: A, B and C as de-fined in EN 61672-1/-2. The A-weighting applies approximately to sound pressures levels up to 55 dB, B-weighting are used for sound pressures levels between 55 and 85 dB, and C-weighting accounts for sound pressure levels above 85 dB.

In order to distinguish between the different sound measures a suffix is used, A-weighted sound pressure level is written as dBA. Unweighted sound pressure level is

also called "linear sound pressure level" and is often written as dBL. Typically, all

HVAC related acoustic comfort values are given in A-weighting sound pressure levels. This means that very low frequencies will

have a minor influence on acoustics com-fort ratings. Special attention should be given to all frequencies between 500 and 5000 Hz.

Figure 2.5. Weighting curves for sound pressure levels.

Different national and international stand-ards provide the following recommenda-tions for sound pressure levels:

 35 to 40 dBA: Difficult mental,

pro-gramming or scientific work (see Ger-man Bundesanstalt für Arbeitsschutz und Arbeitsmedizin „Arbeitswissen-schaftliche Erkenntnisse" AWE 124).  35 to 45 dBA: Mainly mental work

(vgl. DIN EN ISO 11690, AWE 124)  40 to 45 dBA: High level of

intelligibil-ity, discussions with costumers (see DIN EN ISO 9241 part 6)

 40 to 50 dBA: Call center and open

space offices (see German AWE 124, Wissensspeicher CallCenter der Bun-desanstalt für Arbeitsschutz und Ar-beitsmedizin)

 45 to 55 dBA: As matters of routine

office work (see DIN EN ISO 11690)

Air distribution and other personal related factors

Large differences exist between people with regard to a preferred environment. The pre-ferred temperature may differ up to 10°C (Grivel and Candas 1991) and preferred velocity more than 4 times (Melikov et al. 1994, Skwarczynski et al. 2009). Some peo-ple love air movement, others hate it.

Air temperature, relative humidity, pollu-tion and air movement affect ocular fatigue and performance. Decrease of air tempera-ture and relative humidity and increase of air movement velocity increase eye tear film evaporation resulting in increased blink rate (Wolkoff et al. 2003). Indoor air pollution may affect negatively eye func-tion (Wolkoff et al. 2012). The percepfunc-tion of air dryness and irritation increases when the velocity of facially applied airflow in-creases (Tsutsumi et al 2007, Melikov et al. 2011). Air movement when supplied at high velocity directly against the face may cause eye irritation especially for people wearing glasses and eye lenses (Kaczmarczyk et al. 2002). The negative impact on eye was reported at relatively high velocity above 0.5 m/s (Wyon and Wyon 1987). In rooms with mixing ventilation the velocity in the occupied zone is usually kept bellow 0.25 m/s and may not have impact on eye discomfort. However temperature, relative humidity and pollution level may vary over a wide range and may effect eye dryness, tear film quality, etc. The combined effect of air temperature, relative humidity, pollu-tion level and air movement on eye perfor-mance has not been studied in deep (Me-likov et al. 2011).

Air flow with high temperature and veloci-ty and low relative humidiveloci-ty may cause nose dryness. Pressure on the skin exposed

to airflow with high velocity can be reason for discomfort as well (McIntyre 1978). Control of the environment provided to occupants is important for their satisfaction. Of the currently used air distribution meth-ods only the personalized ventilation

pro-vides possibility for controlling the air dis-tribution at workstations (Melikov 2004). The total volume air distribution principles, mixing and displacement, can only provide limited control of the airflow pattern in spaces.