Source book for efficient air duct systems in Europe
Foreword - Introduction - Download - Home page
This document was written within the framework of the European AIRWAYS project (Save II program - Project 4.1031/Z/99-158 – DG TREN).
SAVE is the European Union non-technology energy efficiency programme. One of the goals of this programme is the implementation and completion of Community-wide measures taken to improve energy efficiency in the domain of buildings.
The objective of the AIRWAYS project is to provide guidance for designing and maintain energy efficient air duct systems, and bringing to light energy saving opportunities in parallel to health, safety, and comfort issues.
This book is targeted at decision-makers concerned with indoor climate issues, including policy makers, architects, and designers. It provides condensed information on reasons behind better air duct system design and how this can be achieved.
The participants in the research (also called the AIRWAYS Partners) are:
·
Royal Institute of Technology, KTH,
Sweden
Website : http://www.kth.se/index-eng.html
·
Belgian Building Research Institute,
Belgium
Website : http://www.bbri.be/
·
Ecole Nationale des Travaux Publics
de l'Etat (ENTPE), France
Website : http://www.entpe.fr
·
Scandiaconsult, Sweden
Website : http://www.scc.se
One aim of this source book is to increase HVAC designer awareness of the important role the ductwork plays with respect to function, costs and energy use of the HVAC system. Another aim is to point out the connection and co-operation that is necessary between the HVAC designer and the architect when working with building design and space requirements. To illustrate how this can be done the book provides case studies demonstrating good examples and, in a few cases, less fortunate examples (§ 12).
1.1 Why is it important to design a well-functioning ductwork?
1.1.1 General
This chapter describes some of the philosophies behind the design of a ventilation system, the ways to decide upon correct airflow and the importance of guaranteeing that the air really will be of use. The ductwork thereby plays a most important role in safeguarding air quality, good thermal climate and occupant wellbeing.
As soon as a ventilation system is connected to more than one room, there is a need for a distribution system – a ductwork – to connect the different rooms to air-handling units and extract fans. The airflow that is decided suitable for ventilation and thermal comfort reasons has to be transported to and from the rooms. The air distribution to and from the rooms – the supply and extract air flows – has to be adjusted to the correct values by achieving correct pressure drops through the pressure resistance in ducts, dampers, registers, air terminal devices, and other ductwork components.
As described in this book there are many ways whereby a duct system will function in a less efficient way. The air flow distribution might differ due to influence from wind and outdoor temperature (§ 3.3), air may leak into and out of the ducts through small holes (§ 4.2), high air velocities might create unwanted noise (§ 7.8), dust and other impurities in the duct system might cause health problems unless dealt with (§ 7.4). These and other factors should be taken into consideration during design, installation and maintenance of the duct system and the following chapters will show how this can be done in order to achieve an efficient and well functioning duct system at a low investment and low life cycle cost.
1.1.2 The air should be transported to the areas in the building where it is most needed
Air transport is often necessary for maintaining good air quality in a room. The ventilation calculation is thus normally based on an assumed emission of CO2 and moisture from occupants, dust and gases emitted from furnishings, furniture, interior surfaces and activities. In this case the airflow is needed to dilute the emissions and transport them out of the room.
The other main reason why transporting air to and from a room might be needed is to control the thermal climate. In this case transporting heat to or from the room with the air controls the room temperature. If the room needs to be cooled, the excess heat will be carried out of the room by supplying air at a lower temperature than the desired room temperature. If the room needs to be heated this will be done by supplying air at a higher temperature than the desired room temperature.
In both cases - air quality or thermal climate - the airflow is calculated to correspond to the assumed loads of emissions or, similarly, to the heat/cold load. A given heat/cold load and a suitable temperature difference between the supply and the room temperatures will correspond to a required airflow. A given or calculated emission load and an acceptable emission level increase between the supply and the room concentration levels will similarly also correspond to a required airflow.
It is therefore vital that the correct airflow is transported to and from the rooms accordingly. To be efficient, the air should neither be allowed to leave the supply duct nor be allowed to enter the extract ducts through leakage openings. It is hence important that the airflow is adjusted to the correct values before the plant is taken into operation.
The ways of adjusting the airflow and the different methods to measure airflow in ducts and at registers with an acceptable amount of accuracy is also described in this book (§ 10.4).
1.1.3 Air quality – emissions should be diluted and safely transported from the rooms
“Dilution is not the only solution to pollution”. This means that the first way to reduce high and unhealthy pollution levels in rooms should be by reducing the strength of the emissions sources – by choosing low emitting materials and components wherever possible. There are many national and international research programs in operation for labelling building and interior materials. These take the emission to the room air during normal operation into consideration. Today’s’ knowledge on this still lacks maturity. However, in time, this approach might be used to calculate airflow rates based on IAQ demands. Hereby the cost for higher ventilation airflow could be compared to the initial, operating, and Life Cycle cost of less emitting furnishings and finishing materials
If the emissions are due to activities in a room it is important to prevent the hazardous or disagreeable pollutants from being inhaled by the occupants. The air has thus to be supplied to and extracted from the rooms with this in mind. The air should be supplied to the part or the room where occupants are to be found while the air should be extracted from that part of the room where the highest concentration of pollutants can be expected (e.g. at the kitchen stove, above polluting machines). This safety in preventing hazardous pollutants to enter breathing zones can be still increased if the source of pollution is enclosed to a high degree only leaving small openings for the extract air to enter. The under-pressure in the enclosure or hood compared to the ambient pressure in the room makes it hard for the pollutants to enter the room. There are many articles and handbooks covering this item. One common principle is that the design of the hood should take into account the laws of nature. If the emitted pollutant is warmer than the room temperature, the hood (e.g. a kitchen hood) should be located above the pollution source to be able to take care of the upward air movement. If the pollutant (e.g. particles emitted from a grinding machine) is released with a velocity the hood should mainly be covering the area in the direction of the pollution flow. A commonly used metaphor is the goalkeeper’s glove – to catch the ball where it arrives.
1.2 Thermal comfort – no draught
Ventilation air is used as an aid to creating a better thermal indoor climate by transporting excess, or lack of, heat and moisture out of or to the room respectively. But this advantage is often reduced by simultaneous disadvantages from the same air. It might create disagreeable fast air movements in the room. In wintertime a person is more sensitive to draught than in summer. In winter the acceptable air velocity is normally below 0.15 m/s while in summer – when the air movement is often longed-for and agreeable due to the higher room temperature - the maximum air velocity is normally 0.25 m/s.
This influences the choice of ventilation system. The air is supplied to the room via supply air registers that have to be chosen in such a way that the corresponding air velocity in the occupied zone is acceptable. This determines the size and number of the registers and the distance between them and to the occupants. Displacement ventilation systems, where the supply air is delivered at a lower temperature and at floor level might be more difficult to design than a mixing ventilation system.
The ultimate goal for the design of a ventilation and air handling system is to satisfy the needs and wishes of the occupants without creating any inconveniences like draught or noise. It stands to reason but is not always the case; this book points out some of the problems that should be examined – before they become problems!
The energy use of a ventilation system should be reduced as much as possible without decreasing the benefits of the system regarding thermal comfort and indoor air quality. The annual energy needed for transporting the ventilation air through the system is proportional to the fan power and the number of operation hours per year.
Both these values can be influenced. The fan power is proportional to the airflow and the total pressure difference through the system and inversely proportional to the efficiency of the fan with its motor.
Normally the pressure drop in the system is roughly equally distributed between the air handling unit and the duct system. How the latter is calculated is described more in detail below (§ 7.3), where it is shown that the pressure drop increases with the square of the air velocity. By keeping low air velocities in the ducts, i.e. choosing ductwork with ample dimensions, the energy can thus be reduced which, if the annual number of operation hours is high, will lead to substantial energy savings. Another advantage of low air velocities in the ductwork is that the risk of emitting noise from the ductwork is diminished.
Often the supply air is heated or cooled before being supplied to the room. If the ducts are properly insulated, the temperature difference will be kept between the air in the duct and the cooler or hotter surroundings of the ductwork. This will reduce the need for any extra thermal energy input in the air handling units to cover thermal losses.
In both these cases – i.e. reducing the transport energy by sizing the ductwork and reducing the thermal losses by insulating the ductwork – the investment cost will be higher than the one for a poorer installation.
As the ducts probably will be used for many years these possible energy and cost savings vs. the extra investments should be considered on a Life Cycle Basis – discussed below (§ 6.3).
1.4 Avoid noise transmission through the ductwork
Ducts are normally connected to adjacent rooms which might create an unnecessary path for noise to be transmitted between them. During normal operation when the fans are running this is not normally a problem but should they be stopped e.g. after normal office hours, conversation in one room might be overheard in the other. In cases where there are more strict requirements on privacy between rooms, the ducts have to be designed and installed in a way that corresponds to the chosen sound insulation of the adjacent wall. One of the case studies presented in this book (§12) shows how this can be done in a building with very high demands on privacy between rooms.
1.5 Do the ducts have to be hidden?
There is a trend among some architects today to let part of the building installations be visual to the user. They regard that the installations are necessary for the function of the building and not something that has to be hidden. One of the case studies in this book (§ 12) could be seen as an example of this trend. The brightly coloured circular ducts are running up through atria in the office building. On the different floor levels, the ducts are also visible and not hidden above false ceilings which is normally the case in office buildings.
Besides resulting in lower building costs, this normally also presents an advantage for the thermal climate in the building. The lack of false ceilings results in a larger ventilated room volume. The extra space thus created at the ceiling, where the emissions are normally at a higher concentration, results in a better use of the ventilation airflow. The direct contact between the ventilation air and the bare concrete ceiling also enhances the possibility to use cool night air for comfort cooling of the building.
This visual installation of ductwork in e.g. office buildings is however only acceptable if the workmanship of the installation is of a high standard and should otherwise be avoided.
The ductwork could present a fire hazard in a building when the ducts run through fire classed walls. There are different building code requirements in different countries but they all have one thing in common – the duct penetrating the wall must not lead to a reduction in the fire safety of the building. The technical solution chosen should thus be compared to the case of the wall without any penetrating duct.
Even though the national requirements differ, there are mainly two different demands required for fire safety in this case, namely fire insulation “I” and tightness or “integrity”, ”E”. The first requirement, “I”, is covered if duct penetration through the wall is thermally insulated in such a way and to such a degree that the heat from a fire on one side of the wall will not be able to set fire to anything on the other side. Tightening the space between the outside of the duct and the wall opening fulfils the tightness requirement, “E”. Both these requirements, for E and I, are combined with a figure expressed in minutes during which the construction has to withstand the effect of a standard fire as defined in international standard. A normal requirement for walls in office buildings is fire class “EI 60”.
But there is yet another demand – the ducts on both sides of the fire wall have to stay in place during the fire. The duct hangers thus also have to withstand the strain from a fire during the same time required for the duct itself. This mechanical strength demand during fire is expressed in international standard as an “R”-demand and should thus for the office building above be expressed as “R 60” for the duct hangers.
There are different ways of arriving to a safe solution. The ducts may be fire insulated on both sides of the wall or the duct could be connected to the wall opening via a fire damper tested to fulfil e.g. “EI 60” as in the example given above. The fire dampers are normally officially tested and provided with certificate showing that they close tightly and withstand the heat during the time required. The fire damper can however only provide safety if it works properly and closes when the fire starts. Therefore some countries require that fire dampers are regularly tested and that this requirement is stated in the operation manuals of the installations. The fire dampers normally used today thus have to be equipped with damper motors used to open the damper after the test.
Sometimes the chosen solution is a combination of these alternative ways - duct insulation and fire damper – providing an alternative as safe as the wall itself.
1.7 How are the duct designers, and other participants, working with duct design and requirements today?
Designers of HVAC systems, installers, contractors and building owners in different European countries have been interviewed or asked to answer enquiries sent out to provide a background on what tools and facilities are used. They were also questioned on what the quality requirements on ductwork are and how they are expressed and controlled.
The evaluation of this material shows that there is a certain difference between the way technicians in northern and southern Europe use ductwork. The former seem to be more accustomed to using circular ducts as a standard solution whenever suitable while technicians in southern Europe use more rectangular ductwork. The differences in working with these two types of ductwork are discussed in different following chapters in this source book (§ 8). A third lesser used alternative, the flat-oval ducts, does not seem to be of common use by interviewees and are not available on the market in most of the countries.
The answers mainly show that ductwork in many countries is considered as an important part of the building installations and that this part of the design work is done meticulously. This is gratifying as the ductwork normally accounts for about half of the installation costs of the HVAC plant.
The ductwork is also indirectly involved to a large degree in the life cycle costs if not designed in a proper way. These questions are dealt with in several of the following chapters in this source book (§ 6.3).
In some countries, e.g. in Sweden with its half century old “AMA-system”, which is described in § 5.3.4, quality requirements for duct installations have been specified for many years. These demands are normally stated in building specifications, expressed in controllable units and controlled by testing before the contractor is released from his commitments.
In other countries the awareness is not as clearly expressed. Ductwork tightness requirements of ductwork are. neither expressed in building specifications nor tested before the building is taken into operation. These different philosophies and different methods were also found in an earlier EU SAVE-project “Improving ductwork – A time for tighter air distribution systems” [Ref 2] where ductwork in Sweden was found to be about 25-50 times tighter than ductwork used in Belgium and France (§ 7.10.6).
The present sourcebook comprises the following main content:
Chapter 2 gives an overview of different ventilation principles and components used in such systems.
Chapter 3 explains some reasons why and how a ductwork system should be carefully designed.
Chapter 4 describes how less energy can be used in the duct system.
Chapter 5 gives examples on how better ductwork can be introduced in Europe.
Chapter 6 discusses the cost elements and whether a better ductwork really costs more than one of lower quality.
Chapter 7 shows different ways of integrating a duct system into the building, how to reduce noise transmission and fire hazards, system flow and tightness characteristics, and maintenance requirements.
Chapter 8 compares space requirements and costs for circular and rectangular ducts.
Chapter 9 describes duct manufacture and installation.
Chapter 10 describes how the quality of the system is controlled before being put into operation.
Chapter 11 points at the importance of maintaining the duct systems during its lifetime.
Chapter 12 presents several practical examples and case studies of duct installations, good and bad.
Chapter 13 comprises a large number of ductwork checklists that can be used by those concerned from the programming phase to operation and maintenance.