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Ventilation Guidelines For Design

The purpose of the guidelines (Blomsterberg,2000 ) [Ref 6] is to give guidance to practitioners (primarily HVAC-designers and building managers, but also clients and building users) in how to bring about ventilation systems with good performances applying conventional and innovative technologies. The guidelines are applicable to ventilation systems in residential and commercial buildings, and during the entire life cycle of a building i.e. brief, design, construction, commissioning, operation, maintenance and deconstruction. 

The following prerequisites are necessary for a performance based design of a ventilation systems: 

  • Performance specifications (concerning indoor air quality, thermal comfort, energy efficiency etc.) have been specified for the system to be designed. 
  • A life cycle perspective is applied. 
  • The ventilation system is considered as an integral part of the building. 

The aim is to design a ventilation system, which fulfils project specific performance specifications (see chapter 7.1 ), applying conventional and innovative technologies. The design of the ventilation system has to be co ordinated with the design work of the architect the structural engineer, the electrical enginee and the designer of the heating/cooling system This in order to ensure that the finished building with heating, cooling and ventilation system performs well. Last and not least the building manager should be consulted as to his specia wishes. He will be responsible for the operation of the ventilation system for many years to come. The designer therefore has to determine certain factors (properties) for the ventilation system, in accordance with the performance specifications. These factors (properties) should be chosen in such a manner that the overal system will have the lowest life cycle cost fo the specified level of quality. An economica optimisation should be carried out taking into account: 

  • Investment costs
  • Operating costs (energy)
  • Maintenance costs (change of filters, cleaning of ducts, cleaning of air terminal devices etc.) 

Some of the factors (properties) cover areas where performance requirements should be introduced or made more stringent in the near future. These factors are:

  • Design with a life cycle perspective
  • Design for efficient use of electricity
  • Design for low sound levels
  • Design for use of building energy management system
  • Design for operation and maintenance 

Design with a life cycle perspective 

Buildings must be made sustainable i.e. a building must during its lifetime have a small as possible impact on the environment. Responsible for this are several different categories of persons e.g. designers, building managers. Products are to be judged from a life cycle perspective, where attention must be paid o all impacts on the environment during the entire life cycle. At an early stage the designer, he buyer and the contractor can make environmentally friendly choices. A building consists of several different components with different life spans. In this context maintainability and flexibility have to be taken nto account i.e. that the use of e.g. an office building can change several times during the ife span of the building. The choice of ventilation system is usually strongly influenced by the costs i.e. usually the nvestment costs and not the life cycle costs. This often means a ventilation system that just fulfils the requirements of the building code at he lowest investment costs. The operating cost for e.g. a fan can be 90 % of the life cycle cost. Important factors relevant to life cycle perspectives are:
Life span.

  • Environmental impact.
  • Ventilation system changes.
  • Cost analysis.

A straightforward method used for life cycle cost analysis is to calculate the net present value. The method combines investment, energy, maintenance and environmental cost during part of or the entire operational phase of building. The yearly cost for energy, maintenance and environment are recalculated o a cost at present, today (Nilson 2000) [Ref 36]. With this procedure different systems can be compared. The environmental impact in costs is usually very difficult to determine and is herefore often left out. The environmental impact is to some extent taken into account by including energy. Often the LCC calculations are made to optimise the energy use during the period of operation. The main part of the life cycle energy use of a building is during this period i.e. space heating/cooling, ventilation, hot water production, electricity and lighting (Adalberth 1999) [Ref 25]. Assuming the life span of a building to be 50 years, the operating period can account for 80 – 85 % of the total energy use. The remaining 15 – 20 % is for the manufacturing and transportation of the building materials and construction. 

Design for efficient use of electricity for ventilation 

The use of electricity of a ventilation system is mainly determined by the following factors: • Pressure drops and air flow conditions in the duct system
• Fan efficiency
• Control technique for the air flow
• Adjustment
In order to increase the efficiency of the use of electricity the following measures are of interest: 

  • Optimise the overall layout of the ventilation system e.g. minimise the number of bends, diffusers, cross section changes, T-pieces.
  • Change to a fan with higher efficiency (e.g. directly driven instead of belt driven, more efficient motor, backward curved blades instead of forward curved).
  • Lower the pressure drop at the connection fan – ductwork (fan inlet and outlet).
  • Lower the pressure drop in the duct system e.g. across bends, diffusers, cross section changes, T-pieces.
  • Install a more efficient technique of controlling the air flow (frequency or fan blade angle control instead of voltage, damper or guide vane control).

Of importance to the overall use of electricity for ventilation is of course also the airtightness of the ductwork, the air flow rates and the operational times.

 In order to show the difference between a system with very low pressure drops and a system with up to now current practise an “efficient system”, SFP (specific fan power) = 1 kW/m³/s, was compared with a “normal system”, SFP = between 5.5 – 13 kW/m³/s (see Table 9). A very efficient system can have a value of 0.5 (see chapter 6.3.5 ). 

  Pressure drop, Pa
Component Efficient Current
practise
Supply air side    
Duct system 100 150
Sound attenuator 0 60
Heating coil 40 100
Heat exchanger 100 250
Filter 50 250
Air terminal
device
30 50
Air intake 25 70
System effects 0 100
Exhaust air side    
Duct system 100 150
Sound attenuator 0 100
Heat exchanger 100 200
Filter 50 250
Air terminal
devices
20 70
System effects 30 100
Sum 645 1950
Assumed total fan
efficiency, %
62 15 – 35
Specific fan
power, kW/m³/s
1 5.5 – 13

Table 9 : Calculated pressure drops and SFP values for an “efficient system” and a “current system”. 

Design for low sound levels 

A starting point when designing for low sound levels is to design for low pressure levels. This way a fan running at a low rotational frequency can be chosen. Low pressure drops can be achieved by the following means: 

 

  •  Low air velocity i.e. large duct dimensions
  • Minimise number of components with pressure drops e.g. changes in duct orientation or size, dampers.
  • Minimise pressure drop across necessary components
  • Good flow conditions at air inlets and outlets

The following techniques for controlling the air flows are suitable, taking sound into account:

  • Control of the rotational frequency of the motor
  • Changing the angle of the fan blades of axial fans
  • Type and mounting of the fan is also important to the sound level.

If the thus designed ventilation system does not fulfil the sound requirements, then most likely sound attenuators have to be included into the design. Do not forget that noise can enter through the ventilation system e.g. wind noise through outdoor air vents.
7.3.4 Design for use of BMS
The building management system (BMS) of a building and the routines for following up measurements and alarms, determine the possibilities to obtain a proper operation of the heating/cooling and ventilating system. An optimum operation of the HVAC system demands that the sub-processes can be monitored separately. This is also often the only approach to discover small discrepancies in a system which by themselves do not increase the energy use enough to activate an energy use alarm (by maximum levels or follow up procedures). One example is problems with a fan motor, which does not show on the total electric energy use for the operation of a building. 

This does not mean that every ventilation system should be monitored by a BMS. For all but the smallest and simplest systems BMS should be considered. For a very complex and large ventilation system a BMS is probably necessary. 

The level of sophistication of a BMS has to agree with the knowledge level of the operational staff. The best approach is to compile detailed performance specifications for the BMS. 

7.3.5 Design for operation and maintenance
In order to enable proper operation and maintenance appropriate operation and maintenance instructions have to be written. For these instructions to be useful certain criteria have to be fulfilled during the design of the ventilation system: 

  • The technical systems and their components must be accessible for maintenance, exchange etc.. Fan rooms must be sufficiently big and equipped with good lighting. The individual components (fans, dampers etc.) of the ventilation system must be easily accessible. 
  • The systems must be marked with information as to medium in pipes and ducts, direction of flow etc. • Test point for important parameters must be included

The operation and maintenance instructions should be prepared during the design phase and finalised during the construction phase. 

 

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/313573886
Towards improved performances of mechanical ventilation systems
Aauthors, including:Peter Wouters, Pierre Barles, Christophe Delmotte, Åke Blomsterberg
Some of the authors of this publication are also working on these related projects:
Airtightness of buildings
PASSIVE CLIMATIZATION: FCT PTDC/ENR/73657/2006


Post time: Nov-06-2021