Central or decentralised hall ventilation? A guide to help you decide

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When is central hall ventilation the better choice, and when should decentralised ventilation be used? This neutral comparison by Dipl.-Ing. Jürgen Dorenburg looks at technical aspects, customer benefits and financial considerations.

The first step in the process is to determine the supply and exhaust air flow rates involved. You can then design the ventilation technology. Two systems are available:

  • Central system: a single ventilation unit that conditions air centrally. A network of air ducts brings the air into the hall.
  • Decentralised system: several smaller and less powerful ventilation units. They draw in the outside air directly, filter it, heat or cool it, and then blow it out into specific locations. There is no need for a network of air ducts in this case.
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Each system can offer benefits based on how it is configured, so it is worth considering the key differences between the two options:

  • Central systems weigh around three times as much as all the units required in a decentralised system put together. The central ventilation unit and the network of air ducts, together with flow rate controllers, dampers and air outlets, all have to be weighed up – quite literally – when choosing a system.
  • A lower weight means fewer issues for the hall support structure, allowing it to be constructed more cost-effectively.
  • Integrating decentralised units into manufacturing hall infrastructures is often easier than integrating an air duct network among crane runways, conveyor systems and utilities.
  • If there is no air duct network, there are no leaks. This means that there is no need to increase the swept flow rate in order to compensate for leakage.
  • Additionally, the lack of an air duct network reduces the need for maintenance according to hygiene regulations – for example, VDI 6022 in Germany.

Comparing central systems and decentralised systems means considering more than just the significant weight reduction and the lack of a duct system, however:

  • Decentralised systems allow you to develop new installations gradually or expand existing installations with ease.
  • Decentralised systems can be serviced during operation, as switching off individual units for a short time does not generally cause any problems.

Benefits for planners and installers

  • Planners are required to carry out less configuration work, as decentralised units are supplied ready for ventilation.
  • Installers need less time for installation, as the fully equipped functional units also contain fully wired electrics.
  • The operator benefits from more system reliability because several units are being used.

Integration of different airflow approaches

For a hall to fulfil its potential, the airflow must adapt to its requirements, using mixed or layered ventilation.

Air distribution is integrated in decentralised units. In standard configurations, vortex air distributors are used for mixed ventilation. In the case of layered ventilation, the supply air is introduced through fabric sleeve or radial beam outlets located within or directly above the work space. For this purpose, an air duct running from the unit to the outlet must be installed – a task which is also necessary for central systems.

Jet nozzles can be used to achieve a targeted flow in a particular section.

If various air distribution systems are being used, then the alternative outlets must exhibit the same pressure drop as the integrated standard outlet.

Measuring and control technology approaches

A hall ventilation system is only efficient if it can supply individual areas of the hall with variable air flows and different temperatures. This can create a lot of work in a central system: the air duct network requires flow rate controllers as well as multileaf dampers and, potentially, spot heaters in the sections beyond just the basic heating in the central unit.

Decentralised systems supply areas of the hall via one or more units. The units do not have to be identical and they can also be run in different operating modes. This means that it is possible to heat sections of the hall that are not in use with air recirculation whilst supplying other areas with outside air.

The decentralised arrangement of the units must take into account the architecture of the measurement and control technology. By doing this, two things are ensured:

  • Different types of units can be controlled and regulated in different operating modes with variable supply air flows and temperatures.   
  • If the different hall areas are used at different times, these zones can be ventilated, heated or cooled individually.

The system can therefore be adapted to suit each area and its different requirements, stemming from thermal and pollution loads that are specific to each. Individually adjustable control variables result in a high level of user acceptance.

In order to achieve this flexibility, units require not only the usual functions provided by a single controller, but also a zone concept, zone management software and an operating module:

  • The zone concept for the units and hydraulics determines the function of the individual units in the whole network. A zone regulator serves as an interface with the system environment: it handles the higher-level functions of the units and controls things such as operating modes, times, parameters, fault inputs and error messages.
  • With zone management software that has been adapted to suit the conditions, it is possible to form control zones using units which fulfil the same requirements. Every unit is individually and autonomously controlled by a controller, which means it must be possible to perform hydraulic switching of each unit individually.
  • The operating module and input module make it possible to assign the units in the system structure and enter the operating modes, times and parameters.

The options to integrate a system into a higher-level building automation system, and communicate via modem and Internet, are standard features.

Cost comparison

In order to compare the costs of central and decentralised systems, it is necessary to consider investments, operation and maintenance. The total cost of ownership is therefore the benchmark in this case.

Overall, there is hardly any difference in cost. A detailed analysis shows, however, that the specific areas to which costs are attributed are proportionately very different for each system.

Cost proportions

Central system

Decentralised system

Units, outdoor installation

40%

85%

Ducts and outlets

40%

÷

Measuring and control technology

20%

15%

Complete system

100 %

100%

Table: Proportions of the investment costs involved in central and decentralised systems /1/

The unit proportion includes the installation of the unit in the case of the central system, and the placement of the units on the roof in the case of the decentralised system. The item relating to ducts and outlets includes the installation proportion. The table shows that, in systems with comparable control functions, the cost of the measurement and control technology in central systems is somewhat higher. It is also apparent that comparing individual items does not make it possible to draw any meaningful conclusions.

Summary

Decentralised systems are adaptable. In many cases, they fulfil diverse hall ventilation requirements in a more precise manner than central systems, particularly in manufacturing halls. In order to get the most out of them, decentralised systems require a control system that takes their structure into consideration.

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Author
Tobias Brugger
 
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