Ventilation system

SWEGON AB Indoor Climate Systems 2004 - Air distribution products - Rev. 5 June, 2007
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Ventilation system

TRADITIONAL TYPES OF VENTILATION SYSTEMS

The choice of a suitable technical solution is an important planning factor. The choice of system should be made after considering the following main factors:

Practicability. The ability of the technical solution to meet the required quality demands.

Reliability. The ability of the technical solution to function satisfactorily over time.

Efficiency. The efficiency of the technical solution regarding power, cost-effectiveness etc. When choosing a technical solution one should always strive to attain simplicity, intelligibility and ability to cope with fluctuations of operating factors. Avoid technical solutions that do not allow the layout of the premises to be changed, windows to be opened or which are in any other way sensitive to external disturbances.

Basic principles and characteristic properties:
There are different ventilation solutions that can be implemented to fulfil the demands for correct air flows to all parts of a system. The main categories are:

CAV systems (Constant Air Volume), systems with constant air flows. The simplest and generally the “cheapest” alternatives.
VAV systems (Variable Air Volume), systems with variable air flows, as a rule regulated by thermostats in each room. The fan is fitted with some form of pressure regulation device.
DCV systems (Demand Controlled Ventilation), systems controlled by demand, as a rule regulated via an air quality or presence sensor.
All system solutions can, of course, be designed for either mixing or displacement regulated ventilation.

Both CAV and DCV systems can be combined with different heating and cooling units for regulating the indoor temperature.

CAV-SYSTEM

CAV systems are used where both heat and pollutant production is low and reasonably constant. The flow of supply air is mainly determined by the quality demands on the air. If the hygienic air flow is not sufficient to remove the generated heat then products for waterborne cooling can be added. CAV systems are often designed according to the branching principle with an adjustment damper in each branch. The pressure drops across the terminals is chosen so that these, together with the pressure drops across the dampers, give the correct flow distribution.

The disadvantage of this principle is that the system can easily become unbalanced because of the effects of thermal lift, changes in damper settings etc.

Another disadvantage is the relatively high pressure drop across dampers and terminals that is required to ensure that the flow variations are not too large. In turn, this means that sound problems can be difficult to deal with and power consumption unnecessarily high. Lowering the fan speed, to lower power consumption during certain periods, means that the flow distribution cannot be maintained because the pressure drops across terminals and dampers is reduced.

Figure 29. CAV systems, schematic diagram.
1. Exhaust air
2. Supply air
3. Ventilation equipment (FTX)

VAV/DCV-systems

VAV/DCV systems are used for variable occupancy rates. Heating is best achieved by using radiators. Cooling the room can be achieved by varying the air flow.

VAV/DCV systems differ from CAV systems in a number of ways, one of which is that pressure is regulated in the main ducts for supply and exhaust air. This is necessary from both a power and a sound point of view.

Another difference is that in the immediate vicinity of the supply air terminal there are dampers that regulate the air flow through the terminals. A fundamental problem with this is that when air flows are reduced the pressure drops increase. This can have serious consequences. Increased pressure drops lead, in general, to higher sound levels. The pressure in the main duct must always be sufficient to guarantee that requisite volumes of air can be delivered to the remotest branches. If the flow distribution in the system should cause a temporary lower pressure, the setpoint value must still be maintained. This has a negative effect on the running costs of the system.

Demand controlled ventilation
It is generally accepted that if we, as users of an installation, can easily adjust the system, we perceive the system as better. In homes, for example, this means that residents can easily adjust air volumes to meet individual needs. In terms of the traditional FTX-system (fan controlled supply and exhaust air), we are not accustomed to the luxury of controlling the various air volumes in different spaces. Instead there has been an attempt to keep the air volumes as constant as possible. However, we are convinced that there is a clear advantage if residents can adjust air flows in individual rooms (within reasonable limits). This should be possible without altering the air volumes in other rooms. The minimum air volume in each space must be guaranteed.