The frequency converter controls the fan speed by adjusting its value to the current airflow demand. This solution makes a dynamic system and offers electricity savings. How does the frequency converter ensure that the design parameters are achieved?

The role of the frequency converter in ventilation systems capacity control

Speed control is very often the key [Ventilation systems] naturally require performance control depending on a number of variables (e.g. the number of people in the room, season, etc.). In comparison to other methods of capacity control – such as changing the angle of the rotor blades or using air dampers – the inverter has the highest efficiency. For systems that do not require dynamic capacity control, the question often arises: should an inverter be used when (apparently) its only role will be the reduction of starting currents?

Fan’s capacity characteristics

At the beginning of our considerations, it is worth remembering how the actual operating point of the fan is established. This is because it is placed at the intersection of the pressure gain characteristics and those of the system as a whole. This point is determined automatically during the operation of the machine after reaching a compression equal to the system’s resistance, which in turn results from the flow.

How to establish the exact resistance of the system?

Establishing the exact resistance of the system is not an easy task, even though various methods are applied, including numerical analysis. Most frequently, a certain reserve is added to the calculated resistance to make up for any inaccuracies in the calculations and in the representation of the system in design during its implementation.

How to select a fan for specific parameters?

The next step, when we get an additional available pressure, is to select a fan for the given parameters. In this process, we often end up with another oversizing, because it is very rare that the operating point is exactly the same as the design point. Most frequently, fans are selected for slightly higher parameters determined by the geometry of the impeller and the nominal speed of the fan at 50 Hz. This is because it is extremely rare for a fan to be selected exactly as needed for a rigid speed, as well as selected for the sizes of typical device diameters specified by a Renard series.

Which selection methodology to choose?

The selection of the device for the static pressure gain parameter is another factor that affects the oversizing of fans. For systems where the lines are situated at the discharge side, the phenomenon of the conversion of dynamic pressure to usable static pressure is ignored.
The effect of all the above phenomena is most often as follows: the measured parameters at the facility are often higher than required.

What potential savings are offered by a frequency converter?

The following example shows the impact of an inverter on the required power demand within a facility, one in which we can reduce the capacity to achieve the value that is actually required. Consequently the power demand may be reduced and, in turn, the operating costs.

Fan selection for ventilation purposes – a case study

Guidelines for fan selection:

Operating parameters of the selected fan (from the product data sheet):

Reference characteristics for the assumed grid resistance curve and selection for a given static pressure

During the measurements at the facility, a much higher capacity was measured =~ 83,000 m3/h. There were several contributing factors, and in particular:

• A well-performed system conforming to all the design assumptions,
• initial reserve assumed by the designer for an available compression =~20% (the actual resistance curve runs more smoothly),
• the fan selected with an allowance due to its impeller geometry and constant speed,
• system type D: respectively, the inlet and outlet ducts of the fan supply the fan with air,
• the fan selected without considering the issue of static pressure recovery during dynamic pressure conversion in the event of jet velocity drop at the outlet,
• straight sections upstream and downstream the fan 2.5D

Reference characteristics for the actual grid resistance curve (available compression = total pressure)

In case of using an inverter, we are able to adjust the capacity to the design value and consequently reduce the power demand.
To achieve the values required for the system, the speed must be reduced by approx. 20% to adjust the operating point to the design point.

Reference characteristics for the actual grid resistance at reduced speed

Conclusions:

Oversizing the fan, which is a kind of a security measure, in case of inaccurate calculations or the execution of the ductwork other than that specified in the design, is often required at the design stage, and the selection for static pressure results from Polish best practice, one that offers a certain safety reserve – particularly considering the fact that the catalogue characteristics are most frequently drafted for a specific type of system development (A,B,C,D), which may differ from the actual one.
All these procedures are crucial, considering that the main design goal lies in creating a properly working system with the appropriate parameters.

Ventilation system adjustment possibility

However, during the measurements by means of using an inverter and reducing the fan speed for a case like the one described herein, the user has a chance to adjust the system to operate in conformance with the design assumptions and save an enormous amount of electricity.
Naturally, in the case considered herein, all best practices have been applied and the reserve remained unused, which is not always the case during on-site measurements.

Manipulating the speed

The inverter also offers a chance to help the user in adjusting the parameters when, despite all efforts, the design parameters cannot be met due to errors and inaccuracies at the particular implementation stages. In such a case, the speed can be increased up to the motor power limit in consultation with the device supplier.

Motor efficiency in the ventilation system

It is worth noting the fan’s efficiency, the frequency converter of which is designed to work with the fan, and with the kind of motor it operates. SMAY supplies high-efficiency fans and inverters with the lowest internal losses available and also offers a selection of IE4 motors, which is key especially for standard domestic operation mode, as it greatly reduces operating costs.

Potential savings for the described case