FANS Throttle control


The simplest, but also the most uneconomical control is the throttle control. In this case, an adjustable orifice is installed in the system, with the help of which the system characteristic curve is changed, which leads to new points of intersection with the fan characteristic curve which are then further to the left, i.e. at a lower V̇ but at higher pressure, which then has to be additionally throttled away. Furthermore, the efficiency of the fan decreases with stronger throttling. With a throttling of the volumetric


flow by eg. 25%, the expected change in power consumption will be < 5%, so not an overwhelming effect.


Speed control More economical, but more costly, is the speed control of a fan with the help of controllable electric motors. This type of control has the advantage that the fan can be operated in the most favourable efficiency range.


The system characteristic curve remains unchanged, while the fan characteristic curve changes in accordance with the affinity laws. The disadvantages of this type of control are the higher acquisition costs for the frequency converter and the poor efficiency in partial load.


Using a similar example as for the throttle


control we would observe here a reduction in power consumption of about 60%, a significant advantage.


Blade adjustment


For axial fans with adjustable blades, the flow rate can be controlled by adjusting the blade angle.


Axial fans with variable pitch blades do not


achieve quite the same efficiency in flow rate control as speed control, but the electrical losses are eliminated in this case. The capital expenditure for axial fans with "variable pitch blades" is considerably higher than for "fixed pitch blades". This expense is only worthwhile if the volume flow changes frequently and has to be adjusted for operational reasons. Using a similar example as for the throttle


control we would observe here a reduction in power consumption of about 45%.


Inlet vane control Inlet vane controllers can be used primarily with centrifugal fans, but also with axial fans. They are on the suction side and change the


direction of the inlet velocity into the impeller as adjustable guide vanes. They generate a "swirled flow" at the impeller inlet and thus cause a corresponding change in volume flow.


In the case of large changes in the volume flow rate, inlet vane control is only meaningful in conjunction with a pole-changing motor due to the rapid deterioration in efficiency.


The advantages of inlet vane control are


the lower investment costs and the possibility of using squirrel-cage motors. Using a similar example as for the throttle


control we would observe here a reduction in power consumption of about 40%.


Drive layout The power demand at the shaft of the fan can be easily calculated, we have shown this before. Usually, we add a power reserve which is about 5 to 10 % for directly driven fans, and 10 to 20 % for belt-driven fans, depending on the size of the fan. An important criterion for motor selection is the acceleration torque. This must be in a certain ratio to the mass moment of inertia of the fan to ensure proper start-up behaviour. The mass moment of inertia refers to the


rotating parts of the fan, i.e. impeller, hub, and shaft. It is the product of the mass of the rotating parts multiplied by the square of the 'radius of inertia' and determined experimentally and specified by the fan manufacturer. The motor manufacturers generally allow a start-up time of 10 s. This is only correct for direct driven fans. For belt-driven fans we must use the so- called reduced mass moment of inertia. In ventilation and air-conditioning applications, the belt drive is very common, because they show a very good adhesion due to the wedge effect between belt and pulley. The belt drive should be designed in such a way that the belt speed does not exceed 20 m/s.


Clutches are used to connect rotating machine parts, in this case the motor and fan impeller. They have the task of transmitting a torque at a certain speed. The basis for dimensioning is therefore the fan speed and the torque on the fan shaft or the shaft power.


In ventilation and air-conditioning applications, mainly flexible, direct-acting clutches are used. In special cases eg. if the motor does not reach its nominal


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speed during the maximum start-up time, centrifugal clutches are also used, in which the motor first runs up to nominal speed and then the fan is accelerated by the clutch through frictional forces until it reaches its operating speed.


Selection of fans


When designing a fan, the selection of which is made on the basis of the measured characteristic curves, one should always compare the intended installation situation with the measurement arrangement when determining the characteristic curve. It is not uncommon for fans to be installed in systems with unfavourable flow conditions, so that the conditions are completely different than in theory and therefore the operating point on the characteristic curve cannot be reached at all. In case of free-intake fans without inlet


nozzle we have to consider a different installation situation ie. characteristic curves are always measured on test stands with an inlet nozzle. If this is omitted the flow lines run quite differently due to the sharp-edged flanges without a guiding nozzle. The flow constricts, resulting in an unfavourable impact on the blades. This results in a reduced performance, i.e. the fan characteristic curve of the test stand measurement, which is given in the manufacturer’s data sheet, will not be achieved.


In case of free-blowing axial fans we note that with this type of installation the pressure recovery is lost. This loss must be added to the other system’s resistances when calculating the total resistance inside the system! When calculating the resistance elements located directly behind the fan, for example heat exchangers, are subjected to higher incident flow velocities on the ring surface leading to higher overall resistance values.


An improvement of the conditions can be


achieved by arranging the elements on the suction side or with a diffuser. At the end of this fourth instalment of our series on fans, I would like to refer to the numerous technical publications on the subject, which I reviewed and used for the summary. I would like to highlight the publication by Trox TLT GmbH (see Grundmann, Schönholtz, 2013) on the subject matter, which in addition to my remarks also contains detailed installation instructions for centrifugal and axial fans.


www.acr-news.com • July 2022 15


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