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Manufacturers often invest in two parallel production lines to ensure there is no interruption of output when one line is being maintained or repaired.Atandemdrive is away to avoid that additional investment.

A tandem drive for centrifugal fans comprises two drives: a medium- or high-voltage motor and a steamdriven turbine.Amediumor high-voltage motor serves to start up the centrifugal fan, since steam from the boiler housesmay not be available at the start-up of the plant. As soon as sufficient steam is on hand, the plant switches to the steamturbine.

These drives can switch over automatically, without interrupting the production process from one drive to the other. Changing from the electric motor to the steam turbine calls for the steam turbine to be brought up to the rated speed of themotor through speed regulation. When a defined differential speed between the two drives has been reached, the turbine takes over and the electricmotor can be switched off. The electric motor is removed from the shaft unit via an overrunning one-way clutch.

Coupling configuration

The reverse, switching from turbine drive to the electric motor, is more technically challenging. It is true that relatively simple frequency- controlled motors could be used for medium- and high-voltage. But these are cost-intensive compared to motors with direct on-line starting. In this case, a special coupling configuration is used which limits the torque peaks to a defined,maximum-permissible torque (Graphs 1 & 2). This rules out damage to the drive and shaft assembly.

Typical processes where combustion air and flue gas fans with tandem drive are used are ammonia, methanol, hydrogen and syngas plants.Ahydrogen/steammixture is converted into amixture of H2, CO and CO2 and a remainder of CH4 using a steam reformer.

The overall reaction in the steamreformer is strongly endothermic and thus requires heat input. The necessary reaction heat is provided by burners which are arranged in the furnace ceiling between the tube rows.A combustion air fan must supply the required combustion air to the steamreformer burner.The flowvolume necessary for the combustion air can, depends on the size of the plant. It can be up to 550,000Am³/h.

Downstream of the combustion air fan, the combustion air is preheated to the required temperature by the burners. Combustion air is added in excess of the stoichiometric requirements to ensure complete combustion. The released heat is mainly transferred to the process in the reformer tubes,while thewaste heat is carried with the flue gas at about 1,050°C to a horizontal heat exchanger.

The waste heat is utilized in the convection bank as far as possible, for example:

• Generating and super-heating steam

• Preheating the hydrocarbon/steam mixture for the steam reformer


• Preheating and evaporating process condensate

• Preheating the hydrocarbon feed

• Preheating the combustion air Finally, the flue gas is exhausted through the stack to the atmosphere by means of a flue gas fan.The flue gas flowvolume can be as much as 1,000,000Am³/h.

By designing centrifugal fans with independent drives, it is possible to carry out maintenance work on the drive which is not in service while the plant is in operation. Overrunning one-way clutches, among other things, are used for this purpose to ensure reliable operation. The numerous design regulations for centrifugal fans and their individual components in accordancewith standards such as API 614, API 670, API 671 or API 673 aremet byVenti Oelde for couplings, oil systems and machine monitoring systems as well as for instrumentation, control valves, manometers, and so on.

Technical Data

Operating flow volume 927,000Am3/h

Differential pressure 8,800 Pa

Mechanical design 350°C

Drives 3,300 kW

Rotating speed 1,192 rpm


Joachim Schürmann, a certidied mechanical engineer, is a project manager at Ventilatorenfabrik Oelde GmbH.Venti Oelde plants and components are used for collecting, handling and filtering of air, vapours, gases, dust and airborne solids.