User Manual

Control concept
Closed-loop control strategy
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● Low supply air setpoint for the reverse-acting part of the sequence
● High supply air setpoint for the direct-acting part of the sequence
● Supply air setpoint for energy recovery
● Min/Max setpoint limit control (supply air setpoint)
● Selection of type of operation for heat recovery
● Initial value for the integrator can be defined
Compared with control without a cascade, e.g., cascade control improves the dynamics of the control
process.
If the temperature in a ventilated room is below the setpoint, e.g., the supply air temperature must be
increased, at least for a brief period, in order to raise the temperature to the room setpoint. This can be
achieved by measuring and controlling not only the room temperature, (that is, the value which actually
concerns the user), but also the supply air temperature, whose setpoint depends on the difference between
the room setpoint and the room temperature.
If the room temperature is lower than the room setpoint, the supply air setpoint is adjusted in proportion to
the room control differential, and the supply air temperature is increased via the supply air control loop.
The master controller generates the setpoint for the auxiliary variable (e.g., the supply air temperature) on
the basis of the difference between the primary setpoint and the primary controlled variable (e.g., the room
setpoint and the room temperature).
The master controller must include an integrator function (I component), because even under static
conditions (that is, when the measured value and the setpoint are equal) there is generally a negligible
control deviation, which means that the controller output must be at a different operating point. For
improved control dynamics, a P-component should be connected in parallel with the integrator. This is why
the master controller in this case has a PI control structure.
Even when the primary controlled variable (room temperature) is identical to its setpoint, the auxiliary
controlled variable (supply air temperature) must generally be at a value other than 0, (that is, setpoint ≠
0). This is only possible if the output of the master controller is not equal to 0, even if the P component = 0.
In other words, the master controller must have an I-component which remains constant when the control
differential = 0. This is why the master controller has a proportional and an integral component. It is a
numerical PI controller for use as a master controller in a room/supply air cascade.
To save energy in the ventilation plant, various room set points are selected for different types of air
handling (heating/cooling and humidification/dehumidification). The master controller in the cascade must
therefore be able to generate different supply air set points, depending on how the kind of air treatment
(heating/cooling or humidification/dehumidification).
The supply air controller must determine whether the heating or cooling sequence is to be activated and
the decision-making strategy does not affect the calculation of the two supply air set points. Within the
cascade control loop, the supply air set points always move parallel to each other, and their offset is
determined by the integral component.
If the air handling plant includes an energy recovery aggregate, this aggregate may be either reverse-acting
(e.g., heating) or direct-acting (e.g., cooling) depending on the relationship between the condition of the
outside air and the condition of the exhaust air.
To avoid external calculation of the energy recovery setpoint, this, too, is done by the master cascade
controller, and made available to the energy recovery aggregate, if there is one, at a separate output pin: