The obvious problem which the Dessicant based AHU presents is that instead of the Cooling load being met by one dedicated battery or staged DX coil, the cooling is introduced in stages using a combination of thermal and dessicant wheels and evaporative coolers. The heating is likewise staged. Various interlocks complicate the problem.
This is a standard Controller and it provides control of Fans, interlocks and Supplementary Heating and Cooling batteries as found on the McQuay unit. The internal connection between the Control loops and the drivers passes through a re-scale function thus allowing the internal drivers for the heating and cooling coils to be allocated any portion of the control output range. These settings are accessed using config variables HRMN, HRMX (Heat Rescale MiNimum and HeatRescale MaXimum) for heater battery and CRMN, CRMX for cooling battery. These variables are found in the driver block at C154 onwards.
This is a new type of module which is registered to the AHU controller and takes the address slots which are normally used by actuators. Each module allows portions of the Heat Demand and Cool demand signals to be re-scaled and sent to one of the outputs of the universal driver.
This is based on the Actuator but the control loops are replaced by 're-scale function blocks' which have a passing resemblance to those used in a common brand of Configurable controller.
The network variable is either nviHeatDemand or nviCoolDemand from the AHU controller, a portion of its control authority, defined by AIMN (Ablock Input MiNimum) and AIMX (Ablock Input MaXimum) is re-scaled to normally 0 to 100% in the re-scale function for onward processing by the local driver. The output range of this recaling can be limited, offset or the sense inverted by suitable settings of AOMX (Ablock Output MaXimum) and AOMN (Ablock Output MiNimum).
The addition of an ON DELAY module completes the basic picture. This provides a delay of ADLY minutes to the progression of the demand signal to the driver, this is needed for some outputs to allow time to dry the exposed section of wheel etc.
These modules each have two channels, both with heat and cool inputs. See diagram.
From this general purpose module two function specific versions have been produced, one to control the Dessicant wheel and Re-generative Heater and another to control the Thermal Wheel and Evaporative Cooler. Each of these have additional interlocks incorporated.
This overall control scheme for the McQuay equipment shows that there are quite a number of configuration parameters to be setup and 'tuned' at commissioning. Some Doorway pages have been produced to assist in this task.