Close Humidity Control (HUMx4a1)

Warning: Experience gained at Manchester Tobacco shows that controlling Supply Humidity will not be successful unless the supply temperature can be maintained within very tight tolerance. Controlling the Humidifier based just on Return Humidity provided the most consistant performance. The InterCoil control loop can still be utilised to hold the AHU in Humidity control. The analogue output range parameters (MNVA) can be used to maintain a minimum voltage on the humidifier to prevent it shutting down and thus introducing a long warm up delay.

The Humidity Controller has been upgraded to offer the option of close humidity control. The controller now offers the capabilty of cascading the Supply Humidity control from the Return (Space) Humidity control loop. The Supply and Return Humidity measured values are received over the network from TRH sensors. If both sensors are present then cascaded humidity control will be selected. The output from the Return Humidity loop is re-scaled between MNSH Minimum Supply Humidity and MXSH Maximum Supply Humidity to provide the setpoint for the Supply Humidity control loop. An offset DHOF DeHum Offset may be subtracted from this setpoint to produce a DeHum setpoint, large values for the offset may be used to prevent the DeHum loop becoming active under normal control conditions.

The output from the Supply Humidity loop is passed to the universal driver (HTYP, HSTG, HDLY) which may be set to use analogue or relay outputs.

The Supply DeHum loop acheives control by sending it's output demand to the Main AHU controller where it is used to control the Cooling battery, this needs to be activated when used by setting the CMDE Control Mode to 2.

For high specification Humidity control it is necessary to ensure that the Humidifier is kept running at all times so as to prevent Supply RH fluctuations as the AHU changes from Humidify to DeHumidify modes. This is acheived by monitoring the Supply Humidity loop output and using another control loop to maintain this output at or above a Target value set on TRGT. If the Humidity control output drops then the AHU needs to be controlled to DeHumidify the Supply air more. The current DeHum level can be inferred from the temperature measured between the Cooling and Heating batteries in the AHU. A temperature sensor mounted here is wired into the Humidity controller. The setpoint for the InterCoil temperature control loop is generated by comparing the Humidity Control output with the required target value, if the Humidity control output is too low the InterCoil setpoint is progressively lowered, the rate at which this happens can be changed with ICGN InterCoil Gain. Similarly as higher levels of Humidification are required then the InterCoil temperature setpoint is increased to a maximum of 30 degC. This control approach provides best Humidity control whilst maintaining energy conservation. It is important that the InterCoil setpoint changes slowly, so it may adapt to the mean Humidity output, otherwise the Humidity loop and the InterCoil loop may fight and cause oscillations. Some on site adjustment of ICGN is inevitable.