Wednesday, 18 February 2015

In all applications it’s a good idea to make sure the control of the humidifiers is correct. This ensures the humidifier operates reliably, conditions are maintained and limits are not exceeded. Gibbons continue to supply humidifiers in a variety of applications, including data centres, semiconductor manufacturing facilities, theatres, factories and offices and we understand that good control is essential to all.

The most common mistake we encounter is the control of the supply air leaving the AHU. This is most important and enables the humidifier to provide the highest performance. Relying on a humidistat – perhaps set to trip at 80% RH – to limit humidity is not a good method.  That’s because it can cause limit cycle operation (where the humidifier repeatedly exceeds the trip level of the humidistat, so it turns off and back on again when the humidity drops), which results in unnecessary wear and tear on the humidifier and also reduces the average humidity produced. There’s quite often a delay between the humidifier going off line and coming back on line when the humidistat has recovered.

These problems become particularly acute if the airflow is reduced, at initial start-up with a large humidity demand from the space or when the air conditions only require a small addition of water to reach the permitted maximum humidity within the AHU.

Using a PID control loop to modulate the humidifier output solves these problems. The diagram below illustrates the configuration of the AHU humidity control loop with the RH sensor mounted in the supply air:

(The AHU humidity control loop and humidity sensor are included in all Gibbons humidifiers as standard)

The difference between the control techniques is illustrated in the graphs below, with a humidistat trip level of 80% RH for the no control option and 85% for the PID control option.

It can be seen that the average humidity with the supply air PID control loop in place produces significantly higher average humidities than when no supply air control is used and the system limit cycles with the humidistat.

With the supply air PID control in place, the building humidity can be managed using the normal humidity control method. Even if the demand from the building control is 100%, the control loop will keep the supply air at a steady 80% RH (this level is not essential but is the normal maximum allowed).

Humidity control in data centres

Gibbons Engineering Group has a wealth of experience installing and controlling humidity in data centres, where the main problem with controlling humidity levels is the temperature gradient in server halls.

Conventional humidity controls take the average humidity level in the humidified space to provide feedback to the humidity control loop. The difficulty with this approach is that relative humidity alters considerably as temperature changes, so it is very difficult to obtain a reliable humidity feedback level for the control system.

One method of eliminating unreliable humidity readings is to measure and control moisture content, which can be obtained by combining dry bulb temperatures and relative humidity readings (normally available from a single sensor). The moisture content sensors could be mounted in the server hall, however, it is now possible to control the supply air moisture content because the moisture content will be the same in the supply air as it is in the server hall (assuming minimal moisture leakage). Controlling the supply air moisture content only requires one sensor mounted in the output of the AHU saving considerably on installation costs.

This technique not only enhances the reaction time of the humidification control system in data centres, it can also be employed in industrial humidifiers for offices and factories.  

For more information on humidity control for your building, call Gibbons’ Humidification Managing Director Steve Rix on 07966 423165 or email


  1. Very informative post. The chart and the images made it easy for me to understand the topis - controls of humidifier.



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