Precise control of environmental parameters such as humidity is essential in any cleanroom where stable conditions are critical to the activities within the space. Furthermore, the exact requirements will vary from one cleanroom application to another. This means that choosing the most appropriate solution from the range of humidity control technologies available requires the application of specialist knowledge.
To that end, it is important to select a humidity control specialist that is able to supply every type of humidity control system, rather than being tied to one manufacturer’s particular range.
This principle was clearly illustrated in a recent project for the Department of Physiology, Anatomy and Genetics at the University of Oxford, in the UK. This project was for a new laboratory where it was necessary to maintain extremely strict environmental conditions that are essential for the consistency and success of the experiments taking place.
The specified conditions were that the relative humidity (RH) should remain between 45% RH and 65% RH at a temperature of 20–23°C. The system was therefore set to maintain a target RH of 50%. A further challenge was that this 36m3 cleanroom environment would experience 30 air changes per hour, and the humidity control strategy needed to allow for this.
Consequently, whereas the requirement in most cleanroom environments is to control either the upper limit or the lower limit, in this case it was necessary to control both by using both a humidification system and a dehumidification system.
Complying with the lower limit
Ensuring that the RH did not fall below 45% required the use of a humidifier in the supply air duct serving the room and, as noted above, it was necessary to select the most appropriate type of humidifier for the conditions that would be experienced.
For example, Oxford is a hard water area so it was clear that limescale formation would be a potential issue. To minimise this risk a Neptronic resistive humidifier was selected, rather than an electrode boiler. The reason for this was that, with a resistive humidifier the output does not fall off as the limescale begins to form – whereas scale formation on an electrode boiler would result in reduced output over time and also necessitate a more onerous maintenance schedule.
While regular maintenance is still required, the stainless steel cylinder of the resistive humidifier is easily cleaned and maintained
While regular maintenance is still required, the stainless steel cylinder of the resistive humidifier is easily cleaned and maintained. Furthermore, it does not require the regular replacement of components that would be necessary with an electrode boiler. This is not to say that electrode boilers do not have a use; simply that they were not the most suitable solution for this project.
Another benefit of resistive humidifiers is that they have the ability to provide the best control tolerance as they pulse. As a result, the current/heat through the elements is maintained even when the water level varies as moisture is boiled off.
The inlet water feed can also be set to trickle feed so that the water in the cylinder does not fall below the boiling point even when a large volume of cold water is introduced.
Drainage from the system can be set to suit the quality of the water serving the unit, which includes the ability to stop drainage if the cylinder is served with mineral-free water (e.g. from a reverse osmosis system). Again, this ensures that there is no interruption to the production of steam. This humidification system uses modulation in response to a control signal; in this instance provided by a dedicated HR020 controller, which also controls the dehumidifier. The controller is connected to a duct sensor to provide a signal to the controller, as well as a high level duct humidistat to ensure that the humidity in the supplier never goes too high.
The supply air volume to the laboratory is 0.25m3/s through a 400mm2 duct so the air velocity is relatively low. However, to ensure that the 5kg/hr of steam being introduced to the laboratory is fully evaporated before it enters the space 2m downstream of the steam lance meant that the selection of the type of steam lance was critical.
The most appropriate choice proved to be a SAME2 steam lance which has a double row of steam outlet holes, each of which is fitted with a brass insert. The purpose of the brass insert is to ensure that the steam is taken from the centre of the pipe where it is hottest and driest while removing heavier droplets and condensate. This arrangement ensures that the steam droplets are very small, which facilitates fast evaporation into the pre-heated 20–23°C supply air to achieve the 50% RH set point.
A further challenge in designing the system resulted from the space constraints imposed by the need to install the humidifier above a false ceiling with the point of injection into the duct being lower than the height of the humidifier. Because of the potential for condensation a condensate trap was therefore fitted at the low point of the steam pipe.
Complying with the upper limit
As noted earlier, it was also necessary to include dehumidification to ensure that the RH in the laboratory did not exceed 65% during the summer months, when the humidity of outside air is relatively high.
As a result of the space constraints in the false ceiling mentioned above, it was not possible to install the dehumidifier within the supply air flow. Instead, a stand-alone floor-mounted refrigerant dehumidifier capable of removing up to 4.5 litres/hr with an air flow of 1,180m3/hr was installed. The condensate produced by the dehumidifier could not run to a drain point so a small condensate pump has been used to take the water to the drain installed for the humidifier in the ceiling void.
The Fisair dehumidifier range
This high-performance, high-efficiency unit quietly supplies dry air through a grill at the top of the unit while removing moist air at the bottom of the unit. The capacity of the selected unit ensures that the humidity in the laboratory does not exceed 65% RH during the summer.
Due to the nature of the experiments taking place in the laboratory, the dehumidifier has been boxed-in and uses fine inlet and outlet grilles, with the casing around the dehumidifier manufactured from a special high-pressure laminate (HPL) material called Trespa.
Horses for courses
The key point that this project illustrates is the need to fully understand the requirements of the project and also the factors that can impinge upon the operation of the system. Consequently, choosing the most appropriate form of humidity control goes beyond simply being able to introduce the required amount of moisture into the air. It is important to consider other factors such as water quality, space availability and the configuration of the air supply system.
In today’s cost and carbon conscious world it is also necessary to take account of issues such as energy consumption and environmental impact. So, while meeting the performance criteria of the project is always the primary consideration, it is important to try to meet those criteria efficiently and cost-effectively.
This approach also needs to take account of life-cycle costs such as ongoing maintenance requirements and the potential need to replace components in the future. For example, in hard water areas where limescale formation is a potential problem it may be cost-effective to make an initial investment in demineralisation or reverse osmosis plant to treat the supply water as this will reduce ongoing maintenance costs.
Given all of these factors it is clear that there is no off-the-shelf solution for cleanroom humidity control. The key is to call on the assistance of those with the knowledge and experience to guide you to the best solution.