Understanding desiccator technology
24th April 2006
As components and products become smaller and more sophisticated they become more susceptible to damage by moisture. Once the moisture is absorbed it can have disastrous consequences for the products integrity often making them unserviceable. This is true in many industries from Electronics, Pharmaceutical, Chemical and Medicines.
To prevent this the product must be manufactured and stored in a dry medium normally Nitrogen [N2] which is dry, inert, non reactive and relatively cheap. The introduction of nitrogen into an enclosed environment has a rapid impact on Relative Humidity [RH] Desiccant processes are available but are slow and not easy to control.
High purity N2 is introduced into the top of a desiccator or glove box where it purges out the ambient moisture laden air. In the case of a multi-chamber desiccator the N2 should be first fed into a plenum and then into each chamber to ensure equal distribution. There needs to be a valve in the desiccator to allow for the gas to flow, Relief/Bleed [RB] valves are fitted to each desiccator chamber to ensure that they are bled independently.
Almost any low RH can be obtained using this system, however nature being what it is will constantly endeavour to equalise the RH between the outside world [high RH] and the atmosphere inside the desiccator or glove box [low RH]. This will happen every time a door is opened to remove the product, it will also happen gradually over time because desiccators are made from materials which are Hygroscopic which permit the absorption or passage of moisture. This can be controlled by continually topping up the N2 manually. If this becomes a problem during long term storage or where the specified RH is critical an automated closed loop system must be employed.
An RH sensor is required inside the desiccator along with a controller [NitroWatch] connected to a system to turn on and off the N2 flow [DualPurge]. The systems will work together to sense the internal RH of the desiccator or Glove Box, compare that condition with the specified set point on the NitroWatch and call for more flow of N2 from the DualPurge to returned the system to the desired conditions. The DualPurge is designed to provide N2 at a high flow or purge rate when it is desirable to rapidly recover the set point just after a door has been opened and then to provide a low flow rate to enable the system to toggle around the set point under normal storage conditions. This ensures that the product spends the maximum amount of time stored at the specified RH whilst minimising the consumption of N2.
These systems can be employed using different construction materials and in a wide range of sizes. The technology has advanced to where today the equipment is all ˇ§plug and playˇ¨ taking away from the operator complicated setup and control procedures. Long or short term storage away from the harmful effects of moisture is now within the reach of almost any industrial application.
Author : Alan Waterman-Smith - C.Eng. M.I.E.E. Managing Director Sirus Microtech Ltd.