Cleanrooms are designed to insure the area meets the necessary requirements of the clean production process. But what happens when the product changes and a different manufacturing process must be put in place along with new cleanroom requirements? A similar dilemma can occur if production needs require that the clean manufacturing space double or triple because of product demand. Such needs can be addressed by modular cleanroom constructions that allow revision or expansion to meet the evolving prerequisites.
Unfortunately, many cleanrooms cannot be easily expanded or reconfigured, and they still adhere to the mandatory ISO classifications. Many modular cleanrooms are not supplied with door interlocks and must be retrofitted in the field. As a result, cleanroom manufacturers benefit from utilizing door interlock controls as an option for new and existing installations as a means of addressing these changing conditions.
The addition of door interlocks not only assures the cleanroom meets the necessary ISO classifications by enforcing clean air control, but can also add a level of security within the controlled environment of the cleanrooms. Depending upon the manufacturing process, interlock system considerations may include:
• Air system control
• Traffic controls
• User feedback indication
• Automatic door openers
• Emergency override
• Access control
• Door alarms
The construction of some modular cleanrooms may limit the number and types of interlock devices that may be compatible. This is frequently addressed with factory customized mounting hardware for locks and control switches. Fixed construction cleanrooms may utilize standard locking devices.
Choosing the right door interlock system can involve both construction limitations and building code requirements. Some locations may not allow any doors to be locked to permit free egress at any time. While this unlocked scenario provides for personnel safety, door violations may become a common occurrence. This has been addressed with the use of red/green traffic lights mounted on either side of the doors to indicate when access is allowed. Adding a built-in sounder which annunciates an improper opening can minimize such actions.
Some cleanrooms require a pressurized air system to maintain the sterile environment. This may force doors to utilize a timed delay before the second door can be opened. This re-pressurization of the interlock could be a simple timer or may incorporate a monitored pressure switch. While the air is pressurized, traffic indicator lights might be employed to provide user feedback.
While automatic door openers are frequently used to provide touch-free access or to allow easy movement or materials, special considerations are required if interlocking door controls are involved. Most overhead doors have operators that use open, close and stop push button controls. These doors cannot be opened manually, so if the door should remain closed while another door is open, the interlock controller can inhibit the open circuit while allowing the stop and close actions to remain active. Automatic pedestrian doors can be of the sliding or swinging type. While either are difficult to open manually, secure locations with swing doors often incorporate electric locks to insure that the doors remain closed when necessary. A request to open the door is inhibited and the door remains locked if another interlocked door is open. If access should be allowed the door is first unlocked and then the signal to open is allowed to activate the door operator.
Sometimes traffic controls are necessary to force personnel to use a desired traffic pattern. Limited access through doors with automatic operators or manual doors with electric locks can direct personnel by simply not providing push buttons for access on one side of the door to accomplish the required traffic flow.
Providing a means to override a locked door for emergency is common in interlocked cleanroom doors. Some consideration must be given as to how or when the system should be reset and the doors relocked. Using a push-pull switch in the sealed room allows the user to unlock all doors or a particular door by pressing the emergency button. The designated doors will remain unlocked until the switch is reset to normal. If there is a concern that personnel would forget to reset the interlock, a programmed time delay may be used to automatically relock the doors after a period of time.
Controlled access using card readers, etc., is provided in secure facilities and most access control systems can monitor how long a door is open. However stand-alone door prop alarms can also be used for other installations to maintain the integrity of environmental interlocks. These allow field adjustment of the door held open time and can provide local and remote alarm capability.
Cleaning the cleanroom requires that all electrical devices be sealed or protected from wash-down operations with a Plexiglas switch cover. Likewise, the locks, lighted devices, etc. must be sealed from moisture. Specially rated devices can be supplied that meet these criteria.
Bryan Sanderford is National Sales Manager with Dortronics Systems Inc. in Sag Harbor, N.Y. www.dortronics.com
This article appeared in the March/April 2016 issue of Controlled Environments.