Access conditions and options
In order to seal the cleanroom and also allow access to personnel, vestibules with two or more doors are constructed and only one door is allowed to be opened at a time. A simple two-door airlock will have electric locks on each door—opening either door will cause the other door to lock. The door hardware must cause the doors to close immediately after an entry or exit. If both doors are accessed simultaneously, it will be impossible to prevent possible contamination through the unsealed doorway. One solution is to have one or both doors normally locked. Access can be granted through a locked door by card access or push button controls located adjacent to the door and within arm's reach.
An emergency override switch must be located at each doorway and be tied into the fire alarm system.
Frequently, the construction and/or locations of interlocked doors do not allow visibility of the other controlled doors. Traffic lights can be used in these situations to allow a smooth entry and exit by personnel. A normally unlocked door will have an indicator that turns red when the door is locked. If the door is normally locked then the light should be green to indicate that access is allowed. PLC-based controllers can easily provide this function, but most relay logic interlocks cannot. Large, hi-intensity LEDs are best for these applications and operate for an extended life cycle.
If a door does not fully close, then the other related doors will be inhibited and not allow entry or exit. A door prop alarm function may be incorporated into the PLC programmed logic or can be an independent stand-alone alert device. The door prop function monitors how long the door remains opened—should it not be closed in a predetermined time, an alert is sounded and a supervisor may be summoned.
Overrides for safety
Special lock override controls may be included to allow escape in an emergency. The system must be tied into the fire alarm system to unlock the doors in case of fire. An emergency override switch should be located within the room and at each doorway. Some facilities use an emergency pull station or a latching push-pull switch with a key reset; these devices frequently incorporate a built-in sounder to alert area personnel that the doors are unlocked.
Traffic lights can give personnel visual signals for door access.
Custom cleanroom sequences
Some facilities require automatic door operators to allow rolling carts. Most swing door operators are powered open and spring closed. Sliding doors are nearly always powered open and power-closed. If locks are needed, special programming is required to have the door sequenced to unlock before the door operator attempts to open it. Traffic can be directed by specifying motion sensors and/or push button control switches on only one side of the doorway.
Complex traffic patterns can be accommodated using a PLC-based controller. These can involve dozens of doors, or doors used to pass between two sterile rooms. This situation requires that all doors in each of the two rooms be secured whenever the "shared" door between them is open. The shared door may be normally unlocked for faster traffic if the other doors in the two rooms are normally locked.
A PLC programmed interlock can operate devices and systems other than electric locks. A custom timing sequence may be required to allow exhaust fans to extract contaminated air from the airlock before allowing the door to be unlocked. Similarly, one system uses a special function to inflate and seal the door gaskets before allowing access through another door.