Isolators are normally maintained at a specific pressure differential with respect to the isolator room and to other parts of the isolator system. In sterile or other clean operations, such as semiconductors, the isolator will be held at a positive pressure with respect to the room, so that any leakage, small or large, will be outward from the isolator, thus maintaining clean conditions. In toxic containment applications, the isolator will be held at negative pressure, so that any leakage will be inward, thus protecting the operators. Neutral pressure is an option, given a two-fan system, but one that is rarely employed.
Operating differential pressures range from 30 to 200 Pa (3-20 mm of water gauge), but the range 30-70 Pa gives better operator comfort (see below) and less likelihood of leakage. Systems that have several isolators interconnected, as with pharmaceutical filling lines, can be designed to have a pressure cascade with the most critical unit having the highest differential, falling through the system via the less critical isolators or lockchambers. Where the product is toxic and yet must be maintained in sterile conditions, as with cytotoxic drugs, an interesting dilemma develops, as discussed in Chapter 5.
Another problem that may need to be addressed when considering pressure issues and, indeed, flow issues, is that of exhaust ducts. It is quite common to take the exhaust from a toxic isolator to the atmosphere through a duct and, in the case of sterile isolators, a duct may be needed to remove exhaust-sterilising gas. Safety regulations require ducts carrying hazardous gases to be under negative pressure throughout their length, so that any leakage will be inward. This then requires the addition of a fan at the atmospheric end of the duct, capable of overcoming the pressure drop of the duct, at the working airflow rate.
This sounds like a fairly simple engineering exercise, but there are some difficult issues to address, because the addition of the remote fan creates a double-fan situation. It is worth noting this particular problem here. Where an isolator is fitted with a single fan, be it positive pressure or negative pressure, exhausting to the atmosphere, the isolator pressure and flow will be stable for any given fan speed setting. In marked contrast, if one fan is fitted to the inlet of the isolator and another to the exhaust, an unstable situation is created — one that is not easy to control. This is inherent in the nature of the centrifugal fans used on isolators. Where one centrifugal fan is balanced against another, very slight changes in the speed of one fan make dramatic changes in the dynamics of the system. These changes can be caused by minor factors, such as the temperature or pressure of the atmosphere, or small changes in the electrical control and performance of the fans.
If the two fans in a double-fan system are very close to the isolator, or if the ducts between the fans and the isolator are very wide with little pressure drop, then the isolator control system can cope normally, whether pressure-governed or flow-governed.
If, on the other hand, there is a long and narrow duct between the isolator and the exhaust fan, with a large pressure drop, the system may be unstable to the extent of being uncontrollable. Worse still, the time constants of such a system may be such that all may appear well on the day of commissioning, only to alter significantly some days later. One solution to this problem is to decouple the isolator from the duct by use of an air break or "thimble," but this has further safety implications. Here are some suggestions:
• Avoid double-fan situations if possible.
• If a remote exhaust fan is required, make the duct as short, as straight, and as wide as possible.
• Use something other than a centrifugal fan to drive the incoming air, such as compressed air from a conventional 7 bar supply. This high pressure will need to be dropped through a suitable pressure-reducing valve.
There are implications for the building system when considering the installation of an isolator requiring a duct to the atmosphere, and these should be addressed at the earliest possible time in the project. The next section, on flow regimes, may also have some impact on the building system.
As a final comment on pressure regimes, designers may need to consider fitting some form of pressure-relief device to the isolator. If, for example, the isolator is fitted with a supply of compressed air, the valves controlling the supply may fail, or may leak over time, thus raising the pressure of the isolator to potentially disastrous pressures. In the case of toxic containment isolators, the pressure relief device must exhaust to a safe place, such as the exhaust duct. ISO DIS 14644-7 describes a suitable oil-filled device for use particularly with inert atmosphere isolators.
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