Doublefan turbulent flow isolators

It is common to fit two fans onto an isolator, one on the inlet and one on the exhaust, so that pressure and flow can be controlled by the relative adjustment of the fan speeds. This also obviates the need for very large filters; the exhaust filter of a single-fan, positive-pressure isolator is driven by the canopy pressure, and the converse is true for negative pressure. If a relatively large airflow is required, then a much smaller HEPA filter can be used if fan pressure is available to drive it.

The problem with double-fan systems is that, as mentioned in Chapter 2, they are dynamically unstable. Small changes in the speed of one fan can have a major effect on the pressure in the system as the second fan responds to the alteration. Thus, the control system governing fan speed must be precise and have stability better than 0.1 percent of the full power rating. If the control is manual, then very stable circuitry will be needed, impervious to changes in temperature, input voltage, etc. If the control is electronic, some

Figure 4.2 A Very Simple Monitoring System Fitted to a Biomedical Isolator. The beacon on the top of the enclosure provides a silent alarm if the pressure in the isolator exceeds the setting of 80 Pa on the Photohelic gauge. The smaller gauge gives a visual indication of the airflow through the isolator. (Courtesy of Astec Microflow.)

Figure 4.2 A Very Simple Monitoring System Fitted to a Biomedical Isolator. The beacon on the top of the enclosure provides a silent alarm if the pressure in the isolator exceeds the setting of 80 Pa on the Photohelic gauge. The smaller gauge gives a visual indication of the airflow through the isolator. (Courtesy of Astec Microflow.)

Figure 4.3 The Control and Instrumentation System Fitted to a Zone 1 Flameproof Isolator. Again, the isolator pressure and airflow are the parameters of concern while, in this case, the alarm levels are internally fixed and not operator defined. (Courtesy of Astec Microflow.)

care will be needed to develop algorithms that do not have harmonics similar to any periodicity in the complete system, including any exhaust ductwork. Probably the simplest way to tackle the problem is to fix the speed of one of the fans to give, for example, a known flow rate, and then vary the other to arrive at a set isolator pressure. Some precautions when considering the use of double-fan arrangements are given below in "Flow."

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