Process Description and Challenges

Clean compressed air (CCA) may be used as a pressure source to transfer
shear-sensitive products or materials. It may also be blown through systems
to remove condensate or liquid after CIP or SIP. In some cases, it may
be used to expedite batch cycle times by cooling equipment after SIP. Of
course, compressed air is often used as a source of oxygen for aerobic bioreactors.

The key challenge to delivering clean compressed air is to ensure that it is
free of moisture, oil, and particulate matter. From an engineering design
perspective, the focus is on an oil-free compressor system, combined with
suitable filtering.

Like other utilities, routine monitoring of quality is required. This introduces
the need for sample ports.

With multiple asynchronous concurrent users, the compressed air must
be sized to handle the maximum possible load. A collector/surge tank is
often used to smooth out peak demands for air and to minimize cycling
of the compressor.

Typical Instrumentation Requirements

Pressure indications and controls are the primary requirement for the
compressed air system. The compressor surge tank pressure is the primary
control loop for the CCA system. If there is a larger CCA distribution system,
additional sensors or regulators may be needed for each branch.

As with other utilities, instrument connections and sample ports should
be carefully designed to minimize the possibility of contamination, while
allowing for routine sampling.

Control Strategies

A calibrated spring-actuated regulator may provide all the control you
need in a smaller system. A pressure switch may be used to start and stop
the compressor. Analog surge tank pressure may be used to start or stop
the compressor. A deadband of a few psi will save compressor wear-and-
tear by reducing cycling. Surge tank pressure should be set several psi
above the distribution pressure to allow for pressure drop through the regulator
and distribution piping.

Communications

The primary need for communications is to indicate whether or not CCA
is available for use. In most cases, simple logical evaluation of the surge
tank or distribution pressure can be used to set the status bit for CCA
availability.

Sequences

A deadband of a few percent of pressure may be used to reduce compressor
cycling.

To prevent damage to larger, more complex compressor systems, you may
need to manage the compressor cycling with more sophisticated logic.
Consult with the compressor supplier to ensure that you have addressed
this in the logic design.

If the compressor requires cooling, there may be a small amount of compressor
protection logic.

Control Loops

Generally, there are no PID control loops for the CCA system. See the
"Bioreactors" section for discussion of air flow used with dissolved oxygen
controls.

Calculations

It is rare for the control system to calculate anything for the CCA system.

Tuneables

There are only a few ways to adjust a CCA system:

• Adjust pressure regulators to ensure supply pressure.
• Adjust surge tank pressure setpoint above the regulator pressures.
• Adjust surge pressure deadband to minimize cycling.