HVAC design for pharmaceutical facilities merits a textbook unto itself. This
section covers the basics of automation requirements for bio-pharmaceutical
HVAC controls.

Process Description and Challenges

The ambient environment of a processing facility must be held in tight
control. This includes good control of temperature, pressure, and humidity.
Process cross-contamination is mitigated by maintaining a slight differential
pressure difference between adjacent rooms. This way, when a door is
opened, the general air flow will be in one direction only (from "clean" to
"contaminated"). Specific definition of the desired direction of air flow
between each room must be developed during the building design.

Furthermore, production facilities may have a requirement to maintain a
certain level of airflow through the room. This may typically be established
as a "number of exchanges" of air compared to the volume of the
room.

Excess humidity can result in condensation in processing suites. Stray
water such as this makes an excellent breeding ground for contamination,
so it is important that the HVAC system is capable of reducing humidity.

This is typically accomplished through a series of condenser coils to drop
out the moisture, followed by a re-heater.

For temperature controls, one of the key challenges is selecting the appropriate
location(s) for sensors in the room. The location should be away
from high-temperature equipment, like jacketed tanks or electrical cabinets,
and away from doors. Some plants use the return air duct as a location
for measurement of well-mixed room air. But specific location
selection depends heavily on facility design.

Communications

Typically, HVAC controls are completely separate from the process controls.
Some level of integration to historian and reporting systems may be
desired, for production records.

Typical Instrumentation Requirements

Typically, each processing room will have its own temperature and pressure
indication. Often, analog differential pressure gauges are located near
the doorways.

Control Strategies

Meeting the stringent requirements for airflow and differential pressure is
a delicate balance. During facility commissioning, HVAC engineers or
technicians "balance" the HVAC system. This process involves making
adjustments to airflows and pressures by adjusting HVAC system dampers.
Keep in mind that the HVAC control system may have a limited range of
calculation to affect control of pressure and air flow independently. Optimization
of HVAC and air handling systems is well-covered in Lipták [3.6].

Temperature, pressure, and humidity are typically independently- controlled,
simple regulatory control loops.