Critical Process Air (CPA) systems–also referred to as
Hygenic Air Handling (HAH) units–are systems which clean recirculated or
outside air. These units draw in air, filter out contaminants, adjust the
humidity, and then adjust the temperature, before expelling the clean air into
the facility. They’re common in large scale food processing facilities,
laboratories, and hospitals.
Because CPAs are used within such critical operations, it’s
important that they perform their best. The ductwork leading to and from them
is essential to that performance. Good ductwork allows consistent, even
distribution of clean air throughout a facility–but achieving this requires
addressing several design constraints.
Duct size must be considered at several points. Resistance
must be minimized while retaining sufficient air flow and keeping velocity
within a desired range. Those factors depend not only on length of the ducts,
but also their diameter—and multiple size profiles may be used throughout the
same system to ensure that air distributes evenly as the ducts branch in
Another key consideration is location. Ductwork is typically
installed in either rooftop or interstitial spaces (free spaces between
floors), or in refrigerated spaces. All three have specific concerns.
For rooftop CPA ductwork, the most pressing concern is that
the ducts must be able to withstand environmental conditions such as snow,
rain, and extreme heat. Interstitial ducts, on the other hand, are protected
from such conditions, but must worry about size. And for ductwork in
refrigerated spaces, precautions must be taken to prevent the moist air inside
the ducts from freezing. Ducts in refrigerated spaces also typically have to be
designed around processing equipment.
Specific field installation must also be considered. The
overall size of the duct should be considered to ensure that the duct can be
easily manipulated and properly installed on site. Adequate supporting
structure, such as exterior framing or hanging supports, must be designed to
ensure that duct sections are self-supporting and will not collapse during or
Ductwork must also be air-tight to prevent the introduction of moisture or buildup of condensation or contaminants. Choice of material affects overall hygiene of the ductwork as well. For this reason, stainless steel is used on the interior of CPA ductwork since its resistance to moisture and condensation prevents buildup of mildew and mold.
Other hygiene considerations might include designing the ductwork so that it can easily be accessed and cleaned to prevent surface buildup of pathogens. For example, a system might be designed with service doors along the ductwork length, so that the internal surfaces can be easily sprayed down and wiped.
Other considerations may come into play for certain
ductwork. For example, diffusing plates can be added to distribute air more
evenly and prevent strong exiting velocities. Similarly, air dampeners may be
installed to allow for fine control of room pressure differentials. These air
dampeners can also reduce operating costs over time.
When designing ductwork for a CPA system, size, location,
construction, and hygiene must all be considered. The design constraints vary
wildly between projects, but they must be given careful consideration, given
the crucial role that CPA systems play in medical, food processing, and