In an HVAC system, one or several AHU can operate in parallel, in series, or in tandem, servicing various environments to precondition the air and fine-tune the environmental conditions.
The AHU houses the major components used for moving, filtering, and conditioning the air to be distributed throughout controlled environments. These components include:
- Blowers (fans) deliver the airflows at specified pressures to every point of use throughout the system.
- Heating and cooling coils condition the air to the specified temperature and humidity ranges
- Air filters control the airborne contaminants to pre specified level.
- Humidifiers bring air humidity to specified levels
- Control and monitoring devices control the operation of the mechanical devices installed within the unit and deliver information on the status of these devices
These devices are installed as separate units to reduce and control the humidity in controlled environments.
Typical components include the following:
- Chemical Desiccants. Air-drying or dehumidification can be achieved by the use of chemical compounds. These compounds retain water on their surface and can easily be reactivated by applying heat. Depending on the design of the system, the air stream can be exposed to desiccants such as silica gel beads, granulated activated alumina, lithium chloride impregnated in a rotating drum or liquid lithium chloride solution.
- Regeneration-Heating Elements. This group of devices is used to evaporate the water from the compound surface. Its operation is critical for the proper achievement of the controlled humidity range.
- Blowers. Blowers are used as airflow-moving devices.
- Section Seals. The particular importance of this device is the correct operation of the seals that separate the processing and regeneration sections of the dehumi- difier. Incorrect operation could affect the operation of the devices and in some cases, the pressurization of the environments served by these units.
- Heat Transfer Devices and Mechanisms. Various devices are used to cool or heat the air. This may include compressors, heat exchangers or coils, water chillers of various types, and steam boilers.
These devices provide the fluids (refrigerants, chilled water or steam) to the coils normally located on the AHU.
- Controls. All of these devices are equipped with a series of electric, pneumatic, or electronic devices to control and monitor the operation of its components.
They are interconnected to the control and monitoring system for the facility.
Ductwork-Air Distribution Network
Components include the following:
- Duct network, which is a network of ducts used for distributing the air from the air handlers to the points of use in the controlled environments and from these controlled environments back to the air handlers or to extraction points.
- Insulation materials, used to maintain the air temperature and humidity along the network.
- Dampers or valves, used for the proper balancing of the system.
- Reheat coils installed along the system to maintain system temperature.
- Air diffusers and terminal filter housings, used for the installation of HEPA filters at the point of delivery in clean or controlled environments.
Terminally Installed Air Filters. Such filters are generally high-efficiency filters (US 99.97% DOP test HEPA, MERV 20, MERV 19 (Minimum Efficiency Reporting Value) ASHRAE (3); Eurovent EN 1822 European designation H13 and H14, installed at the point of delivery in clean and controlled environments. They are mounted in a special housing that assures filter integrity.
In some instances, HEPA filters are installed at both supply and return points within a clean room to prevent or control room-generated contamination from entering the ductwork.
Proper operation of these devices is critical for obtaining the desired levels of environmental cleanliness both from the viable and nonviable point of view
Intermediate or Low-Efficiency Filters. These filters (MERV 14 MERV 15 Minimum Efficiency Reporting Value) ASHRAE (3); EN 1822 F5-F9 are generally installed as part of the main or intermediate air handlers (when these are used in tandem). Their purpose is to reduce the particle burden on more efficient and expensive filters, and to reduce the cost of maintenance of the components of the AHU and the duct distribution network.
Other filters, such as adsorbent cells, may be required for specific applications in the control of chemical or gaseous contamination.
Dust Control and Containment Systems. These systems are designed to prevent dust spreading and potential cross-contamination. They are installed in areas where weighing of powdered materials takes place. These systems are also used where compounding and preparation of solutions takes place.
Critical Parameters. The humidity, temperature, pressure differentials, and air cleanliness classification (both for viable and nonviable levels) are affected by this system.