Industrial Air Conditioning and Air Handling Units

What problem it solves

Ventilation replaces the air; air conditioning also defines its properties. If temperature and humidity in a production area must stay within a set range, the supply air has to be treated. The equipment that does this is the air handling unit (AHU).

The need usually comes from the process itself, not from comfort. Bonding and coating work within a set humidity range; print quality is directly linked to the moisture content of the paper; in electronics assembly, low humidity creates a risk of static electricity; in food and pharmaceutical production, temperature and humidity are part of the hygiene requirements. In these areas the answer to "is ventilation enough?" is no: the air must be both replaced and conditioned.

The second function of the AHU is filtration. Supply air enters the space cleaned of outdoor particles. As the filter class rises, the particle load in the room falls, but pressure drop — and with it fan energy — increases. The right choice is a filter staging that delivers the required cleanliness with the lowest energy.

Applications

• Food and beverage production and filling areas
• Pharmaceutical, cosmetics and chemical production areas
• Electronics assembly and quality control rooms
• Textile and garment production halls
• Printing and packaging plants
• Laboratories and measurement rooms

Sections of an air handling unit

Filter stages

Usually two or three stages, starting with a coarse filter (G4) followed by a fine filter (F7–F9). In sensitive spaces a HEPA stage is added on the supply side. The purpose of staging is to extend the life of the fine filter; without a pre-filter the fine filter clogs quickly and operating costs rise.

Heating and cooling coils

They run on water or direct expansion. Coil selection accounts not only for capacity but also for air-side pressure drop and condensate drainage. Downstream of the cooling coil there must be a droplet eliminator and a sloped drain pan.

Humidification and dehumidification

Humidification is done by steam or adiabatic methods. Where hygiene is a high priority, steam humidification is preferred. Dehumidification is achieved by cooling and reheating or with a desiccant wheel; which one fits depends on the target humidity level.

Fan section

Plug fans (free-running impellers) or scroll-housed fans are used. Adjusting airflow to demand with a variable speed drive gives significant energy savings compared with fixed-speed operation.

Heat recovery

Transfers heat from the exhaust air to the fresh air. It comes in plate, rotary and run-around coil types. In food and pharmaceutical areas, where cross-contamination is not acceptable, plate or run-around types are preferred because the two airstreams never mix.

Key questions for selection

Points to watch

Unit location. An air handling unit needs maintenance. Enough clearance must be left around it for filter changes, coil cleaning and fan checks. A unit squeezed into a tight space becomes a unit that is never serviced.

Condensate and drainage. Every unit with a cooling coil produces condensate. The drain line must have a trap, and the trap depth must be calculated from the fan pressure; otherwise water either does not drain or air leaks through the drain.

Duct insulation. Ducts carrying cooled air sweat on the outside if they are not insulated. This leads to dripping water and, over time, corrosion.

Controls. Temperature and humidity are controlled by automation. Sensor placement directly affects the result: a sensor in the return duct measures an average value, a sensor in the room measures the real condition.

Maintenance

The air handling unit is one of the most neglected pieces of equipment, and neglect turns into cost fastest here. A clogged filter reduces airflow, the fan draws more current and coil efficiency drops. Periodic maintenance checks filter condition, belt tension, belt and pulley alignment, fan bearings, coil surfaces, the drain line and damper movement.

How to choose the unit type

Air handling units are classified by installation type, and the choice is usually set by what the existing building allows.

Rooftop units remove the need for a plant room and shorten the duct run. In return they are directly exposed to outdoor conditions; casing insulation and outer skin quality become critical. The roof's load capacity and safe maintenance access must also be solved in advance.

Indoor units sit in a plant room, are easy to maintain and last longer. In return they need space and the duct run gets longer.

Hygienic units are built for food and pharmaceutical areas. The interior has no corners and can be cleaned, all surfaces are stainless steel or specially coated, and the drain pan is sloped so no standing water remains.

The second factor in the choice is the airflow range. For a system that will operate across a very wide airflow range, installing two smaller units instead of one large unit is significantly more efficient at part load.

Operating costs

The purchase price of an AHU is a small part of its ten-year total cost. The real cost is energy and maintenance.

Every item in this table is decided at the design stage. A system with high pressure drop cannot be made cheap to run after it is built; the fan has to overcome that resistance for its whole life. That is why savings on duct cross-section and filter area turn out to be expensive over the years.

The fresh air ratio is the parameter that deserves the most discussion. An unnecessarily high fresh air ratio directly increases the heating load in winter and the cooling load in summer. It should be set by occupancy and process need and, where possible, adjusted to demand with a carbon dioxide sensor.

Project process

1. Site survey. Room dimensions, the existing system, machine heat loads, working pattern and electrical infrastructure are checked on site.
2. Load calculation. Heating, cooling and humidity loads are calculated; the required airflow and coil capacities follow.
3. Selection and engineering. Unit selection, duct routing, grille layout and control scenario are defined.
4. Fabrication and installation. Ductwork is fabricated, the unit is placed, electrics and controls are connected.
5. Commissioning. Airflows are balanced, set points are entered and the measurement report is delivered.