Air handling unit is a device to treat air. It has the ability to perform air circulation, ventilation, heating, cooling, humidification/dehumidification and filtering.
Air Handling unit components:
Air handling unit at minimum consist of supply fan and unit casing. Depending of design requirements AHU may contain other sections for ventilation, heating, cooling, humidification/dehumidification and air cleaning.
1- AHU Casing:
Casing is the body of the air handling unit. Casing consists of unit floor, walls and roof. Here are some important design consideration for Air Handling Units:
- Insolated double skin body is recommended even if AHU is placed in mechanical room inside the building. Provide more insulation for outdoor unit.
- For outdoor units slope the roof to prevent rain water. In cold weather where snow may accumulate, AHU roof shall be designed to bear snow accumulation.
- Size AHU casing at about 500 ft/min velocity through the filter section and coils.
-Casing floors must have stronger construction, checker plate 14 gauge provides durable floor.
-Drains must be provided in cooling coil section and humidifier section. Stainless steel drip pans are long-lasting.
-Marine lights are necessary for maintenance and repairs. Marine lights should be c/w protective metal case and glass seals. it is recommended to provide receptacle on the wall beside the switch for the marine lights mounted adjacent to the supply fan access door.
-Provide access doors (or panels) for inspection and maintenance access. under positive pressure doors are preferred to open inwards, under negative pressure doors are preferred to open outwards.
-Select inside and outside casing surfaces to resist corrosion. Specify weather protection for outdoor units.
- Supply, return, outside and exhaust air duct shall be connected to air handling unit through flexible connection.
- AHU walls and roof seams should have turned inward to provide a clean flush finish.
Also air handling unit may contain:
2- Fans or blowers:
3- Heating and/or cooling elements:
4- Air cleaning (Filters):
6- Mixing box.
7- Stratification eliminator:
8- Heat recovery
10- Vibration isolators
Locating Air Handling Unit:
AHU must be placed in accessible location for maintenance and repair. Always maintain clear space for replacing every piece of equipment like fans, coils, filters and dampers. Unit may be placed inside the building, generally in mechanical room, or outside the building. When placing AHU inside the building, mechanical room is preferable place to locate air handling units; because of noise issues and ease of access without disturbing the other building areas. Small units (up to 1,000 cfm, 500 L/s) may be placed inside ceiling space, with access panels sized properly. This is not always good idea, due to noise and maintenance issues. If AHU have to be placed outdoor, it should be weather protected and well insulated, coils and pipes must be protected against freezing, Outdoor units is recommended only when AHU handling contaminated air, like in pharmaceutical applications.
Always provide house keeping pad to place air handling unit on. Generally 6″ (150mm) height concrete pad or steel stand.
Custom made air handling units:
To obtain design requirements, air handling unit may be built to fit the exact requirements of the building. Custom unit is more expensive, but it has many advantages make it more efficient. Custom air handling provide the exact amount of air, exact amount of cooling, heating and humidification, and it have the flexibility to fit the exact filtration to match the application.