of outlet air flow (front, up, or down) Package model (horizontal or vertical) But, the first method is very usual in HVAC market. In fact, most of the company advertise their products by air handling unit applications: Normal Hygienic Ceiling mounted Components The major types of components are described here in approximate order, from the return duct (input to the AHU), through the unit, to the supply duct (AHU output). Filters Air filtration is almost always present in order to provide clean dust-free air to the building occupants. It may be via simple low-MERV pleated media, HEPA, electrostatic, or a combination of techniques. Gas-phase and ultraviolet air treatments may be employed as well. Filtration is typically placed first in the AHU in order to keep all the downstream components clean. Depending upon the grade of filtration required, typically filters will be arranged in two (or more) successive banks with a coarse-grade panel filter provided in front of a fine-grade bag filter, or other "final" filtration medium. The panel filter is cheaper to replace and maintain, and thus protects the more expensive bag filters. The life of a filter may be assessed by monitoring the pressure drop through the filter medium at design air volume flow rate. This may be done by means of a visual display using a pressure gauge, or by a pressure switch linked to an alarm point on the building control system. Failure to replace a filter may eventually lead to its collapse, as the forces exerted upon it by the fan overcome its inherent strength, resulting in collapse and thus contamination of the air handler and downstream ductwork. Hot (Heat A.K.A Furnace) and/or Cold (Air Conditioning) elements Air handlers may need to provide hot air, cold air, or both to change the supply air temperature, and humidity level depending on the location and the application. Such conditioning is provided by heat exchanger coils within the air handling unit air stream, such coils may be direct or indirect in relation to the medium providing the heating or cooling effect. Direct heat exchangers include those for gas-fired fuel-burning heaters or a refrigeration evaporator, placed directly in the air stream. Electric resistance heaters and heat pumps can be used as well. Evaporative cooling is possible in dry climates. Indirect coils use hot water or steam for heating, and chilled water or glycol for cooling (prime energy for heating and