What are the reasons to prefer Computer Room Air Handler(CRAH) in DATA HALL over Air Handling Unit(AHU)?

CRAH equipment are specially designed and manufactured with inbuilt control system to precision control the temperature and humidity. The data hall design criteria shall be 18 deg C DB and Relative humidity of 50% +/- 5%.

Ordinary AHUs are designed for the same criteria, but cannot be precisely controlled if there is a variation or fluctuation in outdoor air temperature and humidity conditions.

The CRAH or the precision unit or the close control unit have humidifier and dehumidifier components to maintain the humidity precisely within limits.

CRAH are the units specially designed for computer rooms like they have below discharge and above suction plus their accuracy is very high that y they are also called precision units

Normal units can be installed but they shall be customized little so they will become CRAH also

Computer room requires précised control over Temperature & humidity, which is only possible with the precision equipment such as CRAH, CRAC, In-row etc, not with ordinary AHU 

CRAH units have more precise control gear than normal AHU's , moreover they are set to be able to provide low air supply under raised flooring usually found in these rooms. Moreover , they are pretty much capable of working in low ambient conditions( low ambient kits are a standard item in these units)

To avoid heating up of  server and hardware components  in maintain un form temperature to achieve high reliability of components. By maintaining uniform tempearture the life of component is being increased.

CRAH is designed to achieve certain requirement for data centers and server rooms such as accurate temperature and RH and precision control also supply air under raised floor and return from upper level in order to flow air through servers units common CRAH use in large volume data centers and in majority use chilled water type.

CRAC ALWAYS use DX AC  type for small centers and computer rooms  

they are precise in humidity and temperature control with dehumidifier and humidifier plus they also come in dual circuit that is dx as well as chilled water when the chilled water cannot achieve the precission it switches to dx type and control the internal parameters accordingly.

please note that data centers usually employ chilled water network with thermal storage tank to feed the system in case of system failure or electricity outrage.