Generally Precision AC Units, sometimes also called as peripheral cooling units, are kept around the room in the periphery which gives them the name peripheral cooling units. Peripheral cooling units are used for applications having less heat load or Low density heat load application ( load generally 3-5 Kw per rack).
PAC units are available with following cooling medium:
- Chilled Water Based System
- DX refrigerant system
- Hybrid/Dual Fluid system
Chilled Water Based System
Chilled water PAC units runs on water as the cooling medium inside the coil. These systems are generally used in large server rooms/DC’s & bigger halls having dedicated chillers catering them.
DX Refrigerant System
Dx system is refrigerant based system running on refrigerant as cooling medium used in smaller server/UPS rooms. They are the most economical solution for any kind of cooling requirement.
Hybrid or Dual Fluid System
This kind of system runs on both refrigerant as well as chilled water circuits based on the requirement. Basically, it is a kind of redundant/flexible system rolled into one.
The units come with bottom or top air delivery.
Bottom Discharge Units/System
In Bottom discharge, cold air is blown down from the unit the reason is cold air being denser settles down in the room. As it picks up the heat from the server it becomes light & moves up in the room. Hot air is then sucked by the PAC units located along the periphery.. This being a natural cycle (cold air in bottom & hot air in the top of room) consumes less power. False floor acts as pressurized plenum and generally ensures uniform air distribution throughout the room. Grilles are provided in the false floor to distribute the cold air in the room. once cold air passes through servers it becomes hot & the same is discharged in the rear side of server. The portion where cold air opening is provided is called as cold aisle & the rear end where hot air is discharged is known as hot aisle. From the hot aisle the hot air is sucked by the PAC units, it can be done directly or via false ceiling.
A typical layout is as shown below.
Some Advantages of False Flooring System
Load of individual racks can be varying from each other, in case of peripheral cooling units it can vary from 2/3 kw per rack to 10 kw /rack. Air volume required for each rack varies due to this, so for varying the air volume we can go for floor grilles with different amount of opening. False floor can also be used for running the cables.
TOP Discharge Units/System:
In Top discharge units cold air is blown upwards from the unit which is taken to the server area with the help of Plenum & Duct. Then the cold air is distributed in front of the server via diffusers, . The hot air is again sucked back by the PAC. This system eliminates the usage of False floor, & it is best suited for small capacities of servers.
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