Data Center Airflow 1

[newgradme]

Hi All;

I'm working on my first data center cooling project and I'm trying to understand the airflow requirements for the A/C. I know the heat generated by the IT equipment will require 20 tons of A/C (70kW) and I've seen rules of thumb for determining airflow requirements for equipment (~150 cfm per kW). In trying to specify the HVAC system would you assume the unit will provide enough cool air to the space and that the fans on the IT equipment will take care of circulating it through the equipment? We're using a raised floor design with grated panels in front of the intake side of the equipment. I saw a rule of thumb for typical units of 400 cfm per ton of A/C so it doesn't seem like that would meet the cfm requirements of the equipment. Does the CFM from the A/C unit need to match the airflow requirement of the equipment? Sorry if this is really basic, but, I'm just getting into the field. Thanks!

 

[dbill74]

The fans on the IT equipment are drawing air from the space itself and dumping it back into the space. Those fans are specifically for circulating air across the components and chipsets of the IT equipment, something your HVAC system cannot do and thus you do not need to concern yourself with what those fans are doing (except perhaps to know where they are discharging, I will get to that).

Do not use or rely on rules of thumb to design an HVAC system. They are great for getting you pointed in the right direction and indicating if a design is off-base. The CFM of the A/C needs to match the requirements of the space and not the equipment. 400 CFM per ton is a good starting place, but it is acceptable to higher or lower. For a server room, I would not be surprised to see close to 350 CFM/ton.

Typically, computer room air conditioners (CRAC) are a big box sitting along a wall of the server room. Depending on the size and layout of the data center, the CRAC units may or may not be ducted. When they are ducted it is best to locate supply air (SA) diffusers in aisles or near the perimeter with return air (RA) grilles near the center and/or over the server racks. This is because server racks are typically designed to discharge their circulating air out the top of the rack, and you want to pull the warmer air out of the space as quickly as practical.

Server rooms need very little outside air and it is common for CRAC units to have zero (0) outside air while any space OA requirements are provided from other sources.

One last thing to consider when selecting CRAC units is space humidity. Server rooms are not regularly occupied, the load in them is almost entirely sensible and the HVAC system can easily dry out the server rooms. This can create an ideal environment for static electriciy and potentially damage the IT equipment. Thus it may be necessary to provide a humidifier on/with the CRAC units.

I just gave you a lot of "if's" and "maybe's", stuff you need to keep in mind and there is no single right answer without specifics. Hopefully you have a mentor or senior mechanical engineer that can help you learn the ins and outs of designing HVAC systems.

 

[willard3]

Rules of thumb result in bad design. Do a xcareful heat loss/gain analysis.

 

[lilliput1]

The computer room air conditioning (CRAC) unit rating will indicate the supply CFM corresponding to the unit capacity and the design indoor temperature. Spread the CRAC and the floor supply air grille around the room, matching the equipment load. Quantity and size of floor grilles should match capacity of CRAC units. Confirm airflow is not blocked by piping and wires in the under floor plenum. There should be a spare unit.