Recommendations for air conditioning system design for data centres

A modern data centre design needs thorough study due to the fact that the cost of one square meter can reach up to several tens of thousands of dollars. That is why organizing the most suitable server rack layout to maximize the space capacity is becoming one of the biggest issues facing any modern data centre. Trying to solve this challenge companies are moving towards high density layout of IT equipment within within the data centre. Also it is very significant to take into consideration the fact that in case of higher density there will be a huge amount of heat emitted during the work by fully populated server racks and demanding its further removal.

The following IBM and DELL solutions can be taken as an example of calculating Thermal Design Power (TDP):

IBM Solution

DELL Solution

Thus required cooling airflow for a fully populated server rack is 4000 6000 m³/h.

At least three perforated tiles 600 ×600 mm installed in front of the server rack are necessary to ensure equal air supply. In this case it is preferable to have at least a 1.8 m spare space before every sever rack. If server racks are positioned in rows front to front then a distance between them will be 3.6 m. which leads to uneven air distribution and, eventually, lack of cooling in some areas.

A real amount of the heat emitted by the equipment will be less than estimated one due to the fact that server racks are not fully charged at once during the work. However it is necessary to take into consideration the fact that server virtualization increases server load up to 90%. Even a rack with 15 kW TDP due to its height requires special cooling solutions to avoid equipment overheating.

There are several approaches to air cooling solutions for data centres with server-rack TDP 15-25 kW.

Data center with heat load less than 2 kW per m².

Room area 1000 m²
Heat load 1 kW per m² (1 kW/ m²)
Cooling system layout Air conditioners are positioned around the perimeter of the server room (green rectangles in the picture below)
In reserve n + 2
The number of cooling units 15 × 80 kW

A typical airflow distribution for the data center cooling:

Raised floor height 0.6 1.0 m
Ceiling height 3 5 m
Server rack height 2 2.4 m

In case of rack load increasing such a layout will be inefficient, there will be following problems:

Switching over to one server rack TDP estimation.

In case of load increasing in modern data center it is necessary to switch over to estimation and calculating of heat load for a one rack unit what makes it possible to evaluate more accurately the cooling system of the data center.

There are other approaches to the equipment layout design and airflow distribution within the data center which are also required during its design.

Hot-Aisle/Cold Aisle layout for data center racks makes it possible to increase server rack load up to 6kW.

It requires Hot Aisle/Cold Aisle layout solution when data center load is 3 6 kW. In this scheme, the cabinets are adjoined into a series of rows, resting on a raised floor. The fronts of the racks face each other and become cold aisles, due to the front-to-back heat dissipation of most IT equipment. Computer Room Air Conditioners (CRACs) or Computer Room Air Handlers (CRAHs), positioned around the perimeter of the room or at the end of hot-aisles, push cold air under the raised floor and through the cold aisle, perforated raised floor tiles are placed only in the cold aisles concentrating cool air to the front of racks to get sufficient air to the server intake. This concept helps to reach a certain level of separation between cold supply air and hot return air.

Unfortunately this scheme faces some challenges in today's high-density rack environment where TDP is more than 5-6 kW for a rack.

Cold Aisle Containment Solutions. Rack TDP is 10 15 kW.

Even at minor heat loads, there are challenges associated with hot aisle/cold aisle. Some of them include: "bypass air", where cool air fails to enter the IT equipment; and "recirculation", where heated exhaust air flows back into the cold aisle over the tops of the rack or through open rack space. Both of these conditions may lead to data center hot spots and the need for lower temperature set points and significant cooling over-capacity. The cold aisle containment solution can improve cooling performance. Cold aisle containment attempts to isolate the cold air in a "room" of its own. By using containment curtains, metal, or other similar barrier, the cooling air is concentrated at the equipment intake. The cold air must pass through the server racks, cooling the equipment, before entering the rest of the room. Racks will be positioned in rows front to front. The cold aisle in between racks will be covered on the top and at the end of the rows. A full separation between supply and return air is achieved. Cold air will be supplied through the raised floor into the contained cold aisle; hot return air leaves the racks into room and back to the CRAC unit. In the picture below there is an example of cold aisle containment configuration.

Cold Aisle Containment Advantages.

In case of 15 kW load the cold aisle is covered on the top, thereby becoming totally isolated

Picture of a Data Centre with Cold Aisle Containment.

Active Floor solution.

There are situations when it requires the removing of 10 -15 kW from every server rack and at the same time not to increase the number of perforated tiles in the cold aisle. Therefore the following question is raised here: how to ensure the required airflow to server racks through the same number of perforated tiles? An Italian company Uniflair has its own solution for fulfilling this task Active Floor system. The main advantage of this system is its capacity to supply a much better controlled airflow (up to 4500 m³/h instead of 800-1000 m³/h from a standard perforated tile 600 × 600 mm).

Uniflair Active Floor is the flexible solution for cooling the high intensity heat loads associated with modern data servers. Located within the floor void in front of the server rack, Uniflair Active Floor fits exactly into a standard 600mm x 600mm floor grid. Cool air is focused directly at the heat load by means of a unique dual air deflector system linked to independent temperature sensors located in the outlet air from the targeted servers. Active Floor simultaneously minimizes energy consumption by reducing the volume of air flow to exactly match the thermodynamic heat load.

Active Floor Components

1. Floor grille
2a. Adjustable vanes Zone A
2b. Adjustable vanes Zone B
3. Fixing brackets
4. EC fan
5. Suction air grille
6. Electrical panel
7. Microprocessor control
8. Temperature sensor. Zone A
9. Temperature sensor Zone B

In the picture below there are examples of Active Floor system for horizontal and vertical airflows distributed to two zones.

Active Floor System solutions. 15 25 kW/rack.

Active Floor System makes it possible to increase the rack TDP up to 25 kW.

Active Floor and Cold Aisle Containment Solution.

Advantages of the Active Floor System.

Any solution has its advantages and disadvantages. Here are pros and cons of the Active Floor system solution.

Disadvantages

Taking into consideration the above it is possible to make an action plan for Data Center design consisting of three stages:

  1. Equipment layout based on Hot-Aisle/Cold Aisle solutions.
  2. With increasing load it requires cold aisle containment solutions with total cold aisle isolation.
  3. If it requires a greater cooling capacity Active Floor system should be installed in zones with high server rack load.

Of course, the data center should be cooled enough by air precision units at all these stages.