In my previous article, we have seen multiple ways to generate cold air and pull out the hot air. But all these things were showing only the supply/return until the white space. Have you ever wondered how this airflow is managed effectively inside the white space area? If this is not managed properly all your efforts for cooling become worthless. Let us look into the way to distribute/return the air from within the white space area.

Every data center air conditioning system has two key functions: to provide the bulk cooling capacity, and to distribute the air to the IT loads. The first function of providing bulk cooling capacity is the same for room, row, and rack-based cooling, namely, that the bulk cooling capacity of the air conditioning system in kilowatts must exhaust the total power load (kW) of the IT equipment. The various technologies to provide this function are the same whether the cooling system is designed at the room, row, or rack level. The major difference between room, row, and rack-based cooling lies in how they perform the second critical function, distribution of air to the  loads. Unlike power distribution, where flow is constrained to wires and clearly visible as part of the design, airflow is only crudely constrained by the room design and the actual air flow is not visible in implementation and varies considerably between different installations. Controlling the airflow is the main objective of the different cooling system design approaches.

There are three basic cooling configurations for how air is distributed to the IT loads,

• Room based
• Row based
• Rack based

Here we can see the 3 basic configurations depicted in generic floor plans. The orange square boxes represent racks arranged in rows, and the green arrows represent the logical association of the computer room air handler (CRAH) units to the loads in the IT racks. The actual physical layout of the CRAH units may vary. With room-based cooling, the CRAH units are associated with the room; with the row-based cooling the CRAH units are associated with rows or groups, and with rack-based cooling CRAH units are associated with the individual racks.

The conventional room-based(air conditioning units push chilled air into a white space and around the equipment) approach has served the industry well, and remains an effective and practical alternative for lower density installations and those applications where IT technology changes are minimal. However, latest generation high density and variable density IT equipment create conditions that traditional data center cooling was never intended to address, resulting in cooling systems that are oversized, inefficient, and unpredictable. Various measures can be taken to increase power density of this traditional cooling approach, but there are still practical limits. With power densities of modern IT equipment pushing peak power density to 20 kW per rack or more, simulation data and experience show traditional cooling (no containment), dependent on air mixing, no longer functions effectively. To address this problem, design approaches exist that focus on room, row, and rack-based cooling. In these approaches the air conditioning systems are specifically integrated with the room, rows of racks, or individual rack in order to minimize air mixing. This provides much better predictability, higher density, higher efficiency, and a number of other benefits. Let us have the deep dive into each of the cooling strategies.

Room Based Cooling

With room-based cooling, the CRAH units are associated with the room and operate concurrently to address the total heat load of the room. Room-based cooling may consist of one or more air conditioners supplying cool air completely unrestricted by ducts, dampers, or vents, or the supply and/or return may be partially constrained by a raised floor system or overhead return plenum. 

In the below picturization, you can identify that the cold air(blue color) pumped into plenum space beneath floor tiles, vented into cold aisles, returns through a ceiling plenum as hot air(red color). The performance of this layout can be enhanced by managing the return airflow, in this example using a false ceiling. 

The room-based design is heavily affected by the unique constraints of the room, including the ceiling height, the room shape, obstructions above and under the floor, rack layout, CRAH location, and the distribution of power among the IT loads. When the supply and return paths are uncontained, the result is that performance prediction and performance uniformity are poor, particularly as power density is increased. Another significant shortcoming of uncontained room-based cooling is that in many cases the full rated capacity of the CRAH cannot be utilized. This condition occurs when a significant fraction of the air distribution pathways from the CRAH units bypass the IT loads and return directly to the CRAH. This bypass air represents CRAH airflow that is not assisting with the cooling of the loads; in essence a decrease in overall cooling capacity. The result is that the cooling requirements of the IT layout can exceed the cooling capacity of the CRAH despite the required amount of nameplate capacity.

While discussing about this room based cooling, it’s also important to understand the cooling technique aisle containment systems(Cold aisle/hot aisle). For new data centers greater than 200 kW, room-based cooling should be specified with hot-aisle containment(where exhaust air exits the back of the equipment) to prevent the issues we just discussed. This method is effective with or without a raised floor and the cooling units can either be located inside the data center or outdoors. For existing data centers with room-based raised-floor cooling, cold aisle(where chilled air enters the front of the equipment) containment is recommended, since it is typically easier to implement. Both hot and cold aisle containment are being used to reducing the amount of space to be cooled and preventing the mixing of hot and cold air. Each of these solutions has its own unique advantages that are described in further detail.

Row Based Cooling

In a row-based configuration, the CRAH units are associated with a row and are assumed to be dedicated to a row for design purposes. There can be multiple designs for achieving the row based cooling. We will see two of the most popular designs in this era. The first and foremost design could be, the CRAH units may be located in between the IT racks or they may be mounted overhead. Compared with the traditional uncontained room-based cooling, the airflow paths are shorter and more clearly defined. In addition, airflows are much more predictable, all of the rated capacity of the CRAH can be utilized, and higher power density can be achieved. 

Aisle containment coupled with in-row cooling can be particularly valuable because it focuses the chilled air where it’s needed most. Below picture illustrates a row based cooling along with hot aisle containment system. Room based and row based cooling systems shown here can also be configured as a hot-aisle containment system that extends the power density capability. This design further increases the performance predictability by eliminating any chance of air mixing.

Row-based cooling has a number of side benefits other than cooling performance. The reduction in the airflow path length reduces the CRAH fan power required, increasing efficiency. This is not a minor benefit, when we consider that in many lightly loaded data centers the CRAH fan power losses alone exceed the total IT load power consumption. A row-based design allows cooling capacity and redundancy to be targeted to the actual needs of specific rows. For example, one row of racks can run high density applications such as blade servers, while another row satisfies lower power density applications such as communication enclosures. Furthermore, N+1 or 2N redundancy can be targeted at specific rows. 

The second most popular design for row based cooling is that the cooling units can be situated in rows. i.e. the cooling unit is placed between the server cabinets in a row for offering cool air to the server equipment more effectively. In-row cooling systems use a horizontal airflow pattern utilizing hot aisle/cold aisle configurations and they only occupy one-half rack of row space without any additional side clearance space. Typically, each unit is about 12 inches wide by 42 inches deep.

These units may be a supplement to raised-floor cooling (creating a plenum to distribute conditioned air) or may be the primary cooling source on a slab floor. The in-row cooling unit draws warm exhaust air directly from the hot aisle, cools it and distributes it to the cold aisle. This ensures that inlet temperatures are steady for precise operation. Coupling the air conditioning with the heat source produces an efficient direct return air path; this is called “close coupled cooling,” which also lowers the fan energy required. In-row cooling also prevents the mixing of hot and cold air, thus increasing efficiency.

The simple and pre-defined layout geometries of row-based cooling give rise to predictable performance that can be completely characterized by the manufacturer and are relatively immune to the affects of room geometry or other room constraints.

Rack Based Cooling

With rack-based cooling, the CRAH units are associated with a rack and are assumed to be dedicated to a rack for design purposes. The CRAH units are directly mounted to or within the IT racks. Compared with room-based or row-based cooling, the rack-based airflow paths are even shorter and exactly defined, so that airflows are totally immune to any installation variation or room constraints. All of the rated capacity of the CRAH can be utilized, and the highest power density (up to 50 kW per rack) can be achieved. Here we can see an example of rack-based cooling. Rack-based cooling will find application in situations where extreme densities, high granularity of deployment, or unstructured layout are the key drivers.

Similar to row-based cooling, rack-based cooling has other unique characteristics in addition to extreme density capability. The reduction in the airflow path length reduces the CRAH fan power required, increasing efficiency. As we mentioned, this is not a minor benefit considering that in many lightly loaded data centers the CRAH fan power losses alone exceed the total IT load power consumption.

A rack-based design allows cooling capacity and redundancy to be targeted to the actual needs of specific racks, for example, different power densities for blade servers vs. communication enclosures. Furthermore, N+1 or 2N redundancy can be targeted to specific racks. By contrast, row-based cooling only allows these characteristics to be specified at the row level, and room-based cooling only allows these characteristics to be specified at the room level.

As with row-based cooling, the deterministic geometry of rack-based cooling gives rise to a predictable performance that can be completely characterized by the manufacturer. This allows a simple specification of power density and design to implement the specified density. Rack-based cooling should be used in all data center sizes where cooling is required for stand-alone high-density racks. The principal drawback of this approach is that it requires a large number of air conditioning devices and associated piping when compared to the other approaches, particularly at lower power density.

Hybrid Cooling

Nothing prevents the room, row, and rack cooling architectures from being used together in the same facility. In fact, there are many cases where mixed use is beneficial. Specifically, a data center operating with a broad spectrum of power densities could benefit from a mix of all three types. Placing various cooling units in different locations in the same data center is considered a hybrid approach or we can call it a mixed cooling architecture. This approach is beneficial to data centers operating with a broad spectrum of rack power densities.

Another effective use of row and rack-based cooling is for density upgrades within an existing low density room-based design. In this case, small groups of racks within an existing data center are outfitted with the row or rack-based cooling systems. The row or rack cooling equipment effectively isolates the new high density racks, making them “thermally neutral” to the existing room-based cooling system. However, it is quite likely that this will have a net positive effect by actually adding cooling capacity to the rest of the room. In this way, high density loads can be added to an existing low density data center without modifying the existing room-based cooling system. When deployed, this approach results in the same hybrid cooling depicted here.

Another example of a hybrid approach is the use of a chimney rack cooling system to capture exhaust air at the rack level and duct it directly back to a room-based cooling system. This system has some of the benefits of a rack-based cooling system but can integrate into an existing or planned room-based cooling system. Here we can see an example of this equipment.

Let’s summarize the these cooling strategies as below,  

With room-based cooling, the CRAH units are associated with the room and operate concurrently to address the total heat load of the room. Room-based cooling may consist of one or more air conditioners supplying cool air completely unrestricted by ducts, dampers, or vents, or the supply and/or return may be partially constrained by a raised floor system or overhead return plenum.

Contained room-based, row-based, and rack-based cooling provide the flexibility, predictability, scalability, reduced electrical power consumption, reduced TCO, and optimum availability that next-generation data centers require. Rack-based cooling will find application in situations where extreme densities, high granularity of deployment, or unstructured layout are the key drivers. For most users with newer high density server technologies, contained room-based and row-based cooling will provide the best balance of high predictability, high power density, and adaptability, at the best overall total cost of ownership (TCO). 

Knowledge Credits: Schneider Electric

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