Better Ceiling Containment Yields Energy Efficiency and Savings | Data Centers

In a typical air-cooled data center, airflow management is about delivering cool air to the IT equipment and returning warm air to the computer room air handler (CRAH). Cooling is most efficient when hot and cold airstreams are separated to prevent mixing, thereby improving predictability and control. Recognized in modern design, the containment of hot and cold air is one of the most promising energy-efficiency measures available to both new and legacy data centers.

Hot Aisle Containment (HAC) is a popular strategy where cold air is supplied to the room and passed through the IT equipment before returning to the CRAH. Ideally, the air supplied by the CRAH is in balance with the air demanded by the IT equipment so that conditioned air passes once through the racks. In the real world, some amount of air bypasses the IT equipment or recirculates through due to leakage in containment structures. The result is elevated fan power and reduced chiller efficiency.

Figure 1. Cool supply air passes through the server picking up heat and enters the hot aisle to leave through the ceiling plenum. Some cool air can also bypass the server and leak through the ceiling into the plenum.

Much attention is given to reduce leakage in containment aisles and raised floors. These components are known sources of fugitive leaks and distributed leaks that contribute to bypass losses. Raised floors form a supply plenum and help to direct airflow to the IT equipment.  

The discipline given to sealing raised floors and racks should also be applied to ceilings. Ceilings form a return air plenum and, more importantly, separate the warm return air from the conditioned room. Gaps around the tiles and drop rod penetrations allow conditioned air to bypass the IT equipment. The bypassed air must be made up by increasing cool air supply from the CRAH, thereby consuming more energy.

Typical ceiling systems have leakage rates far larger than raised floor systems. At a differential pressure of 0.02 [in WC], a raised floor system may leak about 0.05 to 0.30 [cfm/ft²]. A non-gasketed ceiling with typical drop rod penetrations will leak about 1.44 [cfm/ft²]. When AIRASSURE ceiling tile is combined with DYNAMAX structural grid, leakage reduces to about 0.19 [cfm/ft²] and becomes comparable to raised floor and containment wall systems. 

Figure 2. Leakage resistance curves for typical raised floors (dashed) and AIRASSURE ceiling tile with DYNAMAX structural grid (solid green).

AIRASSURE Ceiling Tile with DYNAMAX structural grid is an engineered solution designed for data centers. AIRASSURE tiles feature a perimeter gasket that reduces ceiling leakage at the tile edges. DYNAMAX structural grid eliminates drop rod ceiling penetrations. Paired together, the system reduces bypass leakage down to levels of the best raised floor systems. The Armstrong ceiling solution delivers accessibility and containment that improves cooling efficiency and saves fan energy.

Figure 3. Typical non-gasketed tile with tee-bar grid and drop rod penetrations. Leakage rate may be 1.44 [cfm/ft²] at 0.02 [in WC]. 

Figure 4. AIRASSURE gasketed tile with DYNAMAX grid reduces leakage at the tile perimeter and eliminates leakage around drop rods. Leakage rate may be 0.19 [cfm/ft²] at 0.02 [in WC].

To illustrate the impact of ceiling leakage and its effect on operating energy, one-dimensional calculations were made for a 25,600 [ft²] data hall. This hall used on-slab, hot aisle containment and included an IT cooling load of 13.2 [MW].  Supply air temperature was fixed at 65 [F] and hot aisle temperature was fixed at 82 [F]. 

System static pressure across the fan was varied from 1 to 4 [in WC]. The pressure difference across the ceiling plane was varied from 0.01 to 0.08 [in WC]. The cost of electricity was varied from 0.08 to 0.17 [$/kWh]. 

Heat balance and airflow calculations were solved to satisfy the governing equations for 4000 random combinations of parameters. The resulting charts clearly show the effects of changing from a typical (high leakage) ceiling to the (low leakage) AIRASSURE with DYNAMAX system. 

Figure 5. Typical high leakage suspended ceilings may have bypass leakage rates of 0.4 to 2.0 [%] for total HVAC system flow (right distribution). Low leakage AIRASSURE with DYNAMAX ceilings reduce that bypass leakage to below 0.5 [%] of total HVAC system flow (left distribution).

Figure 6. Depending on operating conditions and local electricity costs, fan energy savings can range from 0.42 [$/ft²] (median) to 1.30 [$/ft²] (maximum) in this 25,600 [ft²] example. 

Ceiling and grid systems form return air plenums in data centers and separate cold supply air from warm return air. Leaks across this containment plane contribute to bypass losses, increased fan energy, and increased operating cost. The use of a AIRASSURE gasketed ceiling tiles with DYNAMAX structural grid reduces leakage rates to equal to the best sealed raised floor systems.

William Frantz
Senior Principal Scientist, ASHRAE Member

Jason Cavanaugh
Research Engineer, ASHRAE Member