Conventional Technology
There are obviously variations on what is considered to be conventional technology (CT). The baseline CT system is chilled water including raised floor air distribution and elevated floor mounted computer room air conditioning units (ACUs) with top return from the room. The ACUs are generally packaged units with temperature and humidity control provided in each unit. Chilled water is generally supplied from a cooled chilled water plant.

The theory behind this approach is to provide a flexible cooling system, where the cool air supplied from the underfloor plenum could be directed to specific locations through the use of perforated floor tiles. The latest variation is called "hot row/cold row." Using this approach, perforated tiles are only placed in the "cold row," where the rows of cabinets are arranged with the front doors facing each other. The "hot row" has no perforated supply air tiles and the cabinets have the back doors facing each other.

There are many problems inherent with CT:

Return Temperature/ Humidity Control: With this approach, a temperature (T) and humidity (H) sensor is located in the return air stream of each ACU, and modulates the capacity of the chilled water coil, humidifier and reheat coil to maintain set point.

Herein are the problems. Each ACU is doing its own thing. I have been in data centers where three ACUs are in three modes of operation: Cooling, de-humidification and humidification. If you combine this with the fact that most ACUs boil water for humidification using package electric resistance heat and use electric resistance elements for reheat, the result is an energy hog.

Additionally, with all of the local humidifiers and controls comes localized maintenance. Humidifiers require servicing at least every six months. You also need to consider the added risk of domestic water being piped to each ACU for humidifier water supply.

Variable Supply Temperatures: Consider that virtually all data centers have more cooling equipment than is needed for two reasons: First, the need for reliability; and second, the need for oversized equipment. Most facilities have N+1 for every five units installed, and many data centers are designed for future loads. The result is 25% to 50% more air handling capacity than is needed. This sounds good, but it's not.

For example, by combining excess capacity with return T & H control, the result is elevated variable supply temperatures. It's bound to happen. The only ways to modulate cooling capacity are to vary the volume of air or the temperature. In a data center with a room set point of 72-F and a 54DEG.F design supply temperature, and 50% excess cooling capacity, the result will be an average supply temperature of 62�F. The final result is loss of humidity control and an increase in localized hot spots.

By-Pass Air: The single biggest contributor to the inefficiency of CT is by-pass air. We define by-pass air as cooling that returns to the ACU without doing any work or air that does not pass through electronic equipment and gets warmer by extracting heat. Consider that most computers and other electronics equipment use front to back air flow to dissipate the heat generated in the box and 100% of the power into the computer is turned into heat. A typical computer will elevate the air temperature between 30�F and 40�F from the intake to the exhaust.

If the return air is only 10�F to 20�F warmer than the supply, then a significant amount of the supply air is mixing with the hot air coming from the back of the cabinets. This will "dilute the hot air" before it returns the ACU and not do any work to extract heat. In a data center with CT and N+1 for every five ACUs, the by-pass airflow rate is an astounding 60%.

An alternate solution Supply Temperature Control: This solution is based on decoupling temperature and humidity control and maintaining a constant discharge air temperature. Eliminating humidity control from the ACUs on the data center floor allows for the humidifiers and reheat coils to be eliminated from the ACUs. This results in a lower first cost, maintenance and operating cost for the system.

High Delta T Cooling (HDTC): The use of the ceiling plenum as a means of returning the hot air from the hot aisle to the ACUs. This also involves connecting the top return on the ACUs with a duct to the return plenum, effectively returning the warm air from the back of the cabinet directly to the ACUs. Using the constant 52�F supply temperature and 30�F air temperature rise through the electronics equipment translates into an 82�F return temperature to the ACUs.

RTKL has conducted extensive research with all major manufacturers of computer room ACUs. This research has revealed a near linear relationship between coil delta T and cooling capacity. For example, a nominal 30 ton ACU sized on 72�F return air temperature will produce 45 tons of cooling with 82�F return air temperature. The result is a data center with by-pass air reduced to about 20% vs. 60% with CT.

Central Ventilation & Humidity Control: Some outside air is required to ventilate the raised floor environment. Our solution utilizes an ACU in a 2N configuration using 50% outside air, and 50% return air to humidify and dehumidify the data center. The amount of moisture that either needs to be added to or removed from the space is not a function of the total air circulated in the data center, but is purely a function of the condition of the outside air used for ventilation and the moisture migration through the building skin. This can be calculated, and the humidifier and dehumidification coil sized to match the required load.

Ultrasonic Humidification: Ultrasonic humidification systems use high frequency sound waves to evaporate water and operate on 1/10th the energy of conventional electric systems that boil water. Ultrasonic systems use de-ionized water. This requires the installation of a packaged de-ionized water system that some view as a complication not worth dealing with. Modern packaged de-ionized water systems are highly reliable. Also consider the maintenance savings of having two dedicated ACUs with ultrasonic humidification vs. humidifiers and reheat coils in half of the ACUs on the data center floor.

Dispelling a common myth
Elevated chilled water temperatures reduce energy consumption and make the chilled water plant operate more efficiently, thus reducing the cost to cool a data center.

Not Correct! Yes, it is true that elevated chilled water temperatures allow for a chilled water plant to operate more efficiently. But more importantly, elevated chilled water temperatures reduce the capacity of the cooling coil in the ACU's. If a data center can be cooled effectively with 50�F chilled water vs. the normal 44�F chilled water, there are too many ACUs moving a large amount of air. The following chart illustrates the range of cooling capacities for a nominal 30 ton ACU with variable return air temperatures and variable chilled water supply temperatures. Entering chilled water temperatures range from 44�F to 50�F, and entering air temperatures range from 72�F (45% RH) to 82�F. (Please note, the entering air conditions of 77�F and 82�F are at a constant moisture level commensurate with the HDTC approach of heat collection in the hot row. Units in the chart are in Tons of cooling).

What is important to note is the doubling in cooling capacity of 82�F entering air/ 44�F entering water vs. 72�F entering air/ 50�F entering water. The following example illustrates the importance of HDTC vs. elevated chilled water as an energy savings strategy.

In this example, we will compare two versions of a sample data center, looking specifically at chiller plant and fan energy.

- Raised Floor: 10,000 SF @ 150 W/SF
Raised Floor air Distribution, Ceiling Plenum Return - Equipment Cooling Load: 425 Tons, Nominal 30 ton ACU's - ACU's Required (N+1 for each 5 ACU's): - 50 EWT/ 72 EAT: 22 - 44 EWT/ 82 EAT: 11 - Add. Fan Motor Heat for 50/ 72 vs. 44/ 82: 23 Tons

Figure 1 is an energy analysis comparing the cooling plant energy usage for the two conditions highlighted above. In summary, the cost of moving excess air greatly outweighs any savings associated with increased plant efficiency related to elevated chilled water temperatures by about 21%. If you include the use of central ultrasonic humidity control, the energy savings are 41%.

It is important to note that HDTC with central humidity control has a lower first cost and lower operating cost than conventional technology

About the author As vice president and firm-wide director of RTKL's Engineering Studio, Steve Spinazzola offers more than 25 years of experience in both mechanical design and project management on a wide range of corporate, mission-critical, educational, health and science, retail, and institutional projects. With a Bachelor's of Architectural Engineering degree from Pennsylvania State University, Steve specialized in HVAC design and has six patents in advanced data center cooling technology. Contact sspinazzola@rtkl.com