Project Background

A critical systems mechanical diagnostic review conducted earlier this year revealed insufficient humidity control in all five floors of the data center: “holes” in the envelope. Results showed that humidifiers operating at capacity would not be able to maintain required humidity levels (35% rh) on cold winter days. Humidity control in data processing areas is especially critical because hydroscopic (moisture retaining) circuit boards expand and contract with fluctuating humidity levels. Expansion and contraction breaks microelectronic circuits and edge connectors; low humidity causes destructive static electricity discharges. In an attempt to curtail mounting costs (over $500,000 in subsequent projects and $30,000 per year in wasted energy), project managers recently hired Jack Dale Associates (JDA, Baltimore), a research and development engineering firm. JDA’s task was to quantify the amount of air transference between the data centers and their adjacent administration spaces.

Figure 1. Second-floor data center air transference testing. CO₂ decay curves for the data center show very high rates of dilution. The quadrant served by ahu 223.1 (2V04) was the worst, with an air exchange rate (Axc) of 2.20, or 733% over the 0.3 Axc design goal. The average Axc in this data center of 1.93 equates to 24,126 cfm of air transference.

Techniques to Quantify Air Transference

Seeding an area with a tracer gas and then determining the dilution of the tracer gas that occurs from air transfer provides far greater accuracy in quantifying air transference. Since pioneering the use of this technique in 1986, JDA has continuously refined testing protocol. As an example, JDA has replaced sodium hexafloride as a tracer with less expensive “beverage-grade” carbon dioxide. It must be understood that the tracer gas is NOT carbon monoxide gas (CO), which is a harmful gas.

“Seeding” the space to be tested means flooding it with carbon dioxide (CO₂) until the CO₂ concentration in the test area reaches five times the outdoor ambient concentration (Photo 1). Data recording equipment is distributed throughout the test area. These instruments record the level of CO₂ in the air at regular intervals. Data collected produces decay curves. Decay rates determine the air exchange per hour rate (Axc) of air transference.

Any difference in CO₂ concentration from ambient CO₂ levels indicates air transference between the data center and adjacent areas. Therefore, dilution of the seeded areas in the data center indicates administration air, or outside makeup air, being transferred to the data center. The obverse is also true: if CO₂ concentration increases in adjacent areas, air is moving from the data center to these spaces.

The formula for air transference follows the following differential equation:

C(t) = C’ + (Co-C’) x exp[-V(t)/Vo]

Where:

C’ = average CO₂ concentration outside the test area

Co = seeding concentration level at t=0

C(t) = Concentration of tracer gas at time (t) after seeding to an initial concentration level

Vo = Volume of the area under test

V(t) = Volume of gas exchanged between the inside and outside of the space

Note: If V(t)=1 hour = Vo, then one air change has occurred in an hour then (axc = 1)

Note: Zone Volume; there are 4 ahu; each supplies 25% of the space volume

Axc = Air exchanged from the outside to the inside of the space

Figure 2. Second-floor data center CO₂ dilution. This chart shows the impact on air dilution within the data center after administrative ahu-223.2 outside air dampers were opened.

Environmental Parameters

These outdoor conditions are typical for the Southeast. In these conditions, adding humidity to the air would require 6.81 kW of electric humidification for every 1,000 cfm of makeup air (162,695 grains/hr). Makeup air is defined as outside air that enters into the data center directly through outside air dampers, or air that is transferred into the data center from nonhumidified adjacent spaces.

Any prudent design provides filtered and conditioned makeup air for data center environments. Excess air causes positive pressure. This prevents infiltration of dirty, unconditioned air into the center, controls airborne contamination, humidity, and temperature. Existing humidifiers in each of the four air handlers per floor total 80 kW. During a cold design day they are capable of humidifying 10,573 cfm makeup air per floor, assuming a degredation of capacity between cycles of 10%. Calculations showed that existing humidifiers are capable of producing enough steam to satisfy an air exchange per hour of 0.3 Axc.

Figure 3. Third-floor data center testing. CO₂ decay curves for each quadrant of the data center show high rates of dilution. The average air exchange rate is 420% higher than the reasonable goal of 0.3 Axc. The average Axc of 1.27 in this center equates to 15,843 cfm of air transference from unconditioned sources.

Preliminary Findings

Both floors showed construction attempts to resolve air transfer problems. Indeed, some of this work was effective. The amount of transfer between the administration and data center air handlers was negligible in the return air plenum bulkheads. Additionally, the walls between the data center and administration hallways had abandoned fire dampers that have been closed and covered with sheet metal. Field verification confirmed that the return air bulkheads and hallway demising walls were treated in a similar manner on the third floor.

Much of the data center infrastructure, however, showed evidence of successive generations of overlaid design concepts aimed at reducing air transference. Most efforts exhibited a distinct lack of scope, preparation, or cohesion with the existing system. Others appear to have caused additional problems or uncovered problems previously masked by excess capacity.

For adequate exfiltration, each floor of the facility requires 3,750 cfm of conditioned makeup air. At this quantity, the air exchange per hour would will be 0.3 Axc. Since the existing humidifiers are capable of producing enough steam to satisfy 10,573 cfm of outside air (i.e., at the proposed design conditions) modifications to the existing humidification systems should be unnecessary.

Figure 4. Third-floor administration CO₂ concentration. CO₂ from the data center migrated into administrative spaces during testing. Administration CO₂, concentrations increased. This indicates a mixture of return air systems between data center ahu and administrative ahu.

Results of Seeding

Because of the air transfer on the second floor, the humidifiers require the outside air to be more humid at all times year-round than a 34˚F rainy day. The third-floor humidifiers require that the conditions outside never drop below 29˚.

The original focus of study was to determine the air transfer between the data center and administration spaces that was causing humidity control problems. The results of such a study speak to more than just humidity control problems. As mentioned earlier, availability leans on total infrastructure control. Lack of air control can allow dust and particles or even smoke to migrate into the data center. Indeed, JDA found that the excessive amounts of air transference in this case increased the risk of smoke migration. Smoke and other airborne pollutants are dangerous to the extremely sensitive microcircuits found in data centers.

Conclusion