13.3.1.1 Cooling with Ambient Dry‐Bulb Temperature

The simplest form of an air‐side economizer uses ambient air directly supplied to the space to remove heat generated by IT equipment. Figure 13.1 shows a schematic of a typical DASE arrangement that includes a DEC, item 1, and a cooling coil, item 2. Without item 1 this schematic would represent a DASE that uses the db temperature of the ambient air to cool the DC. For this case, ambient air can be used to perform all the cooling when its temperature is below the design cold aisle temperature and a portion of the cooling when it is below the design hot aisle temperature. When ambient temperature is above hot aisle temperature, or ambient dew point (dp) exceeds the maximum allowed by the design, then the system must resort to full recirculation and all mechanical cooling. When ambient temperature is below the design cold aisle temperature, some of the heated process air is returned to the inlet plenum to mix with the incoming OA to yield the desired delivery temperature. In almost all cases, except in extreme cold climates, some level of mechanical cooling is required to meet the space cooling requirements, and, in most cases, the mechanical supplement will be designed to handle the full cooling load. The result is that for most regions of the world, the full‐year energy reduction is appreciable, but the capital equipment cost reflects the cost of having considerable mechanical refrigeration on board. Other factors to consider are costs associated with bringing high levels of OA into the building that result in higher rate of filter changes and less control of space humidity. Also, possible gaseous contaminants, not captured by standard high‐efficiency filters, could pose a problem.

13.3.1.2 Cooling with Ambient Wet‐Bulb Temperature

If a source of usable water is available at the site, then an economical approach to extend the annual hours of economizer cooling, as discussed in the previous paragraph, is to add a DEC, item 1, as shown in Figure 13.1. The evaporative pads in a DEC typically can achieve 90–95% efficiency in cooling the ambient air to approach wb temperature from db temperature, resulting in a db temperature being delivered to space at only a few degrees above the ambient wb temperature. The result is that the amount of trim mechanical cooling required is considerably reduced from using ambient db and in many cases may be eliminated completely. In addition, there is greater space humidity control by using the DEC to add water to the air during colder ambient conditions. The relative humidity within the space, during cooler periods, is controlled with the face and bypass dampers on the DEC. It is important that the system is designed to prevent freeze conditions at the DEC or condensate formation in supply ductwork or outlet areas. There would be no humidity control however during the warmer ambient conditions. In fact, lack of humidity control is the single biggest drawback in using DASE with DEC. As with the db cooling, factors to consider are costs associated with bringing high levels of OA into the building, which results in higher rates of filter changes and less control of space humidity. Also, possible gaseous contaminants, not captured by standard high‐efficiency filters, could pose a problem. Even with these operating issues, the DASE using DEC is arguably the most efficient and least costly of the many techniques for removing waste heat from DCs, except for DASE used on facilities in extreme climates where the maximum ambient db temperature never exceeds the specified maximum cold aisle temperature.

A DASE with DEC cooling process is illustrated in Figure 13.2. In this instance, the cold aisle temperature is 75°F, and the hot aisle is 95°F, which is a fairly typical 20°F temperature difference, or Delta T (ΔT) across the IT equipment. With an ambient design wb of 67.7°F and a 90% effective evaporative process, the supply air (SA) to the space can be cooled to 70°F from 91.2°F, which is lower than specified. Under this type of condition, there are several control schemes that are used to satisfy the space cooling requirements:

1. Reduce the process air flow to maintain the hot aisle temperature at 95°F, which increases the ΔT between the hot and cold aisles. Decreasing the process airflow results in considerably less fan power. This scheme is shown as the process between the two square end marks.
2. Maintain the specified 20°F ΔT by holding the process airflow at the design value, which results in a lower hot aisle temperature. This is shown in the horizontal process line starting from “Out of DEC” but only increasing up to 90°F db return temperature.
3. Use face and bypass dampers on the DEC to control the cold aisle SA temperature to 75°F as shown in the process between the two triangular end marks.

13.3.1.3 Arrangement of Direct Adiabatic Evaporative Cooler

A bank of multiple DASE with DEC units arranged in parallel is shown in Figure 13.3. Each of these units supplies 40,000 cubic feet per minute (CFM) of adiabatically cooled OA during warm periods and a blend of OA and recirculated air, as illustrated in Figure 13.1, during colder periods. The cooling air is supplied directly to the cold aisle, travels through the servers and other IT equipment, and is then directed to the relief dampers on the roof. Also shown in Figure 13.3 is a commonly used type of rigid, fluted direct evaporative cooling media.

13.3.2.1 Air‐to‐Air Heat Exchangers

In many Datacom cooling applications, it is desirable to indirectly cool recirculated DC room air as opposed to delivering ambient OA directly into the space for cooling. This indirect technique allows for much more stable humidity control and significantly reduces the potential of airborne contaminants entering the space compared to DASE designs. When cooling recirculated air, dedicated makeup air units are added to the total cooling system to control space humidity and building pressure. An AHX serves as the intermediary that permits the use of ambient OA to cool the space without actually bringing the ambient OA into the space. The most commonly used types of AHX used for this purpose are plate and heat pipe as shown in Figure 13.4. Sensible wheel heat exchangers have also been used in IASE systems, but are no longer recommended due to concerns with air leakage, contaminant and/or humidity carryover, and higher air filtration requirements when compared with passive plate or heat pipe heat exchangers. Please refer to Ref. [2], Chapter 26, Air‐To‐Air Energy Recovery Equipment, for further information regarding performance and descriptions of AHX. Figure 13.5 illustrates the manner in which the AHX is used to transfer the heat from the hot aisle return air (RA) to the cooling air, com monly referred to as scavenger air (ScA) since it is discarded to ambient after it performs its intended purpose, that of absorbing heat. The effectiveness of an AHX, when taking into consideration the cost, size, and pressure drop, is usually selected to be between 65 and 75% when operating at equal airflows for the ScA and recirculating air.

Referring to the schematic shown in Figure 13.5, the ScA enters the system through a roughing filter at ① that removes materials that are contained in the OA that might hamper the operation of the components located in the scavenger airstream. If a sufficient amount of acceptable water is available at the site, then cooling the ScA with a DEC before it enters the AHX at ② should definitely be considered. Evaporatively cooling the ScA will not only extend the energy‐saving capability of the IASE over a greater period of time, but it will and also reduce the amount of mechanical refrigeration required at the extreme ambient design conditions. The ambient conditions used for design of cooling equipment are generally extreme db temperature if just an AHX is used and extreme wb temperature if a form of evaporative cooling is used to precool the ScA before it enters the heat exchanger. Extreme ambient conditions are job dependent and are usually selected using either Typical Meteorological Year 3 (TMY3) data, the extreme ASHRAE data, or even the 0.4% ASHRAE annual design conditions.

When DEC is used as shown in Figure 13.5, and trim direct expansion refrigeration (DX) cooling is required, then it is advantageous to place the condenser coil in the leaving scavenger airstream since its db temperature, in almost all cases, is lower than the ambient db temperature. If no DEC is used, then there could be conditions where the ScA temperature is above the RA. Under these circumstances, there should be a means to prevent the AHX from transferring heat in the wrong direction; otherwise heat will be transferred from the ScA to the recirculating air, and the trim mechanical refrigeration will not be able to cool the recirculating air to the specified cold aisle temperature. Vertical heat pipe AHXs automatically prevent heat transfer at these extreme conditions because if the ambient OA is hotter than the RA, then no condensing of the heat pipe working fluid will occur (process ② to ③ as shown in Fig. 13.5), and therefore no liquid will be returned to the portion of the heat pipe in the recirculating airstream (process ⑦ to ⑧). With the plate heat exchanger, a face and bypass section to direct ScA around the AHX may be necessary in order to prevent heat transfer, or else the condenser will need to be in a separate section, which would allow the scavenger fans to be turned off.

As an example, when using just an AHX without DEC and assuming an effectiveness of 72.5% (again using 75°F cold aisle and 95°F hot aisle), the economizer can do all of the cooling when the ambient db temperature is below 67.4°F. At lower ambient temperatures the scavenger fans are slowed in order to remove the correct amount of heat and save on scavenger fan energy. Above 67.4°F ambient the mechanical cooling system is staged on until at an ambient of 95°F or higher the entire cooling load is borne by the mechanical cooling system.

When precooling ScA with a DEC, it is necessary to discuss the cooling performance with the aid of a psychrometric chart. The numbered points on Figure 13.6 correspond to the numbered locations shown in Figure 13.5. On a design wb day ① of 92°F db/67.7°F wb, the DEC lowers the ScA db temperature from 92 to 70.1°F ②. The ScA then enters the heat exchanger and heats to 88.2°F ③. During this process, air returning from the hot aisle ⑥ is cooled from 95°F (no fan heat added) to 77.2°F ⑧, or 89% of the required cooling load. Therefore, on a design day using DEC and an AHX, the amount of trim mechanical cooling required ⑨ in Figure 13.5 is only 11% of the full cooling load, and the trim would only be called into operation for a short period of time during the year.

13.3.2.2 Integral Air‐to‐Air Heat Exchanger/Indirect Evaporative Cooler

The previous section used a separate DEC and AHX to perform an indirect evaporative cooling (IEC) process. The two processes can be integrated into a single piece of equipment, known as an indirect evaporative cooling heat exchanger (IECX). The IECX approach, which uses wb temperature as the driving potential to cool Datacom facilities, can be more efficient than using a combination of DEC and AHX since the evaporative cooling process occurs in the same area as the heat removal process. It is important to note that with this process the evaporative cooling effect is achieved indirectly, meaning no moisture is introduced into the process airstream.

Configuration of a typical IECX is illustrated in Figure 13.7. The recirculating Datacom air returns from the hot aisle at, for example, 95°F and enters the horizontal tubes from the right side and travels through the inside of the tubes where it cools to 75°F. The recirculating air cools as a result of the indirect cooling effect of ScA evaporating water that is flowing downward over the outside of the tubes. Because of the evaporative cooling effect, the water flowing over the tubes and the tubes themselves approach ambient wb temperature. Typically, an IECX is designed to have wb depression efficiency (WBDE) in the range of 70–80% at peak ambient wb conditions. Referring to Figure 13.6, with all conditions remaining the same as the example with the dry AHX with a DEC precooler on the ScA, a 78% efficient IECX process is shown to deliver a cold aisle temperature of 73.7°F, shown on the chart as a triangle, which is below the required 75°F. Under these conditions the ScA fan speed is reduced to maintain the specified cold aisle temperature at 75 instead of 73.7°F.

A unit schematic and operating conditions for a typical IECX unit design are shown in Figure 13.8. Referring to the airflow pattern in the schematic, air at 96°F comes back to the unit from the hot aisle ①, heats to 98.2°F through the fan ②, and enters the tubes of the IECX where it cools to 83.2°F ③ on a design ambient day of 109°F/75°F (db/wb). The trim DX then cools the supply to the specified cold aisle temperature of 76°F. At these extreme operating conditions, the IECX removes 67% of the heat load, and the DX removes the remaining 33% of the heat. This design condition will be a rare occurrence, but the mechanical trim cooling is sized to handle this extreme. For a facility in Quincy, WA, operating with these design parameters, the IEC economizer is predicted to remove 99.2% of the total annual heat load.

The period of time during the year that an economizer is performing the cooling function is extremely important because a Datacom facility utilizing economizer cooling has a lower PUE than a facility with conventional cooling using chillers and computer room air handler (CRAH) units or computer room air conditioner (CRAC) units. PUE is a metric used to determine the energy efficiency of a Datacom facility. PUE is determined by dividing the amount of power entering a DC by the power used to run the computer infrastructure within it. PUE is therefore expressed as a ratio, with overall efficiency improving as the quotient decreases toward 1. There is no firm consensus of the average PUE; from a survey of over 500 DCs conducted by the Uptime Institute in 2011 the average PUE was reported to be 1.8, but in 2012 the CTO of Digital Realty indicated that the average PUE for a DC was 2.5. Economizer PUE values typically range from as low as 1.07 for a DASE using DEC to a high of about 1.3, while IECX systems range from 1.1 to 1.2 depending upon the efficiency of the IECX and the site location. For example, if the economizer at a given location reduced the time that the mechanical refrigeration was operating by 99.7% during a year, then the cooling costs would be reduced by a factor of around 5 relative to a DC with the same server load operating at a PUE of 2.0.

Typically, in a DC facility where hot aisle containment is in place, the IECX system is able to provide cooling benefit even during the most extreme ambient design conditions. As a result, the mechanical refrigeration system, if it is required at all, is in most cases able to be sized significantly smaller than the full cooling load. This smaller amount of refrigeration is referred to as “trim DX,” since it only has to remove a portion of the total heat load. A further benefit of the IECX system is that, referring again to Figure 13.8, the ScA leaving the IECX ⑥ is brought close to saturation (and thus cooler than the ambient temperature) before performing its second job, that of removing heat from the refrigeration condenser coil. This cooler temperature entering the condenser coil improves compressor performance with the resulting lower condensing temperature.

Figure 13.9 shows a graph of operating power vs. ambient wb condition for a typical IECX unit, and the shaded area represents the number of hours in a typical year that each condition occurs in a given location, providing a set of bin hours (right ordinate) at which the IECX might operate at each wet bulb. Most of the hours are between about 11 and 75°F. The upper curve, medium dashed line, is the total operating power of the economizer cooling system. The short‐dashed curve is the DX power and the dot‐dash curve is the scavenger fan motor, both of which operate at full capacity at the extreme wb temperatures. The average weighted total power for the year is 117 kW. Typically, the lights and other electrical within the DC are about 3% of the IT load, so the total average load into the facility is 1500 kW × 1.03 + 117 kW or 1662 kW. This yields an average value of PUE of 1662/1500 or 1.108, an impressive value when compared with conventional cooling PUEs of 1.8–2.5. For this example, the onboard trim DX represented 24% of the 452.9 tons of heat rejection, which results in a lower connected load to be backed up with generators, as long as short‐term water storage is provided.

One company that has experienced great success implementing IECX systems is colocation provider Sabey Data Centers Inc. Figure 13.10 illustrates an aerial view of one of several Sabey DC facilities at their Intergate campus located in Quincy, WA. This campus has one of the largest IECX installations in the Western United States with a reported annual PUE of 1.13. Overall, the annual PUE of the campus is less than 1.2, which is impressive considering that these colocation facilities have variable loads and are often operating at partial loads below the design capacity (and higher efficiency points) of the equipment.

In order to give even a better understanding of how the IECX performs at different climatic conditions and altitudes, Figure 13.11 shows the percentage of cooling ton‐hours performed during the year: first the IECX operating wet (warm conditions using wb temperature), then second the IECX operating dry (cool conditions using db temperature), and third at extreme conditions the operation of DX. Fifteen cities are listed with elevations ranging from sea level to over 5,000 ft. The embedded chart gives a graphical representation of the energy saved during each operating mode. The last column is the percentage of time during the year that there are no compressors staged on and the IECX is handling the entire cooling load.

13.3.2.3 Trim Cooling Drain Trap Considerations

When using an indirect economizer in combination with a cooling coil used for trim cooling, there will be extended periods of time when mechanical cooling is not active. This can lead to “dry‐out” of the condensate traps, resulting in air leakage in or out of the recirculating air handler. This situation can impact pressurization control within the DC, and can also increase the amount of conditioned make‐up air required. It is recommended that the cooling coil drain traps be designed to prevent dry‐out and the resulting airflow within the condensate drain line, such as use of an Air‐Trap (which uses the fan pressure to trap air) instead of a P‐Trap (which uses water to trap air).

13.3.2.4 Other Indirect Economizer Types

There are other types of indirect economizers that can be considered based on the design of the DC facility and cooling systems. These are indirect water‐side economizer (IWSE) and indirect refrigerant‐side economizer (IRSE), and the primary difference between these and the IASEs discussed in this chapter is the working fluid from which the economizer is removing the heat energy. Where heat energy is being transported with water or glycol, an IWSE can be implemented. Similarly, in systems where a refrigerant is used to transport heat energy, an IRSE can be implemented. Either of these economizer types can be implemented with or without evaporative cooling, in much the same way an IASE can be, and similarly the overall efficiency of these economizer types depends on the efficiency of the heat exchange devices, efficiency of other system components, and facility location.