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Changes down the pipe for data center chiller operation

What goes into your data center chiller might be warmer than expected, as energy use reduction efforts threaten the capacity of CRAHs and other cooling systems.

Warmer server inlet temperature is just the beginning of your energy-reduction journey in data center cooling....

The next frontier: warm chiller water.

The ASHRAE recommendations for higher temperature inlet air to servers define efficient data center cooling. Air temperatures as high as 80.6° Fahrenheit (27° Celsius) preempt a lot of unnecessary cooling and open up free cooling options without materially increasing equipment failure rates. Even an average inlet temperature of 75° F to 78° F (23.9° C to 25.6° C) saves a lot of energy compared to the legacy 60° F to 65° F (15.5° C to 18.3° C).

But there's another element to cooling -- especially in new construction -- that most IT professionals don't know. Higher-temperature chiller water lessens energy use, but also reconfigures data center cooling.

Most large and many smaller data centers cool with chilled water, whether for computer room air handlers (CRAHs), in-row coolers, rear-door coolers, refrigerant pumps or even direct application on processors. A chiller plant -- essentially a big refrigeration system -- produces cold water, which circulates through the data center cooling units. Most cooling systems specify the maximum cooling capacities, generally based on 45° F (7.22° C) water temperature, in addition to water flow rate, air temperature and humidity level in the data center.

The cooling unit -- CRAH, in-row or another design -- transfers heat from the computer equipment to the circulating water, which returns to the chiller to be re-cooled. Removing waste heat takes a lot of energy, and energy use increases in relation to how cold you need the water.

Many newer buildings incorporate designs, such as chilled beams, based on higher water temperatures. If designers use higher-temperature water in the building to save energy, they probably will in the data center as well, even if the facility runs on a separate cooling plant.

The combination of warmer data center chiller water and warmer inlet air radically reduces energy use. In most regional climates, water cools to 60° F with economizers that circulate it through cooling towers. This approach can be coupled with modern chillers to cool off water during warmer seasons or in moderate climates, and it is still more energy conscious than cooling exclusively via mechanical means.

Most data center cooling devices are not inherently designed for warmer water from the chiller plant. With warmer inlet air temperature increases, the air temperature returning to the CRAHs -- hence the temperature of the water returning to the chiller -- will rise, requiring even more energy to cool down to a legacy temperature.

The problem is that equipment designed to use cold water delivers less cooling capacity when higher-temperature water moves through it (See Table 1).

Table 1 shows the relationship between data center chiller water temperature and water flow rate in cooling.

In each example, 80° F (27.7° C) is the return air temperature to the air conditioners. Humidity is calculated at a 52° F (11.1° C) dew point temperature, rather than relative humidity.

  Constant flow rate (1) Increased flow rate (2) Constant temperature rise (3)
Cooler EWT Flow rate Sens. capacity Flow rate Sens. capacity Flow rate Sens. capacity
° F/° C GPM LPS kW GPM LPS kW GPM LPS kW
CRAH 1 45/7.2 77.0 4.86 97.8 95.0 5.99 100.5 77.0 4.86 97.8
50/10.0 77.0 4.86 86.3 95.0 5.99 88.6 60.2 3.80 82.7
55/12.8 77.0 4.86 71.1 95.0 5.99 73.1 48.0 3.03 64.8
60/15.6 77.0 4.86 55.9 95.0 5.99 57.4 35.5 2.24 46.4
CRAH 2 45/7.2 114.0 7.19 143.4 140.0 88.3 147.2 114.0 7.19 143.4
50/10.0 114.0 7.19 126.6 140.0 88.3 129.7 89.0 5.62 121.9
55/12.8 114.0 7.19 104.2 140.0 88.3 106.8 71.5 4.51 96.0
60/15.6 114.0 7.19 81.8 140.0 88.3 83.8 69.2 4.37 69.2
In-row 45/7.2 22.5 1.42 28.9 30.0 1.89 30.0 22.4 1.41 28.9
50/10.0 22.5 1.42 25.7 30.0 1.89 26.8 17.6 1.11 24.6
55/12.8 22.5 1.42 21.5 30.0 1.89 22.3 14.2 0.90 19.6
60/15.6 22.5 1.42 17.2 30.0 1.89 17.8 10.6 0.67 14.3

Table 1. Temperatures are in Fahrenheit and approximate Celsius. Water flow rates are in gallons per minute and liters per second.

There are three outcomes when chiller water temperature rises, as shown by columns 1, 2 and 3.

Column 1: The cooling capacity lessens when the water flow rate remains constant through conventional computer room cooling units, such as computer room air conditioners (CRACs) and in-row coolers. The cooling capacities drop to a range of 57% to 60% of the rated capacity when the water temperature increases by 15° F (8.4° C). With no other compensation, the data center would need nearly twice as many cooling system units to deliver the same capacity as it did running colder water. That would use a lot of floor space and add significant capital cost, making the long-term ROI for such a design questionable. The data center would require larger pumps and twice as many fans, all of which use power.

Column 2: The water flow rate increases to compensate for the cooling loss. But the capacity only improves by about 3% -- hardly worth the extra pump power to increase flow rate.

Column 3: Water flow rate adjusts so that the water temperature increase is the same at any inlet temperature. It results in an even greater reduction in cooling capacity, but also a significant reduction in water flow rate, which avoids additional power to run the pumps.

Data center chillers: Pumped refrigerant

Another form of data center cooling uses pumped refrigerant where the cooling distribution unit relies on chilled water to remove the refrigerant heat. Although these systems are highly efficient, warmer water significantly reduces their cooling capacities (See Table 2).

    Constant flow rate (1) Increased flow rate (2) Constant temperature rise (3)
Cooler EWT Flow rate Sens. capacity Flow rate Sens. capacity Flow rate Sens. capacity
° F/° C GPM LPS kW GPM LPS kW GPM LPS kW
Refrig-
erant pump cooling
45/7.2 140 8.83 160.0 170 10.73 167.2 140 8.83 160.0
50/10.0 140 8.83 143.0 170 10.73 149.5 111.3 7.02 134.6
55/12.8 140 8.83 126.0 170 10.73 131.7 93.2 5.88 112.3
60/15.6 140 8.83 109.1 170 10.73 114.0 75.2 4.74 89.9

Table 2 shows that data center cooling systems using pumped refrigerant aren't immune to performance loss from warmer water.

The pumped refrigerant system's cooling capacity falls to 68% of the maximum possible level when the water temperature goes up 15° F (8.4° C). This is much better than with conventional cooling units (Table 1), but it is still a significant loss. Increasing water flow rate or maintaining the same temperature rise is no more advantageous than with the other systems.

If we want the advantages of higher-temperature water in the data center chiller, sometimes -- not always -- we need additional cooling units to make up for the loss in capacity.

One option is to select cooling units designed for warmer chiller water. Their coils are engineered for the different water temperatures and flow rates. The energy savings quickly offset the higher initial investment for specially designed units. Rear door heat exchangers (RDhX) illustrate the more effective performance of these tailored warmer-water chiller systems (see Table 3). Under the same conditions as the conventional systems, the RDhX cooling capacity drops to 76% of its maximum potential at a constant flow rate and holds at 70% of maximum when temperature rise is constant. Capacity loss can be reclaimed with only a modest increase in flow rate.

  Constant flow rate (1) Increased flow rate (2) Constant temperature rise (3)
Cooler EWT Flow rate Sens. capacity Flow rate Sens. capacity Flow rate Sens. capacity
° F/° C GPM LPS kW GPM LPS kW GPM LPS kW
RDhX 45/7.2 8.0 0.51 20.0 10.0 0.63 21.0 10.0 0.63 21.0
50/10.0 8.0 0.51 18.4 10.0 0.63 19.4 9.0 0.57 18.9
55/12.8 8.0 0.51 16.8 10.0 0.63 17.7 8.0 0.50 16.8
60/15.6 8.0 0.51 15.2 10.0 0.63 16.0 7.0 0.44 14.7

Table 3 shows RDhX coil designs that accommodate warmer chiller water without prohibitively decreasing cooling capacity.

The technologies for higher chiller water temperatures are installed and successfully used in many locations. The key is to recognize that there are differences in selection, specification and operation of this equipment from chillier chilled water systems.

Author's note: Emerson Liebert, Schneider APC, and CoolCentric assisted in generating these product-neutral charts by supplying the data from which they are derived.

About the author:
Robert McFarlane is a principal in charge of data center design at Shen Milsom and Wilke LLC, with more than 35 years of experience. An expert in data center power and cooling, he helped pioneer building cable design and is a corresponding member of ASHRAE TC9.9. McFarlane also teaches at Marist College's Institute for Data Center Professionals.

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This was last published in October 2014

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How are you reducing energy use in the data center?
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One easy way to reduce energy use in a data center is to use variable speed fan drives. These drives change speeds on the fans based on intensity of use. When the data center server load is low and cooling needs are less, the fan blades slow down, therefore conserving energy. The fan blades then speed up when the server load increases. Studies have shown data centers can save thousands of dollars a year on energy costs using this technique.
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