It’s said that “good things come in threes”, like a DJ’s turntables and a microphone, or those three unit-less constants called the “proportional”, “integral” and “derivative” that dominate the operational scheme of nearly every control system serving the data center; or the famous trio of Moe, Larry, and Curly, for that matter.
Walking through the data center and reflecting on a few simple devices that have made a big difference in being able to optimize its energy and costs, are clusters of a few humble devices, hiding in the dark recesses under the floor or high overhead in the data center or related mechanical spaces. They can be more important to an engineer or data center operator than any other trio of tools. They are “2 Temp Sensors and a Flow Device”.
Individually they measure three rudimentary values, however, collectively they enable the determination of exceptionally important thermal energy production and utilization metrics for data centers using chilled water as a cooling medium.
While all data centers seem to have an adequate number of pairs of supply and return temperature sensors, flow measurement devices seem to be a little rarer; however simultaneously measuring flow and temperature provides an opportunity to gain a greater degree of control of a chiller plant, data center, or an entire building.
Precise thermal energy measurement is sometimes an unexploited opportunity, if you consider that most data center measurements have for a long time focused on the electrical energy of the facility which is largely dominated by (essentially) “mass-less equations”, that more or less say that something modified by something equals something (such as volts times amps equals “watts”; or “volt amps” to be more accurate).
Electrical measurement describes “electricity”, which is (for all practical purposes) a “force”. Chilled water, on the other hand, is “thermal energy”, and has considerable mass, and can “give-up” or “absorb” heat while doing its work in the data center, or when being processed through chillers, heat exchangers, or free cooling devices.
With “two temp sensors and a flow device” the:
-the entire chilled water plant can be measured, and
-chilled water production of a chiller can be measured, and
-the chilled water consumption of the data center’s “white space” can be measured, and
-in multi-use buildings, the consumption of the data center can be separated from the rest of the building.
The units of measure are simple and whether you are partial to measuring chilled water production and consumption in “tons” or “kilowatts” of heat, the procedure can be done manually or (better yet) can be computed by a building automation system, and even updated and displayed in real time.
Allowing the data center to see what the chiller plant is producing and what the data center is using, is very helpful in buildings where the data center shares a chiller plant with other non-data center operations that may be under the same roof.
If you think about cooling as the act of removing heat and not “making cold”, the units of measure of thermal energy seem more intuitive, because it all becomes the “making or rejection of heat”.
To set the record straight, “tons” of cooling refer to the arcane, though still popular, measurement that represents the amount of heat energy that a one ton block of ice would absorb in the process of completely melting.
Kilowatts of cooling (in other words, “kilowatts of heat rejection”) is a more international unit of measurement, though it can be confusing if kilowatts of cooling and kilowatts of electricity are mixed into the same discussion, which often occurs. Consequently, it may be convenient, for the sake of this discussion, to call kilowatts of cooling “HRkW” and electrical kilowatts “kW”.
To provide further perspective, chillers sold in the United States are rated in both Tons and Kilowatts, though also may carry a rating in BTUs.
All these units of measure are related, in so much as there are 3412 Btu’s in an “HRkW” (kilowatt of cooling); and 3.517 HRkW for each Ton of cooling (which incidentally contains 12,000 Btu’s).
The flow meter and temperature sensors may deliver their outputs to a building automation system (BMS) in regionally appropriate (English or metric) units, and the BMS can continually calculate and display chilled water production or consumption. Of course, production and consumption is, for all practical purposes, the same if the chiller plant only serves the data center. However, if the chiller plant serves multiple departments within a building, with sets of these three tools installed, the opportunity to accurately apportion and “cost allocate” the thermal energy becomes easy.
The formulas are simple, and can be expressed as a fraction. The numerator is the product of chilled water flow, the weight of water, and time, further multiplied by the difference between the supply and return temperatures. The denominator is the unit of energy appropriate to the units in the numerator. The weight of water is ordinarily 8.3453 pounds per gallon.
This formula can be run as many times as necessary using the instantly available information from each set of temp sensors and flow devices. The first complete measurement and calculation may determine the production of the chilled water plant, and if necessary, a second or additional number of measurements and calculations may be performed to determine the chilled water consumption of the data center and any other department within the building.
So for US applications (for example):
(GPM x WTwater x 60minutes) x ΔT
= ton hours of chilled water
Since flow is “time-based”, in other words, expressed in gallons per minute (GPM) or liters per second (LPS), the result of the calculation will also be “time-based”, just like the kilowatt hours of electricity (or any other input energy) used to produce the chilled water. These important, related time based measurements, chilled water produced and electricity used can be automatically compared, to assess the efficiency of the plant, building, or data center.
After installation of sets of 2 temp sensors and a flow device, better efficiency will likely result as the information that it produces can be used to optimize pump and chiller performance, restore the originally intended delta-T to the chilled water system, and accurately allocate chilled water use. Costs and cost allocation across the enterprise will likely improve.