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Standby management for CW units

Today, the operator of a Data Center basically has two fundamental concerns: firstly, reliability, i.e. Data Center availability; secondly, energy efficiency, particularly in this age of rising energy prices.

In most cases, larger Data Centers continue to use closed-circuit air conditioning units. These so-called CW units basically consist "only" of an air/water heat exchanger, fans, air filters, control valves and the necessary electrical components, plus a controller. The cooled water supply to these units is provided by a centralized chiller.

To remove the heat load from the Data Center, a certain airflow is required, the amount of which depends on the air-side temperature difference. This airflow is supplied by the closed-circuit air conditioning units.

A certain level of so-called "redundancy" of air conditioning units is created, depending on the size and desired reliability level, to ensure reliable Data Center air conditioning. In other words, more units are installed (standby units) than are actually required for air conditioning. Normally, these units are only brought (automatically) into operation if a running unit switches off due to a fault (passive redundancy).

The latest closed-circuit air conditioning units make use of EC fans for ventilation. These fans are considerably more energy efficient than the older versions with AC motor. Another major advantage of these fans is that as the fan speed decreases, the motor's power consumption does not decline in a linear fashion as a function of the speed, but by the power of three.

If the three parameters above (i.e. the required airflow, redundancy of air conditioning units and EC motor technology) are combined, a simple control concept can save even more energy (and therefore money), based solely on considerations of reliability and energy savings.

With this control concept (STULZ CW standby manager), all installed units (i.e. the units required to remove the heat load, plus the standby units) are operated with an airflow that is the same in total but reduced for each unit. This technique is known as "active" redundancy. If one of these units fails, the speed of all the fans on the other units is raised to close the gap in the airflow.

Thanks to the EC technology principles outlined above, the power consumption of the fans is dramatically reduced. Depending on the number of redundant units, the heat load in the room (100 % full load or partial load) and the operating point, this technology enables savings in fan power consumption of up to 75 %. A further positive "side effect" is the lower overall noise level of the installed air conditioning units, due to the lower fan speed.



Example: Passive redundancy

6 x ASD 1710 CWU air conditioning units: passive redundancy (4 units on, 2 units off)

Airflow: 4 x 33,000 m³/h = 132,000 m³/h => power consumption of fans: 4 x 4.2 kW = 16.8 kW



Example: Active redundancy

6 x ASD 1710 CWU air conditioning units: active redundancy (6 units on)

Airflow: 6 x 22,000 m³/h = 132,000 m³/h => power consumption of fans: 6 x 1.5 kW = 9.0 kW

Savings: 7.8 kW (= 47 %)

Savings at € 0.15 per kWh a year: € 10,250

Furthermore, the CW standby manager "Made by STULZ" is a basic constituent of certain other energy-saving forms of control, such as differential and raised floor pressure control, and Dynamic Indirect Free Cooling.

About the author

Norbert Wenk is head of Product Management. Following a degree in Mechanical Engineering, he joined STULZ in 1999. He started his career at STULZ in the R&D department and later moved to the Export (Sales) department, where he worked as an Area Sales Manager. He therefore now has over 20 years of experience in the Data Center cooling industry.