STULZ worldwide

Closed-circuit cooling via the raised floors allows the data center air conditioning to constantly satisfy the ever present demands for low running costs, high flexibility and redundancy with a well proven system.

However, in most cases the air is not regulated as it exits the raised floor. Here, sensor-controlled raised floor grilles with a variable opening angle are an essential element for restricting the energy consumption of the air conditioning system.

In the majority of cases, the raised floor manufacturer also provides the grilles for the outflow of air. As a rule, the customer can then choose between different degrees of perforation. Some manufacturers also offer restrictor panels, which are fitted to the underside of the floor grille and enable the airflow to be regulated manually.

The actual airflow rate required depends on the current load of the server. However, in this age of server virtualization and cloud technology, load fluctuations can occur as entire racks are switched on and off. Due to this fluctuating server load, flexible solutions are also in demand for closed-circuit air conditioning. The challenge lies in supplying the servers with sufficient cooled air that is targeted in line with demand.

Here, 'sufficient' means that precisely the amount of air needed at that moment by the servers exits the raised floor and 'targeted' means that as far as possible, the air exits the raised floor directly in front of the server rack's air intake. This last point is especially important when the cold and hot aisles are not separated by walls or partitions. The demand-based supply of cold air immediately in front of the server intake keeps the mixing of cold and hot air to a minimum. We can therefore refer to this concept as virtual containment.

With the AirModulator, STULZ offers a solution for a large variety of applications, and with dimensions of 600 mm x 600 mm that make it compatible with commercially available raised floors. The opening angle of the dampers can be regulated to suit demand by the building services management (BMS) system, or by the AirModulator's own controller, based on the temperature or pressure difference. In the event of a power failure, the dampers are opened automatically by a return spring. The unit as a whole is finished with a flow optimized grille, and is therefore also protected from mechanical stress, e.g. from lift trucks.

1. Precision air conditioning unit

2. Raised floor

3. Differential pressure sensor for room/raised floor

4. Containment system/cold aisle enclosure

5. Differential pressure sensor for cold aisle/room

6. Floor grille controlled by pressure or BMS

7. Floor grille controlled by temperature or BMS

Special considerations for the containment system/cold aisle enclosure

Ideally, the static pressure in the raised floor generated by the precision air conditioning units is kept constant by means of differential pressure control. In this case, differential pressure monitors distributed in the raised floor send a control signal to the precision air conditioning units, as a function of which the fan speed is regulated. If the heat load of the servers increases, they require more air for cooling. The result is a change in the differential pressure (room/raised floor), on which basis the outflow of air is regulated for the server room as a whole.

When containment systems are used in data centers where the cold aisle is separated from the hot aisle by a wall or partition, the large server room is divided into several enclosures with highly contrasting loads. If these enclosures are supplied with air from the same raised floor, the pressure inside the enclosures can only be kept constant by another differential pressure control mechanism (cold aisle/room), in conjunction with adjustable floor grilles.

Info Box: Closed-circuit cooling in the data center

Servers in data centers are mainly cooled using the principle of closed-circuit cooling. The air heated by the servers is cooled once more by precision air conditioning units.


The server racks in the data center are normally arranged in a hot aisle/cold aisle layout, involving alternating rows of cold and hot aisles. The air intakes of the server racks are positioned opposite one another in the cold aisle, and the air outlets are in the hot aisles. The air from the hot aisles is fed back to the precision air conditioning units, where it is cooled to the air temperature required by the servers. Next, the cooled air is conveyed through a raised floor to the cold aisle, where it is expelled in front of the servers through holes in the raised floor grilles.


To increase the efficiency of the precision air conditioning units, so-called containment systems can be used to prevent the cold air from mixing with the hot air. Separating the air flows enables the air conditioning units to work with optimum temperature differences.

AirBooster 2

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