VARISTAR with air/water heat exchanger

With the increasing performance loss of components in electronic units conventional air cooling is reaching its limits more and more often. From approx. 8 kW performance loss per cabinet it becomes almost impossible to cool solely with air. In particular with high performance servers the limit is reached very fast.

Server based systems form the core elements of the network infrastructure in the telecommunications sector. In the test centre at Siemens Networks in Munich for example IMS application servers (IMS = IP Multimedia Subsystems) are installed inside cabinets. With new systems, such as FCM (Fixed Mobile Convergence) the network operators are able to introduce multimedia services homogenously and cost effectively for all networks that are supported by the operator (for instance GSM, UMTS, WLAN, DSL, Cable). The user then has access to the same functions via different network types, regardless of which terminal is used. Technical basis for the simple network change and the provision of multimedia services is the IMS.

The cabinet technology deployed with area dependent cooling concepts and air cooling is no longer sufficient for these new high performance systems. With a fully assembled cabinet the limits of air cooling are reached very quickly. From 1.500 W/m² it is no longer economical to use room dependent cooling concepts. An alternative is combined air/fluid cooling with water as medium. The heat capacity of water is higher by a factor of 4000 than that of air (relating to volume) and is therefore far more suited for the cooling of components. The higher density of water allows considerably more heat to be taken out of the cabinet. Furthermore, through the closed cycle of an air/water heat exchanger a cooling solution is created that is independent of the room (Picture 1).

Infra structure - rear cooling unit

The specialists in the test centre, who are responsible for the entire planning, configuration and system testing, have looked at various cabinet solutions with water cooling, which are offered on the market. As the test facilities were erected in an existing climate controlled building without suitable water supply, the required infra structure (200 kW rear cooling unit) had to be retrofitted (Picture 2). Important criteria for the water cooled cabinets, apart from the cooling performance, were the installation area and the noise level. Chosen was the cabinet platform VARISTAR with integrated air/water heat exchanger. Based on a basic configuration, Siemens could consolidate their demands regarding configuration and functionality of the cabinet system. For the installation of the server and further components into the cabinet no special measures were needed. Necessary mechanical assembly parts and accessories are available from the VARISTAR range. For the power supply and signal distribution customer specific solutions were realised.

Horizontal air flow

A huge advantage of the selected solution is the horizontal air flow inside the cabinet, with the heat exchanger positioned on the side. The cool air circulates over the entire cabinet height from the front to the rear through the installed components. With a vertical air flow, the air circulates over the cabinet height. As a rule the heat exchanger is than positioned at the base (=> reduced, usable assembly height) and directs the cool air through the installed components to the top. The hot air is then directed back to the heat exchanger at the base with the aid of fans in the rear door of the cabinet. When comparing both principles under the same framework conditions and equal dimensions, the horizontal air flow requires less of a cabinet depth. Thereby the footprint is kept small despite the high cooling performance.

20 kW performance loss per cabinet

With the VARISTAR LHX 20 Schroff has developed a cooling concept as a complete cabinet system which is indepent from the room. The system is able to deal with a heat loss of up to 20 kW. The air/water heat exchanger is inserted as a complete module either on the left or right inside the cabinet, therefore the 19" platform in usuable to its full height (42 U).

The heat exchanger is in a closed air cycle of the cabinet with an air flow volume of up to 3000 m³/h. Air direction elements and six fans which are spread over the entire height provide the even cooling of all installed components. Cool water runs through the external water cycle and the heat exchanger, it takes up the heat and flows back to the rear cooling unit. Here the water is then cooled down to the required supply temperature again (Picture 3).

Energy saving temperature regulation

The temperature inside the cabinet is regulated with a micro controller; via a three-way valve and a bypass the water supply and return is optimally mixed. The cabinet temperature is predominantly held constant via the water cycle. In order to run the cooling operation as energy efficient as possible, the fans are operated with the required minimum speed until the three-way valve is open 100 %. Only then is the speed of the fans increased to maximum. If the cabinet temperature then drops, first the fans are shut off and then the water cycle is regulated. Different interfaces (RS 232, RS 485) for alarm ports and warning signals, as well as external cabinet monitoring systems ensure interference-free operation.

Noise level

As people are working in the vicinity of the test facilities, noise pollution is an important subject. The noise level generated by the cooling components must not be underestimated. VARISTAR with LHX20 reaches a value of only 50.7 dB(A) with a load of 80 % and 54.8 dB(A) with a 100 % load. Therefore deployment in an office environment would be possible. The limit determined in the specification is 55 dB(A) (DIN EN ISO 11650-1).