Heat exchanger performance

Select the correct water inlet temperature and water flow rate to achieve the necessary heat removal.

A heat removal of 100% indicates that an amount of heat equivalent to that generated by the devices has been removed by the heat exchanger and the average air temperature leaving the heat exchanger is identical to that entering the rack (27°C [80.6°F] in this example). Heat removal in excess of 100% indicates that the heat exchanger not only removed all of the heat that is generated by the devices but further cooled the air. This ensures that the average air temperature leaving the rack is lower than that entering the rack.

Attention:
To help maintain optimum performance of the rear door heat exchanger and provide proper cooling for all rack components, you must always take the following precautions:
  1. Install filler panels over all unoccupied bays.
  2. Route signal cables at the rear of the rack so that they enter or exit the cabinet through the top and bottom air baffles.
  3. Bundle signal cables together in a rectangle so that the upper and lower air-baffle sliders are closed as far as possible. Do not bundle signal cables together in a circular formation.

Figure 1 - Figure 6 can help you decide what is the required water flowrate per rear door heat exchanger so that the facility and CDU can be sized.

As example, pick one of Figure 1 - Figure 5, whichever more closely resembles the expected sustained heat load. Interpolate between graphs if necessary. Pick the curve that defines the water temperature that can be provided to the rear door heat exchanger. Determine the water flow rate that is required to achieve 5 - 10% more heat removal than is required for the installation. This additional capacity allows for less than perfect airflow that is blocking and possible air that bypasses the rear door heat exchanger. For instance, if you want the rear door heat exchanger to make the rack data center neutral, you can select a water flow rate that provides 105 - 110% heat removal to provide some margin to the cooling design. For rack powers other than what is listed in Figure 1 - Figure 5, for racks that are far from uniformly populated with heat generating devices, or for racks with significantly different airflow rates than what the curves show (plus or minus 30%), you must take guidance from IBM Power Systems Thermal Development, which is available through the IBM Sales team.

For all graphs, a given water temperature might be used only if the system that is supplying the water is able to measure the room dew point and automatically adjust the water temperature. Otherwise, the water temperature must be above the maximum dew point that is allowed at that data center installation.

Figure 1. Typical performance of the heat exchanger, 20 kW heat load
Typical performance of the heat exchanger, 20 kW heat load
Figure 2. Typical performance of the heat exchanger, 25 kW heat load
Typical performance of the heat exchanger, 25 kW heat load
Figure 3. Typical performance of the heat exchanger, 30 kW heat load
Typical performance of the heat exchanger, 30 kW heat load
Figure 4. Typical performance of the heat exchanger, 35 kW heat load
Typical performance of the heat exchanger, 35 kW heat load
Figure 5. Typical performance of the heat exchanger, 40 kW heat load
Typical performance of the heat exchanger, 40 kW heat load

After the required water flowrate per rear door heat exchanger is defined per the heat removal information above, the water-side pressure drop of the rear door heat exchanger can be determined from Figure 6.

Using the total water flowrate requirement for all of the rear door heat exchangers and the pressure drop for the entire water flow network, which the rear door heat exchanger is a part of, the facility and CDU can be defined to meet these flow and pressure drop requirements.

Figure 6. Pressure drop (standard units)
Pressure drop (standard units)