Multimedia communications for the simultaneous transmission of voice, data, and video or still images need a number of communications systems and network servers at transmitting stations. To reduce equipment size yet maintain capabilities, these systems are increasingly designed with their printed circuit boards situated in very close proximity, increasing the critical need for an effective means of cooling.
Generally, several fan trays with multiple 4.69 in. sq. fans cool the printed circuit boards that are mounted in standard 19-in. rack enclosures. However, over stuffing an equipment cabinet with fans can create other problems for installation, wiring, noise, and power consumption.
The engineers at Oriental Motor, Calif., thought they had a solution. Through the use of their proprietary computational fluid dynamics (CFD) simulation program, they demonstrated that one low-noise ORIX MRS20 7.87 in. sq. x 3.54 in. thick fan would produce cooling equivalent to multiple fan trays. Plus, use of one fan rather than many solved problems in wiring, noise, and power consumption.
Internal system cooling
The following procedure was used to select the appropriate type of fan:
• Specify the required conditions for fan selection.
• Calculate the amount of heat dissipation required in the system.
• Calculate the amount of heat released through the surfaces of the system.
• Calculate the amount of heat released by the fans.
• Calculate the amount of airflow required by the fans.
• Select the appropriate fans
• Check the temperatures present within the system.
Heat in a system is either dissipated through the cabinet walls and mounting surfaces by means of convection and radiation, or through ventilation provided by one or more fans (Fig. 1).
Fig.1-Heat in a system cabinet must be dissipated through the cabinet walls and mounting surfaces or through ventilation provided by fans.
Using computational fluid dynamics calculations
Computational fluid dynamics software was used to analyze airflow, static pressure and temperature within a system to compare the cooling effects of the MRS20 and fan trays with respect to the printed circuit boards contained in the system (see Fig. 2).
Fig.2- Layout models for fan trays indicate the potential to place up to 24 fan units. But, before selecting the maximum number, it is always wise to try other configurations.
The CFD method took a model and divided it into meshed areas where heat and flow equations could solve for each area. This simulation approach permits the modeling and analysis of individual electronic components. Therefore, it helps in the creation of general designs in applications where internal layouts must accommodate a variety of factors regarding the printed circuit boards and fan trays, heat-dissipating conditions, and others.
Fig.3- This photo shows a system that incorporates four fan trays and how air moves through them.
For the simulations to test temperature contours within the system, four models were assumed. One or more fan trays, each carrying a row of six MU1238 units, were inserted between the shelves in the mounting test fixture, starting from below the bottom shelf.
Six units inserted below the bottom shelf only.
A total of 12 units inserted below the bottom and second shelves.
A total of 18 units inserted below the bottom through third shelves.
A total of 24 units inserted below the bottom through fourth shelves.
Layout model for the MRS20: One MRS20 unit was placed on top of the mounting test fixture.
The mounting test fixture consisted of twenty printed circuit boards W18.9 x D14.2 x H9.4 in., each dissipating 40 watts of heat, and placed in a shelf at 0.24 in. intervals. The shelf was then stacked within a system to a height of four levels, with the adjacent shelves separated by 3.8 in. (total heat dissipation 3.2 kW). Outside the system the ambient temperature was maintained at a constant 68°F.
Fig.4- A curve of airflow/static pressure characteristic was created for each simulation model.
With one and two fan trays installed, temperatures in the upper section rose to 125.6°F and 111.2°F, respectively, and the interior of the system was not cooled uniformly. Temperatures in the upper section were 102.2°F with three fan trays and 89.6°F with four fan trays. Only when four fan trays were installed, did the system’s interior cool uniformly.
Fig.5 – Noise levels can be a key factor in any design. This graph shows calculated noise levels for each simulation model.
When one MRS20 fan was used for cooling, the temperature in the upper section was only 96.8°F, indicating that it produced a cooling effect roughly equivalent to that achieved with four fan trays. Also, analyzing the temperature contours at the center of the shelves in the horizontal direction showed that each shelf was cooled in a relatively uniform manner (see Fig. 3).
Generally speaking, the static pressure of a fan decreases as the airflow it generates increases. Ventilation resistance is created in such a direction as to prevent that airflow. This ventilation resistance, which increases as airflow increases, is given as the internal pressure present in the system.
Figure 4 shows a curve indicating airflow and static pressure characteristics, which demonstrated the relationship between fans and the system’s internal pressure. The intersection between these two curves is referred to as the “operating point,” which represents airflow in a system and the pressure loss that occurs at that particular airflow. To cool a system internally, it is important to select fans offering greater capacity than the operating point would otherwise indicate.
The noise level inherent with each simulation model was predicted by means of a calculation. The noise level of one MRS20 unit was 61 dB, being roughly equivalent to the degree of noise produced by four fan trays (a total of 24 fan units).
A curve of airflow/static-pressure characteristic was created for each simulation model (Figure 4). The airflow/static pressure characteristic curves of the respective models, along with a sample characteristic curve of system pressure loss proved that one MRS20 fan unit is capable of producing airflow at 10.1 m3/min—roughly equal to the 10.5m3/min airflow obtained with four fan trays (a total of 24 fan-units).
The results of the computational fluid dynamics simulation showed that, with a system whose printed circuit boards are mounted close together, thereby generating a large amount of heat, the cooling effect and noise level of one MRS20 fan unit are roughly equal to those of four fan trays (with 6 fan units, each 4.69 in. square).
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