Despite rampant increases in power dissipation, the use of liquid for cooling electronics remains sporadic. Air cooling continues to be the dominant mode for most electronics applications. Higher power dissipation in ATCA enclosures, PICMG telecomm chassis, medical electronics, laser equipment, etc. mean that a single fan is no longer adequate to provide the airflow required for system thermal management. Therefore, an aggregate of fans in a tray is required. Figure 1 shows varied examples of fan trays used for different electronic systems.
Figure 1: Different Fan Trays Used in Electronic Systems (source: various)
Fan Types
While only tube axial fans are shown in Figure 1, there are five broad categories of fans. These are:
- Propeller used where the required pressure is low (less than 187 Pa).
- Tube-axial develop high pressure, but have low efficiency.
- Vane-axial similar to tube-axial fans; tend to be cheaper and require less space than centrifugal fans.
- Centrifugal typically require bends in duct work for flow introduction. They are typically very quiet (noise decreases with the increase in number of blades).
- Impeller typically known as flat packs, result in smaller aspect ratios with good flow rate and pressure drop.
Figure 2: Different Fan Designs [3]
The tube axial fan is by far the most common in electronic equipment. Flat pack and impeller (blower) fans have excellent pressure drop characteristics and are commonly seen in densely packed electronics. Axial and centrifugal fans have limited application for moving air through electronics systems because of their low pressure drop characteristics. However, because of their large volumetric flow rates, they play a pivotal role in cooling buildings or large cabinets that house electronic systems.
Fan Placement Configuration
Before we discuss the design issues involved with a fan tray implementation, we must review the characteristics of fans when placed in parallel and in series:
- Parallel the total system flow is divided among the fans. Pressure drop remains the same, but the flow rate increases according to the number of fans.
- Series the flow through all fans is the same, while the total system pressure drop is divided among them.
Figures 3a and 3b depict the P-Q curves (pressure drop and volumetric flow rate) for fans in series and
Figure 3: P-Q Curve for Fans in Parallel (3a) and in Series (3b) [4]
Fan Trays
This notion can now be transferred to fan trays and their unique characteristics when implemented in a system. First, we’ll define the characteristic lengths. Figure 4 shows a schematic of a system with fan tray and the supporting electronics.
Figure 4: Schematic Definition of Characteristic Lengths
Here, the characteristic lengths used in the design process are
D = fan diameter or width
H = fan thickness
L = distance between the fan and the adjacent object, e.g., PCBs or another fan.
When placing the fan next to another fan (i.e., in series) or a card rack, the simple rule of thumb is: . This means the farther you place the fan from an obstruction, the better it performs. At shorter distances, the airflow out of the fan will meet flow resistance created by obstacles, e.g., filter or PCBs. This will subsequently impact the intake of the fan resulting in diminished performance. One needs to remember that the air flow out of a fan is radial and not axial. Therefore, the larger the plenum space for the fans to breed, the better the performance. The above scenario addresses the issue of fans in series or the location of an obstruction in the exhaust or intake of the fan.
When we place fans in parallel, there is a strong possibility that the flow from one fan will shunt through a neighboring fan. This depends on their proximity and on the fans RPM and design. Another issue is that acoustic noise may result from the fans proximity to each other. Harmonics can be created from flow shunting or flow interference between neighboring fans, and often sounds like a heart beat. And, as indicated above, radial fan flow will cause flow interference in a fan tray environment. Because the fan is an airfoil-driven device, and proper pressure differential across its hub is essential to performance, improper fan placement and fan/exhaust-flow mixing will cause fan tray performance degradation. Figure 5 shows one example of how to remedy such a situation.
Figure 5: Parallel Fan Flow Coupling and Respective Remedy [3]
Therefore, it is highly recommended that when selecting or designing a fan tray, special care be taken to ensure that the above basic design requirements are implemented.
In addition to these requirements, here are some general guidelines for fan and fan tray implementation:
Elbows – When placed close to a fan inlet an elbow can cause non-uniform distribution of air. This results in flow starvation of some parts of the blade.
Sharp-edged entrance – This may result in vena contracta, with a loss of flow area.
Swirling flow at the inlet – The fan intake design may cause swirling airflow. If the direction of the swirl is the same as the blade, it affects upstream pressure, and decreases fan effectiveness (decreases the pressure). If the swirl is in the opposite direction of the blade rotation, more power consumption by the fan is needed to reach the same performance.
Obstruction at inlet – This will reduce the flow area.
When placing fan trays in typical electronics enclosures the question of Push, Pull, or Push-Pull always arises. Push implies that air is being pushed into the electronics enclosure, i.e., the fan tray resides on the top or bottom of the chassis and pushes the air into the PCB cage. Pull means that air is drawn into the system whether the fan tray is on the top or bottom. Push-Pull is when two fan trays are placed on the top and bottom of the fan tray. With a Push or Pull system, there is no acceptable industry best practice. The implementation is very much chassis dependent. However, the Pull option uses the chassis as a plenum and provides a better pressure distribution for the enclosure. When the Push option is used, high turbulent flow exiting the fans is forced through a filter, honeycomb, and the PCB channels (system configurations may vary). If a large plenum is not provided, the fan tray efficiency will drop significantly because highly turbulent flow must negotiate the added resistances.
As apparent from the above, the successful use of a fan tray in any configuration is plenum size dependent. If the plenum and the subsequent resistances are designed properly, the fan tray will operate at its highest performance point and deliver the required airflow for the cooling of the electronics.
Pressure Drop and Resistance
In designing a fan tray for cooling an electronic system, knowing the velocity head loss and the resistance to flow created by the fan tray is required. The following equations can be used for these calculations:
Acoustic Noise
Generally, the noise created by a fan is the collision of air molecules. The subsequent mechanical vibration is the result of mixing in the physical system. When a fan is running, the blade is rotating, moving a mass of air from intake side to exhaust side. There are relative motions between the air and the blades, the air and the housing, and the air and the ribs supporting the motor. Consequently, the air exiting and entering the fan is highly turbulent, and this turbulence is the source of noise in the fan.
The sound created by fans is a major point of concern for system implementation at a customers premises. For example, a fan with 7 blades rotating at 4200 RPM creates a noise with an approximate 490 Hz frequency (7 — 4200 · 60(sec)= 490 Hz). Therefore, much effort is put forth to measure and manage this noise to make the electronics more acceptable on location. By using the following equations, we can measure the noise and its acceptability before the fan tray is designed. Two parameters play a direct role in the acoustic noise generated by a fan: airflow and fan RPM.
The sound power as a function of RPM is calculated by:
A typical acceptable sound level is 58db. Beyond this the noise is considered intrusive to people around the system. Of course, because of todays high power electronics, this limit is clearly violated and often reaches maximum of 68 db.
In conclusion, fan trays clearly play a pivotal role in providing effective cooling solutions for many electronic enclosures. It is imperative that their functionality and operation is clearly understood and designed for a given system. It is often insufficient to simply place a fan tray in the system and assume it will provide the airflow necessary to cool the system to the acceptable operational levels. As seen above the design of the fan tray, placement of the fans in the fan tray, effect of fan RPM, and the airflow rate all play major roles in the successful use of a fan tray in a chassis, and subsequently in system cooling, which is the ultimate objective.
References:
- Qpedia, Fan Curves and Laws, February 2008.
- Qpedia, Selecting a Fan, March 2007.
- Advanced Thermal Solutions, Inc. Tutorial Series, Understanding Air Movers, Types, Implementation and Characterization.
- ACT-Rx Technology Corp., www.actrx.com.