Air cooling continues to be the dominant mode for most electronics applications despite rampant increases in system and component power dissipation. Using liquids for cooling electronics occurs more often at more modern data denters where the liquid cooling facilities are provisioned up front, yet air cooling continues to be the desired mode if it meets the thermal requirements. Higher power dissipation in enclosures such as PICMG telecomm chassis has mandated the migration from a single fan to an array of fans, commonly referred to as "fan trays." Figure 1 shows examples of fan trays used for different electronic systems.

This article discusses the use of fan trays in enclosures and highlights the pertinent issues facing such undertaking.

Fan Types
There are five broad categories of fans:

• Propeller--used where the required pressure is low (less than 187 Pa).
• Tube axial--develops high pressure, but has low efficiency.
• Centrifugal--typically require bends in ductwork for flow introduction. They are typically very quiet (noise decreases with the increase in number of blades).
• Vane axial--similar to tube axial fans; tends to be cheaper and to use less space than centrifugal fans.
• Flat pack--blower design features radial air exhaust through housing port.
• Impeller--also known as flat packs, result in smaller aspect ratios with good flow rate and pressure drop.

The tube axial fan is 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 for zero resistance.
• 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 parallel.

When fans are placed in parallel, Figure 3a, the volumetric flow rate is added together. If the fans are identical, the flow rate at zero pressure will be doubled. When fans are placed in series, Figure 3b, the pressure drop will be added together, and if they are identical at zero flow rate the pressure drop is doubled. Obviously, the system flow rate and pressure drop for a fan or an assembly are determined by where the system curve intersects the P-Q curve.

Based on the above explanation, if the thermal requirements of the system mandate a higher volumetric flow rate, the parallel configuration is used. If the system is flow-resistive, and thus has a larger pressure drop, the series combination is used. If both conditions are present, i.e., a high flow resistance system with high power dissipation, then a combination of parallel and series can be used. This is the so-called Push-Pull system that is commonly seen in telecom and datacom applications.

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.