Radical Redesign of a Desktop PC Enclosure

The following example is based on work carried out by Rolf Konstad of Intel's End User Component Division in Folsom, CA. It describes how a FLOTHERM analysis carried out for a major PC manufacturer showed them how to eliminate a fan and reposition cooling vents in order to:

• improve the thermal performance;
• reduce acoustic noise; and
• cut costs.

Figure 1 - FLOTHERM Model of the Computer as Designed

The PC is a fairly typical desktop system consisting of:

• a motherboard with a processor (including a fan-heatsink);
• a system fan blowing air across the processor site;
• expansion cards;
• a power supply with an integral fan; and
• disk drives.

and figure 1 shows the FLOTHERM model of the system.

Figure 2 - The Computer as Designed: Air Entering through the Side Vent

Figure 2 shows the results as a set of streamlines originating at the side vent. These show how the side vent provides nearly all of the airflow across the processor site and through the expansion cards.

Figure 3 - The Computer as Designed: Air Entering through the Front Vent

Figure 3, in contrast, shows the streamlines for the air which enters through the front vent and the system fan. Despite the position of the fan, directly in front of the processor, most of this air bypasses the important areas and goes straight into the power supply!

Figure 4 - Comparison of the Air Entering through
the Front Vent and through the System Fan

Figure 4 contrasts the amount of air which is entering through the front vent and the amount which passes through the system fan. The desired effect would be for all of the air which passes through the fan to be fresh air from the vent. However, because of the poor positioning of the fan, it is recirculating considerably more stale air from within the enclosure than fresh air from outside.

Figure 5 - Fan On and Fan Off

Finally, Figure 5 shows the most frightening aspect of all of this - when we turn the fan off, the air passing over the processor site is 1.5°C cooler! (These results were confirmed by experiment).

So what's happening?

Figure 6 - System Pressure with Fan On and Fan Off

Figure 6 shows the main reason why we're having problems. The values shown are the pressures within the system enclosure with and without the system fan "working". When the fan is switched off, the pressure within the enclosure is lower and, as a result, the inflow through the side vent will be greater. Because this is the air which cools the processor, the thermal performance is better with the fan off!

So how was the design changed to improve it?

Figure 7 - The Redesigned Computer

Figure 7 shows the (radical) redesign of the enclosure:

• the system fan has been eliminated;
• the front vents have been closed off entirely; and
• the side vents have been repositioned.

How does this affect the results?

Figure 8 - Flow from the Lower Side Vent

The lower side vent has been positioned to provide air for the processor site. Figure 8 shows that there is a well directed flow of fresh air straight from the vent to the processor.

Figure 9 - Flow from the Upper Side Vent

The second side vent has been positioned in order to provide fresh air for the expansion cards. Figure 9 shows that the flow from this second vent is successfully distributed though this region.

Does all of this work? Here are the results (again, verified by experiment):

Figure 10 - Results for the Original Design
(Fan On and Fan Off) vs. the New Design

The chart shows the temperature of the air passing over the processor site for the 3 cases:

1. Original Design - System Fan On
2. Original Design - System Fan Off
3. New Design - System Fan and Front Vents Eliminated; New Side Vents

The new design has clearly superior thermal performance as well as lower acoustic noise and cost.

Important Note:

A radical solution like this is specific to one particular design and this example shouldn't be treated as a generic design recommendation. Good thermal design is system specific. Always get expert advice!