called "negative" airflow). Worth noting is that positive internal pressure only prevents dust accumulating in the case if the air intakes are equipped with dust filters. A case with negative internal pressure will suffer a higher rate of dust accumulation even if the intakes are filtered, as the negative pressure will draw dust in through any available opening in the case The air flow inside the typical desktop case is usually not strong enough for a passive CPU heatsink. Most desktop heatsinks are active including one or even multiple directly attached fans or blowers. Servers Server coolers Each server can have an independent internal cooler system; Server cooling fans in (1 U) enclosures are usually located in the middle of the enclosure, between the hard drives at the front and passive CPU heatsinks at the rear. Larger (higher) enclosures also have exhaust fans, and from approximately 4U they may have active heatsinks. Power supplies generally have their own rear-facing exhaust fans. Rack-mounted coolers Rack cabinet is a typical enclosure for horizontally mounted servers. Air typically drawn in at the front of the rack and exhausted at the rear. Each cabinet can have additional cooling options; for example, they can have a Close Coupled Cooling attachable module or integrated with cabinet elements (like cooling doors in iDataPlex server rack). Another way of accommodating large numbers of systems in a small space is to use blade chassis, oriented vertically rather than horizontally, to facilitate convection. Air heated by the hot components tends to rise, creating a natural air flow along the boards (stack effect), cooling them. Some manufacturers take advantage of this effect. Data center cooling Because data centers typically contain large numbers of computers and other power-dissipating devices, they risk equipment overheating; extensive HVAC systems are used to prevent this. Often a raised floor is used so the area under the floor may be used as a large plenum for cooled air and power cabling. Direct Contact Liquid Cooling has emerged more efficient than air cooling options, resulting in smaller footprint, lower capital requirements and lower operational costs than air cooling. It uses warm liquid instead of air to move heat away from the hottest components. Energy efficiency gains from liquid cooling is also driving its adoption. Laptops Laptops present a difficult mechanical airflow design, power dissipation, and cooling challenge. Constraints specific to laptops include: the device as a whole has