assembled from individually gathered parts. The past few years have seen an increase in the popularity of liquid cooling in pre-assembled, moderate to high performance, desktop computers. Sealed ("closed-loop") systems incorporating a small pre-filled radiator, fan, and waterblock simplify the installation and maintenance of water cooling at a slight cost in cooling effectiveness relative to larger and more complex setups. Liquid cooling is typically combined with air cooling, using liquid cooling for the hottest components, such as CPUs or GPUs, while retaining the simpler and cheaper air cooling for less demanding components. The IBM Aquasar system uses hot water cooling to achieve energy efficiency, the water being used to heat buildings as well. Since 2011, the effectiveness of water cooling has prompted a series of all-in-one (AIO) water cooling solutions. AIO solutions result in a much simpler to install the unit, and most units have been reviewed positively by review sites. Heat pipes and vapor chambers A heat pipe is a hollow tube containing a heat transfer liquid. The liquid absorbs heat and evaporates at one end of the pipe. The vapor travels to the other (cooler) end of the tube, where it condenses, giving up its latent heat. The liquid returns to the hot end of the tube by gravity or capillary action and repeats the cycle. Heat pipes have a much higher effective thermal conductivity than solid materials. For use in computers, the heatsink on the CPU is attached to a larger radiator heatsink. Both heatsinks are hollow, as is the attachment between them, creating one large heat pipe that transfers heat from the CPU to the radiator, which is then cooled using some conventional method. This method is usually used when space is tight, as in small form-factor PCs and laptops, or where no fan noise can be tolerated, as in audio production. Because of the efficiency of this method of cooling, many desktop CPUs and GPUs, as well as high end chipsets, use heat pipes or vapor chambers in addition to active fan-based cooling and passive heatsinks to remain within safe operating temperatures. A vapor chamber operates on the same principles as a heat pipe but takes on the form of a slab or sheet instead of a pipe. Heat pipes may be placed vertically on top and form part of vapor chambers. Vapor chambers may also be used on high-end smartphones. Electrostatic air movement and