Thursday, July 19, 2012

Air conditioning: It's the humidity

From The Economist's Babbage blog:

AS AMERICANS endure yet another heatwave, their sole consolation is that it might have been worse were it not for Willis Carrier. Precisely 110 years ago in Brooklyn, on July 17th 1902, in the middle of a warm and wet summer, Carrier signed off on the final drawings of what within a few weeks became the world's first modern air-conditioning unit. It was not designed to waft a merciful chill to the perspiring masses. Rather, the bulky device was intended to regulate humidity at a printing plant. Improving workers' comfort was a side-effect. It was not until the 1930s that air conditioners became widespread in offices and apartment buildings; it took another two decades before they were common in detached homes. The latest developments in the technology are also gracing industrial spaces to start with.

The first active cooling systems date back to the 1840s, and relied on air blown over stored ice or pipes containing pressurised liquid that absorbed heat. Their aim was indeed to cool interiors (sometimes to freezing for food storage). In printing, however, moisture is key. Paper shrinks, expands and warps in response to changes in humidity. Keeping the amount of moisture constant was crucial, especially in full-colour printing, which requires four passes of the same sheet (one for each of the component colours—cyan, yellow and magenta—and another for black). One firm, Sackett & Wilhelms of Brooklyn, was so tired of having to bin stacks of poorly matched magazine pages that it hired Buffalo Forge Company to fix the problem. Buffalo put the young Carrier, considered something of a whiz kid, on the task.

Carrier figured out that moisture could be extracted from the air by having a system of coiled pipes. Half of the coils were kept inside the building, under low pressure. The other half remained outside the walls. The difference in pressure in the closed loop was maintained using a compressor. The warmer air surrounding the indoor coils heated the refrigerant, picked to boil at a low temperature, converting it from a liquid to a gas. The laws of physics dictate "phase changes" like this absorb heat, cooling the pipes. The moisture in the air surrounding the pipes would condense onto the coils and drip into a drain. The now-gaseous refrigerant passed through a compressor that turned it back into a liquid. The obverse of the physical law is that heat is created in doing this. But the warm, reliquified refrigerant radiates it while passing through the outside set of coils. Heat is conserved, but redistributed from the building's interior to the exterior. Finally, the gas passed through a compression joint which lowered its pressure—and thus temperature—allowing the process to begin anew. (Refrigerators work the same way, and heat pumps provide warmth thanks to a reverse process.)...MORE