Egyptian Cold Passage

The so-called "Cold Passage" in the Great Pyramid of Khufu was designed to generate and store temperatures as low as -10°C (14°F) using flash-evaporative cooling. Mechanism of the Cold PassageThe horizontal passage of the Great Pyramid, often referred to as the "Cold Passage," was engineered to produce flash-evaporative cold. Pressurized liquid water, generated by a piston-like impactor in the inclined well, was transformed into a fog of microdroplets.

As these droplets evaporated, they created a rapid cooling effect, potentially lowering the temperature in the Queen's Chamber to 0°C (32°F) or even -10°C (14°F) depending on the overpressure generated by the central wooden Djed caisson in the Grand Gallery (Coursol, 2021–2025).

Temperature Storage and Use

The Queen's Chamber acted as a cold storage chamber, where a copper plate cold exchanger likely facilitated the cooling of chemical processes, such as the production of sodium carbonate and sodium bicarbonate, essential for mummification. Salt encrustation observed in the passage and chamber supports the presence of evaporative cooling, as modern evaporative coolers also experience similar salt deposits.

Ancient Egyptian Cooling Techniques

Beyond the pyramid, ancient Egyptians employed evaporative cooling in homes and structures. Techniques included hanging wet reeds or cloths in doorways and windows, using wind catchers (malqaf) to direct cool breezes into buildings, and constructing thick mudbrick walls that stabilized indoor temperatures. These methods allowed indoor temperatures to remain below 22°C (71.

6°F) year-round, demonstrating a sophisticated understanding of passive cooling and low temperature resistance in architecture.

Significance

The Cold Passage illustrates that ancient Egyptians not only understood evaporative cooling principles but also engineered large-scale systems capable of producing and maintaining sub-zero temperatures. This knowledge was applied both for practical purposes, such as chemical processing, and for comfort in residential and ceremonial structures. The combination of structural design, water management, and airflow control highlights their advanced engineering capabilities.

In summary, the Egyptian Cold Passage represents a remarkable example of ancient low-temperature engineering, using evaporative cooling to achieve and sustain temperatures far below ambient conditions, with applications ranging from chemical production to environmental control within monumental architecture.