James Watt patented the separate condenser in 1769. The patent, officially titled “A method of lessening the consumption of steam in steam engines,” protected the improvement that made his steam engine far more fuel-efficient than Newcomen's earlier design.
Watt conceived the separate condenser in 1765 after studying a model of a Newcomen engine. In a Newcomen engine, steam was condensed inside the working cylinder after every stroke. This repeatedly cooled the cylinder, so large amounts of heat and fuel were wasted reheating it. Watt proposed condensing the steam in a separate chamber while keeping the main cylinder hot.
The arrangement preserved a vacuum without subjecting the cylinder to repeated cooling. Watt also used an air pump to remove condensed water and air from the condenser. The result was a major improvement in efficiency, especially in mines and other places where coal was expensive.
A common mix-up is to answer 1765, the year Watt developed the idea. The question asks when he patented it, which was 1769. Watt's later patents covered other advances, including rotary motion in 1781 and the double-acting engine in 1782.