One of the images I had of New York was steam rising from manhole covers in the streets. Well, yes, it still happens.
Birdsill Holly (1820-1894) was an inventor who is credited with originating the concept of District Heating. He was born in New York State and the number of patents to his credit is second only to his friend Thomas Edison. By 1882 the Birdsill District Steam Heating System was in use in a number of cities across the United States.
Wallace C. Andrews set up a company to promote Birdsill's system in New York and in 1882 the first customers were supplied. The efficiency of the system lead to steady growth as the New York Steam Company and virtually all the famous buildings became customers. During the 1930s agreements were in place to exchange steam with gas and electric utilities to cope with peaks. In 1954, the steam business was acquired by Consolidated Edison.
Today, Con Edison Steam operates a network of 105 miles of mains and service pipes supplying around 1800 customers in Manhattan with steam for heating, hot water and air conditioning. See the Con Edison site.
Steam is distributed by underground steam mains rated at either 200 or 400 pounds per square inch. Periodic expansion joints and insulation are provided, together with a series of main valves operated manually from street level via manholes. There is no intentional release of steam at the surface - when this occurs it is due to leaks or surface water leaking into manholes and coming into contact with the hot steam main.
In various locations, I came across temporary chimneys erected over street manholes painted in white and orange stripes, presumably where maintenance work is taking place. The photograph above shows one such chimney on Fifth Avenue (with Central Park in the background). But can somebody explain the liquid nitrogen cylinders? I assume it has something to do with temporarily controlling leaks.