Why do manhole covers steam?

October 17, 2004 5:46 PM

I have seen steaming manhole covers in many cities. Big deal, you say, and yet . . . they steam during the summer. Like this one. Can anybody tell me why?
posted by ashbury to Society & Culture (6 answers total)

At least in NYC, many manhole covers, umm cover, transformer vaults. These vaults get really hot - often liquid nitrogen is used to cool the vaults. Last month, manholes exploded in Midtown due to excessive transformer heat. Also older cities used to have centralized steam heating, and those vaults are quite warm. So the steam could also be excessive water vapour due to the liquid nitrogen and/or steam heating.
posted by plemeljr at 6:19 PM on October 17, 2004

IMO, quite likely part of the old building heating systems, apparently still in use. Powerplant steam turbines sent the "waste" steam to buildings for heating, instead of just dumping it straight, and wastefully, into the atmosphere.
posted by five fresh fish at 6:21 PM on October 17, 2004

In NYC, Con Ed (our power company) pipes steam to customers all over Manhattan south of Harlem, and the steam mains have blow-off valves in those manholes. Here's a neat PDF diagram of what's down there. This year, though, they're insulating the manhole covers.
posted by nicwolff at 7:26 PM on October 17, 2004

I would think steam delivery, especially using pipes dating to the 1880s, is super-inefficient.
posted by five fresh fish at 8:50 AM on October 18, 2004

The distribution system is pretty inefficient, no doubt. However, the generation side, assuming they are truly using waste steam from electric generation, is pretty efficient. "Waste" is a term of art here, as the steam is taken out of the cycle somewhere in the middle in most systems like this. Thus, it has the potential to do a little more work generating electricity. It also could be used to preheat things going into the cycle to increase the efficiency. Most steam turbine electric generating plants have cycle efficiencies in the thirty some percent range. By taking a little heat out for steam distribution the overall efficiency can be increased. The energy cost to the generating side is much less than would be the energy cost to just produce steam with a boiler. I am not sure how the net energy balance works out for Con Ed, especially with the distribution losses. There is a district heating system in Jamestown, NY that is very efficient in both generation and distribution.
posted by caddis at 9:43 AM on October 18, 2004

fff, modern district heating (or cooling) doesn't necessarily deliver the heat directly. Most buildings will interface using a heat pump. There is some energy loss in the "transmission" but this is more than made up for by the energy savings in centralizing generation and being able to offset generation to lower-cost, lower-demand periods. The Chicago Loop has a small number of thermal energy plants built in the last decade. (The piping is new.)
posted by dhartung at 11:54 PM on October 18, 2004