Is it just me or does this column only give half the answer?
OK, so now we know why there are steam pipes running under the streets. There are probably gas pipes and water pipes also. But I assume from the lack of continual explosions that the gas pipes don’t leak, and there don’t seem to be upwellings of water everywhere.
So why does steam rise from manholes? Leaky steam pipes? If so, why so widespread?
Water flows downhill and ends up on the floor of the tunnels; you may also have noticed in TV shows and movies that when they go down into the tunnels, there’s water and puddles everywhere. The pipe insulation isn’t perfect, so it’s fairly warm down there as well. The warm, moist air rises out of the holes in the manhole covers to the outside world where it’s cooler and dryer, and you got what looks like steam. It’s more like when you can see your breath on a cold day than actual steam, though.
Ah, so it’s not actually steam from steam pipes at all, it’s just mist from warm humid air rising from around the steam pipes. Makes sense, thanks.
The column kinda misses the key point, it has to be said.
The temperature differential can be quite high, the tunnel can be somewhere in the uncomfortable 100’s so the mist can be hot and abundant.
Though ocassionally the pressurized steam pipe *does * explode, to rather spectacular visuals :eek:
Ah, Manhattan, land of excitement and adventures 
Wet steam explosions (assuming no one and nothing of value is endangered by them, of course) are the best. The slo-mo footage of the Mythbusters bursting hot water heaters is IMHO the best explosion footage they ever produced. High explosives are just too damn quick. But the (relatively) slow but inexorable way wet steam pushes outward is awesome (and no I don’t mean “awesome” in the sense a teenager uses it, I mean “productive of awe”).
My question is how efficient is it? I would think there’s a lot of heat loss between generation and end user, insulated pipes or not. I figure maybe 50-60%?
You misspelled “terrifying”.
It’s often much less, but it depends widely upon the insulation, the length of travel, the steam conditions, etc.
If you want to think about it another way - say a small, municipal coal power plant has a net plant heat rate (NPHR) of about 14,000 Btu/kWh. This means that if the fuel heat at the plant is converted to electricity (for heat, presumably), then every kWh of electricity generated needs about 14,000 Btu of coal heat. This is a net thermal efficiency of about 24.4%.
If this heat is sent to resistance heaters to heat buildings, then you can say maybe you lose 5% in T&D losses, and you get about 0.95 kWh of heat energy per every 14,000 Btu of coal heat, or about a 23.2% net heating efficiency.
If instead the heat comes directly from the steam, then you have the boiler efficiency for converting fuel energy to steam (about 89% for a bituminous coal), and as a result you are able to send about 89% of the steam energy to heat buildings. Even if you lose 50% of it, you still have a net gain over using resistance heat.
Now what about heat pumps? They often are 3-4 times as efficient in heating mode as an electrical resistance heater, but this also depends upon the ambient temperature or heat source they draw from. In the very cold Northern US, there may be many days where the heat pump is poorly effective. In addition, heat pump systems also require a capital investment which is very significant, and lots of moving parts. A steam radiator system can be done cheaply, and has simpler maintenance requirements (patching steam leaks, replacing valves, etc.)
But yet another thing to consider is that building heat is often taken from the waste steam from a power plant. If a coal power plant has a net plant heat rate as shown above, then that means that the 75.6% not used to generate electricity goes into station service, miscellaneous losses, and cooling tower losses. Cooling tower losses can make up 50% of the total energy loss of the steam. But if you take this hot cooling water (which can be between 100-160 F) and instead let it exhaust its heat into buildings, you essentially are getting free heat on top of the normal electrical generation of the plant (less the capital and O&M cost of the steam system). The net effect is that power plants operating in this mode (called “combined heat and power,” or CHP) can have net plant efficiencies which are stunning - Nordjylland power station unit 3, Denmark, has about a 91% efficiency..
If there are steam pipes going somewhere then there are probably steam traps also involved. These separate condensate from dry steam, often imperfectly. It wouldn’t surprise me if some of the vapor rising up comes from this kind of infrastructure.
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