November 28, 2005
Superhot Steam!
We've been obsessed with Con Edison's NYC steam system ever since we discovered that National Geographic diagram last month and Evan sent in that informative email. We decided to do some more research, and stumbled across this great article on steam power from the Gotham Gazette:
If steam is an easier concept to grasp than, say, fiber optics, producing it on this scale is nowhere as simple as turning on a giant teakettle. Some of the steam comes from the Con Edison steam generating plant on 14th Street and the East River. Inside, two massive boilers -- one 95 feet high -- burn natural gas or fuel oil and air. The resulting heat sends the temperature of water inside the boiler's pipes to a blistering 1,000 degrees Fahrenheit, converting it to steam. Workers wear ear protectors to keep out some of the roar of millions of gallons of flowing water and the sound of spinning turbine blades. Little, though, can shield them from the warmth radiating from pipes filled with superheated vapor."When we used to bring people into the plant for tours, their first impression, when we have them look inside of the furnace, is that it's huge. People are taken back by the sheer size of it - eight stories high," says Wilton Cedeno, former manager of Con Edison's Hudson Avenue steam plant. "There are intricate control systems -- systems for treating the water, electrical systems, pressure systems. It's all very complex."
On average, the plant converts a gallon of water into eight pounds of steam -- every hour, approximately 125,000 gallons of water are turned into more than one million pounds of steam. "We use city water, but the water has to be processed and cleaned," says Cedeno. "If it's not pure, you get build up on pipes and can damage the turbine blades."
The East River plant is one of seven Con Edison plants -- five in Manhattan and one each in Queens and Brooklyn. Three of these plants, including the one on the East Side, produce both steam and electricity through a process called co-generation. In these plants, the steam leaves the boiler and then goes through pipes into a turbine generator. The resulting spinning of the turbine blades produces electricity. The remaining steam then goes into the steam system.
From the plants, the steam goes into Con Edison's underground pipes. On a cold winter day, nearly 10 million pounds of steam at 350 degrees Fahrenheit flow each hour through 105 miles of underground mains. The pipes coming out of the plant can be several feet in diameter, but the steam travels through progressively smaller pipes, ending at ones that may be only a couple of inches wide.
Con Edison fills in some more facts:
The New York Steam Company began providing service in lower Manhattan in 1882. Today, Con Edison operates the largest steam system in the world. The system contains 105 miles of mains and service pipes and 3,000 steam manholes. Steam is provided from seven Con Edison steam-generating plants, five in Manhattan, one in Queens, and one in Brooklyn, along with receiving steam under contract from a steam plant at the Brooklyn Navy Yard.Con Edison's steam system provides service to more than 1,800 customers and serves more than 100,000 commercial and residential establishments in Manhattan from the Battery to 96th Street.
Our commercial and residential customers use steam for heating, hot water, and air conditioning.
The winter peak sendout is nearly 10 million pounds per hour.
Steam sales account for about 7 percent of total Con Edison revenues.
Steam traveling through Con Edison's system is used to heat and cool some of New York's most famous addresses – the United Nations complex, the Empire State Building, and the Metropolitan Museum of Art are just a few.
Interesting! Can someone explain how steam power can be used for air-conditioning? That's hugely counter-intuitive! And good news for you who value clean air: despite the fuel burned to create the steam, it sounds like the NYC steam system actually produces less pollution than some of the other power sources.
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it's like magic...but a more scientific explanation of ABSORBTIVE REFRIGERATION can be found here:
http://geoheat.oit.edu/pdf/tp51.pdf
I may be able to offer some infor about steam for airconditioning. I studied Mechanical Engineering as an undergrad before I fled to art school in Britain, and spent some time working the HVAC field. So, I hope what I say makes sense, I am a bit rusty.
The refridgeration/air conditioning cycle usually consists of 4 devices: compressor, heating coils (and fan), turbine or expansion valve, and a coiling coil (with fan). The process is pretty simple, a reridgerant like R134a, or R14 or R12 (Freon, which is no longer used) is used, in gas form. The gas is compressed, then heats up dramatically as the pressure drops, then undergoes further expansion or a drop in entropy - causing a drastic drop in heat, and the cycle repeats with the cool, and low pressured gas returning to the compressor.
Power plants and heat pumps work on this same process, only they work in reverse. Power plants usually use a boiler to heat water and then use steam. Here, pumps are used instead of compressors because compressors imply gas only, and pumps are used for liquid only.
In the case of steam, I imagine that Con Ed could be used in place of a local compressor and radiator fan components, so an air-conditioning system could be set up using steam as refridgerant and most likely a thermo-expansion valve (TXV) and whatever type of cooling fan to distribute the air.
Does that make sense? I have to say, a good source of info for this would be howstuffworks.com, although they only speak about air conditioning and not so much about thermodynamics, power plants. To learn more about thermodynamics relevant to this issue, try to look up: Canot Cycle, Diesel Cycle, Otto Cycle, Rankine Cycle.
I found a simplified diagram that shows the case well.
http://static.howstuffworks.com/gif/ac-cycle.gif
Here, the red heating coils and the compressor would be Con Ed's responsibility, with the expansion valve and blue cooling coils being installed locally. Also, I don't know if the loop would 'complete' itself because most likely the steam is discharged into the water supply as opposed to being piped back to Con Ed. Then again, the steam comes from the city water supply so I guess the cycle does complete.
Someone feel free to correct me. Like I said, I am no longer a practicing engineer. Hope this helps.
Steam can be used for air conditioning with a steam jet cooler, which uses water as a refrigerant. The water is vaporized at a low temp. in a vacuum induced by a jet of superheated steam.
Actually, steam is used for absorption refrigeration, not gas compression refrigeration, which is the type of refrigeration scott explained.
Steam isn't used for gas compression air-conditioning because water doesn't evaporate well at room temperature, which is the the temperature that the blue coils in the Howstuffworks diagram would be. You would have to place the water in a near vacuum to get it to boil at room-temperature.
What really happens is that hot, humid air is blown through a spray of concentrated salt solution. The salt makes the solution absorb water really well, just like putting salt water in your mouth dries it out. The result is that the humidity is sucked out of the air. Then it's blown through a spray of pure water. The water evaporates, cooling the air and also increasing its humidity. It's then passed through another salt water spray to remove the humidity, leaving cold, dry air. The place where the steam comes in is to move water from the salt solution to the pure water supply. Without the steam, the salt water solution gains water and the pure water supply diminishes, so eventually you run out of pure water. The air conditioner is essentially running off the osmotic pressure difference between the pure water and the salt water. The steam is used to heat the salt water, boiling off the water and leaving the salt. The water is then condensed and added to the pure water supply.
A good explanation and diagram is given here:
http://en.wikipedia.org/wiki/Gas_absorption_refrigerator
Alternatively, instead of using steam the salt water could be pumped through a distillation membrane which lets water through but not salt. The pump would have to work against osmotic pressure. This would probably be more efficient than the steam air-conditioner, but I haven't heard of anyone using this method.
If you have waste steam though, steam air-conditioners are very good because they use steam which would otherwise just be dumped into the air.