I was interested to see the article on steam in the last issue, since steam power is one of my favorite old technologies, which may have applications for the future as well as the past. Steam plants can use a wide range of fuels and can have very low emissions, so they may be worthwhile.
Published: April, 2003
Still Steamin’
Dear Editor:
I was interested to see the article on steam in the last issue, since steam power is one of my favorite old technologies, which may have applications for the future as well as the past. Steam plants can use a wide range of fuels and can have very low emissions, so they may be worthwhile.
It is important to understand just a bit about the details of a marine steam plant to understand the opportunities – God (or the devil) is in the details.
The main difficulties with steam occur on the low temperature side of the plant. Once the steam leaves the turbines, it is condensed back to liquid. Since low pressure steam has a lot of volume, this requires a large heat exchanger – the condenser – which has many feet of tubing carrying cold sea water to condense the steam. Low pressure exhaust increases plant fuel efficiency, but the condenser increases rapidly in size, weight, and cost as the steam pressure leaving the turbines goes down, so there is an economic trade off.
The liquid condensate is then deaerated to remove gases, especially oxygen, that might have leaked in. The deaerating feed tank (DFT) does this by heating the water to just under boiling, which drives off the gases (this is why distilled water tastes flat – it is degassed). However, pumping nearly boiling water is a problem since it wants to turn to vapor and cavitate, filling the pump with unpumpable vapor. Regular merchant ships take care of this by having the tank high in the stack with the pump as low as possible in the engine room so the height difference brings up the water pressure without pumping. This solution is impossible for a small vessel because they aren’t high enough.
Alternatively, special pumps can be used, but they are expensive and not as reliable (as gravity) – one ship which had a low DFT was famous for "slugging the feed pumps," which resulted in starving and hence shutting down the boiler and thus the entire propulsion plant practically every time it passed under the Golden Gate bridge. The reason this happened was because as the speed of the ship was changed as it maneuvered, the flow through the pumps was too variable. The DFT and the associated equipment is also high maintenance. In addition, a ship must produce makeup water at sea. This was done by distilling seawater using steam from the plant, but this costs fuel and money – note how much distilled water costs at the grocery store.
An alternative, still used on the Great Lakes, is to run the water through once, since the ship is sitting in fresh water. The condenser is a simple can with a lake water spray on top and a pump on the bottom. Due to the direct contact between steam and water, the temperature of the steam leaving the turbines can be much lower than would be feasible with a heat exchanger, so the engine efficiency increases. The pump empties the lake water and condensed steam back into the lake, and the boiler feed pump takes water from the lake. Most of the problems of feed water dearating and all the equipment disappears.
Likewise, a ferry can use a once through system since it can take on water frequently, and the water can come from the municipal water supply and can be conditioned ashore, if necessary. (Using a water tower – looking at old pictures of the Berkeley ferry terminal you can see just such a tower. The water can even be deaerated with solar heat, if you like.) Another savings for a ferry is reversing power. Because merchant ship engines are so large, shifting gears into reverse is impossible. Instead steam plants have a special turbine which runs in reverse. (Diesel merchant ships stop the engines and restart them running backwards – this can be exciting in an emergency, which is why you should give merchant ships a decent right of way.) The reversing turbine is normally shut off from the steam supply, but some low pressure steam comes out of the condenser backwards into it, so that the reversing turbine is always spinning in steam and producing "windage losses," decreasing efficiency. Reversing gears are common on small ships like ferries, so they don’t need a reversing turbine, so this is another savings. There are other details of turbine design and similar issues that also make steam now much more feasible for a smaller plant, but if emissions and fuel flexibility is important, looking back at steam is worthwhile, especially because it is a very mature and well proven technology.
There is an even more interesting application of steam to ferries – some municipal power systems are "bottoming cycles". Though a bottoming combined plant is possible for a ship, it is fairly complicated.
The real point of this letter, though, is that, besides steam, there are a lot of technologies out there that have been supplanted. (Flywheels and Stirling cycle engines are two more – maybe one or the other could be used in ultra low emission gardening equipment, for instance, though the original inventors probably never even imagined a power mower.) This doesn’t necessarily mean that they are obsolete or useless now – the reasons that they are no longer used may be purely due to some special circumstances which may have changed, or due to problems that other technologies have solved, or because when they were invented they were a solution without a problem. Some solutions to our current problems may already have been developed and used – it is a matter of understanding the details and seeing new opportunities from old technology.
Chris Barry, P.E.
Raise the Damn Bridge Tolls
Dear Editor:
Thanks for your editorial "Raise all the Damned Bridge Tolls to $5." But the tolls should be $7.50 on every bridge! Check the toll on the bridges that feed autos into Manhattan.
The tolls on all the bridges should be raised to $7.50 and the new money used to fund the operations of transit operators in all nine Bay Area counties. None of the new money should be earmarked for capital expenditure. This kind of investment in the service would allow the region to offer very low cost service across the bay in buses in HOV lanes, by BART and by Ferry. At this point every transit operator in the region is reducing service and laying off workers. Perhaps transit workers should pressure legislators to make this tough decision (or give it to the MTC to make) by shutting down transit service in every county around the Bay for one day per month - "a la critical mass." Our transit service is shrinking when we need it more than ever.
Raise all the damned bridge tolls to $7.50.
Christine Zook
President/Business Agent
Amalgamated Transit Union, Local 192
