Heating Help newsletter - District Heating

croydoncorgi

Interesting that Boston is changing from steam to 'combined heat and power', judging by the link in the newsletter.



But what are the technical issues to overcome???



You know where you are with steam!  So long as you get it hot enough to start with and include enough de-watering points on the way round the loop, it's pretty-much BOUND to work!  Usualy plenty of heat available and if worst comes to worst and the temperature drops so condensation occurs, you can get rid of the water to make more room for hot steam.



But if you're using 'waste' heat from a power-plant, that will already be in the form of hot water (??).  The second stage of a 'combined cycle' plant is a steam turbine which takes a lot of heat-energy out of the steam (that's the whole point of combined cycle!), so what's the Flow temperature going into the District Heating part of the system? 



If it's hot water (and not in fact very hot!) instead of steam, what re-engineering is going to be needed to convert steam radiators, controls, etc. to hot water?

DanHolohan

Got this interesting e-mail from subscriber, Ron Schrank:

Regarding your Boston district heating reference, that plant in

Cambridge, Kendall Square Station, has been cogenerating since 1949. It

was built while there was still a lot of industry in Cambridge and a

market for the steam. The first two steam turbine/generators installed 

are called extraction units. Steam passes through the high pressure

turbine and reaches a point where as much as can be used, dictated by

demand, was sent to the distribution system and the rest was condensed.

The cooling water that condensed the steam was the source of heat into

the river.

In later years much of the industry left and more steam

had to go to the condensers creating more waste heat. Some time back, I

believe 1988, a steam line, and condensate return line was run across

the river, under the Longfellow Bridge, and began supplying steam to

Massachusetts General Hospital. It is through that line that the

connection was made to get

more steam into the Boston system, which was supplying steam to their

system from oil and gas fired boilers.





The Cambridge plant has been expanded over the years but

they remain an excellent example of how cogeneration can work. They

presently use a large GE gas turbine to generate electricity. The

exhaust heat goes through a heat recovery steam generator (waste heat

boiler). That steam runs the old steam turbines which continue to make

electricity and extract steam for sale. On another issue worth

mentioning, on the Cambridge side of the system, the plant took back all

the condensate the steam customers could return, saving untold amounts

of water, chemicals and btu's/fuel. The original company, Cambridge

Electric Light Company, also owned a plant farther up the river that

supplied steam to Harvard University through a non condensing turbine

beginning in 1930. I believe they still supply Harvard though I'm not

familiar with the present plant configuration, or owner.

DanHolohan

Each week,

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