Steam Company (SSC) is a district heating steam production plant that
supplies steam to downtown Seattle customers for heating and other energy uses.
Due to corrosion of condensate return lines and the very high cost of maintaining
them (now buried under streets, buildings, and other infrastructure,) SSC has no
condensate return. All condensate is ‘sewer’ed at the point of use. For every pound
of steam produced, a pound of make-up water is pre-heated for boiler injection.
Following filtering and softening stages, primary heating is performed in a de-aerating
heater. Aside from economizers to the boilers, no energy recovery was present.
The Direct Contact LLC (DCLLC) Heat Recovery System started on
November 20, 2000 under manual operation. System characteristics were studied and
were defined over the next month. The system became automatic on January 15, 2001.
SCOPE OF ENERGY RECOVERY
Using water vapor from the boiler flue gas, the DCLLC Heat Recovery System heats
incoming feed-water before feeding it to the de-aerating heater.
The Heat Recovery System consists of a Heat Recovery Vessel, Circulation Pump, Transfer
Pump, Induction Fan and controls. The system ties into the exhaust of two field-built
boilers (the Garret & Schaffer and the Riley Stoker) and exhausts back into the
stack for the same boilers at high enough temperatures to avoid condensation.
The Heat Recover Vessel
The Heat Recovery Vessel is a 9’ diameter, 21’-4” tall vessel located on the rooftop
next to the exhaust stack serving the two boilers. Gas flows through the vessel
and directly heats incoming feed-water that is served by a pump located on the floor
directly below the vessel. The feed-water/hot water return tie-in is located above
the operating floor. The Heat Recovery System receives water from the softener and
returns hot water to the de-aerating heater. A block valve with outlet and return
isolation valves are inserted in the 10-inch boiler feed-water line between the
water softener and de-aerating heater.
Pumps Two horizontal centrifugal pumps serve the Heat Recovery System.
The Circulation Pump circulates 525 gallons/minute at 20 feet total dynamic head
(TDH) through the bottom nozzles in the Heat Recovery Vessel. The Transfer pump
supplies the de-aerating heater with up to 610 gallons/minute at 80 feet TDH.
The Induction fan, located on the Heat Recovery Vessel Outlet, is a variable-speed,
radial-bladed, centrifugal fan. It has a nominal 45,600 ACFM (actual cubic feet
per minute) flow at 8-inches static pressure with a 100 HP motor.
Water supplied to the boilers is maintained by a cascading level-control system.
As water is converted to steam and sent to processes, it is re-supplied to maintain
level in the boiler. The de-aerating heater (DAH) tanks have a reservoir from which
water is pumped to the boiler. Water in the DAH tank is supplied from make-up water
pumps. At SSC, a city-water tank supplies the make-up pumps. The make-up water tank
level is maintained with city water through a control valve with water-main pressure.
The Heat Recovery System becomes another stage within the cascading level control.
Water currently is pumped from the city-water tank to the DAH via multi-media filters
and softeners. The Heat Recovery Vessel return water ties in between the filters/softener
and the DAH tank. The level is controlled in the Heat Recovery Vessel reservoir
by the make-up pumps from the city-water tank, and the level in the DAH is control
by the transfer pumps from the reservoir. The Heat Recovery System may be by-passed
without effecting boiler operations.
Flue gas is the source of heat for the Heat Recovery System and is controlled to
maintain make-up water temperature. The temperature of the transferred water to
the DAH is proportional to the volume of flue gas throughput in the Heat Recovery
Vessel that is, in turn, controlled by fan speed. This was achieved by correlating
the fan speed and volume to the water make-up rate demanded by steam production.
With a period of monitoring the inlet gas temperature to the fan and the exit water,
fan speed data was controlled to match water flow.
Two 36-inch-diameter ducts tie-in to the boiler exhausts and connect to a rectangular
manifold duct leading to the Heat Recovery Vessel. A circular block damper in each
36-inch round duct provides isolation for both boilers. The induction fan draws
cooled flue gas from the Heat Recovery Vessel via a 48-inch, round duct and discharges
the gas back to the boiler stack. The gas diverted through the Heat Recovery Vessel
combines with unused hot boiler exhausts and the combined gas is maintained above
its dew point.
A vane damper on the fan outlet isolates the Heat Recovery System, when necessary.
Apart from the piping for the Heat Recovery Vessel, several hundred feet of stainless
steel pipe supplies cold water to the vessel and carries hot water back to supply
the de-aerator. The entire pipe is insulated for heat isolation, condensation prevention,
and safety (some portions are easily touched.)
The SSC plant has operated at 150,000 to 300,000 pounds per hour since the Heat
Recovery System start-up. This load reflects the incoming feed-water and steam produced.
The inlet gas to the Heat Recovery System varies according to the load. The gas
typically has 350ºF dry-bulb and 145ºF wet- bulb temperatures. The exit gas is typically
108º to 118ºF saturated.
Incoming water temperatures vary from 50ºF to 62ºF, depending on season. The Heat
Recovery System raises the feed to the DAH to 133ºF.
Stack Gas Temperatures
With combined gas re-heating, stack gas temperatures have averaged at 170º to 185ºF.
The initial theory in maintaining the water level in the Heat Recovery Vessel was
that the existing make-up pumps would vary their speed to hold the HRV Level Control
Valve at 60% open. However, operating experience showed that the pumps had insufficient
capacity to provide adequate response with the valve at that setting. By holding
the valve at 25% open, the pumps and level control have operated successfully.
At 100% make-up, each 12ºF increase in the feed water temperature equates to a 1%
increase in the plant efficiency. The calculated efficiency change shows improvement
of 6.3% in overall plant efficiency as reported by plant personnel.
Reported fuel savings
Seattle Steam Company personnel report an average daily savings of $2,500 with current
natural gas prices, based on $6.00 per million BTU using the 6.3% efficiency improvement.
Vapor condensation has recovered an average of 1,975 gallons per hour
of water that recycles to the boiler intake, about 7% of the total intake.