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Boilers

BACKGROUND
SSC PlantSeattle 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 sewered 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.

EQUIPMENT
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.

Induction Fan
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.

Controls
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.

PHYSICAL LIMITS

Duct Connections
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.

Stack Connection
A vane damper on the fan outlet isolates the Heat Recovery System, when necessary.

Piping
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.)

OPERATION

Load
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.

RESULTS

Gas Characterization
The inlet gas to the Heat Recovery System varies according to the load. The gas typically has 350F dry-bulb and 145F wet- bulb temperatures. The exit gas is typically 108 to 118F saturated.

Water Temperatures
Incoming water temperatures vary from 50F to 62F, depending on season. The Heat Recovery System raises the feed to the DAH to 133F.

Stack Gas Temperatures
With combined gas re-heating, stack gas temperatures have averaged at 170 to 185F.

NOTEABLE ADJUSTMENTS

Level Adjustments
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.

ECONOMICS

Efficiency Improvements
At 100% make-up, each 12F 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.

Water Savings
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.