The Seattle Steam Company (SSC) is a district heating steam production plant that supplies steam to downtown Seattle customers for heating and other thermal uses. Due to the cost of maintaining distribution pipes, SSC has no condensate return and 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. The primary heating is performed in the deaerating heater (DAH)fed by live steam following filtering and softening stages. After the economizers in the boilers, there was no other heat recovery present.

The Direct Contact Inc. (DCI) Heat Recovery System started on November 20, 2000 under manual operation. It became automatic on January 15, 2001 when system characteristics became known and entered into the control programming.

SYSTEM DESCRIPTION

SCOPE

Using the water vapor in the boiler flue gas, the DCI Heat Recovery System (HRS) would heat incoming feed water as warm as possible before being fed into the DAH.

PROCESS

The HRS consists of a Heat Recovery Vessel, a Circulation Pump, a Transfer Pump, an Induction Fan, and controls. The HRS ties into the multiclones of two field-built boilers (the Garret & Schaffer and the Riley Stoker) and exhausts into the stack for the same boilers.

Heat Recovery Vessel

The Heat Recovery Vessel (HRV) is a 108 inch diameter by 21’-4” tall vessel located on the old coal bin rooftop next to the exhaust stack serving the two field built boilers. The gas flows through the vessel to directly heat incoming feed water served by a pump located on the floor under the vessel. The diverted gas passes through the HRV cooling and drying the gas and in turn heating the boiler feed water. The feed water and hot water return tie-in is located above the operating floor. The Heat Recovery System receives water from the water softener and returns hot water prior to the deaerating 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 the deaerating heater.

Pumps

Two horizontal centrifugal pumps serve the HRS. The Circulation Pump circulates 525 gpm at 20 feet TDH through the bottom nozzles in the HRV. The Transfer pump supplies the DAH upon level demand in the DAH with up to 610 gpm at 80 feet TDH.

Induction Fan

The Induction fan, located on the HRV Outlet, is a variable speed radial-bladed centrifugal fan. It has a nominal 45,600 ACFM flow at 8 inches static pressure with a 100 HP motor.

Controls

Water supplied to these boilers is maintained by a cascading level control system. As boilers evaporate water on demand, water is supplied to the boiler to maintain level. The deaerating heater tanks have a reservoir from which water is pumped to the boiler. The water in the DAH tank is supplied from the 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 tank via multi-media filters and water softeners. The HRV is inserted between the filters/softener and the DAH tank. The level is controlled in the HRV 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 HRV reservoir. The HRS may be by-passed without effecting boiler operations.

Flue gas, the source of heat for the HRS, 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 HRV that is turn controlled by fan speed. This was achieved by characterizing the fan speed, therefore the fan volume, to the incoming water flow demanded by steam production. With a period of monitoring the inlet gas temperature of the fan inlet and the exit water, a set of characterization fan speeds were recorded in the fan controller to match against water flow data.

BATTERY LIMITS

Duct Connections

Two 36-inch diameter ducts tie-in to the G&S or the RS multiclones 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 the cool flue gas from the Heat Recovery Vessel from a 48-inch round duct and discharges the gas to the stack. The gas diverted through the Heat Recovery Vessel combines with hot flue gas remaining from the boiler exhausts and the combined gas is maintained above its dew point.

Stack Connection

A vane damper on the fan outlet allows for isolation when necessary.

Piping

Apart from the piping local to the Heat Recovery Vessel, several hundred feet of stainless steel pipe brings cold supply water to the Vessel and carries hot return water back to the deaerator’s supply pipe. The entire pipe is insulated for personnel protection (some portions are easily touched), heat isolation, or condensation prevention.

OPERATIONS

LOADS

The SSC plant has operated at 150,000 to 300,000 pounds per hour since the HRS started up. This load reflects the amount of incoming feed water and steam produced.

RESULTS

Gas Characterization

The inlet gas to the Heat Recovery System varies in flow according to the load. The gas typically has a 350ºF dry bulb and a 145ºF wet bulb. The exit gas has typically been 108º to 118ºF saturated.

Water Temperatures

The incoming water temperature varies with the amount of effluent cooling upstream of the tie point with temperatures of 50º to 62º depending on the amount of effluent cooling upstream of the tie-points. The city water temperatures will vary seasonally with the city water supply temperatures. The HRV raises the water temperature to 133ºF feeding the DAH.

Stack Gas Temperatures

With the combined gas re-heating, stack gas temperatures have averaged at 170º to 185ºF.

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 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

SSC 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

The vapor condensation has recovered an average of 1,975 gallons per hour of water that adds to the plant water intake, about 7% of the average intake.