1. Field of the Invention
The subject invention relates to the deaeration of water in a boiler system. More particularly, this invention involves increasing overall boiler efficiency by reducing the temperature of boiler feedwater before the feedwater enters a boiler flue gas economizer.
2. Background of the Related Art
It is well known that dissolved oxygen must be removed from boiler system feedwater to prevent corrosion of steel surfaces that come in contact with the feedwater such as boiler tubes, surge tanks, and connecting conduits. Oxygenated feedwater also causes the formation of iron oxide deposits on the surfaces of the boiler tubes which restrict the flow of feedwater therethrough and consequently, reduces the transfer of heat to the water passing through the boiler.
Fresh water at 55.degree. F. has an oxygen content of approximately 10 ppm. It is preferable to reduce the oxygen content in boiler feedwater to 0.007 ppm or less, thereby maintaining the heat transfer characteristics of the boiler system and reducing maintenance costs.
A method well known in the art for removing oxygen from feedwater, also called feedwater deaeration, is to raise the temperature of the feedwater to its boiling point so that the oxygen will degas from solution. Feedwater deaeration may be accomplished with this method by passing the feedwater through a deaerator nozzle in which pressurized steam and feedwater are mixed together to cause oxygen to separate from the feedwater. The deaerated feedwater is then collected in a pressurized enclosure while the oxygen is removed by the steam as it exits the enclosure through a vent. The diameter of the orifice in the vent is selected to allow all of the oxygen to be removed while sustaining a sufficient back pressure in the tank. A method of deaerating water in a similar manner is disclosed in U.S. Pat. No. 3,362,132 to Schellenberg.
A conventional deaerator system includes a deaerator tank, as described above, and an unpressurized vented surge tank for collecting boiler condensate. Fluid flows in a loop through these components to continually supply deaerated feedwater to the remainder of a boiler system in a manner that will be described hereinbelow. Water is added to the surge tank as necessary to make up for lost fluid and thereby maintain the level of fluid in the boiler system.
The flow of fluid through a boiler system that includes a conventional deaerator system, a flue gas economizer, a boiler, and load components such as, for example, room heaters, autoclave equipment, and laundry equipment, operates as follows. Hot deaerated feedwater (approximately 227.degree. F.) is pumped by a boiler feed pump from a pressurized deaerator tank to a flue gas economizer for preheating. The solution then travels to the boiler and is transformed to steam. The steam then travels to the load components. Some load components lose steam to the atmosphere by venting. Losses also occur through system leaks. Typically, 20% to 30% of feedwater is lost as it circulates through a boiler system. Thereafter the steam condenses.
The condensed feedwater then returns to the vented surge tank of the deaerator system. Relatively cool makeup water, which is typically softened tap water at approximately 55.degree. F., is added to the feedwater in the surge tank to replace the feedwater that was lost due to venting and leaks. Because the surge tank is vented to the atmosphere and oxygen rich makeup water has been added to the surge tank, the solution in the tank becomes oxygenated. Thereafter, the oxygenated feedwater is pumped to the deaerator tank.
All of the fluid that is pumped to the deaerator tank is then deaerated as it passes the deaerator nozzle and enters the tank. The hot deaerated feedwater, the temperature of which is again raised to approximately 227.degree. F., then starts the cycle again as it is pumped by the boiler feed pump from the deaerator tank to the flue gas economizer. A deaerator system for a boiler similar to the conventional system described above is disclosed in U.S. Pat. No. 5,129,366 to Chikamori, et al.
There are several disadvantages associated with the prior art deaerator system described above. For example, the high temperature of the deaerated feedwater pumped directly from the deaerator tank to the flue gas economizer provides a relatively small differential feedwater temperature across the flue gas economizer. It is readily understood by those skilled in the art that boiler efficiency gains are obtained with flue gas economizers by developing a differential temperature between the feedwater inlet and the feedwater outlet. The greater the differential temperature therebetween, the greater the efficiency gain. Therefore, prior art deaerator systems allow only marginal efficiency gains when used with flue gas economizers in a boiler system.
Another disadvantage is that both the makeup water and the deaerated condensate return must be deaerated continuously. This is because the condensate return becomes oxygenated after entering the vented surge tank as it is exposed to the air in the tank and mixed with the oxygen rich makeup water fed into the tank. Also, inasmuch as there is oxygen mixed into the fluid in the surge tank, the tank and conduits connecting the surge tank to the deaerator tank corrode prematurely.
An additional disadvantage is that boiler feedwater pumps in prior art systems fail prematurely since the hot deaerated feedwater that flows through the pumps tend to induce cavitation. Cavitation leads to pitting in the pump impeller--causing seal and bearing failure.
Yet another disadvantage associated with conventional deaerator systems involves the use of high efficiency hot water heaters. Such heaters are designed to extract heat energy from boiler steam in excess of that extracted by standard hot water systems. Consequently, the condensate return coming from a high efficiency hot water heater system is cooler. This apparent gain in efficiency, however, is lost when the cooler condensate return is reheated in the deaerator nozzle, since the deaerator must utilize a greater amount of energy to reheat the cooler condensate to 227.degree. F. In addition, the increase in boiler efficiency that could have been had by providing the cooler feedwater to the evaporator--and thus increasing the differential temperature across the evaporator--is lost because the cooler feedwater in prior art deaerator system is pumped to the deaerator and not the evaporator.
Clearly there is a need in the art for a deaerated water supply system that increases the differential temperature across a flue gas economizer so as to improve boiler efficiency. There is also a clear need for a deaerator system in which feedwater is not continuously deaerated.