As a rule of thumb, it is commonly accepted throughout the water treatment industry that the feedwater temperature should be maintained between 180 - 210°F, but is this justified?
It is important that feedwater is kept at a high enough temperature for mechanical deaeration of dissolved oxygen and other gases. The correlation between water temperature and oxygen content in feedwater can be seen in Figure 1.
Figure 1: Solubility of dissolved oxygen
There is an obvious cost involved with heating the feedwater tank but seeing that water temperature would be increased by the equivalent amount inside the boiler, supplemental energy is not required. The
only real loss is the radiant heat from the feed tank itself. Provided the feed tank is properly insulated, this extra heat loss should be de minimis.
An important diminution of costs is generated by reducing the chemical consumption of sodium sulfite added to the boiler feedwater. A reduction in chemical demand will increase obtainable concentration cycles and will compensate for the small additional heat loss from the boiler feed tank.
Under normal operating conditions, a boiler undergoes thermal shock when cold feedwater is introduced to the hot surfaces of the boiler wall and its tubes. The stark difference in water temperature causes the rapid expansion and contraction of equipment inside the boiler, including tubes, sheets, valves, fittings and piping. Maintaining feedwater at a minimum of 180°F will equate to a lower temperature difference and less risk of thermal shock. A low feedwater temperature will also reduce the heat rate deviation required to produce steam. Although not researched to the extent that low temperatures are given, the feedwater efficiency begins to plateau after 210°F, and maintaining a temperature higher may lead to feed tank malfunctions.
Attached below are chemical savings that would be attained from increasing feedwater from 110°F to 200°F.
I would concur that as a general rule of thumb, the optimal temperature of feedwater tanks should be maintained within 180 - 210°F, and pre-heating the feedwater to the prescribed range will allow the boiler to run most efficiently. Feedwater temperatures outside this control range can lead to a reduction in overall efficiency and an increase in chemical expenditure.