Electrode boilers are used in industrial applications where large amounts of water must be converted to steam often under short notice. The electrode boilers are of two basic types:
Immersion electrode type
Water jet or spray type
In the conventional immersion type boiler, current flows through a resistance wire, which generates heat. The heat is transfered through the electrode sheaf and into the water by conduction. These boiler types are most commonly used to produce hot water and will not be covered in any detail in this article since the water treatment requirements are very similar to a conventional boiler.
JET OR SPRAY TYPE ELECTRODE BOILERS
Electrode boilers utilize the conductivity of water to carry electric current and generate steam. An alternating current flows from the electrode of one phase to an electrode of another phase using the water as the conductor. The resistance of the water to current flow produces heat directly. The heat generated is proportional to current flow.
The boiler consists of a vertical, cylindrical, ASME code pressure vessel. Steam is generated in the upper portion and the boiler water is collected in the lower portion. A central, vertical cylindrical nozzle header or basket is located in the top center of the vessel. A number of vertical electrode plates are arranged around the nozzle header. A counter electrode is positioned below each electrode in the steam space.
A circulating pump moves water into the nozzle header. The boiler water is sprayed through the nozzles, striking the electrodes and falling downwards onto the counter-electrodes a
nd then into the water section. The flow of current through the water generates heat and produces steam. Only 3% of the water flow is converted into steam. And, therefore, no part of the boiler reaches a temperature higher than the water temperature.
The electrical energy output is a function of the current flow. This, in turn, is a function of the amount of water sprayed onto the electrodes. In high voltage spray type steam boilers, a movable sleeve is used to control the number of water jets allowed to contact the electrodes.
Electrode boilers are very efficient in converting power to heat (3,412 BTUH per kW or 293 kWh per million Btu). One hundred percent of the electrical energy consumed is converted into heat energy with no heat transfer loss. By comparison, conventional fossil fuel boilers have a combustion efficiency in the 70 to 85% range.
Nominal boiler ratings are quoted in kilowatts (kW) at 125 psig operating pressure. 1,000 kW is equivalent to about 3,300 pounds per hour steam generating capacity.
Manufacturers state that steam purity will be 99.95% dry.
WATER QUALITY CONSIDERATIONS
As with conventional boilers, water quality and treatment are important maintenance considerations in order to prolong the useful life of an electrode boiler.
The main objectives in the operation of an electrode steam boiler are:
1. Maintain the correct conductivity of the boiler water as specified by the boiler manufacturer.
2. Prevent scale deposition on the electrodes
3. Control corrosion and fouling of the spray nozzles
4. Minimize corrosion and fouling of the electrodes
5. Prevent foaming of the boiler water.
The regulation of the boiler water conductivity is an important criteria since excessively high conductivity can damage the boiler shell. The electrodes are subject to damage as a result of arc-overs if the conductivity is too high.
Notwithstanding the limitation of operating the boiler as excessively high conductivity, increasing the conductivity in boilers of 600 volts or less will improve the speed of response to changes in steam demand. This is accomplished, however, at the decreased life cycle of the electrodes and an increased risk of boiler corrosion.
Conductivity control is also important in high voltage (greater than 600 volts) boilers. Under conditions of excessively high conductivity, foaming may occur. This allows conductance of current in the steam space resulting in arc-overs between the electrodes.
Mineral scale control is an important consideration with this boiler design since scale deposits on the electrodes serve as an insulator resulting in a reduction in current flow. This reduces the boiler output. In the low voltage boiler, scale deposits reduce the output capacity at a given conductivity setpoint. If the conductivity is increased to compensate for the insulating properties of the scale deposits, this may create a condition of high amp density, which, in turn, will cause corrosion damage of the electrodes.
In high voltage boilers, scale tends to originate first at the ends of the spray nozzles. The nozzles are designed to shape the stream of water that flows onto the electrodes. If the scale deposits are severe enough, the water stream loses its design definition, which often leads to arc-overs. Under this condition, the boiler goes off-line. Thus, proper pretreatment of the boiler feedwater is required to remove scale forming impurities such as calcium and magnesium. The recommended hardness control range in low and high voltage boilers is 0.3 ppm max. Total iron should be 2.0 ppm or less.
Foaming is another potential problem. This is an important consideration in high voltage units because the circulating pump produces considerable agitation of the boiler water. Foaming causes arc-overs leading to unscheduled boiler shutdowns. This may also cause a disruption in the electrical supply circuits and switch gear. In low voltage boilers, foaming conditions tend to promote carryover of boiler water into the steam although the potential for arc-over is not as severe.
Regulating the alkalinity of the boiler water (carbonate and hydroxide) is an important consideration because excessive concentrations can lead to the attack of the porcelain insulators used as lead-through bushings to bring the electrical power into the boiler. If porcelain insulators are used, the total alkalinity should be limited to 400 ppm. If higher alkalinity above 400 ppm is required to keep silica in solution, then special high alumina insulators are preferred. With high alumina insulators, the total alkalinity can be maintained up to 600 ppm since they are less prone to alkaline attack as compared to porcelain. The final alkalinity will largely determine the pH, which should be within the 8.5 to 10.5 range.
Because of the sensitivity of the electrodes, counter electrodes and boiler internals to corrosion, dissolved oxygen in the feedwater should be less than 7 parts per billion prior to the addition of a chemical oxygen scavenger. Sodium sulfite may be added to the deaerator to scavenge the last traces of dissolved oxygen, but this will increase the conductivity of the boiler water. If this is a concern, a volatile oxygen scavenger may be used.
Overall, electrode boilers offer advantages over conventional fossil fuel boilers especially when a quick response to variable steam loads is required. Electrode boilers are clean firing with no stack emissions. They are also highly energy efficient with minimal losses. Adherence to proper water quality guidelines and treatment will minimize maintenance costs and prolong the useful life of this equipment.