The removal of dissolved gases from boiler feedwater/heating plants is an essential process in a steam system. The presence of dissolved oxygen in feedwater causes rapid localized corrosion in boiler tubes. Carbon dioxide will dissolve in water, resulting in low pH levels and the production of corrosive carbonic acid. Low pH levels in feedwater causes severe acid attack throughout the boiler system. While dissolved gases and low pH levels in the feedwater can be controlled or removed by the addition of chemicals, it is more economical and thermally efficient to remove these gases mechanically. This mechanical process is known as deaeration and will increase the life of a steam system dramatically.
Deaeration is based on two scientific principles. The first principle can be described by Henry’s Law. Henry’s Law asserts that gas solubility in a solution decreases as the gas partial pressure above the solution decreases. The second scientific principle that governs deaeration is the relationship between gas solubility and temperature. Easily explained, gas solubility in a solution decreases as the temperature of the solution rises and approaches saturation temperature. A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon dioxide, and other non-condensable gases from boiler feedwater. The feedwater is sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation. Spraying feedwater in thin films increases the surface area of the liquid in contact with the steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This process reduces the solubility of all dissolved gases and removes it from the feedwater. The liberated gases are then vented from the deaerator.
A vacuum deaerator is used for treatment of circulating water in heating plants. The circulating water should not contain oxygen as this increases the risk of corrosion on the system. The oxygen is dissolved in the make-up water and thereby enters the tank together with the make-up water, which normally contains 6-8 mg/l oxygen. The oxygen content can be reduced to under 0.2 mg/l with a vacuum deaerator.
Vacuum Deaerator is especially useful when:
1. In installations with many branches and low flow rates
2. For small temperature differences between supply and return. A vacuum degasser is not limited by fluid temperature
3. If a through-flow deaerator cannot be mounted on the installation for practical reasons. A vacuum degasser can be connected in almost every part of an installation.
THEORY OF OPERATION:
The oxygen-containing make-up water, preheated to 40-90 °C, is led to the upper section of the deaeration tank. In order to optimize the removal of oxygen, the deaeration tank is equipped with fillers for division of the water into fine particles. The vacuum pump creates the necessary vacuum so that the make-up water boils. When the water boils, the oxygen is liberated and removed by means of the vacuum pump. The deaerated water is separated into two streams, which are pumped partly into the district heating network, partly recycled over the deaeration tank.
A vacuum deaerator consists of three main components: A vacuum deaerator, a vacuum pump unit and a pump unit.
The deaeration tank is constructed in galvanized or stainless steel. Inside the tank is equipped with an intermediate bottom, under which a reservoir for deaerated water is mounted. Fillers are installed on top of the intermediate bottom. The system is provided with requisite instrumentation for level control and is delivered for foot/wall-mounting.
VACUUM PUMP UNIT:
The vacuum pump unit consists of a vacuum pump (liquid ring pump) as well as a valving arrangement for setting of cooling water quantity and vacuum force. The vacuum pump is delivered on bracket for wall-mounting.
The pump unit consists of a centrifugal pump and of a pipe system with valves for setting of the make-up water quantity and the circulating quantity. The pipe system can be in galvanized or stainless steel, and the pump in stainless steel.