There are various types of air pump used in condensers: 1. Reciprocating Piston or Bucket Pump 2. Rotary Dry Air Pump 3. Steam Operated Air Ejector 4. Water Jet Pumps 5. Steam Jet Air Ejector 6. Modern Air Ejector Plant.
Type # 1. Reciprocating Piston or Bucket Pump:
This is a reciprocating piston or bucket pump known as Edward’s pump. The condensate is gravitated by the conical end of the piston which piercing in the base of the liner where the ports which communicate with the air pump suction pipe.
When the piston moves down, a partial vacuum is produced because the head valves are closed and sealed by water. The piston further moves and uncovers ports, the air and water vapour rush into the space above the piston. Then further motion of the piston causes the conical end to displace the condensate rapidly through the ports.
The rising piston traps the water, air and steam above the piston and raises the pressure to slightly above atmosphere till the head valves open and allow the water vapour and air to pass to the atmosphere. The condensate gravitates to the hot well over the weir which has sufficient water above cover to seal the valves against air leakage. A water sealed relief valve is placed in the base of the cylinder to release the pressure if it exceeds above atmospheric pressure.
Type # 2. Rotary Dry Air Pump:
The reciprocating air pump has restricted application because of limit in speed of operation and becomes bulky for higher vacuum or large power. Therefore rotary pumps are used.
The rotary dry air pumps are similar to the radial flow steam turbines. The revolving vanes provide thin film of water at the velocity of 40 m/sec, to the collecting cone where films act as pistons. The air is entrained between successive sheets of water.
The pump is charged with water even though it is made for handling air. This water and air is being discharged through a diverging cone which raises its pressure to slightly greater than atmosphere. The water and air passes on to the slightly elevated tank in which water is cooled prior to it return to the pump.
Type # 3. Steam Operated Air Ejector:
The steam operated air ejector has more wide field of applications because:
(i) Simple in construction
(iii) No moving parts
(iv) Require less space.
The ejector utilizes the viscous drag of high velocity steam jet for ejection of the air and other non-condensable gases from a chamber. It is mainly used for exhausting the air from the steam condenser. The steam jet flows through an air chamber where it mixes with the air and other gases which are adjacent to the surface.
The kinetic energy of the resulting mixture is then converted to pressure energy when this passes over the mixing zone or diffusor. The increase in pressure obtained allows the mixture to be discharged against a pressure which is higher than the entering chamber.
The steam power plants uses high vacuum pressure, therefore it is necessary to use two or three ejectors in series to obtain sufficient pressure difference in the mixture for discharging it to the atmosphere.
Type # 4. Water Jet Pumps:
These are circulating water pumps used for supplying cooling water using a head for jet condenser and circulates water in case of surface condenser.
The following types of water circulating pumps are used:
(i) Reciprocating type
(ii) Plunger type
(iii) Centrifugal type
(iv) Propeller type.
This kind of air pump is universally adopted for condensers. It has no moving parts and occupies less space. Its efficiency is high and it is simple in construction.
Steam jet air ejector uses a jet or jets of high velocity steam to remove air from the condenser by entraining and compressing it. They are classified as single stage or multi-stage. The single stage air ejector is used for vacuum under 65 cm and is applied condensers for reciprocating engines. The multi-stage air ejector is used for vacuum between 65 cm and 75 cm.
Live steam from boiler is delivered to each stage of the steam jet air ejector. In the first-stage the steam passes through a suitably shaped nozzle where on expansion steam attains a high velocity. When the steam passes the suction opening, it entrains the air.
The mixture of air and steam is compressed and is delivered to the intercooler where the steam is condensed and returned to a suitable point in the feed circuit and the volume of air is reduced.
The second jet takes the air and gases from the intercooler and after further compressing it delivers them to the after cooler in which the steam used for the operation of the second jet is recovered. The quantity of air which can be entrained by the steam jet depends upon the surface of contact between the jet and the air.
The steam consumption is low and thermal efficiency is near about 95%.
Such plants are commonly used with modern condensing plants. A small surface condenser is placed between the first-stage and the second stage to condense the steam used to work the first-stage of air ejector. This surface condenser is known as inter-condenser. The second-stage air ejector takes suction on the inter-condenser shell and discharges to the after-condenser shell.
The temperature of the condensate in the main condenser is less than the saturation temperature corresponding to the pressure in the inter-and after-condensers, the condensate from the main condenser is used as cooling water for the air ejector condensers. The condensate pump discharges through the tubes of the inter- condenser, after-condenser and thence to the boiler feed system.
The steam condensed in the inter-condenser is drained back to the main condenser through the vacuum drain. Since the pressure in the shell of the inter-condenser is greater than that in the main condenser, a seal or trap is used to prevent air or vapour from passing back to the main condenser through the vacuum drain.
If the main condenser has a vacuum of about 70 cm of mercury and the inter- condenser a vacuum of 63 cm, the pressure in the inter-condenser is 7 cm of mercury or about 1 metre of water greater than in the main condenser.
In the loop seal, the pressure in the inter-condenser shell acting against that in the main condenser, forces the water level in the inter-condenser leg of the loop down until the weight of the water in the other leg balances the combined weight of the water in the inter-condenser leg of the loop and the pressure differential.
As more water drains into the inter-condenser leg of the loop, its mass causes an equal amount of water to flow from the main condenser leg of the loop into the main condenser.
The discharge of the second-stage of the air ejector leads to the after-condenser where the steam is condensed and the air, being non-condensable, is vented to the atmosphere. The condensate from after-condenser has got atmospheric drain.