Тексты по специальности ТЭС / Condensers

Condensers

1))The primary function of a condenser is to reduce the exhaust pressure of the prime mover. A reduction in the exhaust pressure will increase the pressure and temperature drop through the prime mover and output. Secondary functions of the condenser are: 1) to reduce the amount of make-up boiler feedwater by condensing the steam in order that it can be returned to the boiler; 2) to remove air or other noncondensable gases which are corrosive.

Like feed-water heaters, condensers are classed as direct-contact or surface types.

The direct-contact condenser is a jet condenser consisting of water nozzles, a steam-and-water mixing chamber, and Venturi-section or a tailpipe. The jet condenser may be used where it is not necessary to reclaim the condensate. Although it requires more cooling water than a surface condenser, the jet condenser has the following advantages: 1) construction and operation are simple; 2) no vacuum pump is required to remove noncondensable gases from the steam.

The jet condenser is used mainly for small prime-mover installation in industry.

The conventional surface condenser is of shell and tube construction. Cooling water passes through the tubes, and steam circulates around the tubes and is condensed and removed. At no time do the steam and condensate come into contact with the cooling water. Condenser like feed-water heaters, are classified as single- or multipass and straight or bent tube.

Generally, condensers used with prime movers are the straight- tube single- or multipass type.

ln a single-pass surface condenser water enters from the bottom left, passes through the tubes, and leaves at the upper right. Steam enters the condenser shell from above, circulates around the nest of tubes, and then flows toward the center or core which is the zone of lowest pressure. Air and other noncondensable gases are removed from one end of the core at the vents. The condensate flows by gravity to the condensate well or hot well. The condensate is then removed from the well by a pump.

Because cooling water is usually corrosive in nature, condenser tubes are often made of special alloys of copper or aluminium. Among these are admiralty metal, muntz metal, arsenical copper, and aluminium brass.

The tubes may be rolled into each end plate. In this case, expansion is taken care of by bowing the tubes. The tubes of some condensers are rolled into and keyed to one end plate and are free to move in the other end plate. Leakage between the tube and end plate is prevented by packing. Expansion and contraction of the condenser shell may be taken care of by providing an expansion joint in the shell wall at one end.

Owing to the expansion and contraction of the exhaust line or nozzle leading from the turbine to the condenser, all condensers are either rigidly suspended from the turbine or connected to turbine by an expansion joint. In the former case, the condenser may be placed on spring supports. The spring supports permit the condenser to rise or fall without overloading the turbine exhaust line. In the latter case, the condenser will be rigidly anchored to the floor. All expansion or contraction in the turbine exhaust line will be taken up in the expansion joint.

There are a number of condenser auxiliaries that are essential to the proper functioning of the condenser: 1) a condensate hot well for collecting the condensate; 2) a condensate pump to return the condensate to a surge tank where it can be reused as boiler feedwater; 3) a circulating pump for circulating the cooling water; 4) an atmosphere relief valve for relieving the pressure in the condenser in case the condenser or auxiliaries do not function properly; 5) an air ejector or a vacuum pump for removing the noncondensable gases from the condenser.

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