What Are Condensers?
As their name implies, condensers are
employed to help condense spent steam into water. In this process
they act as heat exchangers, extracting heat from the steam in
lowering its temperature and further extracting its heat of
vaporization in turning the steam into water.
One other result of this action is to
cause a vacuum at the exhaust end that helps the steam to pass
through the turbine. When engines exhaust steam to the atmosphere,
the exhaust pressure is atmospheric or about 14.7 pounds per square
inch.
When they exhaust in a condenser, the
action is such that the back pressure is reduced to some value below
atmospheric pressure. The difference between atmospheric pressure and
the absolute pressure in the condenser is known as a vacuum.
Absolute pressure is the pressure above
zero pressure; it is the gauge pressure plus the atmospheric
pressure, both expressed in the same units.
The effect of this vacuum is to
increase the effective pressure difference between the pressure of
the incoming steam and that of the exhaust steam; this results in a
considerable saving in the steam requirements to achieve the same
results if no condenser was employed.
Types of Condensers
Condensers may be divided into two
different classes:
1. Jet condensers, in which
condensation of the exhaust steam is affected by direct contact of
the vapor and the cooling water.
2. Surface condensers, in which the
exhaust steam and the cooling water do not come in direct contact
with each other; the heat is extracted from the steam by heat
transfer through metal walls or tubes.
The jet condenser cools the incoming
exhaust steam by causing it to flow under a fine water spray. The
mixture of condensate and cooling water also contains air which is
carried along with the mixture and removed from the bottom of the
condensing chamber. In one type, referred to as a low-level jet
condenser, this residue is removed by means of a pump.
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