Spray Type. Spray condensers utilize mixing or direct contact of cooling
water and steam. Cooling water is distributed inside the condenser in the form of a
fine spray that contacts and condenses the steam. This type has application where dry
cooling towers are used. Part of the condensate from the condenser is circulated
through dry cooling towers and returned to and sprayed into the condenser. The balance
of the condensate, which is equal to the steam condensed, is pumped separately and
returned to the feedwater cycle.
Surface Type. Surface condensers are basically a shell and tube heat
exchanger consisting of water boxes for directing the flow of cooling water to and from
horizontal tubes. The tubes are sealed into fixed tube sheets at each end and are
supported at intermediate points along the length of the tubes by tube support plates.
Numerous tubes present a relatively large heat transfer and condensing surface to the
steam. During operation at a very high vacuum, only a few pounds of steam are contained
in the steam space and in contact with the large and relatively cold condensing surface
at any one instant. As a result, the steam condenses in a fraction of a second and
reduces in volume ratio of about 30,000:1.
Pass Configuration. Condensers may have up to four passes; one and two pass
condensers are the most common. In a single pass condenser, the cooling water makes one
passage from end to end, through the tubes. Single pass condensers have an inlet water
box on one end and an outlet water box on the other end. Two pass condensers have the
cooling water inlet and outlet on the same water box at one end of the condenser, with a
return water box at the other end.
Divided Water Box. Water boxes may be divided by a vertical partition and
provided with two separate water box doors or covers. This arrangement requires two
separate cooling water inlets or outlets or both to permit opening the water boxes on
one side of the condenser for tube cleaning while the other side of the condenser
remains in operation. Operation of the turbine with only half the condenser in service
is limited to 50 percent to 65 percent load depending on quantity of cooling water
flowing through the operating side of the condenser.
Reheating Hotwell. The hotwell of a condenser is that portion of the
condenser bottom or appendage that receives and contains a certain amount of condensate
resulting from steam condensation. Unless the condenser is provided with a reheating
hotwell (also commonly called a deaerating hotwell), the condensate, while falling down
through the tube bundle, will be subcooled to a temperature lower than the saturation
pressure corresponding to the condenser steam side vacuum. For power generation,
condenser subcooling is undesirable since it results in an increase in turbine heat rate
that represents a loss of cycle efficiency. Condenser subcooling is also undesirable
because the condensate may contain noncondensible gases that could result in corrosion
of piping and equipment in the feedwater system. Use of a deaerating hotwell provides
for reheating the condensate within the condenser to saturation temperature that
effectively deaerates the condensate and eliminates subcooling. Condensers should be
specified to provide condensate effluent at saturation temperature corresponding to
condenser vacuum and with an oxygen content not to exceed 0.005cc per liter of water
(equivalent to 7 parts per billion as specified in the Heat Exchange Institute (HEI),
Standards for Steam Surface Condenser, 1970.
Air Cooler Section. The condenser tubes and baffles are arranged in such a
way as to cause the steam to flow from the condenser steam inlet toward the air cooler
section. The steam carries with it the noncondensible gases such as air, carbon
dioxide, and ammonia that leave the air cooler section through the air outlets and flow
to air removal equipment. Any residual steam is condensed in the air cooler section.
The proper size of condenser is dependent on the following