The Condensate Water Systems Condenser: A closed vessel in which steam is condensed by abstracting the heat and where the pressure is maintained below atmospheric pressure is known as a condenser. The efficiency of the steam plant is considerably increased by the use of a condenser. In large turbine plants, the condensate recovery becomes very important and this is also made possible by the use of condenser. Condenser functions: 1) Decreasing the circulating cooling-water temperature (equal to saturation temperature). 2) It creates a low back pressure (vacuum) for the turbine to exhaust and corresponds to the condensing steam temperature. 3) The enthalpy drop, and hence the turbine work, per unit pressure drop is much greater at the low-pressure end of a turbine. 4) By lowering the back pressure by only few psi, increases the work of the turbine, increases the plant efficiency, and reduces the steam flow for a given plant out-put. 5) Condensing power plant much more efficient than non-condensing once. 6) All modern power plants are of condensing type. There are primarily two types of condensers a) Direct-contact b) Surface condensers(more common) Direct-contact condensers Direct-contact, or open, condensers are used in special cases, such as when dry-cooling towers are used, in geothermal power plant, power plant that use temperature differences in ocean water. This type classified into: 1) Spray condenser: used in modern direct-contact condensers. 2) Barometric or jet type. Direct-contact condensers condense the steam by mixing it directly with cooling water.
Spray condensers In the spray condenser this is done by spray the water into the steam directly. From the figure (1), the cycle can be analysis as follow: 1) The turbine exhaust steam at point (2) mix with cooling water at (5) to produce needing saturation condensate at (3) which is pumped at (4). 2) Part of the condensate, equal to the turbine exhaust flow, sent back to the plant as feed water by pump (4). 3) The rest is cooled usually in a dry (closed) cooling tower to point (5). 4) The cooled water at (5) is sprayed into the turbine exhaust and the process is repeated. Mass balance on the system And Energy balance Ratio of circulating cooling water to steam flow is given by: Notes:- I. Circulating-water flow is much greater than steam flow because larger fraction of the large latent heat of vaporization reduced pressure. II. much smaller sensible heat of the liquid. Example: Find the ratio of circulating water to steam flows if the condenser pressure is the cooling tower cools the water to. Assume turbine exhust at quality. and Answer:
At from saturation steam table with and by interpolation: Dry cooling tower Turbine exhaust 5 2 Condenser Non-condensable 4 To plant feed water system Pump 3 Figure 1: Schematic flow diagram of a direct condenser of the spray type.
T 1 4 5 3 2 s Figure 2: T-s diagram of condensate and cooling water in a direct contact condenser system. Barometric and jet condensers Non-condensable Exhaust steam from turbine Cooling water Baffles Cooling water Cascades Mixture Exhaust steam from turbine H Tail pipe Diffuser Mixture Hot water Figure (3a) Hot water Figure (3b) Figure 3: Schematic diagram of early direct-contact condenser (a) barometric, (b) diffuser or jet.
These early types condensers operate on the same principles as the spray condenser, except that no pump is required. The vacuum in the condenser is obtained by virtue of a static heat, as in the barometric condenser as shown in figure (3a) or a diffuser, as in the jet condenser, figure (3b). : Condensate pressure : Friction losses In the jet type condenser, the height of the tail pipe is reduced by section of convergentdivergent nozzle in subsonic flow. It thus helps raise the pressure in a shorter distance than tail pipe. Surface condensers Surface condensers are the most common type used in power plants. They are essentially shell-and-tubes heat exchanger. Heat transfer mechanisms are condensing of the saturated steam on the outside of the tube and the forced convection heating of the circulating water inside the tubes. The early design used simple circular tubes sheets that supported as many tubes that could be tightly packed between them. This design resulted in heat transfer problems, because the upper tubes shielded the steam from the effective condensing. The current design is to have a tube layout in the shape of a funnel with most tubes, and the largest tube passage area where the steam enters the condenser from the turbine. Notes: Number of passes and division, determines the size and effectiveness of a condenser. Single and multi-pressure condenser, the condenser may be divided into corresponding sections or shells, situated below the low-pressure turbine sections. Requirements of a Modern Surface Condenser The requirements of ideal surface condenser used for power plants are as follows: 1. The steam entering the condenser should be evenly distributed over the whole cooling surface of the condenser vessel with minimum pressure loss.
2. The amount of cooling water being circulated in the condenser should be so regulated that the temperature of cooling water leaving the condenser is equivalent to saturation temperature of steam corresponding to steam pressure in the condenser. This will help in preventing under cooling of condensate. 3. The deposition of dirt on the outer surface of tubes should be prevented. Passing the cooling water through the tubes and allowing the steam to flow over the tubes achieve this. Tube size and materials Tubes size a. 5/8 in (OD, outer diameter) used only for small and special applications. b. 7/8 or 1.0 in used in the modern condensers which are adequate for water pressure encountered in condensers. Materials: 1. Early used admiralty metal [70-73% Cu, 0.9%-1.2% tin (Sn), 0.07%Fe, the rest from zinc. 2. Todays used stainless steel 304. Deration The non-condensable gases are mostly air that leaks from atmosphere into those portions of the cycle that operate below atmospheric pressure such as condenser, also, the gases that caused from decomposing of water into oxygen and hydrogen and the water reaction with material of constructions. The undesirable effects of non-condensable gases are: 1. They raise the total pressure of the system, and thus lower plant efficiency. 2. They blanket the heat transfer surfaces such as the outside surfaces and decreasing the condensing heat-transfer coefficient. 3. With presence oxygen, increases corrosion and hydrogen cause diffusing through some solids.
Surface condenser calculations Where: =heat load on condenser J/s. =overall condenser heat-transfer coefficients based on outside tube area(w/m 2.K). : Total outside tube surface area m 2. : log mean temperature difference in the condenser o C and given by: ( ) Where: : Difference between saturation-steam temperature and inlet circulating water temperature o C. : Difference between saturation-steam temperature and outlet circulating water temperature o C. Also called terminal temperature difference TTD. The overall heat transfer coefficients U: Where: : Circulating water velocity in the inlet (cold) conditions m/s. : Dimensionless factor depends on the tube outer diameter. : Dimensionless factor depends on the water inlet temperature. : Dimensionless factor depends on the tube material and gauge. : Dimensionless factor depends on the cleanliness factor. All these coefficients can be found in table 6-2, page 234. The circulating water inlet temperature should be sufficiently lower than steam saturation temperature to result in reasonable values of. It is usually recommended that should between about 11 to 17 o C and that, the TTD, should not be less than 2.8 o C.
T Steam, TTD Circulating water 2 1 Heat transfer length Figure 4: Condenser temperature distribution. Circulating-water flow and pressure drop The water mass-flow rate is simply given by: Where: : is the specific heat of the water and are the inlets and exit temperatures, respectively. The pressure drop in the condenser is composed of (1) the pressure drop in the water boxes and (2) the friction pressure drop in the tubes. The pressure drops are given in the terms of head H, which is related to the pressure loss by: Where is the density, g the gravitational acceleration. Water inlet velocities in condenser tubes are usually limited to a maximum 2.5 m/s to minimum erosion, and a minimum of 1.5-1.8 m/s for good heat transfer. Values between 2.1 to 2.5 m/s are most common. For design process use figure 6-10 and 6-11 page 236.