Fundamentals of Air Cooled Steam Condensers P M V Subbarao Professor Mechanical Engineering Department I I T Delhi After Coal, Water to hit India s Power Sector Is there enough water to fuel India s power expansion? Avoid Usage of More Water and Conserve Water..
Introductory Remarks The production of electricity requires a reliable, abundant, and predictable source of freshwater. A resource that is limited throughout the world. The process of power generation from fossil fuels such as coal, oil, and natural gas is water intensive. In a country where hardly any city gets 24-hour supply of drinking water The per capita water availability is shrinking at an incredible pace.. Water levels of India s dams are falling to record lows.. Agriculture draws approximately 90% of domestic water.. It is inevitable that India will be water stressed not in the medium term but in the immediate short term.
The Scarcity of Water With the present population of over 1,200 million, the per capita water availability is around 1.170 m 3 /person/year. This translates to 1170 litres/person/per year or less than 3 litres per day per person. The urban area consumption is in upwards of 100-150 litres per day.
The Water Requirement The water requirement for coal based plant with cooling tower used to be about 7 m 3 /h per MW without ash water recirculation and 5 m 3 /h per MW with ash water recirculation. In recent past, plants have been designed with water consumption requirement in the range 3.5-4 m 3 /h per MW. This is important for a country like India, which has about 16% of the world s population as compared to only 4 per cent of its water resources. An installed capacity of 130,370 MW of thermal power plants if assumed running even at 75% PLF would consume over 30 bcm of water.
Water Withdrawal in Cooling Towers Coal Power Plants
Water Withdrawal in Cooling Towers Water consumption Vs Power Generation Technology
Alternative Cooling Systems Direct Dry Cooling Systems. Indirect Dry Cooling Systems. Hybrid Cooling Systems.
Direct Dry Cooling Systems.
Indirect Dry Cooling Systems.
Hybrid Cooling System
Air Cooled Steam Condensers Air Cooled Condensers Types Natural Convection: Occurs when a heater with a reduction of dense fluid is pressed left by a cooler denser fluid. Forced Convection: Occurs when an outer force pushes a fluid, such the same as water of air, to create it move about and transfer heat. Geometrical Designs The nearly all accepted style of Air Cooled Condenser is the modularized ('A' frame structured design), used on industries and power plants of all sizes. The 'A' frame structured designs are mostly used in the power plants. Coil Design : Advanced Smart Circuitry Coil Technology.
Natural Convection ACSC
Forced Convection ACSC
Mixed Convection ACSC
A Frame Mechanical Draught ACSC
Coil Type Mechanical Draught ACSC
World Market Evolution for large Power Plants with ACSCs
Development of Design Conditions The minimum amount of information required to establish the simplest design point for an ACSC is: Steam flow, W (tons/hr) Turbine exhaust team quality, x (kg dry steam/kg turbine exhaust flow) Turbine backpressure, p b (mm Hga) Ambient temperature, T amb (deg C) Site elevation, (m---above sea level)
Site Characteristics In addition to these basic quantities, the ACC design (and cost) may be affected by a number of plant and site characteristics which are listed below. Site characteristics Meteorology Annual temperature duration curves Prevailing wind speeds and directions Extreme conditions (hottest day; freezing conditions) Topography and obstructions Nearby hills, valleys, etc. Nearby structures, coal piles, etc.
Nearby heat sources---aux. coolers, plant vents, etc. Other Noise limitations At ACSC At some specified distance---neighbors, sanctuaries, etc. Maximum height restrictions Footprint constraints (length, width) Location restrictions---distance from turbine exhaust
Basic Design Determination Specification of the quantities and characteristics above are sufficient to obtain a budget estimate from ACC vendors. Consider a case study to illustrate the considerations in selecting an appropriate design point. An ACC for installation at a 500 MW (nominal), gas-fired combined-cycle plant located in an arid, desert region selected the following design values: Steam flow, W (tons/hr): 497.2 Quality, x (kg/kg) 0.95 Backpressure, p b (cm Hga) 10.16 Ambient temperature, Tamb (C) 27 Site elevation Sea level (p amb = 75.9968 cm Hga)
Number of Cells : Air-Cooled Steam Condenser The number of cells (also referred to as modules) is clearly an important part of the supplier data. Obviously, the number of cells dictates the amount of mechanical equipment (i.e. fans, motors, gear boxes). Further, many current large-scale designs use components, whose dimensions are optimized for shipping and erection. For instance, use of 10 meter diameter fans and individual tube bundle sections of approximately 11m and with 2.5m bundle and 5 bundles per cell per side for a plan area of 11 m by 12.4 m per cell per side.
As a result, the number of cells often dictates a number of features of the air-cooled condenser, including the mechanical equipment as well as the amount of heat transfer surface. The total number of cells or modules is the sum of the Primary and Secondary Modules. The Primary Modules are responsible for the majority of the heat transfer and condensing, while the Secondary Cells are responsible for residual heat transfer and condensables collection and evacuation
Number of Primary Modules The number of Primary Modules is typically about 80 percent of the total number of modules. Length of Primary Modules - The length of the primary modules is typically on the order of 10-13 m for a Single Row Condenser type system. Number Of Secondary Modules The number of Secondary Modules is typically about 20 percent of the total number of modules and there is typically one module per row (or street). Length of the Secondary Modules these modules are typically shorter than the primaries by about 1 1.5 m. Primary Module Dimensions (Width) must be greater than the fan diameter and typically run on the order of 15-25 percent larger than the fan diameter.
Fan Characteristics Fan diameters for ACC s used on most recent power plant applications are typically 10-12m. The number of blades per fan will minimally be 5 but may be as many as 8-10 depending upon the fan supplier and the performance requirements. Motor Characteristics Fan motor power must be equal to that required by the fan shaft power divided by the motor and gear box efficiencies. Often a margin of 5-10 percent if provided, in addition to service factor margins.
Thermo-hydraulic Specifications of ACSC Overall Heat Transfer coefficient, U, (based on air-side surface area) Total Air-Side Surface Area, A Total Mass Flow Rate of Air at Each Design Condition, m air Fan Static Pressure (p static ) or the total system pressure drop. Log Mean Temperature Difference (LMTD) Steam Duct Pressure Drop Heat Exchanger Bundle Pressure Drop (Steam Side)
Important Global Parameters Thermal Duty It is important to verify that the thermal duty solicited (i.e. the amount of heat to be rejected) is matched or exceeded by the supplier s offering. Heat transfer Area This is calculated knowing the total heat transfer area of the tubes in the ACC s. For a Single Row Condenser (SRC), the ratio of the airside surface area and the total face area is approximately 124. Outlet Air Temperature The outlet air temperature is obviously less than the steam temperature and can be calculated from the following equation. Face Velocity of the Air - The face velocity of the air, can be calculated from the mass of air flow rate, the air density, and the total face area of the ACC. Typical values will run from about 1m/s) to as much as 3 m/s) with the average being about midway between those limits.
Fan Static Pressure - Fan Static Pressures will vary depending upon whether the fan is a low-noise or more standard design. Fan Static Pressure, which in essence is the force required to overcome the system resistance (with the required design air flow rate), will run on the order of ~100 Pa +/- 20%) for a standard fan and system design. Fan Shaft Power or Brake Horsepower - Depending upon the fan static efficiency, one can calculate whether the fan system will deliver the appropriate amount of air. Power Requirements - Total fan power can be calculated using the aforementioned information and assuming nominal gear box efficiencies of ~97% and motor efficiencies ~92-94%.
Operating characteristics of an air-cooled steam condenser - I
Operating characteristics of an air-cooled steam condenser - II
Operating characteristics of an air-cooled steam condenser -III
Performance Impacts Wind Effects :Prevailing ambient winds can be high (>10-20 mph) at some sites, leading to: Flow separation at the fan inlet and poor fan performance, Recirculation of the hot exit air into the air inlet of the ACC, and mal-distribution of the air in the plenum and across the heat exchange surfaces.
Local Interferences The location of the ACSC is necessarily closer to heat sources such as service water cooling systems, turbine exhaust piping, etc. than evaporative cooling towers typically are from the Plant. The entrained air from adjacent sources is very likely to be warmer than design or ambient conditions and therefore the performance of the ACSC is negatively impacted.
ACSC Unit with 30 A frames
Streamline plot: Global flow field
Volumetric effectiveness of fans
Air inlet temperature of the fans in row 6, and the wind in the x-direction
Volumetric effectiveness of ACSC
Thermal effectiveness of ACSC
Ambient temperature and wind effect on saturation temperature of the turbine exhaust steam.
Ambient temperature and wind effect on turbine back pressure
Distribution of dimensionless heat rejection rate under different wind speeds (a) 0 m/s (b) 6 m/s (c) 15 m/s.
Schematic of the deflecting plates
Dimensionless total heat rejection versus wind speed
Identifying the Steam-Condenser Problem A successful air-cooled steam condenser must continuously and completely gather and discharge all of the noncondensables in the system. These are the gases that result from atmospheric air leaks into the vacuum portions of the steam-cycle equipment, and from the chemicals used for boiler feed water treatment. The noncondensables are left behind inside the tubes and headers when the steam condenses. They accumulate if not removed from the system at the release rate. Such trapping of noncondensables is responsible for the steam condenser problem.
Steam-condensing system ties in with turbine and with air-removal package
Trapping of noncondensables causes the steamcondenser problem
Backflow Problems