The steam condenser of a power plant boiler is a critical heat exchanger in the process. Poor performance due to restricted heat transfer can have a dramatic influence on unit efficiency. And, condenser tube leaks will introduce impurities to the condensate that can cause severe steam generator damage.
Part 1 of this article examines fundamental thermodynamics of condenser heat transfer, and how efficiency losses can cost a plant much money. In Part 2, my colleague Kevin Boudreaux and I will examine how waterside fouling and scaling (and excess air in-leakage on the steam-side) can affect condenser performance, and in Part 3 we will review chemistry upsets that occur due to in-leakage of cooling water into condensate.
Too often, plant personnel feel that they can safely operate for several days or even weeks with a condenser tube leak to keep producing power. The negative results of such actions can be monumental.
Basic Condenser Thermodynamics
Quite often those who are new to the steam generation power industry ask why turbine exhaust steam is condensed and not just returned to the boiler as steam. The answer lies in efficiency. For simplicities’ sake, consider the simple power system shown below.
Fig. 1. A basic steam generation system with isentropic turbine.
An explanation of the symbols is as follows:
· QB – Heat input to the boiler from fossil fuel combustion or other sources
· WT – Work done by the turbine
· QC – Exhaust heat from the condenser (no thermodynamic process is possible without the discharge of some heat)
· WP – Work done to raise the pressure of the condensate for return to the boiler
The diagram assumes 100 percent turbine efficiency, when in actuality turbines are typically 80 to 90 percent efficient, but this factor does not need to be included here to show the importance of steam condensation and maintaining condenser cleanliness. Consider the following hypothetical case.
Main Steam (Turbine Inlet) Pressure – 1000 psia
Main Steam Temperature – 1000oF
Turbine Outlet Steam Pressure – Atmospheric (14.7 psia)
Call this Example 1. The steam tables show that the enthalpy of the turbine inlet steam is 1505.9 Btu per pound of fluid (Btu/lbm). Thermodynamic calculations indicate that the exiting enthalpy from the turbine is 1080.9 Btu/lbm (steam quality is 93 percent). The energy transfer can fundamentally be calculated by multiplying the difference between the turbine inlet and outlet enthalpies times the mass flow rate [WT = m(h1 – h2)]. Accordingly, the unit work available from this ideal turbine is (1505.9 Btu/lbm – 1080.9 Btu/lbm) = 425.0 Btu/lbm. To put this into practical perspective, assume steam flow (m) to be 1,000,000 lb/hr. The overall work is then 425,000,000 Btu/hr = 124.5 megawatts (MW).
Now consider Example 2, where the system has a condenser that reduces the turbine exhaust pressure to 1 psia (approximately 2 inches of mercury). Again assuming an ideal turbine, the enthalpy of the turbine exhaust is 923.4 Btu/lbm. The unit work output equates to 1505.9 – 923.4 = 582.4 Btu/lbm. At 1,000,000 lb/hr steam flow, the total work is 582,400,000 Btu/hr = 170.6 MW. This represents a 37 percent increase from the previous example. Obviously, condensation of the steam has an enormous effect upon efficiency. An important point from this very basic example is that the steam quality would be just above 80 percent. A general rule-of-thumb recommends less than 10 percent moisture in exhaust steam to prevent damage to the last stages of the LP turbine. That is a primary reason why most modern steam generators have at least one steam reheating circuit. Not only does reheating improve unit efficiency, but it raises steam quality at the turbine exhaust. Also, note that when discussing steam, quality refers to the percentage of steam in two-phase mixtures. Plant personnel sometimes confuse quality with purity, which, as the name implies, refers to how pure the steam is, where normally it should contain very low concentrations (<2 parts-per-billion) of sodium, chloride, and sulfate.
One can also look at this example from a physical perspective. Calculations indicate that the steam quality at the turbine exhaust in Example 2 is 82 percent. This means that 18% of the steam has condensed to water. However, the remaining steam takes up a specific volume of 274.9 ft3/lbm. The corresponding volume of water in the condenser hotwell is 0.016136 ft3/lbm. Thus, the condensation process reduces the fluid volume over 17,000 times. The condensing steam generates a strong vacuum in the condenser, which acts as a driving force to pull steam through the turbine. (The strong vacuum also pulls in air from outside sources, where excessive air in-leakage can seriously affect heat transfer, as will be discussed later in this series.)
Of importance also is that the work (WP) required by the feedwater pump is fractional compared to other energy flows within the process. The energy required by a compressor to return turbine exhaust steam directly to the boiler would be much, much greater than WP.
Let’s take these energy transfer concepts a step further in Example 3. Consider if waterside fouling or scaling (or excess air in-leakage to condenser steam-side) causes the condenser pressure of Example 2 to increase from 1 psia to 2 psia. Thermodynamic calculations show that the work output of the turbine drops from 582.4 to 546.1 Btu/lbm. So, at 1,000,000 lb/hr steam flow, a rise of 1 psia in the condenser backpressure equates to a loss of 36,300,000 Btu/hr or 10.6 MW of work. This is a primary reason why proper cooling water chemical treatment and condenser performance monitoring are very important, as will be discussed in subsequent parts of this series. Even a very thin coating of an insulating deposit, such as calcium carbonate, magnesium silicate, or others will greatly inhibit heat transfer. Microbiological fouling may be even worse. The microbial colonies and accompanying slime/silt layer not only retard heat transfer but can induce severe under-deposit corrosion.
Figure 2. Microbiologically fouled condenser tubes.
Figure 3. Microbiologically fouled cooling tower film fill.
Some Co-Generation Comments
Ponder again for a moment the conditions outlined in Example 2, where the well-functioning unit extracts 582.4 BTU/lbm of energy in the turbine. The enthalpy of the condensate at 1 psia condenser pressure is 69.7 BTU/lbm. Thus, the heat “wasted” in the condenser is 923.4 – 69.7 = 853.7 BTU/lbm. So, even in this ideal condition only about 40% of the energy in the turbine influent steam is utilized. Most of the wasted heat represents the latent heat required to convert water to steam in the boiler. This is the reason why co-generation is a common process at many industrial plants (and is becoming increasingly popular for combined heat and power (CHP) applications.) These plants often have turbines to produce electrical or mechanical power (for example, large steel mills use turbines to produce the “blast” air for blast furnaces), but rather than condensing the steam it is extracted from the turbine while still dry (100 percent quality) and is utilized for process heating. Very common is to have the steam condense as it flows over heat exchanger tubes or plates, such that the latent heat of the fluid is directly extracted. Overall net efficiencies of such plants may reach 80 percent. A very good exercise at many plants, and which the author has participated in the past, is identifying locations to utilize waste steam or hot water for process improvements.
About the author: Brad Buecker is Senior Technical Publicist with ChemTreat. He has 36 years of experience in or affiliated with the power industry, with nearly two decades of it in steam generation chemistry, water treatment, air quality control, and results engineering positions at City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s La Cygne, Kansas station. This experience also includes 11 years at two engineering firms, and he spent two years apart from power as acting water/wastewater supervisor at a chemical plant. He has authored many articles and three books on power plant water/steam chemistry and air pollution control topics. He is a graduate of Iowa State University. He may be reached at [email protected].