Condenser performance monitoring (Part 1)

The first half of this two-part series examines basic ideas behind the importance of condenser heat transfer.

By Brad Buecker – Buecker & Associates, LLC

A recent Power Engineering article discussed a technology being developed by the University ofIllinois at its Abbot Power Plant to increase steam surface condenser performance by up to 2%.1

This may not sound like much, but such improvement can be very valuable. In this first part ofa two-part series, we will examine basic ideas behind the importance of condenser heat transfer,and in the second part, we will review straightforward methods to monitor condenser performance.Water-side fouling or scaling, or steam-side excess air in-leakage can severely affect condenserefficiency and cooling capacity.

The word “thermodynamics” conjures up visions of complex mathematics to many people(including at times this author). Yet, relatively simple formulas from thermodynamics can explainmuch about steam generator fundamentals, including condenser heat transfer.

Thermodynamics is primarily built around two laws. They are sometimes jokingly referred to as(first law), “You can’t get something for nothing,” and (second law), “You can’t break even.”The first law is based on the conservation of energy. It says that energy used within a system isneither created nor destroyed but only transferred. The classic energy equation for afundamental system (defined as a control volume in textbooks)2,3 is:

Q – Ws = ṁ2[V22/2 + gz2 + u2 + P2υ2] – ṁ1[V12/2 + gz1 + u1 + P1υ 1] + dEc.v./dt Eq. 1

Where,

Q = Heat input per unit timeWs = Shaft work, such as that done by a turbine, per unit timeṁ2 = Flow out of the system per unit timeṁ1 = Flow into the system per unit time(V22 – V12)/2 = Change in kinetic energygz2 – gz1 = Change in potential energyu2 = Internal energy of the exiting fluidu1 = Internal energy of the entering fluidP2υ2 = Flow work of fluid as it exits the system (P = pressure, υ = specific volume)P1υ 1 = Flow work of fluid as it enters the systemdEc.v./dt = Change in energy within the system per unit time

While this equation may look complicated, it can be better understood through a few definitions andsimplifications. First, in many systems and especially steam generators, potential and kineticenergies are very minor compared to other energy changes and can be neglected. Second, in asteady flow process such as a steam generator, the system does not accumulate energy, so dEc.v./dt iszero. Removal of these terms leaves the internal energy of the fluid (u) plus its flow work (Pυ)capabilities. Scientists have combined these two terms into the very useful property known asenthalpy (h). Enthalpy is a measure of the available energy of the fluid, and enthalpies have beencalculated for a wide range of steam and saturated liquid conditions. These values may be found instandard steam tables, where saturated water at 0oC has been designated as having zero enthalpy

Using these simplifications and definitions, the energy equation for steady flow operation reducesto:

Q – Ws = ṁ(h2 – h1) Eq. 2

But this equation represents the ideal scenario with no energy losses, and here is where the secondlaw steps in. Among other things, the second law describes process direction. A warm cup ofcoffee placed on a kitchen table does not become hotter while the room grows colder. Humanbeings grow old. A literally infinite number of examples are possible, but these examples conveythe essence of the second law.

The second law has as a foundation the concept of the Carnot cycle, which says that the mostefficient engine that can be constructed operates with a heat input (QH) at high temperature (TH)and a heat discharge (QL) at low temperature (TL), in which

QH/TH – QL/TL = 0 Eq. 3

This equation represents a theoretically ideal engine. In every process known to humans, someenergy losses occur. These may be due to friction, heat escaping from the system, flowdisturbances or a variety of other factors. Scientists have defined a property known as entropy (s),which, in its simplest terms, is based on the ratio of heat transfer in a process to the temperature(Q/T). In every process, the overall entropy change, of a system and its surroundings, increases.

So, in the real world, Equation 3 becomes

QH/TH – QL/TL < 0 Eq. 4

While entropy may seem like a somewhat abstract term, it is of great benefit in determining process efficiency. Like enthalpy, entropy values are included in the steam tables.

Two important points should be noted about the Carnot cycle, and by logical inference, all real-world processes. First is that no process can be made to produce work without some extraction of heat from the process (QL) in Equation 3.

Second, the net efficiency (η) of a Carnot engine is defined as:

η = 1 – TL/TH Eq. 5

So, in general as inlet temperature goes up and/or exhaust temperature goes down, efficiency increases. Calculations can become rather complicated for complex systems, but the focus of this series is on the steam surface condenser.

For simplicities’ sake, consider the very basic system shown below with a turbine that has no frictional, heat or other losses, which means no entropy change (isentropic).

Per the concepts outlined in Equations 3 and 4, QB, the heat input to the boiler and superheater,represents QH; and QC, the waste heat extracted by the condenser, represents QL.

I have been asked on several occasions throughout the years why turbine exhaust steam must be condensed. Why not transport it directly back to the boiler? Among several answers, a primary reason is that much energy would be required to compress the exhaust steam to return it to boiler pressure. By converting the steam to water, which is essentially an incompressible fluid under normal conditions, the fluid can be returned to the boiler by a feedwater pump with a much lower energy requirement than a vapor compressor.

The benefits of steam condensation can also be illustrated via basic thermodynamics. Let’s return to the isentropic system shown in Figure 1. (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 condenser performance.) Conditions for this first case are:

• Main Steam (Turbine Inlet) Pressure – 2000 psia

• Main Steam Temperature – 1000oF

• Turbine Outlet Steam Pressure – Atmospheric (14.7 psia)

The steam tables give an enthalpy (h1) of the turbine inlet steam as 1474.1 Btu per pound of fluid (Btu/lbm). Thermodynamic calculations at isentropic conditions indicate that the exiting enthalpy (h2) from the turbine is 1018.5 Btu/lbm (steam quality is 86.4%). The first law, steady-state energy equation for work from a turbine is, wT = ṁ(h1 – h2). Accordingly, the unit work available from this ideal turbine is (1474.1 Btu/lbm – 1018.5 Btu/lbm) = 455.6 Btu/lbm. To put this into practical perspective, assume steam flow (ṁ) to be 1,000,000 lb/hr. The overall work is then 455,600,000 Btu/hr = 133.4 megawatts (MW).

Now consider case 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 871.1 Btu/lbm (steam quality is 77.4%). The turbine output becomes 1474.1 – 871.1 = 603.0 Btu/lbm. At 1,000,000 lb/hr steam flow, the total work is 603,000,000 Btu/hr = 176.6 MW. This represents a 32% increase from the previous example. Obviously, condensation of the steam has an enormous effect upon efficiency. Remember, Equation 5? This is a practical illustration of how the condenser lowers TL.

One can also look at this example from a physical perspective. The condensate volume is many times lower than that of the turbine exhaust steam. Thus, the condensation process generates a strong vacuum that acts as a driving force to pull steam through the turbine. (The vacuum also pulls in air from outside sources, where excessive air in-leakage can seriously affect heat transfer. We will address this issue in Part 2 of this series.)

Let’s take this concept a step further in case 3. Consider if waterside fouling or scaling (or excess air in-leakage) causes the condenser pressure of the previous example to increase from 1 psia to 2 psia. In line with the calculations shown above, the work output of the turbine drops from 176.6 MW to 166.5 MW. This is a primary reason why proper cooling water chemical treatment and condenser performance monitoring are very important. Another is protection of condenser tubes from such issues as under-deposit and microbiologically-influenced corrosion.5 Degraded condenser efficiency and loss of generating capacity can cause a plant much money. Problems during peak operating conditions may be enormously expensive, especially if the unit must be de-rated to keep the turbine from tripping due to high condenser backpressure.

Notes: The term steam quality is often confused with steam purity. Steam quality refers to the percentage of steam in a water steam mixture. For example, a mixture having a steam quality of 0.9 is 90% steam and 10% water droplets. Steam purity, as its name implies, refers to the impurities in a steam supply. For instance, a common guideline for steam purity in high-pressure utility units is <2 parts-per-billion (ppb) of sodium, chloride, and sulfate, and <10 ppb of silica.

In the examples shown above, the steam quality for each case is less than 90%. Such a high moisture content would cause erosion of low-pressure turbine blades. A common recommendation is <10% moisture at the turbine exhaust. For this reason, virtually all utility boilers are equipped with steam re-heaters. Reheating improves efficiency but more importantly adds enough heat to the steam to keep the moisture at the last rows of the low-pressure turbine below the 10% level.

The heat lost in the condenser is primarily the latent heat of vaporization and represents that portion of energy input to the boiler that converts feedwater to steam. The next section examines this issue more closely.

Returning again to Figure 1, for simple steam generating systems, the general efficiency can berepresented by the following equation:

η = (wT – wP)/qB Eq. 6

Where,

wT = Work produced by the turbinewP = Work needed by the feedwater pumpqB = Heat input to the boiler

The energy required by the feedwater pump is much less than the work produced by the turbine,so we can neglect wP in Equation 6. The boiler heat input (qB) is equivalent to the difference inenthalpy of the condensate entering the boiler vs. that of the main steam exiting the boiler.

Assuming isentropic conditions again, for case 2 above, qB calculates to 1380.1 Btu/lbm. Fromthe simplified efficiency equation (η = wT/qB), the respective net efficiency is 43.7%. Asconventional steam-based power units evolved in the last century, modifications includedincorporation of (or enhancements to) regenerative feedwater heaters, economizers, superheatersand reheaters, inlet air heaters, and other equipment. But parasitic power requirements for fans,pumps, air pollution control systems, etc., combined with the large heat loss in the condenserlimited state-of-the-art drum boilers to perhaps mid-30% net efficiency. Even ultra-supercriticalsteam units can only achieve net efficiencies of perhaps 45% or slightly above. This is one ofseveral reasons that combined cycle power units gained popularity. The combination of acombustion turbine (operating on the Brayton thermodynamic cycle) and one or more heatrecovery steam generators (HRSGs) on advanced Rankine cycles, can now operate at or a bitabove 60% net efficiency. But even then, much energy is still lost in HRSG condenser(s).

In cogeneration and combined heat and power (CHP) applications, where the steam is extractedfrom a turbine before reaching the saturation point and is then utilized for process heating, muchof the latent heat is recovered rather than wasted. Such turbines are classified as “non-condensing” or “backpressure” turbines.

Non-condensing turbines operate at a design backpressure, set by the process requirements. Theyare common in many heavy industries – petrochemicals, pulp and paper, primary metals, etc., –and are often used to drive centrifugal equipment such as turbo blowers and compressors. Someco-generation processes may approach or perhaps even exceed 80% net efficiency, making ithard to argue against the economics of these processes.4

This installment outlined some of the most important fundamentals regarding condenser heattransfer and the importance of maximizing condenser efficiency. While cogeneration isbecoming increasingly popular at many industrial plants, most dedicated power plants withsteam turbines still have condensing turbines. In the second and final part of this series, we willexamine practical methods for monitoring condenser performance.

References

About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas station. His work also included 11 years with two engineering firms, Burns & McDonnell and Kiewit, and he also spent two years as acting water/wastewater supervisor at a chemical plant. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines and has written three books on power plant chemistry and air pollution control. He may be reached at [email protected].