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Reheat in vapor cycles

Figure 1: An ideal Rankine Cycle

Why we use reheat

Consider the Rankine Cycle shown in Figure 1. While at first glance this appears to be an attractive cycle (a thermal efficiency of 40% is quite good), it has two problems considered as a practical device. First, the pressure ratio assumed across the turbine is unrealistically high (about 1000). Second, the steam coming out of the turbine (at S3) is over 20% wet, which may damage a real turbine.

We could address the pressure ratio problem by splitting the turbine into two sequential turbines, each with a PR of about 32.

The low quality of the steam at the turbine outlet is a more interesting problem, and one that can occur in many vapor power cycles. When the steam at the outlet of a turbine becomes wet, the liquid present is in the form of water droplets. If the steam is not very wet, the amount of water is small and the droplets are not too troublesome to the operation of the turbine. This is because the liquid drops are formed by condensation from the steam to form a kind of fog. Like atmospheric fog, this fog contains extremely small drops and the drops move with almost the same velocity as the surrounding steam.

However, as the quality of the steam decreases, the concentration of these water droplets increases. The turbine blades move rapidly through the steam and tend to collect the water droplets. This is because the denser water droplets do not move with quite the same velocity as the steam, and so get "scooped up" by the blade. Once on the blade, the water forms a film and runs to the back of the blade. Here the water is re-entrained into the steam. But this droplet formation method is completely different to the original one. The droplets are a completely different size: they are much larger and now no longer follow the steam flow. When these large drops impact with the turbine blades they can do much damage and certainly impair the efficiency of the turbine. It is often considered unwise to allow steam with qualities of less than around 85% to 90% to remain in the turbine.

There are two remedies available for this problem.

Mechanically remove the condensate from the turbine. This can be done by sucking water from the turbine casing or from holes in the turbine blades. Notice that decreasing the quality of the steam (i.e., allowing more water to condense) improves overall efficiency, as the sensitivity analysis of Figure 2 illustrates. Since we can accommodate lower quality steam using this technique, mechanical removal seems attractive. Unfortunately, as removing condensate from a moving turbine is not a trivial task, this option significantly complicates turbine design.

Figure 2: Increasing the quality of the steam at the turbine
outlet leads to decreased efficiency in the Rankine Cycle

Design the cycle so that the steam at the turbine exit is not unacceptably wet. This is where the reheat process is used.

We will examine the addition of a reheat process to a Rankine cycle, paying attention to the necessary design assumptions and examining the effect on turbine outlet quality as well as cycle thermal efficiency.

The Reheat Process

Figure 3: Rankine Cycle with reheat

This second option is not as difficult as it may sound. Consider the cycle shown in Figure 3. It is a conventional Rankine cycle except that the turbine has been split and an additional heating process (the "reheat" stage HTR2) has been added between the two turbines.

The new components

Let's take a quick look at the new components and states that we have added to accommodate the reheat process. We will examine some of the assumptions that underlay the modified design.

High-pressure turbine outlet (S5)

Here, we can choose to extract the steam for reheat at a lower pressure than it left TUR1 in the Rankine cycle. We choose 315 kPa, giving a more reasonable (though still high) pressure ratio of 32. We have chosen this pressure primarily to lower the turbine's pressure ratio. However, keeping our original aim in mind, we check that, though this steam is already starting to condense, its quality is still above 90%.

Note that we also could have designed this statepoint by assuming the outlet quality to be 90%. In that case, our outlet pressure would have been near 179 kPa and the high-pressure turbine's pressure ratio would have been about 56.

Reheat Boiler (HTR2)

Since we are still dealing with an ideal cycle here, we assume the second heater to be isobaric, as was the first.

Reheat heater outlet (S6)

Now that we have taken the saturated mixture from the high-pressure turbine outlet and add heated it again in HTR2, we must decide how high to reheat it. In theory, we can reheat the steam to any temperature we want, bounded by practical considerations of our heat source and the turbine properties. Of course, we want the low-pressure outlet state to have a quality of at least 90%. A glance at Figure 4 shows that choosing any temperature over about 220°C will accomplish this.

Figure 4: outlet quality vs. reheat temperature

We know that we have a heat source good for 500 C, so we will reheat to that temperature. This also raises the average temperature of heat addition, something which will tend to improve cycle efficiency. (See Better Efficiency in Reheat Cycles for more discussion of this.) Reheating the steam to this temperature allows it to exit the second turbine still in the gas phase.

CyclePad Design Files

Download the CyclePad design of the Rankine cycle from here, and the Rankine with Reheat cycle from here.

Related Entries

Sources

Whalley, P.B. 1992. Basic Engineering Thermodynamics. Oxford University Press. ISBN: 0-19-856255-1

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Contributed by: Peter B. Whalley, Kenneth D. Forbus, M. E. Brokowski
Initial Entry: 9/5/97
Last Edited: 12/9/97
For comments or suggestions please contact cyclepad-librarian@cs.northwestern.edu