Steam Mollier diagram

Steam Rate Enthalpy data can be obtained from Mollier diagrams or from steam tables (see Sec. 2), from which the theoretical steam rate can be calculated. For example, a throttle inlet condition of 4137 kPa (600 psig) and 399° C (750° F) gives an enthalpy of 3.2 MJ/kg (1380 Btu/lb), and if the end point is at 348 kPa (50 psig), then adiabatic expansion is to 2.69 MJ/kg (1157 Btu/lb). This gives 0.52 MJ/kg (223 Btu/lb) available, and the theoretical steam rate is calculated from the Btu equivalent per Idlowatthour or horsepower-hour ... 

FIG. 29-23 Mollier diagram showing ASME-IEEE steam-turhine standards. To convert British thermal units per pound to kilojoules per kilogram, multiply hy 2.328 to convert British thermal units per pound-degrees Ranldne to joules per gram-Kelvin, multiply hy 4.19 and to convert poiinds-force per square inch to Idlopascals, multiply hy 6.89. 

Equation 2 will yield the same result as the Theoretical Steam Rate Tables (Reference 10). Therefore, this is a handy way of getting theoretical steam rates when only a set of steam tables sans Mollier diagram are available. 

Estimate steam flow. Refer to the Mollier diagram for steam in Figure 14-21, which shows the available enthalpy in Btu/lb of steam. 

The H-S plot is called a Mollier diagram and is particularly useful in analyzing throttling devices, steam turbines, and other fluid flow devices. A Mollier diagram for steam is presented in Figure 2-37 (standard engineering units) and in Figure 2-38 in SI units. 

Dear engineering reader, please recover from your desk drawer, your steam tables. In the back, there will be a Mollier diagram for steam. Figure 17.2 is a representation of your Mollier diagram. We will use... 

Step 2. When steam passes through a turbine, it undergoes an isoentropic expansion. The work that the steam does in transferring its momentum to the turbine wheel exactly equals the shaft horsepower developed by the turbine. The entropy of the system is therefore constant. On this basis, extend a line through point A straight down the Mollier diagram. This line represents a constant entropy expansion. 

Most of the turbines you will encounter in your work are called topping, or extraction, turbines. The idea of such a turbine is to extract much of the potential work from the motive steam, and then use the exhaust steam to reboil towers. Typically, the energy content of the exhaust steam is only 10 to 20 percent less than that of the motive steam. That is the calculation we just did with the Mollier diagram. The rest of the energy of the steam may then be used as the steam condensers, to reboil towers. This sounds pretty efficient. It is the basis for the new cogeneration projects you may have heard about. Of course, this system was used by the British Navy in the nineteenth century. 

For practice, pull out your Mollier diagram. If the motive steam is 400 psig and 650°F, what is the effect of reducing the vacuum in the surface condenser from 27 to 14 in Hg Answer 13 percent loss in horsepower (see Chap. 17). 

Mollier diagram A chart relating enthalpy, entropy, temperature, and pressure of steam. 

On a Mollier diagram like that with Example 4.1, it is clear that expansion to a low pressure may lead to partial condensation if insufficient preheat is supplied to the inlet steam. The final condition after application of the efficiency correction is the pertinent one, even though the isentropic point may be in the two-phase region. Condensation on the blades is harmful to them and must be avoided. Similarly, when carbon dioxide is expanded, possible formation of solid must be guarded against. 

The amount of energy that the steam turbine extracts from the steam depends on the enthalpy drop across the machine. The enthalpy of the steam is a function of its temperature and pressure. One can use a Mollier diagram as a graphic tool to determine the amount of energy available under a particular set of conditions. If in Figure 2.131 the inlet conditions correspond to point and the outlet conditions to point P2, a line drawn between these two points is called the "expansion line" and represents the operation of the turbine as it is extracting energy from the steam. In an ideal turbine, the steam would expand at a constant entropy (isentropically) and the condition of the exhaust steam, from an ideal machine (which has no losses), would correspond to point 3. 

Mollier diagram showing performance of a steam turbine. 

Enthalpy/concentration diagram, 440-447 for sodium hydroxide/water, 444 for sulfuric acid/water, 441 Enthalpy/entropy (Mollier diagram), 183, 185 for steam (see back endpapers)... 

FIGURE 19.1 Simplified Mollier diagram for steam. (Note 1 psia = 6.895 kPa.)... 

Thus, project along the 100-psia (689.5-kPa) pressure curve until it intersects the saturation curve, point g. From here project horizontally to the left-hand scale and read the enthalpy of 100-psia (689.5-kPa) saturated steam as 1187 Btu/lb (2761.0 kJ/kg). (The Mollier diagram in Fig. 19.1 has fewer grid divisions than large-scale diagrams to permit easier location of the major elements of the diagram.)... 

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