Cooling curve

The separation of the solid phase does not occur readily with some liquid mixtures and supercooling is observed. Instead of an arrest in the cooling curve at /, the cooling continues along a continuation of c/ and then rises suddenly to meet the line f g which it subsequently follows (Fig. 1,13, 1, iii). The correct freezing point may be obtained by extrapolation of the two parts of the curve (as shown by the dotted line). To avoid supercooling, a few small crystals of the substance which should separate may be added (the process is called seeding ) these act as nuclei for crystallisation. 

By constructing cooling curves for several mixtures of different composition, the total weight of the mixture being kept the same in each case, a series of cooling curves may be obtained which may be employed for the construction of the equilibrium diagram. Thus in Fig. /, 13, 2, a. 

Fig. 2. Time—temperature—transformation (TTT) diagram where A represents the cooling curve necessary to bypass crystallization. The C-shaped curve separates the amorphous soHd region from the crystalline soHd region. Terms are defined ia text.

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

Fig. 18. Continuous-cooling transfomiation diagram for a Type 4340 alloy steel, with superimposed cooling curves illustrating the manner in which transformation behavior during continuous cooling governs final microstmcture (1). Ae is critical temperature at equiUbrium. Ae is lower critical...

FIG. 11-114 Three- and nine-level cascade cycle cooling curve for natural gas. 

FIG. 12-24 Spray ponds cooling curves for steam-condensing service. 

These relays also possess characteristics similar to those of a bimetallic relay and closely match the motor heating and cooling curves. They are basically made of a low-melting eutectic alloy which has defined melting properties. The alloy, with specific proportions of constituent metals such as tin, nickel and silver, can be made for different but specific melting temperatures. This property of the alloy is used in detecting the motor s operating conditions. 

Figure 12.31 Heating and cooling curves of an intermittent duty motor...

Phase transformations in the solid state (like those in iron), too, have latent heats. They may be small, but with sensitive equipment for measuring cooling curves or heating curves, they are easily detected. 

The shelves of the cooling curve are called arrest points. The two shown in the picture are at the boiling point and the melting point of the material, at the given pressure. 

During cooling, a point D is reached where the internal air temperature decreases less quickly for a period. This represents the solidification of the plastic and because this process is exothermic, the inner air cannot cool so quickly. Once solidification is complete, the inner air cools more rapidly again. Another kink (point E) may appear in this cooling curve and, if so, it represents the point where the moulding has separated from the mould wall. In practice this is an important point to keep consistent because it affects shrinkage, warpage. 

Figure 7.3 Predicted, (a) cooling curves, and (h) transient. supersaturation for potassium sulphate solutions after Jones, 1994)...

The resulting numerical prediction for the size-optimal cooling curve is shown in Figure 7.3. It predicts that in order to maximize the final sizes of the S-crystals, the temperature should be held constant for a period at both the start and end of the operation with a convex curve in between. This has the result of reducing both the early and terminal supersaturation levels and so maximizes solute deposition on the S-crystals and their growth rather than that of the A -crystals. Thus, programmed cooling is strictly sub-optimal , but nevertheless remarkably close to the optimum result in this case to be a practical alternative (Jones, 1974). 

Estimate the maximum product crystal size and determine a simple controlled cooling curve (Jones, 1972, 1974 Mullin and Jones, 1974). 

See the cooling curves in Figure 33.1. The evaporating temperature will now fall to about 0.2°C. Compressor manufacturers tables show 10.3% loss in duty for 1.5% less power at the new condition - an overall power increase of 9%. A more accurate estimate can be obtained by calculating a new basic rating for the reduced air flow. This shows about 8% extra power. 

Figure 9.13. Cooling curve for bodies of various shapes 1, slab (2L = thickness) 2, square bar (2t — side) 3. long cylinder (L = radius) 4. cube (2L = length of side) 5, cylinder (L = radius, length — 2L) ...

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