Exchange-inversion temperature

Components in which water temperature changes abruptly with distance, such as heat exchangers, tend to accumulate precipitates. Heater surfaces also accumulate precipitates if the dissolved species have inverse temperature solubilities. Systems in which pH excursions are frequent may accumulate deposits due to precipitation processes. Plenum regions, such as heat exchanger headboxes, tend to collect deposits. 

Most salts absorb heat when they go into solution, and their solubility increases with a rise in temperature however, calcium carbonate (CaC03), in common with several other anhydrous salts such as calcium sulfate (CaS04) and calcium phosphate [Ca3(P04)2], has an inverse temperature solubility and thus readily precipitates to form deposits in hot water areas (FW tanks, FW lines, and boiler heat exchange surfaces). 

Several common salts have an inverse temperature solubility and readily precipitate to form deposits on hot boiler surfaces and other heat exchange areas. These include ... 

G. Tsitsishvili Our studies of zeolite chromatographic properties indicated that the retention volumes, separation factors, and other characteristics are strongly dependent on cation nature, degree of exchange, and temperature. We have observed the inversion of the sequence of elution by temperature alteration. 

Impuritiesand the a P-quartz tranition. The a- 3-quartz transition was the basis for one of the earliest systematic investigations of the variation of transition temperatures in response to impurities. Pure a-quartz undergoes a first-order transition to a microtwinned incommensurate structure at 573°C, and this modulated phase transforms to P-quartz at 574.3°C with second-order behavior (Van Tendeloo et al. 1976, Bachheimer 1980, Dolino 1990). Tuttle (1949) and Keith and Tuttle (1952) investigated 250 quartz crystals and observed that Tc for natural samples varied over a 38°C range. In their examination of synthetic specimens, substitution of Ge for Si raised the critical temperature by as much as 40°C, whereas the coupled exchange of Ar +Li o Si depressed Tc by 120°C. They concluded from their analyses that the departure of the a-P-quartz inversion temperature from 573°C could be used to assess the chemical environ-ment and the growth conditions for natural quartz. 

If the solute solubility is relatively temperature independent or inversely temperature dependent add heat to remove the solvent (i.e., must use evaporative crystallization) or add an antisolvent to drown out crystals. For evaporation 14 to 20 g vapor evaporated/s nf exchanger area. For the exchangers use 2.5 cm diameter tubes with fluid velocities 1.5 to 3 m/s to minimize plugging. Caution if the vapor pressure rise > 3.4 kPa/°C then potential problems with control. 

Fig. 5 Comparison of calculated and experimental interaction parameter xsans for PIB/DhhPP blends ( pib = 0.475) as a function of inverse temperature. Squares are the experimental data of Krishnamoorti et al. [60], while the solid line is a least-squares fit of the SLOT /SANS for the exchange energy e =- 1.16 K...

The simple Linde cycle may also be used as a liquefier for fluids that have an inversion temperature that is above ambient temperature. Under such circumstances, the refrigeration duty, Q, is replaced by a draw-off stream of mass rhf representing the liquefied mass of fluid that is continuously withdrawn from the reservoir. Note that under these conditions, only the unliquefied mass of fluid is warmed in the counter-current heat exchanger and returned to the compressor. An energy balance around the heat exchanger, expansion valve, and liquid reservoir now results in... 

FIG. 8-48 Temperature leaving a heat exchanger responds as a distributed lag, the gain and time constant of which vary inversely with flow. 

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