Evaporator temperature control

The system shown in Fig. 31.1 has no provision for controlling the evaporator temperature this is controlled solely by the compressor suction pressure. The lower the compressor suction pressure, the lower the evaporator temperature. This is exactly how our home air conditioner works. If the evaporator temperature is too cold, what can we do Well, if this were a steam turbine compressor, gas engine drive, or any other type of variable-speed driver, we could reduce the compressor s speed. This would reduce the flow of refrigerant, and raise both the evaporator and compressor suction pressure. 

But we are working with an ordinary AC (alternating-current) motor—which is a fixed-speed device. There are then three methods available to control the temperature in the evaporator spillback, discharge throttling, and suction throttling. 

Spillback bypasses the compressor discharge gas back to the compressor suction. This is a relatively energy-inefficient way to increase the evaporator temperature. Spillback is discussed in Chap. 41, Centrifugal 

Suction throttling also wastes energy. Whenever a control valve is partially closed in any service, energy is always wasted. 

But with a fixed-speed centrifugal compressor, it is still the best option. As the suction valve is closed, two things happen. The flow of refrigerant vapor to the compressor is reduced, and the compressor suction pressure drops. The reduction in the refrigerant vapor flow reduces the workload on the compressor. But the reduction in the suction pressure increases the compression ratio (discharge pressure divided by suction pressure). This increases the workload on the compressor. 

As we will discuss in Chap. 28, increasing the compression ratio is a small effect on compressor workload, as compared to decreasing the vapor flow. Hence, suction throttling will significantly reduce the horsepower load on the compressor, as well as increase the evaporator temperature. 

Suction throttling is usually done with a butterfly control valve, which has a very low pressure drop when it is wide open. 

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The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

The heated polymer solution emerges as filaments from the spinneret into a column of warm air. Instantaneous loss of solvent from the surface of the filament causes a soHd skin to form over the stiU-Hquid interior. As the filament is heated by the warm air, more solvent evaporates. More than 80% of the solvent can be removed during a brief residence time of less than 1 s in the hot air column. The air column or cabinet height is 2—8 m, depending on the extent of drying required and the extmsion speed. The air flow may be concurrent or countercurrent to the direction of fiber movement. The fiber properties are contingent on the solvent-removal rate, and precise air flow and temperature control are necessary. 

Fig. 8. Longitudinal section of an experimental waste-heat greenhouse in which temperature control in all seasons is provided by evaporation and heat...

In the vacuum mixer, water evaporation is also used for the temperature control, since the evaporation rate can be influenced by the grade of the vacuum. The water vapor, however, does not escape from the mixer, but is condensed and returned into the mix, the composition of which is thus not changed. 

The reaction heat is removed by the vacuum evaporation of dilution water. The resulting water vapors allow complete degassing and stripping of any trace of undesired low boiling by products (i.e., 1,4-dioxane for ethoxy sulfates). The product temperature is accurately controlled with the vacuum level kept in the reactor and by the temperature control in the reactor jacket. The automatic control of the different process parameters, i.e., flow rate of reagents, vacuum degree, temperature of thermostatting water, also allows for accurate control of the product concentration. 

Liquids. Many of the CVD reactants are liquid at room temperature, They must be heated to their evaporation temperature and transported into the reaction chamber by a carrier gas, which may be an inert gas such as argon, or another reactant such as hydrogen. If the vapor pressure of the liquid reactant is known, its partial pressure can be calculated and controlled by controlling the rate of flow and the volume of the carrier gas. 

Vacuum evaporator, Btichi Model RE121, Brinkmann Instruments, Inc. (Burlingame, CA, USA), with temperature-controlled water-bath Nitrogen evaporator, Meyer Model 111, Organomation Associates, Inc. (South Berlin, MA, USA), with temperature-controlled water-bath... 

Next, the two temperature controllers were activated and the sandwich was taken up to 90°C (194°F) for one hour to evaporate the solvents from the liquid Nation 117 catalyst coating. The temperature was then raised to 130°C (266°F) over the next 30 minutes. This is the PEM glass transition temperature. 

Also shown in Figure 23.17 are let-down stations between the steam mains to control the mains pressures via a pressure control system. The let-down stations in Figure 23.17 also have de-superheaters. When steam is let down from a high to a low pressure under adiabatic conditions, the amount of superheat increases. Desuperheating is achieved by the injection of boiler feed-water under temperature control, which evaporates and reduces the superheat. There are two important factors determining the desirable amount of superheat in the steam mains. 

The hydroformylation reaction is highly exothermic, which makes temperature control and the use of the reaction heat potentially productive and profitable (e.g, steam generation). The standard installation of Ruhrchemie/Rhone-Poulenc s aqueous-phase processes is heat recovery by heat exchangers done in a way that the reboiler of the distillation column for work-up of the oxo products is a falling film evaporator... 

Offringa, J.C.A., de Kruif, C.G., Van Ekeren, P.J., Jacobs, M.H.G. (1983) Measurement of the evaporation coefficient and saturation vapor pressure of fraras -diphenylethene using a temperature-controlled vacuum quartz-crystal microbalance. J. Chem. Ther-modyn. 15, 681-690. 

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