Evaporative-Cooled Condensers

For systems where a volatile component can be vaporized and condensed, evaporative cooling is often the most economical means of heat removal. The... 

Relative humidity at which water condenses on the apphed film as a result of evaporative cooling effects. 

Evaporative condensers and water cooling towers with two or more fans on separate drive may he controlled in the same way. If a single motor drives several fans on one shaft, speed control or dampers will he required. Evaporative condensers and cooling towers should he fitted with antifreeze thermostats which will stop all fans before the water reaches freezing point. 

In most climates the wet bulb temperature is well below the dry bulb temperature and there is an advantage in using water or evaporative cooling for larger plant. These options need to be investigated and compared. The present concern over spray-borne diseases may indicate a preference for air cooling in the vicinity of institutions but correct maintenance of water cooling towers and evaporative condensers will permit their use elsewhere. Table 10.2, based on the tentative temperature differences of 15 K and 12 K... 

Air-cooled condensers require a large air flow for a given heat rejection duty and the limitation on their use is reached on account of their size and the need to get enough air. Water or evaporative cooling should always be considered as a possibility, except for smaller sizes or where using packaged condensing units. 

All these solvents are volatile liquids and will have a pressure-temperature characteristic (see Section 1.2), so can he condensed if cooled to their saturation temperature. Finned-tuhe evaporators are generally used, hut the condensation may he at a high pressure, requiring heat exchangers of the shell-and-tuhe type. 

Arc Plasma Method The principle of NPs synthesis in this method is based on evaporation by heating and condensation by cooling. The bulk metal is evaporated by heating with electrical resistance, electron beam, or high-frequency magnetics, and subsequently the vapor of metal atoms is condensed on a substrate as a sohd film or particles. In the AP method, electrical charge filled in an external capacitor... 

There are a number of other types of measurement made in soil that involve electrodes that are not directly in contact with the soil. An example is the thermocouple psychrometer, which involves a Thomson thermocouple in a ceramic cell buried in soil. The thermocouple cools when a current is passed through it, causing water to condense on the thermocouple. When the electricity is turned off, the condensate evaporates at a rate inversely proportional to the relative humidity in the soil. A voltage generated by the cooling junction is measured and related to the soil moisture content. This moisture content is related to both the matrix and osmotic potentials of the soil being investigated. 

Hausbrand, E. Evaporating, Condensing and Cooling Apparatus. Translated from the second revised German edition by A. C. WRIGHT, Fifth English edition revised by B. Heastie (E. Benn, London, 1933). 

Benzyl cyanide. Place 100 g (2 mol) of powdered sodium cyanide and 90 ml of water in a 1-litre round-bottomed flask provided with a reflux condenser. Warm on a water bath until the sodium cyanide dissolves. Add down the condenser a solution of 200 g (181.5 ml, 1.58 mol) of benzyl chloride (Expt 5.27) in 200 g of rectified spirit during 30-45 minutes. Heat the mixture in a water bath for 4 hours, cool and filter off the precipitated sodium chloride with suction wash with a little alcohol. Distil off as much as possible of the alcohol using a rotary evaporator. Cool the residual liquid, filter if necessary and separate the layer of crude benzyl cyanide. (Sometimes it is advantageous to extract the nitrile with ether.) Dry over a little magnesium sulphate, and distil under diminished pressure. Collect the benzyl cyanide at 102-103 °C/ 10 mmHg. The yield is 160 g (86%). 

Figure 2.30 shows the design principle of such a system. Heating and cooling of the shelves are effected indirectly over a heat transfer medium while the condenser evaporator is cooled directly. Contrary to conventional condenser evaporators which... 

These events can be avoided in most cases, if the following start-up rules are used (1) the plant is evacuated down to the lowest operation pressure if this pressure is reached, possible water or ice from the cleaning or sterilization is evaporated (2) the condenser is cooled to the operating temperature and the shelf heated to the maximum temperature during SD (3) if the lowest operation pressure is reached again, the leak test can be done (see Section 2.2.6) (4) the shelves are cooled back to the loading temperature and the plant is vented with a gas as specified (see end of Section 1.2.3 and Figures 2.51.1, 2.51.2 and 2.52). 

If a runaway is detected at such an early stage, a controlled depressurization of the reactor may be considered. As an example, during an amination reaction, a 4 m3 reactor could be cooled from 200 °C to 100 °C within 10 minutes without external cooling, just by using a controlled depressurization allowing evaporative cooling. Obviously, the scrubber and the reflux condenser must be designed to work with independent utilities. 

Thus, the system must be kept open to allow vapor to condense and escape. Since the gas is toxic, a scrubber, which works in the case of cooling failure, must be provided. The condenser must also work after a cooling failure, such as an independent coolant. In order to check the feasibility of these measures, it is important to assess the controllability of the mnaway at MTT The objective is to control the reaction course by providing evaporative cooling. 

Figure 8.1 Air conditioners use the principles of evaporation and condensation to cool rooms.

There are many reactors in industry that use evaporative cooling. The liquid in the reactor boils to remove reaction heat. The vapor leaving the reactor is condensed in an external heat exchanger, and the liquid is returned to the reactor. Clearly the vapor phase is important in these autorefrigerated reactors. In addition to chemical kinetics, the vapor-liquid equilibrium properties influence the design of the reactor-condenser system. 

Evaporative-cooling crystallizers are fed with a liquor whose temperature is such that solvent flashes upon feed entry to the crystallizer. They typically are operated under vacuum, and flashing of solvent increases the solute concentration in the remaining liquor while simultaneously reducing the temperature of the magma. The mode of this operation can be reduced to that of a simple cooling crystallizer by returning condensed solvent to the crystallizer body. 

---