Cooling: evaporative

Cooling by solvent evaporation is an efficient method. On one hand, it is independent of the heat transfer at the reactor wall and, on the other hand, the condenser can be dimensioned independently of the reactor geometry. This reaches relatively high specific cooling powers. In case a reaction cannot be performed at boiling temperature, it is possible to work under partial vacuum to decrease the boiling 

Internal film Stirrer speed and type Water 1000 

Fouling at external wall Thermal conductivity X With d = 0.1mm  

Some technical aspects and limitations must be considered in the design of such cooling systems  

These different points are reviewed in detail in the following subsections. 

The temperatures reached with the optical cooling methods discussed so far are not sufficiently low to obtain Eose-Einstein condensation. Here the very old and well-known technique of evaporation cooling leads to the desired goal. The principle of this method is as follows [14.60]  

The optically cooled atoms are transferred from the MOT to a purely magnetic trap, where the restoring force 

The cooling process must be slow enough to maintain thermal equilibrium, but should be sufficiently fast in order not to lose particles from the trap by collision-induced spin flips. 

The phase transition to EEC manifests itself by the sudden increase of the atomic density (Fig. 14.21), which can be monitored by measuring the absorption of an expanded probe laser beam. 

The EEC can be kept only for a limited time. There are several loss mechanisms that result in a decay of the trapped particle density. These are spin-flip collisions, three-body recombination where molecules are formed, collisions with excited atoms where the excitation energy may be converted into kinetic energy, and collisions with rest gas atoms or molecules. The background pressure therefore has to be as low as possible (typical pressures are 10 to 10 mbar) 

The numerical examples illustrates that the usual techniques of optical cooling are not sufficient to reach BEC at realistic atomic densities unless one uses the experimentally difficult Raman-cooling or the coherent generation of dark states (see Sect. 7.10). 

Fortunately there exists a very old but efficient method for cooling liquids, namely evaporation cooling [1177] which is used in daily life when cooling hot coffee in a cup by blowing over its surface. For achieving BEC the technique is applied as follows The atoms are optically precooled in a MOT. 

The phase transition to EEC manifests itself by the sudden increase of the atomic density (Fig. 9.32), which can be monitored by measuring the absorption of an expanded probe laser beam (Fig. 9.33). As example of EEC in a magnetic trap Eig. 9.34 shows the spatial confinement of the atomic cloud in a Z-shaped Joffe-Pritchard micro-trap (see Sect. 9.1.8b), where a homogeneous magnetic field in y-direction is superimposed to the field produced by the current through the wire. The volume of the atomic cloud is an ellipsoid with rotational symmetry around the z-axis. 

12-5 Mollier chart showing changes in Twb during an adiabatic saturation process for an organic system (nitrogen-toluene). 

12-6a Diagram of psychrometric chart showing the properties of moist air. 

---

Open steady-flow systems, which include almost all air conditioning processes, foUow this law. Examples include the energy flows in a cooling and dehurnidifying coil or an evaporative cooling system. 

Cooling. A compression refrigeration system, driven by an electric motor, suppHes cooling for either direct expansion or ice bank systems (Fig. 12). In the former, the milk is cooled by the evaporator (cooling cods) on the bulk tank liner opposite the milk side of the liner. The compressor must have the capacity to cool the milk as rapidly as it enters the tank. 

Order of preference adiabatic evaporation > isothermal evaporation > cooling >drown-out. 

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

Many organisms are exposed to some of the thermal, chemical, and physical stresses of entrainment by being mixed at the discharge with the heated water this is plume entrainment. The exact number exposed depends on the percentage of temperature decline at the discharge that is attributed to turbulent mixing rather than to radiative or evaporative cooling to the atmosphere. 

Cooling by means of evaporative cooling towers is required to maintain a constant temperature of 30—40°C. At higher temperatures, the deposit is rougher, impurity effects are more pronounced, lead codeposition is favored, and the manganese dioxide formed at the anode iacreases and tends to adhere rather than fall to the bottom of the cell. 

The specific enthalpies ia equation 9 can be determined as described earUer, provided the temperatures of the product streams are known. Evaporative cooling crystallizers operate at reduced pressure and may be considered adiabatic (Q = 0). As with of many problems involving equiUbrium relationships and mass and energy balances, trial-and-error computations are often iavolved ia solving equations 7 through 9. 

Predrag S. Hrnjak, F. C. Standiford, Klaus D. Timmerhaus Psychrometry Evaporative Cooling, and Solids Drying Charles G. Moyers, Glenn W. Baldwin... 

Glenn W. Baldwin, M.S., P.E., Staff Engineer, Union Carbide Corporation Member, American Institute of Chemical Engineers (Section 12, Psyclirometry Evaporative Cooling, and Solids Drying)... 

A nominal resiilt of this techniqne is that the reqnired airflow rate and eqnipment size is abont two-thirds of that when evaporative cooling is not nsed. See Sec. 20 for eqmpmeut available. 

Evaporative Cooling The process fluid can be cooled by using evaporative cooling with the sink temperature approaching the wet-bulb temperature. 

Example 4 Evaporative Cooling Air at 95 F dry-bulb temperature and 70 F wet-bulb temperature contacts a water spray, where its relative humidity is increased to 90 percent. The spray water is recirculated makeup water enters at 70 F. Determine exit dry-bulb temperature, wet-bulb temperature, change in enthalpy of the air, and quantity of moisture added per pound of dry air. 

The contrihiition of Robert W. Norris, Robert W. Norris and Associates, Inc., to material that was used from the sixth edition is acknowledged. (Evaporative Cooling)... 

---