Condensers dehumidifying

Humidity (condensation) Dehumidifiers, insulation, ventilation, warmed surfaces... 

Dehumidifiers, in which air is passed first over the evaporator to remove moisture, and then over the condenser to re-heat and lower the humidity (see Figure 29.1). 

Figure 24.13 Dehumidifier with condenser re-heat - process lines...

In the unit dehumidifier process, all or part of the condenser heat is used to re-heat the air leaving the evaporator (see Figure 29.1a). Since the moisture in the air has given up its latent heat in condensing, this heat is reclaimed and put back into the outlet air. In a typical application, air at 25°C dry bulb and 60% saturation can be dried and re-heated to a condition of 46°C dry bulb and 10% saturation (see Figure 24.13). In this state, it is hot enough to provide the necessary latent heat to dryout the load product. The entire system is in one unit, requiring only an electrical supply and a water drain, so there is no constraint on location. 

The drying load in a swimming pool will vary throughout the year, and dehumidifiers built for this application may have an air-cooled condenser for re-heating the air, and also a water-cooled condenser, so that some proportion of the heat maybe used to warm the pool water. Automatic controls will use the condenser heat to the best advantage. 

Some heat pumps, such as the dehumidifier, serve the double purpose of a useful load both on the evaporator and condenser side. Again, there is no reversal of the refrigerant flow in operation so the components maybe selected without compromise. 

In summer the ventilation air has to be dehumidified to keep the required comfort and to prevent from condensation at cold ceilings. The air dehumidification is done by a liquid desiccant dehumidification and cooling system, sketched in Figure 271. Warm and humid outside air is cooled and dried in a special dehumidifier by a concentrated Lithium Chloride salt solution (LiCl-H20) before it is blown into the atrium of the building. From there several air handling units draw the air into the offices and provide additional cooling on demand. 

Cooling. A gas stream may be dehumidified by bringing it into contact with a cold liquid or a cold solid surface. If the temperature of the surface is lower than the dew point of the gas, condensation will take place, and the temperature of the surface will tend to rise by virtue of the liberation of latent heat It is therefore necessary to remove heat constantly from the surface. Because a far larger interfacial surface can be produced with a liquid, it is usual to spray a liquid into the gas and then to cool it again before it is recycled. In many cases, countercurrent flow of the gas and liquid is obtained by introducing the liquid at the top of a column and allowing the gas to pass upwards. 

Fresh air containing 4.00 mole% water vapor is to be cooled and dehumidified to a water content of l.70mole% H2O. A stream of fresh air is combined with a recycle stream of previously dehumidified air and passed through the cooler. The blended stream entering the unit contains 2.30 mole% H O. In the air conditioner, some of the water in the feed stream is condensed and removed as liquid. A fraction of the dehumidified air leaving the cooler is recycled and the remainder is delivered to a... 

Spray dehumidification. Warm humid air is dehumidified by spraying cold water into it. Pro vided that the liquid temperature is low enough, the air is cooled below its dew point, causing some of the water vapor in it to condense. 

Briefly, air is introduced by a large compressor to a cooling system, where it is cooled down to 3-5°C and most of the moisture is condensed. From there, the air is passed through a dehumidifier, such as silica gel, where the last traces of moisture are retained. Figure 22 shows Ballestra s air drying system. 

Controlling temperature and humidity of process air or ambient air is another unique application of membrane contactors. Membranes are used to humidify or dehumidify air by bringing air in contact with water or a hygroscopic liquid. Mass transfer in such processes is very fast since mass transfer resistance in the liquid phase is negligible. Heat transfer and mass transfer are directly related to these processes, since latent heat of evaporation (or condensation) creates temperature profiles inside the contactor. Some of the references in Literature are shown in Refs. [78-79]. Application of such processes has been proposed for conditioning air in aircraft cabins [80], in buildings or vehicles [81], or in containers to store perishable goods [82]. 

Most commonly, air is humidified by passage through a spray of water. Small quantities of air are easily dehumidified by adsorbing the water vapor with alumina or silica gel arranged in columns. These are mounted in pairs so that one can be regenerated while the other is in use. Alternatively, the air can be cooled below the dew point. Excess water vapor condenses and the cold saturated air is reheated. 

The approximate process path is illustrated in a psy-chrometric chart in Fig. 1. The process shown is for air being cooled and dehumidified from conditions of 95°F dry bulb (db), 75°F wet bulb (wb) to about 77°F db, 58 grains/lbmda. Initially, the dry bulb temperature of the moist air decreases, while the moisture content remains constant, as shown in Fig. 1. The dry bulb temperature continues to decrease as moisture begins to condense out of the air onto the cooling coil. 

DEHUMIDIFYING CONDENSERS. A condenser for mixtures of vapors and noncondensable gases is shown in Fig. 15.9. It is set vertically, not horizontally like most condensers for vapor containing no noncondensable gas also, vapor is condensed inside the tubes, not outside, and the coolant flows through the shell. This provides a positive sweep of the vapor gas mixture through the tubes and avoids the formation of any stagnant pockets of inert gas that might blanket the heat-transfer surface. The modified lower head acts to separate the condensate from the uncondensed vapor and gas. 

Warm saturated gas can be dehumidified by bringing it into contact with cold liquid. The temperature of the gas is lowered below the dew point, liquid condenses, and the humidity of the gas is reduced. After dehumidification the gas can be reheated to its original dry-bulb temperature. Equipment for dehumidification may utilize a direct spray of coarse liquid droplets into the gas, a spray of liquid on refrigerated coils or other cold surface, or condensation on a cold surface with no liquid spray. A dehumidifying cooler-condenser is shown in Fig. 15.9. 

Point A represents the gas that is to be dehumidified. Suppose that liquid is available at such a temperature that saturation conditions at the interface are represented by point B. It has been shown experimentally that in such a process the path of the gas on the humidity chart is nearly a straight line between points A and B. As a result of the humidity and temperature gradients, the interface receives both sensible heat and vapor from the gas. The condensation of the vapor liberates latent heat, and both sensible and latent heat are transfti red to the liquid phase. This requires a temperature difference 7] — 7 through the liquid. 

CONDENSATION - Process of changing a vapor into liquid by extracting heat. Condensation of steam or water vapor is effected in either steam condensers or dehumidifying coils, and the resulting water is called condensate. 

Figure 8.24 shows a typical heat pump FBD. The fluidized bed drying chamber receives wet solids and discharges dried product, whereas dehumidified and heated air is charged into the chamber from the bottom of the chamber. The drying tanperature can be adjusted by monitoring the capacity of condenser, whereas the desired humidity of inlet air can be obtained by controlling the motor frequency of compressor. 

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