Technical college/ Baghdad 4th Year Week No. :- 11. The objectives of this lesson are to: Introduction:

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Gregory Horn
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1 Refrigeration Systems Theoretical hours: 2 Practical hours: 2 Units: 6 COOLING TOWERS First 10 minutes: review the last lecture. Then explain the new lecture, solve an example. Last 10 minutes review the lecture and ask if there are any questions. The objectives of this lesson are to: Technical college/ Baghdad 4th Year Week No. : Discuss basic components of cooling towers. 2. Present classification of cooling towers based on working principle and based on it s components. 3. Explain the Principles of the heat- and mass-transfer process in a cooling tower. Introduction: A cooling tower is a device in which recirculating condenser water from a condenser or cooling coils is evaporatively cooled by contact with atmospheric air. Most cooling towers used in refrigerating plants for commercial buildings or industrial applications for air conditioning are mechanical draft cooling towers. A mechanical draft cooling tower uses fan(s) to extract atmospheric air. It consists of a fan to extract intake air, a heat-transfer medium (or fill), a water basin, a water distribution system, and an outer casing, as shown in Fig. below. According to the location of the fan corresponding to the fill and to the flow arrangements of air and water, currently widely used mechanical draft cooling towers for HVAC&R can be classified into the following categories: 1. Counterflow induced-draft. 2. Crossflow induced-draft. 3. Counterflow forced-draft. Counterflow Induced-Draft Cooling Towers:- In a counterflow induced-draft cooling tower, as shown in Fig.(a), the fan is located downstream from the fill at the air exit. Atmospheric air is drawn by the fan through the intake louver or, more simply, an opening covered by wire mesh. Water from the condenser or recirculating water from the coil, or a mixture of the two, is evenly sprayed or distributed over the fill and falls down into the water basin. Air is extracted across the fill and comes in direct contact with the water film. Because of the evaporation of a small portion of the condenser water, usually about 1 percent of the water flow, the temperature of the water gradually decreases as it falls down through the fill countercurrent to the extracted air. Evaporated water vapor is absorbed by the airstream. Large water droplets entrained in the airstream are collected by the drift eliminators. Finally, the airstream and drift are discharged at the top exit. Drift, or carryover, is the minute water droplets entrained in the airstream discharged out of the tower. The evaporatively cooled water falls into the water basin and flows to the condenser. In a counterflow induced-draft cooling tower, the driest air contacts the coldest water. Such a counterflow arrangement shows a better tower performance than a crossflow arrangement. In addition, air is drawn through the fill more evenly by the induced-draft fan and is discharged at a higher velocity from the top fan outlet. Both higher exhaust air velocity and even velocity distribution reduce the possibility of exhaust air recirculation. Compared with the crossflow induced-draft cooling

2 tower, the vertical height from the installation level to the inlet of the water-spraying nozzles in a counterflow tower is greater and, therefore, requires a higher pump head. Crossflow Induced-Draft Cooling Towers:- In a crossflow induced-draft cooling tower, as shown in Fig.(b), the fan is also located downstream from the fill at the top exit. The fill is installed at the same level as the air intake. Air enters the tower from the side louvers and moves horizontally through the fill and the drift eliminator. Air is then turned upward and finally discharged at the top exit. Water sprays from the nozzles, falls across the fill, and forms a crossflow arrangement with the airstream. The crossflow induced-draft cooling tower has a greater air intake area. Because of the crossflow arrangement, the tower can be considerably lower than the counterflow tower. However, the risk of recirculation of tower exhaust air increases. Cooling towers: (a) counterflow induced-draft; (b) crossflow induced-draft; (c) counterflow forceddraft. 2

3 Counterflow Forced-Draft Cooling Towers:- In a counterflow forced-draft cooling tower, as shown in Fig. (c), the fan is positioned at the bottom air intake, i.e., on the upstream side of the fill. Condenser water sprays over the fill from the top and falls down to the water basin. Air is forced across the fill and comes in direct contact with the water. Because of the evaporation of the water, its temperature gradually decreases as it flows down along the fill in a counterflow arrangement with air. In the air stream, large water droplets are intercepted near the exit by the eliminator. Finally, the airstream containing drift is discharged at the top opening. Because the fan is located near the ground level, the vibration of the counterflow forced-draft tower is small compared with that of the induced-draft tower. Also, if the centrifugal fan blows toward the water surface, there is a better evaporative cooling effect over the water basin. However, the disadvantages of this type of cooling tower include the uneven distribution of air flowing through the fill, which is caused by the forced-draft fan. In addition, the high intake velocity may recapture a portion of the warm and humid exhaust air. Counterflow forced-draft cooling towers are often used in small and medium-size installations. The preceding types of cooling towers, which use the evaporation of water to cool the condenser water, are sometimes called wet towers. There is also a kind of cooling tower called a dry tower. It is essentially a dry cooler, a finned coil and induced fan combination that cools the condenser water flowing inside the tubes. Principles of the heat- and mass-transfer process in a cooling tower:- The explanation of the heat- and mass-transfer process in a cooling tower starts with the recollection of the straight-line law, which is state that when air is in contact with water, the change in air conditions is a straight line on the psychrometric chart directed toward the saturation line at the water temperature. This information is used to examine what happens to the enthalpy (heat content) of the air. If the enthalpy of air increases in the process, the enthalpy and temperature of the water must decrease. Consider first the special case shown in Fig (a) below, where the wet-bulb temperature of the air equals the water temperature. The path of the air moves toward the saturation line at the water temperature, which is along the wet-bulb temperature line. The wet-bulb temperature lines and the enthalpy lines are essentially parallel, so there is no change in the enthalpy of air, and the temperature of water does not change either. This is the process that takes place in evaporative coolers that reduce the air temperature in homes in arid regions. If the temperature of the water is higher than the wet-bulb temperature of the air, as in Fig (b), the enthalpy of the air increases from point 1 to point 2, so an energy balance requires that this heat must come from the water by cooling it from point 1' to point 2'. When these elementary processes are expanded to a complete counterflow cooling tower, they show the pattern of air and water conditions as in Fig (c). The air progressively increases in enthalpy, and while its dry-bulb temperature is shown decreasing in Fig (c) as it rises through the tower, there could be situations where the temperature increases in passing through the tower. 3

4 Fig. (a) When the water temperature is the same as the wet-bulb temperature of the air, there is no change in water temperature. Fig. (b)the enthalpy of air rises and the temperature of water drops when the water temperature is higher than the wet-bulb temperature of the air. Fig. (c) Conditions of air and water in a counterflow cooling tower. The key concept implicit in Figs (a) through (c) is that the leaving water temperature can approach the wet-bulb temperature of entering air. For this reason, catalog data for cooling towers show the ambient condition that affects cooling tower performance as the wet-bulb temperature, and dry-bulb temperatures may not even be indicated. When a constant heat load is imposed on the condenser and its cooling water, the leaving water temperature rides up as the ambient wet-bulb temperature increases in a trend as shown in Fig. below. Because the heat load and the water-flow 4

5 rate are constant, a fixed drop in water temperature (5 C or 9 F in this case) prevails over the entire range shown in the graph. Leaving water temperature from a cooling tower as the ambient wet-bulb temperature changes. The heat load and water-flow rate are constant. 5

Chapter 14, Problem 27.

Chapter 14, Problem 27. A house contains air at 25 C and 65 percent relative humidity. Will any moisture condense on the inner surfaces of the windows when the temperature of the window drops to 10 C?