Cooling tower water rates

Improving control of cooling tower blowdown (see Chapter 24) for evaporative cooling water circuits to increase the cycles of concentration and reduce the cooling tower blowdown rate. 

Characteristics of raw makeup and cooling tower waters, temperatures, maximum allowable temperature, flow rates available, and unit costs. 

To calculate the logarithmic-meaii temperature difference, the terminal temperatures of the condenser must be fixed. Because the condensation is essentially isobaric, the inlet and outlet ten eratures of the ammonia stream are 41.4°C (106.5 °F). From Table 4.1, the inlet cooling-water temperature is 30°C (86.0 °F) if cooling-tower water is used. Also, for thermodynamic considerations the exit water temperature must be less than 41.4°C, and it is calculated from Equation 4.7.6. If the lower value of the approach tenqjerature difference of 5 C (9.0 °F) is selected from Table 4.4, a low cooling-water flow rate will be needed. Thus, exit water temperature is 36.4°C. Therefore, from Equation 4.7.5, the logarithmic-mean temperature difference,... 

Treatment of the spray water is comparable to that of cooling tower water. Chromate treatment is not required polysulfate treatments are adequate. Only a small amount of water is required to maintain the evaporative cooling cycle. The makeup water rate is determined by the amount of water that is lost due to evaporation, blowdown, and entrainment. Higher cycles of concentration can be achieved with evaporative-cooled equipment than with cooling towers. 

Cooling-tower system—includes a cooUng-tower and pipe system to transfer cooled water to the unit and back to the cooling-tower water-distribution system. The cooling tower has a series of complex instrument systems to control ppm, pH, level, temperature, fan speed, and flow rate. 

The thermal design of cooling towers follows the same general procedures already presented. Integration of equation 35 is usually done numerically using the appropriate software, mass-transfer coefficients, saturation enthalpies, etc. In mechanical-draft towers the air and water dows are both suppHed by machines, and hence dow rates are fixed. Under these conditions the design procedure is straightforward. 

For most cooling towers in the United Kingdom, the exit air is saturated at a temperature close to the mean water temperature in the tower. Hence, if the water temperatures and the air inlet conditions are known, AH, AT, and AT can all be calculated, and Tcan be deterrnined. It was found that the quantity C was approximately constant for these towers, ca 0.4—0.5 (34). If the value of C is known for a given tower, then the left side of equation 49 can be computed and, setting this equal to Z9, the allowable Hquid flow rate can be found. Alternatively, when and air-inlet conditions are given, the... 

To simulate the next summer s condition the plant was run at the desired production rate and two cooling tower fans were turned off. It turned out that the cold water temperature rose to slightly above that predicted for the next summer. A thorough inspection of critical temperatures and the plant s operation indicated that the plant would barely make it the next summer. Process side temperatures were at about the maximum desired, with an occasional high oil temperature alarm on the large machines. 

Petrochemical units generate waste waters from process operations such as vapor condensation, from cooling tower blowdown, and from stormwater runoff. Process waste waters are generated at a rate of about 15 cubic meters per hour (m /hr), based on 500,000 tpy ethylene production, and may contain biochemical oxygen demand (BOD) levels of 100 mg/1, as well as chemical oxygen demand (COD) of 1,500 to 6,000 mg/1, suspended solids of 100 to 400 mg/1, and oil and grease of 30 to 600 mg/1. Phenol levels of up to 200 mg/1 and benzene levels of up to 100 mg/1 may also be present. 

Calculating the heat transfer and water evaporation rates are illustrated by the following example. A cooling tower eools 900 gpm of water from 95 to 85 F. The problem is to determine what the heat rejeetion is, and also what is the evaporation rate. The heat rejeetion is ealeulated as follows ... 

Another type of crossflow cooling tower is the wet-dry tower, which consists of a normal crossflow tower over which a few air coils are placed. The hot water is first cooled by an air cooled heat exchanger and then drops to the wet cooling tower where more cooling is obtained by the evaporative mechanism. Figures 5 and 6 provide examples. In contrast, deck-filled towers contain tiers of splash bars or decks to aid in the breakup of water drops to increase the total water surface and, subsequently, the evaporation rate. 

The economics of forced and induced draft cooling tower operation require a study of fan and water pump horsepower and usually dictate a fan static pressure requirement not to exceed 0.75-1.0 in. of water. For atmospheric and natural draft towers the economics of pumping water are still very important. This means that the ground area must be so selected as to keep the height dovm while not dropping the unit rates so low that performance becomes poor. This then, is a balance of ground area versus total deck height. Pritchard [16] presents an... 

Performance, 387 Ground Area vs. Height, 391 Pressure Losses, 393 Fan Horsepower for Mechanical Draft Tower, 392 Water Rates and Distribution, 393 Blow-Down and Continuation Build-Up, 394 Example 915 Determining Approximate Blow-Down for Cooling Tower, 395 Pre-... 

In a cooling tower, cooling of the main mass of water is obtained by the evaporation of a small proportion into the airstream. Cooled water leaving the tower will be 3-8 K warmer than the incoming air wet bulb temperature. (See also Chapters 24 and 25.) The quantity of water evaporated will take up its latent heat equal to the condenser duty, at the rate of about 2430 kj/kg evaporated, and will be approximately... 

It will be seen that the water and air mass flow rates over a cooling tower are roughly equal. 

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