Cooling range

Using Merkel s approximation and knowing the desired thermal performance, the flow rates, and transfer coefficient, can quickly be calculated. The difficulty with this method is that errors of >10% in can arise if the cooling range Tj — T2 is larger than a few degrees. 

Cooling range (hot-water temperature minus cold-water temperature)... 

Example 12 Application of Sizing Chart Assume the same cooling range and approach as used in Example 11 except that the wet-hiilh temperature is lower. Design conditions would then he... 

From Fig. 12-14, the water concentration required to perform the cooling is 1.75 gal/(min-fF), giving a tower area of 1145 fF versus 1000 ft for a 70°F wet-hiilh temperature. This shows that the lower the wet-hiilh temperature for the same cooling range and approach, the larger is the area of the tower required and therefore the more difficult is the cooling joh. 

Heat Load Amount of heat (in Btu) dissipated in a cooling tower. It is equal to the weight of water circulated per unit of time multiplied by the cooling range. 

E = rate of evaporation, gpm (if not accurately known, evaporation can be approximated by multiplying total water rate in gpm times the cooling range (°F) times 0.0008). 

CR = cooling range, °F, difFerence between hot water into tower and cold water from the tower, °F B = rate of blow-down, gpm. (Because an acceptable level of concentration has usually been predetermined, the operator is more concerned with the amount of blowdown necessary to maintain the concentration. Equation 9-132.)... 

Note for use Locate "cold water-cooling range point and connect to seiected wet buib temperature of air. (Line 1.) Then, through LVGa draw iine paraliel to iocate vaiue of KaV/L. (Line 2). The graph may be used in reverse to examine changing conditions on a given tower. 

Cooling range (cooling tower), °F Concentration of solution, lb solute/ft solution... 

Several methods for the evaluation of the MDF have been put forward, notably that processed by Tchebycheff, which gives a high degree of accuracy in the case of large cooling ranges. In the form in which it is most commonly used, it reads ... 

Cooling range Approach Design wet bulb Design dry bulb Water analysis - circulating - make-up... 

Calculations of this sort are only of importance to the tower designer. Manufacturers application data will give the cooling range or capacity in terms of wet bulb, inlet water temperature and mass flow [16, 19]. 

Evaporation losses are 1 % of the circulation for every 100 F of cooling range. Windage or drift losses of mechanical draft towers are 0.1-0.3%. Blowdown of 2.5-3.0% of the circulation is necessary to prevent excessive salt buildup. 

Unless specified, all water is untreated, brackish, bay or sea. Notes H = horizontal, fixed or floating tube sheet, U = U—tube horizontal bundle, K = kettle type, V = vertical, R = reboiler, T=thermosiphon, v = variable, HC = hydrocarbon, C) = cooling range At, (Co) = condensing range At. 

Evaporation losses are 1% of the circulation for evety 10°F of cooling range. Windage or drift losses of mechanical draft toweis... 

Increases in loading, cooling range and humidity all tend to improve cooling tower performance. Two basic types of hyperbolic towers are counterflow and crossflow, as shown in Figure 4.12. 

The selection of the cooling range depends on process characteristics and requirements, as well as the type of cooling tower under consideration. In general, cooling ranges are divided into three categories ... 

Approach and cooling range are also indicated in the diagram. Note that the smaller the approach, the smaller the area A-B-C-D and, consequently, the higher the investment cost. 

Evaporation losses, E, are fairly well predicted. The evaporation of 1 lb of water requires approximately 1000 Btu. This heat will cool 200 lb of water by 10°F. Therefore, the evaporation losses are approximately 1% of the circulation water rates for each 10°F of cooling range ... 

The manufacturer s proposal shall include performance curves of the tower, based on design fan horsepower, showing cold water temperature versus air wet-bulb temperature for 90%, 100% and 110% of design water flow. The curves shall cover cooling ranges of 15°F, 20°F, 25°F, 30°F and design range. 

NOTE Typically the cooling range designed into a cooling tower will be around 15 to 25°F (8.3 to 13.9°C)... 

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