The performance curves consist of a minimum 3 sets of three curves each, which are presented as a plot of wet bulb temperature as abscissa versus cold water temperature as ordinate with the cooling range as parameter. According to CTI ATC 105 (Acceptance Test Code for Water Cooling Tower), graphical scaling shall be incremented so as to provide a minimum of 0.5oF increments and no more than 5oF per inch for both wet bulb and cold water temp.

The curves shall fully cover the range of variables specified in ATC 105 as follows;

  • Wet Bulb Temperature: +/- 15oF from Design WBT
  • Dry Bulb Temperature: +/- 25oF from Design DBT
  • Cooling Range: +/- 20% from Design Range
  • Water Flow Rate: +/- 10% from Design Flow Rate

The performance curves could be prepared by the simple method and detail method, which shall be discussed later. The performance (= cold water temperature) prediction of cooling tower by means of the simple method is made by a few design parameters as well as water flow rate, L/G, KaV/L, range, cold water temp., wet bulb tem., and fan bhp, while the performance prediction by the detail method is requiring all the actual cooling tower dimensions, thermal rating conditions, and all the mechanical rating conditions.

Example 17-1: Plot the performance curve by the method of constant fan pitch and simple method using the example 16-1.

(Solution)
The cold water summaries for the previous result of calculating the tower performance are as follow and the performance curves are being plotted.

1) Performance Curve @100% of Design Water Flow Rate

2) Performance Curve @90% of Design Water Flow Rate

3) Performance Curve @110% of Design Water Flow Rate

The file used here is same as one used for example 16-1.

Example 17-2. Plot a set of performance curves presenting the cold water temperature vs wet bulb temperature by the constant fan pitch and the detail method using the same example 16-1.

(Solution)

The curves are plotted for 80, 100 & 120% of cooling range on 90, 100 & 110% of design water flow rate. Note that these performance curves are based on the constant fan pitch and are plotted by the detail method. For further details of calculation, download the file.

Please compare these results of cold water temperature obtained by detail method and simple method. You will see that the difference between these two results is very minor. So, the simple method is strongly recommended to use in practice.