|
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.
|
