How To Develop Cooling Tower Life Cycle Cost Analysis: Critical Design Condition Values
/By Chad Edmondson
In our last blog we ended with this cooling tower life cycle worksheet (shown below) with one data point complete, the 20-year life expectancy of our “example” cooling tower. Now let’s fill in a few more blanks, starting with information that would easily accessible to anyone endeavoring to select a cooling tower for a particular application. These include items unique to the application such as tonnage/gpm, wet bulb (w.b.) temperature, the entering and leaving water temperatures or range, etc. These values, which are now shown at the top of our worksheet, are based the cooling tower selection at design conditions.
We chose 95 degree entering temperature, 85 degree leaving temperature, and 78 w.b. because they are the basis of a nominal cooling tower ton according to CTI. Needless to say, if your design conditions are different, you will want to adjust these values.
Notice also that we have assigned the fan motor with a horsepower of 40 HP. At full load, the kw of a 40 HP motor is calculated as follows:
40HP x 0.746kw/hp = 29.84kw
This value has also been added to the worksheet.
The last data point (excluding the load profile) is one that is often, at least in part, overlooked as an operating cost for a cooling tower. That is the condenser water pump energy consumption which includes the head loss values for:
(1) the vertical lift between the pump discharge and the point at which water enters the tower, and
(2) the spray nozzles that are used to distribute water to the fill IF the tower is a counterflow tower.
For this example we are assuming that this is a crossflow tower that relies on gravity for the flow of water from the hot water basin to the cold water basin. We are using a lift value of 13.75 ft. and zero head pressure for nozzles since none exists on a crossflow tower. This point is really important to note and capture in the life cycle cost. A counterflow cooling tower requires additional pump head to pressurize the nozzles. For example, a 1500 gpm cooling tower requiring an additonal 2 psi for the spray header will add a considerable amount of energy costs which must be included in your life cycle costs.
All remaining values are dependent on the load profile we develop for the application. But before we go there (as we will in subsequent blogs), let’s step back and think about why we need to even create a load profile in order to compare cooling towers. What’s wrong with comparing cooling towers based solely on design conditions?
Most of us are well aware of the fact that cooling towers typically only operate at design conditions a few days out of the year. But that’s just part of the reason we need to incorporate load profiles into the above information in order to compare lifecycle costs. How you choose to operate cooling towers ( e.g. how you size and stage them), along with the load profile of a given climate, has a dramatic impact on operational costs. The life cycle method we propose captures these and other variables so that the engineer can make completey informed decisions with their clients.
