Cooling Tower Factors: Dry & Wet Bulb Temperature, Range & Approach

When asked, most operators will provide the purpose of a cooling tower: to reduce the temperature of water to the lowest practicable point.  Pressed on the topic, however, most could not explain how the ambient temperatures and humidity affect the performance of the tower.  If the temperature was 70°F outside, the average operator might tell you that they would expect their CT to put out 70°F water.  But is this always true?  Is the tower operating efficiently?

Two important (and often misunderstood) factors that describe cooling tower performance are range and approach.  To understand these terms, one must understand the difference between “dry bulb temperature” and “wet bulb temperature.”

Temperature

Ask a friend to define temperature and you will likely hear something along the lines of “how hot or cold something is;” and this definition is good enough to get most of us through the day.  Regarding ambient temperature, most people perceive it to be what the thermometer reads or what the television weatherperson tells us.  But the television weatherperson always throws in ambiguous qualifiers: humidity, heat index, dew point.  These terms play an important part not only in how it feels outside but in how well cooling towers perform.

While that thermometer on the front porch does a good job of telling you one thing: dry bulb temperature, it doesn’t tell you how comfortable that temperature is right now.  And, we have all heard the people in Phoenix say, “sure it’s 110°F, but it’s a dry heat.”  After you roll your eyes and stop and think about it – what does that mean?

A typical outdoor thermometer is showing you what is referred to as the “dry bulb temperature.”  It does not take into account the relative humidity in the air.  Relative humidity is an expression of how much moisture is actually in the air compared to how much there could be at this temperature.  If the humidity is 100%, the air is completely saturated with water and no evaporation is possible.  This means that neither your facility’s cooling tower nor your body perspiring will enjoy the effects of evaporative cooling.  You will just sit there and sweat and your cooling tower will underperform one located in lower humidity at the same temperature.

To measure the effects of both the temperature and humidity together, we use a psychometric chart.  These charts combine the effects of humidity and temperature to calculate the “wet bulb temperature.”  The wet bulb temperature describes the effects of evaporative cooling on both your body and on cooling towers.  Using these charts, it is easy to see how a 95°F/30% humidity day in Phoenix can feel downright comfortable, while the 80°F/70% day in Atlanta is uncomfortable – you just don’t get the same evaporative cooling effect in Atlanta.  This is why most facilities measure ambient temperature and humidity.

Approach

Cooling tower approach is the difference in temperature of the water entering the basin (cold) and the wet bulb temperature.  For the purpose of tower design, a tower with a smaller approach (small delta between basin water temperature and wet bulb temperature) is considered superior.  Modern towers commonly have approach temperatures as low as 5°F.  While it is possible to have a smaller approach, it becomes cost-prohibitive since the size of the tower grows exponentially as approach lowers, which in turn requires more pumps and fans.  This leads to more auxiliary power usage and diminishing returns.

In a mechanical draft tower, the only variables that change approach are wet bulb temperature and heat load (the amount of heat removed by the tower).  If your facility has variable speed cooling tower fans, approach can be reduced by increasing fan speed and therefore taking advantage of more evaporative cooling.  If fan speed and heat load are maintained constant for a given wet bulb temperature, approach is constant.

Range

Put simply, range is the difference between the temperature of water entering the cooling tower and leaving the cooling tower.  It is determined by the heat load on the tower and the water circulation rate.  If pump speed is constant and heat loads are constant, the tower range does not change.  This means that for a clean, properly functioning tower, wet bulb temperature does not affect cooling tower range.  Consequently, in practice, for a given water flowrate and heat load, if wet bulb temperature increases, tower inlet and outlet temperature increase proportionally.  The result is an unchanged range.

Typically, cooling towers are designed to cool a specified maximum flowrate of water from one temperature to another at an exact wet bulb temperature.  For example, a designed tower may be guaranteed to cool 10,000 gpm of water from 95°F to 80°F at 75°F wet bulb temperature.  In this case, the range is 15°F and the approach is 5°F.  These design calculations are always done using average wet bulb temperatures at the site where the tower will be installed to ensure performance guarantees are met.

Summary

With an understanding of wet bulb temperature, range and approach, the operator should be able to understand the lowest theoretical basin temperature for the current temperature and humidity.  Using the cooling tower range, they can estimate how efficiently the process is functioning in comparison to cooling tower design.  Armed with their newfound knowledge of these properties, the operator can efficiently stage pumps and fans without trying to squeeze out one more degree of approach that is simply not physically possible.