I've looked at clouds from both sides now,
from up and down, and still somehow
it's cloud illusions I recall.
I really don't know clouds at all.

("Both Sides Now," by Joni Mitchell, 1967)

 

RELATIVE HUMIDITY

Acquiring knowledge about clouds is easy for anyone that chooses to exert the gray matter to learn:

All air contains a smidgen of water.

Near the surface of the earth, the water content in air is an invisible water vapor; the vapor is indistinguishable from air.

Above the earth's surface, the temperature drops and the water vapor turns into ice crystals .... which is what clouds are.

The concentration of water in air has a limit.

When the air cannot hold any more water, the logically named  dew point of the air has been reached. The term is logical because the definition of "dew" is "the formation of drops of water."

When the dew point of air has been reached, it starts to rain.

And air does not blow around holding the maximum amount of water that it can absorb all of the time ... otherwise it would be raining everywhere all the time.

The amount of water in air compared to the maximum amount of water that the air can hold is the Relative Humidity of the air.

The effectiveness of a cooling tower to cool hot water via evaporation greatly depends upon the Relative Humidity of the air ... which makes perfect sense:

The amount of room the air has to evaporate water into it depends upon how much water is in the air before the air enters the tower.

In summary,

The amount of water in air compared to the maximum amount of water that the air can hold is very relevant to cooling tower operations.

Low Relative Humidity (for example, 20%) means the air has room to evaporate more water into it.

High Relative Humidity (for example, 70-80-90%)  means that the air already has a lot of water in it and has less room to evaporate more water into it.

For cooling tower efficiency, it's all relative!

 

RELATIVE HUMIDITY AND AMBIENT AIR TEMPERATURE

Relative Humidity changes with the ambient temperature.

At higher temperatures, the Relative Humidity of air is low; there is room for the air to evaporate more water.

At lower temperatures, the Relative Humidity of air is high; there is less room for the air to evaporate more water.

The below graphic attempts to clarify by illustration how Relative Humidity increases as ambient temperature decreases.

PTOA Readers and Students should notice:

1.The amount dry air is the yellow part of the ball; its magnitude decreases as the ambient temperature decreases.

2.The amount of water vapor in the air is the blue part of the ball and its magnitude does not change as the temperature decreases; however, since the dry part of the ball decreases with temperature, the relative amount of water as percentage of the total ball increases.

Otherwise stated, the Relative Humidity increases as ambient temperature decreases and vice versa. 

Hey!

All this increasing and decreasing is confusing!

What does it mean in the real world?

 

The below graph shows how the Relative Humidity of air changes throughout a single day.

The Relative Humidity of the air (red line) changes directly opposite to the ambient air temperature (blue line).

 

At night when the ambient air temperature drops, the amount of water in the air compared to the maximum it can hold (aka the Relative Humidity) increases.

As the ambient air temperature rises in the daylight hours, the amount of water in the air compared to the maximum it can hold (aka the Relative Humidity) decreases.

Process Operators that understand the relationship between Relative Humidity and ambient air temperature are aware that cooling towers work more efficiently when the humidity of the air is lowest and that happens when the ambient temperature is hottest.

Perhaps a fan can be turned down during the hottest part of the day since the decreased Relative Humidity allows more water to be evaporated even at lower rpms.

Lower fan rpms may decrease drift losses which would then help decrease the concentration of heebie jeebies.

Decreasing the heebie jeebie concentration would decrease chemical injection rates and blow down which greatly helps the environment.

The above linked chain of actions and events illustrates how Process Operators who understand and track daily variations in Relative Humidity can operate cooling towers more efficiently. 

 

THE APPROACH DEPENDS UPON RELATIVE HUMIDITY

PTOA Readers and Students already know by heart that the cooling tower Approach provides the driving force for evaporative cooling.

The Approach is the difference in temperature between the cold water in the basin and the wet bulb temperature of the air that flows through the tower.

Approach (expressed in °F/°C) =

Temp of Cold Water in Basin  - Wet Bulb Temp of Cooling Air 

The wet bulb temperature is the normal, everyday temperature of the air corrected for Relative Humidity.

 

WET BULB TEMPERATURES AND RELATIVE HUMIDITY

Wow!

Relative Humidity is obviously important to cooling tower operations.

So how does the Outside Process Operator determine and monitor changes in Relative Humidity?

 

Wet Bulb Thermometers

Outside Process Operators will record ambient air dry bulb and wet bulb temperatures.

The term dry bulb temperature refers to the common, everyday way of measuring a temperature with a thermometer.

A wet bulb temperature is measured by first wrapping a common, ordinary thermometer in muslin and dipping it into water thus making a wet bulb thermometer.

The wet tip of the wet bulb thermometer is exposed to air which evaporates the water in the muslin.

PTOA Readers and Students that are reading the PTOA Segments in sequential order are already aware how evaporative cooling decreases temperatures and are not at all surprised to observe that the wet bulb temperature is lower than the dry bulb temperature.

The closer the wet bulb temperature and dry bulb temperatures become, the higher the Relative Humidity of the air.

When the dry bulb temperature matches the wet bulb temperature that means the air is at 100% Relative Humidity ... meaning the air cannot hold any more water ... and it will start to form droplets.

 

Once the wet bulb temperature is known, The Approach could be calculated by subtracting the wet bulb temperature from the temperature of the water in the basin.

 

HOW TO DETERMINE RELATIVE HUMIDITY 

Charts may be available for the Outside Process Operator to use the inputs of dry bulb and wet bulb temperatures to determine a dew point.

(Dew point might also be determined by local testing if ice is available).

Many Thanks to Ringbell Company in the UK for providing a tool to calculate the Relative Humidity from the Dry Temp and the Dew Point in the below link!  Be sure to use consistent units for Temperature!

Determine RH from Dry Temp and Dew Point

 

THE REAL WORLD

The unfortunate truth is that translating the theory of evaporative cooling into optimal tower operations is difficult and thus does not typically receive much priority at a processing facility.

 

Application of the evaporative cooling theory used to design the tower to daily operations would require every cooling tower Outside Process Operator on shift to be core competent in understanding Approach as well as have the interest of a meteorologist with respect to understanding local atmospheric conditions.

In addition, the Outside Process Operator would need expert knowledge on the total load placed on the tower which varies depending upon the throughput to each process chiller.

As water becomes a more valuable commodity perhaps more attention will be placed upon improving cooling tower operations; significant environmental benefits and decreased cooling tower operating expenses would be the outcome.

Until that time, at least PTOA Readers and Students who become future Outside Process Operators will understand the purpose behind recording wet bulb and dry bulb temperatures.

 

TAKE HOME MESSAGES: The Relative Humidity of air is the amount of water in the air compared to how much water the air can hold.

When the water content in air is at the maximum 100%, the air has reached the dew point and water droplets will start to form.

The Relative Humidity of air significantly impacts cooling tower performance because the amount of water that can be evaporated into the air depends upon how much water the air has before it flows into the tower.

The Relative Humidity of air varies inversely with air temperature; as the temperature increases, the Relative Humidity decreases and vice versa.

A wet bulb temperature is a dry bulb temperature that takes into account evaporative cooling to measure temperature.

When the wet bulb temperature equals the dry point temperature, the air is at 100% Relative Humidity and it will start to rain.

Tower Approach can be determined from the temperature of the cooling water in the basin and the wet bulb temperature.

Using a special chart, the input of wet bulb temperature and dry bulb temperature can determine the dew point of the air.

Using a different specialized chart or tool, the input of dew point and dry bulb temperature can be used to determine the Relative Humidity.

Process Operators who understand daily variances in Relative Humidity may be able to adjust air flow rates yet maintain the rate of evaporative cooling. 

Optimizing evaporative cooling in a cooling tower would no doubt be beneficial environmentally and economically; however, optimizing evaporative cooling would require each Outside Process Operator responsible for the cooling tower to possess core competency in understanding evaporative heat transfer theory as well as have expert knowledge of process load variances.

 

©2015 PTOA Segment 00077

PTOA Heat Transfer Focus Study Area
PTOA Process Industry Static Equipment Operations

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