Well I woke up this morning
With the cold water, with the cold water, with the cold water.
Well I woke up this morning
With the cold water, with the cold water, with the cold.

("Cold Water," by Tom Waits, 1999)

 

Everything is going well in the morning at the process facility if the Cooling Tower is successfully operating and supplying cold water to all the process industry shell and tube HExes that are counting on cool water to pick up the heat from hot process streams.

The PTOA classifies Cooling Towers and Cooling Water Systems as temperature-decreasing process equipment because that is their ultimate functional purpose.

Like the car radiator and fin fan exchanger/condenser, the Cooling Tower injects heat into the atmosphere.

However a Cooling Tower is distinctively different than a car radiator or fin fan in an important respect:

The hot water that is returned to the Cooling Tower is cooled down via heat transfer by direct contact with colder ambient air!

To reiterate:

The Heat of Evaporation is removed from the hot water when it is directly contacted with colder air and the temperature of the water that remains is significantly colder.

 

COOLING TOWER STRUCTURE AND FLOWS

Hot Water Return Header

PTOA Readers and Students have already analyzed the service duty of the six shell and tube HExes that appeared in the familiar Benzene Plant PFD to the right.

PTOA Readers and Students will remember that three of the HExes (E-102, E-104, and E-105) used cold water on the tubeside to pick up heat from their respective hot shellside-flowing process streams.

 

The Hot Water Return Header Brings Hot Water to Tower

The water that exits the tubesides of E-102, E-104, and E-105 is too hot and must be cooled down to be useful as "cooling water" again.

The water that exits the tubesides of E-102, E-104, and E-105 joins the Hot Water Return Header that flows toward the Cooling Tower.

The Hot Water Return Header discharges the hot water into one of the two Hot Water Tanks located on both sides of the Cooling Tower's fan deck.

The below picture shows two uncovered Hot Water Tanks on either side of this Cooling Tower's Fan Deck. Most industrial Cooling Towers have covered Hot Water Tanks (like the one shown at the top of this page).

The Hot Water Flow Path

The hot water flows into Hot Water Pipes that have several Spray Nozzles that point downward.

The hot water is sprayed out over the Cooling Tower Fill.

Cooling Tower Fill literally fills the left and right sides of the wedgy Cooling Tower structure.

The center area of the Cooling Tower is not filled with anything but warm moist air flowing up to the top of the tower (the flow path of the air is illustrated below).

The purpose of Cooling Tower Fill is to provide a surface area that enhances heat transfer between the hot water and colder ambient air.

The corrugated structure of Fill forces the cold air to flow through nooks and crannies and take more time making direct contact with the hot water.

The type of Cooling Tower Fill chosen for use in the Cooling Tower depends upon the expected dirtiness and/or fouling tendency of the hot water. Most all the modern versions of Fill are sections of corrugated metal.

 

Cooling Tower Parts and Air Flow

The left-side graphic below labels the Cooling Tower hardware mentioned above. PTOA Readers and Students should identify the Hot Water Tank, Hot Water Pipe, Spray Nozzles, and Filler as well as the Fan Deck and Fan.

The right-side graphic above shows how the downward vertical flow of hot water (outlined blue arrows)  interacts with the horizontal and upward cross flow of cold air in the wedge-shaped Cooling Tower (hot pink arrows).

PTOA Readers and Students should notice:

• The hot water trickles downward through the Fill on both sides of the wedge-shaped Cooling Tower.
• Cool, dry ambient air is drawn into the Cooling Tower by the huge, whirring Fan located on the Fan Deck. The 'real world' jargon for this kind of air flow...drawing airflow into the fan blades.... is called "induction."
• The cool, dry air enters the side Louvers and flows through the nooks and crannies of the Fill, all the while making direct contact with the trickling water and becoming more moist and warm.
• Before exiting the filled area, the moist warm air flows through a Mist Eliminator that knocks water particles back into the filled area. The Mist Eliminator can be seen in the above left schematic as red slanting lines on the interior sides of the filled area.
• The moist, warm air is drawn into the Fan and is injected into the atmosphere on top of the Cooling Tower.
  

  The Lower Basin/Cement Tank collects the once-gain cold water.

• The now-cold water collects in the Basin/Cement Tank at the bottom of the tower.
• The water is pumped into the Cooling Water Supply Header which might be buried or might be visible at the discharge of the Cooling Water Supply Pump.
  

  Return and Supply Headers.

• The cooling water flows back to all facility users that need it; in the Benzene plant the cool water users are E-102, E-104, and E-105.

 

OK. GOT THE FLOW DOWN...
BUT HOW IS THE COOLING TOWER COOLING?

PTOA Readers and Students already know how a Cooling Tower works because they have felt how it works!

Consider the hot and sweaty dude to the left who is taking a break from playing basketball.

While the sweat evaporates into the atmosphere the body temperature becomes cooler; in northern climates it might even be necessary to jump into the shower before "chilling down" too much!

PTOA Readers and Students already know that thermal energy (aka heat) is needed to change the physical state of a liquid into a vapor...and that's what is happening when sweat is evaporated into the air and when the water in a Cooling Tower is chilled down by evaporation.

The energy required to evaporate the hot water cannot be sensed by a thermometer; nevertheless the remaining water trickling down the tower will be cooler.

Eighty percent of the cooling done in the Cooling Tower is due to evaporation.

That statement does not mean that 80% of the hot water is evaporated!

When 1% of the water in a Cooling Tower is evaporated, the temperature of the remaining water drops 10 °F (5.6 °C).

Wow! A small amount of evaporating water removes a lot of heat! The significant temperature drop compared to the small amount of water evaporated shows that it takes a lot of energy to change from a liquid into a vapor!

The water that evaporates must be replaced by fresh water. All circulation systems require Fresh Water Make-Up and something called "Blow Down" which is covered in detail in the next PTOA segment.

The remaining 20% of heat transferred from the water to the air is done by convective heat transfer which will be covered in future PTOA segments.

 

Take Home Messages: Cooling Towers and Cooling Water Systems are process temperature-decreasing equipment.

The heat is transferred by direct contact between the hot, trickling water and the cool, dry air. Eighty percent of the heat removed from the water is due to removing the Heat of Evaporation while the water is evaporated into a vapor.   

The hardware found in the Induced-Draft, Cross Flow Cooling Tower popular in industry include:

• Hot Water Return Header
• Fan deck and Fan
• Top Water Tank
• Hot Water Pipe with Downward Sprayers
• Side Louvers
• Fill
• Mist Eliminator
• Induced Draft Air Flow Area
• Cement Tank aka Bottom Cold Water Basin
• Fresh water makeup supply (discussed next)
• Blow down line (discussed next)
• Cooling Water Supply Pump
• Cooling Water Supply Header

 

©2015 PTOA Segment 00039
Process Industry Temperature Changing Equipment

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