I've got a master plan, babe
I been workin' on it from the start.
Pluggin' in all of the numbers.
Watchin' it on all of the charts.

("Master Plan," by My Morning Jacket, 2003)

 

DESIGNING FOR EVAPORATIVE HEAT TRANSFER 

PTOA Readers and Students know that the cooling tower featured in the photo to the left is no longer able to evaporate heat from hot water and return cold water to chillers (the category of shell and tube heat exchangers that use cold water as a heat sink).

PTOA Readers and Students recently learned actions Outside Process Operators can take to a prevent cooling tower failures like the one shown in the above picture.

This PTOA Segment covers choices that were made about the cooling tower before the Process Operator began working at the facility ...  yes, perhaps even before the Process Operator was born.

The master plan for cooling tower design decision-making includes:

• The design and size of tower(s) that have the capacity to supply the  expected rate of cooling water to the chillers.
• The velocity of the wind around the tower.
• The temperature of the air drawn into the cooling tower louvers.
• The relative humidity of the air (explained in the next PTOA Segment).

This PTOA Segment features the design criteria used for master plan decision making related to the top two enumerated items.

Process Operators arrive after-the-fact and cannot impact any of the listed design criteria.

Although Process Operators cannot impact the master plan design of the cooling tower, understanding the design criteria can result in more efficient use of rotating equipment to optimize evaporative heat transfer.

 

DECISIONS, DECISIONS

 

Natural Draft or Mechanical Draft Tower?

The volume and  rate of cooling water needed for the chillers or jacketed reactors will determine if the structure should be natural or mechanical draft.

The Capacity of the cooling tower is the volume of water the cooling tower can cool.

In future PTOA Segments, PTOA Readers and Students will learn that the process of creating electrical power also  creates mucho heat.

The heat must be continuously removed and water is a great heat sink for that job.

The kitchen trashcan holds 13 gallons.

The capacity of natural draft cooling towers is 500,000 gallons per minute (gpm) and greater.

That is a lot of circulating water, like 38,462 kitchen-size trashcanfulls every minute!

Most non-power-generating process industries just need a cooling water capacity rate in the range of 500 gpm (38 trashcanfuls per minute) and therefore install mechanical draft cooling towers.

 

How Many Fans Should be in the Tower Bank?

Cooling tower designs include determining the design Approach and Cooling Range.

PTOA Readers and Students recently learned that The Approach is the driving force that determines how efficiently the evaporating process will remove heat from the circulating water.

The Approach is the difference in temperature between the cold water in the basin and the wet bulb temperature of the air used to evaporate the hot water.

The wet bulb temperature is the ambient temperature corrected for Relative Humidity and is discussed in detail in the next PTOA Segment. So stay tuned!

The cooling tower must be designed for an Approach that covers the variances in ambient temperature and Relative Humidity throughout the year.

If the wet bulb temperature of the air is 72°F and the temperature of the cooled water is 85 deg F, then The Approach can be calculated:

APPROACH: 85°F - 72 °F = 13 °F

PTOA Readers and Students already know when the driving force for heat transfer decreases(say, in wintertime) then there is less driving force for evaporation to occur.

Loss of Approach may very well mean that the  the circulating water is at a sufficiently low temperature for the chillers without additional evaporative heat loss.

During those times Process Operators can decrease fan rpms and/or perhaps adjust supply pump circulating rates to re-establish Approach more economically.

In some locations it may even be necessary to insert a heater in the basin water to keep ice from forming during cold winter months.

And what about Cooling Range?

The Cooling Range is the difference between the temperature of the hot water entering the tower and the cool water leaving the tower.

For example, if the temperature of the hot water returned to the tower is 100 °F and the temperature of the cold water pumped into the cold water supply header is 85 °F, then the Cooling Range for the tower is:

Cooling Range is 100 °F - 85 °F = 15 °F.

The number of induction fans needed to successfully cool water is determined from the expected capacity needed for the tower at full load and the expected temperature of the hot water return.

A compatible design that works well for the desired Cooling Range and Approach will likewise establish the desired cooling water supply temperature.

Once the desired cold water supply temperature is known, the size of the tower and number of fans required for the evaporative cooling at maximum planned load can be determined.

 

What about Air Velocity?

Air velocity is dang important considering it contacts the hot trickling water in the process of evaporating it.

For a natural draft cooling tower, the height and construction of the tower neck will establish draft velocity.

At the time this PTOA Segment was written, the picture to the right shows the world's biggest cooling tower.

Process facility cooling towers are lower capacity and rely upon mechanical draft to induce or force air through the structure.

Fans at the top of the cooling tower induct air to flow through the structure and fans placed toward the bottom sides of the cooling tower force air through the tower.

Forced air cooling towers are 20% less efficient than induced draft cooling towers because they  suck the hot air vapor that exits the top of the tower back into the fan.

 

What's the best Air/Water Flow Pattern?

Counter flowing air and water in a natural draft cooling tower.

PTOA Segment 41 showed that the air and water flow counter-current to each other in natural draft towers.

In counter-flow designs, cool air flows upward while hot water trickles downward.

The most efficient mechanical draft cooling tower design is the induced-draft cross-flow model shown below.

 

TAKE HOME MESSAGES: Temperature-decreasing process industry equipment that utilizes evaporative heat loss for cooling has a different "approach" to removing heat (pun intended!).

Although Process Operators cannot impact the design of the cooling tower, understanding the design criteria can result in more efficient use of rotating equipment to optimize evaporative heat transfer.

PTOA Readers and Students need to be aware of the meaning of these new terms that are related to evaporative heat transfer:

• Cooling Tower Capacity
• Cooling Tower Approach
• Cooling Tower Cooling Range
• Wet Bulb Temperature
• Relative Humidity
• chillers

©2015 PTOA Segment 00076
PTOA Heat Transfer Focus Study Area

You need to login or register to bookmark/favorite this content.