Did you forget
Did you forget
'Bout me?

("Did You Forget?," by D. Lovato, N.,J., &K. Jonas, 2008)

 

PTOA SEGMENT 77:  IT'S  ALL  RELATIVE

PTOA Readers and Students learned how the effectiveness of a cooling tower to evaporate heat from water is significantly impacted by the Relative Humidity of the air that flows through the structure.

This PTOA Segment 77 also clarified why the Approach is the driving force for evaporative heat transfer. Approach is defined as the difference in temperature between the cold water in the basin and the humidity-adjusted temperature of the air (aka the wet bulb temperature).

Clouds were used as a visual representation to remind PTOA Readers and Students that all air has a bit of water in it; at higher altitudes the moisture in air changes into ice crystals and forms clouds.

PTOA Readers and Students should not have been surprised to learn that the ability for air to evaporate water vapor in a cooling tower depends upon how much water the air contains before it enters the cooling tower.

PTOA Readers and Students learned that the amount of water in air compared to the maximum amount of water that air can hold is defined as the Relative Humidity of the air.

PTOA Readers and Students also learned that:

Temperature impacts how much water air can hold.

The warmer the air, the lower the Relative Humidity and vice versa.

PTOA Readers and Students were shown a graph that illustrated how Relative Humidity increases during periods of cold temperatures (night) and decreases during periods of warm temperatures (day). A logical extension of this observation would be that Relative Humidity varies from season to season.

The dew point is reached when air is at 100% Relative Humidity ... meaning the amount of water in air is at the maximum for that temperature.

When the dew point is reached, it begins to rain.

PTOA Readers and Students learned in the previous PTOA Segment 76 that prudent cooling tower design must assume that the cooling tower operates at maximum capacity to attain the desired range between hot and cold water temperatures ... with the worst case of air conditions that historic weather data predicts could occur.

Consequently, unless the astute Outside Process Operator(s) make adjustments to fans and pumps to accommodate actual tower load and actual ambient air conditions, more than likely mechanical energy is being wasted on a daily basis.

Process Operators who understand how Relative Humidity impacts the ability for the induced air to evaporate heat from water are more apt to optimize use of induction fans. Air with low Relative Humidity can evaporate sufficient heat at lower induction air flow rates than more humid air can.

Prior to learning how to determine Relative Humidity, PTOA Readers and Students were introduced to the definitions/concepts of wet bulb and dry bulb temperatures and how to measure both of them.

The wet bulb temperature indication is always lower than the dry bulb temperature indication because evaporative cooling adjusts the dry bulb temperature downward in the process of generating a wet bulb temperature indication.

In the event the wet bulb temperature is equal to the dry bulb temperature, Relative Humidity is at 100%, the dew point has been reached ... and it should be raining on the Outside Process Operator.

PTOA Readers and Students learned the following procedure that can be used to determine the Relative Humidity of the air at the processing site:

• Record wet bulb and dry bulb air temperatures.
• Use the recorded wet bulb and dry bulb temperatures to determine dew point from a chart. Alternatively, if ice is available there is a simple experiment that can be done to determine dew point.
• Use the air temperature and dew point to discern the Relative Humidity from a look up table or online tool.

PTOA Readers and Students learned that optimization of cooling tower operations requires all of the cooling tower Outside Process Operators to understand:

• How wet bulb temperatures are used to determine Relative Humidity.
• How to use the intelligence about Relative Humidity to make changes in fan rpms.

This PTOA Segment 77 also included a paragraph that explained how changing one parameter in a controlled system (decreased inducted air flow through the cooling tower) impacts other parameters in the system (decreased drift loss, decreased blow down rate, decreased chemical release to treated water system).

The goal of all Process Operators is to understand how one component change in a system impacts other components in the system.

 

PTOA SEGMENT 78: A DAY IN THE LIFE OF A SHELL AND TUBE HEx

Using the example of one chiller (aka trim cooler) in a cooling water exchanger system, PTOA Readers and Students learned the interweaving, step-by-step mechanics of conduction and convection which swap heat between the hot and cold process streams that flow through all kinds of shell and tube heat exchangers.

PTOA Readers and Students were reminded that ... in the absence of radiant heat transfer and the combustion reaction ... the driving force for heat transfer in a shell and tube HEx is established by the Delta T created between the temperatures of the hot and cold process fluids (in this PTOA Segment the cold process fluid was cold water).

PTOA Segment 78 also included:

• A review of shell side and tube side flow through a shell and tube heat exchanger courtesy of the SHECO animated graphic.
• Illustration of a temperature profile though a heat exchanger; no surprise that the HEx temperature profile shows the hot process stream becoming colder and the cold process stream becoming warmer as both streams flow through the HEx.
• Definition and graphic examples of parallel flow and counterflow shell and tube heat exchangers.
• Examples of temperature profiles for parallel flow and counterflow shell and tube heat exchangers.

Understanding the above mentioned HEx structural designs and their respective temperature profiles was  necessary prior to informing PTOA Readers and Students that the counterflow HEx design is more efficient with respect to heat transfer for this reason: the temperature of the exiting cold fluid can be greater than the temperature of the exiting hot fluid in a counterflow design HEx.

Otherwise stated, the limiting factor to heat transfer in a counterflow exchanger is the temperature of the entering cold fluid; the limiting factor to heat transfer in a parallel flow exchanger is the temperature of the exiting cold fluid.

Process Operators will still experience industrial use of parallel flow HExes even though they cannot transfer as much heat as the counterflow design. PTOA Segment 78 did not clarify the below list of valid reasons to use a parallel flow HEx design (because Segment 78 was already way too long):

• Need for quick heat transfer.
• Need to limit the temperature of the exiting cold water.
• Phase change within the HEx.
• Structural limitations do not allow total counterflow path for  both fluids (a 2-pass tube bundle will have partial parallel flow).

PTOA Readers and Students applied their growing familiarity with conduction and convection heat transfer to ponder each conduction and convection step that is required to successfully transfer thermal energy (aka heat) indirectly from a hot fluid into a cold fluid while both fluids flow through a HEx.

First, the convection heat energy of the hot flowing fluid heats the interior of a tube in a tube bundle.

Second, the heat is conducted through the tube wall to the tube's exterior ... which is exposed to shell side flow.

Then the hot exterior tube temperature provides the Delta T required to transfer heat via convection into the cold fluid that is flowing on the shell side.

PTOA Readers and Students learned that even a small film of heebie jeebie gunk greatly decreases conduction heat transfer by:

• Effectively increasing the inside diameter (d) of a tube in a bundle tube with a low conductivity material (k).
• Effectively increasing the outside diameter (d) of a tube in a tube bundle and/or reducing the surface area (A) for heat transfer when low conductivity gunk (k) builds up on the exterior of tubes and between tubes.

Gunky buildup on tube sheets and inside the tubes of a tube bundle decreases convection heat transfer by reducing the mass flow rate of the flowing fluid.

By recording and comparing inlet and exit temperatures,vigilant Outside Process Operators will identify when heat transfer has been impaired in each HEx.

During turnaround, the tube bundle will be removed to a cement slab and hydroblasted to clean each tube internally and externally so that optimal conduction and convection heat transfer can be restored.

 

©2015 PTOA Segment 00087
PTOA Deja Vu Review 3-8

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