And I can't forget, I can't forget
I can't forget ... but I don't remember what. 

("I Can't Forget," by Leonard Cohen, 1988)

PTOA SEGMENT 75: STILL SITTIN' HERE CHILLIN'

PTOA Readers and Students learned that optimizing heat transfer in temperature-decreasing equipment that relies upon evaporative cooling has a different driving force called The Approach.

The Approach is the temperature difference between the circulating water in the basin and the temperature of the flowing air (adjusted for humidity).

PTOA Readers and Students also learned that Process Operators are limited with respect to optimizing Approach (and ergo evaporative heat transfer) because they cannot impact the temperature of the flowing air nor its humidity.

However, Process Operators that know how to control the concentration of heebie jeebies in the circulating water to levels that do not signficantly impede convection, conduction, and evaporative heat transfer in the tower hardware and cooling water system will favorably impact the temperature of circulating water and ergo The Approach.

In addition, PTOA Readers and Students learned:

Cooling water system heebie jeebies come from a variety of sources which include airborne contaminants, scale from corrosion, scale from fresh water makeup with high concentrations of magnesium and calcium, precipitates, and living organisms.

Heebie jeebies that deposit on cooling tower fill and throughout the distribution piping and exchangers severely inhibit conduction, convection, and evaporative heat transfer.

This PTOA Segment 75 introduced PTOA Readers and Students to chemical injections that are used to eliminate heebie jeebies in cooling water systems.

Cooling water system chemicals include scale inhibitors, corrosion inhibitors, biocides, and anti-foulants.

A chemical injection skid  has a dedicated chemical tank, interconnected piping, pumps, and a distribution header to deliver each chemical where it is needed in the processing complex. For a cooling water system, the injections flow into the basin and/or supply pump suction.

Industrial processing facilities use a wide variety of chemical injections; in PTOA Segment 74, PTOA Readers and Students learned that poorly operated chemical injections can result in causing leaky fin fan tubes.

Industrial processing facilities use so many chemicals that a Chemical Company Representative may be accountable to optimally adjust chemical injection rates; otherwise the Outside Process Operator will be responsible for interpreting lab results and making the required injection adjustments.

The lab results that are used to assess cooling tower status include:

• Circulating water pH.
• The total amount of dissolved solids.
• Corrosion Inhibitor concentration.
• Biocide concentration.
• Precipitate concentrations.

PTOA Readers and Students learned that Outside Process Operators can greatly impact the mechanical readiness of the cooling tower.

Without a functioning fan in the cooling water tower, evaporative heat transfer diminishes to the level of air flow that natural draft can supply.

Without a functioning supply pump, cold water cannot be circulated back to the shell and tube heat exchangers in the cooling water system.

Alert Outside Process Operators must be vigilant for unusual noises, unexplained increases in power demand, and lubricant leaks which can hint that preventative maintenance is required on a piece of rotating equipment.

 

PTOA SEGMENT 76: MASTER PLAN

Way back in PTOA Segment 41, PTOA Readers and Students learned the classifications of cooling tower designs with respect to type of draft (induced, forced or natural), and the air/water interface (counter flow or cross flow).

This PTOA Segment 76 featured the design criteria decision making that is used for cooling towers.

PTOA Segment 76 informed PTOA Readers and Students what the compelling reasons were to choose between the type of draft and air/water interface in a cooling tower:

Natural draft towers have very high cooling water loads. They are typically used to cool the large amounts of water needed for heat sinks in power plants.

Due to their construction, natural draft cooling towers use counter flow with air flowing upward and hot water trickling downward.

Typically smaller industrial cooling water loads can be met by using mechanical draft in cooling towers.

The most efficient mechanical draft design is induced draft; forced draft designs are 20% less efficient because of their tendency to recycle some of the moist, hot air that has been injected into the atmosphere.

Obviously, cooling tower design engineers must determine the volume of cooling water that the tower can successfully cool which is also called the tower's "capacity."

The difference in temperature between the entering hot water and exiting cold water is the tower's "range."  An example of a cooling tower's range was featured in this PTOA Segment 76.

PTOA Readers and Students were already aware that the driving force for evaporative heat transfer is the "Approach" and that "Approach" is the difference in temperature between the temperature of the air (adjusted for relative humidity) and the temperature of the cool water in the basin.  An example of a cooling tower's Approach was featured in this PTOA Segment 76.

Determining the tower Approach requires the design engineers to take into account historical weather data on seasonal variations in wind velocity, wind temperature, and wind relative humidity.

Alert PTOA Readers and Students might have noticed that "Range" and "Approach" both use the temperature of the cooled water at the basin in their definitions.  This tie between "Range" and "Approach" helps cooling water design engineers iterate between tower size and number of fans needed after taking into account the local wind characteristics listed in the above paragraph.

More important than all of the above, this PTOA Segment 76 introduced the concept of determining "relative humidity" from a measurement called a wet bulb temperature which is determined with an instrument called a wet bulb thermometer.

The concept of "relative humidity" is sufficiently important to cooling tower operations to warrant its own PTOA Segment 77.

This PTOA Segment 76 also referred to the shell and tube heat exchangers that are part of a cooling water system as "chillers."

Shell and tube heat exchangers that use water as the cooling media (aka heat sink) are frequently referred to as "chillers" and sometimes "trim coolers."

 

©2015 PTOA Segment 00086
PTOA Deja Vu Review 3-7

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