Yes, I know
Boogie Chillen'

("Boogie Chillen," by John Lee Hooker & Bernie Besman, 1948)

 

OPTIMIZING HEAT TRANSFER IN COOLING TOWERS

PTOA Readers and Students who read the PTOA Segments in sequential order as intended are already aware of the form and function of industrial cooling water towers as described in PTOA Segments 39 through 41.

PTOA Readers and Students are aware that cooling towers are  temperature-decreasing process equipment which primarily cool by evaporating the heat of convection contained within the hot flowing water.

PTOA Readers and Students know that The Universe demands a temperature differential must exist prior to heat transfer taking place.

The new information begins here.

In a cooling tower, the Delta T is 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).

The ability of the cooling tower to keep on chillin' at any one point in time depends upon The Approach and the following list of parameters:

• The design of the tower.
• The velocity of the wind around the tower created by natural or mechanical draft, depending upon the design of the tower.
• The temperature of the air drawn into the louvers of the cooling tower.
• The relative humidity of the air (explained in the next PTOA Segment and part of the definition of Approach).
• The mechanical readiness of the induction fan, supply pump, and tower parts.
• The type and concentration of heebie-jeebies in the circulating water.

 

CAN WE TALK?

Process Operators can only impact the last two parameters on the above list.

Your Mentor has never understood why brick and mortar Process Technology and Operations programs require future Process Operators to become fluent in cooling tower design criteria that they have no control over.

This PTOA Segment will feature how Process Operators can favorably impact cooling tower and cooling water system effectiveness.

To be complete, the next PTOA Segment will cover the remaining parameters that most definitely impact the efficiency of a cooling tower to transfer heat out of hot water and into the atmosphere. However, these parameters are not impacted by Process Operators because they are out of his/her ability to control.

 

PROCESS OPERATORS CAN IMPACT COOLING TOWER MECHANICAL READINESS

When a piece of rotating mechanical equipment breaks down, the finger pointing between the Operations and Maintenance personnel begins.

Rotating mechanical equipment is a piece of equipment that has a moving or rotating part in it like a piston or shaft or a gearbox. Pumps, compressors, and turbines are all examples of rotating equipment.

Operations staff will claim the preventative-maintenance program for the device was inadequate; Maintenance staff will claim the start up, shut down, and normal operations of the equipment was poorly performed and resulted in the failure.

The two critical pieces of rotating equipment in the cooling tower are the cooling water supply pump and the mechanical draft fan (induced or forced). More than likely, there will be a backup source for critical rotating equipment because ... well ... the equipment is critical to successful operations.

The PTOA will thoroughly cover Rotating Equipment in a future PTOA Focus Study.

A general rule that applies at this juncture is for the alert Outside Process Operator to use audio keys to discern if the fan and pump are making other than the usually expected loud noises.  An additional whoosh or intermittent high-pitched sound may be the first warning of mechanical failure.

The alert Outside Operator should also be vigilant to observe if the driver being used for the pump and fan are consuming the expected amount of power given the circulating flow rate (for the pump) or rpms (for the fan).

An increasing demand on the driver is worth writing a work order for Maintenance to check out the rotating equipment while the back up is placed in service.

Increased use of lubricant and lubricant leaks are conditions that also warrant maintenance.

 

PROCESS OPERATORS CAN IMPACT HEEBIE JEEBIE BUILDUP 

Sources of Heebie Jeebies

The buildup of heebie jeebies in cooling towers and their circulating water systems is unavoidable; the best a Process Operator can do is manage the concentration of heebie jeebies.

The total concentration of heebie jeebies in circulating cooling water is derived from several contributing sources:

1. The cooling tower is open to atmosphere and purposefully drafts air to flow through it.

These conditions are ripe to entrain a wide variety of locally produced airborne heebie jeebies. For example, algae growth in the summer, autumn leaves in the fall, pollen in the spring.

The wood used to construct the tower also decays and adds to the debris suspended in the circulating water.

2. On top of that, add the fact that water is "The Universal Solvent" which dissolves anything ... including metal pipes.

The photo to the right is the sauce pan that PTOA Readers and Students boiled out over and over again while learning the difference between sensible and latent heat (see PTOA Segment  25).

Notice that hot water has dissolved some of the interior surface metal; the etched interior of the saucepan is due to missing metal.

In the home kitchen the metal particulates are scoured out.

In the process plant, the scum created by dissolving and corroding metal is continuously circulated as scale until it is blown down and ergo removed from the circulating water system. 

3. Scale buildup is exacerbated when the fresh water makeup ends up being "hard water" with a high concentrations of calcium and magnesium.

4. Always remember and never forget that the purpose of the cooling tower is to evaporate a percentage of circulating water.

Evaporation increases the concentration of suspended solids and then cooling causes some suspended solids to settle out and foul the fill and other surface areas.

PTOA Readers and Students are already aware that the type of fill selected for the cooling tower depends upon the expected dirtiness and/or fouling tendency of the hot water.

All the enumerated paragraphs above describe a system of circulating warm water with dissolved solids and debris ...  an environment that supports growth of living microorganisms!  Legionnaires' disease bacteria has been found in some (negligently operated) cooling water towers.

Left unchecked, all of the contributions to heebie jeebies will impair the rate of convection and conduction heat transfer that supports evaporative cooling.

An upcoming PTOA Segment will quantify the cost of decreased convection and conduction heat transfer that results when shell and tube heat exchangers are fouled by the heebie jeebies contained in circulating cooling water.

 

Managing Heebie Jeebie Concentrations

As stated above, heebie jeebies happen; the goal of the Process Operator is to keep the heebie jeebie concentration at manageable  levels.

Managing cooling water system heebie jeebies is just one of the many uses of chemical injections at a processing facility.

Somewhere in the processing facility a structure will house the chemical injection skids.

The skids have an easy-to-refill tank and pumps and piping to deliver the chemical to the basin of the tower.

The water in the cooling tower basin flows into the suction of the cooling water supply pump, thus the chemicals are circulated with the cooling water.

Adjusting the chemical feed rate may be the duty of the Outside Process Operator or the "Chemical Company Representative."

Processing complexes use such a wide variety of expensive chemicals that a Chemical Company Representative is usually assigned to "optimize" chemical usage.

"Optimize" is in quotes because the compensation for the Chemical Company Representative is usually tied to the consumption of chemicals.

PTOA Students who do not want a desk job, have people skills, and like making presentations would make great Chemical Company Representatives.

The list of chemicals injected into cooling water systems include the following:

Scale Inhibitors:

• Inhibit scale formation by chemical addition or water softening.
• Create chemical reactions that intentionally form precipitates which are more easy to remove from the system.

Corrosion Inhibitors:

• Reduces metal corrosion by laying down a protective film.

Biologic control:

• Bromine (and formerly chlorine) are added as biocides to reduce algae and other living organisms.

 

Anti-foulants:

• Dispersant chemicals that break up suspended solids.
• Filtration that removes precipitates and suspended solids.

 

Monitoring Cooling Tower Operations

The Outside Operator duties may include performing local testing or delivering water samples to the laboratory so that professional lab technicians can perform the water testing.

The typical list of cooling tower water tests includes:

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

The correlation of test results to normal daily cooling tower operations will be featured in a future PTOA Segment that will cover Cooling Tower Normal Operations.

 

TAKE HOME MESSAGES: The driving force for heat transfer in a Cooling Tower is called The Approach.

The Approach is the difference between the temperature of the cooling water at the basin and the temperature of the drafted air (corrected for humidity).

Process Operators can impact the heat transfer efficiency of cooling towers with respect to evaporating hot water into moist air by:

• Monitoring the rotating equipment.
• Monitoring and controlling the heebie jeebies that circulate with the water. 

The fan and supply pump are the critical rotating equipment in a cooling tower.

The heebie jeebies are from several sources and impact heat transfer by reducing convection and conduction.

Heebie jeebies are chemically controlled and monitored by Outside Process Operators and/or Chemical Company Representatives.

Cooling tower chemical injections include:

• Scale and corrosion inhibitors to inhibit corrosion and scale.
• Biocides to inhibit the growth of biofilms.
• Anti-foulants to disperse or intentionally precipitate foulants that are later removed by filtration.

 

©2015 PTOA Segment 00075
PTOA Heat Transfer Focus Study Area
Process Industry Equipment Operations

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