I will be there and everywhere
Here, there and everywhere

("Here, There, and Everywhere," by the Beatles, 1966)

 

PTOA Readers and Students have finished the PTOA Segments that focus on how radiant heat is generated and transferred in a processing complex.

The radiant heat generated in fired heaters, boilers, and reaction furnaces is easy to observe ... just open a peep hole and there it is!

However ...

Here .... there ... and everywhere throughout the industrial complex alert Process Operators will notice how process lines and equipment are intentionally assembled to accommodate first the generation of thermal energy and afterward the efficient management of that heat to attain desired process temperatures.

This PTOA Segment features how heat transfer is managed through process fluid piping and in fin fans.

Here ... there ... everywhere in an industrial complex a high priority is given to managing the Process Variable Temperature via heat transfer because 30 cents of every dollar spent to stay in business is used to attain and maintain the temperatures needed to convert feedstocks into products.

 

THERE IS ENERGY FLOWING THROUGH THOSE PIPES

PTOA Readers and Students already recognize the above picture of a  fluid flowing from left to right through a glass pipe.

Looking through a transparent pipe makes it easy to imagine a process fluid with mucho BTUs or Joules of thermal energy (aka heat) flowing through a process.

The PTOA Department of Redundancy Department thinks the above statement is worthy of expressing a different way:

Process Operators must realize that each of the pipes in an industrial complex is moving thermal energy contained in a flowing fluid from one place to another.

In the real world hot process fluids flow through industrial  pipes that are fabricated of non-transparent metal.

PTOA Readers and Students already know that fluids flowing through pipes cannot convect the heat contained within to the open atmosphere as a cup of coffee would.

Regardless, The Universe still demands that the thermal energy (aka convected heat)  flowing through the pipes must be equalized with the ambient air temperature.

The Delta T between the hot process fluid and the cooler ambient air is a considerable driving force to conduct heat through the process tube wall and into the ambient air.

 

INSULATED PIPES CONSERVE HEAT

To reduce the rate of heat loss via conduction, process pipes will be insulated.

The silver pipes shown in the above paragraphs are wrapped with an insulating material, a material that has a low conductivity factor ("k").

PTOA Readers and Students are already aware that materials with a low conductivity factor will slow down the rate of conductive heat transfer.

The insulated pipe is then covered with an aluminum wrap that helps weatherproof the insulation and is resistant to atmospheric corrosion.

The additional layers of insulation and aluminum weatherproofing also increase the effective thickness of the pipe which also helps decrease the rate of conductive heat transfer from the hot fluid to the ambient air surrounding the pipe.

 

BARE NAKED PIPES CAN HELP CONSERVE OR LOSE HEAT

Not all pipes are insulated, of course.

The tubes in a fired heater/boiler/reaction furnace are not insulated because the whole purpose of purchasing fuel and igniting it is to  to increase the process fluid temperature via the combined flow of energy via radiant → conduction → convection heat transfer.

Guess what?

 

Pipes used for the complete opposite intention of cooling down a process stream via wasting heat will not be insulated either.

The finned tubes of Fin Fans are not insulated because their purpose is to get rid of the heat and lower the temperature of the process fluid flowing through the pipes.

The hot water return header that delivers hot water to the cooling tower will not be insulated; conductive heat loss to the atmosphere is desirable because less electrical energy to spin the fan will be needed for evaporative cooling.

Heat transfer in cooling towers is the subject of upcoming PTOA Segments.

 

HEAT TRANSFER IN FIN FANS

PTOA Readers and Students learned the structure and function of Fin Fans  in PTOA Segment 38.

Now PTOA Readers and Students can combine their knowledge of conduction and convection heat transfer to understand how a fin fan fundamentally accomplishes the task of intentionally wasting heat and lowering the process stream temperature.

 

• The Heat of Convection is contained in the hot fluid that flows through the header and thence into the multiple layers of finned tubes.
• Separating the flow into multiple tubes increases the surface area (A) available for conductive heat transfer from the flowing hot fluid through each tube wall.
• The Delta T between the hot fluid flowing nearest the interior tube wall and the temperature of the ambient air that surrounds the outside of each tube provides the driving force for cooling the flowing fluid.
• Conductive heat transfer is enhanced by using finned tubes; fins increase the surface area (A) available for conductive heat transfer through very thin walls (d).
• The forced air fans (or induction fans) blow the hot air in waves of convected heat into the ambient air above the bank of fin fans.

The heat is thus intentionally wasted because the next processing step requires a lower temperature.

As stated in PTOA Segment 38, even on the coldest of days the convected heat waves can warm the hands of any Process Operator willing to climb the ladder up to the fin fan.

 

IMPAIRED HEAT TRANSFER IN FIN FANS

Over time, Process Operators will notice a bank of fin fans cannot reduce process temperatures as needed for the next processing step or storage. The cause of impaired heat transfer in the fans may be:

Externally Clogged tubes

Atmospheric debris  packed in between the tubes of a fin fan has exactly the same effect as soot packed in between the convection section tubes in a fired heater:

1. The debris increases the effective pipe thickness (d) which all PTOA Readers and Students know by heart will decrease the rate of conductive heat transfer.

2. The atmospheric debris ... be it leaves ... or bird poop ... or pollen will not have a high conductivity factor (k). The effect of wedged in, low conductivity debris is that the finned tubes will be insulated from conductive heat transfer.

3. The surface area available for heat transfer (A) ... the reason fins were separated into tube flow in the first place ... is reduced.

Cleaning the debris out of fin fans cannot be performed while the fan is online and is added to the plant Turnaround maintenance list.

Cleaning fin fans may require physically lifting and moving each bank to a slab for hydro cleaning or can be done in place by specialists that have the know-how to clean fans as shown in the photo to the above right.

 

Too many plugged tubes

Outside Process Operators will notice a leaking tube from time to time.

In some fin fan applications, a leaky tube would warrant a process shutdown in the event the leaky fluid is hazardous or easily combustible.

Most Plant Owners choose fin fan designs that allow intentional plugging of leaky tubes using a procedure that does not require a total plant shutdown.

Logically, process fluid cannot flow through plugged tubes. The capacity of the fin fan bank is permanently reduced until the tubes are replaced.

When too many tubes are plugged there is insufficient surface area for conductive heat transfer even at lower process flow rates.

 

Poor Operations and Corrosion cause Leaky Tubes

Leaky fin fan tubes can be caused by a chronically corrosive environment.

Assuming the metallurgy of the tubes is designed to be compatible with the process, the cause of leaky tubes is probably due to poor operations that originate from chemical injections taking place much further upstream.

Chemical injections are typically added to process stream to adjust pH, mitigate pipe corrosion, maintain the activity of catalysts, etc.

Chemical injection rates must be adjusted when process flow rates are adjusted; elevated levels of chemicals that are not consumed as intended can precipitate at cooler temperatures and initiate corrosion.

 

Loss of fan (driver)

A mechanical failure of the induction or forced air fan is not the fault of Process Operators.

The maintenance department will have preventative maintenance scheduled for fin fan electric motors, reducing gears, belt drives, etc.

 

TAKE HOME MESSAGES:  Managing the Process Variable Temperature requires a Process Operator to understand how heat transfer is managed throughout the processing complex.

The Process Variable Temperature receives much attention because the cost to generate processing temperatures is at least 30 cents of every dollar spent keeping the facility operating.

Each pipe in a processing complex contains flowing fluid with the energy of convective heat.

Insulation is used to reduced the rate of conductive heat loss through pipes.

Insulation is made of a material with a low conductivity factor, k.

Insulated pipes are typically covered with a weatherproofing aluminium wrap.

Insulation and the aluminum wrap also increase the effective pipe diameter which decreases conductive heat transfer.

In temperature-decreasing process equipment, pipes are not insulated so that conductive heat loss to the atmosphere will be optimized.

The heat transfer through fin fans can be reduced by:

• Externally clogged fins.
• Too many leaky tubes that have been intentionally plugged.
• Fan failure.

Leaky tubes may be caused by not adjusting upstream chemical injection rates to accommodate changes in process flow rates. At cooler temperatures injected chemicals can precipitate and cause fin fan tube corrosion.

 

©2015 PTOA Segment 00074
Process Industry Equipment Troubleshooting Operations

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