Heat Exchangers Specific Applications and General Maintenance

 Heat exchangers Specific Applications and General Maintenance.

The heat exchangers we have seen up to now have all been used to heat, a process liquid in some instances.

However, the purpose of heat exchange is to remove heat from a fluid that requires cooling.

Suppose we wanted to convert steam generated by one process into feed water for another process.  To do this a shell and tube exchanger can be used as a condenser.

This cutaway will show how the condenser works.

Water enters the space between the head of the shell and the adjacent tube sheet and flows through the tubes to the opposite end.  Steam enters at the top of the shell and flows down and between the condenser's tubes.

When the steam is cooled it condenses to water and falls into the hot well at the bottom of the condenser.

Air also may be used to cool steam. Here a fan blows air across tubes that carry steam.

Although not as efficient as water the air absorbs heat from the steam and the steam condenses an air-cooled system may be used if water is scarce.

 

Another application for heat exchangers is as a Reboiler.

Reboilers are used in systems that heat and vaporize hydrocarbons.

This one is a kettle type reboiler. It is simply a shell and tube exchanger surrounded by an enlarged shell that accommodates vapor.

Let's examine the system.

This system includes a distillation tower a furnace and a kettle type reboiler.

The furnace heats oil which is pumped through the tubes of the reboiler.

Meanwhile liquid isobutane flows from the bottom of the distillation column into the shell of the reboiler.

The heat from the oil is transferred through the tube walls and vaporizes some of the isobutane.

The isobutane vapors are then channeled into the distillation column.

These vapors provide the heat needed for distillation to take place.

 The isobutane vapors lose heat,  Condense and fall to the bottom of the tower starting the cycle again.

 

Coolers are heat exchangers too.

As the name implies coolers lower the temperature of the liquid or vapor

In the next example kerosene is being cooled before going to storage.

Cooling water passes through the exchanger twice while the kerosene makes only a single pass.

As the kerosene moves through the exchanger it releases some of its heat to the water.

The horizontal segmental baffles keep the flow of the kerosene turbulent ensuring maximum contact between the kerosene and the tubes.

With this contact ensured the kerosene is cooled efficiently and can then be stored safely.

 

The final application that we'll look at is one that converts waste heat into valuable steam.

The heat exchanger used for this purpose is called a waste heat reboiler.

Let's look at a diagram of a typical waste heat system.

The main components are a steam drum a waste heat boiler and a distillation column.

Water from the steam drum is sent through the tubes of the waste heat boiler while hot oil from the bottom of the distillation column moves through the shell as water absorbs heat from the oil part of it is vaporized and turns to steam.

The steam carries small droplets of water through the boiler.

This mixture of steam and water droplets flows up into the steam drum

Here the two fluids are separated from one another.

The steam is sent to the plant steam system while the water is channeled back to the boiler.

 

 

As you know all the equipment involved in these applications requires periodic maintenance.

For this reason we will now discuss some basic operating problems and how to handle them.

Fouling is a common and destructive enemy of heat exchange.

A low-velocity flow can start the build-up of deposits on internal surfaces.

These deposits can cause changes in temperature and pressure.

The deposits have several sources. Process fluids may contain solid sediments.

Once they settle a restriction to the flow develops and the restriction then causes more settling.

 

Corrosion is produced when the metal of the exchanger interacts chemically with the process stream.

These deposits can break free eventually and foul the exchanger tubes.

The warm exchanger provides a comfortable environment for the growth of certain organisms.

The tube surfaces may become contaminated with algae.

 

There are several ways to control fouling in a heat exchanger.

One way is to add dispersants.

These will prevent insoluble materials like dirt from forming solid deposits.

Another control method is to add chemical inhibitors that will keep chemical reactions from taking place  Or anti-foulants can be added to the process streams.

These chemicals prevent biological growth.

After fouling has occurred, removal methods depend on the type and severity of the deposits.

Deposits on the outside of tubes can often be removed by hydro blasting.

A high-pressure stream of water that loosens and washes away the deposits. Streams of water or steam can also move the deposits from the inside of a tube.

If water and steam fail, chemical cleaning may dissolve the deposits.

 

If, however, the deposits resist both chemical cleaning and hydro blasting

The exchanger must be completely dismantled and the deposits scraped off.

At this point it may be best to re-tube the exchanger

As always, during shutdown and startup, workers must follow safety precautions

Nitrogen and other inert gases are used to purge the exchanger of hydrocarbons in air.

These gases can get trapped inside the exchanger where they interfere with the heat transfer process.

These gases can be released by venting according to the procedure specific to your unit.

Condensers are susceptible to problems caused by other gases as well.

One problem occurs when air leaks restrict the flow of water by causing vapor binding.  To eliminate vapor binding, open the vent in the water exit line.

Another problem is a reduction in cooling capacity.

This can occur when non-condensable gases are present in the process side of the condenser.

You can correct this type of problem by venting the process side of the condenser to release the trapped vapors.

Leaks are another important maintenance concern.

Whenever exchangers are down for cleaning,  they are almost always tested for leaks.

But before dismantling the exchanger, there are some preliminary tests you can perform.

One simple way to test for leakage is to take a sample of the lower pressure fluid and check it for contamination, If the fluids are very different in appearance, like oil and water, just looking at the sample should tell you if there is a leak.

This sample, for example, has a small amount of oil in the water line, a sure sign of a leak.

However, if the fluids are very similar in appearance, chemical testing may be needed to detect the results of a leak. This must be done in the lab.

 

If neither the visual nor the chemical tests are conclusive, a test using high pressure water may be done. This is called hydrostatic testing.

Before it can be done, the exchanger must be taken offline and drained.

If you're checking for tube side leakage, the tubes are filled with water under pressure. If the tubes or joints are leaking, the water will be forced through the leak points into the shell.

If tests indicate a leak, the exchanger must be partially dismantled to determine its source.

To find a leaking tube, the shell is filled with water under pressure. This water will enter the tube at the point of the leak and then run out the tube end. By watching the tube sheet, you can often tell exactly which tube is leaking.

These procedures will help ensure proper operations and warn of problems before the situation gets out of hand

In this program, you have seen that heat exchangers can meet the plant's needs efficiently depending on their design for both heating and cooling purposes. You have also seen the importance of regular maintenance.

“Thanks for Reading!”