Basic Problems That Can Affect a Typical Heat Exchanger - Causes and Effects of these problems.

Basic Problems That Can Affect a Typical Heat Exchanger -     Causes and Effects of these problems.

Fouling is a term that's often used to describe the buildup of deposits on the internal surfaces of the heat exchanger.

When fouling occurs, the result is an additional layer of material that heat must pass through. This additional layer reduces the ability of the unit to transfer heat. Also, if the buildup becomes excessive, the flow of fluids through the unit may be restricted.

Fouling can be caused by many things. One common cause is impurities in the fluids passing through the heat exchanger.

For example, in a process that uses water, impurities such as calcium can come out of the water and form an additional layer of material.

Another source of fouling is small plants and animals that enter the heat exchanger. Many forms of algae and bacteria can live and grow inside the unit and form a layer of slime on the internal surfaces of the heat exchanger.

Gases dissolved in the fluids that flow through a heat exchanger can also cause fouling. For example, some gases react with the metal inside a unit to cause a type of corrosion.

The corrosion forms a layer that acts as an insulator, just like impurities or algae.

 

Different techniques can be used to minimize fouling.

For example, filters and screens can be used to remove particles from the fluids before they enter the heat exchanger.

Fouling can sometimes be minimized by adding chemicals to the fluids passing through a heat exchanger.

For example, chemicals such as chlorine are often added to cooling water to reduce the amount of algae or other organisms inside a unit.

In some situations, fouling can become bad enough to restrict the fluid flow. This problem may show up on the heat exchanger's instruments as an increase in the pressure drop or as a gradual decrease in the flow through the affected side of the unit.

Fouling may also affect the temperature of both fluids passing through the heat exchanger.

When fluid flow is restricted, the heat exchanger must be cleaned.

 

One way that heat exchangers can be cleaned is by using chemicals.

When this is done, a chemical solution is passed through the heat exchanger to dissolve the fouling on the walls of the tubes.

Fouling on tube walls can also be removed by scraping or by spraying with high pressure water or steam.

However, these methods require the heat exchanger to be shut down and taken apart.

In some heat exchangers, cleaning can be accomplished using a technique called backwashing. Backwashing is the reversing of flow through the heat exchanger. This technique is effective in temporarily dislodging materials from the ends of the tubes and the tube sheets.

 

Tube leakage is a problem that can seriously affect the operation of a heat exchanger. It's usually caused by the failure of a tube as a result of overheating, erosion or corrosion.

Erosion is the wearing away of tube metal caused by the flow of fluids or by solid impurities in the fluids.

Some fluids that pass through a heat exchanger may contain abrasive particles. As these fluids flow through the heat exchanger, the particles come into contact with the tube metal and erode the tubes.

Eventually, the tube wears away in a spot and a leak forms.

Corrosion chemically deteriorates tube metal to create a leak. Corrosion is caused by a chemical reaction between the metal in the heat exchanger and either the fluid passing through the unit or impurities in the fluid.

The corrosion weakens the metal until a leak forms.

 

The biggest problem that can result from leaks in a heat exchanger is the mixing of one fluid with the other fluid.

For instance, if cooling water mixes with oil in a lube oil cooler, the water could damage the equipment that the oil lubricates.

To prevent this type of damage from occurring, leaks must be detected. One way to check for tube leaks is to take and analyze a sample of the lower pressure fluid.

When a leak occurs, the high pressure fluid leaks into the low pressure fluid.

In some cases, you may be able to tell there's a leak by just looking at the sample. In other cases, a lab test may be required.

To prevent leaks in some applications, the process fluids pass through filters or strainers to remove impurities that can erode the tube metal.

Also, chemicals may be added to the fluids to control corrosion. Another method of controlling corrosion involves using a device called a sacrificial anode.

When a sacrificial anode is used, impurities in water tend to react more readily with the anode than with the metal of the heat exchanger. So the sacrificial anode corrodes while the heat exchanger is less affected. Heat exchangers that are used with water often have sacrificial anodes made up of zinc plates mounted inside.

 

When a leak does occur, some type of corrective action must be taken.

If only a few tubes in a heat exchanger have leaks, it may be possible to plug the affected tubes. This prevents the tube side fluid from passing through those tubes and effectively eliminates the leak.

However, plugging tubes reduces a heat exchanger's heat transfer capacity. If a lot of tubes are leaking, the unit will have to be shut down and the affected tubes will have to be replaced.

 

When air, non-condensable gases, or other vapors are trapped inside a heat exchanger, they can prevent the unit from operating efficiently. This is because the air or gas can either blanket the tubes or block the tubes off and prevent fluid from flowing through them. The effect is the same as the effect created by fouling. Less heat can be transferred across the tubes.

Gas trapped on the tube side of a heat exchanger can block off tubes and prevent fluid from passing through them. Gas trapped on the shell side can displace the shell side fluid at the top of the heat exchanger.

This can reduce the amount of tube surface area that is exposed to the shell side fluid and thus reduce the amount of heat that can be transferred.

There are many ways that air or other gases can get trapped inside a heat exchanger.

For example, this can happen during the startup of the unit. During startup, the heat exchanger should be vented to remove unwanted gases. If the vetting is not complete, gases will remain inside the unit.

Another source of gases is the process itself. In some situations, the process can produce vapor bubbles. As the process fluid passes through the heat exchanger, the vapor collects inside the unit. When maintenance is performed on process equipment, air may be trapped in the piping or shell as the equipment is put back together. When the equipment is restarted, the air can make its way to the heat exchanger and become trapped inside.

No matter how air or other gases get trapped inside a heat exchanger, they can cause it to become air bound or vapor bound. There are several symptoms to indicate this.

For example, when trapped gas blocks flow to some of the tubes, the outlet temperatures of the two fluids may change. This is because the gas is restricting flow into the tubes and the restricted flow causes less heat transfer surface area to be available.

The decrease in heat transfer surface area will cause the outlet temperature of the process fluid to increase since less heat will be removed from the fluid.

There are also situations where the pressure inside a heat exchanger may be affected.

For example, this can occur in heat exchangers that are used to condense process vapors.

In this situation, non-condensable gases partially fill the unit, reducing the amount of heat transfer area that the process vapor can come in contact with.

In turn, this reduces the amount of vapor that can be condensed. The flow of vapor will start to decrease and the pressure inside the unit will start to increase.

Regardless of how gases get into the heat exchanger, the unit must be vented to allow them to escape. Repeated venting may be necessary to ensure that the heat exchanger remains free of air and other undesirable gases.

 

However, excessive venting can cause its own problems. For example, each venting may allow a small amount of the process fluid to be lost from the process. This reduces the efficiency of the process.

Venting may be only a temporary fix. If the source of the gas is the process, there may be a problem with the process or with one of the components in the process.

In this topic, we looked at some of the basic problems that can affect a typical heat exchanger, including fouling, tube leaks, and air and vapor binding. We also looked at the causes and effects of these problems, and we saw how they can be dealt with.

 

Let's take a moment now and revise what we have learned.

Fouling can sometimes be minimized by adding chemicals to the fluids passing through a heat exchanger.

For example, chemicals such as chlorine are often added to cooling water to reduce the amount of algae or other organisms inside a unit.

Corrosion chemically deteriorates tube metal to create a leak. Corrosion is caused by a chemical reaction between the metal in the heat exchanger and either the fluid passing through the unit or impurities in the fluid. The corrosion weakens the metal until a leak forms.

Regardless of how gases get into the heat exchanger, the unit must be vented to allow them to escape. Repeated venting may be necessary to ensure that the heat exchanger remains free of air and other undesirable gases. However, excessive venting can cause its own problems. For example, each venting may allow a small amount of the process fluid to be lost from the process. This reduces the efficiency of the process.

 

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