Start-Up and Shutdown OPERATING AND MAINTAINING HEAT EXCHANGERS

A mixture of flammable fluids and air is dangerous because of the possibility of explosion. So before adding a

liquid or a gaseous flammable fluid to an exchanger, inert gas or steam is used to purge air from the exchanger.

The shell and tube bundle of an exchanger may be made of different metals that react differently to temperature changes. In this case, the shell and tube bundles expand at different rates when heated to a particular temperature. Then, the metals are subjected to stress.

A sudden temperature change causes _______ expansion or contraction.

Moderate.

Rapid.

Answer

The tube bundle and shell experience more stress than usual. As a result, tubes can be loosened from the tubesheets, or tubes can be broken.

Cold fluid should never be introduced suddenly into a hot exchanger. And, a hot fluid should never be introduced

suddenly into a cold exchanger.

During start-up and shutdown, temperature changes should be made slowly. During start-up, introduce the cooling fluid first. Then, gradually add the hot fluid, and the exchanger will be brought down to operating temperature.

During shutdown, the flow of hot fluid is shutdown first. With no input of hot fluid, the exchanger will gradually cool. Then, the flow of cold fluid is stopped.

The exchanger should not be valve closed while it is full of fluid. Just like a solid, a liquid expands when it is heated. This is called thermal expansion. When a liquid expands, its volume increases. If the expanding liquid is enclosed, it exerts force on its container. So, a filled exchanger that is valve closed can be damaged by expanding fluid.

So, the exchanger must be drained before being valve closed.

If the outside temperature is low, water left in the exchanger tubes can

freeze. When water freezes, it:

Contracts.

Expands.

Answer

Expansion puts excess pressure on the inside of the tubes and excess pressure can cause damage. For this reason, you must consider the temperature surrounding the exchanger. It is usually best to completely drain the exchanger.

Water in the tubes can also freeze as a result of the rapid depressurizing of light liquid in the shell-side of the

exchanger. If a light liquid is suddenly depressurized, it evaporates. As a liquid evaporates, it cools the surface from which it evaporates.

The evaporating liquid _______ the inside walls of the shell and the outside

of the tubes.

Heats.

Cools.

Answer

So, the evaporating liquid cools the inside walls of the shell and the outside of the tubes. Sudden cooling causes water in the tubes to freeze.

Just like in the start-up operations, you must take care to avoid potentially explosive mixtures of air and

flammables during shutdown. So, you must purge some exchangers of air with steam or inert gas after shutdown.

 

 

 

 

 

 

 

Operating Pressure and Temperature OPERATING AND MAINTAINING HEAT EXCHANGERS

Every exchanger is designed to operate at a pressure and temperature listed on a plate attached to the

exchanger. When the exchanger is operated at a pressure higher than the rated pressure, chances of tube or shell failure increase.

Suppose a high operating pressure resulted in rupture of a tube or in a tube

being pulled from the tubesheet. This:

Will result in contamination.

Will not result in contamination.

Answer

In a typical recirculating cooling water system, corrosion inhibitors and acid are added to prevent scale from forming. The scale is still dissolved in the fluid.

High temperature causes this scale to precipitate out as solids in the exchanger. Such precipitates collect in the

tubes and restrict flow.

In some fluids, precipitation occurs if the temperature is too low. Temperature must be held within the selected range so that fouling is prevented. In water dropout processes, where heat is used to separate water from product, too low a temperature decreases efficiency.

If the outlet temperature is too low, decreasing the rate of water flow will

_______ the rate at which heat leaves the exchanger.

Decrease.

Increase.

Answer

So, temperature in the exchanger will increase. One way of controlling temperature within the exchanger is to control the rate of water flow.

The velocity must not be allowed to drop too low and the cooling water temperature must not be allowed to get

too high. Otherwise, solids may precipitate and foul the exchanger.

In some cases, the outlet temperature can be raised by bypassing some of the product stream around the exchanger and joining it to the product flow that has passed through the exchanger. In this method, part of the

product is cooled.

The exchanger is part of a system that includes other pieces of equipment. Because they are connected, whatever physical change happens in one piece of equipment affects the operation of each piece of equipment within the system.

You should consider what changes will occur in other parts of the system if a change is made in the operation of an individual exchanger.

You should observe _______ both before and after changes are made.

Temperature.

Pressure.

Flow.

All of these.

Answer

This will give you an accurate idea of how conditions have actually changed and you will be able to pinpoint difficulties in operation.

Also, you should always keep a record of how and where changes are made. If changes produce an unsatisfactory result, the system can be returned to its original operating condition.

 

 

 

 

 

 

 

 

 

 

Exchanger Fouling OPERATING AND MAINTAINING HEAT EXCHANGERS

Fouling is a general term that describes the buildup of various kinds of deposits on the

parts of an exchanger. Because fouling particles adhere to the tube wall, fouling increases the thickness of the tube wall.

Once the wall is fouled, it takes heat longer to pass through the wall. In other words, the time of heat transfer increases. And, the flow of fluids through the exchanger is restricted.

Fouling in an exchanger causes a general loss of efficiency. Trouble in an exchanger is almost always indicated by changes in temperature and pressure.

If fouling restricts the passage of fluid, the drop in pressure across the

exchanger will:

Decrease.

Increase.

Answer

And, the flow rate will decrease. The temperature will also indicate that heat is transferred less effectively.

There are different types of fouling:

• Sedimentation: Sedimentation involves deposits of dirt, clay, and dust. • Corrosion: Corrosion products are formed when exchanger materials interact with fluids. • Organic material growth: Organic material growth includes algae growing in cooling water. This forms an insulating

layer. • Coking, salt deposits, and chemical reaction.

Regardless of the type of fouling, deposits reduce the rate of heat transfer. The kind and degree of fouling are influenced by the materials used in an exchanger. For example, surface roughness provides cavities for the buildup of solids.

Some materials corrode faster than others, introducing corrosion products which decrease heat transfer. The higher the corrosion rate, the ________ fouling occurs.

Later.

Sooner.

Answer

Also, velocity of flow affects fouling rates. The lower the rate of flow, the more sediment is allowed to drop out of the stream. So, up to a point, increasing velocity decreases the fouling rate.

Fouling in an exchanger can be handled in two different ways:

• Antifoulants prevent the formation of deposits. • Inhibitors prevent chemical reactions that might cause deposits to build up.

 

 

 

 

 

 

 

 

 

 

 

 

Maintenance OPERATING AND MAINTAINING HEAT EXCHANGERS

Detailed maintenance is required to protect exchange tubes from fouling. For example, dispersants prevent the

coagulation of insoluble materials that are suspended in the fluids.

The method used for removing the deposits depends on the type and severity of deposits. If a fouling problem has been neglected for some time, mechanical cleaning, such as cutting or scraping, may be necessary. The exchanger must be disassembled to use mechanical cleaning techniques. But, many deposits can be removed without shutting down the exchanger.

Cleaning while the exchanger is operating is called _______ maintenance.

On-line.

Off-line.

Answer

In a typical method used for on-line maintenance, chemicals are added to the fluids flowing through the shell-side or tube-side.

This is how sodium chloride (salt) deposits can be washed from the outside of the

tubes, while the exchanger is in use.

Water is injected into the process product inlet. As the mixture of water and product flows over the tubes, the water dissolves the salt. In the accumulator, the product and the salt water solution are separated. In some cases, it is better to shut down the exchanger for either chemical or mechanical cleaning.

It is not necessary to disassemble the exchanger for _______ cleaning.

Mechanical.

Chemical.

Answer

In chemical cleaning, a cleaning solution is circulated through the tubes or the shell-side. For mechanical methods of cleaning, the exchanger is partially or fully dismantled.

Hydroblasting

Hydroblasting is also a common cleaning method. Water, under high pressure, is

sprayed on the outside or inside of tubes. The force of the water loosens the deposits and washes them away.

Steam jets are also commonly used for heavy deposits. The heat generated by the steam softens the deposits and the force of the steam jet washes them away.

For any kind of hydroblasting, the exchanger must be at least partially dismantled. For example, the end plates (bonnet covers) must be removed to expose the tube sheets.

For the most difficult deposits, which resist chemicals or hydroblasting,

_______ methods are used.

Chemical.

Mechanical.

Answer

The exchanger must be fully dismantled. Drills or other devices are used to cut and scrape the deposits from the parts of the exchangers.

 

 

 

 

 

Condenser OPERATING AND MAINTAINING HEAT EXCHANGERS

A condenser is a type of heat exchanger that removes heat from a fluid that requires

cooling and converts a vapor stream to liquid.

Condensers are used to:

• Recover condensate to be used as boiler feedwater. • Condense the overhead products in distillation processes. • Condense the vapors from a condensing turbine. • Condense vapors from crystallizers or evaporators.

Condensers can use either air or water as the cooling medium.

This is a water-cooled condenser.

The shell-side fluid in this condenser is steam and the tube-side is cool water. As the steam flows across the tubes, it releases some of its heat to the water.

When the steam is _______, it condenses to water.

Heated.

Cooled.

Answer

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

This is an air-cooled condenser. Air-cooled condensers are especially valuable where water is scarce. In this type of condenser, the steam is fed into the tubes. A fan is used to blow air across the tubes. The tubes have fins, which increase the heat transfer across the surface area.

As the cool air blows across the tubes, it absorbs heat from the steam inside the tubes. This lowers the temperature of the steam, causing it to condense to water.

Which exchanger is more efficient?

Air-cooled.

Water-cooled.

Answer

Because water is a better conductor of heat, the water-cooled exchanger is more efficient. But, an air-cooled exchanger is more economical.

 

 

 

 

 

 

 

 

 

Depropanizer and Reboiler OPERATING AND MAINTAINING HEAT EXCHANGERS

Examine this heat exchanger system, which includes a depropanizer, a furnace, and a kettle-type reboiler. The depropanizer supplies the shell-side fluid, which is liquid isobutane. From the reboiler, isobutane flows back to the depropanizer as a vapor. The tube-side fluid is hot process liquid.

Cooled oil from the reboiler flows back to the furnace for reheating. Because it relates to independent parts of the system, the central part of this system is the reboiler. The purpose of this system is to cause isobutane to vaporize, using heat generated by the furnace.

Notice that the tube-side fluid is oil and the tube bundle is U-shaped.

This is _______ flow.

Double-pass.

Single-pass.

Answer

Shell-side fluid (isobutane) enters at one end and exits in liquid form at the opposite end. This is the typical shell-side single-pass arrangement.

The weir at the right of the reboiler functions as a sort of dam. By assuring that the tubes are always fully

submerged in hot liquid, the weir increases the efficiency of the reboiler.

The greater the contact, the greater the rate of heat transfer. The tube-side fluid provides the heat required to cause the shell-side isobutane to boil. The domed area of the reboiler allows the isobutane vapor and liquid to separate.

In this system, vapors from the reboiler are used to heat liquid in trays in the depropanizer. The amount of vapor produced in the reboiler depends on how much is needed in the depropanizer.

Suppose the amount of vapor in the depropanizer needs to be increased. To

do this, you will need to _______ the input of heat to the reboiler.

Increase.

Decrease.

Answer

One way to do this is to increase the temperature of the tube-side oil. Because the tube-side fluid is pumped through the tubes, circulation in the reboiler system is forced.

Another way to add heat to the boiling fluid is to increase the rate of hot liquid flow through the exchanger.

Adding heat to a boiling liquid makes it boil faster.

The faster the fluid boils, the ________ vapor it produces in a given time.

More.

By changing the flow rate of oil through

Less.

Answer

the reboiler, it is possible to control the amount of isobutane vapor going back to the tower.

 

Thermosyphon Reboiler OPERATING AND MAINTAINING HEAT EXCHANGERS

This is an exchanger used as a thermosyphon reboiler. A liquid

(propane and propylene) from the bottom of the fractionating tower is heated in the reboiler and goes back into the tower as a combination of liquid and vapor.

A thermosyphon reboiler functions like a kettle reboiler in that both break down a liquid into vapor and liquid components. But, the liquid and vapor are removed in separate streams only from the kettle-type reboiler. This also represents flow through the thermosyphon reboiler.

The propane and propylene are:

Shell-side.

In the process of giving up heat to

Tube-side.

Answer

vaporize the propane and propylene, the steam condenses. When the shell-side fluid leaves the exchanger, it has condensed and is a liquid.

This represents flow on the outside of one tube and on the inside of another.

As the steam loses heat, it condenses on the outside of the tubes as water. The water collects and runs out the bottom of the exchanger.

Inside the tubes, the heat given up by the steam causes some of the propane and propylene to boil.

The boiling begins closer to the ________ of the tube bundle.

Top.

Bottom.

Answer

A mixture of vapor and liquid is lighter than an equal volume of liquid only.

Because the vapor-liquid mixture is lighter than the liquid entering the inlet, it moves upward in the tubes. So,

moving products through a thermosyphon reboiler does not require a pump. A flow of liquid is produced by the formation of vapor.

Vapor in the tubes flows:

Slowly.

Rapidly.

Answer

Vapor in the tubes flows rapidly, carrying entrained droplets of liquid. Because they leave the exchanger in one stream, a dome space is not necessary.

Shell and Tube Water Coolers OPERATING AND MAINTAINING HEAT

EXCHANGERS

Coolers are used to lower the temperature of a liquid or a vapor. In this

example, a process product is being cooled. Before the product reaches the coolers, it has been pre-cooled in two pre-heat exchangers. The cooling liquid in the coolers is water. The tube-side fluid is also water, and it passes through the exchanger twice.

The hot process product (the shell fluid), makes a ________ pass through

the shell.

Single.

Double.

In the shell-side of the exchanger, there is a series of baffles. These baffles

Answer

continually change the direction of the product flow, maximizing the contact between the product and the tubes. The efficiency of this single-pass exchanger is increased by the baffles.

In many cases, more than one unit is required to do the heat transfer job. In this example,

two heat exchangers are used to pre-heat a feedstock. Two more are used to cool the process product for storage.

This shows two ways a stream can be connected to heat transfer units.

The incoming flow is split in a parallel connection. In a series connection, all of the stream goes through each exchanger. The process product flows first through one cooler, then out and into the next.

The shell-side process product flows in _______ through the coolers.

Series.

Parallel.

Answer

The tube-side water flows in parallel through the coolers. Notice the connection on the preheaters. They are connected in parallel on both the tube- and shell-sides.

 

 

Waste Heat Boilers OPERATING AND MAINTAINING HEAT EXCHANGERS

This system includes a fractionating tower, a steam oil drum, and a waste heat

boiler. Hot bottoms product, the shell-side fluid, is drawn off the bottom of the fractionating tower. The tube-side fluid is water, drawn from the steam drum.

The boiler uses waste heat from the hot bottoms, which must be cooled before it is stored, to produce steam for the plant steam system.

The shell-side of the exchanger or boiler has baffles every six inches. Bottoms product entering the shell flows back and forth across the outside of the tubes.

As it absorbs heat from the bottoms product, some of the water starts to boil in the first half of the tubes.

By the time the water gets to the other end, most of the space in the tubes is taken up by vapor. At the end of the tube, the steam blows along unvaporized water in the form of

small droplets.

The water leaves the boiler as a mixture of steam and droplets of water.

When the vapor-liquid mixture of water enters the steam drum, the droplets

of water fall to the bottom. The lighter steam remains in the ________ half of the steam drum.

Bottom.

Top.

Answer

The steam drum performs a similar function to the dome in a kettle-type reboiler. It permits the steam to be separated from the water.

To maintain a constant amount of water circulating in the system, treated make-up water is added at the bottom of

the steam drum. In the steam drum, the new incoming water mixes with the hot water recirculated from the boiler.

The steam drum is usually positioned above the boiler. When it is overhead, boiling induces a flow upward. The boiler with a steam drum above it creates a natural circulation similar to a thermosyphon reboiler.

For maximum heat transfer in a boiler, it is better to have ________ in contact

with the tube walls.

Steam.

Water.

Answer

Water absorbs heat better than steam does.

In this example, the tubes are extremely hot. If the water does not flow rapidly through the tubes, it is vaporized

near the beginning of the tube. The steam then passes through the remainder of the tube and absorbs less heat than the water would.

If the water boils too soon, much of the heating surface of the tube is wasted. The hot bottoms product simply retains more of its heat and leaves the boiler at a higher temperature.

Natural circulation by the thermosyphon action usually cannot push water

through the tubes fast enough.

True.

False.

Answer

To increase the flow rate of the water, there is a pump in the line between the steam drum and the boiler intake. The pump increases the efficiency of this exchanger system.

The new makeup water is treated at water treatment facilities to remove minerals. Some minerals remain in

the water even after treatment.

As the water is partly boiled and the steam drawn off, the mineral concentration

in the water tends to:

Decrease.

Increase.

Answer

To prevent mineral content of the water from increasing, a continuous blowdown is provided. The blowdown drains off some of the recycled water to control the mineral content.

Testing for Leaks OPERATING AND MAINTAINING HEAT EXCHANGERS

If you suspect a leak inside the exchanger, preliminary tests can be made without dismantling. Such tests can be

run on either the tube-side or shell-side of the exchanger.

If the two fluids in the exchanger have different physical properties (like water and oil), it is usually easy to tell them apart. The easiest way to test for leaks is to take a sample from the lower pressure fluid.

If the fluids are water and oil, then it is easy to see if there is a leak by just looking at the sample.

If the fluids are very similar, a _______ test may be necessary.

Visual.

Chemical.

Answer

If visual or chemical tests do not indicate a leak, further testing may be necessary. These tests are called hydrostatic tests, because they usually involve using water under pressure.

In the case of a tube-side test, the shell-side fluid is drained, and a drain point, like a disconnected lower nozzle

or bleeder valve, is left open.

The tube-side fluid is replaced with water under pressure, which fills the tube bundle. If there are leaks in the tubes or at the tube ends, the pressurized water in the tube bundle will be forced through the leak points into the shell.

The fluid will accumulate in the bottom of the shell and eventually run out of the drain points, and you will be able to see it.

Because the leak may be very small, and because fluid must accumulate in

the shell before it will run from the drain point, this test usually:

Takes some time.

Happens quickly.

Answer

The same kind of test can be made on the shell-side of the exchanger. In that case, the tube-side of the exchanger is drained of fluid, and a tube-side drain point is left open.

The shell is filled with water under pressure. Fluid running from the tube-side drain point indicates a leak in the tube bundle. If

preliminary tests indicate a leak, the exchanger is partially dismantled to determine the source of the leak through further tests.

In the case of a fixed tubesheet exchanger, the end plates or bonnet covers are removed. Then, you can directly observe the tubesheets and tube ends. The shell is filled with water under pressure and the pressurized fluid enters any leaking tube at the point where the tube leaks.

This fluid accumulates in the tube and runs out of the tube end. By observing the tubesheet, it is possible to tell which tube is leaking.

A leaking tube can be plugged at both ends with a tapered plug.

Examine this tube, which has come loose in the tubesheet. You will be able to readily observe

this leak on the face of the tubesheet.

To correct this, the tube must be rerolled or welded back into the tubesheet.

This is one method of testing a partially dismantled floating head exchanger.

This shell cover has been removed, and the tube bundle is filled with water under pressure.

It is possible to observe the leak if it is located:

In the floating head gasket.

In the tube ends at the floating head.

Either of these.

Answer

If the leak is located in the tube walls farther back in the exchanger or at the tube ends in the stationary head, you will observe fluid in the shell. But, you will not be able to locate the source of these leaks because the tube bundle and stationary head are not visible.

If the leak is coming from one of these areas, a different test is required.

The channel cover is removed and tube-side fluid is drained.

Then, the shell is filled with water under pressure. Fluid will enter the tubes at the points where they leak, accumulate in the tubes, and run out the tube ends.

By observing the tubesheet, you can tell which tube is leaking.

Also, you must always be aware of danger from thermal expansion of fluids under test pressure. A fluid expanding in a confined space can rupture the exchanger.

Test pressure is generally 1.5 times the design pressure and stated on the specification plate for the heat exchanger. Test pressure is used during a hydrostatic test that detects leaks on the heat exchanger.

Normal test pressure is usually _______ times the designed operating

pressure.

Answer

Pressures during testing should never exceed the rated test pressure.

Distillation Systems OPERATING AND MAINTAINING HEAT EXCHANGERS

In a distillation, or fractionating tower, feedstock is separated into fractions, or

parts. Light fractions are taken from higher levels, and heavy fractions are taken from the lower levels. The overhead vapors from the fractionator are fed into a condenser. In the condenser, heat is transferred from the vapor to the cooling fluid, in this case, water.

When vapor cools, it condenses, or liquefies. Some of the condensed vapor is drawn off through the product line, and some of it is returned to the top of the fractionator as reflux.

Suppose that the pressure in the fractionator is too high. The problem may

be caused by too ________ a reflux rate.

Low.

High.

Answer

If the reflux rate is too high, the condenser becomes over-loaded with vapor. The condenser is not able to condense the vapor quickly enough. The excess of vapor causes a pressure buildup in the fractionator.

To correct this, the reflux rate must be reduced. Too much pressure can also be caused by a malfunctioning condenser. If the condenser is not transferring heat effectively from the vapor to the cooling water, the vapor will not condense.

An overload of vapor will build up pressure in the overhead vapor line and in the condenser itself.

A fouled condenser is often indicated by a pressure increase on the water side

or a water outlet temperature that is too:

High.

Low.

Answer

If the condenser is fouled by debris accumulating at the tubesheet, the debris can be loosened by backflowing the cooling water. It is also possible that other maintenance must be performed, depending on the nature of the fouling.

Changes in temperature and pressure on the water side may also indicate problems with the water supply.

Condenser malfunction may also be caused by air in the cooling water system. Air causes vapor binding, which reduces the efficiency with which heat is exchanged. To help eliminate vapor binding, a vent is provided with water in the exit line.

Noncondensable vapors in the process side of the exchanger can also cause a

pressure buildup in the fractionator.

True.

False.

Answer

Because the vapors do not condense, they take up surface area in the exchanger. A loss in surface area causes a loss in the cooling capacity of the exchanger. This can be corrected by venting the process side of the exchanger to release the noncondensable vapors.

 

 

Heat Exchange Systems OPERATING AND MAINTAINING HEAT EXCHANGERS

This is the layout of a system involving a furnace, a reactor, two primary

heat exchangers, one secondary heat exchanger, two reboilers, and a condenser.

The reaction in this process unit requires 1,000º F, and the furnace provides this heat. A catalyst promotes the reaction.

Trace the path of the product stream. The product leaves the reactor, flows through the secondary exchanger, through two reboilers and two primary exchangers, and finally, through the condenser.

The system of exchangers, reboilers, and condenser is used to cool the

product stream.

When the product leaves the reactor, its temperature is _______ º F.

Answer

But, when the product leaves the condenser, its temperature has dropped to 100º F.

Now, trace the path of feed stream. The feed leaves the pump and passes through the primary exchangers, the

secondary exchanger, the furnace, and the reactor. During this process, the feed absorbs a large part of the heat given up by the product stream. The product cooling system is used to preheat the feed. Some of the

heat is transferred through the reboilers to other processes.

Because heat is recaptured in the system, the furnace uses less fuel. The cooling problem is also simplified. The entire process is more efficient and economical.

First, you should check temperatures at various points along the system. If

the furnace inlet temperature is stable at 700º F, but the outlet temperature is below the 1,000º F control point, the problem is probably in the:

Condenser.

Furnace.

Primary exchangers.

Answer

The bypass valve also controls the amount of heat available to the reboilers. When the bypass is closed and a maximum amount of heat is transferred to the feed stream through the secondary exchanger, the temperature of the product stream decreases.

This means that there is less heat to the reboilers and primary exchangers. Because of this, when readjusting

the bypass to get maximum heat to the feed stream, be sure to maintain the necessary heat input to the reboilers.

Another cause for difficulty in maintaining the 1,000º F control point might be an overload on the reboilers. Too much heat is being transferred through the reboilers to the stream that leads outside the system. You should reduce the load on the reboilers.

If the inlet temperature has dropped below the 700º F level, the problem

could be with the:

Bypass valve.

Exchanger load.

Either of these.

Answer

The valve controls the flow of the product stream through the secondary exchanger. If the bypass valve is completely closed, all of the product stream will pass through the exchanger. Then, a maximum amount of heat will be transferred to the feed stream.

If the bypass valve is completely open, most of the product stream will bypass the secondary exchanger. So, a

minimum amount of heat will be transferred to the incoming feed stream. When less heat is being transferred to the feed stream through the secondary exchanger, the temperature of the feed stream at the furnace inlet

drops. It is difficult for the furnace to raise the feed stream temperature to the required 1,000º F.

A drop in feed temperature at the furnace inlet might be caused by a

bypass valve that is too far:

Open.

Closed.

Answer

To correct this, you should readjust the bypass.