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Heat exchangers Interview Questions 2 contains following interview questions

1. Draw the temperature profile in counter and parallel flow heat exchanger, why counter flow is more efficient
2. What is the driving force for Heat Transfer?
3. (when to clean)How to choose cleaning moment for the exchanger?
4. Types of fouling
5. Why we use U-type or hairpin construction in double pipe heat exchanger? Why do not we use straight double pipe heat exchanger?

1. Draw the temperature profile in counter and parallel flow heat exchanger, why counter flow is more efficient

Temperature profile in Heat Exchanger

The plot of “flow temperature” Vs “length of the tube” is known as temperature profile in Heat exchanger. From the temperature profile we can conclude, which flow pattern is best in heat exchanger and why it is so.

Parallel flow temperature profile in 1-1 Heat Exchanger:

Sketch of 1-1 Heat Exchanger

1-1 Heat exchanger means, 1-tube side and 1- shell side pass heat exchanger. It is as shown in the below figure

Parallel flow

Temperature profile of 1-1 Heat exchanger – Parallel flow

Counter Flow

In counter flow tube side flow and shell side flow are in opposite direction, and a typical flow scheme is as shown in the figure.

Temperature profile of 1-1 Heat exchanger – Counter flow

If we observe, we can see that, in counter flow there is uniform temperature difference between hot and cold fluids and this difference decreases with the length of the tube in parallel flow.

From the above two temperature profiles we can conclude that counter flow is more efficient than parallel flow.

2. What is the driving force for Heat Transfer?

Temperature gradient (Temperature difference) is the driving force for heat transfer.

According to second law of Thermodynamics heat will flow from high temperature region to the low temperature region under normal conditions.

If we use external force, like heat engine, heat can be transported from low temperature region to high temperature region too.

In case of conduction, Fourier's Law gives that,

q α ΔT

So, we can say that heat flow is directly proportional to the driving force.

3. (when to clean)How to choose cleaning moment for the exchanger?

The precise moment to clean a heat exchanger also strongly depends on the costs, of course.

In the figure below this is illustrated for a simple example for a heat exchanger with a linearly increasing fouling factor, thus linearly increasing energy costs.

The heat exchanger is periodically cleaned offline. A cleaning operation takes a certain amount of money, so when time increases this amount per unit time will drop.

The right time to clean the exchanger is when the sum of the two is at its minimum:

Simple example of choosing the cleaning moment

Automatic Tube Brushing (ATB) Fouling solutions in practice (cleaning of a heat exchanger offline and online)

When it comes to cleaning of a heat exchanger, there are to ways to do this: offline and online.

The advantage of online cleaning is that the heat exchanger will not have to be shut down, and thereby shutdown costs are reduced.

This is why online maintenance is often more preferred and therefore all kinds of online cleaning methods were developed in recent years.

One method to remove foulants, developed by a firm called Advanced Heat Transfer Technologies, is the Automatic Tube Brushing (ATB).

The operating principal will explained in the light of the following picture.

Figure 7 The principal of the ATB system

The system can be applied to almost any heat exchanger and consists of two main parts.

The first is a small nylon bristle brush (white part on the right, within the yellow tube), which is inserted into each tube of the heat exchanger.

The size of the brush is chosen in such a way that there is an appropriate fit within the tube. The second part is a special plastic cage (blue part on the left), which is installed at each side of the exchanger tubes.

When the exchanger fluid flows through the tubes it takes the brush with it from the cage on the one side to the other one. By reversing the flow direction, the brush will then be taken to other side again. This way the brush moves back-and-forth through the tubes, thereby removing all kinds of foulants.

The turning of the flow is done by a third component, a special valve, which is activated by an automatic control panel.

This valve normally turns the flow like two or three times a day, depending on the severity of the fouling in the heat exchanger.

The big advantage of the ATB system lies in the fact that after installation no more extra maintenance is needed, so the shutdown and maintenance costs almost disappear.

The system keeps the fouling factor of the heat exchanger at a constant low value. In practice, fouling factors of 5 till 10 times lower than without ATB are reported.

This way also energy costs are minimized. Furthermore no chemicals are needed to clean the heat exchanger, which is an advantage for the environment.

The disadvantage of the system lies mostly in the installation, which is rather time consuming for big heat transfers and also rather expensive because, partially because of the needed control unit and valves. Capital costs will rise, but a bigger reduction in other costs can be achieved!

The importance of fouling(fouling costs)

First of all, fouling costs can be separated according to how they are generated. Roughly taken, there are four types of costs:

1)      Additional capital costs or costs for special design considerations

Lots of costs in using heat exchangers can be prevented in the R&D departments of a company. Especially when it comes to fouling.

A good design can reduce the effects of fouling and thereby the operational costs of the heat exchanger. But of course research and design costs money.

A way to prevent fouling is to choose a bigger heat transferring surface then needed, as discussed before. The heat exchanger will become bigger and heavier, and thereby also more expensive.

2)      Energy costs

A heat exchanger that suffers from fouling needs additional energy to keep operating at the same level.

This is because the fouling layer decreases the amount of heat transferred as well as it increases the amount of pressure drop needed to maintain the same throughput through the smaller cross-section.

All this additional energy is pure loss.

3)      Maintenance costs

A fouled heat changer has to be cleaned once in awhile, in order to keep the energy needed for operation low. This cleaning can be online or offline, mechanical or chemical, etc.

Sometimes it’s needed to replace some parts of the heat exchanger, for instance because of corrosion.

4)    Costs of loss production or shutdown costs

When a heat exchanger is cleaned or maintained offline, there is no production. No production means no income, so this is considered a loss.

The effect of this shutdown depends on the normal plant capacity and the length of the shutdown.

In recent years many research has been done to know more about the magnitude of the costs mentioned above.

Effect of velocity temperature surface material heat exchanger configuration
Parameters that can influence the fouling factor:

There are even more parameters that can influence the fouling factor. In general, the following conditions are known for their influence:

Velocity of the medium

Increasing medium velocity will in general increase the release rate and therefore decrease fouling.

Temperature of the bulk fluid

Especially precipitation and chemical reaction fouling can depend strongly on bulk temperature, but both in a different way.

Temperature of the heat transfer surface

Lowering this temperature may increase solidification or even precipitation fouling.
Other fouling mechanisms may increase with increasing temperature.

Surface material

The amount of corrosion is strongly dependent on the choice of the surface material.
Surface material may also influence biological fouling, e.g. copper is more sensitive to biological fouling then most other materials.

Heat exchanger configuration

From experience it’s known that shell-and-tube heat exchangers are more sensitive to fouling than for example a plate-and-frame or double-piped heat exchangers.

This is mostly because velocities and turbulence levels are higher for the latter one. Disadvantage of these heat exchangers is that they’re far bigger than a shell-and tube with the same capacity.

It’s clear that the precise influence of each of these conditions on the fouling factor depends on the type of fouling. In some cases this precise influence isn’t known yet, and there’s still a lot of research done.

Still, understanding the impact of these conditions on the fouling resistance is essential to actually control fouling-phenomena, and thereby control the costs.

In the design of a new heat exchanger, one must be aware of all the mentioned conditions and other influences of fouling factors, and choose those conditions that will result in as less fouling as possible.

One must also be aware that it must be possible to clean a heat exchanger once in a while, which will have impact on the chosen construction.

Time dependency of fouling factor(With induction time)

Fouling factors are time-dependent and will always increase with time.

The way it increases depends on the specific situation. It could increase linear with time, or increase asymptotically to a certain limit. The latter case isn’t as worse as the linear case.

When time increases, fouling resistance will reach some constant value. This means that one can design a heat exchanger in such a way, that this constant resistance is compensated.

In the linear case it is necessary to periodically clean the heat exchanger, or else the fouling resistance would reach the sky.

In most cases there is also an initial induction period; for a clean heat exchanger with initial fouling factor zero there is a certain time interval for which the fouling resistance is very low.

This is illustrated in figure 5.

4. Types of fouling(Precipitation fouling or crystallization fouling,Particulate fouling,Corrosion fouling,Chemical reaction fouling,Biological fouling)

Precipitation fouling or crystallization fouling

Also called crystallization fouling.

A fluid or gas used in a heat exchanger can contain dissolved inorganic salts. Given certain conditions, there’s a maximum amount of salt that can be dissolved in this fluid or gas.

When the process conditions inside the heat exchanger differ from the conditions at the entrance, supersaturation may occur.

This means that part of the dissolved salt will crystallize on the heat transfer surface. Figure 1 gives a clear example.

Figure. 1 Percipitation fouling

Particulate fouling

This is when the gas or fluid inside the heat exchanger contains small particles which will attach to the heat transfer surface. Examples are dust or sand.

The deposition occurs mostly as a result of gravity.

Chemical reaction fouling

This type of fouling considers the deposits that are formed as a result of chemical reactions within the fluid.

The heat transfer surface itself is not consumed in the reaction, although it could operate as a catalyst.

This type is a common problem in for example petroleum refining or polymer production.

Corrosion fouling

This fouling is also caused by some chemical reaction, but this time the surface is a reactant and will be consumed.

The surface reacts with the fluid or gas to form corrosion products on itself.

The rusting of steel parts is a well-known example, as can be seen in figure 2.

Figure 2 Corrosion fouling

Solidification fouling

When the heat transfer surface is low enough, a fluid flowing through a heat exchanger can actually freeze at the surfaces.

In case of a multi component fluid, it’s the high melting point constituent that will solidify.

This is easy to imagine for fluids, like water cooling, but in practice this phenomenon can also occur when the medium is a gas.

Biological fouling

It’s also possible for biological micro- and macro-organisms to stick to the heat transfer surface. In this case not only the attaching of the material is a problem, but also it’s growth.

In many cases this will result in a slime layer. This can be seen in figure 3.

Figure. 3 Biological fouling

To understand more about the influence of fouling on the performance of a heat exchanger, one must consider the heat transferred q:

Here LMTD stands for the Logarithmic Mean Temperature Difference, AO is the outer surface area and U stands for the overall heat transfer coefficient.

The influence of fouling can be seen in the coefficient U. In the past various equations for U have been developed to capture fouling factors, but the most widely used is this:

The term inside the first brackets stands for the ordinary heat coefficient, when there is no fouling (or for an unused heat exchanger); h stands for the convective heat transfer  coefficient, RW and AW are thermal resistance resp area of the wall.

The second term is the extra term because of fouling. Here RF is the fouling resistance. The indices I and O stand for inner and outer surfaces.

It’s clear to see that for increasing fouling factors, the thermal coefficient U will drop, causing the transferred heat q to drop too.

One way to compensate this effect is to over dimensionize the heat exchanger, which is increase the heat transfer area. One disadvantage is of course that this will result in a more expensive device.

5. Why we use U-type or hairpin construction in double pipe heat Exchanger? Why do not we use straight double pipe heat exchanger?

Most common are U-type or hairpin constructions.

Due to the need of a removable bundle construction and the need for the ability to handle differential thermal expansions the exchanger is implemented in two parts.

In figure 2 the fluids enter and leave the exchanger by the four nozzles on the right while the exchanger can freely expand to the left which makes the of expansion joints to the other machinery superfluous and makes demounting much easier.

U-type or hairpin construction for a double pipe heat exchanger

Advantages and disadvantages of counter flow in heat exchangers over parallel flow?Why counter flow is effective?

While the temperatures T (of the cooled fluid) and t (of the warmed fluid) in the parallel flow heat exchanger can only approach each other, they can pass each other in the counter flow (Tout < tout) and in this case there has to be more heat been transferred.

One other advantage for the counter flow, since the maximum temperature differences between the two flows are much smaller, they suffer less thermal forces.

(Low thermal stresses will develop in counter flow as temperature difference is uniform throughout the length of the exchanger

Heat exchanger design at minimum overall cost, depreciation and operating cost (Effect of water velocity on annual operating cost of condenser)

The essential requirements in the design of a heat exchanger are, firstly, the provision of a unit which is reliable and has the desired capacity, and secondly, the need to provide an exchange at minimum overall cost.

In general, this involves using standard components and fittings and making the design as simple as possible.

In most cases, it is necessary to balance the capital cost in terms of the depreciation against the operating cost.

Thus in a condenser, for example, a high heat transfer coefficient is obtained and hence a small exchanger is required. If a higher water velocity is used in the tubes.

Against this, the cost of pumping increases rapidly with increase in velocity and an economic balance must be struck.

A typical graph showing the operating costs, depreciation and the total cost plotted as a function of the water velocity in the tubes is shown in Figure

Effect of water velocity on animal operating cost of condenser