Why we pass cold fluid to shell side and hot fluid to tube side?

If we pass hot fluid through shell then we will have a chance of heating loss to the surrounding.


However, practically this statement is not true.

The choice of shell and tube-side fluids for an heat exchanger are often governed by more demanding requirements which have implications on the safety, cost, maintenance time and feasibility of the heat exchanger and cannot be generalized as cold fluid on shell side and hot on tube-side. 

Some examples on how this choice is made might be - 

1-Frequent mechanical Cleaning - Tubeside to more fouling fluid, if mechanical cleaning is required. Note that chemical cleaning can always be done even on shell side. Mechanical cleaning on shells side however would require access to tube outer surface which in turn would require design features such bundle pullout possible / tube-tube gap/tube layout. It is more difficult than tubeside cleaning as it will require dismantling the whole exchanger. eg most Cooling water exchangers in process industry, which are frequently cleaned using hydrojetting.

2-Pressure - Tubeside to high pressure fluids (Shell side high pressure increases the thickness of shell and can have great implications on cost of the exchanger). Remember, shell cost forms the major cost of the exchanger in most cases.

3-Viscosity - Shell side to viscous fluids as turbulence, which can be more easily induced on shell side might be difficult (note viscous and fouling fluids are not essentially the same thing and are often confused as high viscosity implying high fouling). And then there are tube inserts which can give you that effect on tube-side as well.

4-Corrosive - Tubeside to corrosive fluid - Less components for it to see and eat up

5-Metallurgy - Expensive metallurgy on tubeside- Process requirements such as compatibility of your fluid or high or low temperatures might require you to use a particular metallurgy. It is always cheaper to use the more expensive metallurgy on the tubeside as it will require less of that metal.

This is not an exhaustive list and you can find more things in any chemical engineering text on heat exchangers for more info.

It gets tougher than this in real life where you might have competing requirements and this is often a choice between lesser of the two evils. To give you an example would be an  HF - Cooling water exchanger, where you will have to choose between a corrosive or fouling fluid to be kept on the shell side. 

To make things even more complicated, there might be cases such as some plants dismantle the exchangers and transport them to a different place dedicated for cleaning of exchangers for safety concerns. You therefore need much more information for this simple decision if you are designing an actual exchanger.

Source - Ankit Malhotra, Chemical Engineer, Experience in Heat exchanger design for Refining and Petrochem.

By another reference, it's not a good practice to always put cold fluid in the shell.

1- The most general practice is that you calculate the flow areas for both, shell and tube, and put the fluid with higher flow rate in the larger flow area. Otherwise, the pressure drop will shoot up, resulting in a poor design.

2- If any fluid is corrosive, you try and put it in the tube (This way, you only have to clean the inner surface of the tube. On the other hand, if it were in the shell, you'd have to clean the outer surface of tube as well as the inner surface of shell).

3- Generally hot fluids result in scale formation, and because of cleaning reasons explained above, they are put in the tubes.

So, it totally depends on the application, and as such there is no general rule.

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Which liquid should be on shell side of a shell and tube HE and why?

Which liquid should be on shell side of a shell and tube HE and why?

Even though there are no strict conditions on this the following points are taken into consideration normally.

1)The corrosive fluid shall pass through the tube side as the replacement of tubes is easier and cheaper than shell side.

2) The toxic, hazardous fluid shall pass through tube side. Because in case of any leakage it won’t get exposed to atmosphere.

When designing a shell-and-tube exchanger, one of the first issues is deciding which fluid should go on the shell side and which on the tube side. So, let's look at some rules-of-thumb for several key factors, realizing such rough guidelines won't cover all cases.

High pressure:

Put a high-pressure fluid on the tube side. This usually minimizes exchanger cost. The smaller tube diameter has a higher pressure rating for the same metal thickness compared to the larger diameter shell.

Fouling:

A fluid with a tendency to foul generally should go on the tube side. Cleaning straight tubes normally is easier than cleaning the shell — even if a relatively large tube pitch or a square tube pattern is used to make the shell side easier to clean. However, the exchanger configuration significantly influences the choice. Using a fixed tubesheet mandates putting a clean fluid on the shell side; unless expected fouling is easily removed by chemical cleaning, the fixed tubesheet makes the shell side impossible to clean. In contrast, U-tubes are more difficult than straight tubes to clean. So, sending a normally fouling service through the shell side may be better if fouling reduction steps, such as installation of helical baffles, are suitable.

“For fouling services, the exchanger configuration significantly influences the choice.”

Expensive materials:

Put a corrosive fluid on the tube side. That way, only the tubes, tubesheets, heads and channels will need expensive corrosion-resistant alloys. In contrast, a corrosive fluid on the shell side requires the entire exchanger to use the materials.

Low pressure drop:

The fluid should go on the shell side. An appropriate combination of baffle cut, spacing and type (segmental, double segmental, rod-baffle, etc.) can accommodate nearly any pressure-drop requirement. Services under vacuum almost always are on the shell side because of pressure drop sensitivity.

Vapor services:

Because a vapor normally has a higher volume and lower heat-transfer coefficient than a liquid, allocate it to the shell side. This reduces pressure drop for a given volume and typically provides a higher heat-transfer coefficient.

Condensing services

A condensing fluid most often goes on the shell side — but the choice demands careful evaluation. If the shell-side velocity is low enough, the vapor and liquid can separate inside the exchanger. The liquid dropping out makes the vapor leaner, reducing the temperature required to get more liquid to condense from the remaining vapor. For relatively pure mixtures, this effect is unimportant.

For wide-condensing-range mixtures, ensure the overall flow pattern inside the exchanger keeps the liquid and vapor mixed. This may necessitate having the shell-side fluid leave from the bottom (forcing the liquid and vapor to mix) or determine the choice of baffling inside the exchanger (horizontal versus vertical or 45° baffle cut).

Services:

Here, the tradeoffs are complex. A viscous fluid on the tube side tends to have high pressure drop and low heat transfer. That favors shell-side allocation. However, high pressure drop on the shell side can prompt significant flow bypassing around baffles, reducing heat transfer. A shell side with a high pressure drop also may suffer from vibration damage; shell-side modifications (assuming the user is aware of the need for them) can reduce such damage.

Solidifying services:

Generally avoid a shell-and-tube exchanger for any service with a high risk of solidification or freezing. However, if you must use such an exchanger, I suggest putting the fluid with a risk of solidification in the tubes. If the fluid solidifies, you usually can pull out the tube bundle and replace it. In contrast, if the solid is on the shell side, it's often impossible to remove the tube bundle. The entire exchanger may require replacement.

However, allocating to the shell side a fluid that may freeze may offer advantages. In some cases, externally heating the shell with electric tracing may enable melting the fluid enough to get the exchanger back into service.

In either allocation, a freezing fluid can create local plugs. If its velocity drops, the fluid may approach the temperature of the other side of the exchanger, which may cause the fluid to set solid. This can occur on the shell side next to baffles on the shell edge. It also can happen in tubes. Tubes may differ in flow rates. A tube with a low flow rate may reach a lower temperature, making it more likely to freeze or set up. Often, exchangers with freezing fluids may have large areas where the exchanger has set solid.

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When to use double pipe heat exchanger and shell and tube heat exchanger?

When to use double pipe heat exchanger and shell and tube heat exchanger?

Double pipe heat exchangers are used when the heat transfer area is small say up to 14 m2. If we connect them in series to increase the heat transfer area it will require much space as well the pressure drop will be higher due to more fittings. Also, we can’t increase no of passes for either side fluids. In addition, he double pipe HE can’t be used for dirty fluids due to choking and cleaning is tougher. The advantage is it is simple to construct and easy to operate.
However, in shell and tube HE, we can pack a large heat transfer area within a small volume. As the numbers of tubes are more in a shell and tube HE, we can expect a higher turbulence which will result in higher heat transfer rates. Dirty fluids also can be handled owing to easy cleaning.

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SHELL & TUBE HEAT EXCHANGER

SHELL & TUBE HEAT EXCHANGER

Shell and tube heat exchangers are the most widely used type of heat exchanger.

GENERAL INFORMATION

The inside of the exchanger contains many tubes and baffles, as shown in the picture below. These tubes and baffles help direct the two streams flowing through the exchanger.

STRAIGHT TUBE SHELL & TUBE HEAT EXCHENGER

STRAIGHT TUBE SHELL & TUBE HEAT EXCHENGER

A shell and tube heat exchanger consists of several tubes enclosed in a shell. One fluid flows through the tubes while the other fluid is conducted through the shell. Flow through the shell and tubes can be countercurrent, cocurrent, or cross flow. In countercurrent flow, the shell fluid flows in the opposite direction of the tube fluid. In cocurrent flow the shell fluid flows in the same direction as the tube fluid. Lastly, in cross flow the shell fluid flows perpendicular to the flow of the tube fluid. In general, countercurrent flow results in the most efficient heat transfer.

U TUBE SHELL & TUBE HEAT EXCHENGER

The baffles serve two functions: Their strategic positioning supports the tubes, preventing vibration and sagging, and they also channel the fluid in the shell side, resulting in more efficient heat transfer.

U TUBE SHELL & TUBE HEAT EXCHENGER

Static mixers are sometimes placed in the tubes of the shell and tube exchangers to help dissipate heat. Mixing of the fluids in the tube removes temperature, velocity, and material composition gradients. Static mixers also allow fluids to be cooled near their freezing temperature without plugging the tubes.

Static mixer

The diagram below shows the mixing that occurs as the flow in the tube encounters the static mixer. The mixer itself does not move.

Static mixer

ADVANTAGES	DISADVANTAGES
Can handle fluids at high temperatures and pressures. Can handle fluids of all states. Easy to dismantle for cleaning or repairs. Design can be adapted to meet operating conditions.	One unit can only be used for one duty. High amounts of heat loss occur, so insulation is required. Larger space requirements and more expensive than plate and frame. Over time vibrations may damage the heat exchanger. Baffle placement may be optimized to reduce vibrations and help the heat exchanger last longer.

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Plate and Frame Heat Exchanger

PLATE & FRAME

Typically, plate and frame heat exchangers are used for liquid-liquid exchange at low to medium pressures. However, gasket-free plate and frame heat exchangers can safely operate at high temperatures and pressures. Plate and frame heat exchangers offer flexibility because plates can be either added or compressed for each different situation.

Plate and Frame Heat Exchanger

Plate and Frame Heat Exchanger

GENERAL INFORMATION

The gaps between the plates in plate and frame heat exchangers can be adjusted according to the degree of fouling (deposits, corrosion, etc.) that is expected.

Plates of Plate and Frame Heat Exchanger

The counter-current flow of fluids that occurs in plate and frame heat exchangers allows approach temperatures as low as 1 to 2°F.

Gaskets ensure that the cold fluid (blue) and the hot fluid (red) don't mix. Alternatives to the traditional gasket seal include brazing and laser-welding.

The plates are stacked in an alternating manner to cause the counter current flow. The diagram below shows the flow in a heat exchanger. The design allows for the two media to flow in alternate directions and not be mixed. However, heat can be transferred from one medium to the other through the plates. 

ADVANTAGES

DISADVANTAGES

Require less space and are less expensive than shell and tube heat exchangers. Easy to adjust for different liquids by adding or subtracting plates. Pressure can be varied by compressing the plates. One frame can be used for multiple duties by simply changing plates. Working temperatures up to 550°C and pressures of 780 psi are possible with gasket-free versions. High heat transfer coefficients relative to shell and tube heat exchangers. Up to ten times more resistant to fouling than shell and tube heat exchangers.

Gasketed plate and frame heat exchangers have a maximum operating condition of 149°C and 300 psi. Not good for vaporizing fluids or large amounts of vapor. Gasket-free versions are impossible to open for inspection or cleaning.

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Types of fouling in Heat Exchanger

Fouling can be done by the accompanying authors:

1) Scaling is the most common form of fouling and is associated with inverse solubility salts. Examples of such salts are CaCO3, CaSO4, Ca3(PO4)2, CaSiO3, Ca(OH)2, Mg(OH)2, MgSiO3, Na2SO4, LiSO4, and Li2CO3.

2)Corrosion fouling is caused by chemical reaction of some fluid constituents with the heat exchanger tube material.

3)Chemical reaction fouling involves chemical reactions in the process stream which results in deposition of material on the heat exchanger tubes. This commonly occurs in food processing industries.

4)Freezing fouling is occurs when a portion of the hot stream is cooled to near the freezing point for one of its components. This commonly occurs in refineries where paraffin frequently solidifies from petroleum products at various stages in the refining process. , obstructing both flow and heat transfer.

5)Biological fouling is common where untreated water from natural resources such as rivers and lakes is used as a coolant. Biological micro-organisms such as algae or other microbes can grow inside the heat exchanger and hinder heat transfer.

6)Particulate fouling results from the presence of microscale sized particles in solution. When such particles accumulate on a heat exchanger surface they sometimes fuse and harden. Like scale these deposits are difficult to remove.

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