Heat Exchanger: - Factors affecting their efficiency and Design, including internal flow arrangements.

Heat Exchanger: - Factors affecting their efficiency and Design, including internal flow arrangements.

In this blog you will learn about heat exchanger: - factors affecting their efficiency and design including internal flow arrangements.

The program addresses problems that arise when using tube heat exchangers, which are the most prevalent type, and provides solutions to these issues.

There are several designs for heat exchangers in addition these designs are varied as necessary.

Depending upon the characteristics of the process fluid in general there are plate type heat exchangers and tube heat exchangers

Each transfer’s heat from one fluid to another.

This is a plate heat exchanger note its form and size as its name implies.

It's made of corrugated plates the plates are held together with an external pressure plate that can be moved for maintenance and cleaning.

Internally fluids enter each plate compartment the flow is set up so that the plates with hot liquid alternate with those containing cooler liquids.

Large surface areas and the natural turbulence caused by the corrugated metal plates allow for efficient heat transfer.

Plate heat exchangers may be used because of the specific demands put on the system by the characteristics of the process chemicals. For example, for a highly corrosive chemical that must be cooled a titanium plate heat exchanger may be best.

Another type of heat exchanger and the most common is the tube heat exchanger or shell and tube heat exchanger.

It's design consists of a set of metal tubes carrying one fluid and the space around those tubes known as the shell carrying another fluid.  As we'll see the internal arrangement of the tube heat exchanger is critical for efficient heat transfer to increase the amount of heat transfer the tube side fluid can be put through the exchanger more than once.

To do this a channel baffle or pass partition plate is built into the head of the exchanger.

This baffle or partition directs the flow through half of the tubes in one direction and through the other half in the opposite direction.

Here is a channel head baffle that directs the tube side fluid through the exchanger two times.

Every time the hot fluid makes a pass through one channel it gives up more heat.

Increasing the number of tube side passes improves the efficiency of an exchanger.

The shell side flow can also be changed to produce high efficiency.

As these longitudinal baffles force the shell side fluid to flow back and forth over the tube bundle with each pass Shell side fluid absorbs more heat.

Another arrangement involves segmental baffles.

They can be cut vertically or horizontally and their position to face in alternate directions. These baffles will allow the fluid to flow across the tubes a number of times and keep the fluid turbulent.

The more turbulence the more energy is transferred.

Segmental baffles can direct the flow to be horizontal or vertical depending upon the fluid being used.

If the liquid is dirty the sediment will often build up behind a horizontal baffle. This decreases efficiency by blocking the flow.

If the force of the fluid entering the shell is too great, impingement baffles may be used. These baffles direct flow to the sides of the exchanger which reduces internal erosion. In addition, the fluid contacts more surface area and increases efficiency

Despite careful design considerations, there are some problems that occur whenever two fluids with different temperatures run through the same piece of equipment.

This is because metal expands and contracts in length because of changes in the temperature.

Even under normal operating conditions this natural process will stress certain parts of the exchanger.

In a fixed tube sheet exchanger the tubes and tube sheets are permanently attached or fixed to the shell of the exchanger.

Whenever these parts expand and contract the tube joints undergo stress.

Eventually this stress may cause leaks to develop at the tube joints.

There are basically three designs that deal with the possibility of leakage.

Some fixed tube sheet exchangers the tube bundle is anchored between double tube sheets. If a leak does occur the fluid flows into the space between the tube sheets and can be safely drained from the exchanger.  This design may lessen the effect of a leak.

But it cannot lessen the stress that thermal expansion puts on the tubes a fixed tube sheet exchanger can only be used where the temperature difference between the two fluids is small.

The next example is a tube bundle from a U Tube exchanger. In this type, the tubes are free to expand because they're only attached at one end of the tube sheet.

 So a U Tube exchanger can handle fluids that vary greatly in temperature.

Although the U Tube bundle can be removed from the shell the bends in the tubes hinder internal inspection and cleaning.  

A third type of exchanger is designed to allow both thermal expansion and easy maintenance.

Because only one tube sheet is fixed while the other floats horizontally.

This is called a floating head exchanger.

The tubes are free to expand and contract in response to the temperature changes without stressing the joints and the bundle and floating head, can be removed for cleaning an inspection when necessary.

This cutaway view of a floating head exchanger shows you one disadvantage of its design. There is a clearance space between the shell and tubes, it’s needed so that the bundle can be removed for maintenance.  The shell side liquid will tend to flow through this space an area that offers little resistance. When this happens, Efficiency decreases because the shell side fluid does not come into contact with enough tube surface.

We've covered a lot of material in this section dealing with the internal flow arrangements and problems that may occur.

To summarize there are several types of baffles that affect flow - channel head baffles, longitudinal baffles, segmental baffles and impingement baffles.

Expansion and contraction cause stress this can be managed in several ways with the double tube sheet to collect leaks, with a U Tube exchanger to allow for expansion and the floating head exchanger which makes cleaning and inspection easy.

In discussing heat exchangers thus far we've examined factors that affect their efficiency and we've looked at design issues.

"Thanks for reading !"