Heat Exchangers in Process Plants

Introduction

Like pumps, compressors, and vessels, heat exchangers are also used widely in process plants. The control of heat within any facility is important part of the plant operation. The principal application of a heat exchanger is to maintain a heat balance through the addition and removal of heat by exchange with outside sources or between streams of two different operating temperatures.

The most applications for heat exchangers shown in the Process Flow Diagram (PFD) as shown the picture below are

1. Coolers – Cools the process streams by transferring heat to cooling water, atmosphere or other media
2. Exchangers – Exchanges heat from a hot to a cold process stream
3. Reboilers – Boils process liquid in a tower bottoms using steam, hot oil, or a hot process stream
4. Heaters – Heats a process stream by condensing steam
5. Condensers – Condenses vapors by transferring heat to cooling water, atmosphere, or other media
6. Chiller – Cools a process stream to very temperatures by evaporating a refrigerant

Exchanger Construction and Types

The most common type of heat exchangers constructions used in various process plants are as follows:

1. Shell and Tube Exchangers
2. Plate Type Heat Exchangers
3. Spiral Heat Exchangers
4. Double Pipe Exchangers
5. Air Fin Coolers or Fin-fan Cooler

Let us discuss about the heat exchangers.

1. Shell and Tube Exchangers

The most basic and the most common type of heat exchanger construction is the shell and tube type as shown in Figure below. This type of heat exchanger consists of a set of tubes in a container called a shell. The fluid flowing inside the tubes is called the tube side fluid and the fluid flowing on the outside of the tubes is the shell side fluid. At the ends of the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s). The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight seal.

In systems where the two fluids are at vastly different pressures, the higher pressure fluid is typically directed through the tubes and the lower pressure fluid is circulated on the shell side. This is due to economy, because the heat exchanger tubes can be made to withstand higher pressures than the shell of the heat exchanger for a much lower cost. The support plates shown on Figure below also act as baffles to direct the flow of fluid within the shell back and forth across the tubes.

The shells of the most of the exchangers are constructed of seamless pipe for small diameters and shaped welded steel plates for larger sizes. The ends of the shell can be designed to accommodate welded, dished, or flanged shell covers as well as flanged or welded heads. Both the tube side and shell side of the exchangers have inlet and outlet nozzles positioned to provide the required flow through the exchanger. The unit is supported at the shell by attached saddles for horizontal installation and by lugs for vertical arrangements. Tube bundles are made of may small-diameter tubes that are expanded into tube sheets at each end of the bundles. One end is usually fixed; the other end is allowed to float for expansion. For the more simplified U-tube arrangement, only one tube sheet is used, which is integrated wit the channel head.

With many shells, shell covers, and head covers available, exchangers can be arranged in various combinations to provide a wide range of services.

(a) U-Tube Arrangements

(b) Fixed Tube Arrangements

(c) Kettle type Arrangements

Advantages:

• Easy maintenance and repair.
• They have no dimension limit.
• They can be used in all applications.
• They are resistant to thermal shocks.
• They have a very flexible and steady design.
• They can be designed and manufactured to handle extremely high pressures.
• Pressure loss is at a minimum and can be maintained at a minimum in line with the process objectives.
• They can also be designed and manufactured to handle extremely high and extremely low temperatures.
• The shell and heat Tube Exchangers can be easily taken apart and easily put back together for any maintenance such as cleaning and repair.
• The Pipes pitch, arrangement, length, number, and diameter can be modified. In other words, the designs of the shell and tube heat exchangers are very flexible.

Disadvantages:

• Heat exchange effectiveness is less as compared to plate type cooler
• Cleaning and maintenance is troublesome since a tube bundle requires enough clearance at one end to remove the tube nest
• Capacity of tube cooler can’t be increased
• Requires more space in contrast with plate coolers

2. Plate Type Exchangers

A plate heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids are spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperature change.

There are four main types of plate heat exchangers: Gasketed, Brazed Plate, Welded, and Semi-Welded. Each type is suited for a number of applications in various industrial fields. Gasketed plate heat exchangers use high quality gaskets and design to seal plates together and protect against leaks. Plates can easily be removed for cleaning, expansion, or replacing purposes, drastically reducing maintenance costs.

Advantages:

• Effective Heat Transfer
• Easy Maintenance
• Compact Design
• Cost-efficient
• Materials Variety

Disadvantages:

• In situations where there is an extreme temperature difference between two fluids, it is generally more cost efficient to use a Shell & Tube heat exchanger
• In a Plate heat exchanger, there can be a high pressure loss due to the large amount of turbulence created by the narrow flow channels.
• Applications which require a low pressure loss may want to consider a Shell & Tube heat exchanger as well.
• Gasketed Plate Heat Exchangers are limited in high fluid temperatures, by the temperature limitations of the gasket.

Despite these limitations, Plate heat exchangers are the most efficient choice for a wide variety of applications.

3. Spiral Heat Exchangers

Spiral heat exchangers are generally used in chemical plants and are of circular construction, consisting of an assembly of two long strips of plate wrapped to form a pair of concentric spiral passages. Alternate edges of the passages are closed, so that liquid flows through continuous channels. Removable covers are fitted to each side of the spiral assembly for access to the spiral plate. As shown in the figure below, the inlet and outlet nozzles are integral to the plate housing and the covers. The unit is supported by legs attached to the plate housing for horizontal installations and by legs for vertical installations. Similar to the plate exchanger, the spiral exchanger is compact and requires less installation and servicing space than conventional exchangers of equivalent service.

Advantages:

• Minimal fouling, or clogging, in duties involving very dirty, highly viscous or particulate media, ensures uptime
• Easy-to-open design makes cleaning quick and simple, ensuring low maintenance costs
• Reduced pipework and steel structures means lower installation costs
• Increased energy savings and reduced emissions thanks to thermal efficiencies 2-3 times higher than comparable shell-and tube exchangers

Disadvantages:

• Spiral plate heat exchangers require high welding quality
• Difficult maintenance.
• Spiral heat exchangers are difficult to clean and service.
• Heavy weight, poor rigidity,
• Special care should be taken when transporting and installing spiral plate heat exchangers.

4. Double Pipe Exchangers

In double pipe heat exchangers, we have a large pipe with a small pipe inside it concentrically, and all the heat transfer process occurs inside the larger pipe. One fluid flows through the inner of a small pipe, and another fluid is between the two pipes, and that is how the inner pipe acts as a conductive barrier. The outside or shell side includes fluid flow passing on the inner side or tube side.

This type of heat exchanger is also known as hairpin, jacketed pipe, jacketed u-tube, and pipe in pipe exchanger. They can contain one pipe or pipe bundle (less than 30), and the outer pipe must have a diameter of less than 200mm. In some cases, to increase the rate of heat transfer between working fluids, there are longitudinal fins in the inner tube.

Double pipe heat exchanger designs are the simplest type of exchangers suitable for high temperature and pressure applications. They are easy to repair, and due to the straightforward design, they are used widely in many applications and are the first choice for a lot of projects. They can be used in two types of counterflow, and parallel flow depends on the application, and the design of this type is straightforward.

Advantages 

• You can provide good efficiency with lower capital costs.
• They are small compared to the shell and tube and do not need much space for maintenance, while the heat transfer is acceptable.
• As they are very popular, all the parts have been standardized, and it makes the repair and maintenance very easy.
• They have a flexible design, and other addition and removal parts can be done easily.
• You can use this type of exchanger at high pressure and temperature.
• The design of the exchanger allows more thermal expansion without any expansion joint.

Disadvantages

• They are usually used in counter flow designs and can not be used in some applications. It does not mean that they cannot be used in parallel flow.
• They have limitations in heat transfer rather than complicated designs and should be used in low heat duties.
• Leaking is more often in this type (paired with more units)

5. Air Fin or Fin-fan Coolers (AFC’s)

Air Fin or Fin-fan coolers are entirely different from the previously mentioned arrangements in that the cooling arrangement used is air instead of liquid. As seen in figure below, an air cooler unit consists of fin-fan tube bundles with a header box attached to each end, supported horizontally by a steel frame or structure. For the single pass arrangement, the inlet nozzles are mounted on the top of the header box; the outlet nozzles are at the opposite end and mounted on the bottom of the header box. For the double pass arrangement, the outlet nozzles are mounted on same end as the inlet nozzles. For additional surface area, more passes can be added or additional units can be installed or located side by side. For higher elevation installations, platforms are generally provided for access to header box and motors for maintenance and access.

An Air cooled heat exchanger consists of the following components:

• One or more bundles of heat transfer surface.
• An air-moving device, such as a fan or stack.
• Unless it is natural draft, a driver and power transmission to mechanically rotate the fan.
• A plenum between the bundle or bundles and the air-moving device.
• A support structure high enough to allow air to enter beneath the AFC at a reasonable rate.
• Optional header and fan maintenance walkways with ladders to grade.
• Optional louvers for process outlet temperature control.
• Optional recirculation ducts and chambers for protection against freezing or solidification of high pour point fluids in cold weather.
• Optional variable pitch fan hub for temperature control and power savings.

Advantages:

• Very low maintenance and operating costs.
• Do not require an auxiliary water supply because of the lost water due to drift and evaporation.
• It can be used for areas where there is no utilities like service water is available.
• Danger of process fluid contamination is much greater with water-cooled system.
• Air-side fouling can be periodically cleaned by air blowing
• Chemical cleaning can be carried out either during half-yearly or yearly attention. Water-cooled systems may require fre­quent cleaning.

Disadvantages

• AFCs cannot be located next to large obstructions if air recirculation is to be avoided.
• AFCs require large surfaces because of their low heat transfer coefficient on the airside and the low specific heat of air. Water coolers require much less heat transfer surface.
• Noise – Low noise fans are reducing this problem but at cost of fan efficiency & hence higher energy costs.
• Sometimes require more area to be installed and hence more plot area is required.

Conclusion

The articles describes briefly about different types of heat exchangers used in any Process Plant. As a piping engineer, we should have basic to advance knowledge about these heat exchangers for us to design the piping arrangement for these exchangers. Reader is requested to study in detail about these exchangers individually, know about their maintenance requirements. What are the platform and operation requirements? How to arrange valves around these exchangers? How to set elevations for exchangers? Is there any Specific process requirements mentioned in the P&IDs for these exchangers? All these questions need to be kept in mind while preparing conceptual pipe routing design for these exchangers.