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Heat Exchangers

Topic last reviewed: 1 February 2014

Sectors: Downstream, Midstream, Upstream

Heat exchangers are used to transfer heat from one medium to another. These media may be a gas, liquid, or a combination of both. The media may be separated by a solid wall to prevent mixing or may be in direct contact. Heat exchangers can improve a system’s energy efficiency by transferring heat from systems where it is not needed to other systems where it can be usefully used. 

For example, waste heat in the exhaust of an electricity-generating gas turbine can be transferred via a heat exchanger to boil water to drive a steam turbine to generate more electricity (this is the basis for Combined Cycle Gas Turbine technology).

Another common use of heat exchangers is to pre-heat a cold fluid entering a heated process system using heat from hot fluid exiting the system. This reduces the energy input necessary to heat the incoming fluid to working temperature.

  • Specific applications for heat exchangers include:
  • Heating a cooler fluid using the heat from a hotter fluid
  • Cooling a hot fluid by transferring its heat to a cooler fluid
  • Boiling a liquid using the heat from a hotter fluid
  • Boiling a liquid while condensing a hotter gaseous fluid
  • Condensing a gaseous fluid by means of a cooler fluid [Ref 1]

The fluids within heat exchangers typically flow rapidly, to facilitate the transfer of heat through forced convection. This rapid flow results in pressure losses in the fluids. The efficiency of heat exchangers refers to how well they transfer heat relative to the pressure loss they incur. Modern heat exchanger technology minimizes pressure losses while maximizing heat transfer and meeting other design goals like withstanding high fluid pressures, resisting fouling and corrosion, and allowing cleaning and repairs.

To utilize heat exchangers efficiently in a multi-process facility, heat flows should be considered at a systems level, for example via ‘pinch analysis’ [Insert link to Pinch Analysis page].  Special software exists to facilitate this type of analysis, and to identify and avoid situations likely to exacerbate heat exchanger fouling (see Case Study 1).

Application of Technology

Heat Exchangers are available in many types of construction, each with its advantages and limitations. The main heat exchanger types are:

Shell & Tube – The most common heat exchanger design type consists of a parallel arrangement of tubes in a shell [Figure 1]. One fluid flows through the tubes and the other fluid flows through the shell over the tubes. Tubes may be arranged in the shell to allow for parallel flow, counterflow, cross flow, or both. Heat exchangers may also be described as having tube layouts in single pass, multi-pass, or U-tube arrangements. Due to its tubular construction, this type of exchanger can handle large pressures. The exchanger may have one or two heads on the shell and multiple inlet, outlet, vent, and drain nozzles [Ref 2].

Figure 1 : Cross-section of a shell and tube heat exchanger with single pass, counter flow configuration, large segmental baffles, and two shell heads [Ref 3].

Flow deflecting features are often installed in shell and tube heat exchangers to improve the heat transfer between the fluids by creating more turbulent flow of the shell-side fluid and more perpendicular flow across the tubes. Such features must be carefully designed to minimize pressure losses and the formation of ‘dead zones’. Dead zones are regions of slow or stalled fluid flow which can lead to fouling (deposition of solids) in the heat exchanger.

Common flow-deflecting features include:

  • Segmental baffles (staggered perpendicular barriers, each blocking a fraction of the shell side; see Figure 1),
  • Disk and donut baffles – staggered circular and annular barriers force the shell-side flow alternately away and towards the axis of the shell
  • Helical baffles - angled to promote a spiralling flow around the shell side
  • Rod baffles - grids of rods, usually perpendicular to the shell axis. The tubes run axially through the spaces between the rods
  • Tube inserts – inserts, such as long wire coils, are placed inside the tubes to promote turbulent flow and to minimize fouling

Figure 2 – A helical baffles arrangement Note that the baffles would actually have many holes to allow passage of the tubes down the length of the shell. [Ref 4]

 

Another approach to flow deflection is the ‘Twisted Tube’ design, by Koch Heat Transfer Company. With this design the tubes are flattened into ovals and twisted into long spirals, then stacked together. Spiralling flow in both the shell-side and tube-side fluids gives good heat transfer with relatively low pressure drops.

Figure 3 – Tube inserts protruding from tubes in a shell-and-tube heat exchanger5

 

Figure 4 - Twisted Tube heat exchanger tubes and flow pattern6

 

Plate and Frame – thin parallel plates are stacked together to create broad, parallel channels. The hot and cold fluids flow through alternating channels. The plates are separated with a gasket or by welding and may have patterns to promote turbulent flow. The plates are stacked together and additional plates may be added on gasket designs to increase heating capacity. The flow may be arranged in either parallel or counter flow. The large surface area afforded by the plates means that plate and frame heat exchangers can allow more heat transfer between the two fluids, for a given volume relative to shell and tube heat exchangers.

Figure 5: Schematic of a Plate and Frame Heat Exchanger

Other Types – Variations of the previous types of exchangers include plate & fin, plate & shell, spiral, wet surface air cooler, and double pipe. 

The heat exchangers discussed so far all keep both fluids contained separately. Two other categories of heat exchanger exist, however:

  • Open Flow - one fluid is contained and the other fluid is not. Examples include a car radiator, tank immersion heater, fin/fan coolers, or duct coils
  • Direct Contact - immiscible media are brought into direct contact. A cooling tower is used to cool water as it is sprayed into a cooling air stream. The air and water do not mix but heat is exchanged by the evaporative process. The cooled water is then collected and returned to the plant8. Other heat exchangers of this type include rotating wheel regenerative and spray columns. Note that if the two fluids do not separate, then the device is referred to as heater or a cooler. For example, in a steam water tank sparger the steam is absorbed into the water as it cools and condenses.

Figure 6: Crossflow Cooling Tower, a type of Direct Contact Heat Exchanger

A summary of the advantages and limitations of these heat exchanger types is shown in the table below:

Table 1: Comparison of different Heat Exchanger types

  • Type Advantages Limitations
  • Shell & tube High efficiency
  • High operating pressures Large size
  • Double space needed for cleaning
  • Difficult to clean shell side
  • Plate & Frame Highest heat transfer coefficients
  • Low pressure drop
  • Easier to clean than shell & tube
  • Small size
  • Expandable capacity
  • Closer approach temperatures Low operating pressures
  • More prone to fouling by larger particles than shell & tube
  • Direct Contact Large flow rates
  • Low pressure drop
  • High efficiency
  • Less fouling 
  • Large size
  • Requires makeup water
  • Chemical treatment needs
  • Limited applications

 

Heat Exchanger Flow Configurations

Heat exchangers have three (3) primary flow configurations:

Parallel flow – the two fluids enter at the same end of the heat exchanger and flow in the same direction, parallel to one another. In this design, the temperature differences are large at the inlet, but the fluid temperatures will approach a similar value at the outlets.

Counter flow – the two fluids enter at opposite ends of the heat exchanger and flow counter to one another. In this design, the temperature differences are less but more constant over the length of the exchanger. It is possible that the fluid being heated may leave the exchanger at a higher temperature than the exit temperature of the heating fluid. This is the most efficient design because of higher temperature differential over length of the exchanger.

Cross flow – the two fluids flow perpendicular to one another.

There can be more than one method of heat transfer in a heat exchanger. Heat transfer will occur using one or more modes of transfer, conduction, convection, or radiation.

Implementation

Proper implementation of heat exchangers in multi-process systems, like oil refineries, requires a consideration of the network of heat flows on a systems level. This is often performed through ‘pinch analysis’, which matches available heat sources in a system with heat demands, in terms of both the quantity and temperature of the heat. Sophisticated software is available to aid the designer in this process. Fouling mitigation is also a consideration of design and can include the consideration of various technologies, velocities, bypasses for cleaning individual HX during the operation, and the incorporation of spare heat exchangers.

Similarly, software is available for managing heat exchanger fouling. Based on process conditions and component selection, some software packages can predict the rate at which heat exchangers are likely to experience fouling. Software packages are also available to monitor fouling by examining heat exchanger performance over time. Estimates of the costs of cleaning heat exchangers versus the economic benefits (in terms of reduced energy use) are also calculated.

Technology maturity

Commercially available?: Yes 
Offshore viability: Yes 
Brownfield retrofit?: Yes 
Years experience in the industry: 21+ 

Key metrics

Range of application:
Production wells, FPSO facilities, heat recovery from water or crude, heating, cooling, and condensing of water, product media, hydrocarbons, and gases, combustion air heating or cooling, exhaust gas steam production.
Efficiency: 2. 80% to almost 100%
Guideline capital costs: Generic “rules of thumb” for costs are not available due to the wide range of the exchangers available. Costs to consider include the heat exchanger, the skid or foundation, controls, connecting inlet and outlet piping, inlet filters, instruments, valves, fans, pumps, tanks, chemicals, redundancy, as well as installation, start-up and commissioning expenses.
Guideline operational costs: Includes routine maintenance like cleaning of tubes and plates, repairing leaks, rebuilding pumps, cooling towers fill replacement. Additional costs or lost revenue are related to plant outage time while equipment is off line. Operational costs include power for pumps, fans, and controls, and water treatment chemicals.
GHG reduction potential: 
Heat exchangers can greatly reduce the energy input needs of a process, reducing associated GHG emissions.
Time to perform engineering and installation:  1 week – 6 months
Typical scope of work description: Heat exchangers are used in a large variety of industries. A typical project will consider the use of heat exchangers during initial project planning, determine operating conditions, and write equipment specifications. A heat exchanger is generally built by a specialized manufacturer, tested, and delivered to the site ready for installation. Larger exchangers may be shipped in several pieces, or even assembled or constructed on site

Decision drivers

Technical: The pressure ranges of the working fluids and the pressure difference between them
The allowable pressure drop of the fluids across the heat exchanger
The temperature ranges for the working fluids and the required approach temperature
The properties of the working fluids (physical properties, such as density, viscosity, specific heat, thermal conductivity, temperature)
The tendency for the working fluids to cause fouling
Availability of water for cooling
Space available
Governing design codes
Redundancy
Operational: System complexity
Level of automation
Maintenance needs
Commercial: Delivery time
Equipment costs
Parasitic power demands
Material selection
Environmental: Water resources and availability
Discharge temperatures
Plume abatement
Permitting requirements
Noise requirements

Alternative technologies

There are technologies that may be considered as alternatives to using heat exchangers.

Cooling ponds may be used to allow warm water to naturally cool through evaporative loss to the atmosphere. The water in the pond can then be recirculated into the plant as cooling water. These ponds may offer secondary recreational purposes such as fishing, boating, or swimming. Make up water is required to account for evaporative losses. A large amount of land is required for this option.

Direct venting of steam may reduce the need for process water cooling but this option ignores the primary reasons for cooling which is to improve system efficiency and conserve process quality water and results in additional make up water and water treatment chemicals. This option is generally not used except in start-up, emergency venting and shut down operations.

Process design and control modifications may avoid or reduce the need for heat exchangers.

 

Operational issues/risks

Heat Exchangers require regular maintenance to operate at high efficiency and usually require a rigorous overhaul schedule. Much of this effort is aimed at countering the effects of fouling, wherein solids (like foreign particles or precipitates) accumulate on heat exchanger surfaces, inhibiting heat transfer and restricting fluid flow. Chemical additives can also prevent the precipitation of particles and may be a cost effective means of fouling prevention.

Overhauls can range from simple preventative maintenance activities (i.e., flushing) to repairs that require the tube bundle to be removed from the heat exchanger shell for cleaning. This down time should also be taken into consideration when sizing the heat exchangers and designing the process network.

Many heat exchangers operate at high pressures and temperatures or with hazardous fluids and adequate operating procedures must be followed to avoid personnel risks and system outages.

Heat exchangers are typically regulated by industry codes like those of ANSI and TEMA.  New equipment designs and any repairs should comply with applicable codes.

Opportunities/business case

Many heat exchanger designs are available in numerous materials and can be customized for specific applications as well as standard designs that are available with minimal lead time at lower costs. Several benefits of using heat exchangers are listed below:

  • Energy efficiency improvements of plant systems
  • Reduction of fuel usage, GHGs, and emissions
  • Replace existing equipment due to wear and tear
  • Upgrade existing equipment to newer more efficient designs
  • Additional heating or cooling capacity due to increase in plant output

Industry case studies

1. Air to air heat exchanger for waste heat recovery
This study reviews how a food processing facility used a heat exchanger to recover waste heat from a process and used it for heating process make-up air.

Seeking to control odor from its roasting operation, the facility installed an efficient new Regenerative Thermal Oxidizer (RTO).  To help save fuel this unit included supplemental fuel injection (SFI) for periods of low VOC loading. To further reduce operating costs, the company sought to recover waste heat from the RTO to preheat in-coming air. To do this, they contracted a design consultancy to analyze and design the HX solution.

The critical design factors for this project were the air flow rate, temperature of the airstream, permissible pressure drop of the application and the desired heat to be transferred to the heat exchanger. A secondary plate type heat exchanger was chosen because of its versatility and rugged, yet cleanable plates. It has a relatively low pressure drop, a small foot print and low capital cost, making it the most economical option for this application.

The consultancy analyzed the application data using heat exchanger performance modeling software. With this software they performed a boundary layer analysis and adjusted the plate thickness and spacing of the heat exchanger to maximize performance.

The RTO exhaust heat was used to preheat 3.3m3/s of air to approximately 88 degrees C. This hot air mixes without side air to provide 15.6 m3/s of heated air to the makeup air unit. The secondary heat exchanger transfers approximately 1.5MMBTU/hr of heat from the RTO exhaust to the air going back to the makeup air unit and the estimated yearly savings for the project was approximately $45,000.00.

Source: http://www.anguil.com/case-studies/energy-recovery/air-to-air-heat-exchanger-provides-plant-heat-and-big-savings.aspx?alttemplate=PDFCaseStudy&

 

2. Predicting Heat Exchanger Fouling

Build-up of dirt deposits, or fouling, on the metal surfaces of petrochemical plant heat exchangers is a major economic and environmental problem worldwide. Estimates have been made of fouling costs due primarily to wasted energy through excess fuel burn that are as high as 0.25% of the gross national product (GNP) of the industrialized countries. Many millions of tons of carbon emissions are the result of this inefficiency. Costs associated specifically with crude oil fouling in the pre-heat trains of oil refineries worldwide were estimated in 1995 to be of the order of $4.5bn.

This case study examines the use of fouling prediction software by French oil company Total. This software, developed by an industrial design consultancy in conjunction with major oil companies, aims to reduce or even eliminate crude oil fouling in pre-heat train heat exchangers.  In 2002, Total experienced heavy fouling in its preheat train, soon after revamping the refinery to improve efficiency. This led to a significant throughput reduction as the furnace bottlenecked. Total applied the consultancy’s software, which successfully identified the fouling heat exchangers and pointed to retrofit options. These were implemented, solving the problem and restoring normal system operation.

Source: http://www.ihs.com/news/overcoming-effect-oil-fouling.htm


References:

  1. Department of Energy Fundamentals Handbook, Mechanical Science, Module 2, Heat Exchangers, DOE-HDBK-1018/1-93.
  2. Heat Exchange Institute, Basics of Shell & Tube Heat Exchangers.
  3. -removed-
  4. http://en.hx-hr.com
  5. http://www.stamixco-usa.com/products/heat-exchangers/default.html
  6. http://www.oxide.co.il/en/twisted-tube.html
  7. http://www.spiraxsarco.com/resources/steam-engineering-tutorials/steam-engineering-principles-and-heat-transfer/steam-consumption-of-heat-exchangers.asp
  8. www.spxcooling.com/brands/cooling-towers/marley-cooling-tower/