How Do Heat Exchangers Work?
How plate and frame heat exchangers work
Gasket plate and frame heat exchangers are among the most efficient designs so they are also among the most common designs in processing systems. Gaskets between plates guide the flow of product and heating/cooling fluids through alternating channels.
As hot fluids pass over the plates, heat transfers from the hot to the cold side, decreasing the temperature of the hot side and raising the temperature of the cold side.
Key to efficient operations, heat exchangers must maintain sufficient fluid velocity across plates to transfer heat while also controlling pressure drops that can disrupt operation.
Systems typically employ plate and frame heat exchangers for pasteurization, raw milk cooling, and CIP (cleaning-in-place) heating. Given their suitability for products with low to mid viscosity and few to no particulates, plate heat exchangers are also commonly used for beverage, beer, wort, eggs, sauces, and most dairy processing.
Regenerative heating and cooling
In milk processing, chilled milk is heated from, for example, 4 °C to a pasteurization temperature of 72 °C and held at that temperature for 15 seconds and then chilled to 4 °C again.
Heat always transfers from warmer substances to colder ones, so during pasteurization, heat exchangers use heat from the pasteurized milk to warm the cold milk, which saves heating and refrigeration energy. The process is called regenerative heat exchange or heat recovery, typically reaching 90% and achieving up to 95% heat recovery from pasteurized milk. Recovery is lower for higher-fat products such as cream and ice cream mix. Regeneration has a positive impact on energy savings, capital costs, and efficient operation. Heat transfer occurs rapidly when the temperature differential is high. As temperature difference decreases, the rate of transfer slows down and stops altogether when temperatures equalize (Dairy Processing Handbook).
Operators can have multiple sections on one frame to control the flow of hot or cold fluids when products have to be heated in one stage and then cooled in the next stage.
For pasteurization, a multi-section heat exchanger uses connection plates configured by different corner connections for single, double, pass-through or blind channels.
Plate and gasket technology
The design of the corrugated plates creates a large but compact total surface area for transferring heat. The heat transfer area of the plates features a herringbone pattern that creates high turbulence which increases heat transfer and aids cleaning during CIP.
The plate distribution area ensures an even flow of fluid over the entire plate to maximize heat transfer. An optimized flow distribution also reduces uneven temperature zones that contribute to fouling.
While the narrow flow path of plate heat exchangers creates efficient heat exchange, the narrow path also limits its ability to process fluids to those with low to medium viscosity and few suspended particles that can result in fouling from particulates getting caught on plate contact points.
For fluids that contain particles, two solutions are available:
- A low contact point, wide-stream plate that can run product with more particulate
- Wide-gap plates that can run more and larger particulate.
Both allow particles to pass through while minimizing fouling.
How shell and tube heat exchangers work
Instead of transferring heat through parallel plates, shell and tube heat exchangers transfer heat between a bundle of tubes surrounded by a large shell vessel. Fluids that run through the tubes exchange heat with fluids that run over the tubes contained by the shell.
Because the diameter of tubes is typically greater than the gap between plates in plate heat exchangers, shell and tube exchangers are suited to applications in which product is more viscous (resistant to flow), or contains high-density particulates. Maximum particle size depends on tube diameter. Tubular heat exchangers can typically run longer between cleanings than plate heat exchangers in ultra-high-temperature applications.
The basic shell and tube principle moves product through a bundle of parallel tubes with heating fluid between and around the tubes.
A concentric tubular heat exchanger features tubes of different diameters positioned concentrically inside of each other, which is especially efficient in heating or cooling because heating/cooling fluids flow on both sides of the product tubes. Product tubes can be sized to meet the requirements for viscosity and particulates. A concentric tube is especially suited to high- viscosity non-Newtonian fluids whose viscosity changes under pressure (shampoo, nail polish, ketchup).
As with other heat exchanger designs, shell and tube exchangers are set up to have product and heating/cooling fluids flow in opposite directions. For example, cold product fluid travels from right to left in the heat exchanger while the warming fluid travels from left to right over the product tubes. The counter-flow configuration takes advantage of maximized temperature differences for more efficient heat transfer.
One manufacturer’s Pharma-line of shell and tube heat exchanger operates at pressures of up to 10 bar and operating temperatures of 150°C. Typical applications for the shell-and-tube heat exchangers include systems that process water (for injection or purification, for example), and CIP systems.
How Double Tube Sheets Work
In pharmaceutical applications, the risk of mixing between product and the heating or cooling medium is eliminated thanks to a double tube sheet design.
Product flows in the tubes while the service fluid flows around the tubes inside the shell. Service fluid is sealed in the shell by one tube sheet and a second tube sheet seals the product.
Heat exchangers with double tube sheets make leaks easy to spot because they appear at the joint in the outer tube plate. The heating fluid is sealed in the shell by the first tube sheet and the second tube sheet seals the product. In the event of a leak, the leakage of either fluid is easily visually detected.
Shell and tube heat exchangers are especially effective in the pharmaceutical industry where product hygiene and demand for isolating products from heating/cooling fluids are especially high. To meet the industry’s demands, high-quality tubular heat exchangers control microbe growth and prevent cross-contamination.
Some of the newest tube-in-tube designs for pharmaceutical applications feature high shear force and turbulence to maintain efficient transfer of heat while reducing bio-film.
Smaller, lighter-weight heat exchangers designed for tighter spaces can be effective substitutes for larger tube heat exchangers. They feature the same hot and cold fluid flows through alternating channels that create high turbulence for high heat transfer efficiency, while using 50–80% less heat transfer area.
How scraped surface heat exchangers work
The many processes involved in manufacturing food, chemicals, pharmaceuticals, cosmetics, health and beauty products all require reliable heat transfer that prevents fouling from viscous and sticky products. In those processes, scraped surface heat exchangers are the right choice.
Their ability to process fluids with a high number of particulates or high viscosity make them more efficient in those applications.
Scraped surface heat exchangers are more expensive than other exchangers, but they work efficiently when other heat exchangers would be ineffective.
In scraped-surface heat exchanger applications, the product enters the cylinder at the bottom and flows upward. The heating or cooling medium travels through a narrow ring-shaped (annular) channel.
Typical processing applications:
- Ketchup
- Mayonnaise
- Spreads and fillings
- Sauces and puddings
- Baby food
- Skin lotions
- Shampoos
Scraped surface exchangers are fitted with rotating blades that remove product from the cylinder wall to maintain consistent heat transfer.
They’re designed specifically for gentle product handling to avoid interference with product quality and consistency.
Scraped surface exchangers are typically mounted vertically. Inside, an electric motor turns a rotor fitted with scraping blades. To prevent damage to product, rotors and product move through the heat exchanger in the same direction, with product entering at the bottom and exiting at the top.
Scraped surface heat exchangers are common in the food and personal care industries. Ensuring continuous production requires uniform heat transfer, but the consistency or content of some food products hinders efficient heat transfer. Scraped-surface heat exchangers meet the need for efficiency by keeping product off the walls and in the mix where it belongs.
Importance of Cleanability
In dairy processing, products have high protein content that can foul exchangers. Fouling occurs when processed fluids stick to internal surfaces and build up over time, reducing efficiency, so part of a good hygiene program includes using equipment that stays clean for a long time and is easy to clean during CIP.
Fouling can increase pressure, so heat exchangers subject to fouling or scaling should be cleaned periodically. A light sludge or scale coating on the tube reduces its thermal efficiency. Since the difficulty of cleaning increases as scale thickness or deposits increase, operators should perform routine checks to catch fouling sources early.
Advantages and Disadvantages of Each Type of Heat Exchanger
| Plate Heat Exchanger | Tubular | Scraped Surface | |
| Cost per square foot | Low | Low | High |
| Laminar | Low | Low | Medium/High |
| Turbulent | High | Medium | Medium |
| Amount of regeneration | High | Medium | None |
| Maintenance cost | Medium | Low | High |
| Operating pressure | Low | High | High |
| Use with particulates | Poor | Good/Excellent | Excellent |
| CIP ability | Excellent | Excellent | Good |
| Materials of construction available | Good | Good | Good |
| Residence time | Low | Medium | Medium |
| Length of time | Medium/Good | Medium/Good | Excellent |
| Flexibility of process | Fair | Good | Good |
In short, heat exchangers add value to pharmaceutical, food, and beverage operations in several unique ways
Heat exchangers heat the cleaning fluids that remove residues from systems components.
Heat exchangers create consistent temperatures for pasteurizing and CIP.
They heat water for effective rinsing of food production equipment (tanks and piping).
They can be placed on skids for small-footprint, flexible CIP equipment positioning.
Heat exchangers themselves are CIPable because their designs induce turbulence when systems maintain sufficient flow rate.
They transfer heat without contaminating the heated fluids.
Energy savings: regenerative heat transfer conserves energy by re-using heated fluids to heat fluids in repeatable cycles.
Guide to Choosing the Right Heat Exchanger
This guide is designed for processors, production managers, and mechanical engineers to help in the heat exchanger selection process.
Next Steps
As you've learned, heat exchanger styles can vary widely based on a number of variables, which can make the proper selection for your process appear daunting. We're here to help!
Whether you need parts to keep your current units operating, a direct replacement for a worn out or inefficient heat exchanger, or a new unit for a new process, CSI can support you. Our customer service team, engineers, designers, and product specialists provide solutions through a broad range of brands, technologies, and capabilities.
To learn how we can help, contact us today!
ABOUT CSI
Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.