With various heat exchanger designs on the market, selecting the most suitable design for transferring heat between fluids requires some knowledge of differences between heat exchanger styles. Show
In this post, we discuss key characteristics of:
A key question in heat exchanger selection is always, “What type of heat exchanger does my process require?” Process requirements come from fundamental properties of processed fluids such as viscosity and particulates. Processes also have thermal output requirements or amounts of heat that must transfer between fluids, and the temperature change that must occur by the end of the process. When possible, using a plate heat exchanger is the right choice because they’re the most efficient and least expensive option. However, when fluids are highly viscous (thick) or include particulates, tubular exchangers can be more effective. Scraped-surface heat exchangers are the most expensive option; they are also the most effective choice when fluids are incredibly thick or contain large particulates. Scraped-surface can be used for all products, but they are much higher in cost and lower in capacity. Key characteristics of processed fluidsHeat transfer inside a heat exchanger results in temperature changes in both fluids, lowering the temperature of the warmer fluid and raising the temperature of the cooler fluid. Fluid properties partially determine the type of heat exchanger for the application. Fluid types include:
Thermal properties of fluids include:
Plate heat exchangers handle low-to medium-viscosity fluids at higher flow rates than tube or scraped-surface models. Corrosiveness of fluids is an important consideration because of their effects on heat exchanger components. Graphite is used for some industrial and pharma applications if the product is corrosive. When fluids have a high salt content, an even more corrosive resistant material like titanium is recommended for heat exchanger fabrication. Manufacturers of heat exchangers typically ask for additional particulars about fluids before they can provide an accurate cost quotation, including:
Consider phasingHeat exchangers fall into two basic categories based on the fluids (liquids or gases) being processed.
Units configured to cool gases so they condense into a liquid are called condensers. Key design factors of heat exchangersSome heat exchanger designs achieve higher heat-transfer rates and operate at higher temperatures than other designs. High heat transfer rates or specifications with close temperature approaches can include cooling cream to less than 40°F with 34°F and cooling beer wort with city water as far as possible to limit the load on the glycol chiller. Size and shape are important factors as well because space limitations may dictate positioning and footprint requirements. Need for expandability: if a need to expand or contract heat transfer capacity is likely, plate heat exchangers are an excellent choice because you can change capacity easily by adding or removing plates. Considerations when selecting a heat exchangerThe table below outlines several considerations to weigh when selecting a heat exchanger. CharacteristicsPlate-and-FrameShell-and-TubeScraped-SurfaceCost Per Square FootLowLowHIghLaminarLowLowMedium/HIghTurbulentHighMediumMediumAmount of RegenerationHighMediumNoneMaintenance CostMediumLowHighOperating PressureLowHighHighUse With ParticulatesPoorGood/ExcellentExcellentCIP AbilityExcellentExcellentGoodMaterials of Construction AvailableGoodGoodGoodResidence TimeLowMediumMediumLength of TimeMedium/GoodMedium/GoodExcellenceFlexibility of ProcessFairGoodGoodWhen to choose a Plate-and-frame heat exchangerIn this section, we compare shell-and-tube with plate-and-frame heat exchangers based on:
1. Energy usePlate heat exchangers are up to five times more efficient than shell-and-tube designs. In many cases, you can recover more heat by replacing existing shell-and-tube models with compact heat exchangers. The series of plates in a plate-and-frame heat exchanger creates gasketed spaces between plates. These spaces alternate between two fluids—one hot and one cold. The design delivers very high heat transfer efficiency because the plates create a surface area that’s much larger than shell-and-tube designs that would fit in the same space. 2. FootprintShell-and-tube heat exchangers require more floor space than plate heat exchangers, as the comparison on the left illustrates. 3. Ease of maintenanceHigh-quality plate-and-frame heat exchangers can operate efficiently for many years without maintenance. Plates are easy to access for inspection or cleaning. Cleaning intervals depend on amounts of fouling or scaling. Plate heat exchangers are much easier to clean and maintain than shell-and-tube because they’re easier to take apart and inspect. You can easily remove plates for cleaning or repair, whereas shell-and-tube models are more labor-intensive. Sludge or scale coating on surfaces reduces the heat exchanger’s thermal efficiency. Because of plate and frame exchangers’ smaller, modular design, they are faster and easier to clean than larger shell-and-tube units. 4. Capacity adjustmentPlate heat exchangers make capacity adjustment relatively easy. Plates are easy to access, add, and remove. In contrast, shell-and-tube units have fixed capacity at the time of installation. 5. Capital expenseCost of ownership is always a critical factor in the decision-making process. A plate heat exchanger is the lowest cost option because it can achieve high heat transfer coefficients — with pure counter current flow — giving the most efficient heat transfer and lowest surface area. Maintenance cost is also reasonably low, especially compared to scraped-surface heat exchangers. The main expense is the replacement of gaskets and, occasionally, of plates. The close temperature approach, meaning a cold fluid being heated to a temperature that is very close to the temperature of the entering hot fluid, allows more regeneration and heat recovery, making a plate heat exchanger a good option. The compact size of a plate heat exchanger is also important to applications with tight spaces. One key advantage of plate heat exchangers is their modularity. Installers can assemble or disassemble plate heat exchanger units onsite. These units have about 6% the weight of comparable shell-and-tube units while taking up about 10% of the floor space. In addition to the capital expense for the equipment, capital costs can include:
To improve energy efficiency, using a smaller heat exchanger to meet existing requirements translates into savings in transportation and installation. When to choose a shell-and-tube heat exchangerThe basic shell-and-tube principle moves product through a bundle of parallel tubes with heating fluid between and around the tubes. Fluid flowing inside the tubes is called tube-side fluid; fluid flowing on the outside of the tubes is called shell-side fluid. The tube-side fluid is separated from the shell-side fluid by tube sheets. In applications where the two fluids flow at different pressures, the higher-pressure fluid typically flows through tubes, and lower-pressure fluid flows on the shell side. The reason is cost: high-pressure tubes are less expensive than high-pressure shells. Because of the larger clearances between shell-and-tubes than plates in plate heat exchangers, the shell-and-tube design is a good choice for fluids that contain particulates — such as pulpy juices. Because shell-and-tube heat exchangers consist of a large pressure-vessel shell with tube bundles inside, they can conduct heat transfer under higher pressure than some other designs. FoulingAnother advantage of shell-and-tube designs is a lower potential for the fluids you’re processing to foul the equipment. Fouling of heat exchangers is the accumulation of deposits from fluids that pass over or through heat transfer surfaces. The fouling issue comes down to the impact of fouling on exchanger performance and how easily you can clean the equipment. If the shell side is the right option for running a high-fouling liquid, adding baffles to the tube may be an effective way to increase turbulence and reduce opportunities for particles to stick to the inside of the shell or the outside of tubes. Fouling in heat exchangers occurs for several reasons:
When to choose a scraped-surface heat exchangerSome processes require heat transfer that prevents fouling from viscous and sticky products. In those processes, scraped-surface heat exchangers are the right choice because of their ability to process fluids that include a high number of particulates or high viscosity. Using scraped-surface designs can assist processors in moving from batch to continuous processing. While batch processing may be required in pharmaceutical processing to meet safety and purity requirements, continuous flow processing is often preferable in food processing due to its potential for reducing waste and improving productivity. In addition, scraped-surface heat exchangers can do phase changes (for products such as ice cream) and evaporation to cook and concentrate products such as sauces. Applications for scraped-surface heat exchangers
Making Your SelectionTo help with the model-selection process, manufacturers typically provide comparison charts like the one below. Heat exchangerAverage gap (mm)Particle size (mm)Fiber length (mm)Pulp %Viscosity CPSPlate-and-Frame Typical industrial plate 2.4-3.95 Dia 0.5 1 2 2500 Typical sanitary plate 3.95 Dia 0.5 1 3 5000 Low contact point plate 3.95 Dia 0.5 5 7 1000 Shell-and-Tube Monotube Pipes dia 14-97.6 Dia 12-95 Up to 50 mm No limit 1000 Multitube Pipes dia 12-22.6 id Dia 5-10 Up to 18 mm 60 1200 Annular space GAP 5-49 mm Dia 3-47 Up to 35 mm 80 12000 Scraped-Surface Up to 31 mm No limit No limit Guide to Choosing the Right Heat ExchangerThis guide is designed for processors, production managers, and mechanical engineers to help in the heat exchanger selection process. Guide to Choosing the Right Heat Exchanger Read GuideNext StepsWith various heat exchanger designs on the market, selecting the most suitable design for transferring heat between fluids can be difficult! 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. To learn how we can help, contact us today! Contact Us ABOUT CSICentral 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.
Related ArticlesHow Do Heat Exchangers Work? January 4th, 2022 How Do Heat Exchangers Work?Heat exchangers help control fluid temperatures in processing for pasteurization, sterilization, clean-in-place and other hygienic operations. Continue ReadingFouling in Heat Exchangers March 30th, 2021 Fouling in Heat ExchangersFouling is the formation of unwanted material deposits on heat transfer surfaces during process heating and cooling. Continue ReadingWhat Are the Different Types of Heat Exchangers? May 21st, 2022 What Are the Different Types of Heat Exchangers?Heat exchanger types vary to meet requirements for processing efficiency, cost of ownership, and maintenance. Continue ReadingTop 7 Reasons to Purchase a Plate-and-Frame Instead of a Shell-and-Tube Heat Exchanger August 2nd, 2021 Top 7 Reasons to Purchase a Plate-and-Frame Instead of a Shell-and-Tube Heat ExchangerPlate heat exchangers have several advantages over shell-and-tube designs. In this post, we discuss applications that make PHE the right choice. Which heat exchanger is the most efficient?Plate exchanger is the most efficient due to turbulent flow on both sides. High heat-transfer coefficient and high turbulence due to even flow distribution are important. However, a plate heat exchanger regenerator is restricted to low viscosities.
Which is better shellPlate heat exchangers are up to five times more efficient than shell-and-tube designs with approach temperatures as close as 1°F. Heat recovery can be increased substantially by simply exchanging existing shell-and-tubes for compact heat exchangers.
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