Liquid-to-Liquid Heat Exchangers
Liquid-to-liquid heat exchangers are the most common types of heat exchangers. They transfer heat between liquids without the liquids making contact. Aside from this simple description of the liquid-to-liquid heat exchanger process, how the process is completed takes multiple forms and configurations. The main use for liquid-to-liquid heat exchangers is found in industrial applications where the control of the temperature of liquids is necessary for the success of an application.
All forms of heat exchangers have the same objective, which is to transfer heat between sources. They are used as part of cooling processes and heating processes. The exchange of heat can take place through a wall to prevent fluids from mixing or by direct contact between substances. The radiator of an automobile is a heat exchanger where coolant flows through coils and air flows over the coils to cool the coolant by passing heat into the air.
The many forms of heat exchangers can be found in homes, offices, and industrial operations. They are a necessary part of modern applications and take many forms, with the liquid-to-liquid type being the most prevalent and efficient. The basic structure of liquid-to-liquid heat exchangers involves tubes or pipes through which the liquids then transfer heat flow.
Types of Liquid-to-Liquid Heat Exchangers
Liquid-to-liquid heat exchangers are the most efficient and cost-effective method used to transfer heat. The many types of liquid-to-liquid heat exchangers are specifically designed to meet the requirements of an application, process, or other function. Liquid-to-liquid heat exchangers are the most prevalent form of heat exchanger due to the many forms they take, with some models being very compact, yet highly efficient.
Shell and Tube Heat Exchangers
Shell and tube liquid-to-liquid heat exchangers are the most popular form of heat exchanger due to their design flexibility, which allows for a wide range of pressures and temperatures. The two classes of shell and tube liquid-to-liquid heat exchangers are ones approved by the Tubular Exchanger Manufacturers Association (TEMA) for petrochemical industry use and those used in the power industry.
All shell and tube liquid-to-liquid heat exchangers consist of multiple tubes mounted inside a cylindrical shell. The two fluids, process fluid and utility fluid, exchange heat in the cylinder. One of the fluids is contained in the tubes, while the other fluid, located in the cylinder, flows around the tubes such that heat can be exchanged between the two fluids. The exchange takes place as the fluid in the cylinder flows over the tubes.
With shell and tube liquid-to-liquid heat exchangers, the fluids can be single or two-phase and flow in parallel, cross, or counter flow patterns. TEMA has developed a standard method for describing a shell and tube liquid-to-liquid heat exchanger, which consists of three letters, with each letter identifying a characteristic of a particular type of shell and tube heat exchanger. The first letter of the three-letter designation identifies the front header type, while the second letter indicates the shell type. The final letter is a descriptor for the rear header type. The letter set BEM indicates a bonnet front header, one pass shell, and a fixed tube sheet rear header.
The many forms of shell and tube liquid-to-liquid heat exchangers are carefully designed to meet the requirements of a manufacturer. Shell and tube heat exchanger manufacturers work closely with their clients to produce the right shell and tube heat exchanger for an application or process. Their attention to detail in their planning and designs guarantees the quality and effectiveness of the heat exchanger process.
Twisted Tube Liquid-to-Liquid Heat Exchanger
Twisted tube liquid-to-liquid heat exchangers are designed to address the drawbacks of traditional shell and tube liquid-to-liquid heat exchangers. The complex swirl flow caused by the twisted tubes induces turbulence that enhances heat transfer. Turbulence is achieved with any substance, including high viscosity fluids and ones with low velocity. The flow distribution provides a more effective length and surface area than that of shell and tube heat exchangers.
The helix shaped tubes are arranged in a triangular pattern with each tube supported by an adjacent tube. The uniqueness of the support system eliminates vibrations and the need for baffles. The tube design provides for consistent flow distribution and velocity across the entire tube bundle.
The longitudinal swirl flow of twisted tube liquid-to-liquid heat exchangers reduces the pressure drop that is associated with segmental baffles. With the increase in the number of tubes, there can be fewer tube passes for a lower pressure drop on the tube side. In addition, the baffle free design of twisted tube liquid-to-liquid heat exchangers eliminates dead spots, which prevents fouling and helps keep liquid velocity constant and uniform.
Scraped Surface Liquid-to-Liquid Heat Exchanger
Scraped surface liquid-to-liquid heat exchangers have a similar structure to that of shell and tube liquid-to-liquid heat exchangers in that they have a long cylindrical shell. The factor that distinguishes shell and tube heat exchangers from scraped surface heat exchangers is the rotating shaft in scraped surface heat exchangers. Attached to the rotating shaft are blades or paddles that scrape the inner walls of the shell, removing buildup or fouling that occurs during heat exchanging.
The prevention of fouling helps enhance and maintain the heat transfer rate due to fouling, since fouling lowers the heat transfer coefficient and increases energy costs. Although the scraping method of a scraped surface liquid-to-liquid heat exchanger is standard, the process varies in accordance with the requirements of an application. Different tubing, internal and external heating or cooling, and construction materials are modified to meet process requirements.
The common use of scraped surface liquid-to-liquid heat exchangers is in the processing of viscous or fouling fluids that cause fouling during the heat exchanger process. Typical applications include food processing of chocolate, cheese, butter, and semi-solid foods. They are also used in the plastic industry for the manufacture of polymers, resins, and adhesives. The design of scraped surface heat exchangers ensures that surfaces are clean and free from material build-up.
The three liquid-to-liquid heat exchangers described above are a small sampling of the many liquid-to-liquid heat exchangers manufactured for industrial use. There is a liquid-to-liquid heat exchanger designed for any industry that requires efficient transfer of heat between liquids.
Components of Liquid-to-Liquid Heat Exchangers
The basic elements of most liquid-to-liquid heat exchangers are all the same. How the components are arranged is the factor that makes each type of liquid-to-liquid heat exchanger unique. The components that are found in all liquid-to-liquid heat exchangers are tubes or pipes, an enclosure, and plates.
Tubes
The tubes or pipes of a liquid-to-liquid heat exchanger are the keys to the process. They deliver the process and utility fluids that exchange heat. The tubes of liquid-to-liquid heat exchangers have to be exceptionally strong and durable due to the pressure of the process. They are normally made of copper, stainless steel, titanium, copper nickel, or carbon steel.
Plates
The plates of a liquid-to-liquid heat exchanger hold the tubes through which fluids pass. They secure the tubes in place to control vibrations. The types of plates vary in accordance with the design of a liquid-to-liquid heat exchanger and its use.
Enclosure
The types, sizes, and design of liquid-to-liquid heat exchanger enclosures take several forms depending on the types of liquid-to-liquid heat exchanger, the materials used to construct the enclosure, and the application. The design of enclosures is carefully monitored and includes attention to pressure, material strength, durability, and ability to endure the stress of the process. Common metals used for enclosures are carbon steel, stainless steel, and high-performance, highly durable metals.