Heat exchangers are exactly what they are called: devices that transfer heat from one medium to another and heat exchangers find wide use in power generation and other industries. Tubesheets are simply sheets of metal perforated with holes that metal tubes can pass through at either end of a heat exchanger. The product medium typically flows into the exchanger in one large pipe.
When it encounters the tubesheet, the product is split into smaller tubes that pass through the service area where the heat-exchanging medium flows. These many small tubes maximize the surface area exposed to the service fluid, enhancing the heat transfer into or out of the product medium. At the output end of the heat exchanger, the product medium flows through another tubesheet and joins into a single product flow at the desired temperature range.
Double tubesheet heat exchangers add a perforated sheet for the tubes to pass through at the product inflow and outflow sides, usually with a void space between tubesheets still sealed against the external environment. Double tubesheet is considered sanitary because it provides an additional barrier to prevent cross-contamination between the product and the service medium that brings the product to the desired temperature. This is a considerable benefit because many service mediums are toxic or potentially explosive when mixed with the product.
The Benefits of a Double Tubesheet Design
The benefits of a double tubesheet design are best illustrated by their use in power plants where the heat itself is the product. For example, heat is generated in a nuclear power plant from either fission reaction. This heats the water surrounding the reactor. This water is the service medium. When heated, it flows to a type of double tubesheet heat exchanger. The generated heat is transferred to a separate water loop, generating superheated steam that turns the turbine and produces power. In this example, the double tubesheets prevent the external product water loop from being contaminated with radioactive materials.
Natural gas and coal-powered plants frequently have similar setups on the cooling side of the loop. Double tubesheet condensers bring the steam that turns the turbines back into a liquid water state. This steam is the product medium, and the service medium is typically water drawn from an external source like a cooling pond or seawater. In this example, double tubesheet heat exchangers prevent the water used in the power-generating loop from becoming contaminated with external minerals, bacteria, and contaminants.
In both examples, any leaks that occur due to containment failures in the heat exchanger’s service side will be confined to the void space between tubesheets rather than contaminating the product medium. Not only does this keep the product uncontaminated, but it also allows for any leaks in the system to be detected before they can escape the confinement of the heat exchanger. In addition, depending on how they are set up, a double tubesheet is sanitary because the product side of the exchanger can be opened up and easily cleaned or flushed out.
Industries where Double Tubesheet Heat Exchangers are Used
While the power industry provides the simplest example of where double tubesheet heat exchangers are used, they are used in various industries and go by various names depending on where they are in the process loop and what they do there. For example, if one sector requires heating and cooling a product while keeping that product uncontaminated, you will find a double tubesheet sanitary heat exchanger. Some of the industries where they are used are listed below:
- Food and Beverage: Pasteurization uses double tubesheet heat exchangers to bring products like milk to a temperature that kills bacteria and other microorganisms. They are also used to carefully control the cooling of ice cream while keeping things sanitary.
- Pharmaceutical and biotech: The pharmaceutical industry uses double tubesheet heat exchangers like the food and beverage industry. Namely, heat products to the desired temperature so that undesirable microorganisms are killed without denaturing the biological product or pharmaceutical produced.
- Chemical Manufacturing: A lot of chemical production depends on either high heat input or carefully controlled heat input to create the desired molecules. Often this is only possible with certain highly specialized service mediums; for example, chlorosilanes are used to produce silicone polymers, but if allowed to mix with water, they form hydrochloric acids and poisonous chloride gasses. Double tubesheet heat exchangers prevent the water service medium from mixing with the chemical product in these plants.
In addition to these roles in industry, sanitary double tubesheet heat exchangers are very common in heat reclamation. This is where waste heat at the end of the product cycle is captured and transferred back to the beginning to reduce the energy expenditure of bringing or keeping the product at the necessary process temperature.
Double tubesheet are not only sanitary, they are also crucial for efficiency. This combination of properties means they are widely manufactured from a wide range of materials and to a dizzying array of specifications.
Materials and Manufacturing of Double Tubesheet Heat Exchangers
Stainless steel is far and away the most commonly used sanitary metal, and it should be no surprise that most sanitary double tubesheet heat exchangers are made from stainless steel tubing and pipe. However, given the extreme heat conditions and incredibly corrosive materials that are sometimes used as product mediums in these heat exchangers, a variety of superalloys like Inconel, Hastelloy, and Titanium are also used.
Those familiar with industrial stainless steel welding practices know that welding stainless steel can be challenging. Stainless steel welding heat expansion and contraction can be detrimental and interfere with making a strong weld, let alone a weld that prevents leaks over time under extreme conditions. Nickel and nickel-iron superalloys are similarly challenging to weld and meet very high specifications. Not the least because they are costly and thus rare enough that most welders never get to practice with them. Both tube and tube to tubesheet welding are notoriously difficult, and using difficult-to-weld metals does not make production easier.
Orbital welding provides a solution for creating sanitary double tubesheet heat exchangers. This automated welding process simplifies and simplifies autogenous welding like that of tube. Closed weldheads simply enclose the tube and perform a pre-programmed welding parameter with a degree of consistency that manual welders would be hard-pressed to match. The process has been adapted to tube to tubesheet weld as well, making the production of sanitary double tubesheet heat exchangers, if not easy, then at least consistent and reliable.
Arc Machines, Inc. is a long time leader in orbital welding equipment and accessories. Helping industry achieve quality and sanitary requirements for double tubesheet heat exchangers and welding across industries. Contact sales@arcmachines.com. For to learn more or contact us directly.
