Attributes and Uses of ASME Vessel Heat Transfer Surfaces
Knowing the pros and cons of each type of heat transfer surface simplifies selection.
Thermal heat-exchange surfaces are used on vessels for controlling temperature and quality of the contents of the vessel. Heat exchange surfaces can be designed for heating or cooling. Jacketed vessels are used in many industries and can be used to remove the elevated heat of reaction (heat reactor vessel) or reduce the viscosity of high viscous fluids.
- This article will discuss heat transfer methods that use steam or fluid, including:
- Type of heat-exchange surfaces.
- General application information and comparison.
- Optional surface treatments.
Three basic types of external jacketed heat transfer surfaces are used: conventional jacket, dimple jacket and half-pipe jacket. External jackets are welded to the outside of the vessel. Internal coils also can be utilized as a stand-alone option or in combination with any of the other types of external jackets.
Conventional jackets are basically an open jacket with an annular space containing the heat transfer media on the exterior surface of the tank. In some instances, internally welded baffles within the conventional jacket control the flow of the heating or cooling agent. Typically, water, oil or heat transfer fluids are used as heat transfer agents for conventional jackets. Variable coverage areas and pressures are available in conventional-jacket designs. Shells, bottom head, top head or the entire tank can be ASME certified.
Conventional jackets are best used for low pressure applications below 50 psi. They also are used in high fluid volume applications. The major advantage is that this jacket type allows for the lowest pressure drop. Conventional tank designs often are used in small vessels under 100 gal.
While the conventional-jacket design allows the lowest pressure drop, it also can drive up the cost due to the thickness of the material required. Because there is no reinforcement in the design, greater material thickness is required to accommodate the jacket’s external pressure requirements on the vessel wall.
Dimple jackets use a thin-gauge, stainless steel layer that is plug welded to the vessel shell in a regular pattern. The punch and spot-welded areas are called dimples, which create turbulence of the heating or cooling fluid flowing through the jacket. The dimple-jacket design allows for thinner vessel shell walls than a conventional jacket design due to the strength of the dimple pattern design.
Dimple jackets use a thin-gauge stainless steel layer that is plug welded to the vessel shell in a regular pattern.
Dimple jackets are manufactured in several different pressures and patterns. Patterns should be validated and proof tested per ASME regulations. Applications, specifications, cost and contractor experience are important factors when choosing the type of dimple-jacket construction.
The dimple-jacket design provides a large heating or cooling transfer area up to 200 psig. Dimple jackets are versatile. They can be used to provide heat transfer or cooling for nearly any shape or size of vessel. Dimple-jacket technology is not limited to tanks.
Dimple jackets are well suited to steam applications. High jacket pressures are permitted without significant increases of side-structure thickness and provide heat transfer at low media flows.
On large tanks, dimple jackets have a lower price point and maintain a higher pressure drop compared to conventional jackets. On small vessels, conventional jackets have a lower price point, followed by dimple-jacket and half-pipe solutions.
Dimple jackets are not, however, recommended for thermal cycling or when shocking is required. Typically, the stronger half-pipe design is recommended for those applications.
Half-Pipe Coil Jacket
The half-pipe jacketed vessel has a split pipe (split evenly or rolled-formed sheet) wound around the vessel and welded into place. This design provides optimal strength and can be rated up to 500 psig. Half-pipe jackets are recommended for high temperature and liquid heat transfer applications.
Half-pipe jacketed vessels have a split pipe wound around the vessel and welded into place. The design provides strength and can be rated up to 500 psig.
Materials used for half-pipe jackets are commonly 304, 304L or 316 stainless steel and can be welded to a range of alloys. Sound welding practices allow the half-pipe jacket to be welded to high alloys. Half-pipe heat transfer surfaces can contain the entire vessel or part of the vessel, depending upon the applications. Other technologies include inflatable half-pipe designs with laser- or resistance-welded heat sections.
Half-pipe jacket designs often are used when the jacket pressure is the determining factor in vessel wall thickness. In terms of cost and material thickness, half-pipe jackets fall between dimple jackets and conventional jackets on large tanks.
Conventional jackets are an open jacket with an annular space containing the heat transfer media on the exterior surface of the tank.
Internal coils are used inside vessels for transient heating or cooling of the liquid contained in the tank, typically on a batch basis. Coils provide heating and cooling surface contact. They are manufactured as a formed spiral around the inside of the shell, or as a U-shape in the center of the vessel from the top head down. Internal coil heat transfer solutions are found in many industries. However, the ability to clean the coil effectively and efficiently may be an issue in industries that require extreme sanitation or food-safety protocols.
Internal coils are used inside vessels for transient heating or cooling of the liquid contained in the tank, typically on a batch basis.
The internal coil design provides high flow, high internal and external pressures and high pipe ratings. Internal coils often are used in industrial or chemical applications where the product is not corrosive to the coil. It should be noted that, depending on the loads of the application, the required heat transfer may not be attainable with the internal coil.
Internal coils provide efficient heat transfer and are used for heating and cooling surface contact while handling high internal and external pressures.
Food Processing Equipment Application
The stay-bolt, also referred to as a stay-rod, jacket is another design for heat transfer that handles elevated temperatures and pressures that cause stress. Rods are individually welded between the inner shell and outer jacket, providing a robust jacket design.
A stay-bolt jacket can be used on equipment such as a cookers, which are engineered for large-scale industrial batch heating and cooling operations. Due to the extremes in steam heating and cooling processes combined with constant agitation from blending, stay-bolt jackets may be recommended for such demanding applications.
Dimple jackets may be used in equipment requiring heat transfer. In addition, Mixers and auger carts can be engineered with dimple jackets to provide continuous heating or cooling to food processing applications. Temperature control on mixers, cookers and auger carts can also include steam injection and CO2 or N2 injection in addition to ASME dimple jacketing. Requirements for dimple and stay-bolt jackets depend upon the food processing application.
There are several finishing options available, including pickle passivation and Thurmalox.
When required for an application, full-immersion passivation may provide a way to remove free iron and aid in the formation of the stainless material’s passive oxide layer. Full-immersion passivation is an efficient method of color cleaning the exterior welds on a tank. Dimple jackets often are passivated as a byproduct of another requirement that was specified for the tank. Passivation also helps Thurmalox adhere to stainless.
Thurmalox is a high temperature industrial protective coating that is used on conventional, dimple and half-pipe jacket designs. The product is an air-drying, silicone-based heat-resistant coating that protects thermally insulated austentic stainless steel from chloride-induced stress corrosion cracking.
This article was written by Jessica Jacobson, marketing and communications, with Apache Stainless Equipment Corp., with contributions from Nick Buchda, Tom Kosidowski and Tim Reinecke. The Beaver Dam, Wis.-based company can be reached at 920-356-9900 or visit www.apachestainless.com.