Understanding Shell and Tube Heat Exchangers

Shell and tube heat exchangers are one of the most effective heat exchangers employed in various industries such as refineries and chemical industries. These process equipment are widely used in applications, which require cooling or heating a large volume of process fluids or gases. Shell and tube heat exchangers comprise a cylindrical shell with a large number of small tubes. The tubes are positioned into the cylinder using a tube bundle or "tube stack" which can either have fixed tube plates. The tubes are constructed using thermally conductive materials, which enable the exchange of heat between the hot fluids flowing outside the tubes and the coolant flowing through the tubes. These heat exchangers offer an optimal cooling solution in different fields such as Industrial, Hydraulic, Marine, Railways, etc.

Components of Shell & Tube Exchangers

The shell-and-tube heat exchanger is named for its two major components – round tubes & cylindrical shell. The shell cylinder can be fabricated from a rolled plate or from piping while the tubes are thin-walled tubing manufactured specifically to facilitate heat exchange.

● Tubes: The tubing may be seamless or welded. Tubing may consist of ‘Finns’ to provide more efficient heat transfer surface. Fins are commonly found on the outside of the tubes but are also available on the inside of the tubes. Tubes with a special surface, called high flux tubes, are used to enhance heat transfer on either or both sides of the tube wall.

● Tubesheets: Tubesheets are plates or forgings drilled to provide holes (triangular or square) for holding and inserting the tubes. Tubes are properly secured to the tube sheet to prevent the fluid on the shell side from mixing with the fluid on the tube side. The distance between the tube holes, measured from their centers, is called the tube pitch. Triangular pitch provides higher heat transfer and compactness while square pitch facilitates mechanical cleaning of the outside of the tubes.

● Baffles: Baffles are used for 3 reasons

○ To support the tubes

○ To maintain spacing between them

○ To direct the flow of fluid through the shell.

A segment, known as the baffle cut, is chipped in a way so as to permit the fluid to flow parallel to the tube axis as it flows from one baffle space to another.  The spacing between segmental baffles, called as the baffle pitch, along with the baffle cut, is used to determine the cross-flow velocity and hence the rate of heat transfer and the pressure drop.

● Tie Rods and Spacers - Tie rods and spacers are used to hold baffle assembly together & maintain the selected baffle spacing. The tie rods are secured at one end to the tube sheet and at the other end, holding the assembly together.

● Front Header: Also referred to as a stationary header, is the section from where the fluid enters the tube end of the exchanger.

● Rear Header: Is a section from where the fluid from the tube end leaves the exchanger or returns to the front header.

Working of Shell and tube heat exchanger

There are two fluids of different temperatures involved in the cooling operation, one - the process and the other - the cooling medium. The process fluid to be cooled is generally run through small diameter tubes that are housed within the shell. The outer shell, on the other hand, circulates the cooling medium. Both process and cooling fluid are kept in continuous circulation for the heat exchanger to function properly.

Applications of Shell and Tube Heat Exchangers

Shell and tube heat exchanger is used in various industrial applications due to their expertise in performing tasks such as:

● Cooling of hydraulic and lube oil

● Cooling of turbine, compressor, and engine

● Condensing process vapor or steam

● Evaporating process liquid or steam

Benefits of using Shell and tube exchangers

Shell & Tube Heat Exchangers are used in a number of industries such as refineries because of their advantages on other types of heat exchangers. Their benefits include

● Increased efficiency of heat transfer

● Easy to dismantle, clean and repair

● Compact in size

● Capacity can be increased simply by adding plates in pairs

● Affordable as compared to plate type coolers

● Can be used in systems with high operating temperatures and pressures

Praj industries - one of the leading Heat Exchanger Suppliers in the country, offers Critical Process Equipment & Systems to various process industries such as Oil & Gas, Refining, Petrochemicals, Fertilizers, Chemicals, Food, Pharma and Biotech. Praj also offers a range of static equipment like pressure vessels, reactors, shell and tube heat exchangers, distillation columns and other proprietary equipment as per the client design. 

Production of Ethanol using Dry Milling .

Ethanol is produced from biomass, primarily by fermenting the glucose derived from sugars obtained from sugar cane, sugar beet, and molasses, or starches obtained from corn, wheat, grains or using cellulose as raw materials. Industrial production of ethanol is majorly carried on using either the wet mill or dry mill process. Wet milling process involves separating the grain kernel into its constituents of germ, fiber, protein, and starch, before fermentation. Whereas in the dry mill process, the entire grain kernel is directly ground into flour. The starch in the flour is converted to ethanol during the fermentation process, creating carbon dioxide and distillers’ grain. In the USA, around 67% of ethanol is produced by the ethanol industry using the dry-grinding process. The various procedure involved in the ‘dry-milling’ process is discussed below.

Mash Formation

Screening is used to remove debris from the grains mass. These filtered kernels are then ground and mixed with water to form a slurry called ‘mash’.

Cooking

The mash is cooked, followed by the addition of enzyme which converts the starch into sugar. Yeast is then added to ferment the sugar which results in a mixture of ethanol and solids. The ethanol is extracted using the distillation & dehydration process. The solid which remains is dried to find its application as a distiller’s dried grain soluble or DDGS. DDGS is popularly used as a high-protein supplement in cattle, swine, poultry, and fish diets. Starch usually comprises of 25-30% amylose and rest is amylopectin. In order to metabolize this starch, yeast is used break it down into glucose, prior to fermentation.

Liquefaction

The slurry is then pumped through a pressurized jet cooker at high temperatures and held for a few minutes. The mixture is then cooled using a condenser. After the condensation cooling, the mixture is held for a few hours at a predefined temperature to give the alpha-amylase enzyme time to break down the starch. The slurry is then heated to reduce viscosity and to provide mechanical shearing to rupture starch molecule, especially of high molecular weight. The mash is further liquefied for at least 30 min to reduce the size of the starch polymer. This dextrinized mash is further cooked to facilitate the addition of glucoamylase to convert liquefied starch into glucose. To accomplish the saccharification of starch to glucose, glucoamylase is added in enough quantity.

Saccharification

Once inside the fermentation tanks, the dextrins are broken down to form simple sugars. Yeast is then added to convert the sugar to ethanol and carbon dioxide. The mash is afterward allowed to ferment for 48–72 hours, resulting in a mixture that contains about 10% ethanol as well as the solids from the grain and added yeast.

Fermentation 

After saccharification, cooling is done and mash is transferred to fermenter and yeast is added. The whole process requires 48-72 hours and can concentrate up to 10-12% of ethanol. CO2 released during this process can be captured and sold for the use in carbonated soft drinks, dry ice, and some beverages industries.

Distillation and Dehydration

Distillation is the process of separating ethanol from the solids and water in the mash. The fermented mash is pumped into the distillation columns where additional heat is added. The columns utilize the differences in the boiling points of ethanol and water as a milestone to boil off and separate the ethanol. By the time the product stream is ready to leave the distillation columns, it contains about 95% ethanol by volume. To carry out the separation of the remaining 5% water from ethanol, It is passed through a molecular sieve. This step produces 100% pure ethanol.