Multi-tube heat exchanger with annular spaces

Details Multi-tube heat exchanger with annular spaces Multi-tube heat exchanger with annular spaces

2002-09-12

2003-09-30

Flanigan, Allen (Department: 3743)

Heat exchange

Casing or tank enclosed conduit assembly

Manifold formed by casing section and tube sheet of assembly

C165S081000, C165S163000

Reexamination Certificate

active

06626235

ABSTRACT:

FIELD OF INVENTION
The present invention relates to heat exchangers. In particular, it relates to a multi tube heat exchanger for exchanging heat between two fluids.
BACKGROUND OF THE INVENTION
Heat exchangers are used in many industries including food industry. A variety of heat exchangers are used depending upon specific application and so plate heat exchangers, tubular heat exchangers, shell and tube heat exchangers and scraped surface heat exchangers, etc. are used widely in the food industry. Prior art heat exchangers are efficient and cost effective when the fluids passing through them are Newtonian flow and have low viscosities. The capital and operating cost effectiveness depends in large part on the ability to use small diameter tubes to improve heat transfer to the tube side fluid.
However, a number of food products are thick Newtonian fluids, showing linear relationship between shear stress and shear rates and having very high apparent viscosities. Also, a large number of food slurries are non-Newtonian liquids with a very high apparent viscosity. Because of high viscosities, the operating pressure drop through conventional heat exchangers with small tubes rises to uneconomic levels, making high flow very costly due to pumping power and capital costs. The higher pressure drop requires one or more positive displacement pumps, which increase both the capital and operating costs.
One prior art method to reduce high operating pressures is to reduce product flow rates through small tube heat exchangers by arranging product flow in many parallel streams. Lower flow rates on the other hand result in lower heat transfer rates that requires an uneconomic increase in large heat transfer area. Increasing heat transfer area results into more capital investment, more space requirement and significant product loss in cleaning, a rather frequent requirement for food processing. Further, reducing flow rates for non-Newtonian fluids causes higher apparent viscosities at operating pressures, temperatures and flows. Therefore, for non-Newtonian fluids, lowering flow rates does not significantly reduce operating pressure drops.
The “coring effect” is the tendency in a double tube heat exchanger (with the product carried in the inner tube and the cooling/heating fluid in the annular space between the inner and outer tube) for a central portion of the product stream in the inner tube to move faster than the rest of the material at the tube boundary where heat transfer takes place. The central portion or the “core” of the product does not experience any significant mixing and so heat transfer is decreased. The coring effect is more pronounced as the tube diameter of the inner tube is increased and the viscosity of the product increased. Turbulators are inserted to disturb the central portion of thick product flow in the inner tube by creating turbulence. These turbulators have different shapes from an augur or spiral like shapes and run through the length of the inner tube. Another type of turbulator is simple smooth surface insert in the second tube, i.e., a tube turbulator, occupies a small cross section area at the center of the second tube. Placement of tube turbulators thus creates an annular space through which the product flows, but the prior art devices provide for a very wide annular space. For example, a tubular heat exchanger with 4″ outer tube and 3″ inner tube may have a tube core of ¾″ diameter. Thus it will create an annular space of 1.08″ which is very wide. Application of these cores are limited to double tube heat exchangers with inner tube of larger diameter and are not placed inside an inner tube with say 1″ diameter as “coring” or “layering” effect is very insignificant as the inner tube diameter decreases.
Multi-tube heat exchangers provide more heat transfer surface as it employs a bundle of small diameter tubes through which product flows and an outer tube enclosing the bundle of small tubes. It makes the heat exchanger more compact than the double tube heat exchanger and thus require a smaller foot print. The outer shell diameter could vary from 3″ to 8″ while the inner small tubes are predominantly of ½″ to ¾″ diameter and up to 1″ in some cases. These heat exchangers are widely used for heat exchange applications in food industry but they do not work well for thicker food products especially those having very high viscosity at lower shear rate because very high pressure drops are developed and typical slow product velocity results into lower heat transfer.
Scraped surface heat exchangers handle this type of fluids very efficiently but again they are expensive and also require constant maintenance. Some other kinds of tubular heat exchangers with corrugations are offered but they are difficult to clean as they do not drain well and also more expensive because of special. design and fabrication.
One application of tubular heat exchangers in food industry is for energy regeneration where heat exchange takes place between a hot and cold product streams which is termed as ‘product-to-product’ regeneration or direct regeneration. Direct regeneration for thin products is usually carried out employing a double tube or a triple tube heat exchanger. Direct regeneration becomes increasing difficult as the product viscosity increased as prevailing laminar flow situation not only drastically reduces overall heat transfer coefficient but also difficult to clean in place as uneven velocities of cleaning solutions. For these types of application, an indirect regeneration or ‘water-to-product’ regeneration is employed where water in close loop recovers heat energy from a hot product in a cooling regenerator and gives back this heat energy to cold product stream in a heating regenerator. Double tube heat exchangers or multi-tube heat exchangers are used for indirect regeneration. A lower heat transfer rate and high pressure drop limitations for a thick non-Newtonian fluid results in a large heat exchanger surface requirement. A triple tube heat exchanger is not preferred for this types of heat application even though it employs product annular space.
Triple tubes and double tubes have a large exposed surface to heat transfer surface ratio which means that heat loss and refrigeration loss to surroundings is high if the tubes are not properly insulated. This ultimately results in lower thermal effectiveness in comparison to plate heat exchanger and multi-tube heat exchangers. For example a 2½″×1½″ double tube has exposed surface to heat transfer area ratio of 1.66 while a multi tube heat exchanger with 2½″ outer tube and ½″ inner tubes will have this ratio as 0.71. The insulation costs are therefore higher in tubular heat exchanger as compared to multi tube heat exchangers.
In triple tube heat exchanger employed as a regenerator, there are two annular cross sections and one circular cross section through which the hot and cold streams of product flow. In U.S. Pat. No. 3,386,497, a hollow core tube has been inserted in inner round tube of a triple tube regenerator for thick food products like tomato paste to reduce “layering” or “coring” of the product in round cross section of the inner tube. The use of tube turbulators or cores in double tube heat exchanger and triple tube heat exchanger, as described in U.S. Pat. No. 3,386,497, changes the product flow from “flow through round cross section” into “flow through an annular cross section” and thus reaps the benefits of superior heat transfer characteristics of an annular space. However, this arrangement does not address the issue of making tubular heat exchanger more compact. Bulkiness is one of the inherent limitations of tubular heat exchanger and this limitation further gets amplified when thick food products are handled by tubular heat exchangers. This patent shows an example of the prior art thinking to use a tube turbulator, although it is apparent that tube turbulators have not been advan

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