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Dynamic scraped surface heat exchangers

 

 
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Dynamic scraped surface heat exchangers
 
 
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Introduction
The most important technologies for indirect heat transfer
Heat transfer

Heat transfer is the transition of thermal energy or simply heat from a hotter object to a cooler object . When an object or fluid is at a different temperature than its thermodynamic system or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surround...
 use tubes (shell-and-tube exchangers
Heat exchanger

A heat exchanger is a device built for efficient heat transfer from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact....
) or flat surfaces (plate exchangers). Their goal is to exchange the maximum amount of heat per unit area by generating as much turbulence
Turbulence

In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time....
 as possible below given pump
Pump

A pump is a device used to move fluids, such as gases, liquids or Slurry. A pump displaces a volume by physical or mechanical action. One common misconception about pumps is the thought that they create pressure....
ing power limits. Typical approaches to achieve this consist of corrugating the tubes or plates or extending their surface with fins
Fin (extended surface)

In the study of heat transfer, a fin is a surface that extends from an object to increase the rate of heat transfer to or from the environment by increasing convection....
.

However, these geometry conformation technologies, the calculation of optimum mass flows
Mass flow rate

Mass flow rate is the mass of substance which passes through a given surface per unit time. Its physical unit is mass divided by time, so kilogram per second in SI units, and Slug per second or pound per second in US customary units....
 and other turbulence related factors become diminished when fouling
Fouling

Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance ....
 appears, obliging designers to fit significantly larger heat transfer areas.






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Introduction


The most important technologies for indirect heat transfer
Heat transfer

Heat transfer is the transition of thermal energy or simply heat from a hotter object to a cooler object . When an object or fluid is at a different temperature than its thermodynamic system or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surround...
 use tubes (shell-and-tube exchangers
Heat exchanger

A heat exchanger is a device built for efficient heat transfer from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact....
) or flat surfaces (plate exchangers). Their goal is to exchange the maximum amount of heat per unit area by generating as much turbulence
Turbulence

In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time....
 as possible below given pump
Pump

A pump is a device used to move fluids, such as gases, liquids or Slurry. A pump displaces a volume by physical or mechanical action. One common misconception about pumps is the thought that they create pressure....
ing power limits. Typical approaches to achieve this consist of corrugating the tubes or plates or extending their surface with fins
Fin (extended surface)

In the study of heat transfer, a fin is a surface that extends from an object to increase the rate of heat transfer to or from the environment by increasing convection....
.

However, these geometry conformation technologies, the calculation of optimum mass flows
Mass flow rate

Mass flow rate is the mass of substance which passes through a given surface per unit time. Its physical unit is mass divided by time, so kilogram per second in SI units, and Slug per second or pound per second in US customary units....
 and other turbulence related factors become diminished when fouling
Fouling

Fouling refers to the accumulation of unwanted material on solid surfaces, most often in an aquatic environment. The fouling material can consist of either living organisms or a non-living substance ....
 appears, obliging designers to fit significantly larger heat transfer areas. There are several types of fouling, including particulate accumulation, precipitation
Precipitation (chemistry)

Precipitation is the formation of a solid in a solution during a chemical reaction. When the reaction occurs, the solid formed is called the precipitate, and the liquid remaining above the solid is called the supernate....
 (crystallization
Crystallization

Crystallization is the process of formation of solid crystals Precipitation from a solution, melting or more rarely Deposition directly from a gas....
), sedimentation
Sedimentation

Sedimentation describes the motion of molecules in solutions or particle s in suspension in response to an external force such as gravitation, centrifugal force or electromagnetism....
, generation of ice
Ice

Ice is a solid phases of matter, usually crystalline solid, of a non-metallic substance that is liquid or gas at room temperature, such as ammonia ice or methane ice....
 layers, etc.

Another factor posing difficulties to heat transfer is viscosity
Viscosity

Viscosity is a measure of the Drag of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness"....
. Highly viscous fluids tend to generate deep laminar flow
Laminar flow

Laminar flow, sometimes known as Streamlines, streaklines and pathlines flow, occurs when a fluid flows in parallel layers, with no disruption between the layers....
, a condition with very poor heat transfer rates
Heat transfer coefficient

The heat transfer coefficient, in thermodynamics and in mechanical engineering and chemical engineering, is used in calculating the heat transfer, typically by convection or phase change between a fluid and a solid:...
 and high pressure losses
Pressure drop

Pressure drop is a term used to describe the decrease in pressure from one point in a pipe or tube to another downstream. This is usually the result of friction of the fluid against the tube....
 involving a considerable pumping power, often exceeding the exchanger design limits. This problem becomes worsened frequently when processing non-newtonian
Non-Newtonian fluid

A non-Newtonian fluid is a fluid whose flow properties are not described by a single constant value of viscosity. Many polymer solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as ketchup, starch suspensions, paint, blood and shampoo....
 fluids.

The dynamic scraped surface heat exchangers (DSSHE) have been designed to face the aforementioned problems. They increase heat transfer by:

  • removing the fouling layers,
  • increasing turbulence in case of high viscosity flow,
  • avoiding the generation of ice and other process by-products.


Basic description


The dynamic scraped surface heat exchangers incorporate an internal mechanism which periodically removes the product from the heat transfer wall. The product side is scraped by blades attached to a moving shaft or frame. The blades are made of a rigid plastic material to prevent damage to the scraped surface. This material is FDA
Food and Drug Administration

The U.S. Food and Drug Administration is an Government agency of the United States Department of Health and Human Services and is responsible for regulating and supervising the safety of foods, dietary supplements, Medications, vaccines, Biopharmaceutical, blood transfusion, medical devices, Electromagnetic radiation-emitting devices, veteri...
 approved in the case of food applications.

Types of dynamic scraped surface heat exchangers


There are basically three types of DSSHEs depending on the arrangement of the blades:

1. Rotating, tubular DSSHEs. The shaft is placed parallel to the tube axis, not necessarily coincident, and spins at various frequencies, from a few dozen rpm to more than 1000 rpm. The number of blades oscillates between 1 and 4 and may take advantage of centrifugal forces to scrape the inner surface of the tube. Examples are the Waukesha Cherry-Burrell Votator II and the Alfa-Laval Contherm.

2. Reciprocating, tubular DSSHEs. The shaft is concentric to the tube and moves longitudinally without rotating. The frequency spans between 10 and 60 strokes per minute. The blades may vary in number and shape, from baffle-like arrangements to perforated disk configurations. An example is the HRS Spiratube Unicus.

3. Rotating, plate DSSHEs. The blades wipe the external surface of circular plates arranged in series inside a shell. The heating/cooling fluid runs inside the plates. The frequency is about several dozen rpm. An example is the HRS Spiratube T-Sensation.

Evaluation of dynamic scraped surface heat exchangers


Computational Fluid Dynamics
Computational fluid dynamics

Computational fluid dynamics is one of the branches of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows....
 (CFD) techniques are the standard tools to analyse and evaluate heat exchangers and similar equipment. However, for quick calculation purposes, the evaluation of DSSHEs are usually carried out with the help of ad-hoc (semi)empirical
Empirical

The word empirical denotes information gained by means of observation, experience, or experiment, as opposed to theory. A central concept in science and the scientific method is that all evidence must be empirical, or empirically based, that is, dependent on evidence or Logical consequence that are observable by the senses....
 correlations
Function (mathematics)

The mathematical concept of a function expresses dependence between two quantities, one of which is known and the other which is produced. A function associates a single output to each input element drawn from a fixed Set , such as the real numbers , although different inputs may have the same output....
 based on Buckingham’s Pi Theorem:

Fa = Fa(Re, Re', n, ...)

for pressure loss and

Nu = Nu(Re, Re', Pr, Fa, L/D, N, ...)

for heat transfer, where Nu is the Nusselt number
Nusselt number

In heat transfer at a Boundary within a fluid, the Nusselt number is the ratio of convection to heat conduction heat transfer across the boundary....
, Re is the standard Reynolds number
Reynolds number

In fluid mechanics and heat transfer, the Reynolds number is a dimensionless number that gives a measure of the ratio of inertial forces to viscosity forces and, consequently, it quantifies the relative importance of these two types of forces for given flow conditions....
 based on the inner diameter of the tube, Re is the specific Reynolds number based on the wiping frequency, Pr is the Prandtl number
Prandtl number

The Prandtl number is a dimensionless number approximating the ratio of momentum diffusivity and thermal diffusivity. It is named after the German physicist Ludwig Prandtl....
, Fa is the Fanning friction factor
Fanning friction factor

The Fanning friction factor is a dimensionless number used in fluid flow calculations. It is related to the Shear stress at the wall as:where:...
, L is the length of the tube, D is the inner diameter of the tube, n is the number of blades and the dots account for any other relevant dimensionless parameters.

Applications


The range of applications covers a number of industries, including food
Food industry

The food industry is the complex, global collective of diverse businesses that together supply much of the food energy consumed by the world population....
, chemical
Chemical industry

The chemical industry comprises the companies that produce industrial chemicals. It is central to modern world economy, converting raw materials into more than 70,000 different products....
, petrochemical and pharmaceutical. The DSSHEs are appropriate whenever products are prone to fouling, very viscous, particulate, heat sensitive or crystallizing.

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