World Library  
Flag as Inappropriate
Email this Article

Dynamic scraped surface heat exchanger

Article Id: WHEBN0019232416
Reproduction Date:

Title: Dynamic scraped surface heat exchanger  
Author: World Heritage Encyclopedia
Language: English
Subject: Pumpable ice technology, Fouling, Heat exchangers
Collection: Fouling, Heat Exchangers
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Dynamic scraped surface heat exchanger

The dynamic scraped surface heat exchanger (DSSHE) was designed to face some problems found in other types of heat exchangers. They increase heat transfer by: removing the fouling layers, increasing turbulence in case of high viscosity flow, and avoiding the generation of ice and other process by-products. DSSHEs incorporate an internal mechanism which periodically removes the product from the heat transfer wall.

Contents

  • Introduction 1
  • Basic description 2
  • Types 3
  • Evaluation 4
  • Applications 5
  • See also 6
  • References 7

Introduction

The most important technologies for indirect heat transfer use tubes (shell-and-tube exchangers) or flat surfaces (plate exchangers). Their goal is to exchange the maximum amount of heat per unit area by generating as much turbulence as possible below given pumping power limits. Typical approaches to achieve this consist of corrugating the tubes or plates or extending their surface with fins.

However, these geometry conformation technologies, the calculation of optimum mass flows and other turbulence related factors become diminished when fouling appears, obliging designers to fit significantly larger heat transfer areas. There are several types of fouling, including particulate accumulation, precipitation (crystallization), sedimentation, generation of ice layers, etc.

Another factor posing difficulties to heat transfer is viscosity. Highly viscous fluids tend to generate deep laminar flow, a condition with very poor heat transfer rates and high pressure losses involving a considerable pumping power, often exceeding the exchanger design limits. This problem becomes worsened frequently when processing non-newtonian fluids.

The dynamic scraped surface heat exchangers (DSSHE) have been designed to face the above-mentioned problems. They increase heat transfer by: removing the fouling layers, increasing turbulence in case of high viscosity flow, and 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 approved in the case of food applications.

Types

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, Alfa Laval Contherm and Terlet Terlotherm.
  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 Heat Exchangers 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

Computational fluid dynamics (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 correlations based on the Buckingham π 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, Re is the standard Reynolds number based on the inner diameter of the tube, Re' is the specific Reynolds number based on the wiping frequency, Pr is the Prandtl number, Fa is the Fanning friction factor, 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, chemical, petrochemical and pharmaceutical. The DSSHEs are appropriate whenever products are prone to fouling, very viscous, particulate, heat sensitive or crystallizing.

See also

References

This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.