Dynamic scraped surface heat exchangers

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 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 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 (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 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, "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.

References

* cite book
last = Bott
first = T. R.
title = Design of Scraped Surface Heat Exchangers
publisher = British Chemical Engineering
year = 1966
month = May
volume = II, No.5
pages = 338-339

* cite book
last = Bott
first = T. R.
title = To Foul or not to Foul
publisher = CEP Magazine
year = 2001
month = November
pages = 30-37

* cite book
last = Bott
first = T. R.
coauthors = Romero, J. J. B.
title = Heat Transfer Across a Scraped Surface
publisher = The Canadian Journal of Chemical Engineering
year = 1963
month = October
pages = 213-219

* cite book
last = Chong
first = A.
title = A Study of Scraped-Surface Heat Exchanger in Ice-Making Applications, M. Sc. Thesis
publisher = University of Toronto
year = 2001

* cite web
url= http://www.smithinst.ac.uk/Projects/PD/RA-Chemtech/
title= Scraped surface heat exchangers project webpage
authorlink= http://www.smithinst.ac.uk/
publisher= Smith Institute

* cite book
last = Tähti
first = T.
title = Suspension Melt Crystallization in Tubular and Scraped Surface Heat Exchangers, Ph. D. Thesis
publisher = Martin-Luther-Universität
year = 2004

* cite web
url= http://www.personal.soton.ac.uk/cpp/sshe_proj/homepage.html
title= Scraped-Surface Heat-Exchanger Project page
authorlink= http://www.soton.ac.uk/
publisher= University of Southampton

External links

* [http://www.hrs-spiratube.com/en/resources/fouling-factors-in-heat-exchangers.aspx Fouling factors in heat exchangers]