It is a popular misconception that Spiral Heat Exchangers are a recent development. In fact, the Spiral concept was first proposed as long ago as the 19th century. Only the lack of suitable materials and manufacturing techniques delayed its development into a fully-fledged product until the 1930s. Since that time, acceptance of the Spiral has seen steady growth and this exchanger type is used today in many industries including chemical, steel and pulp and paper.
The first Spiral Heat Exchanger was extremely simple in concept. It consisted of two metal strips, bent into a nearly circular shape to form two concentric channels through which media would flow in opposite directions. Channel spacing was achieved by a steel bar along the length.
The heat transfer capacity of the exchanger was dictated by the width of the channels. Right up until the 1960s, this effectively meant a maximum capacity of 200 m2 since steel strip was only available in relatively narrow widths. Attempts to increase capacity by fabricating larger areas met with only limited success, since they resulted in long, thin channels with excessively high pressure drops. Once wider materials became available and wider channels could be formed, however, heat transfer capacity was progressively increased. Today, practical maximum capacity of a standard Spiral Heat Exchanger is 400-600 m2. Currently the spiral is manufactured in a winding process using a D-shaped mandrel with the two strips being welded to a central plate and distance studs have replaced the steel bars. Alternatively tubular centers are becoming more common.
Usually, alternate edges of the passages are closed and covers fitted to both sides of the spiral assembly.
Heat transfer data for spiral heat exchangers is empirically correlated using a conventional Dittus-Boelter type relationship for turbulent flow. A channel curvature component is added in order to take into account the somewhat improved heat transfer generated by secondary flow effects. The equation takes the following form:
where Nu is Nusselt Number, Pr is Prandtl Number, η is bulk fluid viscosity, ηw is fluid viscosity at the wall, dH is channel hydraulic diameter, D is diameter of the spiral, and Re is Reynolds Number.
The term dH/D represents the local channel curvature, which, for a channel of constant spacing, will vary from a maximum at the center of the body to a minimum at the periphery.
Test data and, to a certain extent, results from installed units indicate the existence of two regions for spiral flow: Pure laminar flow:
For nonturbulent flow, therefore, the higher of the two Nu values obtained from the above equations is the one that applies.
The main feature of this exchanger type is that there is a single passage for each fluid. In actual operation, the cold fluid enters at the periphery and flows towards the center where it exits via the cover. The hot fluid goes in the opposite direction, giving countercurrent flow (see Figure 1). The single channel makes the unit well suited to handling fouling liquids with the original design being known as the Type I. Illustrated is an installation at Novo Nordisk A/S in Denmark where cold untreated sludge is heated by hot treated sludge in two spirals. A further unit is used for final cooling of the treated sludge.
Another typical installation is at C. Davidson and Sons, Mugiemoss Mill in Aberdeen, Scotland. Here spiral exchangers cool sealing water for paper and board machine vacuum pumps.
The Type II Spiral was developed to handle the growing demand for vaporizing and condensing capabilities within the process industries. Although it operates on the same basic principle as the Type I, it differs most significantly in terms of channel geometry. It has only one medium flowing spirally. The other flows crosswise, parallel to the axis of the spiral element. The spiral channel is closed on both sides with the crossflow fluid flowing through the spiral annulus (see Figure 2).
Type II Spirals are used in duties involving large volumes of vapor, vapor/gas or vapor/liquid mixtures. The channel geometry makes it possible to combine high liquid velocity in the Spiral passage with very low pressure drop on the vapor/mixture side. They are also occasionally used in liquid/liquid applications where one side has to cope with a much larger volume of liquid than the other, such as some fermenter cooling cases.
When the Type II is used as a condenser it achieves very little subcooling of the vapors or condensate. For applications where this is a necessary part of the process, a different type of Spiral had to be developed–the Type III.
The unit is constructed with (normally) alternately welded channels. The lower face of the body is fitted with a cover, while the upper face is fitted with a distribution cone such that the outer turns are closed and the inner turns are open to the crossflow of the fluid entering the unit. The periphery of the unit is provided with an upper connection for the removal of residual gas/vapor, and a lower connection for the condensate. The cooling medium side is in spiral flow throughout.
The function of the unit is that of condensing a vapor or vapor mixture with or without non-condensable gas in which it is required to cool the residual vapor/gas mixture to as low a temperature as possible and thus obtain maximum possible condensation. A secondary feature is that the condensate is effectively subcooled, the outer turns being in countercurrent flow to the coolant. That the flow is in the spiral mode in the outer turns results in higher heat and mass transfer coefficients than would be obtained with the vapor in crossflow only. The SHE type III is best suited for vapor mixtures at moderate pressure containing small to moderate amounts of noncondensable gas. Operation at very low absolute pressure ("high vacuum") is seldom feasible due to the resulting excessive pressure drop in the outer turns.
The process industry uses columns and reactors extensively and the Type G Spiral was developed to meet the need for a custom built unit which would be vertically mounted onto a column or reactor. The advantage of this arrangement is that it eliminates the need for a separate condenser and, more importantly, all of the large vapor pipework and reflux drum associated with it.
In this model, vapor enters through an open center tube and then rises. In the upper shell extension, its direction of flow is reversed and it condenses downwards in crossflow in the Spiral element. Meanwhile, coolant is pumped through a peripheral connection, flows through the spiral channel towards the center and, finally, exits via a pipe in the upper shell extension.
For minimal subcooling, the condensate is allowed to enter the lower shell extension. However, when subcooling is required, a baffle plate fitted to the lower face of the spiral element forces the condensate to flow in the lower parts of the channels in countercurrent to the coolant.