Editing Heat exchanger (section)

==Spiral==
[[File:Spiral-heat-exchanger-schematic-workaround.svg|thumb|Schematic drawing of a spiral heat exchanger]]

A modification to the perpendicular flow of the typical HCHE involves the replacement of shell with another coiled tube, allowing the two fluids to flow parallel to one another, and which requires the use of different design calculations.<ref>{{cite journal  | author1 = Rennie, Timothy J. | author2= Raghavan, Vijaya G.S. | title = Experimental studies of a double-pipe helical heat exchanger | journal = Experimental Thermal and Fluid Science | pages = 919–924 |volume= 29 |issue= 8 | date = September 2005 | doi=10.1016/j.expthermflusci.2005.02.001| bibcode= 2005ETFS...29..919R }}</ref>  These are the [[Spiral]] Heat Exchangers (SHE), which may refer to a [[helix|helical]] (coiled) tube configuration, more generally, the term refers to a pair of flat surfaces that are coiled to form the two channels in a counter-flow arrangement. Each of the two channels has one long curved path. A pair of fluid ports are connected [[tangent]]ially to the outer arms of the spiral, and axial ports are common, but optional.<ref>{{cite web|url=http://gc3.com/techdb/manual/cooltext.htm |archive-url=https://web.archive.org/web/20090209192326/http://gc3.com/techdb/manual/cooltext.htm |url-status=dead |archive-date=2009-02-09 |title=Cooling Text |access-date=2019-09-09}}</ref>

The main advantage of the SHE is its highly efficient use of space. This attribute is often leveraged and partially reallocated to gain other improvements in performance, according to well known tradeoffs in heat exchanger design.  (A notable tradeoff is capital cost vs operating cost.)  A compact SHE may be used to have a smaller footprint and thus lower all-around capital costs, or an oversized SHE may be used to have less [[pressure]] drop, less pumping [[energy]], higher [[thermal efficiency]], and lower energy costs.

===Construction===
The distance between the sheets in the spiral channels is maintained by using spacer studs that were welded prior to rolling. Once the main spiral pack has been rolled, alternate top and bottom edges are welded and each end closed by a gasketed flat or conical cover bolted to the body. This ensures no mixing of the two fluids occurs. Any leakage is from the periphery cover to the atmosphere, or to a passage that contains the same fluid.<ref>E.A.D.Saunders (1988). ''Heat Exchangers:Selection Design And Construction'' Longman Scientific and Technical {{ISBN|0-582-49491-5}}</ref>

===Self cleaning===
Spiral heat exchangers are often used in the heating of fluids that contain solids and thus tend to foul the inside of the heat exchanger. The low pressure drop lets the SHE handle fouling more easily. The SHE uses a “self cleaning” mechanism, whereby fouled surfaces cause a localized increase in fluid velocity, thus increasing the [[drag (physics)|drag]] (or fluid [[friction]]) on the fouled surface, thus helping to dislodge the blockage and keep the heat exchanger clean.  "The internal walls that make up the heat transfer surface are often rather thick, which makes the SHE very robust, and able to last a long time in demanding environments."{{Citation needed|reason=Who said this?|date=June 2013}}
They are also easily cleaned, opening out like an [[oven]] where any buildup of foulant can be removed by [[pressure washer|pressure washing]].

Self-cleaning water filters are used to keep the system clean and running without the need to shut down or replace cartridges and bags.

===Flow arrangements===
[[Image:Comparison of con- and counter-current flow exchange.jpg|thumb|right|A comparison between the operations and effects of a '''cocurrent and a countercurrent flow exchange system''' is depicted by the upper and lower diagrams respectively. In both it is assumed (and indicated) that red has a higher value (e.g. of temperature) than blue and that the property being transported in the channels therefore flows from red to blue. Channels are contiguous if  effective exchange is to occur (i.e. there can be no gap between the channels).]]
There are three main types of flows in a spiral heat exchanger:
* '''Counter-current Flow''': Fluids flow in opposite directions.  These are used for liquid-liquid, condensing and gas cooling applications. Units are usually mounted vertically when condensing vapour and mounted horizontally when handling high concentrations of solids.
* '''Spiral Flow/Cross Flow:''' One fluid is in spiral flow and the other in a cross flow. Spiral flow passages are welded at each side for this type of spiral heat exchanger. This type of flow is suitable for handling low density gas, which passes through the cross flow, avoiding pressure loss. It can be used for liquid-liquid applications if one liquid has a considerably greater flow rate than the other.
* '''Distributed Vapour/Spiral flow:''' This design is that of a condenser, and is usually mounted vertically. It is designed to cater for the sub-cooling of both condensate and non-condensables. The coolant moves in a spiral and leaves via the top. Hot gases that enter leave as condensate via the bottom outlet.

===Applications===
The Spiral heat exchanger is good for applications such as pasteurization, digester heating, heat recovery, pre-heating (see: [[recuperator]]), and effluent cooling. For sludge treatment, SHEs are generally smaller than other types of heat exchangers.{{citation needed|date=December 2019}} These are used to transfer the heat.