Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a rotor for a regenerative
exchanger for the transfer of moisture and preferably also heat
between two gas streams, particularly air streams.
An ~mportant area of application for a regenerative ex-
changer is in supplying fresh alr to premises,; the incoming fresh
air and the outgoing exhaust air exchanging moisture and heat in
a rotor, so that, for example, in the winter the outgoing exhaust
air provides the incoming fresh air with heat and moisture. Each
stream of air ;s provided with its own individual inlet and outlet
which is separated from the other stream of air so that the res-
pective streams of air are directed to individual zones of the
rotor. The rotor preferably consists of a plurality of sheets
which are alternately flat and corrugated and which form a net-
work of continuous, parallel channels for both streams of air.
The corrugated sheets support the flat sheets along the ridges of
the corrugations which results in the channels being separated
laterally. The rotor is normally cylindrical, the channels run-
ning parallel with the axis of rotation.
Previously such rotors were manufactured of foils or
;20 sheets of fibrous, incombustible material, such as asbestos paper,
or material made highly porous in another way, such as earthen-
ware. Sheets of this type of material then act as carriexs for
a hygroscopic substance, preferably a hygroscopic salt solution,
lithium chloride being the most usual substance used.
A rotor of the type described above incorporating highly
porous foils or sheets can possess exceptional characteristics,
e.g. incombustibility, a high capacity for moisture transfer and
good mechanical strength. However, a large number of operations
are re~uired to provide a rotor with these properties, and this re-
sults in the rotor being relatively expensive and time-comsuming
to manufacture. The material from which the sheets are constructed
has to be impregnated with several substances in order to give the
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mechanical strength re~uired, particularly when wet, and this
impregna-tion has to be perforrmed after -the shee-ting has been cor~
rugated and fashioned into the form of a rotor. Moreover tne ro-
tor's peripheral surfaces have to be finished once these manu-
facturing stages have been completed; grinding and milling being
required to achieve the requisite flatness and accuracy.
If the sheets are made of aluminium foil manufacturi.ng
costs can be reduced considerably, particu]~lrly if -the manufacturing
technique described in our copending ~anadian applica-tion No.
278,159 is used. Such a rotor can be givcn fireproof propcr-ti.es
and rnechan;.cal strength that are complete]y acceptable bu-t it
lacks one property that is important in a moisture exchanger -
hygroscopicity.
An object of this invention is to produce a ro-tor which
is totally or partially made of a]uminium foil and which possesses
exce]lent hygroscopic properties while keepi.ng the rnanufacturing
process simple and the costs of material low.
According to -the prese11t inverlt:i.on l.here :is provi(1ed a
rotor for a regene:rati.ve exchc.,l~(Je:r for l.h(- Ir.lnsfer of mois-ture
and/or heat, between two gas streams, the ro~or comprising layers
of aluminium foil, which are preferably alternately ~lat and cor-
rugated to fo.rm a p:lura:Lity of continuous channels extending through
sai.d rotor for said gas s-treams, the sur:Eace of the foils having
a porous hygroscopic coating oE alurninium hydroxide obtained by
precipitation from a solu-tion of an alkal.i aluminate applied to
the surfaces.
The invention also includes a me-thod of manufacturing a
roLor for a reyel-1era-tive exchanger for -the tral1sfer of moisture,
and/or heat between two gas streams composed of layers of alumi-
nium foil, which are preferably alternately flat and corrugatedto form a plurality of continuous channels extending through said
rotor for said gas streams the method comprising treating the
foils with an aluminate solution so that the surfaces thereof
are given a porous hygroscopic structure.
~ referably the rotor is manufactured using a method in
which two strips of aluminium foil, which can be 0.03 - 0.1 mm
thick, are used. One strip is provided with corrugations of a
height of 1 - 3 mm and is attached to the other strip, which is
flat, using a simple adhesive, e.g. the one described in detail
in the copending patent application ~entioned above, the strips
then being formed into a cylindrical rotor of the required size
by winding.
The aluminium foils are treated with an aqueous solution
of an al~ali aluminate which may be potassium, sodium or lithium
aluminate or a mixture of these. Preferably the treatment with
the alkali aluminate solution consists, either of immersing the
completed rotor into a bath of 15 - 20% aluminate solution, or
such a solution is applied to the rotor in a sufficiently large
quantity to effect the treatment e.g. by pouring the solution over
the rotor.
The concentration is preferably 16 - 17% aluminate in
the solution and, when effected in a bath, the bath i5 suitably
at room temperature or somewhat lower such as 18 and the treat-
ment time is relatively short e.g. 3 minutes. In this way the
channels of the rotor are at least partially filled by the alumi-
nate solution so that the surfaces of the foils are being etched
so that a hygroscopic coating later can adhere to it.
This hygroscopic coating is created in subsequent treat-
ment step as follows.
The rotor is lifted from the impregnation bath or the
supply of solution stopped so that substantially all the solu-
tion leaves the channels except for a film or a skin of the solu-
tion which remains on the surfaces of the rotor. This film is
more easily retained by turning the rotor immediately after empty-
L$~
ing of the solution so that the channels are brought to a horlzon-
tal position. The liquid in the channels is then converted gen-
erating heat and subjecting the foils of the rotor to a substantial
rise in temperature so that aluminium, being a part of the re-
tained treatmentliquid, will be deposited on the surfaces and
adheres as a hygroscopic coating consisting mainly of hydroxides
and at the same time hydrogen gas will escape. This subsequent
step continues for longer than the immersion step, preferably as
long as water remains in the channels. When the reaction has ceased
the rotor is washed to remove unwanted water soluble residuals.
However, before the washing it is advantageous to slowly cool the
rotor in order to prevent separation of the coating and from the
foil carriers as a result of thermal stresses. The coating may
be further strengthened by allowing the foils of the rotor time to
age in a moist atmosphere e.g.during a time oftwo days (48hours). In
this way an increase ofthe grain size in the coating adheringto the
foils is obtained and the risk-of aecomposition is further mini-
mized.
As stated above, in the first treatment step in etching
of the surface of the foil is accomplished to give the same a
possibility to form a secure seat for the porous hygroscopic
coating of aluminium hydroxide. However, whilst the effect on the
thickness of aluminium foil itself is insignificant in the first
step, the second treatment step provides the addition of a porous
hygroscopic coating of about lO - 20 ~m on each side of the foil.
The increase of weight on a foil with a thickness of e.g. 50 ~m
may be about 10%. The main components of the coating are alumi-
nium hydroxides which are taken from the impregnation solution and
not by any conversion of the aluminium foil itself.
The treatment of the invention should preferably be car-
ried out after the rotor has been formed. This method has proved
to have an effect that is of value for the strength of the rotor.
d,
It appears that sorne type of brid~ing takes place between the
porous coatings at the con-tact surfaces between adj~cent foil
layers arld thi:s ~esults in a strengthenin~ oE the joints between
the corrugated sheets.
The hygroscopic coating can be concentrated by repeated
immersion in the aluminate solu-tion in the first -t:reatment step.
In addition a crushed or pulverised solid adsorption medium, e.g.
silic gel, can he in-troduced into the aluminate solution a-t some
stage of the immersion process. Such a powder attaches itself
surprisinyly well to -the surface of -the foil without reducing its
sorpt;on propert;.es -to any greclt extell-t.
'I'he hygroscopic properties obtained usiny -the me-thod
described above are based on adsorption and have proved to re-
sult in moisture transfer properti.es comparable wi-th -those ob-
ta;.ned in ro-tors manufactured using hi.ghly porous foil made of
asbestos or earthenware with :Lithium chloride as the hygro-
scopic substance, as menti.oned in the opening section.
In many cases, where -the rotor is used in velltilal-:i.on
appardtus there is a :ri.sk th~lt pol.ln.ltion ;rl thc e.;l~ st air, e.y.
fats or oils, can cover the surfaces wi.th a thin film which can
restrict the diffusion of the moisture to and from the hygro-
scopic coating, if this consists of a soli.d adsorption medium.
It i.s easy for its fine pores and capillaries, in which the mois-
ture condenses, to become blocked and put out of action. If, on
the other hand, the hygroscopic suhstallce consists of a salt solu-
tion, the entire wet surface becomes dc-tive whi.le the liquid tends
-to seep in and break through the film of con-tclmination.
In si.milar operating sol.utions a hygroscopic capacity
provided by a salt solution is preferableO
3~ Howeyer, untreated aluminium foil does no-t have the capa-
bility of retaining a sufficient quantity of the salt solution on
its surface. It has now been shown that the porous coating ob-
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tained by the treatment of aluminium fo~l with alkali aluminate is
capable of retain~ng a greater quantity of salt solution than an
untreated surface. Moreover, treatment with aluminate provides
a base onto which other coats that will further increase the
porosity can be bonded. ~ description has already been given of
such a coating where the addition of powder to, for example so-
dium aluminate, during the immersion process provided an addition-
al coating. The water absorbency capability of foil suxfaces
can be considerably increased by this method of treatment. The
rotor, once treated with aluminate, can also be immersed in water-
glass and then exposed to carbon dioxide. This gives an addition-
al coating of chemically precipitated silicon dioxide which also
increases porosity.
One vital question when using a salt solution as the
hygroscopic substance is the degree of corrosion that can occur.
Extensive tests have shown that lithium chloride cannot be used;
corrosion attacks far too rapidly. However lithium chloride can
be used in solution with an inhibitor such as lithium hydroxide
which makes the salt solution less corrosive.
Another lithium salt, lithium nitrate, has proved to
result in, practically speaking, no corrosion of aluminium foil
treated with aluminate. At the same time lithium nitrate gives the
foil extremely good hygroscopic properties within the relative
moisture range for the simultaneous moisture and temperature
exchange in question here, i.e. a relative moisture in excess of
10 - 10$. Calcium bromide and sodium chloride have also proved to
be considerably less corrosive than lithium chloride on surfaces
treated with aluminate, although they are not so benign as lithium
nitrate, Calciumbxomide has shown itself to be hygroscopically
part~cularly suitable.
Thé substances and techniques described above which are
intended to provide the rotor with adsorbing and absorbing proper-
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ties are inexpensiVe and of such a nature that the principles
of the manufacturin~ method described in the copending Canadian
patent ~pplication 278,159 can be retained. Together these re-
sult in manufacturing costs which considerably undercut the costs
which were necessary to manufacture high-efficiency transfer ro-
tors in the past.
In this specification the term "aluminium foil" is to
be construed broadly asincluding sheets consisting of a carrier
of non-metallic material such as fibers of glass or cellulose or
plastic foils which are provided with a layer or coating of alumi-
nium.
