U.S. patent number 3,567,117 [Application Number 04/854,102] was granted by the patent office on 1971-03-02 for ice nuclei formation.
This patent grant is currently assigned to Hedco, Inc.. Invention is credited to William E. C. Eustis.
United States Patent |
3,567,117 |
Eustis |
March 2, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
ICE NUCLEI FORMATION
Abstract
A method of making snow by controlling the size and life of ice
nuclei generated in a snow-making machine. Ice nuclei are generated
by homogeneous nucleation in a tunnellike housing through injection
of propane or other refrigerant into a stream of moist air. The
relative humidity about the ice nuclei stream is controlled
relative to the temperature and humidity of the atmosphere to
prevent or inhibit evaporation of the ice nuclei for such time as
to ensure statistically effective mixing with adjacent liquid phase
water droplets which are supercooled by the air mass. The ice
nuclei formed infect the water droplets and they release their
latent heat to the air to form snowlike particles.
Inventors: |
Eustis; William E. C.
(Cambridge, MA) |
Assignee: |
Hedco, Inc. (Bedford,
MA)
|
Family
ID: |
25317736 |
Appl.
No.: |
04/854,102 |
Filed: |
August 29, 1969 |
Current U.S.
Class: |
239/14.2 |
Current CPC
Class: |
F25C
1/00 (20130101); F25C 3/04 (20130101); F25C
2303/0481 (20130101); F25C 2303/046 (20130101) |
Current International
Class: |
F25C
1/00 (20060101); F25C 3/04 (20060101); F25C
3/00 (20060101); A01g 015/00 () |
Field of
Search: |
;239/2,2 (S)/ ;239/14
;62/121 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: King; Lloyd L.
Claims
I claim:
1. A method for the making of snow which method comprises:
a. providing in one zone water droplets having a temperature of
less than 0.degree. C.;
b. forming ice nuclei in another separate zone;
c. preserving the ice nuclei so formed by controlling the relative
humidity of the atmosphere about the ice nuclei to inhibit the
burning off of the ice nuclei prior to the infection of the water
droplets; and
d. infecting the water droplets with the ice nuclei whereby the
latent heat is progressively removed from the ice infected droplets
and the droplets become substantially all snowlike crystals.
2. The method of claim 1 wherein the ice nuclei are discharged from
the separate zone in a nuclei stream and the nuclei are preserved
by controlling the relative humidity between 70 and 95 percent
within the nuclei stream.
3. The method of claim 1 which includes: forming the ice nuclei by
injecting a refrigerant into a moist stream of cold air the
refrigerant adapted to lower the temperature of the airstream to
less than -40.degree. C. and wherein the relative humidity of the
airstream is controlled to provide sufficient humidity within the
ice-nuclei stream formed to inhibit burning off of the ice
nuclei.
4. The method of claim 1 wherein the size of the ice nuclei formed
are between 125 and 250 microns and the refrigerant is selected
from the group consisting of propane, methane, freon, carbon
dioxide and sulfur dioxide.
5. The method of claim 4 wherein the refrigerant is liquid propane
which incurs a phase change from a liquid to a gaseous state.
6. The method of claim 1 wherein the ice nuclei are formed by the
adiabatic expansion of compressed air and the ice nuclei are
discharged from the one zone in a nuclei stream.
7. The method of claim 6 which includes controlling the moisture
content of the compressed air to provide the proper humidity within
the nuclei stream.
8. The method of claim 1 which includes:
a. breaking water into droplets at a temperature of greater than
0.degree. C.;
b. supercooling the droplets so formed to less than 0.degree. C. by
flowing a mass of ambient air about the droplets;
c. discharging the ice nuclei formed from the zone in a separate
nuclei stream;
d. preserving the ice nuclei formed by controlling the relative
humidity about the ice-nuclei stream; and
e. subsequently infecting the supercooled water droplets.
9. The method of claim 7 wherein the ice nuclei are formed by
homogeneous nucleation.
Description
BRIEF SUMMARY OF THE INVENTION
The purpose of a snow-making apparatus is to emit at least
partially frozen particles which will gather on the ground as a
more or less loose snowlike mass suitable for skiing. An essential
property of any snow-making device is that it effects the
initiation of the freezing process in a large proportion of the
water particles generated, otherwise the spray produces an
artificial ice storm.
In the state of the art today, the proper role of nucleation as the
necessary initiation of freezing has not generally been recognized
or clearly identified. The prior art in at least one instance
discloses combining compressed air and pressurized water in a
helically moving chamber and discharging the mixture through a
nozzle. Upon discharge, the mixture forms ice crystals. (See U.S.
Pat. No. 3,301,485). Also a mixture of water and crushed ice
discharged through a nozzle into an ambient stream of air at less
than 0.degree. C. to precipitate snow crystals has been suggested.
(See U.S. Pat. No. 2,968,164).
The primary drawbacks of the present methods and apparatus for snow
making are that the nucleating portion of the snow-making process
has been performed substantially simultaneously with the
disintegration of the water stream into a water droplet spray
without regard to controlling the sets of parameters such as
humidity and temperature conditions which affect the formation of
ice nuclei. This results in uneconomical and inefficient apparatus
for making snow, with the attendant high cost.
To form frozen particles more is required than simply the cooling
of the water to a temperature below 0.degree. C. in the course of
its emission and subsequent fall, for water when it is cooled below
0.degree. C. does not start to freeze until a nucleus of the frozen
phase appears. This nucleus may arise through collision between the
cooling water particles and a bit of natural ice (isomorphic
nucleation), or through the action of a foreign particle of some
other substance, the crystalline, molecular or other structure of
which somewhat resembles ice or otherwise effects the onset of the
ice phase (heterogeneous nucleation), or through cooling of the
liquid phase to such a low temperature that the ice nuclei appears
spontaneously without the introduction of any foreign matter
(homogeneous nucleation), which in water at atmospheric pressure
occurs at about -40.degree. C. Unless the onset of freezing is
initiated in one of these three ways, the water particle below
0.degree. C. remains in a supercooled state until it strikes the
ground, where it freezes in the form of glaze or ice, generally too
hard for skiing and too rough for skating.
In my invention, the production of ice nuclei is separated from the
function of disintegrating the water stream and of cooling the
resulting water droplets. It is accomplished by the cooling of
moist air below the temperature of homogeneous nucleation either by
adiabatic expansion of compressed air in an efficient nozzle, or by
injection into cold moist air of liquid propane or other
refrigerant, or both. Also, the operating conditions are so
controlled as to permit ice nuclei to grow in size rather than
evaporate so that they survive the time period e.g. 0.01--0.9
seconds or the few fractions of a second while they become mixed
with the droplet spray. Generally the humidity about the ice-nuclei
stream is supplemented and controlled to ensure that the ice nuclei
do not evaporate or burn prior to seeding.
The nuclei cloud formed is then mixed with the supercooled water
droplets where the ice nuclei infect the water droplets. The
ice-infected droplets are then discharged into the atmosphere with
which the ice-infected droplets exchange their latent heat during
freezing of the droplets. The ice crystals then formed fall to the
ground as more or less completely frozen snowlike particles.
Accordingly, in my invention, the individual steps required for
controlling the growth and life of the ice nuclei are controlled
separately to ensure economic and efficient results.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic and partially sectional view of my invention
incorporated in a snow precipitator;
FIG. 2 is an alternative embodiment of my invention; and
FIG. 3 is another embodiment of my invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a snow precipitator is shown generally at 10
and includes a housing or tunnellike conduit 12 together with a
propeller or fan 14 secured to the housing by a bracket 16, to move
air through the housing and carry the ice crystals into the
atmosphere. Disposed in the downstream or high pressure side of the
propeller 14 is a nucleating zone 18 used to form the ice nuclei.
The zone 18 comprises a feed conduit 20 for the introduction of a
moist air stream and a diaphragm 22 to provide for the injection of
propane, passing through a liquid to vapor phase change, or other
refrigerant into the moist air stream. The conduit 20 passes
through the housing and as shown is L-shaped. The diaphragm 22 is
installed near the discharge end of the conduit 20 and further is
connected to a conduit 24 which passes through the housing 12. In
communication with the conduit 20 is a feed line 26 to bleed water
into the air stream.
Located downstream of the propeller 14 or on the high pressure side
and axially spaced apart from and generally axially aligned with
the nucleating zone 18 is a second zone which includes a nozzle 28
which faces into the direction of the flow of air created by the
propeller 14. The nozzle is connected to a water supply by conduit
30 which passes through the housing 12. Water flows through the
conduit 30 and the nozzle 28 and is broken up into droplets of a
size to suit the atmospheric conditions.
An air conduit 32 passes through the housing 12 and terminates in
the vicinity of the nucleating zone. A water conduit 34 feeds into
the air conduit 32. The air is discharged into the vicinity of the
ice-nuclei stream to control the humidity about the stream.
In the operation of the snow precipitator, the propeller 14 is
actuated by a motor or other means (not shown) to move ambient air
at less than 0.degree. C., and at atmospheric humidity through the
housing. Compressed air at any humidity, including a high humidity,
and at any convenient pressure, say 10 to 15 p.s.i. is introduced
into conduit 20. Water at a rate determined by the relative
humidity of the atmosphere but typically perhaps of from 0.1 quarts
to 20 quarts/minute is bled into the compressed air through feed
line 26. Liquid propane at a rate determined by the desired
production rate of snow and typically from about 0.5 to 15 lbs/hour
say for example 2 lbs/hour is discharged through the diaphragm 22,
which has an orifice of about 0.010 inches, and into the saturated
airstream. When discharged into the airstream, the propane goes
through a phase change into gas and absorbs heat from the
airstream. If desired, other expanding gases or refrigerants which
have a high heat of vaporization and would lower the temperature of
the air stream to below about -40.degree. C. on expansion or in a
phase change may be used. For example, other hydrocarbons like
butane, ethane, methane, and other liquified gases such as liquid
oxygen, halocarbons like freon, carbon dioxide, etc. may be used,
as well as ammonia, and sulfur dioxide. The refrigerant may incur a
phase change from a liquid to a gas or may simply be an expanding
gas. Other typical refrigerants are set forth in the Handbook of
Chemistry and Physics, 47th Edition, Section E, pages 17--25.
The expanding propane cools a portion of the saturated air below
-40.degree. C. and causes the formation of generally uniform ice
nuclei by homogeneous nucleation within the air stream. The average
particle size of the ice nuclei formed without addition of bleed
water is generally between about 0.1 to 10 microns say for example
less than one micron and with bleed water may be increased to
between about 125 to 250 microns say about 150 microns. The nuclei
are then discharged into the airstream. Water is introduced through
the conduit 34 in combination with the air through conduit 32. The
streams are controlled to provide for a moist airstream to be
introduced into the housing to ensure that the local relative
humidity is sufficient to prevent reduction in nuclei size and in
practice is probably 60 to 95 percent relative humidity.
Water enters the conduit 30 and flows through the nozzle at any
temperature in excess of 0.degree. C. and generally between
1.degree. to 15.degree. C. say about 4.degree. C. at a rate of
between 1--300 gallons/minute for example 15 gallons/minute and is
forced through the nozzle 28 at any rate to produce the desired
particle size between about 40 and 100 p.s.i. for example 80 p.s.i.
This temperature is advantageous in that it prevents anchor ice
from forming in the conduit 30 and in the nozzle. The nozzle breaks
the water into droplets of suitable size say between 100 and 500
microns for example about 200 microns, and they are discharged
countercurrent and directly into the flow of air and ice nuclei.
The droplets are supercooled to below 0.degree. C. by the ambient
air flowing through the housing.
These supercooled droplets are then infected by collision with the
ice nuclei previously formed. When a drop is infected with an ice
nuclei, spicules of ice form very rapidly within it, releasing
latent heat until the droplet becomes an ice water mixture at a
temperature of about 0.degree. C. remaining at this temperature
until the freezing process is completed through the exchange of
heat to the surrounding atmosphere. This exchange of heat with the
surrounding atmosphere progressively continues both while the
droplet is within the precipitator and after the ice crystals are
discharged from the end of the housing into the atmosphere where
the freezing process is continued until the droplet is all or
largely all frozen and the particles fall to the ground as
snow.
Normally and particularly with atmospheric conditions at low
humidity the ice nuclei exchange latent heat with the surrounding
atmosphere (ambient air) at a very rapid rate. These conditions
tend to shorten the life of the ice nuclei and consequently the
probability of achieving successful nucleation infections is
diminished, since in an atmosphere of low humidity, the nuclei
reevaporate very quickly. However, in my invention, the atmosphere
about the ice nuclei is controlled from the time of the formation
of the ice nuclei until they infect the water drops. This provides
enough water content in the nuclei stream to preserve the nuclei
until they infect the water drops. As described in the preferred
embodiment the atmosphere is controlled by the introduction of the
moist airstream through the conduit 32. Of course the humidity or
moisture content about the ice-nuclei stream may be controlled in
other ways. For example, a high enough moisture content may be
provided in the nucleating zone itself through a high moisture
content in the conduit 20. Also, the moisture content of the
ambient air flowing through the housing may be controlled and the
propane or other refrigerant injected directly into the airstream
to form the ice nuclei.
Although the nucleating zone 18 has been shown to include an
L-shaped conduit with the diaphragm located near the discharge end
of the conduit carrying the moist air, it is possible for it to
take other forms. For example, the moist air may flow through a
venturi passage and the refrigerant injected prior to the
constriction as shown in FIG. 3 at the constriction or just beyond
the constriction. Also the nucleating zone may be of a nonunitary
construction and the diaphragm disposed adjacent to the discharge
end of the conduit 20.
When compressed air is allowed to expand adiabatically, the
temperature of the air falls about 10.degree. C. for each 8 percent
decrease in pressure. Therefore, if the air is initially at a
temperature near 0.degree. C. expansion from a modest pressure of
20 lbs/inch is more than sufficient to bring the temperature below
-40.degree. C. If the atmospheric air initially has a relative
humidity higher than about 50 percent, it will become saturated in
the course of this expansion. In this situation, in the initial
instant after the temperature falls below -40.degree. C., very
large numbers of very small ice nuclei, say for example, in the
range of 40 to 60 angstroms form spontaneously. Those nuclei in
this range generally reevaporate or burn off very quickly, but this
can be inhibited by controlling the humidity of the airstream. It
has been found that by expanding compressed air which initially has
a humidity of from 50 to 80 percent say, for example, 70 percent
that this provides that the ice nuclei will survive even when the
humidity of the ambient air is less than 50 percent.
Accordingly, in FIG. 2, in an alternative embodiment of the
invention, the nucleating zone comprises a feed conduit 34 and a
nozzle 36 in communication with the conduit 34. The nozzle 36 is
adapted to release saturated compressed air through a nozzle to
spontaneously form ice nuclei. Again, an efficient economic
operation is provided wherein the formation and life of the ice
nuclei is effectively controlled.
My invention has been described in reference to homogeneous
nucleation when a refrigerant such as propane or carbon dioxide is
injected into a stream of saturated air to lower the temperature to
below about -40.degree. C. to form the ice nuclei as shown in FIG.
1 or when compressed air is cooled by expansion as shown in FIG. 2.
It is also within the scope of this invention that heterogeneous or
isomorphic nucleation may be used to form the ice nuclei in the
nucleating chamber, as for example by the introduction of silver
iodide smoke or comminuted natural ice. Referring again to FIG. 1,
the saturated airstream flows through the feed conduit 20 at a
temperature of less than 0.degree. C. A finely foreign nucleating
agent such as a solid particle or product like a silver salt such
as silver iodide is injected into this airstream such as through
line 26. The silver idodide or other nucleating agent initiates the
formation of ice crystals in a supercooled saturated airstream
Also, the airstream may be treated with foreign crystalline
substances which may normally have a hemimorphic hexagonal
crystalline structure similar to that of ice. For example, wurtzite
has been found to be satisfactory. Also various natural minerals in
a finely divided state may be similarly employed to initiate
crystallization in the supercooled airstream; namely, zincite,
nephelite, and lead iodide may also be used. If desired, ice nuclei
per se may also be injected into the saturated stream. Also,
certain resinous compounds, such as urea, have also been
effective.
Although my invention has been described in reference to particular
embodiments wherein the relative humidity about the ice-nuclei
stream is controlled within certain limits, operating conditions at
a particular time will determine the degree of relative humidity
required about the ice-nuclei stream. Also depending upon the
amount of snow to be generated, the flow rates of compressed air,
propane, bleed water, ambient air, etc., will vary. Further,
although the embodiments described showed a single nucleating zone
and nozzle, there may be employed in a particular operation a
plurality of chambers and nozzles within a housing and the orifice
sizes may vary.
* * * * *