Note: Descriptions are shown in the official language in which they were submitted.
~'~~1~~~
HOT GAS DEFROST SYSTEM FOR
REFRIGERATION SYSTEMS AND APPARATUS '.CHEREFOR
Field of the Invention
This invention is concerned with improvements in or
relating to refrigeration systems, and especially to hot gas
defrost systems for refrigeration systems and heat pumps,
and to apparatus for use in such hot gas defrost systems.
Reyiew of the Prior Art
The cooling coil of any refrigeration system will
gradually collect frost or ice on its surface, due to the
fact that water vapor in the air in contact with the coil
condenses on it, and its temperature is usually low enough
for the moisture to freeze on it. Ice is a relatively good
heat insulator and if allowed to build up will initially
lower the efficiency of the refrigerator, and eventually
cause it to become ineffective. It is standard practice
therefore in all but the simplest refrigerator or
refrigerator.installation to provide a system for
automatically defrosting the coil, usually by arranging that
at controlled intervals it is warmed to a temperature and
for a period that will melt the ice, the resultant water
being drained away. There are two principal methods
currently in use for automatic defrost, namely electrical
and hot gas.
_ 1 _
In an electrical defrost system electric heating
elements are provided in contact with the coil; at the
required intervals the refrigeration system is stopped from
operating and the elements are switched on to provide the
necessary heat. In a hot gas defrost system the hot gas
delivered from the compressor, that normally goes to an
exterior coil to be cooled, is instead diverted into the
cooling coil, again for a predetermined period found from
experience to be satisfactory for the purpose. Both systems
have their advantages and disadvantages.
An electrical system is relatively easy to design and
install, but is more costly to implement and much less
energy efficient than a hot gas system. A hot gas system is
less costly to install but has been difficult to design; a
particular problem of hot gas systems is that the
compressor, the most expensive single component of the
system, is easily damaged if it receives liquid refrigerant
instead of gaseous refrigerant at its inlet. The heat
exchange between the hot gas and the cold ice--laden coil
will tend to liquefy the refrigerant, and the resultant
droplets are difficult to remove from the gas, with
consequent danger to the compressor.
A hot gas system delivers the heat directly to the tube
of the coil and can therefore perform a comparable defrost
with less energy expenditure than an equivalent electrical
system. Moreover, the hot gas system effectively obtains
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CA 02049464 2000-09-19
its power from the compressor motor and requires only the
addition of suitable flow valves and piping for its
implementation; it is therefore the preferred system
provided one is able to ensure that the expensive compressor
S is not damaged by liquid refrigerant.
Another type of apparatus incorporating a refrigeration
system is a heat pump. It is usual practice with such
systems for the outdoor coil to be air-cooled, owing to the
expense of a ground-cooled system, and periodic defrosting
of the outdoor coil is necessary when the system is in
heating mode, because of the tendency of the coil to become
ice-laden, especially when the outside temperature is low.
"Reverse cycle" defrosting is by far the most common method
of defrost employed, and in this method the unit is switched
to the cooling mode and defrost occurs as hot gas from the
compressor condenses in the outdoor coil.
There has been disclosed and claimed in my prior U.S. Patents Nos.
4,798,058; 4,802,339 and 4,914,926, a new liquid refrigerant vaporizer which
is
incorporated in a respective hot gas defrost system between
the outlet of the condenser coil or coils and the compressor
inlet and is supplied with hot gas from the compressor
outlet, the vaporizer ensuring that any droplets in the gas
emerging from the coil outlets are vaporized before they can
reach the compressor inlet. These vaporizers have proven to
be very effective and are now in commercial use.
A typical vaporiser as disclosed in my prior patents
_ 3 _
referred to above consists of three coaxial cylindrical
tubes, all os approximately the same length. The innermost
tube constitutes a first flow passage with an inlet at one
end of the device that is connected to the condenser coil
outlet to receive the refrigerant fluid exiting therefrom.
The other end of this innermost tube is closed and its
cylindrical wall is provided with a number of
radially-extending apertures that direct the refrigerant
fluid radially outwards from the first flow passage into a
second flow passage formed between the innermost and middle
tubes, so as to impinge against the inner wall of the middle
tube, the fluid then passing from the second passage to an
outlet at the other end of the device that is connected to
the compressor inlet. A third flow passage surrounding the
middle tube and formed between the middle and outermost
tubes is provided with hot refrigerant gas from the
compressor outlet and heats the wall of the middle tube so
that the fluid that impinges thereon is fully vaporized.
Since the device is usually inserted into a run of pipe,
often as a retrofit to an existing system, the inlet and
outlet are usually identical and, although the flow
direction may be clearly marked on its exterior, there is
still the possibility that it is connected in reverse,
considerably reducing its effectiveness.
Definition of the Invention
It is therefore an object of the present invention to
provide a new liquid refrigerant vaporizer fox use in a hot
4 _
gas defrost system of a refrigeration system,
It is also an object to provide such a new vaporizer
which is operative independently of the direction in which
refrigerant fluid flows therethrough.
In accordance with the present invention there is
provided a liquid refrigerant vaporizer for use in a
refrigeration system employing hot refrigerant fluid to
defrost a coil or coils thereof, the vaporizer comprising:
a first tubular member having an inlet/outlet at each
end thereof, one of which inlet/outlets in operation is
connected in the refrigeration system to receive refrigerant
fluid exiting from a coil under defrost, and the other of
which is connected in the refrigeration system to deliver
the refrigerant fluid thereto, the member having at leapt
approximately midway along its interior a transverse barrier
dividing the interior into a first chamber connected to one
inlet/outlet and a third chamber connected to the other
inlet/outletp
a second tubular member of heat conductive material
surrounding the first tubular member to form a second
annular chamber between them;
a first set of bores in the first chamber wall
directing fluid from the first chamber into the
second chamber radially outward to impinge against the inner
surface of the second tubular member wall;
a second set of bores in the third chamber wall
directing fluid from the third chamber into the
second chamber radially outward to impinge against the inner
_ 5 _
surface of the second tubular member wall;
fluid that passes from the first chamber inlet/outlet
into the first chamber and through the first set of bores
into the second chamber thereafter moving in turbulent heat
exchange contact with the inner surface of the second
tubular member to the second set of bores, turning radially
inward therethrough into the third chamber, and passing out
of the third chamber inlet/outlet, while fluid that instead
passes from the third chamber inlet/outlet into the third
chamber and through the second set of bores into the second
chamber thereafter moves in turbulent heat exchange contact
with the inner surface of the second tubular member to the
first set of bores, turns radially inward therethrough into
the first chamber, and passes out of the first chamber
inlet/outlet; and
a third ~:ubular member surrounding the second tubular
member to form a third annular chamber between them, the
third chamber having an inlet thereto for hot defrost
refrigerant fluid to contact and heat the second chamber
wall and the surface thereof against which the refrigerant
fluid impinges, and having an outlet therefrom for the
defrost refrigerant fluid.
~ refrigerant fluid flow restriction will usually be
provided at or connected to the third chamber outlet for
producing an increase in back pressure of the refrigerant
fluid in the third chamber.
The vaparizor may be provided with an expansion chamber
downstream of the restriction for re-evaporation of any
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liquid component passing through the flow restriction.
The invention also provides a hot gas defrost system
and a refrigeration system employing such a refrigerant
vaporizer.
Description of the Drawings
Embodiments of the invention will now be described, by
way of example, with reference to the accompanying schematic
and diagrammatic drawings, wherein:
FIGURE 1 is a schematic diagram of a refrigeration
system embodying the invention;
FIGURE 2 is a longitudinal cress-section through a
concentric tubular full flow liquid refrigerant vaporizer of
the invention; and
FIGURE 3 is a schematic diagram of a heat pump system
embodying the invention and employing the vaporizer of
Figure 2.
Description of the Preferred Embodiments
Figure 1 shows a refrigeration system which includes a
compressor 10 having a suction inlet 12 and a high pressure
outlet 14. A refrigerant condenser coil 16 has an inlet 18
connected to the high pressure outlet 14, and an outlet 20
connected to a vessel 22 which is adapted to collect liquid
refrigerant. A refrigerant-conducting line 24 connects the
vessel 22 to a thermostatic expansion valve 26 through a
filter drier 28, a liquid indicator 30 and a
solenoid-controlled liquid valve 32. The cooling coil 34 of
the system has an inlet 36 connected to the expansion valve
26, and an outlet 38 connected to a refrigerant inlet 40 of
a full flow liquid refrigerant vaporizer of the invention
indicated generally by 42. The vaporizer 42 has an outlet
44 connected to the inlet of a suction line liquid
accumulator 46, while the outlet of the accumulator 46 is
connected to the suction inlet 12 of the compressor 10 to
complete the circuit.
In its refrigeration mode of operation hat compressed
gas from the compressor is condensed in coil 16, a fan 48
being provided to circulate air over and through the finned
heat exchange structure of the coil. With the valves 26 and
32 open liquid refrigerant expands in the expansion valve 26
and passes into the coil 34 to cool the coil and therefore
the adjacent space, air being circulated over the coil by a
fan 50. All the expanded refrigerant vapor passes through
the vaporizer 42, whose structure and function will be
described in detail below, to return to the compressor 10
via the accumulator 46. This is of course a standard mode
of operation for a refrigeration system, and this particular
flow is illustrated by the broken line arrows.
The construction of the concentric tubular liquid
refrigerant vaporizer 42 of Figures 1 and 3 will now be
described with particular reference to Figure 2. The device
42 is made of metal, preferably a high conductivity metal
such as copper or brass, and consists of a first innermost
cylindrical pipe 52, provided at least approximately at its
middle point along its leng~:Yi with a transversely-extending
circular disc 54 comprising a barrier extending over its
_ 8 _
entire cross-sectional area and dividing the pipe interior
into two separate cylindrical chambers 56 and 58, called for
convenience in terminology the first and third chambers.
One end of this pipe constitutes the inlet 40, while the
other end constitutes the outlet 44. The disc may be
fastened into the interior of the pipe in any suitable
manner, or alternatively, as illustrated, it may constitute
a connecting member between two coaxial pipe pieces which
together form the pipe 52; it may be noted that the barrier
provided by the disc does not need to be absolutely gas
tight between the first and third chambers. ~ second middle
cylindrical pipe 62 of larger diameter surrounds the first
innermost pipe 52 coaxial therewith and is sealed to the
pipe 52 at both ends which turn radially inwards, thereby
forming an annular cross-section second chamber 64 between
the two pipes.
The fast flowing refrigerant fluid entering the
innermost pipe 52 from the coil 38 impinges strongly against
the transverse barrier 54 and immediately becomes extremely
turbulent within the first chamber 56, far more so than the
low velocity gas involved in the normal refrigeration
cycle. The pipe 52 has a first set of plurality of hales 68
distributed uniformly along the part of its length within
the first chamber 56, and also distributed uniformly around
its periphery, these holes directing the turbulent
refrigerant vapor from the chamber 56, together with any
liquid entrained therein, forcefully into the second middle
chamber 64 against the inner wall of the middle pipe 62.
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~fl~~~~
The pipe 52 has another set of a plurality of holes 70
similarly uniformly distributed along the part o.f its length
within the second chamber 64 and around its periphery, which
holes direct the highly turbulent vapor in the second
chamber 64 back into the third chamber 58 and out of the
outlet 44, the abrupt change of direction of the vapor
required for its passage through the second set of holes 70
considerably increasing its turbulence in the third chamber
64.
A third outermost cylindrical pipe 72 coaxial with the
pipes 52 and 62 encloses at least that portion of the middle
pipe 62 adjacent the location of the holes 68 and 70, and
has its radially inwardly-turned ends sealed to the pipe 62
so as to define a fourth outer annular cross-section chamber
74 surrounding the pipe 62. A hot gas inlet 76 is provided
adjacent to one end of pipe 72 and an outlet 78 adjacent to
the other end, so that hot refrigerant fluid from the
compressor can be passed through the chamber 74 in heat
exchange contact with as much as possible of the outer wall
of the heat-conductive pipe 62, thereby heating the inner
wall against which the refrigerant impinges when emerging
from the holes 68 or 70, and against which the resultant
turbulent fluid moves as it passes along the second chamber
to exit through the other set of holes 70, resulting in
complete and substantially immediate evaporation of any fine
droplets therein. The fluid in the chamber 64, consisting
now entirely of vapor, passes through the holes 70 into the
third chamber 58 and exits through outlet 44 and the
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2~~~~~~~
accumulator 46 to the compressor inlet 12.
The hot gas defrost system of the invention including
the full flow vaporizer 42 has the fourth chamber inlet 76
connected to the hot gas outlet 14 of the compressor via a
control valve 80 and a hot gas solenoid-operated valve 82,
while its outlet 78 is connected via a check valve 84 to the
junction of coil inlet 36 and expansion valve 26. The
operation of the defrost system is under the control of a
defrost timer 86 connected to the fan 50 and the valves 32
and 82. The operation of the expansion valve 26 is under
the control of a thermostatic sensor 88. The remainder of
the controls that are required for operation of the system
will be apparent to those skilled in the art and do not
reguire description herein for understanding of the present
invention.
At predetermined intervals the defrost timer 86
initiates a defrost cycle by closing the solenoid valve 32
so that expanded cold refrigerant is no longer supplied to
the coil 34; the timer deenergizes the fan 50 and opens hot
gas solenoid valve 82, whereupon heated high pressure vapor
from the compressor flows through the chamber 74 and heats
the heat conductive pipe 62. The fluid exits at outlet 78
through a valve 90 constituting a controllable restriction
and an expansion chamber 92 and passes through the check
valve 75 to enter the coil 34. The fluid is still hat and
gives up sensible and latent heat to the coil, warming it
and melting any frost and ice accumulation, the gas becoming
cooler by the consequent heat exchange. The fluid moves
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through the coil at relatively high velocity and only part
of it condenses to liquid, which is however completely
revaporized in the vaporizer, as described above. At the
end of the timed defrost period the timer 86 deenergizes and
closes the hot gas valve 82, opens valve 32 and reenergizes
the fan motor 50, so that the system is again in its normal
cooling mode.
The device will allow refrigerant to flow equally well
in either direction, so that it is immaterial which end is
used as the inlet, and which is used as the outlet, exactly
the same effective heat exchange action being obtained if
the device is reversed. Although the device is illustrated
in horizontal attitude its operation is independent of
attitude and it can be disposed in any convenient location,
unlike the accumulator 46 which must be disposed upright as
shown. It may be noted that the accumulator 46 is not
required for the hot gas defrost cycle and its sole purpose
is to try to protect the compressor in case of a liquid
refrigerant flow control malfunction. As is usual, any
lubricant in the system that collects in the accumulator
bleeds back into the circuit through bleed hole 94 in return
pipe 96.
The dimensions of the three pipes 52, 62 and 72 and of
the apertures 68 and 70 relative to one another axe
2S important for the successful functioning of the vaporizer in
accordance with the invention, as is described in my prior
patents referred to above. Thus, the pipe 52 preferably is
of at least the same internal diameter as the remainder of
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the suction line to the compressor, so that it is of the
same flow cross-sectional area and capacity. The number and.
size of the holes 68 and 70 are chosen so that the flow
cross-section area provided by all the holes together is not
less than about 0.5 of the cross-section area of the pipe 52
and preferably is about equal or slightly larger than that
area. The total cross-section area of the holes need not be
greater than about 1.5 times the pipe cross-section area and
increasing the ratio beyond this value has no corresponding
increased beneficial effect. Moreover, each individual hole
should not be too large and if a larger flow area is needed
it is preferred to provide this by increasing the number of
holes. As described above, the purpose of these holes is to
direct the flow of refrigerant fluid radially outwards into
impingement contact with the inner wall of the pipe 62, and
this purpose may not be f ally achieved if the holes are too
large. Each set of holes is uniformly distributed along and
around its respective portion of the pipe 52 to maximize the
area of the adjacent portion of the wall of pipe 62 that is
contacted by the fluid issuing .from the holes.
rt is also important that the flow cross-section area
of the second annular chamber 64 be not less than about 0.5
of the corresponding flow area of the pipe 52, and again
preferably they are about equal, with the possibility of
that of chamber 64 being greater than that of pipe 52, but
not too much greater, the preferred maximum again being
about 1.5 times. The diameter of the pipe 72 is made
sufficiently greater than that of the pipe 62 that the
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cross-sectional flow area of the annular space 74 is not
less than that of the hot gas discharge line from the pump
outlet 14 to the inlet 76, and can be somewhat larger, to
the same extent of about 1.5 times. The inlet 76 to the
chamber 74 and the outlet 78 are of course of sufficient
size not to throttle the flow of fluid therethrough.
It will be understood by those skilled in the art that
if the vaporizer is constructed in this manner then during
normal cooling operation of the system it will appear to the
remainder of the system as nothing more than another piece
of the suction line, or at most a minor constriction or
expansion of insufficient change in flow capacity to change
the characteristics of the system significantly. The system
can therefore be designed without regard to this particular
flow characteristic of the vaporizer. Moreover, it will be
seen that it can be incorporated by retrofitting into the
piping of an existing refrigeration system without causing
any unacceptable change in the flow characteristics of the
system.
The orifice or flow restrictor constituted by the valve
90 is surprisingly effective in providing consistent
defrosting and self-regulation of the process, the latter
avoiding compressor overload and consequent stress, the
valve being adjusted during operation to provide the
required value of back pressure, for a predesigned and
prebuilt system it can instead be a fixed orifice. The
operation of the vaporizer and the functions of the
restrictor valve 90 and the subsequent expansion chamber 92
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are fully described in my prior patents referred to above,
to which reference can be made.
In a specific embodiment intended for a refrigeration
system employing a 7.5-10 horsepower motor the entire
vaporizer device had a length of about 65 cm (26 in.). The
inner pipe 52 was copper of 3.4 cm (1.325 in.) outside
diameter (O.D.); the middle pipe 62 was also copper of 5.3
cm (2.125 in.) O.D.. The pipe 52 was provided with two
separate sets of 48 uniformly distributed holes each of 4.8
mm (0.1875 in.) diameter f or a total of 96 holes. The
outermost pipe 72 had a length of 60 cm (24 ins) and an O.D.
of 6.56 cm (2.625 ins), while the hot gas line had a
diameter of 2.18 cm (0.875 in).
Unexpectedly I have found that a device as specifically
described, employing three successive chambers with two
abrupt changes of direction through respective sets of
holes, is just as efficient in providing for vaporization of
the fluid refrigerant as my prior device, as described and
illustrated for example in the respective figures 2 of my
above-mentioned prior U.S. Patents, which employs two
successive chambers with only a single abrupt change of
direction through a single set of holes. It is a
substantial commercial advantage of this embodiment that the
installer is able to instal it without having to consider
the direction of refrigerant flow through the device. It
was found with the prior art devices that there was an
unacceptable decrease in performance if it has been
installed reversed, but this cannot happen with the devices
_ 15 _
of the present invention.
The invention is of course also applicable to domestic .
refrigerators which hitherto have normally used electric
defrost circuits, but would be much more energy efficient if
hot gas defrost could be used. The invention is also
particularly applicable to heat pump systems and Figure 3
shows such a system in heating mode, the system being
shifted to air conditioning mode by movement of a
solenoid-operated change-over valve 97 from the
configuration shown in solid lines to that shown in broken
lines. Coil 16 is the outdoor coil which in heating mode is
cooled and in air conditioning mode is heated, while coil 34
is the inside coil with which the reverse occurs. when the
outside temperature falls below about 8°C (45°F) the
temperature of coil 16 in heating mode will be cold enough
to condense and freeze moisture in the air circulated over
it by fan 48, and if this frost is allowed to build up will
quickly reduce the unit's efficiency. The most common
method of defrosting is simply to reverse the cycle to air
conditioning mode by operation of change-over valve 9~,,
every 30 to 90 minutes for a period of from 2 to 10 minutes,
depending upon the severity of the icing conditions. This
valve is normally under the control of room thermostat 98
which causes it to switch from one mode to the other for
heating or cooling as required.
Tn heating mode the hot high pressure vapor produced by
the compressor 10 is fed via the valve 97 to the indoor coil
34 while hot gas solenoid valve 82 is closed. The vapor
_ 16 -
condenses in the coil to heat the air passed over the coil
by the fan 5U, and the condensed refrigerant passes through,
check valve 99, by-passing expansion device 100 which is
illustrated as being a capillary line, but instead can be an
orifice or expansion valve of any known kind. The liquid
however must pass through similar expansion device 102 and
the resultant expanded cooled vapor passes to the outdoor
coil 16 to be heated and vaporized by the ambient air.
Check valves 104 and 106 ensure respectively that the device
lU2 is not by-passed, and that the expanded vapor cannot
enter the vaporization device 42. The vaporized refrigerant
from the coil 16 passes through the device 42 as though it
were simply an open part of the compressor suction line
tubing, and then passes through valve 97 and the accumulator
46 to the compressor inlet 12 to complete the cycle. The
controls required for the operation of the system will be
apparent to those skilled in the art and a description
thereof is not needed herein for a full explanation of the
present invention.
A defrost cycle is initiated by the defrost control 86
without any change required in the position of valve 97, the
control switching off the fan motor 48, so that the coil 16
is no longer cooled by the fan, and opening the hot gas
valve 82 to admit the hot high pressure refrigerant vapor
from the compressor to the vaporizer chamber 74, as well as
to the indoor coil 34. After warming the pipe 62 the hot
gas passes through restrictor valve orifice 90, expansion
chamber 92 and check valve 106 to enter the coil 16 and
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perform its defrost function, as described above with
reference to Figures 1 and 2. The direct pressure of the
hot gas at the end of the expansion device 102 blocks the
flow from the coil 34 so that the refrigerant is trapped in
the line between the two restrictions.
A liquid line solenoid 108 is installed ahead of the
expansion device 102 and is closed during the defrost period
to prevent the liquid refrigerant in the line expanding into
the outside coil 16, which would reduce the defrost
efficiency. The operation of the device 42, the restrictor
90 and the expansion chamber 92 are exactly as described
above, the gas from the outlet 44 passing through valve 97
and accumulator 46 to the suction inlet 12 of the
compressor. After a predetermined period of time set by the
defrost control 86, with or without an override temperature
control provided by a thermostat ll0 adjacent to the coil
outlet 18, whichever arrangement is preferred to ensure that
defrosting is complete, the valve 82 is closed to stop the
direct flow of hot gas to the vaporizer 42 and coil 16. The
solenoid valve 108 is opened and the' fan motor 48 is
restarted. The system then returns to its normal heating
cycle.
The vaporizer 42 is inoperative when the system is in
air conditioning or cooling mode serving as part of the
compressor discharge line due to it being able to pass
refrigerant flow equally in either direction and description
of the cycle in that mode is therefore not required, except
to point out that the expansion device 100 is now operative
while the device 102 is by-passed by check valve 104.
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