U.S. patent number RE29,966 [Application Number 05/831,032] was granted by the patent office on 1979-04-17 for heat pump with frost-free outdoor coil.
This patent grant is currently assigned to Halstead Industries, Inc.. Invention is credited to Otto J. Nussbaum.
United States Patent |
RE29,966 |
Nussbaum |
April 17, 1979 |
Heat pump with frost-free outdoor coil
Abstract
A heat pump having an auxiliary outdoor coil equipped with
heating means preventing the surface temperature during the heating
cycle from falling below 32.degree. C. The heating means comprises
an electrical resistance heater in thermal contact with the fins of
the outdoor coil such that heat is transferred to the fins by
conduction. The system incorporates means to permit functioning
immediately upon return to the heating cycle from a cooling cycle
even though the pressure in the receiver of the system initially
exceeds that in the indoor coil. Additionally, the system
incorporates means for protecting the compressor from liquid
floodback when changeover occurs from heating to cooling.
Inventors: |
Nussbaum; Otto J. (Huntsville,
AL) |
Assignee: |
Halstead Industries, Inc.
(Scottsboro, AL)
|
Family
ID: |
23729321 |
Appl.
No.: |
05/831,032 |
Filed: |
September 6, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
435673 |
Jan 23, 1974 |
3918268 |
Nov 11, 1975 |
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Current U.S.
Class: |
62/150; 62/156;
62/160; 62/324.6; 62/80 |
Current CPC
Class: |
F25B
47/006 (20130101); F25D 21/08 (20130101); F25D
21/04 (20130101); F25B 2400/19 (20130101) |
Current International
Class: |
F25D
21/04 (20060101); F25D 21/00 (20060101); F25D
21/08 (20060101); F25B 47/00 (20060101); F25B
013/00 () |
Field of
Search: |
;62/150,80,156,160,324 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Davis, Jr.; Albert W.
Attorney, Agent or Firm: Murray; Thomas H.
Claims
I claim as my invention:
1. In a heat pump, the combination of:
an indoor heat exchanger,
first and second outdoor heat exchangers,
a compressor,
a receiver,
conduit means interconnecting said heat exchangers, said receiver
and said compressor,
expansion valve means for said indoor heat exchanger and the first
of said outdoor heat exchangers,
valve means in the conduit means, said valve means having a first
condition in which the output of the compressor is connected
through the second of said outdoor heat exchangers, the receiver,
the expansion valve means for the indoor heat exchanger and said
indoor heat exchanger back to the input of the compressor whereby
the indoor and outdoor heat exchangers act as an evaporator and
condenser, respectively, the valve means having a second condition
in which the output of the compressor is connected through said
indoor heat exchanger, the expansion valve means for the first
outdoor heat exchanger and said first outdoor heat exchanger back
to the input of the compressor whereby the indoor and outdoor heat
exchangers act as a condenser and an evaporator, respectively,
.[.electrical resistance.]. heating means in intimate contact with
fins on said first outdoor heat exchanger whereby heat can be
transferred from the heating means to the fins by conduction,
and
means for .[.energizing.]. .Iadd.causing .Iaddend.said heating
means .Iadd.to supply heat to the fins .Iaddend.whenever the
temperature of the fins reaches a point where frost can form
thereon, whereby the fins are maintained free of frost regardless
of the ambient temperature.
2. The heat pump of claim 1 wherein said heating means .[.is
energized.]. .Iadd.supplies heat to the fins .Iaddend.when the
temperature of the fins falls below 32.degree. F.
3. The heat pump of claim .[.2.]. .Iadd.5 .Iaddend.wherein said
means for energizing said heating means comprises thermistor means
in contact with said fins, and means for supplying electrical power
to said heating means when the temperature sensed by said
thermistor means falls below said predetermined value.
4. The heat pump of claim 1 wherein said heating means comprises a
plurality of heating elements spaced along said one outdoor heat
exchanger, each heating element being in intimate contact with the
outdoor heat exchanger surface to transfer heat thereto by
conduction. .Iadd. 5. The heat pump of claim 1 wherein said heating
means comprises an electrical resistance heater, and including
means for energizing said electrical resistance heater to supply
heat to the fins when the temperature of the fins reaches a point
where frost can form thereon. .Iaddend.
Description
BACKGROUND OF THE INVENTION
In a heat pump, compressed refrigerant is evaporated in an outdoor
evaporation coil, the expanded refrigerant being thereafter
compressed and passed through a condenser which extracts heat from
the compressed refrigerant for heating the interior of a building.
During warm weather, the refrigeration cycle is reversed and the
system used as an air conditioner. Heat pumps of this type which
are installed in cold climates must operate at outdoor air
temperatures below 32.degree. F. and sometimes as low as
-20.degree. F. Under these conditions, the evaporating temperature
of the refrigerant in the outdoor coil drops to a point at which
the coefficient of performance of the heating system is
unreasonably low. To make such a system commercially feasible,
electric resistance heaters disposed in front of the fan for the
interior condenser have been used to supply the needed additional
heating capacity; however, this further lowers the overall system
coefficient of performance. Furthermore, operation at outdoor air
temperatures below +32.degree. F. results in frost accumulation on
the outdoor coil surface, which is normally removed by periodically
reversing the system to the cooling cycle for defrosting of the
outdoor coil. In such defrost operations, the supplemental indoor
heating coil must not only supply the entire heating requirement
but must also compensate for the cooling effect of the indoor coil
which temporarily becomes an evaporator. During a defrost
operation, therefore, the power consumption of the system may
nearly double since both the compressor and the maximum resistance
heater demand are imposed on the power supply.
In an effort to overcome this problem, systems have been devised
for heating the outdoor evaporator coil to prevent the accumulation
of frost. One such system, for example, is shown in Trask U.S. Pat.
No. 3,529,659 wherein an effort is made to prevent frost
accumulation with the use of radiant heat from an electrical
heating element. The amount of heat available by radiation,
however, is relatively slight, and, as a result, systems utilizing
this arrangement have not achieved commercial acceptance.
Another problem encountered with heat pumps is their inability to
function immediately upon changeover to the heating cycle since the
pressure in the receiver of the system may initially exceed the
pressure in the indoor coil which becomes a condenser during the
heating cycle. This problem is clearly described, for example, in
Henderson U.S. Pat. No. 2,763,130. If, in fact, the pressure in the
receiver is higher than that in the indoor coil at the start of the
heating cycle, the condensate formed in the indoor coil will
accumulate in the coil which will rapidly fill up with liquid
refrigerant and deprive the remainder of the system of its charge
without performing any useful heating function.
SUMMARY OF THE INVENTION
In accordance with the present invention, a new and improved heat
pump system is provided which overcomes the disadvantages of prior
art systems in that it allows continuous system operation at an
economically practical coefficient of performance and completely
eliminates the need for periodic reverse defrost operations
regardless of outdoor ambient temperature or humidity.
Additionally, the system of the invention incorporates means to
permit the system to function immediately upon changeover to the
heating cycle from a cooling cycle even though the pressure in the
receiver of the system may initially exceed that in the indoor
coil. A still further feature of the invention resides in the
provision of means for protecting the compressor from liquid
floodback when reversal occurs from the heating cycle to the
cooling cycle.
Specifically, in accordance with the invention, two outdoor heat
exchangers are provided, one of which is used as a condenser (heat
sink) during a summer cooling cycle while the other is utilized as
the evaporator (heat source) for a heating cycle. In order to
prevent the accumulation of frost or ice on the outdoor heat source
coil, its surface temperature is always maintained above 32.degree.
F. by means of an electrical resistance heating element which is in
intimate thermal contact with the fins of the coil whereby heat is
transferred from the heating element or elements to the fins by
conduction in contrast to prior art systems wherein radiation was
relied upon. The electrical resistance heating coil can be
controlled by means of a thermistor which senses the surface
temperature of the outdoor coil fins or by means of a pressure
switch which senses a drop in pressure at the output of the outdoor
evaporator coil. In this manner, it is possible to operate at
evaporating temperatures of +20.degree. F. or higher so that a
coefficient of performance of not less than 4.5 can be
anticipated.
With the outdoor coil surface always above the freezing point of
water, no defrost operation will ever be required. Since this will
allow continuous system operation, without frequent interruptions
for defrost, it is feasible to utilize less compressor horsepower
for a greater heating load. In addition, the usual defrost controls
can be eliminated and frequent reversing of the refrigerant cycle
with its inherent danger to the compressors is avoided.
By using two outdoor coil circuits rather than a single coil as
both an evaporator (heat source) and condenser (heat sink),
possible damage to the system compressor is eliminated due to
liquid refrigerant reaching the compressor intake upon reversal
from a cooling to a heating cycle because of the large quantities
of liquid refrigerant held in a condenser in cool weather which are
likely to reach the compressor and its valves immediately upon
reversal.
Further, in accordance with the invention, in order to permit the
system to function immediately upon changeover to a heating cycle
from a cooling cycle, refrigerant from the receiver is permitted to
flow through the heat source outdoor coil for a short period of
time, usually about 2 minutes, at the beginning of the heating
cycle. During this initial short period when the receiver is
connected to the outdoor heat source coil, condensing pressure in
the indoor coil is permitted to build up while the liquid pressure
in the receiver falls. At the same time, while the condensing
pressure is permitted to build up initially, a sufficient
refrigerant charge is provided from the receiver to the active
heating circuit.
The above and other objects and features of the invention will
become apparent from the following detailed description taken in
connection with the accompanying drawings which form a part of this
specification, and in which:
FIG. 1 is a schematic view of the entire heat pump system of the
invention;
FIG. 2 illustrates one manner in which resistance heating elements
may be disposed in intimate thermal contact with the fins of an
outdoor evaporator;
FIG. 3 is a partial schematic diagram of the system of the
invention showing the cooling cycle;
FIG. 4 is a partial schematic diagram of the system of the
invention showing the heating cycle;
FIG. 5 is a schematic diagram showing the operation of the system
of the invention immediately prior to changeover from a heating to
a cooling cycle; and
FIG. 6 is a schematic electrical circuit diagram showing the
controls for the valves, fans, motors and other elements of the
system of FIG. 1.
With reference now to the drawings, and particularly to FIG. 1, the
system shown includes a compressor 10 of conventional construction
having an input or suction intake 12 and an output or discharge
side 14. The compressor discharge 14 is connected through a conduit
16 to the inlet side of a first outdoor heat exchanger 20 which
acts as a condenser during a normal refrigeration cycle. The heat
exchanger 20 is provided with a motor-driven cooling fan 22 in
accordance with usual practice. The exit side of the heat exchanger
20 is connected through a check valve 24 to a receiver 26. The
receiver 26, in turn, is connected through a cut-off valve 28, a
solenoid operated shut-off valve 30 and conduit 80 to an expansion
device 32 connected to the inlet side of an indoor heat exchanger
34 again provided with a motor-driven fan 36. The outlet side of
the indoor heat exchanger is connected through a three-way
solenoid-operated valve 38 and through a pressure regulating valve
40 back to the inlet side 12 of the compressor 10. It will be
appreciated that the system just described is a normal
air-conditioning refrigeration system wherein the indoor heat
exchanger 34 acts as an evaporator and the outdoor heat exchanger
20 acts as a condenser.
During a heating cycle, the heat sink coil 20 is not used. Rather,
a heat source outdoor coil 42 is employed. Coil 42 is again
provided with a motor-driven fan 44 as shown, although it should be
understood that a single fan may be used for both fans 22 and 44
shown herein. To effect a heating cycle, valve 38 is reversed so as
to connect the pressure side 14 of the condenser 10 through a
solenoid-operated valve 46, conduit 48 and valve 38 to the outlet
side of the indoor heat exchanger 34. Refrigerant forced into the
coil of heat exchanger 34 will now flow through a check valve 50
and a solenoid-operated valve 52, which is opened during a heating
cycle, to an expansion valve 54 connected to the inlet of the heat
source heat exchanger 42. As will be explained hereinafter, the
solenoid-operated valve 30 is initially maintained open at the
beginning of a heating cycle to permit refrigerant to flow from the
receiver 26 to the heat source coil 42; and is thereafter
closed.
The outlet side of the heat exchanger 42 is connected through a
conduit 56 back to the inlet side 12 of compressor 10 through
pressure-regulating valve 40. With this arrangement, refrigerant
will first flow through the indoor coil or heat exchanger 34 via
valves 46 and 38; whereupon the heat of the refrigerant is
transferred to the indoor atmosphere by the fan 36. From the heat
exchanger 34, the compressed refrigerant flows through check valve
50 and open solenoid valve 52 to the expansion device 54 at the
second outdoor heat exchanger or heat source 42 where the
refrigerant expands, thereby absorbing heat. The expanded
refrigerant is then returned back to the inlet side of the
compressor 10 via the conduit 56.
In the condition just described, heat from the exterior atmosphere
is transferred to the interior via the indoor heat exchanger 34
which now acts as a condenser rather than an evaporator. When,
however, the outdoor temperature drops to 32.degree. F. or lower,
frost and/or ice will form on the fins of the outdoor heat
exchanger or heat source 42, thereby reducing its heat exchange
efficiency materially. Accordingly, it is desirable to provide some
means for preventing the surface temperature of the fins from
dropping below 32.degree. F.
For this purpose, and in accordance with the invention, an
electrical resistance heating means 60 is provided in intimate
contact with the fins 62 of the heat exchanger 42 whereby heat from
the resistance heater will be transferred to the fins by
conduction. The resistance heater 60 may be connected to a power
source 64 through normally-open contacts 66 of a relay 68. The
relay 68, in turn, is controlled by a thermistor 70 or the like in
contact with the fins 62, the arrangement being such that when the
temperature falls to 32.degree. F., the current through the
thermistor 70 will increase to the point where relay 68 is
energized, thereby connecting the power source 64 to the heater 60.
The thermistor 70, of course, can be connected to additional
control circuitry, not shown herein for purposes of simplicity, or
can be used to control an SCR power supply for the heater 60.
Alternatively, instead of controlling the resistance heater 60 by
means of the thermistor 70, it also can be controlled by means of a
pressure switch 72 connected to the outlet side of the heat
exchanger 42, the arrangement being such that when the pressure of
the refrigerant falls as a result of falling ambient air
temperature, the switch 72 will close to energize the heating
element 60.
One possible arrangement for placing the heating element 60 in
contact with the fins of the heat exchanger 42 is shown in detail
in FIG. 2. Winding through the fins 62 is a serpentine coil 74. To
insure good thermal contact between the heating means and the fins,
a plurality of electrical resistance heaters 76 is disposed
throughout the length of the heat exchanger 42 in thermal contact
with the surfaces of fins 62 and in-between the turns of the coil
74. These, then, are all adapted to be connected in parallel to the
common power source 64 via lead 78.
The flow of refrigerant through the system during a normal cooling
cycle is shown in FIG. 3. Valves 52 and 46 are closed at this time;
solenoid valve 38 is in the position shown so as to connect the
inlet side 12 of the compressor 10 to the outlet of the indoor coil
34; and valve 30 is open. Under these circumstances, refrigerant
from the outlet side of the compressor 10 flows along the direction
the arrows through the heat exchanger 20, which now acts as a
condenser, the receiver 26, open valve 30, expansion device 32, and
the indoor heat exchanger 34 which now acts as an evaporator back
to the compressor through valves 38 and 40. This, of course, is a
conventional and normal refrigeration cycle.
The flow of the refrigerant during a heating cycle is shown in FIG.
4. As was explained above, at the beginning of a heating cycle, the
pressure in receiver 26 may initially exceed the pressure in coil
34. That pressure, which would exist in conduit 80, would cause the
condensate formed in coil 34 to accumulate. The result would be
that the coil 34 would rapidly fill up with liquid refrigerant and
deprive the remainder of the system of its charge without
performing any useful heating function. Accordingly, in the present
invention, the valve 30 is maintained open at onset of the heating
cycles for a predetermined short period of time during which liquid
refrigerant is permitted to flow from receiver 26 through open
valve 52, expansion valve 54, heat source coil 42 and conduit 56
back to the inlet 12 of the compressor 10, thereby forming a
complete cycle. During this short period, which typically may be
set for 2 minutes, the condensing pressure in the indoor coil 34 is
permitted to build up while the liquid pressure in receiver 26
falls. At the same time, a sufficient refrigerant charge is
provided for the active heating circuit.
At the end of this "charging cycle" of about 2 minutes, the liquid
solenoid valve 30 closes and liquid refrigerant condensed in the
indoor coil 34 is now free to flow through check valve 50 and open
valve 52 to the heat source coil 42. To prevent any possibility of
overloading the compressor when the ambient temperature at the heat
source is relatively high, the suction pressure regulating valve 40
is provided and is adjusted for an outlet pressure not to exceed
the maximum suction pressure for which the particular compressor is
designed.
As was explained above, the invention incorporates means for
protecting the compressor from liquid floodback when changeover
occurs from the heating cycle to the cooling cycle. Conditions
which exist during the pump-down operation before changeover to
cooling are illustrated in FIG. 5. At this time, the liquid
solenoid valve 30 and the suction shut-off valve 38 are still in
the position shown in FIG. 4 for the heating cycle. Under these
conditions, a changeover pressure switch, hereinafter described,
permits the hot gas solenoid valve 46 to close but prevents the
liquid solenoid valve 30 from opening until the pressure in the
indoor coil 34 has been reduced to a predetermined low value,
indicating that no liquid has remained in the indoor coil 34.
During this changeover period, refrigerant proceeds from the
discharge 10 to the conduit 16, condenser 20 and check valve 24
back to the receiver 26. At the same time, the indoor coil 34 is
evacuated and its refrigerant pressure is reduced through the check
valve 50, open solenoid valve 52, expansion valve 54, heat source
coil 42 and conduit 56 back through the suction pressure regulating
valve 40 to the compressor 10. Once the pressure in the indoor coil
34 has been reduced to a satisfactory low level, however, liquid
solenoid valve 30 opens and suction shut-off valve 38 reverses to
the position shown in FIG. 3, while heat source valve 52
closes.
The electrical control for the refrigerant system just described is
shown in FIG. 6. It includes two power leads 82 and 84 connected to
a source of potential, now shown. The control operation will first
be explained for the cooling cycle as shown in FIG. 3. The
changeover pump-down switch CH is in the "low" position. Switch CH
is in the low position when the pressure in indoor coil 34 is below
a predetermined value and in the "hi" position when the pressure is
above that value. If there is a demand for cooling, a thermostat CT
makes contact, thereby energizing the liquid line solenoid valve
30. This causes an increase in pressure in the evaporator 34, which
is transmitted to a low pressure cutout LPC which closes, thereby
energizing the compressor motor contactor C. An auxiliary interlock
contact C-1 of contactor C energizes the condenser fan contactor
CFC, thereby energizing the fan 22 of FIG. 1 providing that the
condenser pressure switch FCC is closed. In intermediate climates,
it is likely that the switch FCC will be open upon start-up;
however it will eventually close after the compressor has operated
for a short period of time and an adequate condenser pressure has
been built up to close the switch FCC. All other controls shown in
FIG. 6 to the right of a heating thermostat MHS are deenergized and
inactive during the cooling cycle. Suction shut-off valve 38 and
solenoid valve 52 are deenergized and assume the positions shown in
FIG. 3 since the changeover switch CH is in the low position and
the contactor C3 is deenergized such that contacts C3-1 are
open.
When the cooling thermostat CT is satisfied, it breaks contact,
thereby deenergizing solenoid valve 30. As a result, the evaporator
34 is rapidly evacuated causing a drop in pressure at the control
LPC which breaks and deenergizes the compressor contactor C.
Contactor interlock contacts C-1 break, deenergizing the fan motor
contactor CFC at the condenser or heat sink coil 20. After a
sufficient shut-down period, the pressure at the compressor
discharge will drop sufficiently to also open condenser pressure
control switch FCC.
If the temperature in the conditioned space drops further, say
10.degree. below the setting of cooling thermostat CT, heating
thermostat MHS makes contact, thereby energizing contactor C3 which
closes contacts C3-1. This reverses the position of suction cut-off
valve 38 and energizes solenoid valve 52 to open the same. At the
same time, when the heating thermostat MHS makes contact, current
is supplied to a time delay relay TD; however it is not actuated
for a period of about 2 minutes. As a result, and since contactor
C3 is now energized, contacts C3-2 are closed to complete a circuit
to solenoid valve 30 through normally-closed contacts TD-1 of the
time delay relay TD and contacts C3-1. Thus, the solenoid valve 30
remains open at this time and feeds liquid refrigerant through
solenoid valve 52 which is also open at the same time to heat
source coil 42 and back to the suction line, raising the suction
pressure sufficiently to actuate the switch LPC which, in turn,
energizes the compressor contactor C. Contactor FSC is also
energized at the onset of the heating cycle so that the contactor
CFD is energized to energize or start the motor for heat source fan
44 through contacts C-2 of contactor C and contacts FSC-1 of
contactor FSC which is also energized.
After a short period of time, approximately 2 minutes, time delay
relay TD deenergizes, thereby opening contacts TD-1 and
deenergizing or closing the solenoid valve 30. In the meantime, the
discharge pressure in indoor coil 34 has been raised sufficiently
to condense liquid in this coil and feed it through check valve 50,
open solenoid valve 52, expansion valve 54, outdoor heat source
coil 42 and conduit 56 back to the compressor 10 from where the
refrigerant is discharged through the open solenoid valve 46 and
valve 38 back to the indoor coil 34, completing the cycle.
Changeover switch CH has now switched to the high position since
the refrigerant pressure in the indoor coil 34 has been raised
sufficiently to cause this action. If the ambient temperature at
the outdoor coil drops near 32.degree., thermostat LAT
(schematically illustrated as thermistor 70 in FIG. 1) will make
contact, thereby energizing heating contactor HC which will close
contacts HC-1. Now, current flows from the heating thermostat MHS
through an air-flow switch contact AFS which is maintained closed
by the outdoor coil fan 44 to the contactor 68 for the heating coil
60 which transmits heat to the surface of the fins 62.
When the heating thermostat MHS is satisfied, it breaks contact,
thereby deenergizing the contactors HC and 68 for heating coil 60
as well as the hot gas valve 46 so that the supply of refrigerant
to the indoor coil 34 is interrupted. Since the compressor 10
continues to operate temporarily, both the indoor coil 34 and the
heat source coil 42 will shortly be evacuated, causing the pressure
at the compressor suction intake to drop below the setting of low
pressure switch LPC. This switch now breaks, deenergizing the
compressor contactor C and through its interlock contacts C-2 the
outdoor heat source fan contactor CFD. After the indoor coil 34 and
heat source coil 42 are fully evacuated, the changeover switch CH
will switch to its low position, deenergizing heat source solenoid
valve 52 and suction line valve 38, restoring them to their
positions shown in FIG. 3. If, on the other hand, the changeover
from the heating cycle to the cooling cycle takes place suddenly
and before the changeover pump-down switch has reached its low
position, refrigerant flow temporarily will proceed through coils
34 and 42 as shown in FIG. 5 (since valves 38 and 52 are still
energized) until the indoor coil has been fully evacuated and the
changeover switch has been signaled to switch to its low
position.
Although the invention has been shown in connection with a certain
specific embodiment, it will be readily apparent to those skilled
in the art that various changes in form and arrangement of parts
may be made to suit requirements without departing from the spirit
and scope of the invention.
* * * * *