U.S. patent application number 12/401136 was filed with the patent office on 2010-09-16 for defrost system and method for heat pumps.
This patent application is currently assigned to SHAW ENGINEERING ASSOCIATES, LLC. Invention is credited to David N. Shaw.
Application Number | 20100229575 12/401136 |
Document ID | / |
Family ID | 42729027 |
Filed Date | 2010-09-16 |
United States Patent
Application |
20100229575 |
Kind Code |
A1 |
Shaw; David N. |
September 16, 2010 |
DEFROST SYSTEM AND METHOD FOR HEAT PUMPS
Abstract
A system and method for defrosting an air source heat pump are
presented. A defrost conduit extends from the discharge from a
system compressor to the outdoor coil. The defrost conduit includes
a normally closed defrost valve. When the need for defrost exists,
the defrost valve is opened, and warm refrigerant is delivered from
the compressor discharge to the outdoor coil to effect defrost of
the outdoor coil.
Inventors: |
Shaw; David N.; (East
Falmouth, MA) |
Correspondence
Address: |
CANTOR COLBURN, LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
SHAW ENGINEERING ASSOCIATES,
LLC
East Falmouth
MA
HALLOWELL INTERNATIONAL, LLC
Bangor
ME
|
Family ID: |
42729027 |
Appl. No.: |
12/401136 |
Filed: |
March 10, 2009 |
Current U.S.
Class: |
62/81 ; 62/151;
62/259.1; 62/264; 62/275; 62/324.5; 62/324.6 |
Current CPC
Class: |
F25B 2400/01 20130101;
F25D 21/02 20130101; F25B 2313/02741 20130101; F25B 47/022
20130101; F25B 13/00 20130101 |
Class at
Publication: |
62/81 ; 62/151;
62/264; 62/275; 62/259.1; 62/324.5; 62/324.6 |
International
Class: |
F25D 21/06 20060101
F25D021/06; F25D 27/00 20060101 F25D027/00; F25D 23/00 20060101
F25D023/00; F25B 13/00 20060101 F25B013/00 |
Claims
1. A heat pump system including: at least one compressor; an indoor
coil for delivering heat energy to an indoor space to be heated; an
outdoor coil for extracting heat energy from outside air to be used
to heat the indoor space; a conduit system connected between said
one compressor and said indoor and outdoor coils for circulating
refrigerant from said one compressor to said indoor coil and said
outdoor coil and from said outdoor coil back to said one
compressor; a defrost conduit connected between said one compressor
and said outdoor coil; and a defrost flow control valve in said
defrost conduit, said defrost control valve being normally closed,
and said defrost control valve being opened to deliver refrigerant
through said defrost conduit from said one compressor to said
outdoor coil to defrost said outdoor coil.
2. A heat pump system as in claim 1, further comprising: a detector
in said outdoor coil, said detector detecting accumulation of ice
on said outdoor coil and generating a defrost signal calling for
defrost operation; a controller, said controller receiving said
defrost signal from said detector and generating a signal to open
said defrost valve to initiate defrost operation.
3. A heat pump system as in claim 2 wherein said detector
comprises: a light emitting diode; a phototransistor positioned to
receive light from said light emitting diode and generating said
defrost signal to said controller as a function of intensity of
light from said light emitting diode sensed by said
phototransistor.
4. A heat pump system as in claim 1 wherein; said defrost flow
control valve is a combination back pressure and on-off flow
control valve.
5. A heat pump system as in claim 4 wherein; said defrost flow
control valve prevents discharge pressure from said one compressor
from dropping below a predetermined equivalent temperature
level.
6. A heat pump system as in claim 1, further comprising: said
defrost conduit is connected to deliver refrigerant to a bottom of
said outdoor coil during defrost operation to defrost said outdoor
coil from the bottom toward a top thereof.
7. A heat pump system as in claim 1, further comprising: an
accumulator connected in said conduit system between said outdoor
coil and said one compressor; and a heating element in said
accumulator, said heating element being activated during defrost
operation to increase heat energy of said refrigerant delivered to
said outdoor coil to defrost said outdoor coil.
8. A heat pump system comprising: at least one compressor; an
outdoor coil for extracting heat energy from outside air to be used
to heat an indoor space; an indoor coil for delivering heat energy
to an indoor space to be heated; a conduit system for circulating
refrigerant fluid from said one compressor to said indoor coil and
said outdoor coil and back to said one compressor; a 4 way valve in
said conduit system for reversing flow of refrigerant in said
conduit system to change a mode of operation of the heat pump
system from heating operation to cooling operation; a defrost
conduit connected between said one compressor and said outdoor
coil; and a defrost flow control valve in said defrost conduit,
said defrost control valve being normally closed, and said defrost
control valve being opened to deliver refrigerant through said
defrost conduit from said one compressor to said outdoor coil to
defrost said outdoor coil.
9. The heat pump system as in claim 8 wherein; refrigerant is
delivered to said outdoor coil to effect defrost operation without
operation of said 4 way valve.
10. A heat pump system as in claim 8, further comprising: a
detector in said outdoor coil, said detector detecting accumulation
of ice on said outdoor coil and generating a defrost signal calling
for defrost operation; a controller, said controller receiving said
defrost signal from said detector and generating a signal to open
said defrost valve to initiate defrost operation.
11. A heat pump system as in claim 10 wherein said detector
comprises: a light emitting diode; a phototransistor positioned to
receive light from said light emitting diode and generating said
defrost signal to said controller as a function of intensity of
light from said light emitting diode sensed by said
phototransistor.
12. A heat pump system as in claim 8 wherein; said defrost flow
control valve is a combination back pressure and on-off flow
control valve.
13. A heat pump system as in claim 12 wherein; said defrost flow
control valve prevents discharge pressure from said one compressor
from dropping below a predetermined equivalent temperature
level.
14. A heat pump system as in claim 8 wherein: said defrost conduit
is connected to deliver refrigerant to a bottom of said outdoor
coil during defrost operation to defrost said outdoor coil from the
bottom toward a top thereof.
15. A heat pump system as in claim 8, further comprising: an
accumulator connected in said conduit system between said outdoor
coil and said one compressor; and a heating element in said
accumulator, said heating element being activated during defrost
operation to increase heat energy of said refrigerant delivered to
said outdoor coil to defrost said outdoor coil.
16. The method of effecting defrost operation in a heat pump
system, comprising: operating at least one compressor to circulate
a refrigerant to a first coil to deliver heat energy to an interior
space to be heated, and to a second coil to extract heat energy
from outside air; connecting a defrost conduit between said one
compressor and said outdoor coil; positioning a defrost flow
control valve in said defrost conduit, said defrost control valve
being normally closed; and opening said defrost control valve to
deliver refrigerant through said defrost conduit from said one
compressor to said outdoor coil to defrost said outdoor coil.
17. The method of effecting defrost operation as in claim 16,
further comprising: positioning a detector in said outdoor coil to
detect accumulation of ice on said outdoor coil and generate a
signal calling for defrost operation; delivering said defrost
signal to a controller, said controller receiving said defrost
signal from said detector and generating a defrost signal to open
said defrost valve to initiate defrost operation.
18. The method of effecting defrost operation as in claim 17
wherein the positioning a detector in said outside coil comprises:
positioning a light emitting diode in said outdoor coil; and
positioning a phototransistor in said outdoor coil to receive light
from said light emitting diode and generating said defrost signal
to said controller as a function of intensity of light from said
light emitting diode sensed by said phototransistor.
19. The method of effecting defrost operation as in claim 16,
further comprising: connecting said defrost conduit to deliver
refrigerant to a bottom of said outdoor coil during defrost
operation to defrost said outdoor coil from the bottom toward a top
thereof.
20. The method of effecting defrost operation as in claim 16
wherein; said defrost flow control valve is a combination back
pressure and on-off flow control valve to prevent discharge
pressure from said one compressor from dropping below a
predetermined equivalent temperature level.
21. The method of effecting defrost operation as in claim 16,
further comprising: positioning a heating element between said
outdoor coil and said one compressor; and activating said heating
element during defrost operation to increase heat energy delivered
to said outdoor coil to defrost said outdoor coil.
Description
FIELD OF THE INVENTION
[0001] This invention relates to the field of heat pumps. More
particularly, this invention relates to the field of defrost
technology for air source heat pumps.
BACKGROUND OF THE INVENTION
[0002] Air source heat pumps operate by extracting heat energy from
outdoor air and delivering that heat energy to heat an interior
space. In periods of high outdoor relative humidity combined with
an outdoor temperature close to the freezing point of water, frost
and/or ice will develop on the outdoor coil of the heat pump, thus
impeding heat transfer to the coil surface areas. Defrost operation
is required to remove the accumulated frost and/or ice. Defrosting
may be accomplished on a timed schedule (time defrost), and/or by
sensing the need to accomplish defrosting (demand defrost).
[0003] Whether defrosting is accomplished on a time basis or a
demand basis, defrosting is typically accomplished by reverse cycle
defrosting. In reverse cycle defrosting, a four-way valve is
operated so that the heat pump is, in effect, operated in a cooling
mode of operation wherein heat energy is extracted from the
previously heated interior space to heat the refrigerant fluid
circulating in the system, and the heat energy is transported to
the outdoor coil to melt and remove the frost/ice that has
accumulated on the outdoor coil. The interior of the previously
heated space is thus impacted in two ways. One is by removal of
significant amounts of heat energy that has just been delivered to
the interior space. The other is by creating an uncomfortable flow
of cool air felt by occupants of the previously heated space.
[0004] Another problem with reverse cycle defrosting is that the
warm refrigerant is delivered to the top of the outdoor coil (the
vapor outlet side in heating operation) and melts the frost/ice
accumulation from the top of the coil downward. This can result in
refreezing on the lower parts of the coil.
[0005] Reverse cycle defrosting impairs the efficiency of operation
of heat pump systems, especially as the frequency of defrost
operation increases, and it also results in discomfort of the
occupants of the interior space to be heated.
SUMMARY OF THE INVENTION
[0006] The above-discussed problems of prior art heat pumps are
overcome or alleviated by the present invention. In accordance with
the present invention, an electrically operated flow control
defrost valve is positioned downstream of the discharge of the
compressor(s) of the heat exchange system, with the defrost valve
being in a defrost refrigerant flow line connected between the
compressor discharge and the bottom of the outdoor coil. The
defrost valve is normally closed, so that refrigerant does not flow
through the defrost line during normal (i.e. non-defrost operation)
of the system. However, when defrost operation is desired, the
defrost valve is opened, and warm refrigerant vapor is delivered
from the discharge of the compressor(s) to the bottom of the
outdoor coil to heat the outdoor coil and melt accumulated
frost/ice. The defrost valve is closed when defrost operation is
completed, and normal operation of the heat pump system is
resumed.
[0007] An important feature of this invention is that it eliminates
the need for the conventional reverse cycle defrosting heretofore
used in heat pump systems. Accordingly, this invention eliminates
the need to extract energy from the previously heated interior
space, since warm indoor air is no longer used as the source of
energy for defrosting the outside coil, and cold drafts, which
discomfort occupants of the interior space, are also
eliminated.
[0008] In the present invention, the frost/ice accumulation on the
outdoor coil is melted from the bottom of the coil up to the top,
thus eliminating or reducing the problem of possible refreezing of
the coil when melted from the top down as in the prior art reverse
cycle defrosting.
[0009] Instead of extracting energy from the heated interior space
for defrost operation, the energy source for defrost operation in
the present invention is a combination of the electrical input to
the compressor(s) and an accumulator heating element in the heat
pump system.
[0010] The present invention also incorporates a sensor to detect
the build-up of frost/ice on the outdoor coil and initiate defrost
operation when required. The sensor includes a light emitting diode
(LED) and a photo-transistor secured between adjacent fins in the
outdoor coil. A build-up of frost/ice between the LED and the
photo-transistor partially blocks light transmission, and this
blockage of light is used as a defrost initiating signal by the
heat pump system microprocessor to initiate defrost operation for a
predetermined time period, the time of defrost operation being a
function of outdoor air temperature as sensed by the microprocessor
at the initiation of defrost operation.
DESCRIPTION OF THE DRAWINGS
[0011] Referring to the drawings, where like elements are numbered
alike in the several figures:
[0012] FIG. 1 is a schematic view of the system of the present
invention in a heat pump system having heating and cooling
capabilities.
[0013] FIG. 2 is a view showing the frost/ice accumulation detector
of the present invention.
[0014] FIG. 3 is a schematic view of the system of the present
invention in a heat only heat pump system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] Referring to FIG. 1, a heat pump system is shown which is
capable of heating and cooling operation. The heat pump system
preferably is a boosted air source system having a primary
compressor 10 and a booster compressor 12 connected for operation
in series. As shown, primary compressor 10 is operating, i.e., on,
and booster compressor 12 is inoperative, i.e., off. However, it
will be understood that both compressors can be operating, i.e.,
on, if desired. The system also has an outdoor coil 14 that
functions as an evaporator in the heating mode of system operation
to extract heat from the outside air, an indoor coil 16 that
functions as a condenser in the heating mode of operation to
deliver heat to a interior space to be heated, and a 4 way flow
control valve 18 that functions to switch the system between the
heating to the cooling modes of operation depending on the position
of the 4 way valve. The solid line position of 4 way valve 18 is
the heating position; the position shown in the dashed lines is the
cooling position. Outdoor coil 14 can be, but need not be, an
inverted "A" coil.
[0016] The system also has a refrigerant circulation conduit system
including: a conduit segment 20 which receives compressed
refrigerant from the discharge from primary compressor 10 and
delivers the refrigerant through 4 way valve 18 as shown; a conduit
segment 22 which delivers the compressed refrigerant to indoor coil
16 to heat the indoor space by heat transfer exchange with air
(indicated by arrows flowing over the coil) supplied by a fan 16A;
a conduit segment 24A that delivers the refrigerant through a check
valve 24B and around a closed thermal expansion valve (TXV) 25 to a
conduit segment 26; a conduit segment 26 including a thermal
expansion valve 27 through which the refrigerant flows to a
refrigerant distributor 14B and then to the bottom of outdoor coil
14 and through outdoor coil 14 to extract heat from the outdoor air
flowing over coil 14 (indicated by arrows flowing over the coil)
supplied by fan 14A; a conduit segment 28 that delivers the
refrigerant through 4 way valve 18 as shown to conduit segment 29
and then to accumulator 30 where oil entrained in the refrigerant
is separated for return to the compressor sumps (separation and
return of the oil not shown); and the refrigerant is then delivered
to conduit segments 31 and 32 and through check valve 34; and the
refrigerant then flows to conduit segments 36 and 38 which receive
the refrigerant from check valve 34 and deliver the refrigerant to
the inlet to primary compressor 10. The flow of refrigerant through
the conduit system for the heating mode of operation is indicated
by the arrows in the various conduit segments.
[0017] If booster compressor 12 is also operating, check valve 34
is closed by the discharge pressure from booster compressor 12, and
the refrigerant flows from branch conduit 31 to branch conduit 40,
and then to the inlet to booster compressor 12; and the refrigerant
discharged from booster compressor 12 is delivered via branch
conduit 42 and branch conduit 38 to the inlet to primary compressor
10.
[0018] For operation of the system in the cooling mode, 4 way valve
18 is moved to the position shown in by the dashed lines, whereby
the direction of flow of refrigerant in the system is reversed. In
that case, the system operates as in a cooling mode. Heat is
extracted from the indoor space at indoor coil 16, and the heated
refrigerant is delivered through 4 way valve 18 and conduit segment
29 to accumulator 30 and to conduit segments 31, 32, 34, 36 and 38
to the inlet to primary compressor 10 if primary compressor 10 is
on and booster compressor 12 is off (or if both compressors are on,
the refrigerant from indoor coil 16 is delivered via conduit
segment 22 and through 4 way valve 18 to conduit segment 29 and
accumulator 30 and conduit segment 40 to the inlet to booster
compressor 12, and then from the discharge from booster compressor
12 to the inlet to primary compressor 10). The compressed
refrigerant discharged from primary compressor 10 then flows via
conduit segment 20 and through 4 way valve 18 and via conduit
segment 28 to the top of outdoor coil 14 and through coil 14 to
discharge heat at the outdoor coil. The refrigerant then flows
around closed TXV 27 via bypass line 26A and check valve 41 to
conduit segments 26 and 24 to TXV 25 and to indoor coil 16 where
heat is removed from the space to be cooled. The refrigerant is
then delivered by conduit segment 22, 4 way valve 18, conduit
segment 29, accumulator 30 and conduit segments 31, 32, 36 and 38
and check valve 34 to the inlet to primary compressor 10.
[0019] In prior art heat pump systems when operating in the heating
mode, frost and/or ice can accumulate on the outdoor coil 14,
necessitating the need to effect a defrost operation. That defrost
operation typically involves reversing the operation of the system
to the cooling mode, whereby warm refrigerant is delivered to flow
from the top of outdoor coil 14 to the bottom of outdoor coil 14 to
melt to frost and/or ice accumulated on the coil.
[0020] This is sometimes referred to as "reverse cycle defrost"
operation. However, this reverse cycle defrost operation has
several problems well known in the art. Perhaps foremost among
these problems is that heat energy is extracted from the previously
heated space by indoor coil 16 to heat the refrigerant flowing
through the system, and that heat energy is transported to outdoor
coil 14 to defrost the accumulated frost/ice on outdoor coil 14.
The interior of the previously heated indoor space is thus
negatively impacted in two ways. One is by removal of significant
amounts of heat energy that has just been delivered to the interior
space; the other is by creating an uncomfortable flow of cool air
felt by occupants of the previously heated space. Another problem
with the prior art reverse cycle defrosting is that the warm
refrigerant is delivered to the top of the outdoor coil (the vapor
outlet side in heating operation) and melts the frost/ice
accumulation from the top of the coil downward. This can result in
refreezing on the lower parts of the coil. Especially in situations
of high humidity and a temperature at or near freezing for the
outside air, frequent defrosting can be required, and system
efficiency is impaired. These problems of the prior art are
eliminated or substantially reduced in the present invention.
[0021] In accordance with the present invention, a defrost branch
conduit 50 is connected from the discharge from primary compressor
10 to refrigerant distributor 14B at the bottom of outdoor coil 14.
An electrically operated flow control defrost valve 52 in conduit
50 controls the flow of refrigerant in conduit 50 to outdoor coil
14. Valve 52 is a combination back pressure regulator/on-off
control valve. When called upon to open, it will prevent the
compressor discharge pressure from dropping below the equivalent of
about 70 degrees F., thus ensuring that indoor coil 16 remains
sufficiently pressurized to quickly resume heating upon completion
of defrost. Defrost valve 52 is closed during normal heating
operation of the heat pump system, so no refrigerant flows through
conduit 50 to outdoor coil 14 during normal heating operation of
the system. However, when frost and/or ice accumulates on outdoor
coil 14 requiring defrost, a signal is delivered from the system
microprocessor controller 54 to open defrost valve 52, whereby warm
refrigerant vapor discharged from primary compressor 10 is
delivered to distributor 14B and then to the bottom of outdoor coil
14 and flows through outdoor coil 14 to melt the accumulated ice
and/or frost. The outlet from distributor 14B connects to each one
of the circuits in outdoor coil 14 to evenly distribute the flowing
refrigerant to the outdoor coil circuits. Distributor 14B has an
internal orifice at its entry which further expands the refrigerant
exiting TXV 27 during heating operation. Defrost conduit 50 is
connected to distributor 14B downstream (in the direction of
refrigerant flow during heating operation) of the internal orifice
in distributor 14B so that the internal orifice does not restrict
the flow of defrost refrigerant to outdoor coil 14. After passing
through outdoor coil 14, the defrost refrigerant passes through
conduit segment 28 and through 4 way valve 18 to conduit segment 29
and through accumulator 30 and is delivered to the inlet of primary
compressor 10 (or to the inlet to booster compressor 12 if both
compressors are on) for compression and delivery of warm
refrigerant vapor through defrost conduit 50 to outdoor coil 14 to
continue the defrost cycle.
[0022] Most importantly, defrost of outdoor coil 14 is accomplished
without moving 4 way valve 18 to the cooling position, and without
extracting heat energy from the interior space being heated (as
happens in the prior art reverse cycle defrost operation), so the
major problems of the prior art reverse cycle defrost operation are
eliminated. Also, the warm refrigerant vapor delivered to outdoor
coil 14 for defrost is delivered to the bottom of the coil (which
previously received liquid refrigerant in the heating cycle), so
the frost/ice on outdoor coil 14 is melted from the bottom to the
top of the coil, thus reducing or eliminating the prior art problem
of refreezing of the coil during defrost operation.
[0023] Accumulator 30 has a heating element 56 which is activated
by controller 54 to provide additional heat input to the defrost
refrigerant if the warm refrigerant vapor discharged from the
compressor(s) is not sufficient to accomplish defrost operation. In
effect, the electrical input to drive the primary compressor (or
the electrical input to drive both compressors if both are
operating) plus the electrical input to the heating element 56 in
accumulator 30 is the energy source for defrost operation. It
should be noted that signals from controller 54 to fans 14A and 16A
stop the operation of those fans during defrost operation. Some
prior art accumulators have included a heating element used to boil
off liquid refrigerant in the accumulator to return the refrigerant
to circulation in the system. In the present invention, the heating
element in the accumulator performs the function of adding heat
energy to the refrigerant to increase the heat energy content of
the refrigerant vapor for defrost operation.
[0024] Referring now to FIG. 2, a detector system for detecting the
need for demand defrost operation is shown. FIG. 2 shows a partial
detail of the outdoor coil 14 of FIG. 1. Outdoor coil 14 has a
series of refrigerant tubes 60, with heat exchange coil fins 62
mounted on the tubes 60. A defrost sensor 66 is mounted between a
pair of adjacent fins to detect the accumulation of frost/ice on
outdoor coil 14 and trigger operation of the defrost cycle. Defrost
sensor 66 includes a light emitting diode (LED) 68 and a
phototransistor 70 for sensing the light emitted by LED 68. Prior
to the accumulation of frost/ice sufficient to require defrost
operation, enough of the light emitted by LED 68 is sensed by
phototransistor 70, and phototransistor 70 delivers a signal to
microprocessor 54 to keep defrost valve 52 closed However, when an
amount of frost/ice accumulates on outdoor coil 14 sufficient to
require defrost operation, the light received at phototransistor 70
drops below a threshold level, and the signal from the
phototransistor 70 to microprocessor 54 also drops below a
threshold level. When this happens, microprocessor 54 delivers a
signal to defrost valve 52 to open valve 52 and initiates defrost
operation. Microprocessor 54 can be programmed either to terminate
defrost operation after a predetermined period of time, or to
continue defrost operation until the light received at
phototransistor 70 from LED 68 is sufficient to indicate that
defrost operation has been completed, at which time microprocessor
54 closes defrost valve 52 to terminate defrost operation. A
similar detector for ice build-up in aircraft carburetors is the
"Iceman" probe of Lamar Technologies Corporation.
[0025] With the use of the detector system of FIG. 2, defrost
operation is initiated early in the frost/ice accumulation process.
Accordingly, the deposited frost/ice is removed quickly from the
outdoor coil before any significant impeding of heat transfer from
outdoor air into the coil is encountered.
[0026] Operation of the defrost system of the present invention
can, if desired, be modulated by sensing the humidity and
temperature of the outdoor air to adjust the operation of the
defrost system for conditions of high humidity and low
temperature
[0027] Referring now to FIG. 3, an embodiment of the present
invention is shown for a "heating only" system, i.e., a system
which has the capacity to heat, but does not have the capacity to
cool. The components of the system of FIG. 3 are numbered as in
FIG. 1. "Heating only systems are known in the prior art. However,
even though such systems do not need a 4 way valve to switch to the
cooling mode of operation, most prior art "heating only" systems
still require the presence of a 4 way valve to reverse the flow of
refrigerant to accomplish reverse cycle defrosting. However, with
the presence of defrost conduit 50 and defrost valve 52 to deliver
warm refrigerant vapor to defrost outdoor coil 14, a 4 way valve is
not required for the heating only system of FIG. 3.
[0028] While preferred embodiments of the present invention have
been shown and described, various modifications and substitutions
may be made thereto without departing from the spirit and scope of
the invention. Accordingly, it is to be understood that the present
invention has been described and shown by way of illustration and
not limitation.
* * * * *