U.S. patent application number 13/003942 was filed with the patent office on 2011-05-19 for methods and systems for compressor operation.
This patent application is currently assigned to Carrier Corporation. Invention is credited to Frederic Brisset, JeanPhilippe Goux, Fabienne Peyaud, David Veillon.
Application Number | 20110113797 13/003942 |
Document ID | / |
Family ID | 40242583 |
Filed Date | 2011-05-19 |
United States Patent
Application |
20110113797 |
Kind Code |
A1 |
Peyaud; Fabienne ; et
al. |
May 19, 2011 |
METHODS AND SYSTEMS FOR COMPRESSOR OPERATION
Abstract
There is provided a refrigeration system (10) comprising a
compressor (12) having a suction (11) and a discharge (13), a heat
rejecting heat exchanger. (14), an expansion valve (16), and a heat
accepting heat exchanger (18). Preferably the system (10) comprises
any one or more of: a pressure equalisation valve (4O3 42) for
equalising the pressure differential between the compressor suction
(11) and compressor discharge (13); a liquid valve (44), preferably
a liquid solenoid valve or an electronic expansion valve, the
liquid. valve (44) arranged in a flow line (24) between the heat
rejecting heat exchanger (14) and the expansion valve (16); and a
check valve (46), preferably a solenoid valve or an electronic
expansion valve, arranged in a flow line (22) between the heat
rejecting heat exchanger (14) and the compressor (12). The valves
(40, 42, 44, 46) are operated in a variety of manners upon
compressor shutdown and startup to avoid damage to the components
of the compressor (12). Preferably the system further comprises
means for heating at least one component of the compressor (12) and
preferably also control means for activating the heating means when
appropriate, such as when compressor startup is required, and
starting the compressor after heating.
Inventors: |
Peyaud; Fabienne; (Saint
Pierre De Chandieu, FR) ; Goux; JeanPhilippe;
(Toussieu, FR) ; Veillon; David; (Amberieu En
Bugey, FR) ; Brisset; Frederic; (Druillat,
FR) |
Assignee: |
Carrier Corporation
Farmington
CT
|
Family ID: |
40242583 |
Appl. No.: |
13/003942 |
Filed: |
July 23, 2008 |
PCT Filed: |
July 23, 2008 |
PCT NO: |
PCT/IB2008/001908 |
371 Date: |
January 13, 2011 |
Current U.S.
Class: |
62/115 ; 62/498;
62/84 |
Current CPC
Class: |
F25B 41/20 20210101;
F25B 49/02 20130101; F25B 2500/26 20130101; F25B 2600/025 20130101;
F25B 2500/31 20130101; F25B 2400/01 20130101 |
Class at
Publication: |
62/115 ; 62/498;
62/84 |
International
Class: |
F25B 1/00 20060101
F25B001/00; F25B 43/02 20060101 F25B043/02 |
Claims
1. A refrigeration system comprising: a compressor having a suction
and a discharge; a heat rejecting heat exchanger; an expansion
valve; a heat accepting heat exchanger; and a pressure equalisation
valve for equalising the pressure differential between the
compressor suction and compressor discharge.
2. A refrigeration system as recited in claim 1, wherein the
pressure equalization valve comprises a bypass passage connecting
the compressor, suction to the compressor discharge to enable the
compressor to be bypassed and a valve to control flow of
refrigerant therethrough.
3. A refrigeration system as recited in claim 1, further
comprising: a liquid valve, preferably a liquid solenoid valve,
arranged in a flow line between the heat rejecting heat exchanger
and the expansion valve.
4. A refrigeration system as recited in claim 1, further
comprising: control for operating the system in at least one of a
plurality of predetermined sequences; and at least one sensor,
wherein the control operates the system in a particular one of the
plurality of predetermined sequences based on at least one
parameter of the system measured by the sensor.
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. A method of optimizing startup of a compressor of a
refrigeration system comprising the steps of: providing a
refrigeration system comprising a compressor, a heat rejecting heat
exchanger, an expansion valve, a heat accepting heat exchanger, and
a pressure equalisation valve that connects a suction and a
discharge of the compressor for equalising the pressure
differential between the compressor suction and compressor
discharge; preheating at least one component of the compressor;
opening the pressure equalisation valve to thereby reduce the
pressure differential between the compressor suction and discharge;
and starting the compressor, preferably at substantially the same
time as opening the pressure equalisation valve.
14. A method as recited in claim 13, wherein the step of starting
the compressor comprises operating the compressor at a
predetermined frequency f.sub.1 that is less than the operating
frequency f.sub.n of the compressor during normal operating
conditions of the system.
15. A method as recited in claim 14, further comprising the steps
of: providing a liquid valve in a refrigerant flow path between the
heat rejecting heat exchanger and the expansion valve; opening the
liquid valve in response to a first event; closing the pressure
equalization valve in response to a second event; and increasing
the operating frequency of the compressor.
16. (canceled)
17. (canceled)
18. A method as recited in claim 13 further comprising the steps
of: providing at least one system sensor and measuring at least one
parameter of the system with the sensor; and operating the system
in at least one of a plurality of predetermined sequences based on
the at least one parameter measured by the sensor.
19. A method as recited in claim 13, wherein the compressor has a
compressor body, a compressor motor and oil in the compressor and
the step of preheating at least one component of the compressor
comprises the step of: heating at least one of the compressor body,
the oil in the compressor, and the compressor motor when it is
determined that compressor startup is required.
20. (canceled)
21. A method as recited in claim 19, further comprising the steps
of: providing at least one sensor; measuring at least one parameter
of the system with the sensor; and heating the at least one
component of the compressor for a predetermined period of time
based on the at least one parameter.
22. A method as recited in claim 21, wherein the predetermined
period is based on at least one of a temperature of the oil in the
compressor, a compressor shell temperature, a compressor discharge
temperature, an ambient temperature and a length of time for which
the compressor has been inactive.
23. A method as recited in claim 13, further comprising the step
of: measuring the temperature of oil in the compressor; determining
the saturated discharge temperature of refrigerant in the
compressor; and heating at least one component of the compressor
such that the oil is maintained at a temperature above the
saturated discharge temperature.
24. A refrigeration system comprising: a compressor; a heat
rejecting heat exchanger; an expansion valve; a heat accepting heat
exchanger; a liquid valve, preferably a liquid solenoid valve or an
electronic expansion valve, the liquid valve arranged in a flow
line between the heat rejecting heat exchanger and the expansion
valve; and a check valve, preferably a solenoid valve or an
electronic expansion valve, arranged in a flow line between the
heat rejecting heat exchanger and the compressor.
25. A system as recited in claim 24, wherein the check valve is
configured such that a pressure differential between an inlet of
the valve and an outlet of the valve opens the check valve.
26. (canceled)
27. A system as recited in claim 25, wherein the check valve
comprises resilient means, preferably a spring or the like, that
biases the valve into a closed position when the pressure at the
inlet and the outlet of the check valve is balanced.
28. (canceled)
29. A method of controlling a refrigeration system comprising the
steps of: providing a refrigeration system comprising a compressor,
a heat rejecting heat exchanger, an expansion valve, a heat
accepting heat exchanger, a liquid valve between the heat rejecting
heat exchanger and the expansion valve, and a check valve between
the heat rejecting heat exchanger and the compressor; initiating
shutdown of the compressor; closing the check valve and the liquid
valve, preferably substantially simultaneously with shutdown of the
compressor.
30. (canceled)
31. A method as recited in claim 29, wherein the liquid valve and
the check valve comprise solenoid valves and the step of closing
the check valve and the liquid valve comprises activating the
solenoid valve(s).
32. A method as recited in claim 29 31, further comprising the
steps of: starting the compressor thereby causing a pressure
differential between the condenser and the compressor and opening
the check valve provided in the flow line therebetween; and opening
the liquid valve.
33. (canceled)
34. (canceled)
35. A method of controlling a refrigeration system comprising the
steps of: providing a refrigeration system comprising a compressor,
a heat rejecting heat exchanger, an expansion valve, a heat
accepting heat exchanger, and a pressure equalization valve that
connects a suction and a discharge of the compressor for equalizing
the pressure differential between the compressor suction and
compressor discharge; initiating shutdown of the compressor; and
opening the pressure equalization valve for equalizing the pressure
differential between the compressor suction and discharge.
36. A method as recited in claim 35, wherein the step of opening
the pressure equalization valve comprises opening the valve as or
substantially immediately after compressor shutdown is effected.
Description
[0001] The present invention relates to methods and systems for
compressor operation before and during compressor startup and/or
shutdown and in particular to methods and systems for reliable
startup of a compressor, even at low ambient temperatures.
[0002] Conventional refrigeration or airconditioning systems
typically comprise a compressor, a heat rejecting heat exchanger or
condenser, an expansion valve or device and a heat accepting heat
exchanger or evaporator. In operation, refrigerant is circulated
through these components in a closed circuit. The pressure and
temperature of the refrigerant vapour is increased by the
compressor before entering the heat rejecting heat exchanger where
it is cooled. The high pressure, lower temperature liquid is then
expanded to a lower pressure by means of the expansion valve. In
the heat accepting heat exchanger, the refrigerant boils and
absorbs heat from its surroundings. The vapour at the heat
accepting heat exchanger outlet is drawn into the compressor,
completing the cycle.
[0003] However on compressor shutdown and restart the components of
the system, particularly the compressor, can be damaged or fail
particularly in low ambient temperatures.
[0004] It is therefore an object of the present invention to
provide a refrigeration system and a method of operating a
refrigeration system, particularly although not exclusively for a
transport refrigeration unit, the refrigeration system comprising a
compressor and being operable such that failures of the system due
to or on compressor start up are minimised, particularly when the
compressor is started at low ambient temperatures.
[0005] In accordance with the present invention, from a first broad
aspect, there is provided a refrigeration system comprising a
compressor, a heat rejecting heat exchanger, an expansion valve and
a heat accepting heat exchanger. The system further comprises a
pressure equalisation valve (PEV) for equalising the pressure
differential between the compressor suction and compressor
discharge. Preferably the pressure equalisation valve comprises a
bypass passage connecting the compressor suction to the compressor
discharge and bypassing the compressor. Preferably the system
further comprises a liquid valve (LV), preferably a liquid solenoid
valve (LSV), the liquid valve preferably arranged in a flow line
between the heat rejecting heat exchanger and the expansion
valve.
[0006] Therefore there is provided a refrigeration system
comprising a main refrigerant flow path from the compressor to the
heat rejecting heat exchanger, from the heat rejecting heat
exchanger to the liquid valve, from the liquid valve to the
expansion valve and from the expansion valve to the heat accepting
heat exchanger. A bypass refrigerant path or other pressure
equalisation valve is provided across the compressor, i.e. from the
compressor suction to the compressor discharge that, when opened,
bypasses refrigerant flow around the compressor and enables the
pressure differential between the compressor suction and the
compressor discharge to be minimised, or more preferably the
suction and discharge pressures to be balanced, particularly on
compressor shutdown and most preferably at or shortly before
compressor startup.
[0007] Preferably the refrigeration system is operable in at least
one of a plurality of predetermined sequences, and the particular
sequence (or sequences) used is preferably determined based on at
least one parameter of the refrigeration system. Preferably the
parameter (or parameters) comprises a system parameter measured by
at least one sensor.
[0008] In preferred embodiments, when it is determined that
compressor startup is required, a preheating sequence is initiated
in which one or more components of the compressor is heated.
Preferably the compressor body or shell, and/or the oil in the
compressor, and/or the compressor motor, and/or any other suitable
component, is heated. Preferably the component(s) is heated for a
predetermined period of time, which may be determined based on one
or more system parameters. In a preferred embodiment, the period
for which the component is heated is determined based on the
temperature of the oil in the compressor, and/or the compressor
shell temperature, and/or the compressor discharge temperature,
and/or the temperature of the environment in which the compressor
is located (i.e. the ambient temperature), etc. The period for
which the component is heated is additionally or alternatively
determined based on the length of time for which the compressor has
been shut down (for example if the compressor was only recently
shut down, it may only be necessary to heat the component for a
relatively short time, as the component may have retained some of
the heat from its normal operating conditions and temperature).
[0009] Heating of the component(s) of the compressor is carried out
in any suitable manner. In a particularly preferred embodiment, the
stator windings of a motor associated with the compressor, for
example the internal electric alternating current motor
(synchronous or asynchronous) of the compressor are electrically
connected to an electrical source, e.g. a direct current source, to
thereby heat the windings and thus heat the compressor.
[0010] The pressure equalisation valve can be opened prior to
compressor start up, but in preferred embodiments the pressure
equalisation valve is opened when the compressor is started
(preferably at substantially the same time as the compressor is
started). For example in the embodiment where the pressure
equalisation valve is a bypass passage, the passage is opened as
the compressor is started to allow pressure balancing between the
compressor suction and discharge by bypassing the compressor.
Preferably the pressure equalisation valve is opened after the
preheating steps discussed above.
[0011] Preferably the compressor is started slowly, e.g. at a speed
or frequency considerably lower than a standard operating
frequency. Starting the compressor and opening the pressure
equalisation valve allows oil in the compressor to be mixed. This
is advantageous because in a shutdown compressor the temperature of
the oil is not homogeneous in the compressor shell. When the
compressor is started slowly with the pressure equalisation valve
open (and therefore with very low refrigerant flow), the hot oil
from the motor is mixed with the cold oil from the other parts and
the oil is warmed. Furthermore the oil and other parts of the
compressor are heated by the refrigerant that bypasses the
compressor via the pressure equalisation valve, the vapour
refrigerant from the discharge valve in the compressor being hotter
than the actual suction gas refrigerant and when passing through
the bypass and the compressor suction, the vapour heats the
mechanical parts of the compressor and the oil. That is the bypass
line generally emits heat to the compressor and heats the oil that
is circulating in the compressor. Pressure in the compressor body
or shell is limited by the bypass.
[0012] In particularly preferred embodiments the oil temperature of
the compressor is maintained above the saturated discharge
temperature of the refrigerant in the compressor shell. At
temperatures below the saturated discharge temperature the vapour
refrigerant condenses and if the oil temperature is below the
saturated discharge temperature, refrigerant will condense into the
oil. Preferably the compressor shell temperature is also maintained
above the saturated discharge temperature of the refrigerant. If
the oil, and preferably also the mechanical components and the
shell of the compressor, are above the saturated discharge
temperature refrigerant will not condense in the compressor.
[0013] Preferably the speed of the compressor at startup is lower
than the normal running speed of the compressor as previously
mentioned. For example in preferred embodiments the compressor at
startup operates at a frequency of 30 Hz. Low compressor speed is
desirable at startup because a low flow rate through the compressor
minimises refrigerant condensation in the compressor.
[0014] Preferably the liquid valve is closed prior to and during
compressor startup. In particularly preferred embodiments the
liquid valve closes as the compressor stops and remains closed
during compressor shutdown. Closing the liquid valve on compressor
shutdown limits the flow of refrigerant into the compressor
limiting condensation in the compressor oil.
[0015] Preferably at an appropriate time after compressor startup
the liquid valve is opened. Therefore the system is operating in
some states with both the pressure equalisation valve and the
liquid valve open and the compressor operating at low frequency.
This enables increased flow of refrigerant at the compressor
suction, although the refrigerant flow is still lower than during
normal system operation because the pressure equalisation valve is
open (i.e. the compressor remains bypassed at this stage).
Preferably the liquid valve is opened when it is determined that a
system parameter is at a desired level. For example in a preferred
embodiment the system parameter is the oil temperature and when the
oil temperature is determined (for example by measurement with a
temperature sensor) to be sufficiently high (for example above a
predetermined limit, and/or above the saturated discharge
temperature of the refrigerant in the compressor, etc.), then the
liquid valve is opened. Other suitable parameters and limits could
of course be used. For example in a preferred embodiment the
parameter is alternatively or additionally the pressure in the
compressor shell. Furthermore it is envisaged that the liquid valve
could instead or additionally be opened in response to other
events, for example after a predetermined period of time (e.g. from
compressor startup, and/or from opening of the pressure
equalisation valve, or from any other action or event, etc.).
[0016] Preferably after the liquid valve is opened the pressure
equalisation valve is closed. This occurs in response to any one or
more of the following: after a predetermined period of time since
opening the liquid valve; after a period of time has elapsed
following any other suitable event; after a period of time has
elapsed following one or more system parameters being determined to
have reached a particular level; immediately after opening the
liquid valve; etc. In a preferred embodiment the system parameter
comprises either the compressor discharge temperature or the oil
temperature and when the temperature is determined (for example by
measurement with a temperature sensor) to be sufficiently high (for
example above a predetermined limit, and/or above the saturated
discharge temperature in the compressor, etc.), then the pressure
equalisation valve is closed. The pressure of the suction and
discharge of the compressor are therefore no longer balanced and
refrigerant passes through the compressor at a greater flow rate
than when the pressure equalisation valve was open (e.g.
refrigerant no longer bypasses the compressor).
[0017] The compressor speed is preferably then increased, either
immediately or preferably in response to a measured system
parameter reaching a predetermined limit and/or after a period of
time has elapsed, etc. Preferably the compressor speed is slowly
increased, preferably by a predetermined amount and/or at a
predetermined rate, until a maximum or optimum speed is achieved
and/or a predetermined time period has elapsed. Alternatively or
additionally, when a measured system parameter is determined to
have reached a predetermined level, the compressor speed may be set
to the maximum (i.e. standard) operating speed (which is preferably
after a period of slow increase in compressor speed from the
initial startup speed). In a preferred embodiment, when the
compressor discharge temperature and/or oil temperature of the
compressor has reached a predetermined level, the compressor is
controlled to operate at normal operating speeds.
[0018] The above preferred systems and operational steps provide
compressor starting sequences that enable a compressor to be
started with minimal risk of failure which otherwise might occur
due to condensation of refrigerant in the compressor, particularly
at low ambient temperatures, after compressor shutdown. Refrigerant
condensation is detrimental in a compressor because condensed
refrigerant can become mixed with oil in the compressor sump, and
if the oil temperature in the compressor is below the saturated
discharge temperature of the refrigerant then refrigerant can
condense in the oil. When the compressor is started refrigerant in
the oil is pumped by the oil pump and may fail. Furthermore the oil
viscosity is affected by the presence of refrigerant and therefore
may be inappropriate for compressor operation causing damage to
components that should be lubricated. These problems are solved by
the preferred embodiments of the present invention and by the
preferred methods discussed below.
[0019] In a further broad aspect of the present invention, there is
provided a method of optimising compressor startup for a compressor
of a refrigeration system comprising the steps of: preheating at
least one component of a compressor in a refrigeration system;
opening a pressure equalisation valve that connects a suction and a
discharge of the compressor to thereby reduce the pressure
differential therebetween; starting the compressor, preferably with
a predetermined frequency f.sub.1, preferably at substantially the
same time as opening the pressure equalisation valve. Preferably
the method further comprises the steps of opening a liquid valve
that is located in a refrigerant flow path of the system in
response to a first event, closing the pressure equalisation valve
in response to a second event, and increasing the frequency of
operation of the compressor.
[0020] Preferably the starting speed of the compressor f.sub.1 is
less than the compressor standard operating speed f.sub.s.
Preferably f.sub.1 is about 30 Hz. Preferably fs is about 60 Hz,
preferably about 65 Hz or greater. Preferably the method comprises
the further steps of: further increasing the frequency of operation
of the compressor to a standard operating frequency, preferably in
response to a third event.
[0021] Preferably the first event comprises at least one of a first
predetermined period of time elapsing and a measured compressor oil
temperature being determined to be above a predetermined threshold.
Preferably the second event comprises at least one of a second
predetermined period of time elapsing and a measured compressor oil
temperature being determined to be above a predetermined
threshold.
[0022] Preferably the method further comprises the steps of
providing at least one system sensor and measuring at least one
parameter of the system with the sensor, and operating the system
in at least one of a plurality of predetermined sequences based on
the at least one parameter measured by the sensor.
[0023] Preferably the step of preheating at least one component of
the compressor comprises the steps of providing means for heating
at least one component of the compressor, and activating the
heating means to heat the component when it is determined that
compressor startup is required. Preferably the means for heating at
least one component of the compressor comprises means for heating
at least one of the compressor body or shell, the oil in the
compressor, and the compressor motor. In a particularly preferred
embodiment the heating means comprises means for supplying DC
electricity to the stator windings of an internal AC motor of the
compressor. Preferably the method further comprises providing at
least one sensor and measuring at least one parameter of the system
with the sensor, and the heating means is activated for a
predetermined period of time based on the at least one parameter.
Preferably the predetermined period is based on at least one of the
temperature of the oil in the compressor, the compressor shell
temperature, the compressor discharge temperature, the ambient
temperature and the length of time for which the compressor has
been inactive.
[0024] Preferably the method further comprises the steps of
measuring the temperature of oil in the compressor, determining the
saturated discharge temperature of refrigerant in the compressor,
and heating at least one component of the compressor such that the
oil is maintained at a temperature above the saturated discharge
temperature.
[0025] As discussed above, preheating a refrigeration system prior
to compressor start up and controlling a liquid valve and a
pressure equalisation valve before and during start up can
advantageously reduce or eliminate refrigerant condensation
problems, particularly in low ambient temperatures. However even
the above system may experience failure or other problems on
compressor startup for further reasons on or after compressor
shutdown.
[0026] Therefore in accordance with a further broad aspect of the
present invention, there is provided a refrigeration system
comprising a compressor, a heat rejecting heat exchanger, an
expansion valve and a heat accepting heat exchanger. The system
further comprises a liquid valve (LV), preferably a liquid solenoid
valve (LSV) or an electronic expansion valve (EXV), the liquid
valve arranged in a flow line between the heat rejecting heat
exchanger and the expansion valve, and a check valve arranged in a
flow line between the heat rejecting heat exchanger and the
compressor.
[0027] The refrigeration system is advantageous in that when the
compressor is shut down the liquid valve is actuated to close and
the check valve is also closed. Preferably the check valve is
closed by the pressure differential between the high pressure side
of the system and the compressor. In other preferred embodiments
the check valve is a solenoid valve and is actuated to close. In
further embodiments the check valve comprises a combination of a
solenoid valve and a pressure actuated valve. In a standard circuit
without this valve arrangement the pressure differential would
cause refrigerant to migrate to the compressor from the high side,
e.g. from the condenser and also from the evaporator, particularly
during long periods of compressor shutdown, for example 12 hours or
more. Any refrigerant that migrates to the compressor during
shutdown can condense in the compressor or otherwise migrate into
the compressor oil, which can lead to compressor failure on start
up, for example due to the low oil viscosity.
[0028] Therefore the system of the embodiments of this aspect of
the present invention advantageously reduces the amount of, or even
prevents, refrigerant reaching the compressor during shutdown and
therefore little or no refrigerant can mix with the compressor oil.
This is advantageous because the amount of refrigerant in the oil
is minimised and therefore the oil viscosity will remain
sufficiently high, whereas low viscosity oil is dangerous for the
compressor.
[0029] Preferably the check valve is configured such that a
pressure differential between the valve inlet and the valve outlet
is required to open the check valve, preferably a significant
pressure differential. The system of this embodiment is further
advantageous in that the check valve in combination with the liquid
valve will maintain the refrigerant in the condenser (and in any
other component that may be present between the check valve and the
liquid valve, such as in preferred embodiments an accumulator
and/or a dryer, etc.). Preferably the check valve is configured
such that if the pressure at the inlet and the outlet of the check
valve is balanced, which may occur over time after the compressor
is shut down, the valve remains closed. That is in preferred
embodiments the check valve is configured such that refrigerant can
pass through the valve only when the valve inlet pressure is higher
than the valve outlet pressure, for example when the compressor is
started. Preferably the check valve comprises a resilient means
such as a spring or the like that biases the valve into a closed
position when the pressure at the inlet and the outlet of the check
valve is balanced. This is advantageous because prior art valves
leak refrigerant when the pressure at the inlet and the outlet is
balanced, whereas in the embodiment of the present invention having
the inventive check valve, a greater inlet pressure is required to
open the valve and the amount of refrigerant leak under other
conditions is minimised.
[0030] The above preferred refrigeration system prevents or
substantially reduces refrigerant migration from the condenser and
other high side components of the system, and/or from the low side
of the system, to the compressor after compressor shutdown and thus
enables the compressor to be started, even after a long shutdown
period, with minimised risk of failure from refrigerant having
mixed with the compressor oil.
[0031] From a further broad aspect of the present invention, there
is provided a method of controlling a refrigeration system
comprising the step of: initiating shutdown of a compressor of the
system, closing a check valve provided in a flow line between the
compressor and a heat rejecting heat exchanger of the system, and
closing a liquid valve of the system that is located in a flow path
between the heat rejecting heat exchanger and an expansion device
of the system. In preferred embodiments a pressure differential
between the condenser and the compressor that occurs on shutdown is
such that the pressure on the condenser side of the check valve is
higher than the pressure on the compressor side of the check valve
after shutdown and this closes the check valve to prevent flow of
refrigerant therethrough. In other preferred embodiments, the check
valve comprises a solenoid valve and preferably both valves are
closed as soon as possible after compressor shutdown and preferably
at substantially the same time as each other to prevent refrigerant
migration.
[0032] Preferably the method further comprises the steps of:
starting the compressor thereby causing a pressure differential
between the condenser and the compressor and opening the check
valve provided in the flow line therebetween, and opening the
liquid valve. The pressure differential between the condenser and
the compressor is such that the pressure on the condenser side of
the check valve (e.g. the outlet) is lower than the pressure on the
compressor side of the check valve (e.g. the inlet) after the
compressor is restarted. This opens the check valve to enable flow
of refrigerant therethrough. Preferably the method further
comprises the step of biasing the check valve into a closed
position, wherein the biasing force must be overcome in order to
open the valve.
[0033] As discussed above, providing a liquid valve and a check
valve of the present invention enables the system to be operated to
prevent or substantially reduce refrigerant migration from the
condenser and other high side and/or low side components of the
system to the compressor after compressor shutdown. The compressor
can therefore be restarted and the risk of compressor failure due
to the presence of refrigerant in the oil after refrigerant
migration is minimised. However even the above system may
experience failure or other problems on compressor start up due to
a further effect that also arises from compressor shut down.
[0034] Therefore in accordance with a further broad aspect of the
present invention, there is provided a refrigeration system
comprising a compressor, a heat rejecting heat exchanger, an
expansion valve and a heat accepting heat exchanger. The system
further comprises a pressure equalisation valve (PEV) for
equalising the pressure differential between the compressor suction
and compressor discharge. The pressure equalisation valve is
operable to open after or at the same time as the compressor is
shutdown, preferably substantially immediately after compressor
shutdown is effected. Preferably the pressure equalisation valve
comprises a bypass passage connecting the compressor suction to the
compressor discharge and bypassing the compressor.
[0035] By opening the pressure equalisation valve at the same time
as or just after the compressor is shutdown or stopped, the
pressure differential between the compressor suction and discharge
is balanced as quickly as possible. This is advantageous because a
high pressure differential between the compressor suction and
discharge on compressor shutdown causes oil from the compressor to
migrate towards the suction side of the compressor, leaving the
compressor and entering the suction line, whereas in the present
invention, equalisation or balancing of the discharge and suction
pressures prevents such migration. It is undesirable for oil to
leave the compressor as upon restarting the compressor, little or
no oil is available for compressor lubrication. Furthermore upon
restart the compressor has a mixture of oil and refrigerant in the
suction line, which can lead to compressor failure as the mixture
is sucked into the compressor. Still further the oil that leaves
the compressor and enters the suction line can migrate to the
evaporator which can cause further failure and also if the
expansion valve is controlled in relation to the evaporator
temperature (e.g. the evaporator outlet temperature) then the
presence of oil at the sensor can lead to opening of the expansion
valve even though the system is shut down.
[0036] However the embodiments of the present invention provide an
improved refrigeration system in which migration of oil from the
compressor to the low side of the system is minimised or prevented
after compressor shutdown by equalising the pressure differential
between the compressor discharge and suction just after compressor
shutdown. A method of carrying out the invention is also envisaged
and therefore from a further broad aspect of the present invention,
there is provided a method of controlling a refrigeration system
comprising the steps of: initiating shutdown of a compressor of the
system; and opening a pressure equalisation valve (PEV) for
equalising the pressure differential between a compressor suction
and a compressor discharge. The pressure equalisation valve is
opened preferably as or just after the compressor is shutdown,
preferably substantially immediately after compressor shutdown is
effected, thereby substantially equalising the high side discharge
pressure and the low side suction pressure of the compressor.
Preferably the pressure equalisation valve comprises a bypass
passage connecting the compressor suction to the compressor
discharge and bypassing the compressor.
[0037] The above-mentioned and other features of the various
embodiments of the present invention will now be described, by way
of example only and with reference to the accompanying drawings, in
which:
[0038] FIG. 1 shows a schematic representation a refrigeration
system in accordance with an embodiment of the present
invention;
[0039] FIG. 2 shows a flow diagram illustrating the modes of
operation of a refrigeration system in accordance with an
embodiment of the present invention;
[0040] FIG. 3 shows a first, standard operating sequence for
starting a compressor of a refrigeration system in accordance with
an embodiment of the present invention;
[0041] FIG. 4 shows a second, long operating sequence for starting
a compressor of a refrigeration system in accordance with an
embodiment of the present invention;
[0042] FIG. 5 shows a third, short operating sequence for starting
a compressor of a refrigeration system in accordance with an
embodiment of the present invention;
[0043] FIG. 6 shows a schematic representation of another
refrigeration system in accordance with an embodiment of the
present invention;
[0044] FIG. 7A shows a schematic representation of another
refrigeration system in accordance with an embodiment of the
present invention after compressor shutdown;
[0045] FIG. 7B shows a system, that does not have the pressure
equalisation valve of the embodiment of FIG. 7A, in two states, the
first being shortly after compressor shutdown and the second being
some time after compressor shutdown; and
[0046] FIG. 8 shows a schematic representation of a refrigeration
system in accordance with another embodiment of the present
invention.
[0047] The principles of the present invention can be incorporated
within any suitable system. Examples of such suitable systems
include refrigeration and airconditioning systems and particularly,
although not exclusively, transport or truck refrigeration systems.
For ease of reference, the specific embodiments discussed herein
are described with reference to a refrigeration system suitable for
a transport refrigeration unit or the like.
[0048] FIG. 1 schematically shows a refrigeration system 10 having
a refrigerant cycle or circuit 20 such that refrigerant can flow
around the system. The system comprises a compressor 12 connected
from an outlet or discharge 13 thereof via flow path 22 to a heat
rejecting heat exchanger, which in this embodiment is a condenser
14. The condenser 14 is connected via flow path 24 to expansion
device 16, which is connected via flow path 26 to a heat accepting
heat exchanger, which in this embodiment is an evaporator 18. The
evaporator 18 is connected to the compressor 12 at an inlet or
suction 11 thereof via flow path 28. The expansion device 16 is
preferably a thermostatic expansion valve and in this embodiment is
controlled in response to conditions of the system 10 via control
line 36. The system condition which controls opening of the
expansion valve 16 could for example be the temperature of the
evaporator 18, or a related temperature such as a bulb temperature
at the evaporator outlet, etc. In this embodiment, additional
optional components accumulator 32 and dryer 34 are also provided
on flow path 24 between the condenser 14 and the valve 16.
[0049] The system 10 further comprises a pressure equalisation
valve (PEV) across the compressor 12, i.e. connecting the
compressor suction 11 to the discharge 13. The PEV comprises a
bypass passage 40 and means 42, such as a valve, for opening and
closing the passage 40.
[0050] The system 10 further comprises a liquid valve (LV), which
in preferred embodiments is a liquid solenoid valve 44, in the flow
path 24 between the condenser 14 and the expansion valve 16. The LV
44 can be energised to open or close as required, thereby opening
or closing the flow path 24 to enable or disable refrigerant flow
around the circuit 20.
[0051] In operation, high pressure and high temperature refrigerant
vapour exits the compressor 12 and enters the condenser 14 where it
is cooled to a lower temperature, high pressure liquid refrigerant.
This liquid is then expanded to a lower pressure by the expansion
valve 16 and passes to the evaporator 18 where the refrigerant
boils and absorbs heat from its surroundings. The vapour at the
evaporator 18 outlet is drawn into the compressor 12, completing
the cycle.
[0052] When the compressor 12 is shut down, refrigerant may be
present in the compressor 12 and, particularly if the compressor 12
is shut down for prolonged periods, additional refrigerant can
migrate from the condenser 14 to the compressor 12 as discussed in
more detail below.
[0053] The refrigerant in the compressor 12 may condense on the
compressor shell, particularly at low ambient temperatures, and the
condensed refrigerant will mix with the compressor oil which has an
affinity for refrigerant. If the compressor oil temperature is
below the saturated discharge temperature of the refrigerant, the
refrigerant can condense in the oil. The refrigerant dilutes the
oil and, when the compressor 12 is restarted, the diluted oil is
less effective at lubricating the components of the compressor 12,
which may lead to damage. Furthermore the compressor oil pump will
draw in refrigerant which may also lead to damage.
[0054] Therefore in accordance with embodiments of the present
invention, one or more compressor startup sequences are employed to
minimise or eliminate refrigerant condensation in the compressor
12. FIG. 2 is a flow diagram of one embodiment of the present
invention in which a control means or the like determines the state
of a system 10 (for example the system 10 of any of FIG. 1, 6, 7A
or 8) and in particular the length of time T.sub.stop that the
compressor has been shut down and the discharge temperature
T.sub.ref of the compressor 12. If the compressor 12 is shut down
for a reasonably long period of time, the discharge temperature
T.sub.ref is substantially equal to the ambient temperature. In
other embodiments the ambient temperature may be measured. The
control means determines from these parameters what steps before
and during compressor startup should be taken to minimise or
eliminate the problems of refrigerant condensation in the
compressor 12. Of course, other parameters could alternatively or
additionally be used in this determination, such as the temperature
of the oil in the compressor 12, the temperature of the refrigerant
in the compressor 12, and/or the pressure inside the compressor
shell, etc. The parameters used may depend on the sensors that are
present in the system 10 and so what can be measured for making
this determination.
[0055] In the FIG. 2 embodiment, the sequence begins at step 1.1
and the time since the compressor 12 stopped T.sub.stop is
determined in step 1.2. If it is less than 1 hour, the time is
further determined in step 2.1 and still further in step 3.2 if
T.sub.stop is less than 1/2 hour. For shutdown periods less than
1/2 hour, it is determined unnecessary to preheat the compressor 12
and a normal starting sequence (for example as shown in FIG. 3)
begins in step 4.3. For shutdown periods between 1/2 and 1 hour,
the discharge temperature, T.sub.ref is determined in step 3.1 and
if it is low (less than 20.degree. C.), a short (3 minute) preheat
of the compressor 12 is initiated in step 4.2 and as discussed
below, before the normal starting sequence begins in step 5.2.
However if T.sub.ref is sufficiently high already, no preheat is
required and a short starting sequence (for example as shown in
FIG. 5) begins in step 4.1.
[0056] When the compressor 12 has been shut down for longer than 1
hour, the discharge temperature T.sub.ref is determined in step 1.3
and dependent on the temperature, also in steps 1.4, 1.5, 1.6, 1.7
and 1.8. Furthermore, dependent on T.sub.ref, the compressor 12 is
preheated for 12, 9, 6 or 3 minutes (steps 2.2, 2.3, 2.4 and 2.5
respectively) before a long starting sequence (for example as shown
in FIG. 4) begins in step 3.3. However if T.sub.ref is sufficiently
high (between 0 and 20.degree. C.) as determined in step 1.7, a 3
minute preheat is initiated in step 2.6 before the normal starting
sequence begins in step 3.4. If T.sub.ref is already even higher
than 20.degree. C. as determined is steps 1.8 and 1.9, then no
preheat is required and either a normal starting sequence is
initiated in step 2.7 or for very high temperatures (greater than
40.degree. C.) a short starting sequence is initiated in step
2.8.
[0057] The above sequences ensure that if the discharge temperature
of the compressor 12 is low, the compressor 12 is heated,
preferably prior to compressor startup, to raise the compressor
temperature, including the oil temperature. This is advantageous
not only because the viscosity of the oil is improved making the
oil more suitable for lubricating the compressor components on
startup, but also because a sufficiently high oil temperature
(greater than the saturation discharge temperature of the
refrigerant) reduces or eliminates refrigerant condensation in the
compressor 12 that occurs when the compressor shell and oil are
cool.
[0058] FIGS. 3, 4 and 5 schematically illustrate preferred
embodiments of the starting sequences for starting a compressor 12
after shutdown. FIG. 3 shows a "normal" or default starting
sequence, FIG. 4 shows a long starting sequence and FIG. 5 shows a
short starting sequence. In preferred embodiments the starting
sequences disclosed in FIG. 2 correspond with the FIGS. 3, 4 and 5
sequences, but it is also envisaged that this could differ or be
modified by the skilled person. Furthermore the preheat sequences
disclosed in FIGS. 3, 4 and 5 may correspond with the preheat
sequences of FIG. 2, or may differ or be modified.
[0059] FIG. 3 shows a normal starting sequence for a compressor 12.
In preferred embodiments, some or all of the steps of the FIG. 2
sequence are carried out as the first step of the normal starting
sequence. Therefore the discharge temperature of the compressor 12
is preferably at least 20.degree. C. or the compressor shutdown
period was less than 1/2 hour. Referring to a system 10 as
exemplified in FIG. 1, the pressure equalisation valve (PEV) 40, 42
is initially closed and so is the liquid valve (LV) 44. When the
compressor 12 is to be started, the PEV is opened thereby opening a
bypass of the compressor 12. The compressor 12 is started, but with
a relatively low frequency of about 30 Hz (which is significantly
less than the full operating speed of the compressor 12). Heat from
the bypassed refrigerant gas that passes through the PEV is
transferred to the compressor 12 and to the compressor oil when the
compressor 12 is started. Thus the oil in the compressor 12 is
(further) heated and the condensation risk is further minimised,
particularly as the refrigerant bypassing the compressor 12 cannot
condense in the compressor 12. Furthermore the LV is closed to
limit the flow of refrigerant into the compressor 12 thus further
reducing the risk of condensation.
[0060] When the oil temperature in the compressor 12 is
sufficiently high, e.g. when it is measured, determined or
otherwise expected to be greater than the saturated discharge
temperature of the refrigerant, and/or in this embodiment after the
oil has been heated for a sufficient period of time which in the
normal starting sequence is 20 seconds, the liquid valve is opened
and the PEV remains open. The refrigerant flow at the compressor
suction 11 increases slightly, but is still relatively low as the
compressor 12 is still bypassed by the open PEV. When the oil
temperature in the compressor 12 is sufficiently high, e.g. when it
is again measured, determined and/or expected to be greater than
the saturated discharge temperature of the refrigerant, and/or in
this embodiment after it has been heated for a sufficient period of
time which in the normal starting sequence is a further 20 seconds,
the PEV is closed whilst the liquid valve remains open. Refrigerant
therefore flows around the circuit 20 of system 10 under the
influence of the compressor 12, which is no longer bypassed.
[0061] When the oil temperature in the compressor 12 is
sufficiently high, e.g. when it is yet again measured, determined
or otherwise expected to be greater than the saturated discharge
temperature of the refrigerant, and/or in this embodiment after it
has been heated for a sufficient period of time which in the normal
starting sequence is a still further 20 seconds, the speed of the
compressor 12 is gradually increased, preferably by 5 Hz per second
until an optimum or normal operating frequency is reached, after
which standard compressor speed control is applied as is known in
the art. In other embodiments, the standard operating speed control
can be started after it is again determined or otherwise expected
that the oil temperature is still higher than the saturated
discharge temperature.
[0062] However a normal starting sequence may not be appropriate
under certain circumstances, for example when the temperature of
the shutdown compressor 12 is low (e.g. less than about 5.degree.
C.) and/or when the compressor 12 has been shutdown for a long
period (more than about one hour). Instead a long starting sequence
as shown in FIG. 4 may be more appropriate. The long starting
sequence differs from the normal starting sequence in that the
periods between events are generally significantly longer. For
example the LV is kept closed after compressor startup for 5
minutes rather than 20 seconds, thereby allowing the oil additional
time to heat up. The delay before closing the PEV is also longer
and is about 2 minutes thus allowing the system 10 to operate at a
reduced flow rate for longer. The period of time before the
compressor frequency is increased is also longer and is about 21/2
minutes after which the frequency is increased more slowly than the
normal starting sequence, at about 1 Hz per 5 seconds. The long
starting sequence differs at this stage from the normal starting
sequence in that an additional step is included before standard
compressor speed control is initiated, during which the compressor
is operated at a maximum frequency of 60 Hz for 1 minute. The long
starting sequence is significantly slower than the normal starting
sequence thereby allowing the system temperature to increase
gradually before fully loading the compressor 12, which is
appropriate in colder conditions, particularly if the compressor 12
has been inactive for a long period of time. Furthermore it may be
appropriate to heat the oil for a longer period prior to initiating
the long starting sequence, as discussed in relation to FIG. 2.
[0063] However under other circumstances neither a normal nor a
long starting sequence may be appropriate, for example when the
temperature of the shutdown compressor 12 is relatively high (e.g.
more than about 40.degree. C.) and/or when the compressor 12 has
been shutdown for only a brief period (less than an hour). In such
circumstances a short starting sequence as shown in FIG. 5 may be
more appropriate. The short starting sequence differs from the
normal starting sequence in that the periods between events are
generally much shorter and in some embodiments, little or no delay
between events is needed. For example the LV is not kept closed
after compressor startup but instead is opened up quickly as the
oil perhaps does not need any additional time to heat up at the
lower operating speed. The delay before closing the PEV is also
short or may not even be required and the PEV can be closed quickly
after compressor startup. The period of time before the compressor
frequency is increased is also short and is about 5 seconds, after
which the frequency is increased at a slower rate than the normal
starting sequence, at about 1 Hz per second. The short starting
sequence is significantly faster than the normal starting sequence
as the system temperature does not need to increase gradually and
the compressor 12 is capable of operating under full load
relatively quickly. Furthermore it may not even be necessary to
heat the oil prior to initiating the short starting sequence, as
shown in FIG. 2.
[0064] FIG. 6 schematically illustrates an alternative embodiment
of the present invention, although this system 10 could, be and
preferably is combined with the system 10 shown in FIG. 1 simply by
adding the PEV of FIG. 1 (for example as shown in FIG. 8) or indeed
with FIG. 7A. In the FIG. 6 embodiment, the components are mostly
the same as those of the FIG. 1 embodiment and have like reference
numerals. However the FIG. 6 embodiment further comprises a check
valve 46 which is a one-way valve that prevents flow or migration
of fluid in one direction (from the condenser 14 towards the
compressor 12) but permits flow of fluid in the other direction
(towards the condenser 14 from the compressor 12).
[0065] The system 10 of FIG. 6 operates as normal when the
compressor 12 is running. However, in prior art systems when the
compressor is shut down refrigerant migrates from the condenser
and/or evaporator to the compressor due to the pressure and
temperature differences, and the refrigerant can condense in the
compressor and mix with the oil which is undesirable as discussed
above. In the FIG. 6 embodiment however, refrigerant is effectively
trapped between the check valve 46 and the liquid valve (LV) 44 and
therefore does not reach the compressor 12. This embodiment
operates as follows. When the compressor 12 is stopped, the LV is
closed (preferably the LV is a liquid solenoid valve and the valve
is closed by energising the solenoid) and the check valve 46 is
closed (preferably the check valve is also a solenoid valve and the
valve is closed by energising the solenoid, or the check valve may
be closed by the pressure differential between the condenser 14 and
the compressor 12. Refrigerant, that would otherwise migrate to the
compressor 12, is therefore retained in the refrigerant circuit 20
between the two valves 44, 46. Even if the pressure at the inlet
and outlets of the check valve 46 are balanced, the check valve 46
will not open, because the check valve 46 in this embodiment
comprises a spring inside (not shown) so that no leak occurs if the
pressure is balanced. The inlet pressure of the check valve 46 must
be above the outlet pressure to permit circulation of the fluid.
Any additional components such as an accumulator 32 and a dryer 34
in the circuit 20 help to store the refrigerant during compressor
shutdown. When the compressor 12 is restarted, the LV is energised
to be opened and the check valve 46 is energised or opens due to
the changed pressure differential.
[0066] FIG. 7A illustrates another embodiment of the present
invention, which although as shown is a separate embodiment, it is
within the scope of the invention for this system 10 to be combined
with any one or more of the systems 10 shown in the other figures.
The system 10 comprises similar components as the other
embodiments, including the pressure equalisation valve 40, 42
discussed with regard to FIG. 1 and the FIG. 1 embodiment can be
operated in accordance with the following disclosure as well.
[0067] A conventional refrigeration system 110 is shown in FIG. 7B
and is shown in a first state shortly after compressor shutdown and
in a second state a longer time after shutdown. When the compressor
112 is shut down a large pressure differential exists between the
compressor suction 111 and the compressor discharge 113, which
effectively pushes the compressor oil 100 out of the compressor 112
on the suction side of the circuit 120, into line 128 and towards
the evaporator 118. Therefore on compressor startup there is less
oil than should be present, and in some cases little or no oil, in
the compressor 112 and the compressor components are likely to be
damaged.
[0068] Furthermore as shown in the second diagram of FIG. 7B, after
a period of time the oil 100 can migrate and begin to fill the
evaporator 118 and also the relatively hot oil can fill the bulb of
the expansion valve 116 control means 136 that is located at the
exit of the evaporator 118 (i.e. in line 128). This can cause the
expansion valve 116 to open even if that is not desired, further
affecting the system performance on compressor startup and liquid
in the compressor suction line can damage the compressor.
[0069] The refrigeration system 10 of the embodiment of FIG. 7A
overcomes this problem by provision of the PEV, which is opened
during or preferably just after compressor shutdown. This equalises
the pressure differential between the compressor suction 11 and
discharge 13 and thus prevents migration of oil from the compressor
to the low side of the system 10.
[0070] FIG. 8 schematically illustrates another embodiment of the
present invention. In this embodiment, the system 10 comprises a
check valve 46, a liquid valve 44 and a pressure equalisation valve
40, 42. Therefore all of the advantages disclosed in relation to
the other embodiments and discussed above are provided by this
system having the combination of all the valves.
* * * * *