FIELD OF THE INVENTION
[0001] The invention relates to a method for controlling of gas pressure in a heat rejecting
heat exchanger of a cooling plant. The invention also relates to a control unit operating
according to the method of the invention, and to a plant with such a control unit.
BACKGROUND OF THE INVENTION
[0002] In cooling plants comprising heat rejecting heat exchangers, such as a gas cooler,
gas cooler control may not succeed, when faults in the pressure being measured and/or
faults in the temperature being measured at the outlet of the gas cooler exceeds normally
expected values. Fig. 1 is a log P-h diagram showing how a read-out of a slightly
too high pressure (upper limit of Δp) may result in the controller registering the
pressure and/or the temperature at the outlet of the heat rejecting heat exchanger,
for instance the gas cooler, to be at the optimal curve in point B in Fig. 1, while
the physical situation, or the actual condition, is shown in point A in Fig. 1. Thus,
while it is believed that the cooling plant is operated at near optimal conditions,
it is in fact operated in a very energy inefficient manner. The problem is sometimes
referred to as gas loop operation and may occur, where the isothermal lines are approximately
horizontal in the log P-h diagram, as illustrated in Fig. 1.
[0003] Gas loop operation reduces the cooling capacity to almost zero. It will result in
the controller increasing the capacity of the compressor to 100%, and after a couple
of minutes, the controller will increase the reference of the gas cooler.
SUMMARY OF THE INVENTION
[0004] It is an object of the present invention to provide a method for detecting and for
recovering from gas loop operation. It is also an object to provide a control unit
and a plant with such control unit having limited gas loop operation.
[0005] According to a first aspect the present invention provides a method for monitoring
gas pressure in a heat rejecting heat exchanger in a cooling circuit, said method
comprising the steps of:
- in the heat rejecting heat exchanger, controlling pressure by means of a control unit,
said control unit controlling at least one valve,
- establishing the present capacity of one or more compressors in the cooling circuit
compared to a maximum capacity of the one or more compressors,
- if the present capacity of the one or more compressors is at least at a level corresponding
to a pre-set percentage of the maximum capacity, establishing a period of time elapsed
from a point in time where the compressor capacity reached said level,
- if the established period of time has a duration which is longer than a pre-set period
of time, then concluding that the cooling medium is in a gas loop operational mode.
[0006] In the present context the term 'cooling circuit' should be interpreted to mean a
refrigerant path in which refrigerant is alternatingly compressed and expanded. To
this end a compressor, a heat rejecting heat exchanger, an expansion device and a
heat consuming heat exchanger are arranged in the refrigerant path. The compressor
may be in the form of a single compressor or in the form of a rack of two or more
compressors. The heat rejecting heat exchanger may be in the form of a condenser or
a gas cooler, depending on whether the cooling circuit is operated subcritically or
transcritically. The heat consuming heat exchanger may be an evaporator.
[0007] Refrigerant is compressed in the compressor before being supplied to the heat rejecting
heat exchanger. In the heat rejecting heat exchanger, heat exchange takes place with
a secondary fluid flow across the heat rejecting heat exchanger, in such a manner
that the temperature of the refrigerant flowing through the heat rejecting heat exchanger,
via the refrigerant path, is reduced. In the case that the heat rejecting heat exchanger
is a condenser, gaseous refrigerant which enters the heat rejecting heat exchanger
is at least partly condensed. In the case that the heat rejecting heat exchanger is
a gas cooler, gaseous refrigerant which enters the heat rejecting heat exchanger remains
gaseous, but the temperature of the refrigerant is reduced. In any event, the cooling
circuit is capable of providing heating for a closed volume, via the heat rejecting
heat exchanger.
[0008] The refrigerant is then supplied to the expansion device, where it is expanded before
being supplied to the heat consuming heat exchanger. In the heat consuming heat exchanger,
heat exchange takes place with a secondary fluid flow across the heat consuming heat
exchanger, in such a manner that the temperature of the refrigerant flowing through
the heat consuming heat exchanger, via the refrigerant path, is increased. In the
case that the heat consuming heat exchanger is an evaporator, liquid refrigerant entering
the heat consuming heat exchanger is at least partly evaporated in the heat consuming
heat exchanger. Furthermore, the evaporated refrigerant may be heated further, in
which case superheated refrigerant leaves the heat consuming heat exchanger. In any
event, the cooling circuit is capable of providing cooling for a closed volume, via
the heat consuming heat exchanger. The refrigerant is then returned to the compressor,
and the cycle is repeated.
[0009] According to the method of the first aspect of the invention the present capacity
of one or more compressors in the cooling circuit compared to a maximum capacity is
established. In the present context the term 'capacity' should be interpreted to mean
the work provided by the compressor(s). The capacity may be expressed in terms of
the electrical energy or power consumed by the compressor(s), in terms of the amount
of refrigerant being compressed and displaced by the compressor(s), in terms of the
cooling load on the cooling plant, or in any other suitable manner.
[0010] The present compressor capacity may be measured. Alternatively, information regarding
the present compressor capacity may be inherently present in the compressor controller,
in which case the information is simply provided by the compressor controller.
[0011] The maximum capacity may be the rated capacity of the one or more compressors, the
rated capacity being the maximum work which the compressor(s) is/are designed to deliver.
As an alternative, the maximum capacity may be a capacity limit which is defined by
various operating conditions of the cooling circuit, such as the outdoor temperature,
the indoor temperature, the design of the cooling plant, the installation site, the
desired temperature of the equipment and/or goods to be cooled, etc. For instance,
the cooling plant may be over dimensioned in the sense that the compressor(s) is/are
potentially capable of providing a significantly higher cooling capacity than will
ever be necessary in order to meet the cooling load on the cooling plant. In this
case the maximum capacity may be a specified percentage of the rated capacity, such
as 75% of the rated capacity, 80% of the rated capacity, 90% of the rated capacity,
or the like. The maximum capacity may advantageously be a compressor capacity which
is required when the cooling load on the cooling plant is at a maximum level.
[0012] If it is established that the present capacity of the one or more compressors is
at least at a level corresponding to a pre-set percentage of the maximum capacity,
it is investigated when this level was reached, and for how long the compressor capacity
has been at or above this level. In other words, it is investigated whether the one
or more compressors is/are operating at a relatively high capacity, at or close to
the maximum capacity as defined above, and has/have been doing so for a while. If
the compressor(s) is/are operating at a high capacity level during a continuous time
period, it is an indication that the cooling plant is operating inefficiently, and
that the cooling medium may be in a gas loop operational mode.
[0013] Accordingly, if the established period of time has a duration which is longer than
a pre-set period of time, it is concluded that the cooling medium is in a gas loop
operational mode.
[0014] Thus, the method according to the first aspect of the invention allows a gas loop
condition to be identified in an easy manner. This, in turn, allows the operation
of the cooling plant to be adjusted in such a manner that the cooling medium is brought
out of the gas loop operational mode. Furthermore, since the compressor capacity has
to be at a high level for at least a pre-set period of time, it is ensured that short
spikes of high capacity are not reacted upon. This is an advantage, because such short
spikes may be the result of fluctuations, rather than indicating a gas loop operational
mode.
[0015] The method may comprise the further step of increasing the pressure of the cooling
medium inside the heat rejecting heat exchanger, if it is concluded that the cooling
medium is in a gas loop operational mode.
[0016] As shown in Fig. 1, when the pressure in the heat rejecting heat exchanger (gas cooler)
increases, the pressure and the enthalpy of the cooling medium are displaced into
a region, where the isothermal curves have a more pronounced slope. Thereby the operation
of the cooling plant is pulled away from gas loop operational mode.
[0017] The pressure of the cooling medium may be increased by 5-20 bars, such as by 7-15
bars, such as by 8-10 bars, such as approximately 10 bars, or approximately 5 bars,
or approximately 20 bars.
[0018] Alternatively, the pressure of the cooling medium may be increased by 1 %-15%, such
as by 2%-12%, such as by 5%-10%, such as by approximately 7%, or approximately 5%,
or approximately 10%.
[0019] The step of increasing the pressure may result in a pressure increase which causes
the present capacity of the one or more compressors to decrease to below 95% of the
maximum capacity, possibly to below 90% of the maximum capacity, even possibly to
below 80% of the maximum capacity. According to this embodiment, the pressure increase
moves the cooling medium out of the gas loop operation mode, and thereby operation
of the compressor(s) is moved away from the very high capacity level.
[0020] The method may further comprise the step of decreasing the pressure of the cooling
medium inside the heat rejecting heat exchanger, if it can be concluded that the cooling
medium is no longer in a gas loop operational mode. According to this embodiment,
the increased pressure of the cooling medium inside the heat rejecting heat exchanger
is only maintained as long as required in order to move the cooling medium out of
the gas loop operational mode. Once this has been obtained, the pressure is once again
reduced, and the cooling plant is returned to normal operation.
[0021] The duration within which the pressure of the cooling medium is increased may differ
depending on which percentage of the maximum capacity is decided as being relatively
high or full capacity of the one or more compressors. For instance, if the percentage
of the maximum capacity is decided as being 90%, the increase in pressure will be
terminated, when the present capacity decreases to below 90% of the maximum capacity.
[0022] The pre-set percentage of the maximum capacity of the one or more compressors may
be at least 80% of the maximum capacity, possibly at least 90% of the maximum capacity,
even possibly between 95% and 100% of the maximum capacity of the one or more compressors,
even more possible 100% of the maximum capacity. In any event, the present capacity
of the one or more compressors should be very close to the maximum capacity for a
significant period of time before it can be concluded that the cooling medium is in
a gas loop operational mode.
[0023] The pre-set period of time of a certain duration may be at least one minute, preferably
at least two minutes, possibly at least three minutes, even possible at least four
minutes, and even more possible at least five minutes, possibly at the most 15 minutes.
[0024] The duration within which the one or more compressors of the cooling plant operate
at a capacity decided as being maximum capacity, or close to maximum capacity, may
differ depending on the installation site, the outdoor and indoor temperature, the
desired temperature of the equipment and/or goods to be cooled, etc. Therefore, under
some conditions, a shorter period of time is enough for concluding that the cooling
medium is in a gas loop operational mode, and under other conditions a longer period
of time is needed for concluding that the cooling medium is in a gas loop operational
mode.
[0025] The present capacity of the one or more compressors may be established commonly for
all the compressors of the cooling circuit. According to this embodiment, the combined
capacity of all of the compressors is established in one go, and compared to the maximum
combined capacity. Thus, the sum of capacity of the entire cooling plant is established
for obtaining the capacity of all compressors, if the cooling plant comprises more
than one compressor.
[0026] As an alternative, the present capacity of the one or more compressors may be established
individually for each compressor of the cooling circuit. According to this embodiment,
the capacity of each of the compressors is established and compared to the maximum
capacity for that compressor.
[0027] According to a second aspect the present invention provides a control unit for monitoring
pressure in a heat rejecting heat exchanger of a cooling circuit comprising one or
more compressors,
- said control unit comprising a pressure measuring unit and a capacity establishing
unit for measuring the pressure of the cooling medium inside the heat rejecting heat
exchanger and for establishing the capacity of the one or more compressors, respectively,
and
- said control unit also comprising a timer for measuring a period of time having elapsed
from a point of time, said point of time being when the present capacity of the compressors
reaches a pre-set percentage of a maximum capacity, said timer communicating with
said capacity establishing unit for establishing, by means of the method according
to the first aspect of the invention, whether the cooling medium is in a gas loop
operational mode.
[0028] The control unit according to the second aspect of the invention is adapted to establish
whether or not a cooling medium of a cooling circuit is in a gas loop operational
mode, by means of the method according to the first aspect of the invention. Therefore
the remarks set forth above are equally applicable here.
[0029] According to a third aspect the present invention provides a plant with a cooling
circuit comprising one or more compressors, said plant also comprising at least one
heat rejecting heat exchanger and a controller for controlling pressure in the heat
rejecting heat exchanger, and
- said plant also comprising at least one valve for adjusting the pressure in the heat
rejecting heat exchanger, and the plant also comprising a pressure measuring unit
and a capacity establishing unit for measuring the pressure of the cooling medium
inside the heat rejecting heat exchanger and establishing the capacity of one or more
compressors, respectively, and
- said plant also comprising a timer for measuring a period of time having elapsed from
a point of time, said point of time being when the present capacity of the compressors
reaches a pre-set percentage of a maximum capacity, said timer communicating with
said capacity establishing unit for establishing, by means of the method according
to the first aspect of the invention, whether the cooling medium is in a gas loop
operational mode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The invention will now be described in further detail with reference to the accompanying
drawings in which
Fig. 1 is a log P-h diagram illustrating a gas loop operational mode of a cooling
medium, and
Fig. 2 is a log P-h diagram illustrating the effect of increasing the pressure of
the cooling medium in a gas cooler.
DETAILED DESCRIPTION OF THE DRAWINGS
[0031] Fig. 1 is a log P-h diagram illustrating a gas loop operational mode of a cooling
medium in a cooling plant. The cooling plant is operated transcritically, i.e. no
phase transition takes place during heat exchange in the heat rejecting heat exchanger.
Fig. 1 illustrates that when the cooling medium is operated in a region where the
isothermal curve is relatively flat, small variations in pressure results in large
variations in enthalpy. Therefore a measurement of the pressure of the cooling medium
leaving the gas cooler may lead the controller to believe that the cooling medium
is at the optimal operating point B. However, due to a small error in the measurement
(Δp), the cooling medium may in fact be at the very inefficient operating point A.
As a consequence, the cooling plant is not operated in an optimal manner. Since this
is not registered by the controller, the operation of the cooling plant continues
to be inefficient. This situation is sometimes referred to as gas loop operation.
[0032] Fig. 2 is a log P-h diagram, similar to the diagram of Fig. 1. Fig. 2 also illustrates
the gas loop operational mode described above with reference to Fig. 1. In Fig. 2
a small error (Δp) in the measurement of the pressure of the cooling medium leaving
the gas cooler may lead the controller to believe that the cooling medium is at the
optimal operating point A, while it is in fact at the very inefficient operating point
B.
[0033] However, increasing the pressure level in the gas cooler by ΔP changes the situation
dramatically, because the operating points are thereby moved to a region of the isothermal
curve which is much steeper. Thus, it is clear from Fig. 2 that a small error (Δp)
in the measurement of the pressure of the cooling medium leaving the gas cooler leads
to only a small difference in enthalpy. In other words, operating the cooling plant
at operating point A' or at operating point B' has no significant impact on the efficiency
of the cooling plant. Thus, it can be seen that increasing the pressure of the cooling
medium in the gas cooler brings the cooling medium out of the gas loop operational
mode.
1. A method for monitoring gas pressure in a heat rejecting heat exchanger in a cooling
circuit, said method comprising the steps of:
- in the heat rejecting heat exchanger, controlling pressure by means of a control
unit, said control unit controlling at least one valve,
- establishing the present capacity of one or more compressors in the cooling circuit
compared to a maximum capacity of the one or more compressors,
- if the present capacity of the one or more compressors is at least at a level corresponding
to a pre-set percentage of the maximum capacity, establishing a period of time elapsed
from a point in time where the compressor capacity reached said level,
- if the established period of time has a duration which is longer than a pre-set
period of time, then concluding that the cooling medium is in a gas loop operational
mode.
2. A method according to claim 1, said method comprising the further step of increasing
the pressure of the cooling medium inside the heat rejecting heat exchanger, if it
is concluded that the cooling medium is in a gas loop operational mode.
3. A method according to claim 2, wherein the pressure of the cooling medium is increased
by 5-20 bars.
4. A method according to claim 2 or 3, wherein the pressure of the cooling medium is
increased by 1%-15%.
5. A method according to any of claims 2-4, where the step of increasing the pressure
results in a pressure increase which causes the present capacity of the one or more
compressors to decrease to below 95% of the maximum capacity, possibly to below 90%
of the maximum capacity, even possibly to below 80% of the maximum capacity.
6. A method according to any of claims 2-5, further comprising the step of decreasing
the pressure of the cooling medium inside the heat rejecting heat exchanger, if it
can be concluded that the cooling medium is no longer in a gas loop operational mode.
7. A method according to any of the preceding claims, where the pre-set percentage of
the maximum capacity of the one or more compressors is at least 80% of the maximum
capacity, possibly at least 90% of the maximum capacity, even possibly between 95%
and 100% of the maximum capacity of the one or more compressors, even more possible
100% of the maximum capacity.
8. A method according to any of the preceding claims, where the pre-set period of time
of a certain duration is at least one minute, preferably at least two minutes, possibly
at least three minutes, even possible at least four minutes, and even more possible
at least five minutes, possibly at the most 15 minutes.
9. A method according to any of the preceding claims, where the present capacity of the
one or more compressors is established commonly for all the compressors of the cooling
circuit.
10. A method according to any of claims 1-8, where the present capacity of the one or
more compressors is established individually for each compressor of the cooling circuit.
11. Control unit for monitoring pressure in a heat rejecting heat exchanger of a cooling
circuit comprising one or more compressors,
- said control unit comprising a pressure measuring unit and a capacity establishing
unit for measuring the pressure of the cooling medium inside the heat rejecting heat
exchanger and for establishing the capacity of the one or more compressors, respectively,
and
- said control unit also comprising a timer for measuring a period of time having
elapsed from a point of time, said point of time being when the present capacity of
the compressors reaches a pre-set percentage of a maximum capacity, said timer communicating
with said capacity establishing unit for establishing, by means of the method according
to any of claims 1-10, whether the cooling medium is in a gas loop operational mode.
12. Plant with a cooling circuit comprising one or more compressors, said plant also comprising
at least one heat rejecting heat exchanger and a controller for controlling pressure
in the heat rejecting heat exchanger, and
- said plant also comprising at least one valve for adjusting the pressure in the
heat rejecting heat exchanger, and the plant also comprising a pressure measuring
unit and a capacity establishing unit for measuring the pressure of the cooling medium
inside the heat rejecting heat exchanger and establishing the capacity of one or more
compressors, respectively, and
- said plant also comprising a timer for measuring a period of time having elapsed
from a point of time, said point of time being when the present capacity of the compressors
reaches a pre-set percentage of a maximum capacity, said timer communicating with
said capacity establishing unit for establishing, by means of the method according
to any of claims 1-10, whether the cooling medium is in a gas loop operational mode.