Method for controlling gas pressure in cooling plant

Abstract

A method for monitoring gas pressure in a heat rejecting heat exchanger in a cooling circuit is disclosed. The present capacity of one or more compressors in the cooling circuit compared to a maximum capacity of the one or more compressors is established. 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, a period of time elapsed from a point in time where the compressor capacity reached said level is established. If the established period of time has a duration which is longer than a pre-set period of time, then it is concluding that the cooling medium is in a gas loop operational mode, allowing an operator or a controller to adjust operation of the cooling plant such that the cooling medium is brought out of the gas loop operational mode.

Claims

1. A method for monitoring gas pressure in a heat rejecting heat exchanger in a cooling circuit, said method comprising the steps of: a control unit controlling pressure in the heat rejecting heat exchanger by controlling at least one valve; the control unit establishing a present capacity of one or more compressors in the cooling circuit compared to a maximum capacity of the one or more compressors; in response to the control unit establishing 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, the control unit establishing a period of time elapsed from a point in time where the compressor capacity reached said level; in response to the control unit determining that the established period of time has a duration which is longer than a pre-set period of time, the control unit concluding that the cooling circuit is in a gas loop operational mode; and in response to the control unit concluding that the cooling circuit is in the gas loop operational mode, increasing the pressure of the cooling medium inside the heat rejecting heat exchanger.

2. The method according to claim 1, wherein the pressure of the cooling medium is increased by 5-20 bars.

3. The method according to claim 1, wherein the pressure of the cooling medium is increased by 1%-15%.

4. The method according to claim 1, wherein 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.

5. The method according to claim 1, further comprising the step of decreasing the pressure of the cooling medium inside the heat rejecting heat exchanger, in response to the control unit concluding that the cooling circuit is no longer in the gas loop operational mode.

6. The method according to claim 1, wherein the pre-set percentage of the maximum capacity of the one or more compressors is at least 80% of the maximum capacity.

7. The method according to claim 1, wherein the pre-set period of time of a certain duration is at least one minute.

8. The method according to claim 1, wherein the present capacity of the one or more compressors is established by the control unit commonly for all the compressors of the cooling circuit.

9. The method according to claim 1, wherein the present capacity of the one or more compressors is established by the control unit individually for each compressor of the cooling circuit.

10. 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 claim 1, whether the cooling medium is in a gas loop operational mode.

11. 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 claim 1, whether the cooling medium is in a gas loop operational mode.

12. The method according to claim 2, wherein the pressure of the cooling medium is increased by 1%-15%.

13. The method according to claim 2, wherein 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.

14. The method according to claim 3, wherein 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.

15. The method according to claim 2, further comprising the step of decreasing the pressure of the cooling medium inside the heat rejecting heat exchanger, in response to the control unit concluding that the cooling circuit is no longer in the gas loop operational mode.

16. The method according claim 3, further comprising the step of decreasing the pressure of the cooling medium inside the heat rejecting heat exchanger, in response to the control unit concluding that the cooling circuit is no longer in the gas loop operational mode.

17. The method according to claim 4, further comprising the step of decreasing the pressure of the cooling medium inside the heat rejecting heat exchanger, in response to the control unit concluding that the cooling circuit is no longer in the gas loop operational mode.

18. The method according to claim 2, wherein the pre-set percentage of the maximum capacity of the one or more compressors is at least 80% of the maximum capacity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in further detail with reference to the accompanying drawings in which

(2) FIG. 1 is a log P-h diagram illustrating a gas loop operational mode of a cooling medium, and

(3) 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

(4) 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 (p), 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.

(5) 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 (p) 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.

(6) However, increasing the pressure level in the gas cooler by AP 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 (p) 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.

(7) Although various embodiments of the present invention have been described and shown, the invention is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims.