A METHOD FOR MONITORING A REFRIGERANT CHARGE IN A VAPOUR COMPRESSION SYSTEM

Abstract

A method for monitoring a refrigerant charge in a vapour compression system (1) is disclosed, the vapour compression system (1) including a compressor unit (2), a heat rejecting heat exchanger (3), a high pressure expansion device (4), a receiver (5), at least one expansion device (9, 10), and at least one evaporator (11, 12) arranged in a refrigerant path. A change in net mass flow into or out of the receiver (5) and/or a change in net enthalpy flow into or out of the receiver (5) is detected, and a pressure inside the receiver (5) is monitored as a function of time, following the detected change in net mass flow and/or in net enthalpy flow. A time constant being representative for dynamics of the receiver (5) is derived, based on the monitored pressure as a function of time, and information regarding a refrigerant charge in the vapour compression system (1) is derived, based on the derived time constant.

Claims

1. A method for monitoring a refrigerant charge in a vapour compression system, the vapour compression system comprising a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, a high pressure expansion device, a receiver, at least one expansion device, and at least one evaporator arranged in a refrigerant path, each expansion device supplying refrigerant to one of the evaporator(s), the method comprising the steps of: detecting a change in net mass flow into or out of the receiver and/or detecting a change in net enthalpy flow into or out of the receiver, monitoring a pressure inside the receiver as a function of time, following the detected change in net mass flow and/or in net enthalpy flow, deriving a time constant being representative for dynamics of the receiver, based on the monitored pressure as a function of time, and deriving information regarding a refrigerant charge in the vapour compression system based on the derived time constant.

2. The method according to claim 1, wherein the step of deriving information regarding a refrigerant charge comprises estimating the refrigerant charge in the vapour compression system.

3. The method according to claim 1, further comprising the step of causing a change in net mass flow into or out of the receiver and/or a change in net enthalpy flow into or out of the receiver.

4. The method according to claim 3, wherein the step of causing a change in net mass flow into or out of the receiver and/or a change in net enthalpy flow into or out of the receiver comprises changing a temperature and/or a pressure of refrigerant supplied to and/or leaving the receiver.

5. The method according to claim 3, wherein the step of causing a change in net mass flow into or out of the receiver and/or a change in net enthalpy flow into or out of the receiver comprises increasing or decreasing a flow of gaseous refrigerant leaving the receiver.

6. The method according to claim 1, further comprising the step of repeating the steps of detecting a change in net mass flow into or out of the receiver and/or detecting a change in net enthalpy flow into or out of the receiver, monitoring a pressure inside the receiver and deriving a time constant, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system is performed on the basis of a series of derived time constants.

7. The method according to claim 1, further comprising the step of obtaining a measure for an initial amount of refrigerant in the receiver, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system comprises deriving an absolute estimate for a charge level in the receiver, based on the derived time constant and on the initial amount of refrigerant in the receiver.

8. The method according to claim 2, further comprising the step of causing a change in net mass flow into or out of the receiver and/or a change in net enthalpy flow into or out of the receiver.

9. The method according to claim 2, further comprising the step of repeating the steps of detecting a change in net mass flow into or out of the receiver and/or detecting a change in net enthalpy flow into or out of the receiver, monitoring a pressure inside the receiver and deriving a time constant, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system is performed on the basis of a series of derived time constants.

10. The method according to claim 3, further comprising the step of repeating the steps of detecting a change in net mass flow into or out of the receiver and/or detecting a change in net enthalpy flow into or out of the receiver, monitoring a pressure inside the receiver and deriving a time constant, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system is performed on the basis of a series of derived time constants.

11. The method according to claim 4, further comprising the step of repeating the steps of detecting a change in net mass flow into or out of the receiver and/or detecting a change in net enthalpy flow into or out of the receiver, monitoring a pressure inside the receiver and deriving a time constant, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system is performed on the basis of a series of derived time constants.

12. The method according to claim 5, further comprising the step of repeating the steps of detecting a change in net mass flow into or out of the receiver and/or detecting a change in net enthalpy flow into or out of the receiver, monitoring a pressure inside the receiver and deriving a time constant, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system is performed on the basis of a series of derived time constants.

13. The method according to claim 2, further comprising the step of obtaining a measure for an initial amount of refrigerant in the receiver, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system comprises deriving an absolute estimate for a charge level in the receiver, based on the derived time constant and on the initial amount of refrigerant in the receiver.

14. The method according to claim 3, further comprising the step of obtaining a measure for an initial amount of refrigerant in the receiver, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system comprises deriving an absolute estimate for a charge level in the receiver, based on the derived time constant and on the initial amount of refrigerant in the receiver.

15. The method according to claim 4, further comprising the step of obtaining a measure for an initial amount of refrigerant in the receiver, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system comprises deriving an absolute estimate for a charge level in the receiver, based on the derived time constant and on the initial amount of refrigerant in the receiver.

16. The method according to claim 5, further comprising the step of obtaining a measure for an initial amount of refrigerant in the receiver, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system comprises deriving an absolute estimate for a charge level in the receiver, based on the derived time constant and on the initial amount of refrigerant in the receiver.

17. The method according to claim 6, further comprising the step of obtaining a measure for an initial amount of refrigerant in the receiver, and wherein the step of deriving information regarding a refrigerant charge in the vapour compression system comprises deriving an absolute estimate for a charge level in the receiver, based on the derived time constant and on the initial amount of refrigerant in the receiver.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention will now be described in further detail with reference to the accompanying drawings in which:

[0041] FIG. 1 is a diagrammatic view of a vapour compression system for performing a method according to an embodiment of the invention;

[0042] FIG. 2 illustrates mass flow and enthalpy flow into and out of a receiver of a vapour compression system;

[0043] FIG. 3 illustrates deriving of information regarding a refrigerant charge based on a time constant being representative for dynamics in a receiver, in accordance with a method according to an embodiment of the invention; and

[0044] FIG. 4 is a graph illustrating refrigerant charge as a function of time, as a result of performing a method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0045] FIG. 1 is a diagrammatic view of a vapour compression system 1 for performing a method according to an embodiment of the invention. The vapour compression system 1 comprises a compressor unit 2 comprising a number of compressors, two of which are shown, a heat rejecting heat exchanger 3, a high pressure valve 4, and a receiver 5. A gaseous outlet 6 of the receiver 5 is connected to the compressor unit 2 via a bypass valve 7. A liquid outlet 8 of the receiver 5 is connected a medium temperature expansion device 9 and to a low temperature expansion device 10. The medium temperature expansion device 9 supplies refrigerant to a medium temperature evaporator 11, and the low temperature expansion device 10 supplies refrigerant to a low temperature evaporator 12. The medium temperature evaporator 11 may, e.g., be arranged in thermal contact with a cooled volume in which a medium temperature is required, e.g. a cooling display case in a supermarket, which should normally be maintained at a temperature of approximately 5° C. The low temperature evaporator 12 may be arranged in thermal contact with a cooled volume in which a low temperature is required, e.g. a freezer display case in a supermarket, which should normally be maintained at a temperature of approximately −18° C. Accordingly, the evaporating temperature at the low temperature evaporator 12 is lower than the evaporating temperature at the medium temperature evaporator 11, and therefore the pressure of the refrigerant passing through the low temperature evaporator 12 is also lower than the pressure of the refrigerant passing through the medium temperature evaporator 11.

[0046] The medium temperature evaporator 11 is connected directly to the compressor unit 2. However, the low temperature evaporator 12 is connected to a low temperature compressor unit 13, where the pressure of the refrigerant leaving the low temperature evaporator 12 can be increased before it is mixed with the refrigerant leaving the medium temperature evaporator 11.

[0047] When performing the method according to an embodiment of the invention by means of the vapour compression system 1 of FIG. 1, a change in net mass flow into or out of the receiver 5 and/or a change in net enthalpy flow into or out of the receiver 5 is initially detected. The change in net mass flow and/or in net enthalpy flow may, e.g., be caused actively and deliberately, e.g. by opening or closing the bypass valve 7, by changing an opening degree of the high pressure valve 4, by adjusting a secondary fluid flow across the heat rejecting heat exchanger 3, e.g. by manipulating a fan or a pump driving such a secondary fluid flow, and/or in any other suitable manner which changes a mass flow and/or an enthalpy flow into or out of the receiver 5.

[0048] Following the detected change in net mass flow and/or in net enthalpy flow into or out of the receiver 5, a pressure inside the receiver 5 is monitored as a function of time. Thereby measurement data is obtained, which provides information regarding how the pressure inside the receiver 5 changes over time, in response to the detected change in net mass flow and/or in net enthalpy flow.

[0049] The obtained measurement data is then analysed in order to derive a time constant being representative for dynamics of the receiver 5. Finally, information regarding a refrigerant charge in the vapour compression system 1 is derived, based on the derived time constant. This is possible because unused refrigerant is stored in the receiver 5, and dynamics of the receiver 5 are therefore representative for the refrigerant charge in the vapour compression system 1.

[0050] The time constant being representative for dynamics of the receiver 5 may, e.g., be derived in the following manner.

[0051] A mass balance for the receiver 5 may be identified, assuming that gas and liquid in the receiver 5 are saturated, that the liquid in the receiver 5 is furthermore assumed to be incompressible, and that changes in density of the refrigerant before the high pressure valve 4 and after the bypass valve 7 are negligible. The change in refrigerant mass in the receiver 5 depends on the difference between mass flow into the receiver 5 and mass flow out of the receiver 5, i.e.:

[00001]$\begin{array}{c}\frac{{\mathrm{dM}}_{\mathrm{rec}}}{\mathrm{dt}}={\stackrel{.}{m}}_{\mathrm{HPV}}-{\stackrel{.}{m}}_{\mathrm{BPV}}-{\stackrel{.}{m}}_{\mathrm{MTe}}-{\stackrel{.}{m}}_{\mathrm{LTe}}\\ =f\left({\mathrm{OD}}_{\mathrm{HPV}}\right)\sqrt{2{\rho }_{\mathrm{gc}}\left(P_{\mathrm{gc}}-P_{\mathrm{rec}}\right)}-\\ f\left({\mathrm{OD}}_{\mathrm{BPV}}\right)\sqrt{2{\rho }_{\mathrm{MTc}}\left(P_{\mathrm{rec}}-P_{\mathrm{MT}}\right)}-\\ {\stackrel{.}{m}}_{\mathrm{MTe}}-{\stackrel{.}{m}}_{\mathrm{LTe}},\end{array}$

[0052] where {dot over (m)}.sub.HPV is the mass flow through the high pressure valve 4, {dot over (m)}.sub.BPV is the mass flow through the bypass valve 7, {dot over (m)}.sub.MTe is the mass flow towards the medium temperature evaporator 11, {dot over (m)}.sub.LTe is the mass flow towards the low temperature evaporator 12, p is the density of the refrigerant, P designates pressure of the refrigerant, and ƒ(OD) is a function including valve characteristics of the respective valves 4, 7.

[0053] Furthermore:

[00002]$\frac{{\mathrm{dM}}_{\mathrm{rec}}}{\mathrm{dt}}=\frac{d\left({\rho }_l{V}_l+{\rho }_g{V}_g\right)}{\mathrm{dt}}=\left(V_l\frac{d{\rho }_l}{{\mathrm{dP}}_{\mathrm{rec}}}+\left(V_t-V_l\right)\frac{d{\rho }_g}{{\mathrm{dP}}_{\mathrm{rec}}}\right)\frac{{\mathrm{dP}}_{\mathrm{rec}}}{\mathrm{dt}}+\left({\rho }_l-{\rho }_g\right)\frac{d{V}_l}{\mathrm{dt}},$

where V.sub.l is the volume of liquid refrigerant in the receiver 5, V.sub.g is the volume of gaseous refrigerant in the receiver 5, and V.sub.t is the total volume of refrigerant in the receiver 5, i.e. V.sub.t=V.sub.l+V.sub.g.

[0054] Furthermore, an energy balance for the receiver 5 is calculated as:

[00003]$\frac{{\mathrm{dU}}_{\mathrm{rec}}}{\mathrm{dt}}={\stackrel{.}{m}}_{\mathrm{HPV}}{h}_{\mathrm{HPV}}-{\stackrel{.}{m}}_{\mathrm{BPV}}{h}_{\mathrm{BPV}}-{\stackrel{.}{m}}_{\mathrm{MTe}}{h}_{\mathrm{MTe}}-{\stackrel{.}{m}}_{\mathrm{LTe}}{h}_{\mathrm{LTe}},$

where h designates enthalpy. In other words, the change in internal energy of the receiver 5 is a function of energy entering the receiver 5 via the high pressure valve 4, and energy leaving the receiver 5 via the bypass valve 7 and the evaporators 11, 12, respectively.

[0055] Furthermore:

[00004]$\frac{{\mathrm{dU}}_{\mathrm{rec}}}{\mathrm{dt}}=\frac{d\left(U_l+U_g\right)}{\mathrm{dt}}=\left(V_l{\rho }_l\frac{{\mathrm{dh}}_l}{{\mathrm{dP}}_{\mathrm{rec}}}+\left(V_t-V_l\right){\rho }_g\frac{{\mathrm{dh}}_g}{{\mathrm{dP}}_{\mathrm{rec}}}+V_l{h}_l\frac{d{\rho }_l}{{\mathrm{dP}}_{\mathrm{rec}}}+\left(V_t-V_l\right)h_g\frac{d{\rho }_g}{{\mathrm{dP}}_{\mathrm{rec}}}-V_t\right)\frac{{\mathrm{dP}}_{\mathrm{rec}}}{\mathrm{dt}}+\left({\rho }_l{h}_l-{\rho }_g{h}_g\right)\frac{d{V}_l}{\mathrm{dt}}.$

[0056] From the two equations above,

[00005]$\frac{d{V}_l}{\mathrm{dt}}$

can be isolated and substituted into the equation regarding the mass balance, and the resulting combined mass balance and energy balance can be linearized. A time constant, τ.sub.c, being representative for dynamics of the receiver 5 can then be derived from the linearized mass and energy balance, and the liquid volume of refrigerant in the receiver 5 can be estimated from the derived time constant, τ.sub.c.

[0057] As an alternative, the time constant may be derived based on non-linear approach, e.g. applying higher order models or estimation methods.

[0058] FIG. 2 illustrates mass flow and enthalpy flow into and out of a receiver 5 of a vapour compression system. The vapour compression system could, e.g., be the vapour compression system of FIG. 1. It can be seen that mass flow, {dot over (m)}.sub.HPV, and enthalpy flow, h.sub.HPV coming from a high pressure valve, enter the receiver via an inlet 14. It can further be seen that mass flow, {dot over (m)}.sub.BPV, and enthalpy flow, h.sub.BPV, leave the receiver 5 via a gaseous outlet 6, and flow towards a bypass valve and further on towards a compressor unit. Finally, it can be seen that mass flow, {dot over (m)}.sub.e, and enthalpy flow, h.sub.e, leave the receiver 5 via a liquid outlet 8, and flow towards expansion devices and further on towards evaporators.

[0059] The mass flows and enthalpy flows to and from the receiver 5 result in balances inside the receiver 5 with respect to gaseous mass, M.sub.g, gaseous enthalpy, h.sub.g, liquid mass, M.sub.l, and liquid enthalpy, h.sub.l. The balances may, e.g., be calculated in the manner described above with reference to FIG. 1.

[0060] FIG. 3 illustrates deriving of information regarding a refrigerant charge based on a time constant being representative for dynamics in a receiver, in accordance with a method according to an embodiment of the invention.

[0061] The left graph illustrates dynamics of a receiver as a function of time, following a change in net mass flow and/or enthalpy flow into or out of the receiver. It can be seen that the dynamics are generally changing as a function of time, and this takes place in ‘steps’ at substantially regular time intervals. These time intervals define a time constant, t, which is representative for the dynamics of the receiver.

[0062] A function, f, defines a relationship between the dynamics of the receivers, notably the time constant, τ, and the liquid refrigerant charge, V.sub.l, in the receiver. Thereby, based on the derived time constant, τ, and the function, f, an estimate for the liquid refrigerant charge, V.sub.l, can be derived, as illustrated in the right graph, which shows the estimated charge as a function of time. Since the liquid refrigerant charge in the receiver is closely related to the total refrigerant charge in the vapour compression system, as described above, an estimate for the total refrigerant charge can also be derived.

[0063] FIG. 4 is a graph illustrating refrigerant charge in a receiver as a function of time. The graph of FIG. 4 is based on data obtained during a test, and the graph includes measurement data obtained by means of a liquid level sensor arranged in the receiver, Vl meas in black, and estimated refrigerant charge derived by means of a method according to an embodiment of the invention, Vl hat in white. It can be seen that the estimated refrigerant charge, Vl hat, closely follows the measured liquid level, Vl meas. Accordingly, it can be concluded that the method according to the invention provides an accurate estimate for the refrigerant charge in the receiver.

[0064] While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.