COOLING SYSTEM FOR EFFICIENT OPERATION

Abstract

The invention relates to a cooling system and operating method therefor with a direct expansion cooling circuit for an ammonia refrigerant. A compressor 12 is provided to compress ammonia vapor 11. A condenser is provided to condense the ammonia vapor to obtain liquid ammonia 20. An evaporator 32 is provided to evaporate the liquid ammonia. A superheat vapor quality sensor 40 is arranged at a conduit 34 between at least a portion of the evaporator 32 and the compressor 12. The superheat vapor quality sensor 40 comprises a heating element 48 and a temperature sensing element 52. The superheat vapor quality sensor 40 is disposed to deliver a sensor signal S indicative of a superheat vapor quality X of refrigerant flowing through the conduit 34 from an output of the temperature sensing element 52. The superheat vapor quality sensor 40 is arranged on a wall of a horizontally arranged portion of the conduit 34 in a position forming an angle of more than 120° to a vertical upward direction.

Claims

1. A cooling system, comprising a direct expansion cooling circuit for an ammonia refrigerant, including at least a compressor to compress ammonia vapor, a condenser to condense said ammonia vapor to obtain liquid ammonia, and an evaporator to evaporate said liquid ammonia, wherein a superheat vapor quality sensor is arranged at a conduit between at least a portion of said evaporator and said compressor, said superheat vapor quality sensor being arranged in thermal contact with a wall of said conduit, said superheat vapor quality sensor comprising a heating element and a temperature sensing element, said superheat vapor quality sensor being disposed to deliver a sensor signal indicative of a superheat vapor quality of said refrigerant flowing through said conduit from an output of said temperature sensing element wherein said superheat vapor quality sensor is arranged on a wall of a horizontally arranged portion of said conduit in a position forming an angle of more than 120° to a vertical upward direction.

2. The cooling system according to claim 1, further comprising a controllable evaporator inlet valve connected to an inlet of said evaporator, and controller means disposed to control said evaporator inlet valve depending on said sensor signal.

3. The cooling system according to claim 2, wherein said controller means are configured to reduce an opening of said evaporator inlet valve in response to a sensor signal indicative of a lower superheat vapor quality value, and to increase an opening of said evaporator inlet valve in response to a sensor signal indicative of a higher superheat vapor quality value.

4. The system according to claim 1, wherein said evaporator comprises a plurality of pipes having a first portion exposed to an air flow and a second portion located outside of said air flow, said superheat vapor quality sensor being arranged on said second portion of one of said pipes.

5. The system according to claim 4 wherein one pipe of said pipes of said evaporator has the lowest thermal load, and said superheat vapor quality sensor is arranged on said one pipe.

6. The system according to claim 1, wherein for providing a reference temperature, a reference temperature sensor is arranged to measure a temperature of said liquid refrigerant and/or a pressure sensor is arranged to measure a pressure at an outlet of said evaporator.

7. The system according to claim 1, wherein said sensor signal is dependent on a difference between a reference temperature and a temperature measured by said temperature sensing element.

8. The system according to claim 1, wherein said superheat vapor quality sensor comprises a sensor body made of a metal material, wherein said heating element and/or said temperature sensing element are arranged embedded within said sensor body in thermal contact therewith.

9. The system according to claim 1, wherein said superheat vapor quality sensor comprises a concave portion, said conduit being partially received within said concave portion.

10. The system according to claim 1, wherein an insulating element is provided to thermally insulate said superheat vapor quality sensor and at least a portion of said conduit.

11. The system according to claim 1, wherein an accumulator is provided between said evaporator and said compressor to accumulate a liquid portion of said ammonia refrigerant, wherein said accumulator is arranged in thermal contact with a conduit arranged between said condenser and said evaporator.

12. A method of operating a cooling system, comprising operating a direct expansion cooling circuit with an ammonia refrigerant, including the repetitive steps of compressing an ammonia vapor, condensing said ammonia vapor to obtain liquid ammonia, and evaporating said liquid ammonia, said method further comprising obtaining, from a superheat vapor quality sensor, a sensor signal indicative of a superheat vapor quality of said evaporated ammonia flowing within a conduit, wherein said superheat vapor quality sensor is arranged on a wall of a horizontally arranged portion of said conduit in a position forming an angle of more than 120° to a vertical upward direction, wherein said superheat vapor quality sensor is operated by operating a heating element and sensing a temperature to deliver said sensor signal.

13. The system according to claim 2, wherein said evaporator comprises a plurality of pipes having a first portion exposed to an air flow and a second portion located outside of said air flow, said superheat vapor quality sensor being arranged on said second portion of one of said pipes.

14. The system according to claim 3, wherein said evaporator comprises a plurality of pipes having a first portion exposed to an air flow and a second portion located outside of said air flow, said superheat vapor quality sensor being arranged on said second portion of one of said pipes.

15. The system according to claim 2, wherein for providing a reference temperature, a reference temperature sensor is arranged to measure a temperature of said liquid refrigerant and/or a pressure sensor is arranged to measure a pressure at an outlet of said evaporator.

16. The system according to claim 3, wherein for providing a reference temperature, a reference temperature sensor is arranged to measure a temperature of said liquid refrigerant and/or a pressure sensor is arranged to measure a pressure at an outlet of said evaporator.

17. The system according to claim 4, wherein for providing a reference temperature, a reference temperature sensor is arranged to measure a temperature of said liquid refrigerant and/or a pressure sensor is arranged to measure a pressure at an outlet of said evaporator.

18. The system according to claim 5, wherein for providing a reference temperature, a reference temperature sensor is arranged to measure a temperature of said liquid refrigerant and/or a pressure sensor is arranged to measure a pressure at an outlet of said evaporator.

19. The system according to claim 2, wherein said sensor signal is dependent on a difference between a reference temperature and a temperature measured by said temperature sensing element.

20. The system according to claim 3, wherein said sensor signal is dependent on a difference between a reference temperature and a temperature measured by said temperature sensing element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Embodiments of the invention will be described with reference to the drawings, in which

[0054] FIG. 1 shows a schematic representation of an embodiment of a cooling system;

[0055] FIG. 2a shows a schematic representation of a longitudinal sectional view of the flow of a medium through a vertical portion of a conduit of the cooling system of FIG. 1;

[0056] FIG. 2b, 2c show in schematic representation a longitudinal section and a cross-section of a flow of a medium through a horizontal portion of a conduit;

[0057] FIG. 3a, 3b, 3c show schematic representations of different types of evaporators;

[0058] FIG. 4 shows a perspective view of a sensor arrangement in the cooling system of FIG. 1;

[0059] FIG. 5 shows a longitudinal sectional view of the sensor arrangement of FIG. 4;

[0060] FIG. 6 shows a cross-sectional view of the sensor arrangement of FIG. 4,5 with the section along A . . . A in FIG. 4;

[0061] FIG. 7 shows a cross-sectional view of a part of the sensor arrangement of FIG. 4-6;

[0062] FIG. 8 shows a diagram of temperature curves for the sensor of FIG. 4-6;

[0063] FIG. 9 shows a diagram of a dependency of a sensor signal on a vapor quality value for the sensor of FIG. 4-6;

[0064] FIG. 10 shows a diagram of the dependency of a sensor signal and a superheat value on a circulating rate;

[0065] FIG. 11 shows alternative embodiments of sensor arrangements.

DETAILED DESCRIPTION

[0066] FIG. 1 shows an embodiment of a cooling system 10.

[0067] The cooling system 10 is a direct expansion (DX) cooling system operated with ammonia as refrigerant.

[0068] The cooling circuit of the cooling system 10 comprises a compressor 12 to compress ammonia vapor 11 contained in the upper portion of a suction accumulator 14 filled with gaseous ammonia 11 and a rest of liquid ammonia 20 accumulated at the bottom. Compressed ammonia vapor 13 obtained from the compressor 12 is supplied through a conduit 16 to a condenser 18, where it condenses at least partly to collect as liquid ammonia 20 in a collector 22.

[0069] The hot liquid ammonia 20 is supplied through a first conduit 24 and a second conduit 28 to evaporators 32. The first conduit 24 comprises a heating spiral 26 in the suction accumulator 14 for the heated liquid refrigerant to aid in evaporating liquid ammonia 20 there.

[0070] In the example shown, the cooling system 10 comprises two identical evaporators 32 connected in parallel. The skilled person will recognize that different embodiments of the cooling system 10 may comprise a different number of evaporators 32, such as only one or more than two evaporators. In the following, only one of the evaporators 32 connected in parallel will be described.

[0071] The liquid ammonia 20 is supplied through a controllable evaporator inlet valve 36 to the evaporator 32. The evaporator 32 comprises a plurality of evaporator tubes 34 in thermal contact with an air flow 33 of a ventilator 35.

[0072] The evaporator 32 is preferably a DX evaporator type with one common liquid inlet 37 and one common outlet 39. The evaporator 32 has at least one pass, i.e. one evaporator tube 34 passing from inlet 37 to outlet 39. Preferably, the evaporator 32 has multiple parallel passes, e.g. 6-8 parallel evaporator tubes 34 connected between the inlet 37 and outlet 39. Different evaporator types can be used e.g. as shown in FIG. 3a-3c.

[0073] FIG. 3a shows a bottom feed evaporator where the refrigerant is supplied through a lower feed line 37, distributed to flow along the evaporator tubes 34 in thermal contact with the air flow 33, collected and returned in an upper return line 39.

[0074] FIG. 3b shows a top feed evaporator in which the refrigerant is supplied through an upper feed line 37, distributed into the evaporator tubes 34, collected and returned in a lower return line 39.

[0075] FIG. 3c shows a side/bottom feed evaporator. The refrigerant is supplied through a lower feed line 37 at the evaporator front, distributed into the evaporator tubes 34, collected and returned in an upper return line 39.

[0076] In each case the air flow 33 is directed traverse to the refrigerant flow through the evaporator tubes 34. Through the thermal contact between the air flow 33 and the evaporator tubes 34, heat from the air flow 33 is transferred to the refrigerant flowing within the evaporator tubes 34, such that the refrigerant is evaporated.

[0077] The degree to which the ammonia medium flowing through the evaporator tubes 34 is evaporated may be expressed in terms of the vapor quality value X. The liquid ammonia supplied through the feed line 37 will have a low vapor quality value X of e.g. 10-20%. As the ammonia refrigerant flowing through the evaporator tubes 34 receives heat transferred from the air flow 33, more and more will evaporate such that the vapor quality will rise.

[0078] As the system 10 is direct expansion (DX) cooling system, the refrigerant will be fully evaporated, i.e. the mass flow of refrigerant supplied through the feed line 37 will be less than the evaporator capacity C.sub.E, such that the vapor quality in the return line 39 will be high. The cooling system will be operated with a certain amount of superheat, i.e. within the evaporator 32 the refrigerant will not only absorb sufficient heat to evaporate fully (vapor quality X=100%), but will absorb more heat to enter a superheated state, i.e. assume a temperature above the saturation temperature T.sub.sat.

[0079] The vapor quality value X, as defined above, indicates within the refrigerant the mass ratio of gas to the total gas/liquid mixture. It is generally different from a void fraction, i.e. the volume fraction of the flow-channel volume that is occupied by the gas phase. While the void fraction is determined by the relative volume, the vapor quality value X is the thermal dynamic vapor quality based on mass fraction.

[0080] For a medium flowing through a conduit, the vapor quality X usually cannot be measured directly, as this would require a separation of vapor and liquid to weigh the respective mass, which is not possible in a flowing medium. Moreover, the liquid phase portion and the vapor phase portion may distribute differently within the conduit and may travel at different velocities.

[0081] The state of the refrigerant at the outlet of the evaporator 32, both with regard to the vapor quality X and to the amount of superheat, may be expressed in terms of the superheat vapor quality value X.sub.S.

[0082] For X<100% the superheat vapor quality value X.sub.S is equal to the vapor quality value X. For fully evaporated refrigerant, the superheat vapor quality X.sub.S assumes values of 100% and above, indicating the amount of superheat. Referring to the definition above, since the flowing refrigerant may not be in an equilibrium state and may comprise e.g. both superheated vapor and remaining liquid particles, the superheat vapor quality value X.sub.S should be understood to represent the energy equivalent to the equilibrium state.

[0083] Within the evaporator 32, the heat transfer from the air flow 33 to the individual parallel evaporator tubes 34 will differ. For example, in the top and bottom feed evaporators 32 shown in FIG. 3a, 3b, the first evaporator tube 34a will carry the highest heat load, i.e. at the outlet of the evaporator tube 34a before it enters the collecting conduit 31, the superheat vapor quality X.sub.S will be the highest of any of the evaporator tubes 34. The last evaporator tube 34b will have the least heat load, i.e. the superheat vapor quality X.sub.S at its outlet will be the lowest of all evaporator tubes.

[0084] The evaporated refrigerant from the evaporator tubes 34 is collected in a common collecting conduit 31 and returned through a return line 39.

[0085] Back in FIG. 1, the ammonia vapor 11 returned from the evaporator 32 through the return line 39 is guided through an individual first return conduit portion 38a and a common second return conduit portion 38b back into the suction accumulator 14, where the gas velocity will be reduced, and any rest of liquid ammonia 20 contained within the flow collects in the lower portion.

[0086] For a cooling system 10 with multiple evaporators 32, each evaporator 32 comprises a separate evaporator inlet valve 36 branching off from the conduit 28 and a separate first return conduit portion 38a for the partly evaporated ammonia. The first return conduit portions 38a from the evaporators 32 merge at the common second return conduit portion 38b.

[0087] For each of the evaporators 32, a superheat vapor quality sensor 40 is provided at one of the evaporator tubes 34 to deliver a sensor signal S indicative of the superheat vapor quality of the ammonia medium flowing through the evaporator tube 34.

[0088] Depending e.g. on the flow speed and on the vapor quality, the flow of mixed liquid/vapor ammonia through a conduit such as an evaporator tube 34 may follow different flow regimes. FIG. 2a schematically illustrates an annular flow in a vertically arranged portion of a conduit 34. A liquid film 42 flows on the conduit wall and a two-phase flow 44 of ammonia liquid and vapor flows near the center. As the thickness of the fluid film 42 will be equally distributed in a vertically oriented conduit 34, it will appear as a circle in cross-section. An annular flow regime may e.g. be expected in a conduit 34 at a usual flow speed of 5-15 m/s.

[0089] In a horizontally arranged portion of the conduit 34 as shown in FIG. 2b, 2c, the fluid film 42 will be thicker at the bottom and thinner at the top due to the influence of gravity.

[0090] The sensor 40 provides a sensor signal S which is indicative on the superheat vapor quality value X. As will be described in detail, the sensor 40 derives the signal S based on temperature measurements in response to heat supplied to the conduit 34 and to the refrigerant medium flowing therein.

[0091] An embodiment of a sensor arrangement 50 including a sensor 40 attached to an evaporator pipe 34 is shown in FIG. 4-7.

[0092] The sensor 40 comprises a sensor body 46 with a heating element 48 and a temperature sensor 52 arranged embedded within the sensor body 46. The sensor body 46 is clamped to the outer wall of the conduit 34.

[0093] The sensor body 46 is a solid piece of a metal material of good heat conduction such as copper or aluminum. It is positioned on the outside of the conduit 34 in contact with an outer tube wall thereof. The sensor body 46 has a contact surface 58 in direct contact with the tube wall of the conduit 34. The sensor body 46 and the contact surface 58 extend over a length L in longitudinal direction of the conduit 34. The contact surface 58 has a concave shape to conform to the curved shape of the outer tube wall of the conduit 34.

[0094] The portion of the conduit 34 to which the sensor 40 is mounted is arranged horizontally. As shown in particular in FIG. 7, the sensor body 46 is arranged at the bottom of the outer tube wall of the conduit 34. An installation angle measured between a line from the center of the conduit 34 to the center of the contact surface 58 and an upward vertical direction is 180°. The contact surface 58 in the embodiment extends over a contact angle α of about 50°. Therefore, in the example the sensor body 46 is in direct contact with the tube wall over an angular range of 155°-205° to the vertical axis.

[0095] An insulation 55 is provided to surround the sensor body 46 and a portion of the conduit 34 to thermally insulate it. The heating element 48 arranged within the sensor body 46 is an electric heating element, e.g. an electrical resistor of defined electrical resistance, connected to a driver circuit 56. The temperature sensor 52 is also an electrical temperature sensor such as e.g. a PT100 element, electrically connected to the driver circuit 56.

[0096] The driver circuit 56 operates the heating element 48 to deliver a defined amount of heat, constant over time. The heat from the heating element 48 distributes within the sensor body 46 and to the wall of the conduit 34. Due to the good heat conduction and high mass of the massive sensor body 56, the heating element 48, temperature sensor 52, and the adjoining portion of the wall of the conduit 34 are all thermally closely coupled so that they will assume a common temperature T with only minimal temperature gradient. Due to the insulation 55, the temperature T will be an equilibrium temperature dependent on the constant power H of the electrical heating and an amount of heat per time transferred to the refrigerant medium within the conduit 34.

[0097] In a preferred embodiment, the area of the contact surface 58 may e.g. be about 5 cm.sup.2, and an electrical heating power H may e.g. be 25 W, such that the specific power per cm.sup.2 is 5 W/cm.sup.2.

[0098] The transfer of heat from the wall of the conduit 34 to the refrigerant flowing within the conduit 34 is dependent on the phase of the refrigerant in contact with the wall. If the wall portion in contact with the contact surface 58 is wetted with liquid ammonia, the heat transfer is very high, and heat from the heating element 48 and conducted through the sensor body 46 and the wall of the conduit 34 is absorbed by the refrigerant at a high rate. If the inside of the wall is “dry”, i.e. not in contact with a substantial amount of liquid ammonia, the rate of transfer of heat is significantly lower.

[0099] Under the constant supply of heat H from the heating element 48, the wall of the conduit 34 and the sensor body 46 will assume different equilibrium temperatures T in response to different vapor quality values X and different amounts of superheat.

[0100] FIG. 8 schematically shows curves of the temperature T over a radial distanced in the region of the interface between the sensor body 46, conduit wall 34, and interior 43 of the conduit 34. The diagram shows as T.sub.Sat the saturated temperature and T.sub.Med as the temperature of the ammonia refrigerant in the interior 43 of the conduit 34, which is above T.sub.Sat in a superheat case.

[0101] In FIG. 8, the lowest curve (solid line) shows the temperature curve for a case where a significant amount of liquid ammonia is present in the interior 43 of the conduit 34, and in particular in contact with the wall of the conduit 34 (e.g. for a superheat vapor quality X.sub.S of 30% or less). In the center of the interior 43, the ammonia is at the saturated temperature T.sub.Sat. Due to the heating power supplied to the sensor body 46, a temperature gradient establishes between the sensor body 46 and the interior 43 of the conduit 34, leading to the curve shown. Following the curve from right to left in FIG. 8, the temperature T starts from T.sub.Sat and increases towards the wall of the conduit 34. Within the wall of the conduit 34, the temperature further increases. Within the sensor body 46, the higher temperature T.sub.h_1 is reached.

[0102] The middle curve (dashed line) in FIG. 8 shows the temperature curve if the interior 43 is filled only with gas at a superheat vapor quality value X.sub.S of 100% but no superheat is present. As for the above described dotted line, the ammonia in the center of the interior 43 is at the saturated temperature T.sub.Sat. Following the dashed curve from right to left, the temperature T increases towards the wall of the conduit 34 and further within the wall of the conduit 34 up to a temperature T.sub.h_2 of the sensor body 46. Due to the much lower heat conduction at the inner surface of the wall of the conduit 34, the temperature T.sub.h_2 of the sensor body 46 is much higher than in the case of liquid refrigerant.

[0103] The top curve (slash-dotted line) in FIG. 8 shows the temperature curve if the interior 43 is filled with gas at a superheat vapor quality value X.sub.S of above 100%, i.e. the refrigerant is fully evaporated (vapor quality X=100%) and a certain amount of superheat is present. The ammonia in the center of the interior 43 is at the temperature T.sub.Med above the saturated temperature T.sub.Sat. As for the curves explained above, the temperature T increases towards the wall of the conduit 34 and further within the wall of the conduit 34 up to a temperature T.sub.h_3 of the sensor body 46, which due to the higher temperature of the ammonia within the conduit 34 is higher than the temperature T.sub.h_2 for X=100% but no superheat.

[0104] The value considered indicative of the superheat vapor quality is the temperature difference ΔT between the temperature of the sensor body 46 and the saturated temperature T.sub.Sat. For low vapor quality (e.g. X<30%), the temperature difference is T.sub.h_1. The corresponding temperature difference for this case ΔT1 as shown in FIG. 8 is relatively small. For a superheat vapor quality value X.sub.S=100% (no superheat), the temperature of the sensor body 46 is at T.sub.h_2, higher than T.sub.h_1, and the temperature difference is ΔT2, which is higher than ΔT1.

[0105] In the superheat region with a superheat vapor quality value X.sub.S>100% the temperature of the sensor body 46 will be at T.sub.h_3, higher than T.sub.h_1 and T.sub.h_2, such that the temperature difference ΔT3 will be high.

[0106] Thus, a sensor signal S derived from the temperature difference ΔT is indicative of the superheat vapor quality value X.sub.S, i.e. show a further variation beyond X.sub.S=100% indicating the amount of superheat.

[0107] Thus, the temperature reading T from the temperature sensor 52 processed in the driver circuit 56 of the sensor 40 is indicative of the superheat vapor quality X.sub.S. The sensor signal S is derived from the measured temperature value T by calculating the temperature difference ΔT to the saturated temperature T.sub.Sat, which may be calculated e.g. based on a measurement of the temperature at the inlet of the evaporator 32, or alternatively a measurement of pressure at the outlet of the evaporator 32 is made and the saturation temperature calculated using the known relation between pressure and saturation temperature.

[0108] The sensor signal S may be provided differently from the driver circuit 56, e.g. as a digital signal or as an analog electrical signal. In one preferred embodiment, the sensor signal S is a current signal, for example with a current in the range of 4-20 mA.

[0109] As explained above with reference to FIG. 2b, 2c, the distribution of liquid and vapor ammonia refrigerant within the conduit 34 is not homogenous. In particular for annular flow in a horizontally arranged portion of the conduit 34, there will be a distribution with more of the liquid portion of the refrigerant arranged at the bottom and less on top.

[0110] The position of the sensor 40 has an important influence on the temperature reading T and derived sensor signal S obtained for different superheat vapor quality values X.sub.S. FIG. 9 shows curves of the sensor signal S dependent on the superheat vapor quality X.sub.S.

[0111] The solid line shows the sensor signal S of the sensor 40 arranged at the bottom of the conduit 34 as shown in FIG. 4-7. Due to liquid ammonia accumulating within the conduit and the interior of the wall of the conduit being in contact with the liquid ammonia, the sensor signal for superheat vapor quality values X.sub.S up to about 80% remains constant. From about 85% on, the sensor signal S shows a strictly monotonous rise. The sensor signal S continues to rise in the superheat region of X.sub.S>100%, such that the sensor signal S is indicative of the superheat vapor quality value X.sub.S.

[0112] For alternative arrangements of a sensor 40 on the conduit 34 as shown in FIG. 11 either to the side (under an angle β1 of 90° to the upward vertical direction) or on top (under an angle β2 of 0° to the upward vertical direction), the curve of the sensor signal S in dependency on the vapor quality value X differs. In FIG. 9 the dashed line shows the sensor signal S for a sensor 40 arranged under an angle of β1=90° and a dotted line shows the sensor signal S for a sensor 40 arranged under an angle of β2=0°. The smaller the angle of arrangement β is, the lower the threshold of the superheat vapor quality value X.sub.S required to obtain a rising sensor signal S. However, for the sensor arranged on top (dotted line) or horizontally (dashed line), the curve of the sensor signal rising earlier than in the case of the sensor arranged at the bottom may reach a maximum value and not show a desirable sensitivity for the superheat range of X.sub.S>100%.

[0113] Therefore, the bottom arrangement of the sensor 40 under an angle of β=180° as in FIG. 4-7 is preferred for the sensor 40 to obtain a well usable signal S for high values of the superheat vapor quality X.sub.S extending into the superheat region X>100%.

[0114] It should, however, be recognized that the sensor signal S does not necessarily provide an exact measurement of a specific superheat vapor quality value X.sub.S. While in an effective working range of the sensor 40 there is a strictly monotonous dependency of the sensor signal S on the superheat vapor quality value X.sub.S as shown in FIG. 9, the actual curve may also be dependent on other parameters, such as the distribution of liquid and vapor within the conduit 34, the flow speed, the specific effect of the heating element 48. Thus, obtaining exact measurements of the superheat vapor quality X.sub.S from the sensor signal S may require additional information or assumptions, such as to the flow regime. Taking the additional information into account e.g. by calculations or by calibration, it is possible to obtain a value for the superheat vapor quality X.sub.S. However, as will be shown below, due to a monotonous dependency of the sensor signal S on the superheat vapor quality X.sub.S, even without such calibration the sensor signal S may nevertheless be used to observe operation and to effect control of the cooling system 10 based on the sensor signal S.

[0115] The sensor 40 may be arranged in different positions within the cooling system 10 of FIG. 1. In the most preferred embodiment, the sensor 40 is arranged at the evaporator tube 34b with the least heat load (FIG. 3a-c). The sensor 40 is further preferably arranged at the end of the evaporator tube 34b (although preferably outside of the air flow 33), i.e. the outlet of the tube 34 before entering the collecting conduit 31.

[0116] At this position within the system 10 and evaporator 32, the superheat vapor quality X.sub.S will generally be the lowest. Therefore, this position is well suited to obtain the sensor signal S to ensure that the system 10 is operated in the superheat range X.sub.S>100%. Alternatively, the sensor 40 may be arranged in a different position, or a plurality of sensors 40 may be arranged at different evaporator tubes 34. For evaporators 32 where the load distribution between each of the evaporator tubes 34 is known, the sensor 40 may alternatively be arranged e.g. on another evaporator tube 34 in order to obtain a different sensitivity. Also, it is possible to adjust the sensitivity by using a different mounting position of the sensor 40 as explained above with reference to FIG. 11.

[0117] In alternative embodiments, the sensor 40 may be arranged on the return conduit 38a.

[0118] In the direct expansion ammonia cooling system 10 of FIG. 1, the sensor signals S from the superheat vapor quality sensor 40 of each evaporator 32 are supplied to a controller 80. The controller 80 is a computer programmed to execute a control program to derive a control signal C from the sensor signal S. The control signal C is supplied to the control valve 36 of each evaporator 36 and controls the degree of opening of the control valves 36, and therefore the mass flow of refrigerant through the control valves 36. The control valves 36 are for example solenoid valves controllable by control signals C.

[0119] The control objective pursued by the controller 80 is to operate the system 10 stably with a minimum required superheat, however sufficient to maintain the cooling capacity required.

[0120] The degree of opening of the control valves 36 determines the amount of liquid ammonia refrigerant supplied to each evaporator 32. A circulation rate N indicates the ratio of the mass flow of ammonia supplied to an evaporator 32 and the rated/nominal capacity of the evaporator.

[0121] In direct expansion systems such as the cooling system 10 shown in FIG. 1, the circulation rate N is below 1, i.e. the mass flow of liquid ammonia to each evaporator 32 is lower than the capacity of the evaporator 32, such that the ammonia is fully evaporated and the superheat vapor quality X.sub.S in the evaporator conduits 34 is above 100%. The superheat vapor quality value X.sub.S will be lowest in the evaporator tube 34b with the least heat load, where the sensor 40 is arranged.

[0122] The evaporator capacity is not constant, but dependent on the superheat vapor quality X.sub.S. As more of the ammonia refrigerant is evaporated, less of the inner surface of the walls of the evaporator tubes 34 will be in contact with liquid ammonia. The heat transfer from “dry” tube walls to the refrigerant medium, however, is significantly less than the heat transfer from tube walls in contact with a liquid film 42 as shown e.g. in FIG. 2a-2c. Therefore, the evaporator capacity decreases for low circulation rates N, corresponding to high values of the superheat vapor quality X.sub.S, since the superheat vapor quality X.sub.S is generally the reciprocal value of the circulation rate N.

[0123] To ensure complete evaporation, the evaporators 32 of the direct expansion cooling system 10 are designed and operated to superheat the ammonia refrigerant, i.e. obtaining a positive temperature difference by which the gas temperature of the ammonia vapor is above the saturation temperature. Superheat is obtained e.g. by increasing the evaporator surface, or the temperature difference, or both, having a negative effect on operation and/or installation cost.

[0124] As a consequence, it is desirable to operate the cooling system 10 with a reduced amount of superheat while ensuring complete or near complete evaporation.

[0125] The cooling system 10 is operated by the controller 80 to reduce the amount of superheat, even allowing to extend the control range into the two-phase region. Since this may entail an amount of liquid droplets to be carried through the return conduits 38a, 38b with the vapor, the suction accumulator 14 is provided with the heating spiral 26 through which the hot condensate is conducted, such that any liquid ammonia 20 accumulated there will be evaporated.

[0126] Operation and control of the ammonia direct expansion cooling system 10 of FIG. 1 will be explained with reference to FIG. 10.

[0127] In FIG. 10, the sensor signal S and a superheat value T.sub.S are shown in dependence on the vapor quality value X. Since the cooling system 10 is operated in the superheat range near a vapor quality of X=100%, only high vapor quality values X of 80-110% are shown on the x-axis of FIG. 10.

[0128] The sensor signal S as shown increases with increasing vapor quality X. An amount of superheat T.sub.S increases linearly from Zero at X=100%.

[0129] While previously known direct expansion cooling systems are operated with a relative high superheat, the cooling system 110 is operated by the controller 80 in a region of low superheat, extending down up to the two-phase region close to X.sub.S=100%. In the example shown, a control range R may be e.g. 98%<X<107%.

[0130] While the amount of superheat may be determined by measuring the temperature of the evaporated refrigerant, such a temperature measurement proves difficult in the low superheat region, e.g. X<102%. In this region, there will still be a certain amount of liquid drops contained within the flow of evaporated refrigerant. A temperature sensor provided at the conduit 34 however will show very different temperature readings depending on whether at the point of contact between the sensor and the ammonia flow the ammonia is in liquid (e.g. droplet) or in vapor phase. For this reason, a temperature reading may not reliably be used for control of the cooling system 10 in the control range R.

[0131] However, as shown in FIG. 10, the sensor signal S provides information about the superheat vapor quality X.sub.S throughout the control range R.

[0132] Thus, an operating point P may be chosen which may be e.g. at or near X.sub.S=100. At the operating point, the sensor signal S may take a known reference value S. The controller 80 controls the cooling system 10 depending on the sensor signal S. If the sensor signal S is below the reference sensor signal S.sub.P, i.e. if the superheat vapor quality value X.sub.S is below the operating point P, the controller 80 will provide a control signal C to decrease opening of the evaporator inlet valve 26 to increase the amount of superheat. If the sensor signal S is above the operating point sensor signal S.sub.P, i.e. the superheat vapor quality value X.sub.S is above the operating point P, the controller 180 will provide a control signal C to increase opening of the evaporator inlet valve 26, reducing the amount of superheat.

[0133] Thus, the controller 80 will continuously monitor the sensor signal S, which may be the temperature difference ΔT (or, alternatively, the temperature T so that the controller 80 may calculate ΔT by subtracting the reference temperature T.sub.Sat). Based on the defined setpoint P and the sensor signal S compared to the setpoint sensor signal S.sub.P, the controller 80 will reduce or increase mass flow into the evaporator 32.

[0134] The controller 80 may further incorporate an anti-windup for fast recovery after substantial load variations, i.e. after sudden high heat loads when the setpoint P cannot be achieved, e.g. if the evaporator 32 “overheats” or if insufficient liquid refrigerant is available. In such cases indicated by a high sensor signal S, the control 80 may be disposed to abandon closed-loop control and supply a control signal C to fully open the evaporator inlet vale 36. After the sensor signal S returns to the usual range, the controller 80 may resume closed-loop control.

[0135] In case of a reduced capacity of the evaporator 32, e.g. when the surface of the evaporator 32 is covered with ice, the controller 80 will detect a reduced sensor signal S and react by controlling the evaporator inlet valve 36 to reduce the mass flow of refrigerant.

[0136] It should be kept in mind that the above embodiments are merely examples of the cooling systems, sensor arrangements, operating methods, and sensing methods according to the invention. The invention is not limited to the disclosed embodiments.

[0137] For example, the control strategy and parameters, in particular the specific values of the control range R are given as examples only. The sensor design may differ, and the sensor may e.g. be applied in a different position within the cooling system or within the evaporator. The skilled person will recognize further possible modifications to the disclosed embodiments.