GB2524441A

GB2524441A - Liquid level detecting device

Info

Publication number: GB2524441A
Application number: GB1513256.6A
Authority: GB
Inventors: Yasutaka Ochiai; Makoto Saito; Fumitake Unezaki
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2013-01-30
Filing date: 2013-01-30
Publication date: 2015-09-23
Anticipated expiration: 2033-01-30
Also published as: GB2524441B; GB2524441C; GB201513256D0; JP6021954B2; DE112013006546T5; DE112013006546B4; CN203964987U; JPWO2014118904A1; WO2014118904A1

Abstract

A liquid level detecting device comprises: a plurality of sensors (10) that are installed, so that the position heights are mutually different, on the surface of a container (9), in which the level of liquid is to be detected, and that measure the temperature of the installed locations by means of a temperature measuring element (103); and a liquid level detecting unit (204) that specifies, as the position of the sensor with the lowest measured value of the plurality of sensors (10), the liquid level position in the container (9) that has a temperature difference between the interior and exterior such that the interior temperature of the container (9) is lower than the exterior temperature and that is in a state where a fluid flows into and out of the container (9).

Description

DESCRIPTION

Title of Invention

LIQUID LEVEL DETECTION DEVICE AND REFRIGERATING AND AIR-

CONDITIONING APPARATUS

Technical Field

[0001] The present invention relates to a liquid level detection device and refrigerating and air-conditioning apparatus, which identifies the position of a liquid level in a container.

Background Art

[0002] A liquid level detection device which includes a sensor attached to an outer surface of a container containing liquid therein to be able to detect the position of a liquid level inside the container has been available (for example, Patent Literature 1).

[0003] The liquid level detection device described in Patent Literature 1 includes a strip-shaped sensor main body formed by stacking a temperature measurement layer that measures the temperature of the container surface and a heating layer for heating the container. The sensor main body is used by being attached to the outer surface of the container in such a manner that the longitudinal direction of the sensor main body is equal to the vertical direction of the container and the temperature measurement layer is positioned on the container side.

[0004] Heat of the heating layer reaches the container surface via the temperature measurement layer Due to the influence of the difference in heat transfer rate between gas and liquid in the container, a temperature difference occurs inside the container between a part which faces liquid and a part which faces gas. That is, the container surface temperature of a part in which liquid, which has a high transfer rate, exists is close to the temperature of liquid refrigerant inside the container, whereas the container surface temperature of a part in which gas, which has a low heat transfer rate, exists is close to the container outside temperature (heating temperature of the heating layer).

[0005] In Patent Literature 1, by using the above temperature difference, that is, by comparing temperatures in the vertical direction of the temperature measurement layer when heating is done from the outside, the position of the liquid level is detected by determining a relatively high temperature part as a gas part and a relatively low temperature part as a liquid part.

Citation List Patent Literature [0006] Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2008-39726 (Page 1, Figure 1)

Summary of Invention

Technical Problem [0007] However, when the liquid level inside the container varies due to fluid flowing into and flowing out of the container, the heat transfer rate inside the container varies compared to the case where liquid inside the container is stationary. Since a variation in the heat transfer rate inside the container affects the container surface temperature, the method of Patent Literature 1 for identifying the position of the liquid level without taking into consideration a variation in the heat transfer rate inside the container is problematic in that the position of the liquid level cannot be identified.

[0008] The present invention has been made in view of the foregoing circumstances, and an object of the present invention is to provide a liquid level detection device and refrigerating and air-conditioning apparatus, which is capable of identifying the position of the liquid level of liquid flowing inside a container, from outside the container.

Solution to Problem [0009] A liquid level detection device according to the present invention includes a plurality of sensors that are installed at different positions in height on a surface of a container as a target of liquid level detection and that measure temperatures at the installed positions by using temperature measurement elements; and a liquid level detection unit that identifies a position of a liquid level inside the container in a state in which fluid flows into and flows out of the container, as a position of a sensor having a lowest measured value among the plurality of sensors.

Advantageous Effects of Invention [0010] A liquid level detection device according to the present invention may identify the position of the liquid level of liquid flowing inside a container, from outside the container.

Brief Description of Drawings

[0011] [Fig. 1] Fig. 1 is a schematic diagram illustrating a state in which a liquid level detection device 1A according to Embodiment of the present invention is installed at a container 9, which is element equipment of a refrigerating and air-conditioning apparatus.

[Fig. 2] Fig. 2 is a schematic diagram illustrating a schematic configuration of the liquid level detection device lAof Fig. 1.

[Fig. 3] Fig. 3 is a block diagram schematically illustrating an electric configuration of a control measurement device 20 which forms the liquid level detection device lAof Fig. 1.

[Fig. 4] Fig. 4 is a diagram illustrating the relationship between the fluid velocity and the heat transfer rate of air, water, a liquid refrigerant (R41OA, 20 degrees Centigrade), and a gas refrigerant (R41OA, 20 degrees Centigrade).

[Fig. 5] Fig. 5 is a diagram illustrating measured values of individual sensors 10 (container surface temperatures) after heating is done by the heating elements 102 of the liquid level detection device lAwhen the refrigerating and air-conditioning apparatus is stopped.

[Fig. 6] Fig. 6 is a diagram illustrating estimation of a flow of fluid inside the container in the state of Fig. 5.

[Fig. 7] Fig. 7 is a diagram illustrating measured values (container surface temperatures) of the individual sensors 10 after heating is done by the individual sensors 10 of the liquid level detection device lAwhen the refrigerating and air-conditioning apparatus is in operation.

[Fig. 8] Fig. 8 is a diagram illustrating estimation of a flow of fluid inside the container in the state of Fig. 7.

[Fig. 9] Fig. 9 is a diagram illustrating the relationship between the liquid flow velocity and the height.

[Fig. 10] Fig. 10 is a diagram illustrating the relationship between sensor measured values (container surface temperatures) after heating is done by the heating elements 102 of the individual sensors 10 and the container height when the refrigerating and air-conditioning apparatus is in operation.

[Fig. 11] Fig. 11 is a diagram illustrating the relationship between measured values after heating is done by the heating elements 102 of the individual sensors 10 and the container height when the refrigerating and air-conditioning apparatus is in operation.

[Fig. 12] Fig. 12 is a flowchart illustrating the flow of a process for liquid level detection by the liquid level detection device 1A according to Embodiment of the present invention.

Description of Embodiment

[0012] Hereinafter, a configuration, an installation method, a principle of liquid level detection, and a gas/liquid determination method of a liquid level detection device according to Embodiment of the present invention will be described with reference to drawings. In Embodiment described below, a description will be provided based on an example in which as an element part of a refrigerating and air-conditioning apparatus, a container which is provided on a low-pressure side to accumulate refrigerant is used as a measurement target. In the drawings provided below including Fig. 1, the size relationship of individual component members may differ from the actual size relationship. In the drawings provided below including Fig. 1, parts referred to with the same signs correspond to the same parts or parts equivalent to the parts. The same applies throughout the description. Furthermore, forms of component elements illustrated in the description are merely exemplifications and the present invention is not limited to the described forms.

[0013] Fig. 1 is a schematic diagram illustrating a state in which a liquid level detection device 1A according to Embodiment of the present invention is installed at a container 9, which is element equipment of a refrigerating and air-conditioning apparatus. Fig. 2 is a schematic diagram illustrating a schematic configuration of the liquid level detection device lAof Fig. 1. The liquid level detection device lAwill be described with reference to Figs. 1 and 2. Arrows illustrated in Fig. 1 represent a direction of the flow of refrigerant.

[0014] <Container serving as target of liquid level detection> First, the container 9 serving as a target of liquid level detection will be explained with reference to Fig. 1. As described above, the container 9 is one of component element parts of the refrigerating and air-conditioning apparatus.

Apart from the container 9, the refrigerating and air-conditioning apparatus includes at least a compressor, a condenser (radiator), an expansion device, and an evaporator (none of them are illustrated in figures), and includes a refrigerant circuit in which refrigerant circulates sequentially through the devices mentioned above. The container 9 is installed on the low-pressure side of the refrigerating and air-conditioning apparatus (a portion from the expansion device via the evaporator to the compressor). The container 9 is installed for two purposes.

[0015] One of the two purposes of the installation of the container 9 is to accumulate lubricating oil for lubrication of the compressor. This purpose is designed to accumulate oil in the container 9 on the upstream side of the compressor and return a certain amount of lubricating oil to the compressor since the compressor is installed on the downstream side of the container 9 and lubricating oil is necessary to operate the compressor.

[0016] The other one of the two purposes of the installation of the container 9 is to accumulate excess liquid refrigerant of the refrigerating and air-conditioning apparatus. In the refrigerating and air-conditioning apparatus, the amount of refrigerant necessary for the refrigerating and air-conditioning apparatus varies according to the operation state and the control state. Therefore, the maximum required amount of refrigerant is normally filled in the refrigerating and air-conditioning apparatus. Thus, if the required amount of refrigerant decreases due to the operation state and the control state, the remainder of the liquid refrigerant is excess. The surplus refrigerant is defined as excess liquid refrigerant, and the container 9 has a function for storing the excess liquid refrigerant.

[0017] Furthermore, the container 9 is made of metal for the purpose of pressure resistance and has a wall thickness of, for example, 3 mm to 4 mm, and the liquid level inside the container 9 cannot be viewed from the outside. Furthermore, in general, the container 9 includes a cylindrical main body. That is, the outer surface of the container 9 is a cylindrical surface.

[0018] As illustrated in Fig. 1, two pipes, that is, an inlet pipe 9a and an outlet pipe 9b, are installed at the container 9. The inlet pipe 9a and the outlet pipe 9b are arranged in an upper part of the container 9 so as to vertically penetrate inside and outside the container 9. The inlet pipe 9a allows refrigerant to flow into the container 9. The outlet pipe 9b allows refrigerant to flow out of the container 9.

[0019] The entire outlet pipe 9b has substantially a J-letter shape in a front view.

An oil return hole 9c is formed at the lowest part of the bent portion of the J letter, and the upper end part of the outlet pipe 9b protrudes from above the container 9 and is connected to the compressor Furthermore, a leading end of the outlet pipe 9b which is located inside the container 9 serves as a suction port 9d which sucks refrigerant existing inside the container 9. The outlet pipe 9b has such a structure because it is necessary to return a certain amount of lubricating oil to the compressor In the container 9 configured as described above, the pressure inside the outlet pipe 9b decreases due to the suction flow velocity of gas refrigerant which has been sucked through the suction port 9d, and oil is sucked through the oil return hole 9c and returned to the compressor.

[0020] <Configuration of liquid level detection device 1A> Next, a configuration of the liquid level detection device lAwill be described with reference to Figs. 1 and 2.

[0021] The liquid level detection device IA includes a plurality of sensors lUa to 1 Od (if there is no need to distinguish among the sensors 1 Oa to 1 Od, these sensors will be collectively referred to as sensors 10) which are installed on the surface of the container 9, and a control measurement device 20 which controls the sensors 10 and measures sensor signals sent from the sensors 10.

[0022] The sensors ba to lOd have configurations similar to one another The sensors ba to lOd each include a heating element 102 which heats the container 9 and a temperature measurement element 103 which functions as a temperature measurement layer and are installed on the surface of the container 9 so as to be covered with an insulating material 16. Furthermore, the sensors 1 Oa to 1 Od are connected to the control measurement device 20 via feeder lines and signal lines. As illustrated in Fig. 1, the sensors 1 Oa to 1 Od are installed at different positions in height on the outer surface of the container 9.

[0023] Hereinafter, members forming the sensors ba to lOd, the insulating materials 16, and the control measurement device 20 will be sequentially described.

[0024] (Heating element 102) The heating elements 102 generate heat when power is supplied through electric wires. In order to prevent variations in the sensor measured value among the sensors 10, the heating elements 102 that achieve the same resistance value and the same amount of heat generation among the sensors 10 are used. Furthermore, the heating elements 102 are, for example, rectangular resistors. The outer surface of the container 9 is a curved surface, and it is therefore desirable to use a small resistor, considering ease of close contact.

Furthermore, the heating elements 102 themselves may be resistors or may be elements including a resistor protected with ceramic material or the like.

[0025] (Temperature measurement element 103) The temperature measurement elements 103 include thermoelectric conversion elements which are typified by thermocouples or resistance thermometer sensors which are typified by thermistors. The temperature measurement elements 103 are connected to the control measurement device via signal lines. In order to reduce variations in the sensor measured value among the sensors 10, it is desirable that the temperature measurement elements 103 are as small as possible in size and as small as possible in heat capacity.

[0026] (Insulating material 16) The insulating materials 16 are provided to prevent heat from entering into sensors from outside. For example, foam insulating materials formed by foaming a synthetic resin, such as polystyrene foam, phenolic foam, or urethane foam, or fibrous insulating materials which are typified by glass wool may be used as the insulating materials 16.

[0027] The sensors 10 configured as described above each have a structure in which the heating element 102 and the temperature measurement element 103 are arranged in this order from the side of the container 9, the heating element 102 and the temperature measurement element 103 are covered with the insulating material 16, and the heating element 102 generates a temperature difference between inside and outside the container 9. With this configuration, the insulating material 16 prevents heat from exiting and entering from outside the container, and the flow of heat may be limited to a direction from the heating element 102 toward the container 9.

[0028] (Control measurement device 20) Fig. 3 is a block diagram schematically illustrating an electrical configuration of the control measurement device 20 which forms the liquid level detection device lAof Fig. 1.

The control measurement device 20 is a device which controls the entire liquid level detection device 1A on the basis of a program stored in a storing unit 203, which will be described later. The control measurement device 20 includes a heating element control unit 201, a sensor measurement unit 202, the storing unit 203, and a liquid level detection unit 204. An input unit 205 and an output unit 206 are connected to the control measurement device 20.

[0029] The heating element control unit 201 is a unit which controls the plurality of heating elements 102 which form the plurality of sensors 10 to be simultaneously turned ON/OFF The sensor measurement unit 202 is a unit which simultaneously measures the plurality of temperature measurement elements 103 which form the plurality of sensors 10. The storing unit 203 stores the control program and a program corresponding to a flowchart of Fig. 12, which will be described later, and stores each of the measured values obtained by the sensor measurement unit 202. The liquid level detection unit 204 is a unit which analyzes each of the measured values obtained by the sensor measurement unit 202 and data stored in the storing unit 203 to identify the position of the liquid level in the container 9.

[0030] The input unit 205 is a unit which inputs external information and is used, for example, to input sensor information of the refrigerating and air-conditioning apparatus. The output unit 206 is used to output information processed at the control measurement device 20, such as the position of the liquid level, to the outside. With the provision of the output unit 206, a remote monitoring function for remotely transmitting information and the like may be added.

[0031] <Installation method of liquid level detection device 1A> Next, an installation method of the liquid level detection device lAwill be described. The liquid level detection device 1A may be used in accordance with a method in which the liquid level detection device 1A alone confirms the position of the liquid level and information of the position of the liquid level is output by the output unit 206, a method in which the liquid level detection device 1A is incorporated with an apparatus such as the refrigerating and air-conditioning apparatus when the apparatus is assembled, a method in which the liquid level detection device 1A is incorporated with an existing apparatus so that the liquid level detection device 1A is connected to the existing apparatus at the time of maintenance, and the like.

[0032] As for a specific installation method of the liquid level detection device 1A, the liquid level detection device 1A is installed at a portion on the surface of the container 9 with no irregularities or corrosion. The sensors 10 may be independently installed or may be collectively mounted using a jig.

[0033] It is desirable that the plurality of sensors 10 are installed with regular spaces therebetween. This is because installing with regular spaces allows easier association between the position of the sensors 10 and the liquid level height. However, when a varying liquid level position is limited, when the range of the liquid level position to be detected is limited, or the like, the spaces between the sensors 10 may be changed according to the required resolution, instead of installing the sensors 10 with regular spaces therebetween. That is, a narrower space may be provided for a portion which is measured at a high resolution and a wider space may be provided for a portion which is measured at a low resolution. Furthermore, the number of the sensors 10 may be reduced by installing only the necessary number of sensors at necessary positions.

[0034] Next, the relationship between the fluid velocity and the heat transfer rate which affects the container surface temperature when the container in which the internal fluid is flowing is externally heated, will be explained.

[0035] <Relationship between fluid velocity and heat transfer rate> Fig. 4 is a diagram illustrating the relationship between the fluid velocity and the heat transfer rate of air, water, liquid refrigerant (R41 OA, 20 degrees Centigrade), and gas refrigerant (R41 OA, 20 degrees Centigrade). A represents the relationship between the fluid velocity and the heat transfer rate of air, B represents the relationship between the fluid velocity and the heat transfer rate of water, C represents the relationship between the fluid velocity and the heat transfer rate of the gas refrigerant, and D represents the relationship between the fluid velocity and the heat transfer rate of the liquid refrigerant. Portions of C and D in which the heat transfer rate changes greatly compared to an increase in the flow velocity are portions in which the flow state of fluid changes from a laminar flow into an eddy flow.

[0036] In Fig. 4, for all the fluids A to D, the heat transfer rate increases as the fluid velocity increases. Furthermore, when the gas refrigerant C is compared with the liquid refrigerant D, the heat transfer rate of the liquid refrigerant is higher than the heat transfer rate of the gas refrigerant at the same flow velocity, and the ratio of increase in the heat transfer rate caused by an increase in the flow velocity, that is, the inclination, of the liquid refrigerant is greater than that of the gas refrigerant.

[0037] Furthermore, the difference in the heat transfer rate between the gas refrigerant C and the liquid refrigerant D is smaller than the difference in the heat transfer rate between the air A and the water B. It is clear that when the gas flow velocity is sufficiently faster than the liquid flow velocity, the gas refrigerant and the liquid refrigerant have the same heat transfer rate, or the gas refrigerant exhibits a higher heat transfer rate than the liquid refrigerant. Specifically, the heat transfer rate when the velocity of the liquid refrigerant is 0.4 mIs as represented by a dotted line a is equal to the heat transfer rate when the velocity of the gas refrigerant is 0.7 mIs as represented by a dotted line b. Furthermore, when the velocity of the gas refrigerant reaches 0.7 mIs or more as represented by the dotted line b, the heat transfer rate of the gas refrigerant is higher than that of the liquid refrigerant at 0.4 mIs.

[0038] <Principle of measurement of container surface temperature of sensors 10 in liquid level detection device 1A> Next, the principle of temperature measurement of the sensors 10 in the liquid level detection device lAwill be explained. In the sensors 10, as described above, the temperature measurement elements 103 are installed outward relative to the heating elements 102, that is, installed at positions farther away from the container surface compared to the heating elements 102. Since the amount of heating (amount of heat generation) of the heating elements 102 of the sensors 10 are the same, the temperature measurement elements 103 of the sensors 10 are regarded as detecting the same temperature, when simply considered. In actuality, however, since the temperatures of the heating elements 102 differ from one another due to the influence of the container surface temperature (in other words, in accordance with the condition of fluid inside the container), the measured values of the temperature measurement elements 103 of the sensors 10 differ from one another [0039] That is, the temperature of a part of the container surface that is easy to transfer heat due to the influence of the internal fluid is lower than the temperature of a part of the container surface that is difficult to transfer heat.

Therefore, the temperature of the heating element 102 installed at a portion of the container surface that is easy to transfer heat is lower than the temperature of the heating element 102 installed at a portion of the container surface that is difficult to transfer heat. Accordingly, the measured value of the temperature measurement element 103 provided at the heating element 102 installed at a portion of the container surface that is easy to transfer heat also decreases.

[0040] In contrast, the temperature of a part of the container surface that is difficult to transfer heat due to the influence of the internal fluid is higher than the temperature of a part of the container surface that is easy to transfer heat. Due to the influence of the increase in the temperature of the part of the container surface that is difficult to transfer heat, the temperature of the heating element 102 and the measured value of the temperature measurement element 103 also increase.

[0041] As described above, the temperature of the heating element 102 differs depending on whether the part at which the heating element 102 is installed is a part that is easy to transfer heat or a part that is difficult to transfer heat.

Therefore, the measured values of the temperature measurement elements 103 also differ from one another [0042] <Measured values of sensors 10 and expected phenomenon inside container 9 when container is heated> Regarding measured values of the sensors 10 and an expected phenomenon inside the container 9 when the container 9 is heated, the state in which the refrigerating and air-conditioning apparatus is stopped and the state in which the refrigerating and air-conditioning apparatus is in operation will be separately described with reference to Figs. 5 to 9.

[0043] (When refrigerating and air-conditioning apparatus is stopped) Fig. 5 is a diagram illustrating measured values (container surface temperatures) of the individual sensors 10 after heating is done by the heating elements 102 of the liquid level detection device lAwhen the refrigerating and air-conditioning apparatus is stopped. The horizontal axis represents temperature and the vertical axis represents container height. Line (1) of Fig. 5 is a line connecting plot points of the individual measured temperatures, which represents measured values of the individual sensors 10 after heating is done by the heating elements 102 in the case where the refrigerating and air-conditioning apparatus is stopped and the liquid fluid is accumulated up to a container height Z. Fig. 6 is a diagram illustrating estimation of a flow of fluid inside the container in the state of Fig. 5.

[0044] As illustrated in Fig. 5, when the stopped container 9 is heated by the heating elements 102, the measured values exhibit substantially a constant temperature in an upper portion of a gas part (a). When the height approaches the liquid level Z by a certain amount, a measured value starts to relatively decrease compared to the upper portion of the gas part. In a lower portion (f3) below the liquid level Z, a relatively low temperature is measured compared to the upper portion. That is, the surface temperature of the gas part is relatively higher than the surface of the liquid part.

[0045] This may be assumed as described below. Fluids, both gas and liquid, inside the container during stoppage of the refrigerating and air-conditioning apparatus are in a stationary state (natural convection), as illustrated in Fig. 6.

When the heat transfer rate is compared between fluids with reference to Fig. 4, the heat transfer rate of the gas part is lower than the liquid part (that is, heat transfer from the container wall face to the gas part is difficult to occur).

Therefore, a measured value at the gas part is close to the container surface temperature (heating temperature by the heating element 102).

[0046] Furthermore, in the gas part, when the height approaches the liquid level, the amount of heat transfer toward inside the container increases due to the influence of the heat conduction of the container 9, which is made of metal, and the high heat transfer rate of the liquid part. Therefore, as the height approaches the liquid level, much more heat of the heating element 102 is transferred to the inside the container via the container surface, and a measured value decreases.

[0047] (When refrigerating and air-conditioning apparatus is in operation) Fig. 7 is a diagram illustrating measured values of the individual sensors 10 (container surface temperatures) after heating is done by the individual sensors 10 of the liquid level detection device lAwhen the refrigerating and air-conditioning apparatus is in operation. The horizontal axis represents temperature, and the vertical axis represents container height. Line (2) of Fig, 7 is a line connecting plot points of the individual measured temperatures, which represents measured values of the individual sensors 10 after heating is done by the heating elements 102 when the refrigerating and air-conditioning apparatus is in operation and the liquid fluid is accumulated up to the container height Z. For comparison, (1) of Fig. 5 is expressed as a dotted line (1) of Fig. 7. Fig. 8 is a diagram illustrating estimation of a flow of fluid inside the container in the state of Fig. 7.

[0048] As illustrated in Fig. 7, when the container surface is heated by the heating elements 102 of the sensors 10, the measured values of the individual sensors exhibit substantially a constant temperature in an upper portion of a gas part (a'). When the height approaches the liquid level Z by a certain amount, a measured value starts to decrease, and the relatively lowest measured value is obtained at the liquid level Z. Then, the measured value relatively increases as the height is lowered than the liquid level Z of the container. When measured values (line (1)) at the time when the refrigerating and air-conditioning apparatus is stopped are compared with measured values (line (2)) at the time when the refrigerating and air-conditioning apparatus is in operation, in the gas part (a') and an upper liquid part (ft), the measured values at the time when the refrigerating and air-conditioning apparatus is in operation are relatively lower than the measured values at the time when the refrigerating and air-conditioning apparatus is stopped, whereas in a lower liquid part (n/), the measured values at the time when the refrigerating and air-conditioning apparatus is stopped are equal to or substantially equal to the measured values at the time when the refrigerating and air-conditioning apparatus is in operation.

[0049] This is due to fluid flowing in through the upper part of the container 9 when the refrigerating and air-conditioning apparatus is in operation, since a flow inlet is provided in an upper part of the container 9. That is, this is considered to be caused by a phenomenon described below. As illustrated in Fig. 8, in the gas part and the upper liquid part of the upper part of the container, due to the influence of fluid which flows into the container, forced convection is generated, and the velocity of the fluid increases compared to the time when the refrigerating and air-conditioning apparatus is stopped. However, since a lower liquid part is less susceptible to the influence of the inflow fluid, the flow velocity decreases (has a velocity close to that of when the refrigerating and air-conditioning apparatus is stopped).

[0050] That is, as illustrated in Fig. 9, which illustrates the relationship between the liquid flow velocity and the height, it is considered that velocity distribution is generated in the vertical direction inside the liquid part. In terms of the above point, during operation, the flow velocity in the gas part and the upper liquid part is faster than that during stoppage. Therefore, during operation, the heat transfer rate increases on the whole compared to that during stoppage.

Accordingly, during operation, measured values are lower than those during stoppage. However, since the flow velocity in the lower liquid part is not different from that during stoppage, it may be considered that the heat transfer rate does not change and the measured values are thus not different from those during stoppage. Furthermore, when explaining with reference to Fig. 4, during operation, when the flow velocity in the lower liquid part is not different from that during stoppage, that is, for example, 0.4 [mIs], and the flow velocity increases to, for example, 1.0 [m/s] in the gas part, due to the increase in the flow velocity on the gas part side, the heat transfer rate on the gas part side becomes close to the liquid part side and increases. Due to the influence of the increase in the heat transfer rate on the gas part side, the measured value of the sensor 1 Od becomes lower than that during stoppage. Therefore, it may become difficult to distinguish between the measured value of the sensor 1 Od and the measured value of the sensor 1 Oa.

[0051] <Principle of liquid level detection> The temperature of fluid inside the container is basically the same between the liquid part and the gas part and there is no difference in temperature. When there is no difference in temperature or there is a small difference in temperature between inside and outside the container, a difference in the outer surface temperature of the container does not occur or cannot be discerned between the liquid part and the gas part. However, by forcibly applying heat from outside the container, a temperature difference occurs in such a manner that the temperature of outside the container becomes higher than the temperature of inside the container by an amount required for detecting the liquid level. Thus, by making the difference in heat transfer of the container surface more distinct, the difference (in temperature) is measured to identify the liquid level.

[0052] Based on the phenomenon of inside the container, a principle of liquid level detection by the liquid level detection device lAwill be described.

[0053] In a known liquid level detection method, a liquid level is detected by using a principle that on the surface part of the container 9 a temperature difference occurs between a part corresponding to the gas part and a part corresponding to the liquid part inside the container 9. Specifically, a threshold value of temperature is set, and the liquid level is detected by defining a part having a temperature equal to and higher than the threshold value as the gas part and defining a low temperature part below the threshold value as the liquid part.

[0054] In this method, however, since the temperature of the lower liquid part is relatively higher than the other parts, the lower liquid part is falsely detected as a gas part, and accurate liquid level detection is not achieved. Specifically, explaining with reference to Fig. 7, when the threshold value for during operation is set to TO, the sensor 1 Oa is determined as a gas part. Seeing the sensors from the bottom of the container, measurement results indicating 1 Oa as a gas part, 1 Ob as a liquid part, 1 Oc as a liquid part, and 1 Od as a gas part are obtained, and determination of the position of the liquid level is thus inaccurately conducted.

[0055] Therefore, in the case where the fluid inside the container flows, it is necessary to take into consideration the phenomenon inside the container when detecting the liquid level. That is, it is necessary to perform detection taking into consideration distribution of the heat transfer rate of the flowing part. When the fluid inside the container flows, the heat transfer rate of the upper liquid part of the whole inside the container, that is, in the vicinity of the liquid level, is relatively higher than the gas part and the lower liquid part. The reason is as described below.

[0056] That is, when the flow velocity of the gas part increases as the inlet velocity increases, the flow velocity of the upper liquid part also increases as in the gas part in proportion to the increase in the inlet velocity As illustrated in Fig. 4, the liquid refrigerant has, as a whole, a higher heat transfer rate than the gas refrigerant and a larger variation width (inclination) of the heat transfer rate with respect to a variation in the inlet velocity. Therefore, the upper liquid part (in the vicinity of the liquid level) has the highest heat transfer rate in the whole inside the container.

[0057] Accordingly, a measured value by the sensor 10 (in this example, the temperature of the sensor bc) in the upper liquid part (in the vicinity of the liquid level) exhibits the relatively lowest temperature compared to measured values of the sensors 10 installed in the other parts.

[0058] <Liquid level detection method 1> From the above, the liquid level is determined as described below in accordance with a liquid level detection method 1. That is, to detect the liquid level without false detection, the position of the sensor 10 whose measured value is the lowest among the plurality of sensors 10, which are installed in the vertical direction on the outer surface of the container, is identified as the liquid level.

Thus, the position of the liquid level can be detected without false detection.

[0059] <Liquid level detection method 2> The liquid level can also be determined as described below in accordance with a liquid level detection method 2. The liquid level detection method 2 is a method in which a position close to the liquid level can be identified as the liquid level even when there are variations in the measured value among sensors.

First, the outline of the liquid level detection method 2 will be simply described below. A threshold value is set in accordance with a method described later, and the liquid level is identified by defining the position of the sensor 10 that is installed at the highest position among the sensors 10 whose measured values are below the threshold value as the liquid level.

[0060] The variations in the measured value among the sensors are caused by a sensor installation method, for example, reasons such as differences in the pressing force among the sensors 10, variations in the surface temperature among the heating elements 102, and differences in the heat resistance between the sensors 10 and the container 9 for each sensor 10. The variations in the measured value among the sensors may also be caused by sensor errors of the temperature measurement elements 103, deterioration overtime, or the like.

[0061] Hereinafter, a point in which the liquid level may be detected with high accuracy even when there are variations in the sensor measured values will be described using specific images.

[0062] Fig. 10 is a diagram illustrating the relationship between sensor measured values (container surface temperatures) after heating is done by the heating elements 102 of the individual sensors 10 and the container height when the refrigerating and air-conditioning apparatus is in operation. The horizontal axis represents temperature and the vertical axis represents container height. Z represents the height of the liquid level inside the container 9. Measured values in a normal state are represented by (a) in which there is no variations in the sensor measured value and the container surface temperature is detected accurately, and measured values with measured value variations ±a from the normal state of (a) are represented by (b) and (c). A threshold value is represented by (d).

[0063] Hereinafter, the reason why the liquid level detection method 2 is effective when there are variations in the measured value among the sensors (in the order of 1 Oa, 1 Ob, and 1 Oc from the bottom) arranged at or below the liquid level position Z, will be described in comparison with the liquid level detection method 1. To make the difference between the liquid level detection method 1 and the liquid level detection method 2 most distinct, an example will be described in which the sensor ba measures Ta, with a measured value shifted toward a -a side (a'), the sensor lOb measures Tb, with the normal measured value (b') obtained, and the sensor 1 Oc measures Tc, with a measured value shifted toward a +a side (e).

[0064] According to the liquid level detection method 1 the position of the sensor whose measured value is the lowest is identified as the liquid level. Since the relationship Ta <Tb cTc is obtained in this case, the sensor position of the sensor lOa is determined as the position of the liquid level.

[0065] On the other hand, according to the liquid level detection method 2, a threshold value is set and the position of the sensor that is installed at the highest position among the sensors whose measured values are below the threshold value is determined as the position of the liquid level. Since a', b', and c' are all below the threshold value in this case, the sensor position of the sensor 1 Oc is determined as the liquid level position.

[0066] Thus, in the liquid level detection method 1, the position of the sensor 1 Oa is determined as the position of the liquid level, whereas in the liquid level detection method 2, the position of the sensor bc is determined as the position of the liquid level. Since the actual position of the liquid level is the position represented by Z in Fig. 10, the liquid level detection method 2 allows a more accurate determination of the position of the liquid level than the other by determining the position close to the actual liquid level as the position of the liquid level, that is, more precise than the other, when there are variations in the measured value among the sensors 10.

[0067] The liquid level detection methods for use in the liquid level detection device lAhave been described above. The liquid level detection device lAis also able to detect, based on sensor measured values, absence of a liquid fluid inside the container. This will be described below.

[0068] In the case where the inside the container is filled with gas with no liquid, the measured values of all the sensors 10 of the container surface temperature tend to be uniformly higher than the container surface temperature of the liquid part in the case where liquid is present in the container. Therefore, by setting a threshold value at a temperature that is lower than the container surface temperature of the gas part and that may be clearly discriminated from the container surface temperature of the liquid part, the absence of liquid fluid in the container can be detected when the measured values of all the sensors 10 are higher than the threshold value.

[0069] Therefore, as a detection method on a specific device, first, at least one of the plurality of sensors 10 is installed at a high position (that is, a part which is definitely a gas part (an upper part of the container)) at which liquid is not accumulated, and the sensor is defined as a reference sensor. Next, a temperature that is lower than the measured value of the reference sensor by a set temperature Ts set beforehand is defined as a threshold value. The position of the sensor 10 that is installed at the highest position among the sensors 10 whose measured values are below the threshold value is determined and output as the liquid level.

[0070] In the case where liquid is absent in the container, the measured values of all the sensors 10 are equal to or higher than the threshold value. Therefore, none of all the sensors 10 corresponds to the sensor 10 that is installed at the highest position among the sensors whose measured values are below the threshold value. Thus, the result in which none of all the sensors 10 corresponds to the sensor 10 that is installed at the highest position among the sensors whose measured values are below the threshold value is output as the position of the liquid level, and a user can therefore determine that the liquid fluid is not present in the container [0071] As described above, the liquid level detection device 1A according to Embodiment is capable of detecting the absence of liquid fluid in the container as well as detecting the liquid level. To achieve such detections, the threshold value is, as described above, set at a temperature lower than the measured value of the reference sensor by the set temperature Ts set beforehand. The set temperature Ts is determined taking into consideration the difference in the container surface temperature between the gas part and the upper liquid part and variations in the measured value among the sensors 10.

[0072] For example, when the difference in the container surface temperature between the gas part and the upper liquid part is 5 degrees Centigrade and measured value variations among the sensors 10 are ±1 degree Centigrade, the set temperature Is is set at 2 to 3 degrees Centigrade to avoid false determination. The reason will be described below.

[0073] Fig. 11 is a diagram illustrating the relationship between measured values after heating is done by the heating elements 102 of the individual sensors 10 and the container height when the refrigerating and air-conditioning apparatus is in operation. The horizontal axis represents temperature and the vertical axis represents container height. "77" represents the height of the liquid level inside the container 9. Measured values in a normal state (normal values) are represented by (i) in which there are no variations in the sensor measured value, and measured values with temperature variations ±1 degree Centigrade from the normal value of (i) are represented by (ii) and (iii).

[0074] The sensors 10 are installed at container heights aa, bb, and cc, and it is assumed that the position of the container height aa is in a liquid part and the position of the container heights bb and cc are in a gas part. Then, the sensor 10 at the container height cc is defined as a reference sensor, and a threshold value based on the measured value of the reference sensor is set for determining the position of the liquid level. The threshold value which does not cause a false determination on gas and liquid even if there are variations in the sensor measured value will be considered.

[0075] First, a threshold value by which the gas part is not falsely determined as the liquid part will be considered. In the case where the reference sensor (the sensor at the container height cc) represents a measured value (that is, 84 degrees Centigrade) lower than the normal value by I degree Centigrade, in order to avoid a false determination that the sensor 10 at the container height bb is positioned in the liquid part, the threshold value is set below the measured value (that is, below 84 degrees Centigrade). By setting the threshold value below the measured value (that is, below 84 degrees Centigrade), even if the measured value of the sensor 10 at the container height bb represents a measured value (that is, 84 degrees Centigrade) lower than the normal value by 1 degree Centigrade, the measured value is still higher than the threshold value.

Therefore, the position of the container height bb is correctly determined as the gas part, without falsely determining the position of the container height bb as the liquid part.

[0076] Furthermore, in the case where the measured value of the reference sensor is higher than the normal value by 1 degree Centigrade (that is, 86 degrees Centigrade), in order to avoid a false determination that the sensor at the container height bb in the gas part is positioned in the liquid part, the threshold value may be set below 84 degrees Centigrade, which is different from the measured value (86 degrees Centigrade) of the reference sensor by less than the variation range temperature (the variation range temperature is 2 degrees Centigrade since the variation range is ±1 degree Centigrade). That is, the threshold value may be set at a temperature lower than a temperature which is lower than the measured value of the reference sensor by 2 degrees Centigrade. By setting the threshold value as described above, even if the measured value of the sensor at the container height bb represents a measured value lower than the normal value by 1 degree Centigrade (that is, 84 degrees Centigrade), the measure value is still higher than the threshold value.

Therefore, the position of the container height bb is not falsely determined as the liquid part.

[0077] Next, a threshold value by which the liquid part is not falsely determined as the gas part will be considered. The sensor 10 positioned at the container height aa has variations in the measured value within a range from 79 degrees Centigrade to 81 degrees Centigrade. In order to avoid a false determination that the sensor 10 with such variations in the measure value is positioned at the gas part, that is, in order to ensure that the measured value of the sensor 10 at the container height aa is below the threshold value, the threshold value is set as described below. That is, the threshold value is set equal to or more than the upper limit value within the variation range (that is, 79 to 81 degrees Centigrade).

That is, if the threshold value is set equal to or higher than 81 degrees Centigrade, the measured value of the sensor 10 at the container height aa is equal to or lower than the threshold value, and accordingly is not falsely determined as the gas part. Thus, when viewed from the measured value of the reference sensor, if the threshold value is at or higher than a value that is lower than the lower limit value (that is, 84 degrees Centigrade) of the variation range of the measured value by 3 degrees Centigrade, the liquid part is not falsely determined as the gas part.

[0078] Summarizing the above, in the case where the difference in the container surface temperature between the gas part and the upper liquid part is 5 degrees Centigrade and a sensor measured value varies within a range of ±1 degree Centigrade, the set temperature Ts may be determined in a range of 2 to 3 degrees Centigrade. The temperature to be set in the range of 2 to 3 degrees Centigrade may be determined in a desired manner by the manufacturer of the liquid level detection device 1A. When it is assumed that the temperature is set at 3 degrees Centigrade, the liquid level detection device IA dynamically sets, at the time of detecting the liquid level, the temperature lower than the measured value of the reference sensor by 3 degrees Centigrade as the threshold value, and outputs the result indicating that the position of the sensor 10 that is installed at the highest position among the sensors 10 whose measured values are below the threshold value is determined as the position of the liquid level. The setting of the threshold value is not limited to the method of dynamically setting at the time of detecting the liquid level based on the measured value of the reference sensor. The threshold value may be set as a fixed value which is set beforehand according to an operation state of the refrigerating and air-conditioning apparatus, that is, according to the temperature of the refrigerant which flows inside the refrigerating and air-conditioning apparatus. However, dynamically setting the threshold value at the time of detecting the liquid level, based on the measured value of the reference sensor, allows the threshold value to be set by taking into consideration multiple variable factors such as an installation state, surrounding environment such as outside wind and the outside air temperature, and the temperature of the refrigerant inside the container.

Therefore, an effect that the liquid level can be detected with a higher accuracy without a false determination can be achieved.

[0079] It can be estimated beforehand around how much difference in temperature of the container surface between the gas part and the upper liquid part will become, and around how many plus or minus degrees Centigrade in variation from sensor measured values will be. Thus, based on the above information, how many degrees Centigrade is to be lower than the measured value of the reference sensor is set, and thereby the threshold value may be determined.

[0080] As described above, an example has been explained above in which the threshold value is set taking into consideration both the difference in the container surface temperature between the gas part and the upper liquid part and the measured value variations among the sensors 10. However, the threshold value may be set by taking into consideration at least the measured value variations among the sensors 10. That is, the threshold value may be set at a temperature lower than the measured value of the reference sensor by a set temperature which is set by taking into consideration the measured value variations among the sensors 10.

[0081] The liquid level detection method 2 has been clearly explained above.

Hereinafter, the flow of a process for liquid level detection by the liquid level detection device lAwill be described below.

[0082] <Liquid level detection flow> Fig. 12 is a flowchart illustrating the flow of a process for liquid level detection by the liquid level detection device 1A according to Embodiment of the present invention. Hereinafter, the flow of the liquid level detection will be described with reference to Fig. 12.

[0083] First, the control measurement device 20 performs data measurement by using all the sensors 10(5101). The measured values (that is, the measured values before heating is done by the heating elements 102) here are used for abnormality detection of the temperature measurement elements 103. Next, the control measurement device 20 performs confirmation of whether or not all the measured values obtained in SlOl are the same (S102). When different measured values are obtained (S102; No), there is a possibility of sensor abnormality such as detachment or disconnection of the sensor 10, and the control measurement device 20 transmits a notification indicating that there is a possibility of sensor abnormality (Si 04).

[0084] On the other hand, when all the measured values are the same (Si02; Yes), the control measurement device 20 performs heating by the heating element 102 of each of the sensors 10 (Si 03). Then, after starting heating by the heating element i02, the control measurement device 20 determines whether or not a certain time (for example, two minutes) has passed (Si05). If the certain time has not passed, the process returns to Si03. If the certain time has passed, heating by the heating element 102 is stopped (Si 06). Then, after heating by the heating element 102 is stopped, the control measurement device performs data measurement by using all the sensors 10 again (Si07). The temperature measurement is performed at this timing because immediately after stopping heating by the heating element 102, the temperature difference between inside and outside the container 9 is the largest, and the difference in heat flux is most distinct between the gas part and the liquid part. That is, the temperature variations among the heating elements 102 appear remarkably.

[0085] Then, the liquid level detection is performed using the measured values obtained in S107 in accordance with the liquid level detection method 1 or the liquid level detection method 2 (SlOB), and the liquid level detection ends.

[0086] As described above, according to Embodiment, since the position of the liquid level is detected by the liquid level detection method 1 or the liquid level detection method 2 mentioned above, the position of the liquid level can be identified even in the case where fluid inside the container flows with the fluid inflow to and outflow from the container. Furthermore, with the liquid level detection method 2, the liquid level may be detected even if there are variations in the sensor measured value, and the liquid level detection may be achieved with high accuracy.

[0087] Modifications described below may be added to the configuration of the liquid level detection device 1A illustrated in Fig. 1. Similar effects can also be achieved by the modifications described below. Hereinafter, modifications will be described sequentially.

[0088] In the flowchart of Fig. 12, an example in which temperature is measured after heating by the heating element 102 is stopped is described. However, temperature measurement is not limited to this. Temperature may be measured before heating by the heating element 102 is stopped. This is because at a temperature in the time range immediately before or immediately after heating by the heating element 102 is stopped, that is, at a point in time when sufficient heating has been performed by the heating element 102 or a point in time immediately after sufficient heating is finished and the influence of the outside air temperature is small, a difference in heat flux is likely to be remarkable between gas and liquid phases.

[0089] The method for determining the liquid level in accordance with the liquid level detection method 1 or the liquid level detection method 2 by using obtained measured values has been described above. However, the method is not limited to the above. The liquid level may be determined by comparing the time periods by the time when the measured values of the temperature measurement elements 103 reach a certain temperature (an index related to measured values by the temperature measurement elements 103). In this case, the determination of whether a sensor is positioned in the gas part or the liquid part is performed by using the fact that the measured values of the sensors 10 corresponding to the gas part and a part below the liquid level are likely to be high at the time of heating by the heating elements 102, while the measured value of the sensor 10 corresponding to the upper liquid part (in the vicinity of the liquid level) is less likely to increase.

[0090] In the example described above, all of the sensors 10 are installed on the side face of the container 9. However, the position of the sensors 10 is not limited to this. The upper liquid part (in the vicinity of the liquid level) may be determined by installing the reference sensor 10 at an upper part of the container 9 and the other sensors on the side face of the container 9 and by using the measured value of the reference sensor and the measured values of the sensors installed on positions other than the upper part of the container 9, in accordance with the liquid level detection method 1 or the liquid level detection method 2.

[0091] Furthermore, heating by the heating elements 102 may be constantly performed. Alternatively, heating by the heating elements 102 may be performed only in the time range during which liquid level detection is performed using the control measurement device 20 and heating may be stopped during the other time ranges. In the case where heating by the heating element 102 is performed only in the time range during which liquid level detection is performed, unnecessary heating can be avoided in the time range during which liquid level detection is not performed.

[0092] For the temperature measurement elements 103 used in the sensors 10, thermoelectric conversion elements or resistance thermometer sensors are used as described above. However, the temperature measurement elements 103 may each have a configuration including a self-heating thermistor which is a self-heating resistor. In the case where self-heating thermistors are used, there is no need to separately provide the heating elements 102 in addition to the temperature measurement elements 103. Furthermore, the use of the self-heating thermistors eliminates signal lines, thereby allowing fabrication of compact sensors. Furthermore, a smaller number of wires enhances the efficiency in the work of installing sensors.

[0093] Furthermore, the container 9 is configured to be heated by the heating elements 102. However, the configuration of the container 9 is not limited to this. For example] in the case of the following (A), (B), or the like, there is no need to provide the heating elements 102: (A) case where there is a large difference in temperature between inside and outside the container; and (B) case where the temperature of fluid outside the container is different from the temperature of fluid inside the container and the flow velocity of the fluid outside the container is high.

[0094] Even in the case of (A) or (B) mentioned above, when the internal fluid flows, the container surface temperature in the vicinity of the liquid level is the lowest, and therefore the position of the sensor whose indicates the lowest temperature can be identified as the position of the liquid level.

[0095] Furthermore, the container 9 is configured to be heated by the heating elements 102. However, the configuration of the container 9 is not limited to this. The container 9 may be configured to be cooled by cooling elements. In the case where the container 9 is cooled, the container surface temperature in the vicinity of the liquid level is the highest. Therefore, in the case where the position of the liquid level is determined in accordance with the liquid level detection method 1, the position of the sensor 10 that indicates the highest temperature can be identified as the position of the liquid level.

[0096] In the case where the container 9 is configured to be cooled by cooling elements and the position of the liquid level is determined in accordance with the liquid level detection method 2, the sensor installed at a part (upper part of the container) which is definitely the gas part is defined as the reference sensor as described above, and a threshold value is set at a temperature higher than the measured value of the reference sensor by a set temperature set beforehand.

The set temperature is set taking into consideration at least variations in the measured value among the sensors 10, as described above. Then, among the sensors 10 which indicate higher temperatures than the threshold value, the position of the sensor that is installed at the highest position may be determined as the position of the liquid level.

[0097] The container 9 installed at a low-pressure side of the refrigerating and air-conditioning apparatus has been described as an example of a target of liquid level detection by the liquid level detection device 1A. However, the target of liquid level detection is not limited to this. The target of liquid level detection may be a container that is installed at a high-pressure side of the refrigerating and air-conditioning apparatus. Even in the case of the container installed at the high-pressure side, liquid level detection can be performed in accordance with a method similar to the method described above.

[0098] In the case of the container installed at the high-pressure side of the refrigerating and air-conditioning apparatus, basically liquid fluid flows into and flows out of the container, and therefore the flow of the liquid part is larger than the case where the container is installed at the low-pressure side and gas fluid flows into and flows out of the container described above. The fact that the container surface temperature of the liquid part is lower than that of the gas pad is the same as the container installed at the low-pressure side.

[0099] Furthermore, even in the case of the liquid level detection of the container installed at the high-pressure side of the refrigerating and air-conditioning apparatus, similar to the container installed at the low-pressure side, there is an issue that] depending on the structure or size of the container, the liquid level cannot be detected only on the basis of the difference in the container surface temperature at the time of heating the container from outside as in a known technique. The case where detection of the liquid level cannot be achieved only on the basis of the difference in the container surface temperature at the time of heating corresponds to, for example, the case where the container is long in a longitudinal direction and has a configuration in which the upper part is affected by inflow fluid while the lower part is not affected by inflow fluid, that is, the case where a flow condition of the fluid differs between the upper part and the lower part of the container, and the like.

[0100] Furthermore, the container surface temperature is also affected by physical properties of the internal fluid. For example, in the case where a liquid with a high viscosity is accumulated in the container, the lower part of the container is less susceptible to the inflow fluid. Therefore, while in the gas part the flow velocity increases and that helps to increase the heat transfer rate, in the lower liquid part the flow velocity is unchanged and the heat transfer rate is unchanged. Accordingly, the temperature difference of the container surface temperature between the gas part and the lower liquid part decreases or the temperature of the gas part becomes lower than the temperature of the lower liquid part.

[0101] Thus, even the container installed at the high-pressure side of the refrigerating and air-conditioning apparatus has an issue similar to that of liquid level detection for the container installed on the low-pressure side. This issue can be solved by performing liquid level detection in accordance with the method described above.

[0102] Furthermore, in Embodiment described above, the container that is used for the refrigerating and air-conditioning apparatus and that stores refrigerant has been described as the target of liquid level detection. However, the target of liquid level detection is not limited to the above. Any type of container that is capable of storing liquid may be used as the target of liquid level detection. The liquid level detection device 1A according to Embodiment is effective in particular for use in liquid level detection in the case where internal liquid flows.

Reference Signs List [0103] 1A: liquid level detection device, 9: container, 9a: inlet pipe, 9b: outlet pipe, 9c: oil return hole, 9d: suction port, 10: all sensors, 10 (ba to lOd): sensor, 16: insulating material, 20: control measurement device, 102: heating element, 103: temperature measurement element, 201: heating element control unit, 202: sensor measurement unit, 203: storing unit, 204: liquid level detection unit, 205: input unit, 206: output unit.