REFRIGERATION SYSTEM CONTROL AND PROTECTION DEVICE

Abstract

A device to protect a compressor against liquid flooding, oil heater malfunction, low refrigerant charge, high superheat. The system includes a device that measures two temperatures separated by a heat source (the electric compressor or the suction heat exchanger or both). The temperature difference can detect a liquid return to the compressor, a high superheat, a low refrigerant charge or a crankcase heater malfunction and the temperature difference can control the electronic expansion valve.

Claims

1. A multi-functional refrigeration system protection device comprising: a sensor that measures the temperature just before compression, another sensor that measures the temperature at the suction line or upstream of the suction gas heat exchanger, a device that measures the difference in temperature (DT) between the two sensors, capable of generating one or more signals to perform one or more functions such as stopping the compressor, or trigger an alarm, or a defrost cycle, or a superheat alarm, or generating a signal to indicate that the system is running safely or controlling the electronic expansion valve.

2. A refrigeration system protection device according to claim 1 capable of generating a signal to stop the compressor in case the difference in temperature (DT) is less than 25% of the normal running temperature difference (NTD), and preferably in case the difference in temperature (DT) is less than (UTD).

3. A refrigeration system protection device according to claim 1 capable of generating a signal to trigger an alarm in case the difference in temperature (DT) ranges between 25% and 50% of the normal running temperature difference (NTD), and preferably in case the difference in temperature (DT) is between (ATD) and (UTD).

4. A refrigeration system protection device according to claim 1 capable of generating a signal to trigger a defrost cycle in case the difference in temperature (DT) ranges between 50% and 75% of the normal running temperature difference (NTD), and preferably in case the difference in temperature (DT) is between (DTTD) and (ATD).

5. A refrigeration system protection device according to claim 1 capable of generating a signal to trigger a superheat alarm in case the difference in temperature (DT) ranges between 125% and 150% of the normal running temperature difference (NTD), and preferably in case the difference in temperature (DT) is between (OTD) and (UOTD).

6. A refrigeration system protection device according to claim 1 capable of generating a signal to stop the compressor in case the difference in temperature (DT) is greater than 150% of the normal running temperature difference (NTD), and preferably in case the difference in temperature (DT) is greater than (UOTD).

7. A refrigeration system protection device according to claim 1 capable of generating a signal to indicate that the system is running safely if the difference in temperature (DT) ranges between 75% and 125% of the normal running temperature difference (NTD), and preferably in case the difference in temperature (DT) is between (OTD) and (DTTD).

8. A refrigeration system protection device according to any of the preceding claims, fitted with a timer in order to delay the monitoring of the difference in temperature (DT).

9. A device for controlling the electronic expansion valve comprising a PID controller and a refrigeration system protection device according to any of the preceding claims, by maintaining a difference in temperature (DT) close to the normal running temperature difference (NTD).

10. A refrigeration or heat pump system comprising a device according to any of the preceding claims.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The various features of the present invention and the manner of attaining them will be described in greater detail with reference to the following description, claims, and drawings, wherein reference numerals are reused, where appropriate, to indicate a correspondence between the referenced items, and wherein:

[0051] FIG. 1, shows a convenient position of the sensors on a semi-hermetic compressor. The upstream temperature sensor is preferably installed on the suction line as far as possible from the compressor while preferably staying in the same ambient temperature of the compressor. In case the semi-hermetic compressor is equipped with a suction gas heat exchanger, the upstream temperature sensor is preferably installed upstream of the suction gas heat exchanger.

[0052] FIG. 2, shows a convenient positioning of the sensors on a hermetic compressor. The upstream temperature sensor is installed on the suction line as far as possible from the compressor while preferably staying in the same ambient temperature of the compressor.

[0053] FIG. 3, shows positioning of the sensors in an open compressor using a suction gas heat exchanger.

[0054] FIG. 4, shows a suction gas heat exchanger for compressors without crankcase and crankcase heater (i.e. open type screw compressors), prewired to be installed on the suction line.

[0055] FIG. 5, shows a miniaturized bypass with two sensors and a small heater to be connected to the invention device and to simulate a heat source. This setup is to be used specially for open compressors where a suction gas heat exchanger is not recommended. Only a portion of the gas stream will be heated. The position of the inlet is recommended to be after an elbow to collect effectively by centrifuge the liquid droplets. The heater could be an electric resistance of 20 Watts or less. Its power could be calculated to heat the diverted gas a maximum of 15 C.

[0056] FIG. 6, shows the definitions and the graphical presentation of the parameters used in the specification of the present invention.

[0057] FIG. 7, shows an example of a control algorithm for the present invention. It shows the definition of some variables used in the controller program and their sequence compared with the normal running temperature difference.

[0058] FIG. 8, shows the performance table of a Bitzer semi-hermetic compressor. The table is generated by Bitzer selection software. The DT is the temperature increase of the refrigerant gas thru the electric motor.

[0059] FIG. 9, shows the Temperature difference with an 80% efficiency electrical motor.

[0060] FIG. 10, shows the Temperature difference with a 95% efficiency electrical motor.

[0061] FIG. 11 shows the experimental data of the temperatures at compressor inlet and piston inlet according to a study conducted at Purdue University, Thermal Analysis of a Hermetic Reciprocating Compressor. Authors: A. Cavallini, L. Doretti, G. A. Longo, L. Rossetto, B. Bella, and A. Zannerio published in the International Compressor Engineering Conference on 1996.

[0062] FIG. 12, shows DT across suction gas heat exchanger; using HE 8.0 Danfoss suction heat exchanger, connected to 4DC-5Y Bitzer compressor, condensation at 30 C. and using refrigerant R410A.

[0063] FIG. 13, shows suction gas heat exchanger selection software output with gas and liquid parameters at entrance and exit.

[0064] FIG. 8, is an example of controller settings depending on the type of compressor and the compressor working range.

[0065] FIG. 9, shows some examples of the possible position of the upstream and downstream temperature sensors. The evaporator superheat is what is measured by the expansion valve. Again all the temperatures can vary depending on the system configuration and system running conditions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0066] The present invention will be further understood from the following description given by way of example only. The invention consists of two sensors positioned for example as shown in FIG. 1, FIG. 2, or FIG. 3.

[0067] A temperature sensor that measures the temperature just before compression, referred to as downstream temperature sensor.

[0068] Another temperature sensor that measures the temperature at the suction line, referred to as upstream temperature sensor.

[0069] A device that monitors difference in temperature (DT) between the two sensors and stops the compressor when the temperature difference drops to a predetermined set point (UTD).

[0070] The monitoring of the temperature difference of the refrigerant gas is made when the gas flows:

[0071] In case of semi-hermetic or hermetic compressor, the monitoring of the difference in temperature (DT) is when the refrigerant gas goes thru the compressor electrical motor and inside the compressor casing. When the compressor is equipped with a gas heat exchanger, the monitoring is made thru both of them.

[0072] In case of an open type compressor, the monitoring of the difference in temperature (DT) is thru a suction gas heat exchanger.

[0073] In the above two cases the temperature rise in normal operation between the two sensors can be beyond 35 C. for the hermetic and the semi-hermetic compressors (see FIG. 8 and FIG. 11), and beyond 10 C. for the suction gas heat exchanger (see FIG. 12). This increase in temperature depends on the system operating range, the electric motor efficiency and the refrigeration components selection.

[0074] In case of liquid flood-back to the compressor, the difference in temperature (DT) between the two sensors drops to zero. This is due to the fact that the heat added to the gas stream is evaporating the liquid droplets instead of heating the gas. As long as there are liquid droplets in the gas stream, the gas temperature will not rise between the two sensors. This substantial variation in temperature, up to 30 C. or more, occurring between the desired dry refrigerant gas condition, and a liquid floodback condition (i.e. wet refrigerant gas condition containing non evaporated liquid to the compressor), can be easily detected due to the drastic change in temperature between the two states.

[0075] All the embodiments have in common two sensors separated by a substantial heat source preferably inherent to the system. The difference in temperature (DT) is monitored by the device according to the present invention to detect the saturation condition of the gas at the downstream sensor. This sensor is installed close to the internal suction port of the compressor.

[0076] The first embodiment consists of a device with one level of temperature difference including a relay that will shutdown the compressor when the difference in temperature (DT) drops to the (UTD) value. This is the simplest embodiment.

[0077] A second embodiment is adding a second level of temperature difference including a relay that will send an alarm when the difference in temperature (DT) drops to the (ATD) value.

[0078] A third embodiment is adding a third level of temperature difference including a relay that will send an alarm when the difference in temperature (DT) reaches the (OTD) value. This excessive superheat could indicate in general a low refrigerant charge, a thermal expansion valve malfunction or any restriction on the refrigerant circuit.

[0079] A fourth embodiment is adding a fourth level of temperature difference including a relay that starts a defrost cycle when the difference in temperature (DT) reaches the (DTTD) value. This embodiment is useful in refrigeration and in heat pump systems.

[0080] A fifth embodiment is adding a fifth level of temperature difference including a relay that signals a safe operation of the compressor when the difference in temperature (DT) ranges between (DTTD) and (OTD) values.

[0081] A sixth embodiment is adding a sixth level of temperature difference including a relay that stops the compressor when the difference in temperature (DT) reaches the (UOTD).

[0082] For an open compressor with a crankcase and an oil heater. See FIG. 3. In order to be able to use the same device according to the invention, a heating source is needed to replace the heat dissipated by the electric motor. Normally a suction gas heat exchanger can increase the suction gas temperature by at least 5 C. at running design conditions. See FIG. 13 and FIG. 12.

[0083] For open compressors without crankcase (like open screw compressors, with an external oil separator and oil tank and an external oil heater), a prewired suction gas heat exchanger with the downstream temperature sensor and a small heater at one end, and the upstream temperature sensor at the other end could be used. See FIG. 4. This small heater will provide the necessary temperature difference to allow the compressor to start. It can eventually be replaced by a timer to bypass the controller imbedded in the device according to the present invention for a certain time (DBCP, to ensure that the temperature between the two sensors reaches its normal running value after the compressor starts). The timer has the same function as the one used in the oil differential controller for protecting the refrigeration compressors in case of lubrication failure. The heater alternative gives better results than a timer, because in case there is a liquid flood-back to the compressor, the temperature will drop quickly and the controller will stop the compressor without delay. If a timer is used, the controller will have to wait till the end of the timer to stop the compressor.

[0084] This same embodiment can be used for a cooling system where an additional superheat due to the use of the suction gas heat exchanger is not recommended (i.e. cooling system in a car). The compressor in a car is subject to high evaporation and condensation temperatures. To overcome this limitation, a bypass can be installed in parallel to the main suction gas pipe, see FIG. 5. This will reduce the superheat compared to a full flow suction gas heat exchanger and enables the use of the embodiment of the present invention. Note that the electric resistance is installed next to the downstream sensor to create enough temperature difference to avoid the use of a start-up timer as explained above.

[0085] For all above embodiments except the first one that stops the compressor, it is preferable to add a timer for each embodiment, or one general timer for all. The purpose of this timer is to provide a delay after the compressor starts, to suspend the difference in temperature (DT) monitoring. This will ensure that the monitoring for all other embodiments starts when the system is running at normal running conditions. Each timer can be adjustable from few seconds to few minutes depending on the refrigeration system configuration. This is very simple to implement using a microcontroller such as Siemens Logo 8 series. See FIG. 7. In this algorithm one general timer is used.

[0086] Examples of more sophisticated controller responses are:

[0087] If the difference in temperature (DT) has not reached the set temperature for compressor cutoff (UTD), but the temperature is decreasing at a fast rate (e.g. one degree per second), it stops the compressor.

[0088] If the difference in temperature (DT) is persistent for a long time close to the (UTD) (e.g. 5 minutes at 5% above set temperature), it stops the compressor.

[0089] This can be easily programmed using a microcontroller such as Siemens Logo 8 series or an OEM microcontroller embedded in the device. All these parameters could be adjustable for a specific compressor model working in a specific refrigeration range.

[0090] An extra embodiment to control the expansion valve in a single evaporator system, can be included in the setup shown in FIG. 4. Suction gas heat exchanger for compressors w/o crankcase. A (PID) circuit using the same two temperature sensors can be added to the device according to the present invention to control the expansion valve by maintaining a temperature difference between the two sensors, close to the (NTD) (the normal temperature difference across this device).

[0091] The extremely low pressure drop of the gas stream across the suction heat exchanger gives a better result for controlling the expansion valve than measuring the superheat across the evaporator using two thermal sensors, one at the evaporator inlet and one at the evaporator outlet. The substantial pressure drop across the evaporator decreases the accuracy of the evaporator superheat reading.

[0092] This is the reason why in order to determine exactly the superheat across the evaporator to control an electric expansion valve, a pressure sensor is normally used near the temperature sensor at the evaporator exit, or in case of a mechanical thermal expansion valve, a pressure equalizer line is used.

[0093] All above embodiments can be integrated in one device with one single power supply and a microcontroller with two analogue inputs, one for each thermal sensor, and multiple outputs one for each selected embodiment. Also the device can be fitted with two digit LED display to indicate the difference in temperature (DT). A more sophisticated display can be programmed by the microcontroller to show all the parameters in sequence and alarms status. Also a log of all the last events with a time stamp can be either scrolled or downloaded.

[0094] FIG. 7 shows an example of a control algorithm for the proposed invention. The programmable controller will first check if the compressor is off. In this case, the device according to the present invention checks if the difference in temperature (DT) (the measured temperature difference), is higher than the (MTDC) (the minimum temperature difference that a running crankcase heater should bring between the two sensors when the compressor is not running).

[0095] If the difference in temperature (DT) is not higher than (MTDC), the relay to stop the motor will be kept in its off position for a predetermined time i.e. 10 minutes. If (DT) is higher than (MTDC), the controller program will be directed to the program start.

[0096] If the compressor has started, the controller will immediately start checking if the difference in temperature (DT) is greater than the (UTD), if not, the controller will shut down the compressor immediately for a certain time i.e. 5 minutes. If (DT) is greater than the (UTD), the controller will start (DBCP) delay timer and will wait for this timer to end. Meanwhile the controller will keep checking if (DT)>((UTD).

[0097] Once the (DBCP) timer is over, the controller will check if (DT) is greater than the (OTD), in this case, the controller will signal a high superheat alarm, and can also shutdown the motor if desired. If (DT) is less than (OTD), the controller will check if the (DT) is greater than the (DTTD) (the defrost triggering temperature difference). In case (TDT) is less than (OTD) the controller will indicate that the system is running normally.

[0098] If the difference in temperature (DT) is less than (DTTD), the controller will check if the (DT) is greater than the (ATD) (alarm temperature difference), if Yes it will check if the (TSLD) (Time since last defrost) is greater than the (MTBD) (minimum time between two consecutive defrost cycles) if Yes, it will trigger a new defrost cycle.

[0099] If the difference in temperature (DT) is less than the (ATD) (the alarm temperature difference) the controller will check if the (DT) is greater than the (UTD) (indicating a dangerously low superheat). If Yes, it will trigger an alarm indicating a dangerously low superheat. If No, it will shutdown the compressor.

[0100] All parameters are adjustable, depending on the compressor type, working range and the temperature sensor positions. The difference in temperature (DT) can be set as a function of the incoming gas temperature measured by the upstream temperature sensor. To make the setting of the parameters easier, a two-digit display could be added to the device according to the present invention to show the measured temperature difference. Once the refrigeration system has reached its normal running conditions, the temperature can be recorded and used for setting up all set-points as shown in the legend of FIG. 6.

[0101] A short way to adjust the set point for the different temperatures (UTD), (DTTD), (ATD), as defined in paragraphs above, is to divide the (NTD) into four equal parts in order to maximize the gap between each setting. The (UTD) can be set at 25%, the (ATD) at 50% and the (DTTD) at 75% of the (NTD) value.

[0102] In case the refrigeration system is not equipped for a defrosting cycle, the (NTD) can be divided into three equal parts. The (UTD) can be set at 33% and the (ATD) at 66% of the (NTD).

[0103] With the same logic, the (OTD) can be set at 125% of the (NTD) value and the (UOTD) can be set at 150% of the (NTD) value.

[0104] With system observation these percentages values can be fine-tuned by the manufacturer by following the setting recommendations as explained in paragraphs above.

[0105] Moreover, the (UTD) and the (UOTD) can be replaced by timers that will stop the compressor if the corresponding alarms (ATD) and (OTD) persist for i.e. 5 minutes.

[0106] FIG. 8 summarizes all discussed parameters settings at different ranges (Air conditioning, cold storage and freezer) using semi-hermetic compressors fitted with different electric motor efficiencies. Still for optimum performance, these values should be checked by bench testing the refrigeration machine.

[0107] For better regulation for large compressors, a low precision pressure sensor can be added in order to change the set point according to the suction pressure that defines the working range of the compressor (High pressure, medium pressure or low pressure) equivalent to (Air-conditioning range, cold-storage range or freezer range).

[0108] In both cases, whether the temperature or the pressure sensors are used, their primary function is to detect whether the compressor is working in the freezer range where the temperature difference is expected to be high, or in the cold storage range where the temperature difference is expected to be medium, or in the air conditioning range where the temperature difference is expected to be minimal.

[0109] In any case all setting points should be based on actual measurements of the refrigeration system running at design temperatures.

[0110] The temperature difference, especially in case of hermetic compressors, is difficult to predict due to the gas flow passageways and compressor internal configuration. Each compressor model should be tested at normal running conditions and the normal running temperature difference should be recorded.

[0111] Furthermore, the sensor's position on the compressor can also be optimized depending on compressor models. Especially in hermetic compressors, where the downstream temperature sensor can be factory installed close to the piston inlet valve.

[0112] In order to have a reliable non drift measurement system, the temperature difference can be measured by two temperature sensors connected in a one Wheatstone bridge configuration, or by using two thermocouples connected in series.

Advantages Compared to Prior Art

[0113] Protection against liquid flooding by detecting liquid in the refrigerant gas downstream of all heat producing components (i.e. electric motor in case of hermetic and semi-hermetic compressors, piston body in case of hermetic compressors, and suction gas heat exchanger, in case of an open compressor). All those heat producing components are capable of evaporating a great amount of liquid and protect the compressor in case there is not much liquid in the gas stream. This will prevent frequent non-critical compressor shutdown in comparison to a system that checks the gas condition upstream of the compressor.

[0114] Simple and accurate detection of liquid floodback with a device consisting of two temperature sensors with one comparator and one relay. There is no need for sophisticated electronics and start-up timers. The cost can be so low, that it can be installed even on the cheapest small compressors.

[0115] The main measurement is differential using two thermocouples or any two thermal sensors installed in a single Wheatstone bridge. A differential measurement is less prone to drift with time.

[0116] In case a pressure sensor and a temperature sensors are used to measure the refrigerant gas superheat, the pressure sensor should be capable to measure with a precision of 0.1 bar and yet should be capable to resist a pressure up to 20 bars and at varying temperatures from 40 to +20 C. without drift with time. The total error is the sum of the errors coming from the pressure sensor, the error coming from the temperature measurement, and the error from the pressure temperature saturation table or function.

[0117] No need for periodic calibration. In the device according to the present invention the main temperature measurement is a temperature difference, known to be very stable with time.

[0118] No need for expensive temperature sensors, or expensive electronic comparators. One or two degrees' Celsius error in the measurement will not reduce the effectiveness of the protection feature of the device.

[0119] The device according to the present invention runs with different refrigerants without having to input refrigerant saturated pressure-temperature tables, or refrigerant saturated pressure-temperature function. This is due to the fact that the saturation condition is depicted if the difference in temperature (DT) is zero.

[0120] This is true for any refrigerant either single component or a mixture.

[0121] The device according to the present prevents the compressor from running in case of crankcase heater failure. One protection device even in its simplest embodiments is protecting the compressor against liquid return to the compressor and crankcase heater malfunction. By judiciously installing the two sensors, and in case there is no temperature difference between the two sensors when the compressor is not running due to the crankcase heater failure. The device will prevent the compressor from running.

[0122] It is even possible to use a mechanical differential thermostat, like the mechanical mechanism used in the Trafag DTS 391, and to embed it inside the compressor. In this case no need for electrical power to run the device. This is similar to the mechanical thermal protection installed on most compressors to protect the electrical motor coil, and in some mono-phase compressors, the electric contact is in series with the motor coil, and all is wired inside the compressor.

[0123] The device according to the present invention can be used to trigger the defrost cycles much more efficiently since the invention device is monitoring the result of the ice buildup. Usually, a defrost cycle is triggered:

[0124] By a clock independently of the system condition. In this case many defrost cycle will be triggered early or too late. The clock or fixed timer is used very often in refrigerators and freezers.

[0125] By a low evaporation pressure pressostat based on the low pressure which is not always an indication to start a defrost cycle. Because the low pressure could be due to a low fluid temperature thru the evaporator or a low refrigerant charge.

[0126] By an ice thickness controller, knowing that the ice thickness could be uneven and the ice thickness can give an erroneous indication to trigger a defrost cycle.

[0127] The device according to the present invention can detect an excessive superheat condition and can send an alarm or even shutdown the compressor, if desired. The compressor shutdown can be set at a higher superheat condition than the alarm set point, or by using a timer if the alarm condition persists for more than a certain predetermined time. (i.e. 5 minutes).

[0128] This is an added protection to the discharge temperature and motor winding temperature protections which are, in almost all compressors, installed with a fixed setting. The setting is fixed at the maximum temperature that either the compressor discharge valve, refrigerant oil or the electric motor winding can tolerate. In the device according to the present invention the (OTD) value is adjusted according to the refrigeration system designed operating temperatures. In most cases the refrigeration system designed operating temperatures are lower than the maximum operating temperatures of the compressor. Using the parameters of the system designed operating temperatures will give the opportunity to send an alarm or even shut-down the compressor before reaching excessive temperatures at the discharge valve or at the motor windings. For example, the same semi-hermetic compressor can be used in a freezer system and in a chiller system. The discharge temperature and motor winding protection are set by the manufacturer at the freezer operating temperatures, in general more than 120 C. When the compressor is used as a chiller the discharge temperature can be set less than 100 C., and in case the temperature exceeds 100 C., this means that there is something wrong with the system and the system should be checked.

[0129] The device according to the present invention can extend the low temperature range of compressors, especially the hermetic and semi hermetic compressors. When the compressor is working at low evaporation temperature, thus at low evaporation pressure and reduced mass flow of refrigerant (to cool-down the electric motor), a high superheat will increase dangerously the discharge temperature and the electric motor winding temperature. By controlling the superheat near the inlet valve of the piston, the superheat can be minimized. A low superheat will decrease the discharge temperature and the motor winding temperature. To get the benefit of this feature, the embodiment with a PID to control the expansion valve should be used.

INDUSTRIAL APPLICATIONS

[0130] This invention can be mainly used in refrigeration and heat pump systems. Examples of refrigeration systems are:

[0131] Refrigerators

[0132] Split system air conditioners, cooling and heat pump

[0133] Chillers

[0134] Cold stores and freezers

[0135] Blast coolers and blast freezers

[0136] Water coolers and ice making machines

[0137] Car air conditioning systems

[0138] It is to be understood that the specific embodiments of the invention that have been described are merely illustrative of certain applications of the principle of the present invention. Numerous modifications may be made to the present instruments and methods described herein without departing from the spirit and scope of the present invention.