A Method for Measuring the Energy Levels of Screw Compressors for Refrigeration Air Conditioning Units

Abstract

The present invention discloses a method for measuring the energy level of a screw compressor for a refrigeration and air-conditioning unit. By collecting the evaporation temperature, condensation temperature, actual operating current and voltage of the refrigeration and air-conditioning unit during the partial load operation of the screw compressor, and according to the comparison relationship between the measured current during the partial load operation of the screw compressor and the assumed full load operating current under the same working conditions, the current energy level of the screw compressor is measured. The method of the present invention can avoid the energy level measurement error caused by the change of the operating current with the evaporation temperature and the condensation temperature, ensure the accuracy of the energy level measurement (the deviation is less than 2%), and can effectively solve various problems caused by the mismatch between the output cooling capacity and the actual required cooling capacity. At the same time, the present invention does not require additional hardware specifically for measuring the energy level of the screw compressor, which can not only reduce the cost of measuring the energy level of the screw compressor, but also ensure the reliable and stable operation of the refrigeration and air-conditioning unit.

Description

Technical Field

[0001] The present invention belongs to the technical field of control of refrigeration and air-conditioning units, and particularly relates to a method for measuring the energy level of a screw compressor for a refrigeration and air-conditioning unit. Background Art

[0002] Known refrigeration and air-conditioning units (hereinafter referred to as refrigeration units) are all equipped with a compressor, a condenser, a throttle valve and an evaporator. The working process is that the compressor sucks in the low-pressure refrigerant vapor coming out of the evaporator and compresses it into high-pressure vapor, forms high-pressure liquid through the condenser, becomes low-pressure liquid after passing through the throttle valve and is sent into the evaporator, absorbs heat and evaporates in the evaporator to become low-pressure vapor, and then is sent into the inlet of the compressor, thus completing the refrigeration cycle. At the same time, the refrigeration unit is also equipped with a control system such as an evaporation pressure sensor (high-range pressure sensor), a condensing pressure sensor (low-range pressure sensor), a current transmitter, a voltage transmitter and a control module to monitor its operating state in a timely manner. Existing refrigeration units basically measure the real-time operating energy level of the compressor, and then the control system gives appropriate compressor loading and unloading instructions according to the difference between the current operating energy level (output cooling capacity) and the actual required cooling capacity, so as to avoid adverse situations such as increased operating energy consumption caused by excessive cooling capacity output of the unit or / and alarm shutdown triggered by excessive reduction of the temperature of the controlled object, and at the same time can also prevent the phenomenon that the cooling capacity output of the unit is too small to meet the cooling demand.

[0003] At present, large and medium-sized refrigeration units equipped with screw compressors are widely used in industrial refrigeration and central air-conditioning fields. Screw compressors mainly have two structural forms: semi-hermetic screw compressors and open-type screw compressors. Since the upper and lower limit positions of the energy regulation slide valve of the screw compressor are determined by the compressor structure, no matter what energy level measurement method is adopted in the prior art, it is not difficult to determine the 100% energy level and the lowest energy level of the compressor, but there are different degrees of defects in the measurement of the intermediate energy levels between the 100% energy level and the lowest energy level of the screw compressor.

[0004] The energy control methods of semi-hermetic screw compressors are mainly divided into two types: stepped regulation and stepless regulation. The stepped energy regulation method generally sets multiple oil return pipelines at appropriate positions of the piston cylinder that controls the movement of the slide valve. Solenoid valves are installed in the oil return pipelines. When the solenoid valves are energized and opened, the lubricating oil in the piston cylinder will be discharged to the suction side of the compressor through this circuit. At this time, the piston drives the slide valve to move to a predetermined position, and the control system judges the compressor energy level according to the pre-set solenoid valve position data. This energy level measurement method has the following two drawbacks: First, after the solenoid valve corresponding to the energy level at a certain position is energized, since the tightness between the sealing ring and the inner wall of the piston cylinder of the compressor energy level slide valve is unknown, the time required to move to the predetermined position cannot be predicted (according to actual test experience, the required time may range from within 10 seconds to more than 8 minutes), and even the compressor energy level slide valve may be stuck in a certain part and unable to move. That is, the energization of the solenoid valve corresponding to the energy level at a certain position does not mean that the current energy level of the compressor has reached the predetermined energy level, and the accuracy of energy level measurement cannot be judged; Second, the compressor energy level can only be adjusted in stages, generally divided into four stages: 25%, 50%, 75%, and 100%. The discontinuous energy level makes it difficult to accurately calculate the output cooling capacity of the unit, resulting in various problems caused by the mismatch between the output cooling capacity and the actual required cooling capacity. The stepless energy regulation method generally calculates the energy level of the compressor according to the current. The specific method is that the control system compares the measured current of the compressor with the pre-set maximum current and minimum current of the compressor. When the measured current is not less than the maximum current setting value, it is judged that the compressor is at 100% energy level; when the measured current is not greater than the minimum current setting value, it is judged that the compressor is at the minimum energy level; when the measured current is between the maximum current and the minimum current setting values, the linear interpolation method is used to calculate the current energy level of the compressor. However, the results of theoretical analysis and actual tests both show that even if the compressor maintains a certain fixed energy level unchanged, the operating current of the compressor will change with the changes in the condensation temperature and evaporation temperature of the refrigeration unit. For example, when the condensation temperature increases or decreases by 1°C, the operating current of the compressor will increase or decrease by about 2%. Therefore, there is a large error in measuring the compressor energy level only according to the current, which has a greater impact on the control accuracy of the unit energy level.

[0005] Open-type screw compressors generally adopt stepless energy regulation methods. Their energy level measurement devices mainly include components such as spiral tubes, angular displacement sensors, and linear displacement sensors. The spiral tube converts the linear displacement of the slide valve into an angular change. The angular displacement sensor feeds back the angular change to the control system in the form of current or voltage signals. Alternatively, the linear displacement of the slide valve is detected by linear displacement sensors, and the displacement change is fed back to the control system in the form of current or voltage signals. The minimum value of the current or voltage signal corresponds to the minimum energy level of the compressor, the maximum value corresponds to the 100% energy level of the compressor, and the intermediate value between the maximum and minimum values corresponds to the intermediate energy level between the 100% energy level and the minimum energy level of the compressor according to a linear relationship. This energy level measurement method has many components and low reliability. Once faults such as the spiral tube getting stuck or the angular displacement sensor or linear displacement sensor being damaged occur, it will affect the stable operation of the compressor. At the same time, through actual measurement, it is found that only the 100% energy level and the minimum energy level are accurate. The measured values of the intermediate energy levels often deviate greatly from the actual energy levels. When the energy level is relatively high, the deviation between the measured energy level and the actual energy level can even reach more than 20%. For example, when the measured energy level is 90%, the actual energy level is only 70%; when the measured energy level is 85%, the actual energy level is only 60%. The large deviation between the measured energy level and the actual energy level makes the measured energy level unable to be used to calculate the cooling capacity of the unit group, and can only be used to judge the result of the loading and unloading action by measuring the change amplitude of the energy level. Summary of the Invention

[0006] The present invention is to solve the above-mentioned technical problems existing in the prior art, and provides a method for measuring the energy level of a screw compressor for a refrigeration and air-conditioning unit.

[0007] The technical solution of the present invention is as follows: A method for measuring the energy level of a screw compressor for a refrigeration and air-conditioning unit is carried out according to the following steps:

[0008] Step 1. Collect the evaporation temperature te , condensation temperature tc , actual operating current at part load IA_p and voltage V_C of the refrigeration and air-conditioning unit when the screw compressor is operating at part load;

[0009] Step 2. Taking the evaporation temperature te and condensation temperature tc as operating condition variables, calculate the theoretical current value IA_B when the screw compressor operates at full load assuming the refrigeration and air-conditioning unit operates at the corresponding operating conditions;

[0010] Step 3. Collect the evaporation temperature te0 , condensation temperature tc0 , actual operating current at full load IA_f and voltage V_C ;

[0011] Step 4. Using the evaporation temperature te0 and the condensation temperature tc0 as the operating condition parameters, determine the full-load theoretical current value IA_B0 of the screw compressor when the refrigeration and air-conditioning unit operates under this condition;

[0012] Step 5. According to the formula IA_C0 = IA_B0 * 380 / V_ C , obtain the full-load actual calculated current value IA_C0 of the screw compressor when the refrigeration and air-conditioning unit operates under this condition;

[0013] Step 6. Determine whether the deviation between the full-load actual operating current value IA_f of the screw compressor and the full-load actual calculated current value IA_C0 exceeds 4%;

[0014] If not, obtain the full-load actual calculated current value IA_C of the screw compressor when the refrigeration and air-conditioning unit is assumed to operate under the corresponding condition according to Formula ①;

[0015] IA_C = IA_B * 380 / V_C ①

[0016] If yes, obtain the full-load actual calculated current value IA_C of the screw compressor when the refrigeration and air-conditioning unit is assumed to operate under the corresponding condition according to Formula ②;

[0017] IA_C = c0 * IA_B * 380 / V_C ②

[0018] The said c0 is the correction coefficient, c0 = IA_f / ( IA_B0 * V_ C / 380 )

[0019] Step 7. Calculate the current energy level u of the screw compressor according to Formula ③;

[0020] u = E * s 2 +F * s + G ③

[0021] In the formula s = IA_p / IA_C ,E , F , G is a set of coefficients that meet the calculation accuracy determined during the process of fitting formula ③;

[0022] Step 8. Except for the following several situations, the calculated energy level value u is used as the measured energy level value;

[0023] When the calculated energy level value u is greater than 100%, set the measured energy level value to 100%;

[0024] When the calculated energy level value u = 95 - 100% and after continuously executing the load increasing instruction 3 times, judge whether the change in the actual operating current is less than 1%. If yes, set the measured energy level value to 100%. Otherwise, the calculated energy level value u is used as the measured energy level value;

[0025] When the calculated energy level value u is less than the minimum energy level + 5% and after continuously executing the load decreasing instruction 3 times, repeat steps 1 - 7 to calculate the energy level u＇ , and judge u and u＇ whether the difference is less than 1%. If yes, set the measured energy level value to the minimum energy level. Otherwise, the calculated energy level value u is used as the measured energy level value.

[0026] The theoretical current value of step 2 IA_B is calculated according to the following formula:

[0027]

[0028] where c1 - c10 is a coefficient related to the compressor displacement.

[0029] The theoretical current value of step 4 is IA_B0 substitute IA_B , te0 substitute te, tc0 substitute tc

[0030] Calculated according to the formula of the theoretical current value IA_B .

[0031] The present invention measures the current energy level of a screw compressor by collecting the evaporation temperature, condensation temperature, actual operating current, and voltage of a refrigeration and air-conditioning unit during the part-load operation of the screw compressor. According to the comparison relationship between the measured current during the part-load operation of the screw compressor and the assumed full-load operating current under the same operating conditions, the current energy level of the screw compressor is determined. The method of the present invention can avoid the energy level measurement error caused by the variation of the operating current with the evaporation temperature and condensation temperature, ensure accurate energy level measurement (the deviation is less than 2%), and effectively solve various problems caused by the mismatch between the output cooling capacity and the actual required cooling capacity. At the same time, the present invention does not require additional hardware specifically for measuring the energy level of the screw compressor, which can not only reduce the cost of measuring the energy level of the screw compressor but also ensure the reliable and stable operation of the refrigeration and air-conditioning unit. Description of the Drawings

[0032] Figure 1 is the compressor energy level fitting diagram of the embodiment of the present invention. Detailed Embodiments

[0033] a. First, before the formal energy level measurement, select a screw compressor with moderate cooling capacity as the basic compressor (for example, select a semi-hermetic screw refrigeration compressor with a displacement of 1130 m³ / h). Based on standards such as "EN12900 Refrigerant Compressors - Rating Conditions, Tolerances and Presentation of Performance Data by the Manufacturer" and "ARI540 Performance Ratings of Positive Displacement Refrigerant Compressors and Compressor Units", and according to the data provided by the selected compressor performance calculation software, obtain the full-load current of the compressor as shown below IA_B and the evaporation temperature te and the condensation temperature tc The calculation formula of the relationship:

[0034] Where c1 - c10 is the coefficient related to the compressor displacement, which can be obtained through the compressor selection software.

[0035] The specific method for obtaining c1 - c10 the coefficient is as shown below:

[0036] Taking a screw compressor with a displacement of 1130 m³ / h of a certain brand as an example. In the compressor selection software, select the compressor model as RC2 - 1130B (RC2 - 1130B represents a displacement of 1130 m³ / h), the refrigerant type as R22, and other parameters can be set as default values. After routine operations, select the "Polynomial Display" tab at the top of the interface in the subsequent interface, and obtain the value of c1 - c10 when the refrigerant is R22 as shown in Table 1.

[0037] Table 1

[0038]

[0039] If the refrigerant type is selected as R134a, following the similar steps shown above, the values when the refrigerant is R134a c1 - c10 are shown in Table 2.

[0040] Table 2

[0041]

[0042] b. Select the typical operating conditions of the refrigeration and air-conditioning unit (evaporation temperature te = 6°C and condensation temperature tc = 36°C) in the performance software of the basic compressor, calculate the compressor current at intervals of 5% of the compressor energy level, and at the same time calculate its ratio IA_B to the full-load current s , and record the data as shown in Table 3.

[0043] Table 3

[0044]

[0045] The corresponding relationship between s and the compressor energy level u can be fitted into a polynomial as shown in the following formula using the "scatter plot" function in the excel software:

[0046] u = E * s 2 +F * s + G

[0047] where s = IA_p / IA_C , E , F , G is a set of coefficients that meet the calculation accuracy determined in the process of fitting the formula.

[0048] To determine E , F , G , the specific operation method is as follows: After selecting the corresponding data of the current ratio s and the compressor energy level u shown in Table 3, select the "scatter plot" option in the "chart" button in the "insert" drop-down menu in the excel software. In the subsequent scatter plot, select the scatter points, right-click the mouse to pop up a menu, select "add trendline", select a polynomial of order 2 in the pop-up tab, and check the options of "display polynomial" and "display R-squared value" to obtain a chart as shown in Figure 1 .

[0049] Figure 1 Inu, x The axis is the current ratio s .

[0050] Through trial calculation, it can be known that using a quadratic polynomial to fit the current ratio s and the compressor energy level u has a smaller error in the corresponding relationship formula (the closer the R 2 value is to 1, the higher the coincidence degree of the fitted formula and the selected data). Therefore, select Figure 1 the polynomial shown in the upper right corner in Figure 1 as the final fitting result. The reference values of each coefficient can be excerpted from

[0051] E =-0.6282, F =2.2909, G =-0.6679

[0052] The above data are the reference values of the coefficients when the refrigerant is R22.

[0053] When the refrigerant is selected as R134a, after the same above calculation and fitting process, the reference values of each coefficient are as follows:

[0054] E =-1.038, F =3.574, G =-1.537.

[0055] c. Write the obtained theoretical full-load current value calculation formula IA_B and the energy level calculation formula of the screw compressor into the control program of the refrigeration and air-conditioning unit, and measure the energy level of the screw compressor in an operating refrigeration unit according to the following steps:

[0056] Step 1. Collect the evaporation temperature te , condensation temperature tc , actual partial-load operating current IA_p and voltage V_C when the screw compressor of the refrigeration and air-conditioning unit is operating at partial load; the evaporation temperature te , condensation temperature tc can be calculated respectively from the pressure signals collected by the evaporation pressure sensor (high-range pressure sensor) and the condensation pressure sensor (low-range pressure sensor). The current IA_p and voltage V_C are sent to the controller by the current transmitter and voltage transmitter;

[0057] Step 2. Using the evaporation temperature te and the condensation temperature tc as the operating condition variables, calculate the theoretical full-load operating current value of the screw compressor assuming that the refrigeration and air-conditioning unit operates under the corresponding operating conditionsIA_B ;

[0058] Step 3. Collect the evaporation temperature, te0 condensation temperature, tc0 and the actual full-load operating current IA_f and voltage V_ C ;

[0059] Step 4. Taking the evaporation temperature te0 and the condensation temperature tc0 as the operating condition parameters, determine the theoretical full-load current value of the screw compressor when the refrigeration and air-conditioning unit operates under this condition IA_B0 ;

[0060] Step 5. According to the formula IA_C0 = IA_B0 * 380 / V_ C , obtain the actual calculated full-load current value of the screw compressor when the refrigeration and air-conditioning unit operates under this condition IA_C0 ;

[0061] Step 6. Determine whether the deviation between the actual full-load operating current value IA_f of the screw compressor and the actual calculated full-load current value IA_C0 exceeds 4%;

[0062] If not, obtain the actual calculated full-load operating current value of the screw compressor when the refrigeration and air-conditioning unit is assumed to operate under the corresponding condition according to Formula ① IA_C ;

[0063] IA_C = IA_B * 380 / V_C ①

[0064] If yes, obtain the actual calculated full-load operating current value of the screw compressor when the refrigeration and air-conditioning unit is assumed to operate under the corresponding condition according to Formula ② IA_C ;

[0065] IA_C = c0 * IA_B * 380 / V_C ②

[0066] The said c0 is the correction coefficient, c0 = IA_f / ( IA_B0 * V_ C / 380 );

[0067] Step 7. Calculate the current energy level of the screw compressor according to Formula ③ u ;

[0068] u = E * s 2 +F * s + G ③

[0069] In the formula s = IA_p / IA_C , E 、 F 、 G are a set of coefficients that meet the calculation accuracy determined in the process of fitting formula ③;

[0070] Step 8. Since there may be deviations in the data collected by the pressure sensor, current transmitter, and voltage transmitter, data correction is required except for the following situations. Therefore, except for the following situations, the rest are based on the energy level calculated value u as the energy level measured value:

[0071] When the energy level calculated value u is greater than 100%, the energy level measured value is set to 100%;

[0072] When the energy level calculated value u = 95 - 100% and after continuously executing the load increase instruction 3 times, judge whether the change in the actual operating current is less than 1%. If so, the energy level measured value is set to 100%. Otherwise, the energy level calculated value u is the energy level measured value;

[0073] When the energy level calculated value u is less than the minimum energy level + 5% (for example, if the minimum energy level is 25%, then the minimum energy level + 5% = 30%) and after continuously executing the load decrease instruction 3 times, repeat steps 1 - 7 to calculate the energy level u＇ ,judge u and u＇ whether the difference is less than 1%. If so, the energy level measured value is set to the minimum energy level. Otherwise, the energy level calculated value u is the energy level measured value. Due to hardware errors, it may occur that when the actual energy level of the compressor can no longer be reduced, but the calculated energy level still has not reached the minimum energy level setting value. At this time, the compressor may continue to execute the load decrease operation, resulting in a shortened service life of the load decrease solenoid valve. Therefore, in this case, the minimum energy level value is assigned to the current calculated energy level, indicating that the current has reached the minimum energy level and the load decrease is in place, and there is no need to execute the load decrease instruction anymore.

[0074] To verify the accuracy of the screw compressor energy level measurement method described in the present invention, an LSC285B1A screw water chiller equipped with an RC2-930BW type compressor (theoretical displacement is 930 m³ / h) was selected. The full-load cooling capacity at different evaporation temperatures te and condensation temperatures tc was tested respectively. By unloading the compressor, the measured cooling capacity of the water chiller was adjusted to 100%, 80%, 60%, 40%, and 25% of the full-load cooling capacity under their respective operating conditions, and various test data were recorded. The data was calculated according to the energy level measurement method described in the present invention as shown in Table 4.

[0075] Table 4

[0076]

[0077] The intermediate value of the calculated energy level of the compressor in Table 4 is the result calculated according to the above formulas ①~③. Among them, after the calculated energy levels of 98.7% and 28.4% are corrected according to the above step 8, they become 100% and 25% respectively. The test results show that the deviation between the energy level measured in the embodiment of the present invention and the actual energy level of the compressor is less than 2%.

Claims

1. A method for measuring the energy level of a screw compressor used in a refrigeration air-conditioning unit, characterized in that Proceed as follows: Step 1. Collect the evaporation temperature, condensation temperature, actual operating current at part load, and voltage of the refrigeration and air-conditioning unit during the part-load operation of the screw compressor te , condensation temperature tc , actual operating current at part load IA_p and voltage V_C ; Step 2. Taking the evaporation temperature te and the condensation temperature tc as the operating condition variables, calculate the theoretical current value of the full-load operation of the screw compressor when the assumed refrigeration air-conditioning unit operates under the corresponding operating conditions IA_B ; Step 3. Collect the evaporation temperature of the refrigeration and air-conditioning unit when the screw compressor is operating at full load te0 , the condensation temperature tc0 and the actual operating current at full load IA_f and the voltage V_ C ; Step 4. With the evaporation temperature te0 and the condensation temperature tc0 as the operating condition parameters, determine the full-load theoretical current value of the screw compressor when the refrigeration and air-conditioning unit operates under this condition IA_B0 ; Step 5. According to the formula IA_C0 = IA_B0 * 380 / V_ C , obtain the actual calculated full-load current value IA_C0 of the screw compressor when the refrigeration and air-conditioning unit operates under this working condition IA_C0 ; Step 6. Determine the actual operating current value of the screw compressor at full load IA_f and the actual calculated current value of the screw compressor at full load IA_C0 whether the deviation exceeds 4%; If not, the actual calculated current value when the screw compressor operates at full load under the corresponding working conditions of the hypothetical refrigeration air-conditioning unit is obtained according to Formula ① IA_C ; IA_C = IA_B * 380 / V_C ① If so, the actual calculated current value of the full-load operation of the screw compressor when the assumed refrigeration air-conditioning unit operates under the corresponding working conditions is obtained according to Formula ② IA_C ; IA_C = c0 * IA_B * 380 / V_C ② The said c0 is the correction coefficient, c0 = IA_f / ( IA_B0 * V_ C / 380 ); Step 7. Calculate the current energy level of the screw compressor according to Formula ③ u ; u = E * s 2 + F * s + G ③ wherein s = IA_p / IA_C , E 、 F 、 G is a set of coefficients that meet the calculation accuracy determined in the process of fitting formula ③. Specifically, in the performance software of the basic compressor, typical operating conditions of the refrigeration and air-conditioning unit are taken, and the compressor current is calculated at intervals of 5% of the compressor energy level. At the same time, the ratio of it to the full-load current IA_B is calculated s . Then, the corresponding relationship between s and the compressor energy level u is fitted into a polynomial; Step 8. Except for the following cases, the calculated energy level value u is taken as the measured energy level value; When the calculated energy level value u is greater than 100%, set the measured energy level value to 100%; When the calculated energy level value u = 95~100% and after continuously executing the load increasing instruction three times, determine whether the change in the actual operating current is less than 1%. If so, set the measured energy level value to 100%. Otherwise, the calculated energy level value u is the measured energy level value; When the calculated energy level value u is less than the minimum energy level + 5% and after the load shedding instruction is continuously executed 3 times, repeat steps 1 - 7 to calculate the energy level u＇ , judge u and u＇ whether the difference is less than 1%. If yes, set the measured energy level value to the minimum energy level; otherwise, the calculated energy level value u is the measured energy level value.

2. The method for measuring the energy level of a screw compressor for a refrigeration and air-conditioning unit according to claim 1, characterized in that The theoretical current value of step 2 IA_B is calculated according to the following formula: wherein c1 - c10 is a coefficient related to the displacement of the compressor.

3. The method for measuring the energy level of a screw compressor for a refrigeration and air-conditioning unit according to claim 2, wherein The theoretical current value in Step 4 is IA_B0 substituted by IA_B , te0 substituted by te, tc0 substituted by tc calculated according to the formula of the theoretical current value IA_B .

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