Energy-saving method of screw refrigerating unit based on oil path pressure difference precision control

Abstract

The application discloses a screw refrigerating unit energy-saving method based on oil path pressure difference precision control, and creatively changes the traditional indirect control based on suction and exhaust pressure difference into direct control based on oil low pressure difference. Through real-time monitoring of the oil low pressure difference, the unit is adaptively switched between emergency shutdown, risk avoidance mode, safety mode and energy-saving mode control, and through the cooperation of the cooling water flow bypass regulating valve, the cooling tower fan frequency converter and the electronic expansion valve, the real state of the oil path pressure is accurately matched, the indirect safety protection is changed into safety energy-saving collaborative optimization control, the safety blind area caused by the existing control mode is solved, the compression ratio is reduced through adjustment, the comprehensive energy efficiency ratio (COP) of the unit is obviously improved, and the problem of continuous energy waste caused by the forced artificial increase of the suction and exhaust pressure difference for the sake of safety under the high-efficiency working condition of the prior art is fundamentally solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of refrigeration, and particularly relates to a refrigerating unit energy-saving method based on oil path pressure difference precision control. BACKGROUND

[0002] The refrigerating unit is mainly composed of a screw compressor, a condenser, an electronic expansion valve and an evaporator connected with each other, lubricating oil in the screw compressor returns after being processed by an oil separator and an oil filter, and suction and exhaust ports of the screw compressor are respectively provided with a suction pressure monitoring unit and an exhaust pressure monitoring unit for monitoring suction and exhaust pressures of the screw compressor, and a cooling water flow bypass regulating valve and a cooling tower fan frequency converter are installed in a cooling water pipeline of the condenser. Meanwhile, the refrigerating unit is also provided with a control module to collect key data signals of the refrigerating unit and perform corresponding control.

[0003] The existing screw refrigerating unit generally adopts a control mode of monitoring a suction and exhaust pressure difference and maintaining a fixed high pressure difference threshold (for example, 4 bar which is generally set in the industry to cope with extreme dirt and blockage of the oil filter), and when serious blockage or other faults occur in the oil path, a mechanical oil filter pressure difference switch is provided to provide alarm and other safety protection. However, the following defects exist.

[0004] Firstly, the fixed high pressure difference threshold cannot adapt to actual needs of different units and different working conditions, so that the unit is forced to artificially increase the suction and exhaust pressure difference to run in the natural high efficiency working condition, causing great and continuous waste of energy. For example, in the natural suction and exhaust pressure difference low (usually only 2-3 bar) high efficiency working condition of low ambient temperature (cooling water temperature ≤15℃ in winter), the unit cannot help but artificially increase the exhaust pressure by opening the cooling water flow bypass regulating valve and reducing the cooling tower fan speed to meet the fixed high pressure difference threshold, resulting in a decrease in the theoretical efficiency of the compressor. Taking R134a refrigerant as an example, the exhaust pressure is increased from 5.5 bar (a) to 7.5 bar (a), and the exhaust saturation temperature is increased from 19℃ to 29℃, and the temperature rise is 10℃. According to the inverse Carnot cycle and other thermodynamic principles, the increase in the exhaust saturation temperature directly increases the compression ratio of the compressor, resulting in a decrease in the theoretical efficiency of the compressor by about 30%.

[0005] Secondly, the output of the mechanical oil filter pressure difference switch is a fixed-point and hysteresis switch signal, which cannot reflect the continuous change of the oil path state in real time, cannot actively adjust the operating parameters to maintain the best oil supply when the oil path resistance increases, and can only passively protect when the switch is finally triggered due to serious blockage or other faults in the oil path, so there is a safety protection blind area, and the predictive and cooperative control ability based on the real state of the oil path is lacking. In order to ensure safety, the suction and exhaust pressure difference is forced to be increased, further causing a decrease in the theoretical efficiency of the compressor.

[0006] In summary, the existing control mode taking the suction and exhaust pressure difference as the core not only sacrifices the operation energy efficiency of the unit under most working conditions, but also has the problems of response lag and disconnection with energy efficiency control in the safety protection mechanism, resulting in the long-term dilemma of high energy consumption and safety protection of the refrigeration unit. SUMMARY

[0007] The present application is to solve the technical problems of high energy consumption and inaccurate safety protection in the prior art, and provides a screw refrigeration unit energy saving method based on accurate control of oil pressure difference.

[0008] The technical solution of the present application is: a screw refrigeration unit energy saving method based on accurate control of oil pressure difference, real-time monitoring of oil injection pressure P1 and suction pressure P2, based on oil low pressure difference ΔP1=P1-P2, adaptive emergency shutdown, risk avoidance mode, safety mode or energy saving mode control, the risk avoidance mode is to preferentially adjust the cooling tower fan frequency converter and then adjust the cooling water flow bypass regulating valve when the oil low pressure difference ΔP1 is greater than the oil low pressure difference alarm threshold; the safety mode and the energy saving mode both aim to minimize the total energy consumption of the system, and cooperatively adjust the cooling water flow bypass regulating valve, the cooling tower fan frequency converter and the electronic expansion valve, wherein the safety mode is constrained by being greater than the oil low pressure difference alarm threshold, and the energy saving mode is constrained by being greater than the energy saving threshold, and the oil low pressure difference alarm threshold is less than the energy saving threshold.

[0009] Preferably, the following steps are performed:

[0010] Step 1. Set the shutdown threshold Ts, the oil low pressure difference alarm threshold Ta, the energy saving threshold Te and the duration threshold Tt, the shutdown threshold Ts < the oil low pressure difference alarm threshold Ta < the energy saving threshold Te, and control the unit to run;

[0011] Step 2. Real-time acquisition of oil injection pressure P1 and suction pressure P2, calculation of oil low pressure difference ΔP1=P1-P2;

[0012] Step 3. Based on the oil low pressure difference ΔP1, corresponding mode control is performed respectively:

[0013] When the oil low pressure difference ΔP1 < the shutdown threshold Ts or the shutdown threshold Ts ≤ the oil low pressure difference ΔP1 ≤ the oil low pressure difference alarm threshold Ta, and the duration > the duration threshold Tt, control the unit to emergency shutdown, end the program;

[0014] When the shutdown threshold Ts ≤ the oil low pressure difference ΔP1 ≤ the oil low pressure difference alarm threshold Ta, and the duration ≤ the duration threshold Tt, the risk avoidance mode control is performed, and step 4 is performed;

[0015] When the oil low pressure difference alarm threshold Ta < the oil low pressure difference ΔP1 ≤ the energy saving threshold Te, the safety mode control is performed, and step 4 is performed;

[0016] When the oil low pressure difference ΔP1> energy saving threshold Te, the energy saving mode control is performed, and step 4 is performed.

[0017] Step 4. Determine whether to end the control. Yes, end. No, return to step 2.

[0018] Preferably, the safety mode is to minimize the total energy consumption of the system, preferentially increase the opening degree of the cooling water flow bypass regulating valve, and then adjust the frequency of the cooling tower fan frequency converter and / or the opening degree of the electronic expansion valve with the constraint that the oil low pressure difference ΔP1 is greater than the oil low pressure difference alarm threshold Ta. The energy saving mode is to minimize the total energy consumption of the system, preferentially reduce the opening degree of the cooling water flow bypass regulating valve, and then adjust the frequency of the cooling tower fan frequency converter and / or the opening degree of the electronic expansion valve with the constraint that the oil low pressure difference ΔP1 is greater than the energy saving threshold Te.

[0019] Preferably, the step 1 also needs to set the oil filter pressure drop threshold Tp, and the step 2 also needs to collect the exhaust pressure P3 in real time, calculate the oil filter pressure drop ΔP2 = P3-P1 in real time, and determine whether the oil filter pressure drop ΔP2 is greater than the oil filter pressure drop threshold Tp. Yes, issue an oil filter dirty block alarm signal, and perform step 3. No, directly perform step 3.

[0020] Preferably, the shutdown threshold Ts = 0.8~1.3bar, the oil low pressure difference alarm threshold Ta = 1.4~2.4bar, the energy saving threshold Te = 1.6~2.5bar, the duration threshold Tt = 15~90 seconds, and the oil filter pressure drop threshold Tp = 0.5~1.5bar.

[0021] Preferably, the real-time monitoring of the oil injection pressure P1 of the oil filter is to install an oil injection pressure sensor at a distance of ≤50mm from the oil filter outlet, collect the pressure signal value of the oil injection pressure sensor in real time, the range of the oil injection pressure sensor is 0-30bar, and the accuracy is ±0.2% FS.

[0022] Preferably, the single adjustment range of the electronic expansion valve does not exceed 5% of the total opening degree.

[0023] The present application is the first to change the traditional indirect control based on the suction and exhaust pressure difference to direct control based on the oil low pressure difference. By monitoring the oil low pressure difference in real time, the unit adaptively switches between emergency shutdown, risk avoidance mode, safety mode and energy saving mode control. Through the coordinated execution of the cooling water flow bypass regulating valve, the cooling tower fan frequency converter and the electronic expansion valve, the real state of the oil circuit pressure is accurately matched, realizing the collaborative optimization control from indirect safety protection to safety and energy saving. It not only solves the safety blind area caused by the existing control mode, but also significantly improves the overall energy efficiency ratio (COP) by adjusting and reducing the compression ratio, and fundamentally solves the problem of continuous energy waste caused by forcibly increasing the suction and exhaust pressure difference in the high efficiency working condition of the existing technology.

[0024] Compared with the prior art, the specific technical effects are as follows:

[0025] 1. Excellent energy saving effect: The present application releases energy efficiency by reducing the compression ratio of the compressor, and the overall energy efficiency ratio (COP) of the unit is significantly improved (more than 25%). For example, a 110kW unit can save 264,000kWh of electric energy per year, which fundamentally solves the problem of continuous energy waste caused by forcibly increasing the suction and exhaust pressure difference in the high efficiency working condition of the existing technology.

[0026] 2. Precise and reliable safety protection: The present application directly monitors the oil low pressure difference, realizes the leap from indirect protection to direct protection, and fundamentally eliminates the safety blind area caused by the dirty and blocked oil filter by combining the multi-dimensional threshold preset by the control module. It realizes accurate identification and protection of oil risk, effectively avoids insufficient oil supply, and prolongs the service life of the compressor core component by more than 30%.

[0027] 3. Low implementation cost and good compatibility: The equipment of the present application does not change the main machine structure and the original refrigeration circuit, only adds a low-cost oil injection pressure sensor (cost about 200 yuan), and does not need to change the core program of the original control system. Only simple parameter configuration is needed for the control module, without additional debugging cost, easy to implement on new and old units, with an investment return rate of more than 10 times.

[0028] 4. Intelligent self-adaptation and strong adaptability: The present application can automatically switch between safety and energy saving mode according to the oil circuit state and external working condition without manual intervention. At the same time, different threshold values can be set in the threshold range to adapt to the needs of different power units, different refrigerant types and different operating conditions (such as higher alarm threshold for low temperature working condition and lower threshold for normal temperature working condition), further improving the practicality and promotion value of the technical scheme, and reducing the operation and maintenance cost. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 is a partial structure schematic diagram of the equipment of the embodiment of the present application.

[0030] Figure 2 is a flow chart of the control method of the embodiment of the application.

[0031] Figure 1 Interpretation of reference numerals in the figure: 1 oil separator, 2 oil filter, 3 control module, 4 oil injection pressure sensor, 5 screw compressor, 6 exhaust pressure monitoring unit, 7 suction pressure monitoring unit, 8 electronic expansion valve, 9 condenser, 10 evaporator. DETAILED DESCRIPTION

[0032] Part of the structure of the equipment of the screw refrigerating unit energy-saving method based on oil pressure difference precision control of the application is shown in Figure 1 The same as the prior art, the oil separator 1, the oil filter 2, the control module 3, the screw compressor 5, the exhaust pressure monitoring unit 6, the suction pressure monitoring unit 7, the electronic expansion valve 8, the condenser 9 and the evaporator 10 are provided, the exhaust end of the screw compressor 5 is provided with the exhaust pressure monitoring unit 6, the suction end of the screw compressor 5 is provided with the suction pressure monitoring unit 7, the screw compressor 5, the exhaust pressure monitoring unit 6, the suction pressure monitoring unit 7, the electronic expansion valve 8 and the control module 3 are electrically connected, and the cooling water flow bypass regulating valve and the cooling tower fan frequency converter are also electrically connected with the control module 3.

[0033] Taking a rated power 110kW screw water chiller unit (refrigerating capacity 660kW, refrigerant R134a) as an example, the system is specifically configured as follows:

[0034] The control module 3 adopts Siemens S7-200SMART, is connected with various sensors and actuators through RS485 communication, and is provided with built-in oil low pressure difference calculation, threshold value judgment and energy-saving optimization programs.

[0035] The cooling water flow bypass regulating valve selects a DN150 electric ball valve (adjusting accuracy ±1%); the cooling tower fan frequency converter selects a Hui Chuan CA300 series (power 5.5kW); and the electronic expansion valve 8 selects a Kalor E6V model (bore diameter DN40), with an adjusting accuracy of ±1%, and the single-time adjusting range thereof can be limited to ≤5% total opening degree through the control module.

[0036] The screw compressor is of RE-920A type, the exhaust pressure monitoring unit 6 and the suction pressure monitoring unit 7 both select Kalor SPKT series pressure sensors (range 0-20bar), the oil separator 1 has a volume of 120L, the oil filter 2 has a filtering accuracy of 10μm, the condenser 9 is a shell-and-tube type (heat exchange area 42m²), and the evaporator 10 is a falling-film type evaporator (heat exchange area 35m²).

[0037] Different from the prior art, the oil filter 2 is provided with an oil injection pressure sensor 4 downstream, the oil injection pressure sensor 4 is electrically connected with the control module 3, preferably the oil injection pressure sensor 4 selects a Kal SPKT series pressure sensor (range 0-30bar, accuracy ±0.2% FS), the installation position is preferably at a distance of ≤50mm from the outlet of the oil filter 2, too close (<20mm) is easy to be affected by the internal flow field disturbance of the oil filter, too far (>100mm) will introduce unnecessary pipeline along the pressure drop error, the embodiment of the application is provided with the oil injection pressure sensor 4 at a distance of 40mm from the outlet of the oil filter 2.

[0038] The screw refrigerating unit energy-saving method based on oil pressure difference precision control of the application is to monitor the oil injection pressure P1 and the suction pressure P2 of the oil filter in real time, based on the oil low pressure difference ΔP1=P1-P2, to adaptively carry out emergency shutdown, risk avoidance mode, safety mode or energy-saving mode control, the risk avoidance mode is to preferentially adjust the cooling tower fan frequency converter and then adjust the cooling water flow bypass regulating valve, with the target that the oil low pressure difference ΔP1 is greater than the oil low pressure difference alarm threshold value; the safety mode and the energy-saving mode both have the target that the total system energy consumption is the lowest, and cooperatively adjust the cooling water flow bypass regulating valve, the cooling tower fan frequency converter and the electronic expansion valve, wherein the safety mode is constrained by being greater than the oil low pressure difference alarm threshold value, and the energy-saving mode is constrained by being greater than the energy-saving threshold value, and the oil low pressure difference alarm threshold value is less than the energy-saving threshold value.

[0039] The specific flow is as shown in Figure 2 The following steps are carried out:

[0040] Step 1. Set the shutdown threshold Ts, the oil low pressure difference alarm threshold value Ta, the energy-saving threshold value Te and the duration threshold value Tt, the shutdown threshold Ts < the oil low pressure difference alarm threshold value Ta < the energy-saving threshold value Te, and control the unit to run; the above threshold values can be selected in the threshold value range to adapt to the needs of different power units, different refrigerant types and different operating conditions, and the threshold value range is as follows: the shutdown threshold Ts=0.8~1.3bar, the oil low pressure difference alarm threshold value Ta=1.4~2.4bar, the energy-saving threshold value Te=1.6~2.5bar, the duration threshold value Tt=15~90 seconds, and the oil filter pressure drop threshold value Tp=0.5~1.5bar; the threshold values selected in the embodiment are respectively the shutdown threshold Ts=1.3bar, the oil low pressure difference alarm threshold value Ta=1.8bar, the energy-saving threshold value Te=2.0bar, the duration threshold value Tt=45 seconds, and the oil filter pressure drop threshold value Tp=1.0bar;

[0041] Step 2. Collecting the fuel injection pressure P1, the suction pressure P2 and the exhaust pressure P3 in real time, calculating the oil low pressure difference ΔP1 = P1-P2 and the oil filter pressure drop ΔP2 = P3-P1 (ΔP2 is used to assist in judging the dirty blockage state of the oil filter), judging whether the oil filter pressure drop ΔP2 is greater than the oil filter pressure drop threshold Tp, yes, issuing an oil filter dirty blockage alarm signal to remind replacing the oil filter, proceeding to Step 3, no, directly proceeding to Step 3;

[0042] Step 3. Based on the oil low pressure difference ΔP1, the corresponding mode control is carried out respectively:

[0043] When the oil low pressure difference ΔP1 <1.3 bar or 1.3 bar≤oil low pressure difference ΔP1≤1.8 bar and the duration >45 seconds, controlling the unit to emergency shutdown, ending the program;

[0044] When 1.3 bar≤oil low pressure difference ΔP1≤1.8 bar and the duration ≤45 seconds, performing the risk avoidance mode control, that is, first reducing the frequency of the cooling tower fan frequency converter, if the oil low pressure difference ΔP1 is still greater than 1.8 bar, adjusting the opening of the cooling water flow bypass regulating valve, until the oil low pressure difference ΔP1 is greater than 1.8 bar, proceeding to Step 4;

[0045] When 1.8 bar<oil low pressure difference ΔP1≤2.0 bar, performing the safety mode control;

[0046] The safety mode control is to minimize the total energy consumption of the system, preferentially increasing the opening of the cooling water flow bypass regulating valve, and then maintaining the oil low pressure difference ΔP1 greater than the alarm threshold Ta as a constraint, adjusting the frequency of the cooling tower fan frequency converter or / and the opening of the electronic expansion valve 8 by the extreme value search method, that is, adjusting the frequency of the cooling tower fan frequency converter by 1-2 Hz small step or / and adjusting the opening of the electronic expansion valve 8 by a single adjustment of not more than 5% of the total opening; monitoring ΔP1 and the exhaust pressure in real time during the adjustment process, when the exhaust pressure is stable, locking the current frequency of the cooling tower fan frequency converter or / and the opening of the electronic expansion valve 8; then judging whether the oil filter pressure drop ΔP2 is greater than the oil filter pressure drop threshold Tp, yes, issuing an oil filter dirty blockage alarm signal to prompt replacing the oil filter, proceeding to Step 4, no, directly proceeding to Step 4;

[0047] When the oil low pressure difference ΔP1>energy saving threshold Te, performing the energy saving mode control, proceeding to Step 4;

[0048] The energy-saving mode control aims to minimize the total energy consumption of the system, and preferentially reduces the opening of the cooling water flow bypass regulating valve (reduces the cooling water bypass flow, increases the cooling water flow into the condenser, and reduces the exhaust pressure). Then, with the constraint of maintaining the oil low pressure difference ΔP1 greater than the energy-saving threshold Te, the frequency of the cooling tower fan frequency converter and the opening of the electronic expansion valve 8 are adjusted by the extreme value search method. The frequency of the cooling tower fan frequency converter is adjusted by 1-2 Hz small step amplitude, the total energy consumption of the fan itself and the compressor is monitored in real time, and when the total energy consumption is minimized and the exhaust pressure is stable, the current frequency is locked to optimize the exhaust pressure. The opening of the electronic expansion valve 8 is adjusted by a single adjustment of not more than 5% of the total opening, and the evaporation pressure and the unit COP are monitored in real time, and when the unit COP rises to the highest and the suction pressure is stable, the current opening is locked to optimize the evaporation pressure. Due to the inevitable flow resistance in the suction pipeline, the suction pressure is usually slightly lower than the evaporation pressure in value, but the change trend is consistent. Therefore, by adjusting the electronic expansion valve 8 to optimize the evaporation pressure, the effect will be equivalent to the optimization of the suction pressure, thereby improving the energy efficiency of the unit.

[0049] The extreme value search method includes but is not limited to small step iteration method, PID self-tuning method, gradient descent method and other conventional optimization means in the field of refrigeration unit control. The core logic is "small step adjustment-real-time monitoring-feedback locking", and those skilled in the art can flexibly select the appropriate method according to the unit model and working condition without creative labor to realize it through the control module 3. The embodiment of the application adopts the "small step iteration method", which has simple logic and strong adaptability, and can be implemented by programming to set the adjustment step, dwell time and monitoring parameter threshold without complex algorithm development.

[0050] Step 4. Determine whether to end the control. Yes, end. No, return to step 2.

[0051] The control process of step 3 of the embodiment of the application is described in detail below in three cases of ΔP1=5.2bar, ΔP1=1.9bar and ΔP1=1.5bar calculated in step 2 and the duration <45 seconds.

[0052] 1. ΔP1=5.2bar

[0053] Since ΔP1=5.2bar is greater than the energy-saving threshold Te=2.0bar, the energy-saving mode control is executed, as follows:

[0054] Preferentially, the cooling water flow bypass regulating valve is closed from 30% opening to 0%, and after the cooling water flow bypass regulating valve is adjusted to 0%, the constraint of maintaining the oil low pressure difference ΔP1 greater than the energy-saving threshold (2.0bar) is adopted, and the "small step iteration + energy consumption feedback" extreme value search method is used for adjustment:

[0055] Cooling tower fan frequency converter adjustment: starting from 30 Hz, increasing by 2 Hz each time, staying for 30 seconds to monitor total energy consumption (fan power consumption + compressor power consumption). The monitoring data is as follows: total energy consumption is 152 kW at 30 Hz → 148 kW at 32 Hz → 142 kW at 35 Hz → 139 kW at 38 Hz → 138 kW at 40 Hz → 139 kW at 42 Hz → 141 kW at 45 Hz. When the frequency is increased to 40 Hz, the total energy consumption is the lowest (138 kW), and the exhaust pressure is stable at 6.8 bar (a), so the frequency is locked at 40 Hz;

[0056] Electronic expansion valve 8 adjustment: starting from 40% opening, increasing by 5% each time, staying for 60 seconds to monitor COP. The monitoring data is as follows: COP is 8.2 at 40% → 8.9 at 45% → 9.5 at 50% → 9.3 at 55%. When the opening is increased to 50%, the COP is the highest (9.5) and the evaporation pressure is stable at 4.1 bar (a), so the opening is locked at 50%.

[0057] After adjustment, the oil low pressure difference ΔP1 is maintained at 2.7 bar (2.0 bar higher than the energy saving threshold), and the total energy consumption of the system is reduced to 138 kW, which is 14 kW lower than the initial state.

[0058] Under this optimized operating state, due to the significant reduction in exhaust pressure and moderate increase in evaporation pressure, the compressor compression ratio is reduced, and the power consumption is greatly reduced, resulting in the COP of the unit increasing from 6.7 to 9.8. The fundamental improvement of this energy efficiency level is due to the significant reduction of the operating compression ratio: in this example, based on the monitored exhaust pressure from 8.9 bar (a) to 6.9 bar (a), the suction pressure from 3.7 bar (a) to 4.0 bar (a), the compression ratio is reduced from 2.41 (8.9 / 3.7) to 1.73 (6.9 / 4.0). According to the principle of thermodynamics, the reduction of compression ratio directly leads to the significant reduction of compressor theoretical power consumption, thereby fully releasing the natural energy efficiency potential of the unit under superior working conditions.

[0059] 2. ΔP1 = 1.9 bar

[0060] Since ΔP1 = 1.9 bar, which is greater than the oil low pressure difference alarm threshold Ta = 1.8 bar and less than the energy saving threshold Te = 2.0 bar, the safety mode control is executed, which is as follows:

[0061] Preferentially open the cooling water flow bypass regulating valve from the current opening (such as from 0% to 20%); after the cooling water flow bypass regulating valve is adjusted, the oil low pressure difference ΔP1 is greater than the oil low pressure difference alarm threshold Ta=1.8bar is maintained as a constraint, and an "small step iteration + energy consumption feedback" extremum search method is used for adjustment: the frequency of the cooling tower fan frequency converter is reduced from 45Hz to 30Hz in a small step amplitude of 1~2Hz, the exhaust pressure is increased from 6.5bar (a) to 6.7bar (a), and the oil low pressure difference ΔP1 is increased to 2.1bar; then it is judged whether the oil filter pressure drop ΔP2 is greater than the oil filter pressure drop threshold 1.0bar, no, step 4 is performed.

[0062] 3. ΔP1=1.5bar

[0063] Since ΔP1=1.5bar and the duration is <45 seconds, it belongs to when 1.3bar≤oil low pressure difference ΔP1≤1.8bar, and the duration is <45 seconds, so the risk control mode is performed, the frequency of the cooling tower fan frequency converter is adjusted first, and the result is ΔP1=1.8, since it is still not greater than the oil low pressure difference alarm threshold Ta=1.8bar, the opening of the current cooling water flow bypass regulating valve is continuously adjusted until ΔP1=1.9bar.

[0064] Implementation effect

[0065] The embodiment of the application is continuously operated for 12 months, in the actual operation scene covering a large number of low cooling water temperature and high chilled water temperature conditions, the average COP of the unit is increased from 6.25 to 9.2, the annual power saving amount is 264,000kWh, the energy saving rate is 32.1%; the oil low pressure difference is stabilized between 1.8-4.0bar, and no oil related failure occurs; the oil filter replacement period is extended from 5 months to 10 months, and the operation and maintenance cost is reduced by 50%; the invalid adjustment time of the cooling water flow bypass regulating valve is reduced by 60% (because the bypass valve is preferentially adjusted, the invalid operation of the prior art "first adjust the fan / electronic expansion valve" is avoided), the invalid operation time of the cooling tower fan is reduced by 40%, and the auxiliary energy consumption is further reduced. During the implementation, the core components such as compressor bearings and rotors have no abnormal wear, and the service life is prolonged by more than 30% compared with the prior control technology.

Claims

1. An energy-saving method for screw refrigerating unit based on oil pressure difference, the screw refrigerating unit mainly comprises a screw compressor, a condenser, an electronic expansion valve and an evaporator connected with each other, lubricating oil in the screw compressor is returned after being treated by an oil separator and an oil filter in sequence, a suction pressure monitoring unit and a discharge pressure monitoring unit are respectively arranged at suction and discharge ports of the screw compressor, and are responsible for monitoring suction and discharge pressures of the screw compressor, a cooling water flow bypass regulating valve and a cooling tower fan frequency converter are installed in a cooling water pipeline of the condenser, and the energy-saving method is characterized in that: Real-time monitoring of oil filter injection pressure P1 and suction pressure P2, based on oil low pressure difference ΔP1=P1-P2, adaptive emergency shutdown, risk mode, safety mode or energy saving mode control, the risk mode is to take the oil low pressure difference ΔP1 greater than the oil low pressure difference alarm threshold as the target, priority adjustment cooling tower fan frequency converter, adjustment of cooling water flow bypass valve; the safety mode and energy saving mode are both with the lowest total system energy consumption as the target, coordinated adjustment of cooling water flow bypass valve, cooling tower fan frequency converter and electronic expansion valve, wherein the safety mode is greater than the low pressure difference alarm threshold as the constraint, the energy saving mode is greater than the energy saving threshold as the constraint, the oil low pressure difference alarm threshold is less than the energy saving threshold; Specifically, the following steps are taken: Step 1. Set the shutdown threshold Ts, oil low pressure difference alarm threshold Ta, energy saving threshold Te and duration threshold Tt, the shutdown threshold Ts < oil low pressure difference alarm threshold Ta < energy saving threshold Te, control unit operation; Step 2. Real-time acquisition of injection pressure P1 and suction pressure P2, calculation of oil low pressure difference ΔP1=P1-P2; Step 3. Based on the oil low pressure difference ΔP1, corresponding mode control is carried out respectively: When the oil low pressure difference ΔP1 < shutdown threshold Ts or shutdown threshold Ts ≤ oil low pressure difference ΔP1 ≤ oil low pressure difference alarm threshold Ta, and the duration > duration threshold Tt, control unit emergency shutdown, end program; When the shutdown threshold Ts ≤ oil low pressure difference ΔP1 ≤ oil low pressure difference alarm threshold Ta, and the duration ≤ duration threshold Tt, risk mode control is carried out, step 4 is carried out; When the oil low pressure difference alarm threshold Ta < oil low pressure difference ΔP1 ≤ energy saving threshold Te, safety mode control is carried out, step 4 is carried out; When the oil low pressure difference ΔP1 > energy saving threshold Te, energy saving mode control is carried out, step 4 is carried out; Step 4. Determine whether to end control, yes, end, no, return to step 2.

2. The screw refrigerating unit energy-saving method based on oil path pressure difference precision control according to claim 1, characterized in that The safety mode is to take the lowest total system energy consumption as the target, priority to increase the opening of the cooling water flow bypass valve, then adjust the frequency of the cooling tower fan frequency converter or / and adjust the opening of the electronic expansion valve with the constraint that the oil low pressure difference ΔP1 is greater than the oil low pressure difference alarm threshold Ta; the energy saving mode is to take the lowest total system energy consumption as the target, priority to reduce the opening of the cooling water flow bypass valve, then adjust the frequency of the cooling tower fan frequency converter or / and adjust the opening of the electronic expansion valve with the constraint that the oil low pressure difference ΔP1 is greater than the energy saving threshold Te.

3. The screw refrigerating unit energy-saving method based on oil path pressure difference precision control according to claim 2, characterized in that: The step 1 also needs to set the oil filter pressure drop threshold Tp, and the step 2 also needs to collect the exhaust pressure P3 in real time, calculate the oil filter pressure drop ΔP2 =P3-P1 in real time, and determine whether the oil filter pressure drop ΔP2 is greater than the oil filter pressure drop threshold Tp, yes, oil filter dirty block alarm signal is sent, step 3 is carried out, no, step 3 is carried out directly.

4. The screw refrigerating unit energy-saving method based on oil path pressure difference precision control according to claim 3, characterized in that: The shutdown threshold Ts=0.8-1.3bar, the oil low pressure difference alarm threshold Ta=1.4-2.4bar, the energy saving threshold Te=1.6-2.5bar, the duration threshold Tt=15-90 seconds, and the oil filter pressure drop threshold Tp=0.5-1.5bar.

5. The screw refrigerating unit energy saving method based on oil pressure difference precision control according to claim 1, 2, 3 or 4, characterized in that The injection pressure P1 of the real-time monitoring oil filter is installed at a distance of ≤50 mm from the oil filter outlet, and the pressure signal value of the injection pressure sensor is collected in real time, the range of the injection pressure sensor is 0-30 bar, and the accuracy is ±0.2% FS.

6. The screw refrigerating unit energy-saving method based on oil path pressure difference precision control according to claim 5, characterized in that: The single adjustment range of the electronic expansion valve is not more than 5% of the total opening degree.

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