Variable Frequency Centrifugal Chiller Speed Control Method, Device and Air Conditioner

Abstract

The present invention discloses a speed control method, device and air conditioner for a variable-frequency centrifugal chiller. By setting a temperature difference limit coefficient negatively correlated with the heat exchange temperature difference, when the refrigerant amount entering the evaporator changes, the maximum speed of the centrifugal compressor is limited, which can prevent the evaporator temperature difference from being too large and resulting in too low evaporation pressure, so that the unit cannot operate normally, extend the service life of the unit, make the unit operate in the high-efficiency range, reduce the energy consumption of the unit under the demand of the same refrigerating capacity, and improve the overall energy efficiency of the unit.

Description

Technical Field

[0001] The present invention relates to the technical field of air conditioning and refrigeration, and in particular to a method and device for controlling the speed of a variable frequency centrifugal chiller and an air conditioner. Background Art

[0002] During the operation of a centrifugal chiller, the chilled water outlet temperature TLo is used as the control target. If the water temperature does not reach the set target, the compressor will continue to load until the cooling demand is met. However, during the operation of the chiller, problems such as throttle valve damage and jamming, refrigerant leakage, etc. may occur, resulting in insufficient evaporator liquid supply, thereby increasing the evaporator heat exchange temperature difference. At this time, if the chilled water temperature is still used as the target and the load is continuously increased, the compressor speed will increase, the compressor operating pressure ratio will increase, the power will increase, the overall energy efficiency of the unit will decrease, and the cooling capacity may even decrease. If the evaporator liquid supply is insufficient, it will cause the evaporation pressure to be too low, which may cause malfunctions such as frozen evaporator pipes and the unit will not operate normally. Summary of the invention

[0003] Technical purpose: In view of the shortcomings of existing centrifugal chillers, such as reduced energy efficiency caused by continuous loading when the refrigerant is reduced and the heat exchange temperature difference increases, which easily leads to the evaporation pressure being too low and the unit being unable to operate, the present invention discloses a variable frequency centrifugal chiller speed control method, device and air conditioner, which can limit the upper limit of the unit speed and reduce the heat exchange temperature difference when the evaporator is insufficiently supplied with liquid and the evaporator heat exchange temperature difference increases, while ensuring the cooling capacity and energy efficiency of the unit.

[0004] Technical solution: To achieve the above technical objectives, the present invention adopts the following technical solution:

[0005] A method for controlling the speed of a variable frequency centrifugal chiller comprises the following steps:

[0006] S01. Fitting the correlation curve L between the speed and pressure ratio of the centrifugal compressor when the chiller maintains the chilled water outlet temperature of the evaporator;

[0007] S02. According to the actual heat exchange temperature difference of the current evaporator, the maximum speed n of the centrifugal compressor is lim Limit, n lim =n min +a(n max -n min ), where n min = fl(ε), the minimum speed equation of the centrifugal compressor, n max =fh(ε) is the maximum speed limit equation of the centrifugal compressor, ε is the current pressure ratio of the centrifugal compressor, a is the temperature difference limit coefficient, a is negatively correlated with the actual heat exchange temperature difference of the evaporator, a∈[0,1] and decreases monotonically with the increase of the heat exchange temperature difference.

[0008] S03. Associated curve L and n lim The rotational speed value corresponding to the intersection point of the associated curve L and n in the same coordinate system under the current heat exchange temperature difference is the highest rotational speed at which the centrifugal compressor operates under the current heat exchange temperature difference.

[0009] Preferably, in step S02, a reference difference c between the actual heat exchange temperature difference ΔT and the set heat exchange temperature difference ΔT0 is set, and ΔT0 > c > 0. The temperature difference limiting coefficient reaches the first threshold value a1 at ΔT = ΔT0 - c and reaches the second threshold value a2 at ΔT = ΔT0 + c. Among them, the first threshold value a1 approaches 1, and at the first threshold value a1, n lim approaches n max ; the second threshold value a2 approaches 0, and at the second threshold value a2, n lim approaches n min , and within the range of ΔT from ΔT0 - c to ΔT0 + c, the temperature difference limiting coefficient decreases linearly from the first threshold value a1 to the second threshold value a2.

[0010] Preferably, the first threshold value a1 is between 0.9 and 1, and the second threshold value a2 is between 0 and 0.1.

[0011] Preferably, the temperature difference limiting coefficient is:

[0012]

[0013] where ΔT is the actual heat exchange temperature difference of the evaporator, ΔT = TLo - Te, TLo is the chilled water outlet temperature, Te is the saturation temperature corresponding to the evaporation pressure Pe, and ΔT0 is the set heat exchange temperature difference of the evaporator.

[0014] A rotational speed control device for a variable-frequency centrifugal chiller includes a centrifugal compressor, an inverter, an evaporator, a condenser, an evaporation pressure sensor and a chilled water outlet temperature sensor arranged on the evaporator, and a condensing pressure sensor arranged on the condenser. A controller is provided in the inverter. The evaporation pressure sensor, the chilled water outlet temperature sensor and the condensing pressure sensor are all electrically connected to the controller. The controller controls the rotational speed of the centrifugal compressor through the inverter according to the feedback data of each sensor according to the above-mentioned rotational speed control method for the variable-frequency centrifugal chiller.

[0015] An air conditioner, the air conditioner includes the above-mentioned rotational speed control device for a variable-frequency centrifugal chiller.

[0016] Beneficial effects: The rotational speed control method for the variable-frequency centrifugal chiller provided by the present invention has the following beneficial effects:

[0017] 1. When the refrigerant in the evaporator is insufficient, the present invention limits the maximum loading rotational speed of the centrifugal compressor through the temperature difference limiting coefficient, which can prevent the temperature difference of the evaporator from being too large and resulting in too low evaporation pressure, so that the unit cannot operate normally, and the service life of the unit is prolonged.

[0018] 2. By restricting the rotational speed of the centrifugal compressor, the present invention enables the unit to operate within the high-efficiency range. Under the requirement of achieving the same refrigerating capacity, the energy consumption of the unit can be reduced and the overall energy efficiency of the unit can be improved.

[0019] 3. By setting the reference difference between the actual heat exchange temperature difference and the set heat exchange temperature difference, the rotational speed of the centrifugal compressor is controlled in three levels. When ΔT < ΔT0 - c, the values of the temperature difference limit coefficient are all greater than the first threshold a1, and at this time, the rotational speed n lim approaches n max , the chiller can operate normally and is not affected by the restricted rotational speed. When ΔT > ΔT0 + c, the values of the temperature difference limit coefficient are all less than the second threshold a2, and n lim approaches n min . In the interval from ΔT0 - c to ΔT0 + c, the temperature difference limit coefficient tends to decrease linearly, and the maximum restricted rotational speed can be adjusted correspondingly for different working conditions, thereby avoiding too low evaporation pressure and maintaining the normal operation of the unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following briefly introduces the drawings required for the description of the embodiments or the prior art.

[0021] Figure 1 Structural schematic diagram of the variable-frequency centrifugal chiller of the present invention;

[0022] Figure 2 Restricted rotational speed curve diagram of the centrifugal compressor when ΔT < ΔT0 - c in the present invention;

[0023] Figure 3 Restricted rotational speed curve diagram of the centrifugal compressor when ΔT > ΔT0 + c in the present invention;

[0024] Figure 4 Variation trend diagram of the temperature difference limit coefficient a in the embodiment of the present invention;

[0025] Figure 5 Restricted rotational speed curve diagram of the centrifugal compressor in the embodiment of the present invention;

[0026] Among them, 1 - centrifugal compressor, 2 - frequency converter, 3 - evaporator, 4 - condenser, 5 - evaporation pressure sensor, 6 - chilled water outlet temperature sensor, 7 - condensation pressure sensor, 8 - throttle valve. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The following more clearly and completely describes the present invention by way of a preferred embodiment in combination with the drawings, but the present invention is not limited to the scope of the described embodiments.

[0028] As shown Figure 1 in the figure is a control device for a variable-frequency centrifugal chiller disclosed by the present invention, including a centrifugal compressor 1, an inverter 2, an evaporator 3, a condenser 4, an evaporation pressure sensor 5 and a chilled water outlet temperature sensor 6 disposed on the evaporator 3, and a condensation pressure sensor 7 disposed on the condenser 4. A controller is provided in the inverter 2. The evaporation pressure sensor 5, the chilled water outlet temperature sensor 6 and the condensation pressure sensor 7 are all electrically connected to the controller. The evaporator 3 and the condenser 4 are connected through a throttle valve 8. In the evaporator 3, the heat of the chilled water is transferred to the refrigerant, and the liquid refrigerant becomes gaseous. The centrifugal compressor 1 compresses the low-temperature and low-pressure gaseous refrigerant into a high-temperature and high-pressure gaseous refrigerant. In the condenser 4, the high-pressure gaseous refrigerant liquefies after releasing heat, and the high-pressure liquid refrigerant enters the evaporator 3 through the throttle valve 8 and evaporates and absorbs heat again.

[0029] When the refrigerant leaks or the throttle valve fails, the amount of refrigerant entering the evaporator is insufficient, which increases the heat transfer temperature difference of the evaporator, and the heat transfer amount between the evaporator and the chilled water also decreases. If the loading continues with the chilled water temperature as the target without limitation, the compressor speed increases, the operating pressure ratio of the compressor increases, the operating power increases, and the overall energy efficiency of the unit decreases. Since the amount of refrigerant in the evaporator decreases, the refrigeration capacity may instead decrease.

[0030] The present invention discloses a method for controlling the speed of a variable-frequency centrifugal chiller, including the steps of:

[0031] S01. Fitting the correlation curve L between the speed and the pressure ratio of the centrifugal compressor when the chiller maintains the chilled water outlet temperature of the evaporator;

[0032] S02. According to the actual heat transfer temperature difference of the current evaporator, restricting the maximum speed n of the centrifugal compressor lim , n lim = n min + a(n max - n min ), where n min = fl(ε) is the minimum speed equation of the centrifugal compressor, n max = fh(ε) is the maximum speed limit equation of the centrifugal compressor, ε is the current pressure ratio of the centrifugal compressor, a is the temperature difference limit coefficient, a is negatively correlated with the actual heat transfer temperature difference of the evaporator, a ∈ [0, 1] and monotonically decreases as the heat transfer temperature difference increases.

[0033] S03. The rotational speed value corresponding to the intersection of the correlation curve L and n lim in the same coordinate system is the maximum rotational speed at which the centrifugal compressor operates under the current heat transfer temperature difference.

[0034] When the heat transfer changes due to the change in the amount of refrigerant entering the evaporator, the above control method is used to control the speed of the centrifugal compressor, avoiding the continuous ballasting of the centrifugal compressor. Although the speed increases, it will cause the refrigerating capacity to decrease instead, resulting in the problem of reducing the overall energy efficiency of the unit. The relevant control program is preset in the frequency converter, and the frequency converter controls the speed of the centrifugal compressor according to the feedback data of each sensor according to the above-mentioned speed control method of the variable-frequency centrifugal chiller.

[0035] In order to avoid the influence of speed limitation on the operation of the chiller during normal operation of the chiller, the present invention sets a reference difference c between the actual heat transfer temperature difference ΔT and the set heat transfer temperature difference ΔT0, and ΔT0 > c > 0. The temperature difference limitation coefficient reaches the first threshold a1 at ΔT = ΔT0 - c, and reaches the second threshold a2 at ΔT = ΔT0 - c, where the first threshold a1 approaches 1, n lim approaches n max ; the second threshold a2, a approaches 0, n lim approaches n min , and within the interval of ΔT from ΔT0 - c to ΔT0 + c, the temperature difference limitation coefficient decreases linearly from the first threshold a1 to the second threshold a2.

[0036] The change of the temperature difference limitation coefficient a by the actual heat transfer temperature difference includes three stages, as Figure 3 and Figure 4 When the heat transfer temperature difference is within the normal range, the speed n lim approaches n max is limited. After the actual heat transfer temperature difference reaches or exceeds the set heat transfer temperature difference, the limited speed changes synchronously with the heat transfer temperature difference situation, so that the operating condition of the centrifugal compressor changes synchronously with the heat transfer temperature difference. And when the actual heat transfer temperature difference exceeds the upper limit, n lim approaches n min , so that the unit operates at the lowest state to avoid unnecessary losses.

[0037] For the above three-stage speed control method, a function setting program that satisfies the change trend requirements can be used for control. By detecting the actual heat transfer temperature difference corresponding to the first threshold a1 and the second threshold a2, the frequency converter judges the operating tooling of the unit at this time according to the data measured by the sensor, and then applies the corresponding limited speed.

[0038] Preferably, the temperature difference limitation coefficient of the present invention:

[0039]

[0040] Among them, ΔT is the actual heat exchange temperature difference of the evaporator, ΔT = TLo - Te, TLo is the chilled water outlet temperature, which can be measured by the chilled water outlet temperature sensor 6, Te is the saturation temperature corresponding to the evaporation pressure Pe, and the evaporation pressure is measured by the evaporation pressure sensor 5. Thus, the corresponding evaporation pressure can be obtained by looking up the table according to the evaporation pressure. When applied to the control program, the corresponding relationship can be preset in the system. When the frequency converter controls the speed of the centrifugal compressor, the corresponding data can be directly retrieved. ΔT0 is the set heat exchange temperature difference of the evaporator. The temperature difference limit coefficient control of the present invention only requires one formula to obtain the corresponding temperature difference limit coefficient throughout the process, which can simplify the control logic compared with the segmented control method.

[0041] The present invention also provides an air conditioner, which includes the above-mentioned speed control device for a variable-frequency centrifugal chiller.

[0042] The following uses a specific embodiment to illustrate the implementation process of the control method of the present invention.

[0043] (1) Given the minimum speed limit equation of the centrifugal compressor:

[0044] n min = fl(ε) = 1777ε 3 - 13303ε 2 + 36820ε - 21399

[0045] (2) Given the maximum speed limit equation of the centrifugal compressor. Since the compressor has a speed operation upper limit, the maximum speed limit equation is a piecewise function:

[0046] n max = fh(ε) = - 1359.8ε 4 + 13224ε 3 - 48225ε 2 + 81339ε - 42829, ε < 2.4;

[0047] n max = fh(ε) = 14909, ε ≥ 2.4.

[0048] (3) Temperature difference limit coefficient:

[0049]

[0050] ΔT0 is 2.5°C, and the reference difference c is 0.5°C. As Figure 4 shown, the temperature difference limit coefficient corresponding to the position of ΔT = 2°C is the first threshold a1, and the temperature difference limit coefficient corresponding to the position of ΔT = 3°C is the second threshold a2.

[0051] When the pressure ratio ε = 2.6 and the heat transfer temperature difference of the evaporator is ΔT = 1, it indicates that the heat transfer of the evaporator is normal at this time. The limited speed n lim = 0.999n max = 1.150n min , that is, it is close to the choking region and far from the surge region.

[0052] When the pressure ratio ε = 2.6 and the heat transfer temperature difference of the evaporator reaches ΔT = 3, it indicates that the heat transfer of the evaporator is poor at this time. The limited speed n lim = 0.889n max = 1.02n min , that is, it is far from the choking region and close to the surge region.

[0053] When the amount of refrigerant entering the evaporator changes, the actual heat transfer temperature difference of the evaporator will increase, and the corresponding limited speed curve will move from the maximum speed curve to the minimum speed curve. To keep the refrigerating capacity of the whole machine the same, the operating trajectory speed of the centrifugal compressor will rise with the increase of the pressure ratio. During this process, the operating trajectory of the centrifugal compressor will intersect with the speed limit curve at "Point 1". At this time, the speed corresponding to Point 1 is the limited speed of the centrifugal compressor under the current refrigerating capacity. At this time, the heat transfer temperature difference is 2.7°C, the operating pressure ratio reaches 2.74, and the limited speed is 14040 rpm. The compressor will stop accelerating when it runs to this speed.

[0054] If there is no limit, the compressor will continue to accelerate and will eventually run to "Point 2" and reach a speed of 14909 rpm.

[0055] Make the following table to compare the operating energy consumption and efficiency of the centrifugal compressor under the two working conditions:

[0056] Operating conditions With limited speed Without limited speed Rotational speed rpm 14040 14909 Actual heat transfer temperature difference of evaporator ℃ 2.7 5 Cooling capacity kW 1582 1582 Operating pressure ratio 2.74 2.98 Power consumption kW 260.1 284.3

[0057] Thus, it can be seen that after increasing the limited speed, on the premise of achieving the same refrigerating capacity, the energy consumption of the centrifugal compressor is reduced and the efficiency is improved.

[0058] The above are only the preferred embodiments of the present invention. It should be noted that: for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and retouches can still be made, and these improvements and retouches should also be regarded as the protection scope of the present invention.

Claims

1. A speed control method for a variable-frequency centrifugal chiller, characterized in that, Including the steps: S01. Fit the correlation curve L between the rotational speed and the pressure ratio of the centrifugal compressor when the chiller maintains the chilled water outlet temperature of the evaporator; S02. Limit the maximum speed of the centrifugal compressor according to the actual heat exchange temperature difference of the current evaporator Limit , where is the minimum speed equation of the centrifugal compressor is the maximum speed limit equation of the centrifugal compressor is the current pressure ratio of the centrifugal compressor a is the temperature difference limit coefficient a is negatively correlated with the actual heat exchange temperature difference of the evaporator a ∈[0, 1] and monotonically decreases as the heat exchange temperature difference increases S03. The rotational speed value corresponding to the intersection point of the correlation curve L and in the same coordinate system is the highest rotational speed at which the centrifugal compressor operates under the current heat exchange temperature difference; In step S02, the actual heat exchange temperature difference is set and the set heat exchange temperature difference to obtain a reference difference value c, and > c > 0. The temperature difference limit coefficient reaches the first threshold at , and reaches the second threshold at , where the first threshold approaches 1. At the first threshold , approaches ; The second threshold approaches 0. At the second threshold position, it approaches . In the interval from to , the temperature difference limit coefficient decreases linearly from the first threshold to the second threshold ; The temperature difference limit coefficient: ； Among them, is the actual heat transfer temperature difference of the evaporator, Δ T = TLo - Te , TLo is the chilled water outlet temperature, Te is the saturation temperature corresponding to the evaporation pressure Pe , is the set heat transfer temperature difference of the evaporator.

2. The speed control method of the variable frequency centrifugal chiller according to claim 1, wherein The first threshold is between 0.9 and 1, and the second threshold is between 0 and 0.

1.

3. A speed control device for a variable-frequency centrifugal chiller, characterized in that, It includes a centrifugal compressor (1), an inverter (2), an evaporator (3), a condenser (4), an evaporation pressure sensor (5) and a chilled water outlet temperature sensor (6) provided on the evaporator (3), and a condensation pressure sensor (7) provided on the condenser (4). A controller is provided in the inverter (2). The evaporation pressure sensor (5), the chilled water outlet temperature sensor (6) and the condensation pressure sensor (7) are all electrically connected to the controller. The controller uses the inverter (2) to control the rotational speed of the centrifugal compressor according to the feedback data of each sensor and the rotational speed control method of the variable-frequency centrifugal chiller described in any one of claims 1-2.

4. An air conditioner, characterized in that, The air conditioner includes the rotational speed control device of the variable-frequency centrifugal chiller described in claim 3.

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