Performance analysis of centrifugal chillers, single-unit cooling capacity of centrifugal chillers It is the most critical parameter to determine whether a centrifugal chiller can be used normally. It refers to the cooling capacity of the unit under design conditions, which can be provided from the manufacturer. Found in the product sample. Pay attention to the following points when selecting the single unit cooling capacity of the centrifuge: a) Since the efficient operating range of each centrifuge is between 40% and 100% of its cooling capacity, the load distribution of the building should be carefully analyzed during unit selection. Features, try to make the selected unit operate in the high efficiency zone. b) When the above conditions are met, the unit with large single unit cooling capacity should be selected as much as possible. c) In the selection of the building should refer to the building load distribution characteristics, and strive to avoid the unit operating in its surge zone (the surge point of the centrifuge is related to its energy regulation mode, the author will analyze in detail below), such as building load distribution In the case of low load, a screw-type refrigeration compressor or a piston-type refrigeration compressor can be selected to meet the air conditioning requirements at low load.

The value of the COP value of the centrifugal chiller is an important parameter to determine the performance of the centrifugal chiller. Try to select a unit with a high COP value. For the centrifugal chiller for air conditioning, the unit cooling capacity consumption under air conditioning conditions is below 0.2 kW/kW (the power consumption of some brand centrifuges sold locally in the country can be as low as 0.14 kW/ kW, of course, the price is also higher), Table 1 is the COP value of several imported brand units under air conditioning conditions.

Since the chillers are operated under partial load most of the time in actual use, in addition to considering the COP value under air conditioning conditions, they should also analyze their COP values ​​at partial load. At present, the partial load COP curves of the chillers provided by various manufacturers are in the design of cooling design. Tian Guoqing, male, born in October 1960, university, bachelor, associate professor, director of teaching and research department 310015, Zhoushan Road, Hangzhou, Zhejiang, China Under the water temperature condition, the temperature change of the cooling water in the actual use is very small. Therefore, when analyzing the partial load efficiency, the relevant manufacturer should be provided with the partial load COP curve at the constant water temperature.

In addition, the water resistance of the condenser and evaporator of the centrifuge has a great influence on the power consumption of the pump. It is calculated that the motor power of the 2110 kW centrifuge will increase by 1.5 MPa when the condenser water resistance increases by 0.1 MPa. For every 0.1 MPa increase in water resistance, the motor power will increase by 1.25 kW. This should also be taken into account when selecting equipment.

Noise is an important parameter to measure the performance of a centrifugal chiller. It is directly related to the impact of the centrifugal chiller on the surrounding environment. At present, the centrifugal sound chillers used in foreign countries generally have a sound level noise of 85. When understanding the noise index of any brand product, the following two points must be clarified: The first is the test environment of the noise value, which is actually tested in the field. It is still tested in the silent room; the second is to understand the distance between the measuring point and the chiller (generally divided into 1m and 3m). The main factors affecting the noise of the centrifuge are the transmission gear and the exhaust of the unit (the exhaust noise of the centrifuge is mostly derived from the high-speed flow of the gas in the exhaust pipe). For the former, the third-stage centrifugal can be used to solve the problem better. The following two methods are very effective: a) Optional compressor discharge side refrigerant spray device. The device sprays the refrigerant liquid to the exhaust side through the radial port, the mist-like refrigerant generated by the spray has a sound absorbing function, and the flashing vapor can cool the compressor exhaust. b) Select the noise reduction pad made of sound absorbing material and wrap it on the exhaust pipe, generally reducing the A level noise by 24dB.

The size and quality of the centrifugal chillers are mostly indoors. When selecting the equipment, it must be considered whether the selected equipment size meets the layout requirements. For units with the same performance, the smaller size should be preferred to reduce the floor space of the equipment. Heavy equipment will increase the load bearing requirements of the foundation, wasting materials and increasing costs. Some manufacturers claim that their products are larger in size and heavier in quality in order to improve cooling efficiency, but in fact it is not obvious from the comparison of external dimensions, quality and energy efficiency ratio. Dimensions and quality of several imported brand products (using R134a working fluid).

Structural Analysis of Centrifugal Chiller The analysis of the structure of the centrifugal chiller is to analyze its compressor type, energy regulation mode and self-control and safety protection of the refrigeration system, and compare its advantages and disadvantages in system configuration. The single head and the multi-header think that it is more reasonable to use the multi-head unit from the perspective of use. First of all, its reliability is higher than that of a single head unit. If the compressor multi-head unit is damaged during use, the other compressor heads can be used normally as long as the corresponding suction valve is closed. If the head unit has a damaged head, it will have to be shut down for maintenance, which will affect the normal use of the entire unit. Secondly, the multi-head unit has higher cooling efficiency under partial load. Since most of the operation time of the unit is under partial load conditions, it is more efficient when operating under 40% 100% load, and the efficiency will be greatly reduced when the load is below 40%. It is easier to use 40% in single unit. The following load operation situation. For example, if a multi-head unit has two heads, each assumes 50% of the load. When the entire unit is operated at 30% load, one of the heads is stopped and the other head is operated under its own 60% load condition, then it is still operating in the high efficiency zone.

The open type and closed type so-called open type unit is that the motor and the compressor of the centrifugal refrigeration compressor are separated from each other, and the coupling is connected in the middle. The compressor and motor of the closed unit are directly connected to each other and are housed in one casing.

In order to reduce the mechanical loss of the transmission gear, many companies are currently designing the mechanical loss of the gear to be less than 0.5%. In a centrifugal chiller, the compression ratio of the condensing pressure to the evaporating pressure is ensured by the tip speed of the impeller, and the life of the impeller is determined by the tip speed of the impeller. The multi-stage compression can greatly reduce the compression ratio of each stage, so that the tip speed of the impeller is reduced, which can fully improve the service life of the impeller.

The main factors affecting bearing life are its surface speed and its load capacity. For multi-stage centrifuges, because of its low tip speed, the bearing surface speed is also low, which is beneficial to extend its life. However, the multi-stage centrifuge uses multiple impellers, which undoubtedly increases the bearing capacity of the bearing. In addition, the three-stage centrifuge with R123 as the working fluid has a larger diameter, which also increases the bearing capacity of the bearing and affects the life of the bearing. . Therefore, the impact of single-stage and multi-stage centrifuges on bearing life needs to be considered comprehensively.

At present, the multi-stage centrifuge for air conditioners with economizers on the market has a refrigerant of R123. According to theoretical calculations, the efficiency of multi-stage centrifuges using R123 working fluid under the same working conditions can be increased by 7% compared with single-stage centrifuges. However, in actual work, since the suction and exhaust of each stage of compression have a certain resistance, the actual energy saving effect is affected to some extent.

Energy regulation and surge At present, the energy adjustment methods adopted by various manufacturers in centrifugal chillers are as follows: inlet guide vane adjustment, compressor frequency control, and bladeless diffuser width adjustment. Adjust together for adjustment. Different energy regulation methods are a very important factor in determining the surge point of a centrifuge.

a) Adopt ordinary imported guide vane adjustment, the surge point is between 20% and 30% load, and the different brand units are different.

b) Adopting the guide vane adjustment and supplementing the compressor impeller with variable frequency speed regulation, the surge point can be controlled at about 10% load.

c) The inlet guide vane adjustment is supplemented by the compressor diffuser width adjustment, and the surge point can be controlled at about 10% load.

d) For the multi-stage centrifugal unit, the inlet guide vane adjustment is adopted for each stage of the inlet, and the surge point can also be controlled to about 10% load.

Like the partial load COP value, the above-mentioned surge point is measured under the condition of changing the inlet water temperature of the cooling water, but the temperature of the cooling water in the actual use does not change much, which is the load when the surge occurs during actual use. The rate is always higher than the manufacturer's promise. Therefore, when selecting the unit, the manufacturer should be provided with the surge point under the cooling water temperature, which is of real significance.

Throttling devices Current throttling devices for centrifugal chillers mainly include electronic expansion valves and orifice plates. The main features of electronic expansion valves are: a) adaptive control logic can be used to form adaptive PSD ( That is, the control law of proportional P, sum S, and differential D). Stable operation under constant changes in refrigeration loop load and pressure.

b) The electronic expansion valve is driven by the stepping motor, which not only responds quickly but also accurately, and the expansion valve can realize full opening or full closing process within 30s.

c) It is not affected by the condensing temperature and can work under very low condensing pressure, greatly improving the COP value under partial load.

d) Smooth operation when near zero superheat, no oscillation, thus fully improving the efficiency of the evaporator.

e) The sealing performance is good. When the valve is fully closed, the pipeline can be completely sealed, and no separate solenoid valve is required.

Therefore, using it as a throttling device can improve the efficiency and reliability of the entire unit. However, the manufacturing cost of the electronic expansion valve is relatively high, and the use in the centrifugal refrigeration compressor is not yet widespread.

The main advantage of using the orifice plate as a throttling device for the chiller is the lower manufacturing cost. Its disadvantage is that as the heat load increases, the liquid supply of the evaporator is required to increase, and at this time, the amount of refrigerant supplied to the evaporator is reduced due to the decrease of the difference between the condensing pressure and the evaporation pressure, which forms a contradiction. . Therefore, this type of throttling is not suitable for occasions where the working conditions vary greatly.

Many companies have realized the disadvantages of orifice plates, so in the centrifuges using R134a as the working medium, various manufacturers basically use electronic expansion valves or the like. However, in the centrifuge with R123 working fluid, the difference between the condensation and evaporation pressure of R123 working fluid air conditioning conditions is small, and the mass flow rate of R123 and R134a in the air conditioning working condition is basically the same, such as the system using electronic expansion The valve as a throttling device is bound to make the expansion valve have a large diameter. Therefore, the current throttle device with R123 as the working medium adopts an orifice plate.

To evaluate the performance of a working fluid, there are three main points, namely efficiency (impact on unit efficiency), safety (toxicity, flammability) and environmental impact.

At present, the refrigerants used in the working fluid of centrifugal refrigeration compressors mainly include R22, R123 and R134a. The manufacturers all claim that the refrigerant selected is the most reasonable.

With the concern of more and more people to protect the ozone layer and global warming, there are already two clear goals in the field of air conditioning and refrigeration: a) to use alternative refrigerants with high theoretical cooling efficiency and to design and manufacture energy efficient equipment. To reduce the indirect warming impact of energy consumption and emissions of CO. b) Improve the sealing of equipment and systems, improve the operation and rules and regulations of maintenance and repair, in order to reduce the direct climate warming caused by the leakage and unintentional release of alternative refrigerants (from this point of view, it is more advantageous to use closed units) .

Measurement of Critical Angle The measurement of the critical angle in this experiment is divided into two different side wall conditions, namely a semi-constrained sub-scattering stream and a free-divided scattering stream. For the semi-restricted case, the tuyere is on the plane wall. It can be seen that, according to the difference of the apex angle of the wedge, the axis of the jet has different degrees of curvature. When the apex angle is small, the reflow zone is small, and the jet is bent inward under the pressure difference between the inner and outer sides until it is closed. As the apex angle increases, the curvature of the axis becomes smaller and smaller, and the curve to 87 is already quite small. At the apex angle of 90, the axis of the scattering flow is almost a straight line, and when the apex angle is increased, the influence of the side wall of the tuyere is increased. It may also occur on the outside side to cause the axis to bend, which is illustrated by the upward bending of the axis when the apex angle is 95. Experiments show that the critical angle of the semi-restricted wedge-shaped scattering flow without bending is 90.

For the free wedge-shaped partial scattering flow under the influence of the tuyere without wall, the experimental results are shown in Fig. 6. For the same reason, the critical angle of the free wedge-shaped scattering flow axis does not bend. For this apex angle, the axis will have different degrees of downward bending. .

Comparing the critical angles in the two cases is quite different, the critical angle of the free-segregating flow is 114, which is larger than the critical angle of the semi-restricted jet. This is because the semi-restricted jet is limited by the side wall of the tuyere, the airflow is diffused in the area below the side wall, and the free wedge-shaped jet is limited by the side wall, and its airflow diffusion domain is larger than the semi-constrained condition, thus causing the free splitting flow. The critical angle is larger.

Conclusion This experiment shows that the size of the wedge apex has a decisive influence on the stability of the scattering flow. Regardless of whether the side wall of the tuyere is restricted or not, there is a critical angle that does not bend along the jet axis of the guide vane. Regarding the critical angle of the sub-scattering flow, the critical angle at which the Awbi cone-shaped scattering flow remains unattached is 120. Since the cone-symmetric flow field is different from the wedge-shaped plane flow field of this experiment, the critical angle is different.

Since no relevant papers have been seen, the discussion of this experiment is preliminary. Only two main factors are considered. As for the influence of the size of the wedge-shaped guide vane, the position of the wedge-shaped apex leaving the tuyere and the uniformity of the outlet of the tuyere, etc., it remains to be seen. Further discussion.

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