Compression chillers can be divided into centrifugal chillers, screw chillers, piston chillers and scroll chillers according to their compression forms. Centrifugal chillers are widely used because of their compact structure, large refrigerating capacity of a single machine, small floor area, stepless automatic regulation and high efficiency. To study the influence of operating conditions on the performance of centrifugal chillers and realize the efficient operation of chillers has always been the focus of researchers in the air conditioning and refrigeration industry. The efficiency of the chiller is affected by many factors. In addition to improving the efficiency through its own manufacturing process, attention should also be paid to the impact of operating conditions on its energy efficiency.
1. Influencing factors of chiller performance
The factors that affect the actual operation performance of water chillers can be divided into two categories: internal factors and external factors. Among them, compressor type, unit design, manufacturing process, refrigerant type and filling amount are internal factors. In addition, there are obvious differences in the performance of water chillers under different operating conditions (external factors).
The influence of operating conditions on COP of chiller includes the following factors: condensation temperature, evaporation temperature and load rate. The condensation temperature depends on the condenser water flow, the condenser water inlet temperature and the condenser heat exchange efficiency (the heat exchange temperature difference between water and refrigerant); The evaporation temperature depends on the heat exchange efficiency of the evaporator, the outlet water temperature of the evaporator and the water flow of the evaporator. Fouling coefficient, non condensable gas content and heat exchanger configuration will have a certain impact on heat exchange efficiency.
Generally, the cop indicated on the nameplate of the chiller is the efficiency under the nominal working condition of the national standard. According to this cop, the operating performance of the chiller can be compared in a certain range. However, in the actual operation of chillers, the operating conditions vary greatly, and cop under nominal conditions is difficult to reflect the actual operating energy efficiency of chillers. Taking a centrifugal chiller of a certain brand as an example, this paper analyzes the regular relationship between different factors and chiller efficiency.
2. Effect of load rate on coefficient of performance
The cop of the centrifugal chiller under nominal working conditions is 6.6. Under the conditions of constant flow of cooling water and chilled water, chilled water outlet temperature of 7 ℃ and cooling water inlet temperature of 30 ℃, the performance characteristics of chillers under different load rates are obtained.
The partial load performance curve of the centrifugal chiller starts from 100%. With the decrease of the load rate, the cop slowly increases. In the partial load section of 75% ~ 85%, the cop reaches the highest. Cop at the highest point of the curve is about 1.05 times of cop under full load condition. The partial load performance curve in Figure 1 shows the general performance law of centrifugal chillers, that is, the cop peak of centrifugal chillers often does not appear at full load.
When the constant frequency centrifugal chiller operates at partial load, the cooling capacity can be adjusted by adjusting the guide vanes and inlet throttling. When the guide vane is slightly closed, the refrigerant flow is reduced. At the same time, the heat exchanger has sufficient heat exchange under partial load (equivalent to enlarging the heat exchanger), so the unit efficiency is usually the highest under partial load (the highest cop often occurs in the load section of 70% ~ 90%). However, when the guide vane is opened too small, the throttling effect is significantly increased and the efficiency is greatly reduced. It is worth noting that the load rate corresponding to the highest cop (i.e. the optimal load rate) also changes dynamically with different operating conditions.
Therefore, the on-site operating personnel found and analyzed the optimal operating load section of the chiller, and reasonably controlled the number of chillers to be started according to the change of end load demand, which is of great significance to improve the energy efficiency and safety of the whole machine room.
3. Effect of chilled water outlet temperature on coefficient of performance
At the same condensing temperature, different evaporator outlet water temperatures will also have an impact on the COP of the chiller. Under the conditions of constant flow of cooling water and chilled water, and the inlet water temperature of the condenser is 30 ℃, the performance characteristics of the chiller at different outlet water temperatures of the evaporator are obtained.
The performance COP of the chiller increases with the increase of the outlet water temperature of the evaporator. Through analysis and comparison, for every 1 °C increase in the outlet water temperature of the evaporator, the COP of the chiller increases by 1.5% to 3%, and the specific improvement effect is related to the actual operating conditions. The COP peaks of the three chiller performance curves all appear at 75% to 85% load rate.
Increasing the outlet water temperature of the evaporator can increase the COP value of the chiller, because the increase of the evaporating temperature means that the compression ratio of the compressor decreases. At the same time, the volumetric cooling capacity of the refrigerant in the refrigeration cycle changes with the suction state of the compressor, and the specific volume of the refrigerant vapor in the refrigeration cycle increases with the decrease of the evaporating temperature. When the condensation temperature is determined, the cooling capacity of the centrifugal chiller will increase with the increase of the evaporating temperature, and the COP of the chiller will increase with the increase of the evaporating temperature.
Although increasing the outlet temperature of the chilled water to improve the performance of the chiller has a significant effect, due to the process requirements in the factory engineering, especially the clean room workshop of the semiconductor factory has strict requirements on the temperature and humidity of the air conditioner and the number of air changes, the outlet temperature of the chilled water is adjusted. The range is relatively narrow. This requires the operator to optimize the outlet water temperature as much as possible on the premise of ensuring the process requirements, so that the chiller operates at the peak COP point, and further improves the performance of the chiller. In commercial buildings, resetting the outlet temperature of chilled water is an important way to effectively improve the overall energy efficiency of the air-conditioning chiller room. In the transitional seasons of spring and autumn or during the low-load period at night, appropriately increasing the outlet temperature can improve the efficiency of the chiller.
4. Influence of condenser inlet water temperature on performance coefficient
The condensation temperature of the chiller is determined by the heat exchange process on the cooling side of the chiller. The heat is discharged from the chiller to the outdoor environment and undergoes three heat exchange processes in turn: the condensation heat of the refrigerant in the condenser is transferred to the cooling water, and the cooling water transfers the heat. It is transported from the chiller to the cooling tower, and the cooling water in the cooling tower exchanges heat with the outside air.
Condenser inlet water temperature is affected by cooling tower output and wet bulb temperature. Under the condition of constant flow of cooling water and chilled water, and the outlet water temperature of the evaporator is 7 ℃, the curve of the performance of the chiller with different inlet water temperature of the condenser is shown in Figure 3. Figure 3 shows that as the inlet water temperature of the condenser decreases, the COP of the chiller gradually increases. For every 1 °C decrease in the inlet water temperature of the condenser, the COP of the chiller increases by 2% to 5%, and the energy saving effect of the chiller is obvious.
Many automatic control manufacturers or energy-saving companies on the market have achieved certain energy-saving effects through the optimal control of cooling water temperature. The principle is that when the inlet water temperature of the condenser decreases, the condensing pressure decreases, the enthalpy of the compressor outlet gas decreases, and the input electric energy decreases, thereby increasing the COP of the chiller. In the actual operation control, it is also necessary to reasonably allocate the energy consumption of the chiller and the cooling water transmission, distribution and heat removal equipment according to the outdoor wet bulb temperature and the actual configuration of the cooling tower and the chiller, combined with the conditions such as the minimum allowable inlet water temperature of the cooling water of the unit. weight to improve the overall energy efficiency of the equipment room.
5. The effect of variable flow rate of chilled water on the coefficient of performance
In the variable flow control of chilled water, there are three common methods:
1) Differential pressure control method with constant pressure difference between supply and return water main pipes;
2) The most unfavorable end-loop differential pressure control method in which the pressure differential at the end of the loop is kept constant;
3) Constant temperature difference control method for constant supply and return main pipe temperature difference.
The difference between the three variable flow control will not be discussed here, and the following analysis will be carried out using constant temperature difference variable flow control for chilled water. Under the conditions of constant cooling water flow, condenser inlet water temperature of 30 °C, and evaporator outlet water temperature of 7 °C, the characteristics of the chiller under different inlet and outlet water temperature differences of the evaporator are shown in Figure 4.
It can be seen from Figure 4 that when the partial load of the evaporator of the chiller is greater than 80%, the three variable flow performance curves are very close. Small.
The reason is that, on the one hand, the reduction of the water flow rate on the evaporator side leads to a decrease in the heat exchange efficiency on the evaporator side, which reduces the COP of the unit; The difference in temperature will cause changes in the evaporation temperature. The average temperature of the inlet and outlet water at the evaporator side is higher than the average temperature of the inlet and outlet water at the constant flow rate under partial load, which means that the evaporation temperature of the chiller is higher, so the COP of the unit is higher. improve. The combined effect of the above two aspects keeps the COP of the unit basically unchanged under different temperature differences.
6. The influence of variable flow rate of cooling water on COP
When the inlet water temperature on the condensing side is the same, with the decrease of the cooling water flow rate, the heat rejection of the system will decrease and the heat exchange efficiency of the condenser will decrease. Under the conditions of constant flow of chilled water, condenser inlet water temperature of 30 °C, and evaporator outlet water temperature of 7 °C, the characteristics of the chiller under different inlet and outlet water temperature differences of the condenser are shown in Figure 5. It can be seen from Figure 5 that when the conditions on the evaporator side are the same and the inlet water temperature of the condenser is constant, the variable flow rate of cooling water has a greater impact on the performance of the chiller. Under the constant temperature difference variable flow control, with the decrease of the load rate, the cooling water flow on the condensing side of the chiller gradually decreases. Compared with the constant flow operation, the COP of the chiller drops by 3% to 8%.
Similar to the chilled water pump, after the cooling water pump changes the flow rate, the energy saving often accounts for 3% to 8% of the operating energy consumption of the chiller. The change direction of the specific energy saving rate is consistent with the trend of the decrease of the COP of the chiller, that is, the smaller the load rate, the more energy saving the cooling water pump will save, and the more the COP of the chiller will drop.
In summary, the frequency conversion control of the cooling water pump has a great impact on the performance of the chiller. Whether the energy saved by the frequency conversion of the cooling water pump offsets the increase in the energy consumption of the chiller is still surplus, which is the key factor in deciding to adopt this technology.
In this paper, the five main factors that affect the performance of the chiller, namely the load rate of the chiller, the inlet water temperature of the condenser, the outlet water temperature of the evaporator, the water flow rate of the evaporator and the water flow rate of the condenser, are quantitatively analyzed, and the following conclusions are drawn:
1) The COP peak value of centrifugal chillers often does not appear at full load. In more cases, when the load rate is 70% to 90%, the performance of the chiller is the highest;
2) For every 1 °C increase in the outlet water temperature of the evaporator, the COP of the chiller increases by 1.5% to 3%. On the premise of meeting the end heat exchange requirements, increasing the outlet water temperature of the evaporator can effectively improve the efficiency of the chiller room;
3) When the inlet water temperature of the condenser decreases by 1°C, the COP of the chiller increases by 2% to 5%. In actual operation control, it is necessary to reasonably allocate the energy consumption weight of the chiller and the cooling water transmission, distribution and heat removal equipment to achieve the overall chiller room. energy efficiency improvement;
4) Reasonable adjustment of the water flow of the evaporator can greatly save the energy consumption of the chilled water pump. There is almost no difference in the performance of the chiller under different chilled water flows, so the chilled water flow can be reduced as much as possible on the premise of ensuring the end demand;
5) The variable flow of cooling water has a great impact on the performance of the chiller. The variable flow of cooling water needs to fully understand the performance of the chiller and the performance of the cooling water pump and cooling tower;
6) The energy efficiency of each equipment in the chiller room affects each other. Excellent operation, control and management of the machine room should be based on the basic operating characteristics of the actual equipment, and take into account the overall energy efficiency of the machine room and improve energy utilization.