Discussion on the design and control method of the

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Discussion on the design and control method of secondary pump system

1. Introduction

in recent years, with the extensive use of central air conditioning, building energy consumption in China has increased rapidly. According to statistics, the average annual growth rate of building energy consumption between 1990 and 2000 was 5.8%, significantly exceeding the growth rate of 2.4% of energy production in the same period. In the energy consumption of air conditioning, The conveying energy consumption of the system accounts for about 5mm1/3[1 between the upper pressure plate and the test piece. Therefore, variable flow technology has been paid more and more attention in the energy-saving design of air conditioning system.

for air conditioning water system, the proportion of conveying energy consumption in the total energy consumption increases with the increase of system scale. Variable water volume system (VWV) By changing the cold water flow in the transmission pipe to meet the user's load requirements, the transmission energy consumption of the system can be effectively reduced

2. Design of secondary pump system

as mentioned above, the change of user load can be achieved by changing the cold water flow of the system. However, in order to ensure the stability of hydraulic and thermal conditions, the allowable flow fluctuation range of the chiller is very small. There are usually two ways to solve this contradiction

Figure 1 shows the effective cost reduction method of differential pressure bypass control, which is widely used in domestic design. When the load is reduced, the user's valve is turned down, the differential pressure of the water collector increases, the electric control valve is turned up, and part of the cold water is short circuited through the bypass to maintain the unit flow unchanged. When the user's load is increased, the action is reversed

Figure 1 primary pump system

Figure 2 shows the common bridge pipe bypass control method in foreign design [2 [3. By setting bridge pipes, the whole system is divided into two circuits with relatively independent hydraulic conditions: cold water production and cold water transmission. Circulating pumps are set in each area to provide circulating power in this area. When the cooling load is equal to the user load, the flow in the bridge pipe is zero; when the user load is reduced, the flow in the bridge pipe flows from the water supply to the return water.

Figure 2 secondary pump system

for large regional cooling systems, tertiary pumps are often used System (PST: primary secondary tertiary pumping system), As shown in Figure 3 [4. From the perspective of system form, the tertiary pump system only extends the application of bridge pipe and still belongs to the category of secondary pump system.

Figure 3 tertiary pump system

the tertiary pump system separates cold water into three independent circuits: production, transmission and distribution. From the perspective of circulating water pump settings, the tertiary pump system belongs to a distributed booster pump system [5. The primary pump is responsible for the generation of cold water, the secondary pump is responsible for the transmission of cold water, and the tertiary pump is responsible for the distribution of cold water. The hydraulic conditions between circuits are relatively independent, the hydraulic coupling between users is small, there is no most unfavorable user, and the hydraulic stability of the system is good [6.

users of the tertiary pump system can configure the corresponding circulating water pump according to their own needs, and match the load requirements by adjusting the pump speed. The setting of the bridge pipe effectively avoids the interference of the regulating working conditions between users. Under the ideal working condition, the sum of the heads of the primary pump and the secondary pump is equal to the pump head of the primary pump system. Therefore, the pump energy consumption of the tertiary pump system will not be higher than that of the primary pump system.

3. The negative energy consumption of the secondary pump system Load regulation

the secondary pump system is a variable water volume system, which realizes the load regulation of users by changing the circulating water volume. There are two common methods of variable water volume regulation: number regulation and variable speed regulation

3.1 number regulation

the number regulation of traditional primary pump system mostly adopts differential pressure control, the number regulation of secondary pump system mainly adopts flow control, and load control is often used in occasions with high control accuracy

differential pressure control uses the parallel characteristic curve of water pumps to set a fluctuation range of water supply and return pressure. When the load changes and causes the change of pipe flow, the water supply and return pressure also fluctuates. When the set upper limit is exceeded, increase the pump; Reduce the pump when it is lower than the set lower limit

flow control is to control the start and stop of water pump and corresponding cooler according to the direction and size of water flow in the bridge pipe. When the user load decreases and the secondary flow decreases, the primary flow is excessive, and the cold water in the bridge pipe flows from the water supply to the return water. When the flow is greater than 110% of the flow of a single pump, turn off a cold machine, spray free bumper and the corresponding water pump; When the user load increases, the primary flow is insufficient, and the cold water in the bridge pipe flows in reverse. When the flow is greater than 20% of the flow of a single pump, start a water pump and the corresponding cooler. The purpose of starting the refrigerator in advance is to avoid large fluctuations in the temperature of secondary water supply

Figure 4 Relationship between cooling capacity and flow of fan coil tube

Figure 4 is the relationship between cooling capacity and flow of fan coil unit, a common end equipment of air conditioning system [7. Because the thermal characteristics of end equipment have nonlinear characteristics [8 ~ [10. When the flow demand is reduced to one water pump, it does not mean that the user's load is also reduced to the capacity of one chiller. Therefore, load control should be used in occasions with high control requirements. Load control is to calculate the cooling demand by detecting the temperature difference and flow on the primary side supply and return pipe. When the cooling demand is reduced to the capacity equivalent to one chiller, stop one water pump and the corresponding chiller. Compared with flow control, load control can effectively solve the problem of water Force The problem of uncoordinated thermal conditions [1.

3.2 variable speed regulation

the resistance overcome by the head of the secondary pump includes pipes, coils, balance valves and control valves. In the current system of constant speed and variable water volume, when the flow decreases, the pressure drop of pipes, coils and balance valves also decreases, but the head of the circulating pump does not decrease, but also increases. The difference between the two must be controlled by the control valve (two-way valve) To bear the burden, see Figure 5. Therefore, the energy-saving effect of the constant speed variable water volume system is not obvious. At very low load, the control valve will be out of control due to excessive differential pressure, causing excessive cold water to pass through the coil

Figure 5 pressure on control valve of constant speed variable water volume system

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