Air conditioning circulating pump capacity, number of units, the choice of the best pump operating point and technical and economic analysis to discuss how to choose the pump to ensure that the air-conditioning system is running well and reduce power consumption.
Keywords: circulating pump power consumption cycle resistance hydraulic balance value engineering
1 circulating pump capacity is too large
Circulating pump capacity is too large in our country is a common problem, its capacity is often 2-4 times the actual needs, resulting in a serious waste of project investment and operating costs. The main reasons are as follows:
1.1 design cold load is too large
Design cooling load is the main basis for selecting equipment, so the correct calculation of building cooling load on the entire air conditioning system design is very important. At present, the calculation method of air conditioning load provided in textbooks and design manuals is not only calculating the wall load of the envelope, but also the load of windows and doors, and the calculation result is for a specific room. However, the capacity of air conditioning system equipment is determined by the cooling load of the entire building. Due to the different orientation, location, use function and heat source of each room in the building, the time that the maximum cooling load in each room often appears is not the same. Therefore, the maximum building cooling load should be the maximum value of the hourly load stack per room. According to the survey, some designers in our country simply calculate the maximum cooling load of each room when calculating the cooling load of the building, resulting in the calculation result far exceeding the actual required load. Therefore, we must pay enough attention to this to make the design load more reasonable and correct.
1.2 system cycle resistance is too large
In the calculation of system cycle resistance, due to lack of experience of designers, making some of the calculation parameters are too conservative, resulting in the calculated value of the circulation resistance is too large, even worse, in the construction design phase estimation method to determine the cycle resistance, resulting in calculation cycle Resistance more than double the actual value.
1.3 system static pressure problem
Air conditioning system is full of water to run, pump inlet and outlet to withstand the same hydrostatic pressure. Therefore, the head of the selected pump can only overcome the pipeline system resistance. However, some designers consider the hydrostatic pressure within this circulation resistance, which of course increases the capacity of the circulating water pump.
1.4 system hydraulic balance problem
As the design does not seriously carry out the system of hydraulic balance calculation, after the completion of the project did not complete the required commissioning, often resulting in system hydraulic imbalance, the phenomenon of uneven heating system. Some technicians mistakenly believe that the cause of this phenomenon is that the circulating pump capacity is too small, the result is simply to increase the pump to solve the problem, of course, will increase the pump capacity.
2 pump characteristic curve and the best working point
2.1 pump flow - lift characteristic curve
Related pictures of this topic are as follows:
Pump flow - lift characteristic curve generally has three types: flat, steep drop, hump type (Figure 2.1). Pumps for air-conditioning water circulation system should have flat features, the zero flow and the maximum flow between the lift range should not exceed 10% -15%; steep drop characteristics of the pump due to its maximum flow and minimum flow between the lift too Large, so it should not be used; hump characteristics of the pump can not be used because of the parallel operation of the two pumps may cause load and lift cycle changes, and when the frequency of this change is equal to the natural frequency of the system will produce dangerous " Oscillation phenomenon ", and this phenomenon will have some impact on the normal operation of the system.
2.2 The best working point
Related pictures of this topic are as follows:
As shown in Figure 2.2: The best working point of the circulating pump is the intersection point of the pump characteristic curve and the system pipe network characteristic curve. However, due to various reasons, the actual flow of the system is always greater than the designed calculated flow rate. As a result, the designed pump operating point is shifted to the right along the pump characteristic curve (as shown in Figure 2.2B point).
When the working point of the pump is shifted to the right, the lift produced by the circulating pump is reduced, which is extremely unfavorable to the normal operation of the system. In particular, the most unfavorable loop in the system will further reduce the flow of the loop and affect the normal Use function
There are mainly two reasons for the shift to the right of the work point. The first is that the hydraulic calculation in the design adopts an excessive safety factor and an unrealistic pressure drop calculation method. The second is that the designed system does not perform serious hydraulic balance calculation. After construction, Have not been rigorous system debugging. Therefore, in order to make the system run according to the design conditions, in addition to the relevant calculations should be carefully and carefully, but also in the selection of the pump when the pump operating point at the left side of the optimal choice of the appropriate location to prevent the pump actual working point Beyond a certain range in uneconomical operating conditions, affecting the normal operation of the system.
3 circulating water pump technical and economic analysis
3.1 The number of circulating pump selection
Article 6.1.11 of the Code for Design of Heating, Ventilation and Air Conditioning (GBJ19-87, 2001 edition) stipulates that the number and flow of chilled water pumps (primary pumps) should correspond to the number of chillers and the flow rate under design conditions . Secondary pump settings should be based on the size of the cold water system, the pressure difference between the parallel loop of the degree of difference, conditions of use and regulatory requirements through technical and economic comparison. However, in practical work, designers often fail to conduct a comprehensive technical and economic analysis on the comprehensive disposition of various equipments of the air-conditioning system. As a result, the initial investment of the project and the waste of resources such as the "Malaysian car" are caused. To avoid this phenomenon, the majority of designers in the program design stage should be based on the use of features, peak load time, system characteristics and other conditions, for air-conditioning systems, chillers, circulating pumps, cooling towers and other equipment comprehensive configuration full Technical and economic analysis, with a view to meet the use of functional under the premise of reducing project cost and operating costs.
3.2 project life cycle costs
I believe that in the process of circulating water pumps, chillers and other equipment, technical and economic analysis should introduce a concept - engineering life cycle costs. The life cycle cost of a project is the expenses incurred in the process of engineering design, development, construction, use, maintenance and scrapping, that is, the design fee, construction fee, construction fee, maintenance fee and other expenses incurred by the project during its definite life cycle or within the scheduled validity period. Operation and maintenance costs, the sum of scrap recovery fees. Life-cycle costs are also very different for different projects and different project phases (as shown in Figure 3.1). Often, operating and maintenance costs are often greater than the one-time investment in project construction. Therefore, during the technical and economic analysis, it should be made clear that the cost of life cycle includes the cost items, the content and scope of each fee, and their interrelationship in the cost constitutional system, which is very important for our technical and economic comparison.
3.3 Value Engineering
Value engineering aims to improve the value of a product or a job and seeks to achieve a management technique that reliably realizes the user's desired function with the lowest life-cycle cost through organized and creative work, as expressed in Equation 3.3.1.
V = F / C (3.3.1)
Where V - the value of the research object F - the function of the research object C - the cost of the research object
Related pictures of this topic are as follows:
Value engineering technology has been widely used in R & D, design, construction and other industries, the core idea is to make the product with the lowest life-cycle costs it must have the function. In the air conditioning equipment selection and technical and economic analysis, designers should make full use of value engineering concepts, and strive to achieve the minimum necessary investment in the use of functions. Of course, the current situation, to achieve such a design level will take time, but the majority of designers should work in this direction, with a view to achieve good social and economic benefits.
4 Conclusion
â‘ In the air conditioning design should be objective and accurate calculation of cooling load and system resistance, to avoid resulting equipment selection is too large;
â‘¡ select the circulating pump, pay attention to the pump operating point shift to the right to protect the pump head changes within the allowable range of normal operation of the system;
â‘¢ project life cycle cost and value engineering are good tools for engineering economic evaluation, technical and economic analysis should make full use of them.
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