Why should the air volume be determined first when building a new air conditioning system? It affects air ducts, static pressure and energy consumption
Direct answer: The design sequence of a new air conditioning system should be to determine the air volume first, then determine the air duct, then calculate the resistance, and then select the fan box. Air volume is the common basis for duct size, system static pressure, and equipment selection. If the air volume is set wrong, every subsequent step will be biased. DC speed regulation can optimize operation, but it cannot make up for errors in early air volume calculations. If the air duct is designed with the wrong air volume and the speed is adjusted later, either the noise will be high or the air volume at the end will be insufficient.
Aili Industry is discussingWhen selecting a fan box , the project is usually required to provide a zone air volume balance table and the most unfavorable path pressure loss, and then check the fan, motor and control configuration.
As the air volume changes, the size of the air duct must also change.
The air volume determines the wind speed in the air duct. For air ducts with the same cross-section, as the air volume increases, the wind speed increases; as the air volume decreases, the wind speed decreases. Wind speed directly affects the selection of pipe diameter: when the wind speed is high, the pipe diameter can be small, but the resistance is large and the noise is high; when the wind speed is low, the pipe diameter is large, but the resistance is small and the operation is quiet.
If the equipment is first locked according to the nominal air volume of a certain type of fan box, and then the demand for each room is calculated, two situations may occur. The first is that the air volume is estimated to be too large, the air ducts are selected to be thick, and the equipment is also selected to be large. If it is operated in a low load area for a long time, the energy consumption and initial investment are both high. The second is that the air volume is underestimated and the air duct is selected to be thin. Later, the terminal demand increases and the resistance rises rapidly. Even if the speed is increased, it may cause noise and extra power consumption.
The reasonable sequence is to formulate the zoning requirements first, and then complete the air duct size calculation and equipment matching. Once the construction of the air duct size is completed, subsequent changes will be very costly. For new projects, setting the air volume accurately during the drawing stage is more cost-effective than adjusting it after installation.
Air volume affects system resistance, and resistance determines static pressure.
The resistance in the air duct is approximately proportional to the square of the wind speed. As the wind volume increases and the wind speed increases, the resistance along the way and the local resistance will increase significantly. The local resistance of elbows, reducers, valves and filter sections also increases with the increase in wind speed. This means that if the air volume is set on the high side, the system static pressure demand will also be on the high side, and the selection pressure of the fan box will increase accordingly.
System static pressure is determined by the most unfavorable path. The least favorable path is not necessarily the physically longest path, but the path with the greatest pressure loss. Although some branches are short, they have many elbows and valves or the resistance of the end filter is large, and the pressure loss may exceed that of a long straight pipe. When calculating, the calculation should be carried out section by section along the most unfavorable path, rather than just looking at the length of the total pipe.
If the end of the system is configuredFor HEPA/ULPA high-efficiency filters , the initial resistance of the filter and the resistance at the end of use should be included in the total resistance of the system. If the model is selected only according to the clean state, the terminal air volume may not be satisfied after running for a period of time. When the process requirements are higher, it can be evaluatedDoes the ULPA ultra-high efficiency filter meet the requirements?
Static pressure determines the fan box selection, not the maximum air volume
The working point can be formed only after the design air volume and the most unfavorable path resistance are obtained. The operating point is the intersection of the fan performance curve and the system resistance curve. When selecting the model, it should be confirmed that the intersection point falls near the high efficiency area of the fan, and that the capabilities of the motor and driver can be covered.
The nominal maximum air volume is mostly obtained under specific test conditions and does not necessarily correspond to the external static pressure of the project. The selection table should contain at least three parameters: 'air volume, external static pressure, and machine input power' at the same time. If necessary, a performance curve should also be provided. Only the maximum air volume and motor rated power are given, and it is impossible to judge how the equipment will operate after it is installed in the system.
If the project adopts a DC speed regulation scheme, the speed regulation range, control signal type and driver installation location must also be confirmed. Clean room ceiling air supply is comparableFFU fan filter unit ; centralized management of multiple devices can be matchedThe FFU computer group control system realizes zone speed regulation and status monitoring. Protection of single process equipment can be consideredEquipment-side fan filter unit (EFU) . The final filtered air supply at the end of the air duct can beHigh-efficiency air supply outlet (HEPA BOX) . The division of labor between these equipment and the fan box should be clearly stated on the system flow chart.
Air volume affects energy consumption throughout the year, not just equipment selection
The air volume not only determines the size of the equipment, but also affects the energy consumption throughout the year. From the fan law, the air volume is proportional to the rotational speed, and the shaft power is proportional to the cube of the rotational speed. The rotation speed is reduced by 20%, the air volume is reduced by approximately 20%, and the shaft power is reduced to approximately 51% of the rated value. This is the fundamental reason why speed regulation can save energy.
However, if the air volume calculation itself is too large and the equipment is selected according to the large working conditions, even if the speed is adjusted later, it may still operate in the low-efficiency zone for a long time. On the contrary, if the air volume is small, the terminal demand cannot be met, and the operator may increase the rotation speed to compensate, resulting in increased noise and power consumption. In both cases, speed adjustment cannot replace the previous air volume calculation.
The design document should list both peak operating conditions and common operating conditions. Peak working conditions are used to check the air supply capacity, and common working conditions are used to judge energy consumption. If the system operates at full load for a long time, the benefits brought by speed regulation will be limited; if there are obvious on-duty or partial load periods, the value of the speed regulation solution will be reflected. If there is a high-efficiency filter at the end of the system, the filter status and valve opening should be noted under common working conditions.
Speed adjustment deals with operational changes, not design errors
Even if a DC fan chassis is used in a newly built system, the air supply capacity must be guaranteed under the maximum design conditions; when the load is low, the output will be reduced based on the regional activation status and pressure difference target. Speed regulation can change the operating point, but it cannot compensate for unreasonable air duct size or errors in calculating the air volume at the most unfavorable end.
If the pipeline is too small in the design stage, the resistance will increase later, and even if the DC fan box can increase the speed, it may cause noise and extra power consumption. If the pipeline is too large, the initial investment will increase and it will be difficult to adjust during low load operation. Duct sizes, fan curves and control intervals should be coordinated during the design phase rather than waiting until the equipment arrives on site.
Leave the two working conditions to the manufacturer
Determining the air volume of a new air conditioning system is not a one-time calculation, but a process of aligning end demand, system resistance, equipment capabilities and installation conditions. The project party provides the zone air volume balance table, the most unfavorable path pressure loss and the operating period. The manufacturer can check the fan, motor and control configuration accordingly to form an executable plan.
Aili Industrial can discuss based on the design and common working conditions.DC fan box . Motor, fan and control combination for Specific structure and configuration options can be found inIt is confirmed in the fan box product catalog that non-standard projects should provide equipment or machine room drawings, and then shape them based on the determined working points and on-site interfaces.
