After the installation location of the HVAC system is determined, the selection work of the AHU begins. For a new general contracting project, the design institute may have included all the selection and confirmation work when drawing out the drawings. However, for the modified drawing design, the designer may not have designed the relevant parameters of the air conditioning unit, or the starting point of its consideration may not be well understood at the beginning. Combined with the four cycles of purification air conditioning construction, I will summarize my relevant knowledges about the selection of AHU.
Selection of AHU
The types of air conditioning units installed in clean workshops can be divided into three categories: clean air conditioners, non-clean air conditioners. As the name implies, clean air conditioners are areas that require control of cleanliness levels, such as D/C/B/A-level areas; non-clean air conditioners generally refer to general area corridors, outsourcing areas, CNC areas, etc.; comfortable air conditioners are for office use or other.
According to the system impact assessment of equipment and facilities, clean air conditioning is a directly affected system, so the daily management of clean air conditioning equipment is one of the most important management tasks for public facilities.
According to the conventional design starting point, the selection of purification AHU is generally carried out from the following aspects:
(1)Size/layout of purification AHU
The installation space of AHU after the construction of the air-conditioning room directly determines the size requirements of the air-conditioning unit or the layout design of the functional section of the air-conditioning unit.
(2)Air volume of AHU
The air volume of the AHU is based on the volume and cleanliness level of the control area of the unit. Generally, the ventilation frequency of D-level area is 15 times/h, that of C-level area is 25 times/h, B-level area is 40 times/h, and the wind speed of A-level area is 0.36-0.45m/s. And combined with the possible loss of air volume such as air ducts and workshops, to confirm the selection requirements for the total air supply volume of the air-conditioning unit.
(3)Functional section of AHU
An air conditioning unit is the result of a combination of several functional segments, which include:
① Fresh air outlet (fresh air network): It is mainly used to supplement the fresh air for the entire control area. According to the fresh air requirement of 40m³/person/h, determine the appropriate fresh air ratio and fresh air outlet size.
②Primary filter: Its function is mainly to carry out preliminary treatment of air. Generally, the specification of the primary effect filter is G4, and the common ones are plate and frame type and bag type. Since the primary filter needs to be cleaned and replaced regularly, lighting, differential pressure monitoring and replacement space need to be considered when installing the air-conditioning unit.
③Return air section: According to the different uses of cleanroom, the return air is a exhaust facility used in some cleanroom which mainly ensure the air circulation of the entire control area and the control of the pressure difference in the area. For the HVAC system, its function is the same as the return air function. Generally combined with the return air volume, it is necessary to consider the size of the return air outlet and the return air duct, which has ensured the resistance of the return air duct and the redundancy of subsequent return air duct reconstruction.
④Surface cooling section: It is mainly used to input the cooling capacity of the air conditioning unit in summer. Combined with the requirements for humidity, the surface cooler can be dehumidified, but it should be noted that the consideration of the dehumidification water basin in the air-conditioning unit, including the material and slope, whether it is convenient for the discharge of condensed water. At the same time, it is necessary to pay attention to the heat exchange area of the surface cooler.
⑤Heating section: The function of the heating section is the same as that of the surface cooling section. In winter, the ahu needs to be heated. Considering the form of the public medium, the heating methods include electric heating and steam heating. According to the heat exchange principle of heating, it is also necessary when selecting Consider the heat exchange area of the heat exchanger. The heat exchange area of the heat exchanger also needs to be considered when selecting ahu.
⑥ Humidification section: For areas with humidity control, humidification operation is generally required in winter. The humidification methods include steam humidification and pure steam humidification. For aseptic production, if the medium allows, pure steam humidification can be choosed.
⑦ Medium-efficiency filter: It is mainly used for further processing of the air. Generally, the specifications and models of the filter are F6, F7, etc., and the common bag filter. Since the medium-efficiency filter needs to be cleaned and replaced regularly, lighting, differential pressure monitoring and replacement space need to be considered when installing the air-conditioning unit.
⑧ Rotary dehumidifier: In some workshops for the production of traditional Chinese medicines/proprietary Chinese medicines, there are relatively low requirements for humidity. Therefore, when selecting the control area, it is necessary to consider installing a rotary dehumidifier for further humidity control. The control principle is to use silicon to adsorb moisture, and then use heat to remove the moisture.
⑨ Air supply section: It is mainly for the air supply of the entire area in the clean area. The two key points that need to be considered when selecting fans in the air supply section are the air volume and pressure head. When selecting air supply pressure head, the selection of the external residual pressure after the loss of the air-conditioning unit and the air duct should be fully considered.
⑩High-efficiency filter: It is mainly used for further processing of the air. Generally, the specifications and models of the filter are F9, etc., and the common bag filter. Since high and medium efficiency filters need to be cleaned and replaced regularly, lighting, differential pressure monitoring and replacement space need to be considered when installing the air conditioning unit.
After the air-conditioning unit is installed, the HVAC system also includes air supply pipelines, return air pipelines/exhaust pipelines, static pressure boxes, HEPA filters and diffusers to form the entire purification air conditioner.
(4)Process flow of AHU
Combined with the environmental requirements of the clean workshop, the confirmation of the function process of AHU was completed, and the selection of the purification AHU was completed according to the configuration of the use function.
(5)Control system
The control system of HVAC system can be divided into two aspects: one is that a single unit independently controls the operation of the air-conditioning unit, which can be divided into local control and remote control; the other is combined with the operation of the entire clean area, including pressure difference, temperature and humidity , air volume and other integrated control systems, such as BMS/BCS and other systems. The other is combined with the operation of the entire clean area, including integrated control systems such as pressure difference, temperature and humidity, and air volume, such as BMS/BCS and other systems.
For the relevant content of the control system, there will be an opportunity to describe it later, including the CSV management requirements of the control system.
(7)Other key point
In the process of choosing an AHU, there are also some common points to pay attention to.
①Cold bridge: When the surface cooling section of the air-conditioning unit operates in summer, condensation is likely to occur on the outer surface of the unit, resulting in a loss of cooling capacity. Therefore, the connection of the functional section should be considered in the design.
②Shock absorption: When selecting an AHU, it is necessary to consider the installation location of the unit and the relevant measures for shock absorption during operation.
③Door opening method: According to the installation position of AHU, the AHU can be divided into left-opening type and right-opening type. Combined with the selection of air hardware and control system, and combined with the relevant requirements of control area and site, the technical selection of the air-conditioning unit can be completed. And then combined with relevant business requirements, complete the selection of the brand, specification, installation method and control system of the AHU.