The function of the air filter section is to filter the dust in the air. The dust in the air has an impact on human health and product quality, especially some special industries, such as precision instruments, electronics, the pharmaceutical industry, hospitals, etc., have extremely high requirements for air cleanliness. . This requires the selection of combined air handling units according to the requirements of different industries. The air used in a comfortable air-conditioning environment has certain requirements for dust content. The filter section of this type of air handling unit only needs to be equipped with primary and medium-efficiency filters. For the above-mentioned special industries, it is also necessary to configure high-efficiency filters to achieve ultra-purification, and such purification requirements are very high. The primary filter has a plate filter and a non-woven bag filter, and the medium-efficiency filter usually uses a non-woven bag filter.
The surface cooler is the core part of the combined air handling unit, and it is the place where the air and the refrigerant exchange heat, which is used for air cooling and dehumidification. This section is usually equipped with coils of copper tubes and aluminum fins, and cooling coils with 4 rows, 6 rows, and 8 rows of tubes are available for users to choose from. The wall thickness of the copper tube and the thickness of the aluminum foil varies slightly with different manufacturers. Generally, the wall thickness of the copper tube is 0.2-0.6mm, the diameter is 7-16mm, and the thickness of the aluminum foil is 0.15-0.20mm. It is worth noting that the fin spacing selected by different manufacturers in the calculation of the surface cooler is quite different. Taking a system as an example, the processing air volume is 79 000m3/h and the cooling capacity is 831kW. The plan adopted by Plant A is to reduce the fin spacing of the surface cooler (about 1.8mm) and increase the heat exchange area; the plan adopted by Plant B is to select conventional fin spacing (about 2.5mm) adopt the serial form of the two-stage surface cooling section to prolong the heat exchange time. Both schemes have their own advantages and disadvantages in terms of fan power consumption, water resistance, and section size. The former may cause larger wind resistance and increase the power consumption of the fan, while the latter may increase the unit size and water flow resistance. After a comprehensive technical comparison and comprehensive consideration of cost factors, it was decided to use the products of Plant B. This example shows that the rationality of the selection and calculation of the surface cooler is related to the performance and comprehensive cost of the entire air handling unit, and it should be paid enough attention to. The wind speed on the surface of the cooler is generally not greater than 2.5m/s, and if the wind speed is too large, the cooling air will entrain water droplets and increase the air humidity. Generally, when the oncoming wind speed is greater than 2.5m/s, a water baffle is provided on the air outlet side of the surface cooling section to prevent water droplets from being entrained in the air.
There are many ways to humidify, and the humidification section in the combined air conditioner unit has various forms to choose from. Commonly used humidification methods are: (1)Spray steam for humidification, spray steam directly in the air. Air-conditioning units currently use dry steam humidifiers, which can avoid entraining water droplets, and humidify quickly, uniformly, and steadily, without water droplets, and the amount of humidification is easy to control. It is used in places with strict humidity control. But it can only be used in buildings with steam sources. (2)High-pressure spraying, using a water pump to pressurize the water to 0.3-0.35Mpa (gauge pressure) for spraying, water droplets with an average particle size of 20-30μm can be obtained, and they will absorb heat and vaporize in the air. The advantages are large humidification, low noise, low power consumption, and low operating costs. The disadvantage is that water droplets are precipitated, and impurities such as calcium and magnesium will be precipitated when using unsoftened water. This is a humidification method that is widely used in air conditioners at present. (3)Wet film humidification uses the surface of the wet material to evaporate water vapor into the air for humidification. The advantages are that the structure of the equipment is simple, the volume is small, the packing layer has the function of filtering dust, the packing has the function of blocking water, and water droplets will not be entrained in the air. The disadvantage is that the wet surface is prone to microbial growth and the filler layer needs to be replaced regularly. (4)Moisture-permeable membrane humidification is a humidification technology that utilizes the principle of membrane distillation in the chemical industry. The humidification equipment has the advantages of simple structure, low operation cost, energy saving, and clean humidification. (5)Other humidification methods include electric humidification, infrared humidification, centrifugal humidification, etc.
There are three types of hot water coil, steam coil, and electric heater. The hot water coil has the same structure as the cooling coil, but only 1-row, 2-row, and 4-row coils are available. Steam coil heat exchange components have copper tube sleeves with aluminum fins or wound fin tubes, with 1 row or 2 rows for selection.
The fan is the only energy-consuming part in each functional section of the air handling unit. The same as the general fan, the model, speed, power, and matching motor of the fan is usually selected according to the total air volume and total resistance required by the system. Fans are generally centrifugal fans with backward-curved blades or forward-curved blades. Backward curved blades have high efficiency and low noise and should be preferred. For systems that require high wind pressure, forward curved blade fans should be selected. With the development of science and technology and the improvement of automatic control levels, the variable air volume system is widely used in office buildings, clean workshops, hospitals, and other occasions. Therefore, higher requirements are put forward for the selection of supply and return fans in air-conditioning units. Firstly, the characteristic curve of the fan should have a flat characteristic, so that when the air volume is reduced, the system can avoid unnecessary increase of static pressure. Secondly, when selecting a fan, its working range should be in a relatively stable and high-efficiency area. Thirdly, the return/exhaust fan should be of the same model or type as the supply fan, and have the same or similar performance characteristics, so as to ensure the effective matching of the entire system during operation. The box of the return fan section is provided with the interface of the return air duct, and the air outlet side is generally connected to the split section. The return air passes through the diversion section so that part of the air is discharged to the outside, part of the air participates in the recirculation, and fresh air is also introduced from the diversion section. The proportion of new, return, ed, and exhaust air is controlled by the damper.
In addition to the above main function sections, there are also some auxiliary function sections. There are: (1)Mixing section – the upper part and side of this section are provided with air duct interfaces to connect the return air and fresh air ducts and adjust the proportion of new return air through the damper at the entrance. (2)Middle section – there is an inspection door in this section, which is used for maintenance and repair inside the unit, but the main equipment of the unit produced by some factories can be pulled out, and the middle section may not be set. (3)Secondary return air section – this section has a connection with a return air inlet. (4)Muffler section – this section is used to eliminate the noise of the fan.
The wall panels of the air handling unit are generally powder-coated with double-layer colored steel plates or cold-rolled steel plates, and the insulation material in the middle is mostly polyurethane foam or flame-retardant centrifugal glass wool. If you look at thermal conductivity, water absorption, and density alone, both are good insulation materials. However, due to the high-speed operation of the supply and return fans during the operation of the air handling unit, the box body vibrates greatly. However, the glass wool is light in weight, and there are large gaps in the structure that are easy to fall off due to long-term vibration, resulting in a sharp increase in thermal conductivity and loss of thermal insulation effect, resulting in condensation on the wall of the air handling unit and high supply air temperature. However, it is not necessary to use polyurethane foam to have a good thermal insulation effect. For example, inferior polyurethane foam thermal insulation materials will also affect the use effect. Therefore, when choosing an air handling unit, the wall insulation material cannot be selected lightly.
The box structure of most air handling unit manufacturers is composed of panels and aluminum alloy frames or welded angle steel frames. The sealing treatment of the box is also an aspect that should be paid great attention to when selecting models. Because if the air leakage rate is too high, there will be two consequences: one is that the air supply is insufficient due to excessive air leakage; the other is the air leakage of the air treatment room under negative pressure. In summer, untreated hot and humid air will enter the air supply system, so that the dew point of the machine cannot meet the design requirements, and the air supply temperature parameters cannot be guaranteed. The “Combined Air Conditioning Unit” (GB/T14294-2008) manufacturing specification stipulates: “When the static pressure in the unit is maintained at 700Pa, the air leakage rate of the unit shall not be greater than 3%”. Some manufacturers have a dovetail-shaped sealing groove on the frame of the air handling unit and are equipped with corresponding rubber sealing strips. Under the rated working conditions, the air leakage rate can be guaranteed to be less than 2%, which meets the requirements of the specification. This is a good way to solve the air leakage problem of the unit.
Considering the convenience of replacement, cleaning, and maintenance of the functional section of the combined air-conditioning unit, an intermediate section should be appropriately added. Especially for parts such as surface coolers and filters that often need to be maintained and cleaned, the intermediate section is even more indispensable. However, sometimes due to the limited building size of the air-conditioning room, when the length of the air-conditioning unit is limited and enough intermediate sections cannot be designed, when selecting and ordering equipment, the manufacturer can be required to improve the structure of the air-handling unit itself according to the actual situation of the machine room. For example, the surface cooler section and filter section are designed to be drawn out from the side, or an inspection door is provided in the upstream functional section. In addition, during construction, attention should be paid to reserving maintenance space and passages as much as possible.