Key points for selecting a packaged air handling unit include compact size, multiple functions, low noise, low energy consumption, aesthetic design, and easy installation and maintenance. Designers can flexibly combine functional sections based on parameters such as cooling load, heating load, humidity load, supply air temperature, supply air humidity, and noise. However, due to the multiple functional sections and complex structure, it is necessary for designers and construction units to compare various aspects such as material, manufacturing process, structural characteristics, and selection calculations to achieve satisfactory results. The following points have been summarized from long-term market feedback and production experience:
1. Air filtration section
The function of the air filtration section is to filter out dust from the air. Dust in the air can have an impact on human health and product quality, particularly in certain industries such as precision instruments, electronics, pharmaceuticals, hospitals, etc., where air cleanliness requirements are extremely high. This requires the selection of a packaged air handling unit based on the specific requirements of each industry.
2. Surface cooler section
The surface cooler is the core component of a packaged air handling unit and is the place where air and refrigerant undergo heat exchange to cool and dehumidify the air. This section generally contains coils made of copper tubes with aluminum fins. Customers can select from coils with 4, 6, or 8 rows of tubes. The thickness of the copper tube walls and aluminum foil may vary slightly depending on the manufacturer, but generally the copper tube wall thickness is 0.2-0.6 mm with diameters of 7-16 mm, and the aluminum foil thickness is 0.15-0.20 mm. It should be noted that different manufacturers often use different spacing between fins for their surface coolers. As an example, for a system with an air flow rate of 79,000 m3/h and a cooling capacity of 831 kW, one company chose to reduce the spacing between fins (to about 1.8 mm) to increase the heat exchange area, while another company used conventional spacing (about 2.5 mm) and two levels of surface coolers in series to extend the heat exchange time. Each approach has its own advantages and disadvantages in terms of fan power consumption, water resistance, and section size. After thorough technical comparison and taking cost factors into account, the second company’s product was chosen. This example illustrates the importance of selecting the appropriate surface cooler calculation method to ensure the performance and overall cost-effectiveness of the packaged air handling unit.
The surface cooler entrance velocity generally should not exceed 2.5 m/s, as higher velocities can cause water droplets to be carried along with the cooled air, thereby increasing the air humidity. If the entrance velocity is >2.5 m/s, a water-blocking baffle is installed on the outlet side of the surface cooler to prevent water droplets from being carried along with the air.
3. Humidification Section
There are several methods for humidification, and there are various forms of humidification sections in packaged air handling units. Common methods of humidification include:
(1) Direct injection of steam into the air. Packaged air conditioning units currently use dry steam humidifiers, which can prevent water droplets from being carried along. The humidification is fast, even, stable, without water droplets, and the amount of humidification is easy to control. This method is suitable for places with strict humidity control but can only be used in buildings with a steam source.
(2) High-pressure spraying. Water is pressurized to 0.3-0.35MPa (gauge pressure), which results in the generation of water droplets with an average size of 20-30μm. The water droplets absorb heat and evaporate into the air. The advantages are a large humidification quantity, low noise, low power consumption, and low operation cost. The disadvantage is that water droplets may form, and impurities such as calcium and magnesium will precipitate when using untreated water. This is a commonly used humidification method in packaged air conditioning units.
(3) Wet film humidification. This method uses a humid material surface to evaporate water vapor into the air for humidification. The advantages are simple equipment structure, small volume, and filter dust on the filler layer. The filler layer also prevents water droplets from being mixed with air. The disadvantage is that the wet surface is prone to the growth of microorganisms, and the filler layer requires regular replacement.
(4) Vapor-permeable membrane humidification, which uses the principle of membrane distillation from chemical technology as a humidification technique, this method features a simple device structure, low operating cost, energy efficiency, and clean humidification.
(5) Other humidification methods include electric heating, infrared radiation, and centrifugal humidification.
4. Heating Section
5. Fan Section
6. Other functional sections
7. Insulation performance
8. Air leakage rate
9. Convenient maintenance