Purification Air Conditioning MAU+FFU+DC System

Aug 13, 2024

Purification Air Conditioning MAU+FFU+DC System It is a combined fresh air unit (Make-up Air Unit) + fan filter unit (Fan Filter Unit) + dry coil (Dry coil) system.Due to its characteristics of temperature zone control, flexible layout, FFU negative pressure sealing, and energy saving of return air delivery, it is very suitable for the characteristics of large heat-humidity ratio (extremely large heat load and small heat dissipation), large return air volume, and large-area purification in electronic factories, and is therefore widely used in the design of electronic clean factories. In the actual design and engineering installation process, the author has come into contact with many such systems and found that many similar projects currently lack clear basis in design engineering, and even many projects have conflicts between design and specifications. Therefore, it is necessary to further explore some issues.

----------------------------Pressure problem in FFU static pressure box----------------------------

Main factors of negative pressure formation

The upper static pressure box of FFU is negative pressure relative to the room, which can play the role of negative pressure sealing. This is also a recognized advantage of FFU system. Of course, it is also true in MAU+FFU+DC.

However, in general engineering projects, the static pressure box of FFU is also negative pressure relative to the outside, especially in the static pressure box of FFU in MAU+FFU+DC system. Because the return air passes through the dry coil, there is a large wind pressure loss (there is data or it is generally believed to be 30Pa ~50Pa, which will be analyzed in detail in Section 2); under the condition of ensuring a certain positive pressure value indoors, it is bound to cause the static pressure box of fan filter unit for sale to be negative pressure relative to the outside. For example: if the clean room is guaranteed to be +10Pa (relative to the outdoor non-clean area), the return air orifice plate and the return air channel wind pressure loss is 5Pa, and the wind pressure loss of several coils is 40Pa, then it can be known that the pressure in the static pressure box is -35Pa (relative to the outdoor non-clean area).

Therefore, we can get a conclusion, but also a problem. First, the main reason for the negative pressure in the static pressure box is the wind pressure loss of the dry coil (the return air duct and orifice plate resistance loss are relatively much smaller). Second, is the negative pressure in the static pressure box (relative to the outdoor non-clean area) reasonable.

----------------------------Discussion on negative pressure in static pressure box----------------------------

The static pressure box on the upper part of FFU is negative pressure relative to the indoor area, which can play the role of negative pressure sealing, which undoubtedly plays a positive role in ensuring the cleanliness in the clean area. However, the negative pressure relative to the outdoor non-clean area (generally the technical interlayer) will inevitably cause the outdoor air to leak directly into the static pressure box, affecting the service life of the FFU filter, which is undoubtedly harmful or unfavorable.

In this case, it is actually equivalent to placing the ffu fan in the negative pressure section (relative to the outdoor non-clean area).

First, Article 6.4.1 stipulates that medium-efficiency (high-efficiency) air filters should be concentrated in the positive pressure section of the air-conditioning system. The reason is that the negative pressure section is prone to air leakage (see the article explanation); what's more, FFU generally contains high-efficiency or ultra-high-efficiency filters. Second, the fresh air of the MAU+FFU+DC system is filtered by high-efficiency filters and mixed with clean return air in the static pressure box, indicating that the static pressure box is still a clean area, at least a quasi-clean area (this can be known through calculation). Article 6.2.2 requires that the pressure difference between clean rooms of different levels and between clean areas and non-clean areas should not be less than 5Pa, and the pressure difference between clean areas and the outdoors should not be less than 10Pa.

In engineering design, it is necessary to make a comprehensive comparison from multiple aspects such as economy (including cost and operation), advancement (technical aspects), and energy saving to determine whether it is reasonable or not. It cannot be viewed from a single aspect. Most of the specifications in design engineering designs follow this principle when selecting technical means and determining certain parameters. Therefore, it is reasonable for the FFU static pressure box to have a certain negative pressure relative to the outdoors (see the specific calculation analysis below), which is also consistent with the guiding ideology of engineering design. The key to the problem is how to determine this negative pressure value.

The static pressure box on the top of the ffu filter fan unit cannot be negative pressure relative to the outside, and it should also ensure a positive pressure of not less than 10 Pa.

Assuming that the clean room is 15Pa (relative to the non-clean area outside), reversely, to ensure that the static pressure box is 10Pa (relative to the outside), the sum of the return air dry coil, return air channel, and ground grille pressure losses must be less than or equal to 5Pa. If the return air channel and ground grille losses are not considered first, and only the dry coil resistance loss is calculated as 5Pa (the calculation method is shown in Section 2 of this article), the headwind speed is about 0.34kg/(m2.s). According to this wind speed, if the dry coil is reselected, at least 13 dry coils of the same model are required, which is more than 3 times the original design. To ensure such a small pressure loss, the ground grille (return air effective) area and the return air channel area must also be greatly increased.

From the above example, if the positive pressure in the static pressure box is maintained in accordance with the specification, this situation will inevitably cause a great increase in initial investment, a reduction in purification area (increase in return air ducts), making the MAU+FFU+DC system more expensive, and not conducive to the promotion and use of the MAU+FFU+DC system, so this is very unreasonable.

Because the negative pressure section is prone to air leakage, which affects the service life of the filter. In the design of domestic clean room air conditioning systems, the service life of the system configured with primary, medium and high efficiency filters is only 1 to 3 years. Therefore, if the air volume leaked into the static pressure makes the service life of the ffu's filter within the normal design life range, the author believes that this leakage air volume is acceptable.

The setting of the pressure difference value here does not need to comply with the provisions of the "Specifications"; or the specifications should be adjusted or more detailed and reasonable provisions should be made here according to different system forms. It must also be explained here that in the above example, without increasing the number of DCs, by increasing the indoor pressure setting value, it is still possible to ensure that the hepa filter ffu static pressure box is positive relative to the outdoor pressure. For example, if the clean room is guaranteed to be +45Pa (relative to the outdoor non-clean area), the return air orifice and return air duct pressure loss is 5Pa, and the pressure loss of several coils is 30Pa, then it can be known that the pressure in the static pressure box is +10Pa (relative to the outdoor non-clean area).

----------------------------However, there are three problems in this way----------------------------

First, this will inevitably greatly increase the amount of fresh air, and the cooling capacity and delivery energy of the fresh air table will increase. Obviously, the MAU+FFU+DC system has lost one of its advantages. Second, the excessive pressure difference in the clean room also affects the opening of the door, which is generally set to less than 50Pa. Third, if there are several clean areas of different levels in a purification area, according to the above, the lowest level relative to the non-clean area pressure difference should be maintained at +45Pa, and other high-level areas should increase by 5Pa in accordance with the specifications. Obviously, this makes the first and second problems more serious.

----------------------------Reasonableness and economic value----------------------------

Based on the above ideas, let us theoretically calculate the theoretical service life of the hepa ffu static pressure box at different pressures. The service life of the FFU, that is, the service life of the high-efficiency filter in the ffu clean room, can be calculated using the following formula:

hepa ffu

----------------------------Among them----------------------------

N1= M(1-S)(1-ηn)+NrS(1-ηr)(2)

Where: M is the dust concentration in the atmosphere, mg/m3; S is the circulating air ratio; Nr is the return air concentration, which will not exceed 0.001 mg/m3~0.01 mg/m3 for the highest concentration of 100,000-level clean rooms; ηn is the filter weight efficiency on the fresh air duct before the filter; ηr is the filter weight efficiency on the return air duct before the filter.

For different systems, ηn and ηr are different, and N1 is also different. For the MAU+FFU+DC system (the typical form is shown in Figure 1), the dust concentration of the air before the filter should include 3.

parts: one is the part brought in by the return air, the second is the part brought in by the fresh air, and the third is the part where the outside air directly leaks in because the FFU static pressure box is at negative pressure relative to the outside. Therefore,

----------------------------it can be expressed as----------------------------

N1= M(1-S)(1-ηn)+NrS(1-ηr)+ MSl

According to the definition of S, we can call Sl the leakage ratio; the ratio of leakage to total air volume.

Since the fresh air passage of the MAU+FFU+DC system is equipped with a high-efficiency filter, it is generally considered that η≈1 for the high-efficiency filter, so ηn = 1, and formula can be written as: N1= NrS(1-ηr)+ MSl (4);

Formula is the calculation formula for the dust concentration of the air in the ffu clean room static pressure box of the MAU+FFU+DC system.

There are generally two calculation methods for calculating the air leakage of the FFU static pressure box: the ventilation frequency method and the gap method. The gap method is used to calculate the infiltration air volume, which takes into account both the air tightness of the clean room maintenance structure and the positive pressure air volume required to maintain different pressure difference values in the room, so it is more accurate and reliable. Secondly, the FFU static pressure box is generally made of a guaranteed width purification color steel plate, and its gap length can be calculated more accurately. Therefore, the gap method can be used to calculate the air leakage of the FFU static pressure box.

----------------------------FFU filter service life----------------------------

Calculation of the gap length of the ffu fan filter unit static pressure box. Note that the gap length here can only calculate the part of the static pressure box that is in direct contact with the non-clean area, not including the part that is in direct contact with the clean room. As shown in Figure 2, the static pressure box top plate uses a color steel plate with a width of 1.150m and a length of 3m. There are 12 vertical joints on the top plate, each with a length of 6.2m, and 7 horizontal joints, each with a length of 32.6m. The length of the top surface joint of the static pressure box is 302.6m. The height of the static pressure box is 2m, and the same 1.15m wide color steel plate is used, and the length of the joint is 204m. The total length of the static pressure box joint is 506.6m.

The width of the color steel plate used in general clean rooms is 1.15m. Due to the influence of strength, each plate generally does not exceed 4m. Therefore, the calculation results of this example are universal, and the data can be used as a reference.

Calculation of the air leakage of the price ffu static pressure box. The joints are obtained from Table 2 and Section ①.

----------------------------Room equipment layout plan----------------------------

Calculation of FFU filter service life: The weight concentration of fresh air (outdoor air) is generally taken as M = 0.3mg/m3, and Nr is taken as 0.005mg/m3.

Result analysis: It can be seen from the calculation results that when the negative pressure value of the Fan filter unit manufacturer static pressure box is greater than or equal to 20Pa, the service life of the FFU filter is less than 2 years. Therefore, from the perspective of the FFU service life alone, the author believes that the negative pressure value of the static pressure box is best controlled within the range of less than or equal to 20Pa. When the negative pressure value is within this range, it can be considered that the FFU service life can theoretically reach the normal value.

fan filter unit ffu

----------------------------Dry coil selection----------------------------

From we know that dry coil is the main factor causing negative pressure of FFU static pressure box (relative to outdoor non-clean area), and the negative pressure value determines the service life of FFU filter and the size of dry coil area. Through the above analysis, the negative pressure value of FFU static pressure box needs to be controlled within the range of less than or equal to 20Pa. At this time, the dry coil area is the smallest (that is, the investment is the least) while ensuring the service life of FFU filter.

The air flow resistance is related to factors such as the type, structure and air mass flow rate of the surface air heater, and its empirical formula is usually determined by experiment.

----------------------------Dry coil selection----------------------------

Air flow resistance is related to factors such as the type, structure and air mass flow rate of the surface air heater, and its empirical formula is usually determined by experiments.

Where: C, y are experimental data related to the structure of the surface air heater; vρ is the air mass flow rate outside the tube kg/(m2.s); vρ=G/f; G is the air mass flow rate kg/s; F is the effective ventilation area of the surface air heater, m2.

If the clean room is 10Pa (relative to the outdoor non-clean area), the negative pressure value of the FFU static pressure box is controlled to be less than or equal to 20Pa, and the air resistance loss of the dry coil should be less than or equal to 30Pa.

By back-calculating from formula (6), the maximum allowable flow rate of the dry coil is: 0.97kg/(m2.s). Still using this flow rate of 1.2 as an example to calculate the selection of dry coils, only 6 dry coils of the same model are needed (that is, only 2 more than the original). From the perspective of increased investment, it is far less than the calculation result of 1.2, so it is reasonable. Under the premise of ensuring the normal service life of the FFU filter, it is economical and reasonable to control the dry coil wind speed at around 0.97 kg/(m2.s).

Therefore, it is worth noting that: it is generally believed that in the design of air conditioning projects, the mass flow rate of air is usually taken as 6~10kg/(m2.s). Through the above analysis and calculation, this value cannot be used for the selection calculation of the dry coil of the MAU+FFU+DC system.

----------------------------Conclusion----------------------------

In the MAU+FFU+DC system, the main reason for the negative pressure in the HEPA Fan Filter Units static pressure box (relative to the atmosphere in the non-clean area) is the wind pressure loss of the dry coil. In the design process, the reasonable determination of the relative pressure value of the FFU static pressure box affects the life of the FFU filter and the system cost.

Through calculation and analysis, this paper controls the negative pressure value of the FFU HEPA Filter H14 static pressure box within the range of less than or equal to 20Pa, which can save the number of DCs to the greatest extent while ensuring the normal service life of the FFU, and can be used as a reference for engineering design.

For the situation where the negative pressure in the Cleanroom Fan Filter Unit static pressure box conflicts with the specification, through theoretical calculation and analysis, adjustments or more detailed and reasonable regulations are made here.

In the process of engineering design, when selecting dry coils, the wind resistance of dry coils must not be ignored.

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Purification Air Conditioning MAU+FFU+DC System