Analysis of Fab Process Exhaust Systems

The effects of fab pollutants on human health and air quality are growing in Taiwan. This study shows how best to prevent air-duct leakage from causing performance losses or the escape of toxic gases.

Rong-Hua Ma, ROC Military Academy, Kaohsiung, Taiwan -- Semiconductor International, 3/1/2007

Semiconductor production has become a major industry in Taiwan. Consequently, the effect of pollutants on human health and air quality are growing. Within semiconductor fabs, process exhaust systems serve to eliminate excess heat, toxic gases, acidic gases, organic solvent vapors, etc., produced by equipment. Sources of such pollutants are becoming more diverse and, because the discharged pollutants are dispersed throughout the vicinity via convection and diffusion, the effects are becoming more severe.

There are several issues to consider, including a fab's compliance with company requirements and the government's increasingly strict duct exhaust system regulations and enforcements. This can impede foreign orders and contracts for ducting production, while at the same time could encourage a lack of confidence in the ducting produced by domestic manufacturers.

To add to this, duct ventilation systems must be sufficiently airtight to ensure economic efficiency and meet noise standards. At high pressure, air leaking from small openings tends to cause high-frequency noise, which grows louder as the leak grows in size. Leaks will, of course, also reduce the performance of a duct system, and the leak of toxic gases will in turn reduce the working efficiency of personnel and ultimately lessen productivity. In severe cases, toxic gas leaks may threaten the quality of the public environment, and even cause injury and death. Manufacturers must consider these points in connection with their processing operations.

There are currently no domestic experimental facilities meeting international standards or domestic measurement equipment standards governing measurements involving duct ventilation systems and their components. Literature published by the foreign measurement method and standards organizations ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers)¹ and SMACNA (Sheet Metal and Air Conditioning Contractors' National Association)² specify the geometry and dimensions needed when experimental measurement equipment is used in production situations. This can ensure that the design and assembly process meets flow field measurement quality and stability requirements, and yields accurate measurements. Specialists Swim and Griggs³ studied the difference in leakage before and after seal repair, and also investigated leaks caused by different ducting processing methods. This study concluded that the recommended design value of the leak rate coefficient for a plurality of duct pieces is 6-8 CFM, while the power should be maintained at 0.58. In fact, there is very little research concerning leakage in duct ventilation systems.

Leakage model

A duct system with a limit on air leakage (where the air leak is kept within a specified limit) can ensure that system design parameters are maintained, and can also ensure optimal energy and operating costs. In addition, when air pressure is 4 in-Wg or greater, air escaping through a small opening will cause noise. This noise will increase with the amount of leaking air. Working practice has verified that total air leakage must be kept below 1% of system capacity to reduce average ducting noise to a level that most people can accept.² This implies that the maximum reasonable air leakage threshold in an air leakage experiment should be 1% of system capacity.

Therefore, we used a continuous equation to derive the leakage. Assume that a 1-D flow form is applied in an arbitrary leak path, the fluid is stable and incompressible, the system is adiabatic and does no work, and any height change of the fluid is negligible. In this case, after simplification, the energy equation of the fluid moving from the inside (point 1) to the outside (point 2) of the duct is:

If the velocity of the leaking air V₁ is zero on the inside of the duct, the following equation is obtained:

Here (P₁ - P₂) is the pressure difference inside and outside the duct, and represents the total fluid losses along this path. The continuous equation Q = A₂ V₂ can be used to calculate the leak rate through the pinch point along the flow path. Nevertheless, if we want to calculate total losses along a specific leak path, we must then consider the duct's surface conditions, the geometric shape of the flow path, and the partial Reynolds number. Furthermore, there is a power relationship between fluid energy losses and velocity in laminar flow and speed, while fluid energy losses are the square of partial velocity in turbulent flow or separated flow.³ Consequently, the following equation is obtained when we consider the form of losses, including laminar flow and turbulent flow, in Equation 2:

Here e_l is the coefficient of total losses produced by laminar flow, and e_t is the coefficient of total losses produced by turbulent flow. Equation 3 signifies that leak velocity is proportional to the Nth power of the pressure difference. N≈0.5 when e_t >> e_l, which signifies that gas flow through the pinch point approaches turbulent flow. For instance, N≈0.5 for a leak through a small gap with diameter >0.5 mm between three or more iron plates. N≈1 when e_l >> e_t + 1, which signifies that gas flow through the pinch point approaches laminar flow. N≈1 when the amount of leakage through the pinch point is controlling the flow. By the same principle, leakage, Q_L, is proportional to the Nth power of the static pressure difference between the inside and outside of the duct (ΔP = P₁ - P₂). When the constant of proportionality C_L has been obtained, the leakage Q_L can be expressed as:

Q_L = C_LA(ΔP)^N                               (4)

In addition, P₂ = 0 when point 2 communicates with the atmosphere; when Q_L = FA, then:

F = C_LP_s ^N                                      (5)

Here, F is the leakage rate per unit of duct surface area (CFM/ft²). The constant,C_L, is the leakage class; P_s is the static pressure of the duct (in-Wg); and N is an index with a scope of 0.5-1.0. In accordance with the recommendations of SMACNA² and the ASHRAE Handbook,¹ N is ordinarily taken to be 0.65.

Leakage is directly proportional to the total surface area of the duct system, as can be seen from Equation 4. While leakage is also correlated with the pressure of a process exhaust system, there is no precise formula that can be used to calculate losses. It is generally accepted that the increase in leakage is directly proportional to the 0.65th power of the pressure.

Experimental equipment and procedures

This project was designed in accordance with SMACNA and ASHRAE standards, and established a ducting performance measurement system meeting international standards to serve as the experimental measurement unit. In conformance with SMACNA standards,² the performance testing unit consisted of a main chamber, flange, flow settling plate or flow straightener, static pressure tap, an orifice plate, and a variable air supply. An auxiliary blower and air volume adjustment device were essential to simulating a variety of inlet airflow conditions and obtaining complete performance curves. In addition, measurement instruments, data acquisition and storage equipment, and other auxiliary equipment were needed to obtain the desired measurement results. The main instruments and equipment used included a U-type pressure gauge, wind velocity gauge, flow gauge, an electronic pressure gauge, a digital inverter, an intelligent multifunctional measurement instrument, an auxiliary blower, and PCs (Fig. 1 ).

Generally speaking, rectangular ducting is much more prone to leakage than threaded or oval ducting. In accordance with Reference 4, we took leakage to be ~15% of transported air volume. Since leakage will increase as a system is scaled up, it is best to fully seal rectangular ducting from the point of view of energy conservation. The geometry and size of each duct section are shown in the Table .

Because air leakage is often very great, even in low-velocity ducting systems with an internal static pressure of 5-20 mm-Aq, ordinary plant leakage tests require considerable manpower and expenses to find and repair leaks. Since it was necessary to perform leakage testing of the whole system and partial systems after the completion of assembly, this study's test process exhaust system was temporarily referred to as a "test duct system." After the required working pressure was reached, a fluid pressure gauge (or electronic pressure gauge) was used to read the pressure difference across the two sides of the orifice plate.^2,6 The obtained pressure difference was substituted into Equation 7 to derive duct leakage. It indicates that the larger the opening of a crack, the greater the pressure difference across the orifice plate, and the greater the leakage. The pressure difference will be zero if there are no leaks. The final step was to calculate the leakage rate per unit surface area in the test duct system using Equation 5:

Q = 21.8K (D₂)²√ΔP                      (6)

If D₁ = 400 mm = 15.748 in. and D₂ = 240 mm = 9.45 in., then K = 0.65

Q = 1265.4 √ΔP                              (7)

The tests described above yielded the necessary data. Leakage was determined as follows:

(1) Total leakage cannot exceed 1-10% of total system. The actual value ordinarily depends on the company's demands of the system. When leakage tests are performed on separate sections of ducting, the sum of the leakage in all sections may not exceed the total allowable leakage. The following judgment formula is used:

Here, Q_L is the total leakage from the test duct system in CFM; and Q_sys. is the maximum airflow entering the test duct system.

(2) After all audible leaks have been repaired, the sum of the remaining leakage will not ordinarily tend to exceed 1-10% of system capacity. If the total leakage does exceed this amount, great care must be taken to locate remaining leaks.

Results and discussion

The results of analyzing leakage in the test system indicate that, when pressure is applied to different duct systems, the pressure difference across the orifice plate can be measured, and the value substituted into Equation 6 to obtain the airflow, Q, across the orifice plate, which is also the total system leakage Q_L. In addition, because the total surface area of the test system was 202.975 ft², Equation 5 was used to derive the leakage class, C_L.

The static pressure difference between the inside and outside of the system increases when the working pressure in the system rises, and the pressure difference and airflow across the orifice plate likewise increase. Furthermore, the pressure difference across the orifice plate increases steadily as the working pressure (static pressure) in the system continues to rise (Fig. 2). Leakage from the test system increases as the working pressure rises because the gaps tend to expand, and the pressure difference of flow across gaps also increases. Figure 3 shows the relationship between the pressure difference across the orifice plate and the airflow. The diagram shows that the flux is directly proportional to a certain power of the pressure difference both before and after repairs. This proves that the data resulting from the measurement process will remain on the curves derived from Equation 6 regardless of the magnitude of system leakage. This also shows that experimental error was very small and verifies the accuracy of the experiment.

Figures 4 and 5 , respectively, depict the relationship between the total leakage and static pressure and the relationship between the leakage rate per 100 ft² of surface area and the static pressure. The diagrams reveal that there was a significant difference in the system before and after repair. When total airflow in the system was ~5000 CFM, and maximum leakage could not exceed 5% of the system design airflow, leakage before repairs was 4.38% of system airflow when working pressure was 1 in-Wg (Q_L = 219.17 CFM). When total airflow in the system was ~10,000 CFM, and maximum leakage could not exceed 5% of the system design airflow, leakage before repairs was 4.73% of system airflow when working pressure was 3 in-Wg (Q_L = 473.469 CFM). Since the resulting leakage would fail to meet the system leakage requirements when the working pressure exceeded 3 in-Wg, the system had to be repaired. After repair, leakage was 4.90% of system airflow when working pressure was 13 in-Wg (Q_L = 490.09 CFM), and leakage would fail to meet system requirements if the pressure exceeded 14 in-Wg.

Conclusions

The study conducted process exhaust system performance measurements using the main chamber that had been designed, and provided a detailed account of the measurement methods. As far as the measurement of system leakage was concerned, the results showed that the difference in leakage before and after repair was 670.25 CFM (0.32 CMM) when the working pressure was 15 in-Wg (3735.32 Pa). Furthermore, when total airflow in the system was ~10,000 CFM and maximum leakage could not exceed 5% of the system design airflow, the leakage before repair could not meet this restrictive condition when working pressure exceeded 3 in-Wg. Likewise, leakage after repair could not meet this condition when working pressure exceeded 13 in-Wg. This shows that, after a system has been installed, subsequent processing is necessary to prevent leakage from causing performance losses or the escape of toxic gas.

Author Information

Rong-Hua Ma is an assistant professor in the Department of Mechanical Engineering at the ROC Military Academy . E-mail: rh.ma@msa.hinet.net

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References

ASHRAE Handbook — Fundamentals , Ch. 32, Atlanta, ASHRAE Inc., 1997. HVAC Air Duct Leakage Test Manual , 1st ed., Chantilly, Va., Sheet Metal and Air Conditioning Contractors' National Association Inc., 1985. W.B. Swim and E.I. Griggs, "Duct Leakage Measurement and Analysis ," ASHRAE Trans., 1995, Vol. 101, Pt. 1, p. 274. HVAC Duct Construction Standards — Metal and Flexible , 2nd ed., Chantilly, Va., Sheet Metal and Air Conditioning Contractors' National Association Inc., 1998. Handbook of Air Conditioning System Design, 1960, Pt. 2, Ch. 2, Syracuse, N.Y., Carrier Corp., 1960. G. Gan and S.B. Riffat, "Numerical Determination of Energy Losses at Duct Junctions ," Applied Energy, 2000, Vol. 67, No. 3, p. 331.

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