Fab Process Exhaust Systems Washington DC

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 el is the coefficient of total losses produced by laminar flow, and et 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 et *<# el , 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 el *<# et + 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, QL , is proportional to the Nth power of the static pressure difference between the inside and outside of the duct (?P = P1 - P2 ). When the constant of proportionality CL has been obtained, the leakage QL can be expressed as:

QL = CLA(?P)N

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

F = CLPs N

Here, F is the leakage rate per unit of duct surface area (CFM/ft2). The constant,CL , is the leakage class; Ps 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

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??P (6)

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

Q = 1265.4 ??P (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, QL is the total leakage from the test duct system in CFM; and Qsys. 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 QL. In addition, because the total surface area of the test system was 202.975 ft2, Equation 5 was used to derive the leakage class, CL.

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