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By S.K.Jaiswal & P.B.Kulkarni,
Technical Services Division
Bhabha Atomic Research Centre, Mumbai
P.B.Kulkarni is a Mechanical Engineer working in BARC, Mumbai in the capacity of Chief Engineer and Associate Director, Engineering Service Group He has more than 32 years experience in mechanical and electrical utility systems for various applications such as nuclear, toxic and clean room. He is also involved as an expert in the clean room requirements for Radiopharmaceuticals for overseas projects through IAEA, Vienna.
S.K.Jaiswal is also a Mechanical Engineer working in TSD, BARC in the capacity of engineer Grade-F. He has 17 years experience in HVAC systems including clean rooms. He is also involved as an expert in the clean room requirements for Radiopharmaceuticals for overseas projects through IAEA, Vienna.
A HEPA filter frame is one of the most critical components both in a clean room and in a nuclear / toxic application area and plays a major role in achieving the required parameters. Even then it is the most neglected component in the whole system. Most suppliers or contractors in the clean room industry think that they need not spend time, money and energy in making an “engineered” filter frame. They manage to achieve efficiency of the bank and in turn the class of cleanliness by using some sealant or tape around the HEPA filter or by employing a liquid-gel type seal. If users insist on an “engineered” dry / liquid type frame for class 100 and better application, the vendor will simply use imported components.
The only industry or organization in India which closely looks into the design of the frame is the nuclear industry. This may be because of the toxicity of the exhaust air which is to be treated and passed through a HEPA filter bank before discharging to atmosphere through a stack. The HEPA filter bank is the main “engineered safety feature” (ESF) in nuclear installations.
In nuclear applications, proper embedment of the frame structure in a concrete shell is initially ensured. Integrity of the frames and perfect sealing of the filters with the frames is ensured during the design and execution. While efficiency of filters is 99.97% down to 0.3 micron, efficiency of the entire bank also, is expected to be around 99.95%. The integrity of the frame with a concrete shell on one side and a HEPA filter on the other side must therefore be close to 100%. This feature of a nuclear HEPA filter bank is wholly applicable to the banks of clean rooms as well.
Design of filter banks for nuclear application aims at quick removal of filters to minimise the man-rems (exposure to radio activity). This demands reasonable space between filters for the clamping mechanisms. While such space does not pose any problem in nuclear applications, it becomes a deterrent in clean room applications. Such space, termed as attic space in clean room terminology, is always a cause of concern for class 100 and better applications. HEPA filter banks used in the nuclear industry have therefore to be suitably modified for clean room application.
The evolution of HEPA filter frames from nuclear application to clean room application is shown in Figures 1 to 6.
The engineering up-gradation of a HEPA filter frame will be clear from Table 1.
| Table 1 : Progressive improvement in design of a HEPA filter frame | ||
|---|---|---|
| S.No. | Description | Attic w.r.t filter effective area |
| 1.0 | Unused space in conventional nuclear HEPA filter bank. |
24.1 % |
| 2.0 | Attic space in standard clean room application. |
18.6 % |
| 3.0 | Attic space in a design (by BARC) meant for an international assignment. |
8.6 % |
| 4.0 | Attic space in a design (by BARC) displayed at Clean Room India 2001 workshop organized by SSPL/DRDO, Delhi. |
6.2 % |
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Whether it is a nuclear industry or a clean room application the integrity and compatibility of a filter frame with a HEPA filter is a must. If the filter frame is not properly designed and fabricated, the bank efficiency and required class of cleanliness can never be achieved even if we use HEPA filters of very high efficiency.
A compressive force is required for gasket compression of 60 to 80% for a reasonably good fabrication quality of filter frame and casing. The compressive force reduces considerably if we are able to achieve ±0.5 mm or better tolerances in the flatness of filter frame and casing. For a better finished surface the gasket compression reduces to even 20%. Generally 6-mm thick gasket of soft neoprene rubber with 6 to 10 shore hardness is used on both sides of the filter. An upstream side gasket may or may not be used as it does not play any role in achieving filter bank efficiency. Generally 8 to12 bolts of size, varying from 10 mm to 6 mm diameter, are used for giving compressive force.
Very low compressive force was allowed for an application in BARC using FRP filter casing and FRP frames. This could be managed by moulding the frames to an accuracy of ±0.25 mm.
All the embedment parts of a filter frame should have the same life as the building or structure. Therefore the embedment parts should be necessarily fabricated out of stainless steel material. The filter frame itself can be fabricated out of any material such as aluminium, mild steel duly powder coated or stainless steel. For class 100 and below, the material of construction should necessarily be either stainless steel or an extruded aluminium section, duly anodised.
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A filter frame was designed and fabricated by BARC, mainly for clean room application but with certain changes, it can be employed for nuclear and toxic application as well. This frame was designed for achieving class 100 and better cleanliness level and was installed in a clean room tent which was displayed at the workshop "Clean Room India 2001" at Delhi, organized by SSPL / DRDO. The filters were supplied by Filter Manufacturing Industries, Calcutta. Details of the set-up are as under:
1 Clean room tent - 1225 (L) x 1225 (W) x 2100 (H) mm dimensions
2 HEPA Filter
Quantity - 4 Nos.
Type - Mini pleat HEPA filter
Size - 610 x 610 x 68mm
Manufacturer - FMI Calcutta
Velocity - 90 FPM
Flow - 360 CFM
Special features of HEPA filters:
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3 Material of construction of tent
Filter casing - Extruded aluminium section
Filter frame - Stainless steel
Duct plenum - Anodized aluminium
Tent partitions - PVC
4 Fan details
Flow - 1500 CFM (approx.)
Static pressure - 40 mm of w.g.
5 Salient Features of tent
6 Details of particle counter & measurements
Make - Air Techniques Industrial, USA.
Model - Met One-228
Sampling volume - 0.1cft
Sampling time - 1 minute
No. of locations - 9
Elevation - Three different levels
Click to view the clear picture
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7 Test results
The details of the particle count carried out on 12/04/2001 at SSPL/DRDO, Delhi
is shown in Tables 2 and 3.
Pre lunch readings
| Table 2 : Particle counts inside the clean area | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| S.No. | Level | Particle Size | Particle count / Cu ft volume at various locations as per Figure 7 |
||||||||
| A | B | C | D | E | F | G | H | I | |||
| 1 | Near filter |
0.5 micron | 0 | 0 | 0 | 0 | 0 | 0 | 20 | 30 | 800 |
| 2 | 1.5 ft. below filter | 0.5 micron | 0 | 0 | 0 | 0 | 0 | 0 | 20 | 10 | 890 |
| 3 | 5 ft. below filter | 0.5 micron | 170 | 160 | 140 | 190 | 140 | 150 | 140 | 180 | 1060 |
| Table 3 : Particle counts inside the clean area after re-clamping | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| S.No. | Level | Particle Size | Particle count / Cu ft volume at various locations as per Figure 7 |
||||||||
| A | B | C | D | E | F | G | H | I | |||
| 1 | Near filter |
0.5 micron | 0 | 10 | 0 | 20 | 0 | 0 | 40 | 40 | 260 |
| 2 | Working level | 0.5 micron | 20 | 0 | 10 | 0 | 0 | 0 | 30 | 10 | 80 |
| 3 | Close to floor | 0.5 micron | 140 | 120 | 140 | 180 | 120 | 190 | 150 | 130 | 320 |
It is clear from Table 2 that the count at location "I" i.e. at the junction of all the 4 filters was high. Therefore clamping was redone and second set of particle counts was taken in post-lunch session. The counts are as per Table 3.
Post lunch readings
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A HEPA filter frame with very low attic was designed and fabricated by Bhabha Atomic Research Centre. This frame was tested for the cleanliness level at the workshop "Clean Room India 2001". The results are very encouraging and as intended. The readings were taken for a once-through system and there should be an improvement in particle counts for a recirculatory system. This frame should meet the requirement of class 100 and better cleanliness level. We are confident that this can be a substitute for installations where imported frames are required. This will encourage our local vendors to develop other components also which are required for class 100 and better applications.
The authors would like to thank the officers and staff of Central Workshop, BARC who took a lot of pain in order to fabricate the frame in a very short span of time. We would also like to thank Filter Manufacturing Industries, Calcutta for providing mini-pleat HEPA filter for the clean air tent and Associated Engineers, Mumbai for providing a filter plenum for the set-up.
Thanks are also due to the organisers of "Clean Room India 2001" for giving us a chance to exhibit the clean air tent during the workshop.
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