Ambika Industries

G/1/687, Phase II, RIICO Industrial Area,

Bhiwadi, Rajasthan – 301019

Ph. No      00911493512568

Telefax:    00911493223042

Mob. No.  +919354541359

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In India, ducting clearly features high on any list of project headaches. Yet, for all the effort the end-result in most cases satisfies no one – the quality of ducting continues to be poor, project completion times unduly long and most contractors claim to lose money on this activity.

From the owner/end-user's perspective, the economic penalties are even more acute. Ducting typically represents less than 1% of the overall building costs but is almost always on the project's critical path. As such, the potential savings for the end-user on interest charges alone – from faster completion of the work – are generally more than the entire cost of installed ducting. Further, excessive air leakages from poorly constructed ductwork result in energy losses that verge on being shameful.

The irony is that it is easily possible to obtain good ductwork in India even today. One has merely to look at a site-fabricated duct installation (Photo 1 - indeed one of the better examples) and compare this with a typical modern factory-fabricated ductwork erected under a professional approach to site management (Photo 2).


So why does the industry permit the situation to continue?

The consultants fault the contractors for not doing "a good job"; the latter point to their dependence on unreliable tinkers and margins squeezed so tight that they cannot "afford" to do it any better. Unfortunately, this reflects an industry more distracted by apportioning blame rather than seeking workable solutions.

And the solution is quite straightforward. To raise the level of ducting in India, the industry must bring about a change concurrently at three basic levels, viz: how ductwork should be fabricated, how assembled and erected and the standards under which the ducting system is to perform. We examine each of these below.
Standards and specifications - a core Issue

In my considered opinion, the most important factor inhibiting change from taking place fast enough is a lack of a robust, industry-wide standard covering ductwork.

A frequent argument against change is that the newer ducting solutions are said to be more expensive than the conventional practice of site fabrication. This is indeed perverse. It would be perfectly reasonable if such a practice gave the desired end-result but when it clearly doesn't, the appropriate reference point must be a re-defined standard of performance acceptable to the industry and not existing practice.

Consider the evidence. The only "domestic" standard covering ducting (IS 655-19631) available today is unfortunately silent on virtually every important fundamental performance criteria related to ducting. These include air leakage, noise, vibration and structural rigidity to name a few. It is therefore not uncommon to read tender specifications that include statements such as “leakage should be as low as possible”. This is obviously meaningless as an engineering specification and serves only to perpetuate an industry grossly under-performing its potential.

Fortunately, there is a clear recognition by the industry for the need to radically revamp the BIS standards on ducting. Some progress has also been made on this with a few leading consultants having prepared drafts of revised specifications.

The change required, however, is formidable and the process of change also needs to be accelerated substantially. Indeed, a good case can be made for the industry adopting a well-tested, comprehensive international standard such as SMACNA2.

Either way, it is time for a body such as ISHRAE to put its engineering, organizational and financial muscle behind the effort in a concerted manner.

Ultimately, of course, the quality of the project specifications and how well it is enforced that will shape the quality of the installed ductwork. Till such a time as an acceptable industry-wide standard is available for ducting, our recommendation is for the industry to base its tender specifications on SMACNA duly modified for the specific project environment.
Construction and fabrication- today's technology

Sheet metal fabrication today draws on a wide array of specialty machines such as coil-handlers, folders, notchers, shears, presses, profile cutters, edge-formers, lockformers, flangers and punches amongst others. Further, many of these are CNC controlled (see Photo 3). To not use such equipment and technology would be to miss out on the full extent of the benefits available for quality and speed.

At the same time, it is necessary they be operated in a controlled environment of a factory rather than be carted from site to site. As such, the days of moving some basic finishing machines such as lockformers to site are clearly dated. In addition, there are several other benefits from carrying out the fabrication away from the job site, such as lower scrap and wastage, cleaner job sites (Photo 4) which permits greater efficiency in work, lower noise and less pilferage.

A few years ago it became fashionable to specify "factory-made" ducting. Of course this resulted in some operators demanding qualification on the basis of merely having a garage-type workshop with very basic equipment. In today's context, it is therefore necessary to clearly define the fabrication technology acceptable.

Typically, today's state-of-art technology will involve:
Raw material in coil-form (Photo 5) rather than sheet stock – this permits all longitudinal joints to be formed at the duct corners necessary for a good, strong construction. Also, facilitates the use of prime factory-delivered material rather than the typical condition of commercial sheets found at sites, often with water mark and of ‘hard’ quality.
CNC machines for accuracy in measurement and speed of fabrication (particularly for coil handling and profile-cutting)
Special-purpose machines for the edge finishing operations
Pre-specified tolerances on all finished dimensions (usually within +/– 1.0 mm for all dimensions to 500 mm, +/– 0.2% for larger sizes)
3rd party certification of tests for critical parameters such as duct leakage and strength/quality assurance programmes
Documentation of all supplies
Site management & erection- the professional way

The traditional approach essentially involves the HVAC contractor dumping raw material at site and the ducting sub-contractor essentially taking it from there. The assembly and erection of factoryfabricated ducting, however, imposes a much greater requirement for the contractor's direct involvement in two important areas.

First, in the planning, co-ordination, and control of the job with some of the key activities:
The final duct design and layouts must be checked carefully against site plans and potential fouling with other utilities such as plumbing and electrical lines prior to sending them to the factory for fabrication
Incoming material to be checked against packing lists. There are potentially hundreds or even thousands of finished profiles/parts in a truckload – as opposed to just one stack of raw materials – and a practical system for accounting for them is necessary
For the same reason, incoming profiles must be stacked vertically (both for sorting as well as a space-saving measure) simultaneously as the truck is being unloaded – typically there can be up to 10 tons of semi-finished material per shipment and re-handling and re-sorting such material is just not practical (Photo 6)

Second, to raise the productivity of the work at site: productivity gains at site translate to both speed of job completion and cost savings. The potential for dramatically improving this with pre-fabricated profiles is so vast that one just cannot afford to miss on it. We have seen several instances of jobs that would take some weeks, being completed in as many days and by a fraction of the number of people involved.

To achieve this, however, will require both proper tools and tackles appropriate for the work at hand and training in their use. This includes small tools (both electrically operated and manual ones (Photo 7) as well as some larger material handling equipment and rigs such as hoists (Photo 8) and scaffolding. A few of the more progressive contractors already maintain such equipment.

Putting it all together…

Change, as we all know, is never easy. But is it achievable? In this case, most certainly yes – and the opportunities are just amazing. Believe me, I've seen it.

The pharma manufacturing industry is one of the largest, if not the largest, users of HVACR equipment in India. The present pharma market size is approximately 12,000 crores which is only one percent of the world market size. However our growing population and increasing emphasis on healthcare will be a rapid growth in the industry along with a similar growth in the need for air conditioning and refrigeration equipment.

The emphasis in air conditioning system design in the pharmaceutical industry lies in providing a clean and aseptic environment with controlled relative humidity. The industry in itself is governed by a Good Manufacturing Practice (GMP) The goal of GMP is to provide guidelines for establishing proper environment and repeatable method of producing sterile products free from microbial and particulate contaminants. The GMP embraces a number of issues starting with the selection of building materials and finishes, planning the flow of equipment, personnel and products, determination of key parameters like temperature, humidity, pressures, airflow parameters and classification of clean rooms. It also governs the level of control of various parameters for quality assurance, regulating the acceptance criteria, validation of the facility, and documentation for operation and maintenance.

Various countries have formulated their own GMPs. In the United States it is regulated by several documents such as Federal Standard 209, code of Federal regulations 210 & 211 etc, which are revised and updated from time to time. The European Community has a "Guide to Good Manufacturing Practice for Medicinal Products and in the United Kingdom it is BS 5295, Good Pharmaceutical Manufacturing practice." In India the GMP follows largely the country of the principal technology provider. In addition regulations of the Food and Drugs Administration (FDA) need to be complied with if the products are to be released for sale in the United States.

Building Design

Proper building design and planning of the flow of personnel, material and equipment is essential for achieving and maintaining the design levels of cleanliness and pressure gradients. The building construction itself has to be "tight" with minimum of uncontrolled infiltration of outside air into the building. This is very important in the case of buildings for formulation and sterile production. On the other hand, the bulk drug plants are more open except for some sections for finishing, packing etc. Some of the principles generally followed in the construction and operation in the pharmaceutical industry are:
Minimum seams and joints
Avoid crevices and moldings
Round off all joints
All material used is non chipping and cleanable
Wall and floor finishes should not shed particulates and should provide self-cleaning surfaces
Ceilings are to be flush as far as possible
Provide airlocks & air showers at the entry points of all important areas
Operations producing particulates are enclosed and exhausted
Personnel wear lint-free overalls, head covers etc. And do not wear cosmetics
Each manufacturer develops over a period of time their own GMP in all these aspects.

Design Goals

Room temperature is not critical as long as it provides comfortable conditions Normal temperatures range between 20 to 25 deg C + 2 deg C, Relative humidity (RH) on the other hand, is of greater importance in all the production areas. While most of the areas could have a RH of 50 ± 5% facilities designed for handling powders need to be at 40 ± 5% Automatic control of he RH is essential for maintaining continued product quality.
Of all the design goals, it is the quality of air cleanliness of the space and prevention of contamination which are of utmost importance. All GMP's aim towards achieving a clean aseptic space suitable for production. Currently the normally accepted air quality standards are:
Manufacturing areas:- 20,000 particles of 5 micron or larger per cum of air
- 500 per cum of viable organisms
Large Volume Systems: - 3500 particles of 0.5 micron or larger per cum of air
- 5 particles of 5 microns or larger per cum - 5 viable organisms per Cum

The pressure gradients, roughly are:
Change rooms
Non-aseptic areas 0 Pa
25 Pa
25 Pa
Aseptic areas
Cooling corridors
Access corridors
Manufacture laboratory
Filling rooms
Change rooms
45 Pa
35 Pa
55 Pa
55 Pa
25 Pa
(approx. 10 Pa = 1mm wg)

There should be a net airflow from change rooms to the non-aseptic areas even if the pressures are stated as equal. Regular adjustments are necessary to compensate for drift and filter clogging

Good filtration regime is a precondition for achieving the above design goals. For many areas and processes Clean Room facilities must be provided to meet the GMP. Filtration is mainly required to control the atmospheric contamination reaching the production area. It is also used to manufacture of products and also to protect the operator and environment. In dusty production areas such as grinding, granulation, coating, tabletting etc., the filters not only control the atmosphere contamination but also hold the internally generated particulates.

Atmospheric dust is a mixture of dry particles, fibres, mist, smoke, fumes, live or dead organisms. The air-borne particle size varies from 0.01 micron to as much as 100 microns. Less than 2.5 micron particles are considered as fine and particles over 2.5 micron are regarded as coarse. Fine particles are considered as fine and particles over 2.5 micron are regarded as coarse. Fine particles are airborne for a longer time and could settle on vertical surfaces. Coarse particles, products of mechanical abrasion like in grinding and granulation departments, have lower airborne life time and are subject to gravitational settlement. The air conditioning systems in the pharmaceutical industry have to handle both fine and coarse particulates depending on the production pattern and the filter regime has to be appropriate.

Table 1 is a guide line to filter selection.
Table 1 Air Filter Selection
Areas Efficiency Arrestance Type
Non-aseptic Areas
Pre-filter 1
Pre-filter 2

20-40% dust spot
80-85 dust spot
95% DOP
75 to 85%
Panel or bag
Aseptic Areas
Pre-filter 1
Pre-filter 2
20-40% dust spot
80-85% dust spot
99.97% DOP
75 to 85%
Panel or bag

All filters are dry type with synthetic and glass fiber. While pre-filters could be cleanable, the final filters are disposable.

Filters are distinguished by their efficiency, airflow resistance and dust holding capacity. Filter testing and rating are complicated issues since the performance of a filter varies very the performance of a filter varies very widely on different basis of testing as well as the composition of test dust. Arrestance is a mass fraction of dust removed from a pre-composed synthetic test dust fed into the filter. As the dust is fed, the resistance across the filter increases and the dust holding capacity is determined as the arrestance value when the filter resistance reaches its main maximum specified. Filter efficiency is defined as the ratio of the loaded for pre-filter as per BS 2831 Test Dust No. 2

Other tests are the dust spot and DOP tests. The dust spot test is a measure of the ability of the filter to reduce soiling and discoloration. The DOP test is conducted by counting upstream and downstream particulates through a light scattering photometer or any other particulate counter. Test particulates are of uniform 0.3 micron diameter with a density of 80mg/cum produced by condensation of DOP vapour (Dioctyl phthalate or bis - 2 ethylexyl) or any other suitable substitute British standard 3928 for HEPA filters, and 2831 for other filters and ASHRAE standard 52-1 govern the testing of these filters.

Clean Rooms

Following the development of High Efficiency Particulate Air (HEPA) filters for the nuclear industry, the concept of clean spaces was first developed for the aerospace electronics industry. The same concept was appropriately applied to the semi conductor, pharmaceutical and many other industries where a clean particulate-free environment is essential for maintaining standards of product quality.

A clean room is a defined area where the critical parameters such as temperature, humidity, air changes room pressure, particulates variables etc. are closely controlled to maintain product strength, identity, operator safety and product quality. This may be a requirement of the law or by virtue of the product development data. Clean rooms are divided into several classes based on the particulate concentration limits. The current classification is shown in Fig.1 & Fig.2.

Class Names Class Limits
0.1 um 0.2 um 0.3 um 0.3 um 5 um
SI Inch - pound
Particles per m 3 Particles per m 3 Particles per m 3 Particles per m 3 Particles per m 3
M1 350 75 30.9 10.0
M1.5 1 1240 265 106 35.3
M2 3500 757 309 100

10 12400 2650 1060 353
M3 35000 7570 3090 1000
100 26500 10600 3530
M4 75700 30900 10000
1000 35300 247
M5 100000 618
10000 353000 2470
M 6 1000000 6180
10000 3530000 24700
M7 10000000 61800
Sources : Federal Standard 209E, Airborne Particulate Cleanliness in Clean Room and Clean Zones.
Fig.2: Classes of Clean Rooms

Particle Control

Particle sources in a clean room are either external or internal. External source is atmospheric dust which is a mixture of dry dust particles, fibers, mist, smoke, fumes, bacteria and live or dead organisms. Entry is largely through outside make-up air but could also be by infiltration through doors, windows, wall and ceiling joints and penetrations. Hence a right building with minimum infiltration will help minimize uncontrolled contamination. Good filtration regime controls the flow of contaminants through the make-up air.

Internal sources of contamination are personnel, room surface, process equipment and he process itself. Personnel are by far the largest source of fenestration of internal particles and are to be controlled through lint-free clean garments, head wear, entry through air showers and by suitably designed airflow. Surface shed particulate are to be controlled through proper surface applications. But, there is no way of predicting the particle generation from process and the equipment.
Air Flow Pattern

There are only two basic air flow patterns viz. Non-unidirectional and unidirectional employed in a clean room. Non-unidirectional air flow is clean rooms of class M4.5 and above. In this airflow pattern, there will be considerable amount of turbulence and it can be used in rooms where major contamination is from external source that is the make up air. The contaminants are filtered out in the air handling unit filters and also terminal HEPA filters. If internal contaminants are a concern, a clean work station is provided inside the clean room.

The unidirectional air flow patter is a single pass, single direction air flow of parallel streams. It is also called 'laminar' airflow although it is not truly laminar but the parallel streams are maintained within 18 deg - 20 deg deviation. The velocity of air flow is maintained at 0.46 m/s to 0.65 m/s as specified in Federal Standard 209 version B although later version E does not specify any velocity standards.

Unidirectional air flow clean rooms are vertical down-flow or horizontal types. Vertical down-flow type has the ceiling of HEPA filters. In class M3.5 rooms the entire ceiling requires to be covered with HEPA filters with low level returns or solid or perforated floor plenums. The parallelism of streams is maintainable right up to the working height (say 900mm) but stream lined flow gets degraded by obstacles by way of people, equipment and work tables. Thus the room gets divided into an area of stream lined flow and some area of turbulent flow which causes undesirable particle trajectory Horizontal flow systems are same as the vertical except that the airflow is horizontal from a wall of HEPA filters to a receiving wall on the opposite side. In this the parallelism of stream lines of flow is not as maintainable as in the down flow and further the space becomes more and more contaminated towards the return wall. Fig.4 shows such a clean room. Between the two, the vertical flow pattern yields better results and is more adaptable to pharmaceutical production.


The air conditioning system for a clean room is tested and validated based generally on US Federal Standard 209 and/or BS 5295 and no production can start until the clean room is validated. For proper evaluation of the facility, the system is tested before commissioning, while at rest, and during production. Pre-commissioning test procedures cover the following parameters:
HEPA filter integrity by cold DOP testing for pinhole leaks in the filter media, across sealants and frame gaskets, supporting frame and wall. This has to be done on the upstream and terminal filters if two banks are used
Air stream velocity under each filter panel is to be established
Establish a spectrum of particulates from appropriate air samples
Pressure differentials between room to passage to change rooms
Pressure drop across the final filters
Room temperatures and relative humidity
Smoke testing for establishing flow patterns if possible and if required similar test are desirable with the clean room in operation and at rest for a complete validation. A comprehensive documentation of the testing procedures and test readings is prepared before the facility is handed over for production.

System Design
Cooling Loads & Equipment selection

Pharmaceutical buildings as a rule are totally enclosed without any fenestrations. This is to maintain a 'tight' building to minimise uncontrolled infiltration of atmospheric air. As a result, the room sensible loads are essentially a contribution from process equipment, lighting and personnel. The density of equipment loads is low excepting in the tablet manufacturing facility covering granulation, drying and tabletting. A major contribution of the cooling load comes from outside air entering the air handling unit. There is also considerable diversity in the equipment loads based on the production patterns. All these result in a low room sensible load density varying from as low as 40 Kcal/h/sqm to 100 Kcal/h/sqm. Hence the system design lays emphasis on control land maintenance of relative humidity. The room temperature is normally held between 20 deg. C, whereas the relative humidity is held at 50± 5% in most of the areas. In a few areas it is maintained at 40± 5% or lower depending on the product characteristics.

Equipment selection i.e., cooling coils, hearing coils and air-to-air plate exchangers are prone to degradation over a period of time. Hence it is a normal practice to oversize the cooling and reheating coils. Similarly in order to have a control on the air borne particulates, the number of air changes are considerably more than required for normal comfort air-conditioning. The minimum air changes are 15 to 20/hr going to as high as 300/hr in M3.5 class of clean rooms. To avoid cross contamination independent air handling systems are provided for various discrete operations like manufacturing, coating, tabletting, inspection and packing. In some departments there is further segregation of operations which requires a certain degree of control, if not an altogether independent air handling unit. In other words, the equipment selections and segregation's of the air handling systems needs to be done after through analysis of the process requirements. Fig.5 is a standard airconditioning flow diagram for a facility without any RH control.

Relative Humidity Control

As stated above, pharmaceutical air-conditioning is characterised by low room sensible loads, low and controlled RH and high air changes. All these characteristics contribute to problems of RH control in the industry. A more or less standard configuration of the air handling system is shown in Fig. 6.1 It consists of a cooling coil, a heater array, return air filters, fresh air filters and supply air filters. This configuration will meet the requirements of clean facilities upto class M 5.5. The modified version using variable air volume is shown in Fig. 6.2 This system takes advantage of diversity in the air-conditioning air requirements in different departments served by the same air handling unit. In these cases, the fan is driven by a variable frequency drive. Such a system brings about considerable savings in the energy consumption of the fan system and pays for itself anywhere from 1.5 to 2 years.

Spaces where a low RH of 40±5% is required, an additional brine cooling coil is incorporated. Fig 7 shows a typical diagram using additional brine cooling and hot water coils for a strip packing facility. The brine coil further dehumidifies the supply air which is repeated in the hot water coils for a strip packing facility. The brine coil further dehumidifies the supply air which is reheated in the hot water coil. This is to ensure lower moisture content in the supply air and hence maintain the desired low RH in the space.

Sometimes plate type air to air heat exchangers are interposed between the cold air from the air handling unit and the return air from the room. A part of the sensible heat from the return air (24 deg C) will get transferred to the supply air raising it from 6 deg C to 14 deg. C thus providing free reheat.

Such a system saves the cost of energy spent in reheat coils which is substantial if electrical re-heaters are used. However, one should be cautious in applying a plate heat exchanger for this purpose as the heat transfer capability deteriorates over a period of time. Fig 8 shows the two reheat systems as applied to low RH areas.

Sometimes packaged dehumidifiers are used in rooms where low RH is required. The dehumidifiers are specially designed self contained type air-conditioners which will draw humid room air, dehumidify in the evaporator coil and push back into the room through the condenser coil at a higher temperature. In the process the moisture content in the air is reduced and sensible heat added in the condenser coil. The room humidity will progressively reduce to the desired levels, chemical adsorption dehumidifiers are incorporated for reliable control on the relative humidity. The chemical dehumidifiers are continuously regenerated type. Fig. 9 is an example of an air-conditioning system with the chemical dehumidifier and a thyristor drive to reduce power input to the electric re-heaters on reduction of internal latent load in the production facility.

Clean Rooms

All pharmaceutical facilities belong to one or other class of clean room. General acceptance is:
Tabletting facilities Class M 6.5 (Class 100,000)
Topical & Oral liquids Class M 5.5 (Class 10,000)
Injectables Class M 3.5 (Class 100)

Clean rooms are designed for aseptic as well as non-aseptic applications. Pharmaceutical facilities designed for sterile products will have aseptic clean rooms while manufacturing of tablets and oral liquids will have non-aseptic clean rooms. Besides bulk drug plants designed for handling products in finishing suites will also employ non-aseptic clean rooms.

The entrance to the aseptic clean room should be through a regime of change rooms generally designated as black grey and a white change rooms in order of increasing pressures towards the sterile filling facility

Aseptic clean rooms have the following distinct characteristics:
Highest pressure in the most critical zone is progressively reduced at the rate of 15 Pa towards the outside. Generally these are of class M5.5 but the environment within 600mm of the terminal filters will be as good as class M3.5.
All Critical operations will be in a laminar flow work station.
Air handling system is provided with a central HEPA filter bank along with mandatory terminal filters in order to extend the life of terminal filters.
Low RH say 35±5% is through chemical dehumidifiers.
Supply air outlets are provided flush at the ceiling level with perforated stainless steel grilles and terminal absolute filters
Return air grilles are provided at the floor level with a return air riser for better scavenging
The filtration regime is generally three stages with two stages of pre-filters, 10 micron (EU 4), 3 micron (EU 8) and one central final filter 0.3 micron (EU 12) along with terminal HEPA filter. See Fig.10

Air Distribution

The air distribution has to be appropriate with the class of clean room. Class M5.5 and above are generally non-unidirectional with the supply air outlets at the ceiling level and the return air at the floor level. Class M3.5 and lower invariably have unidirectional down flow type. Air distribution is designed so that the aseptic filling room will have a pressure of 50 Pa or above with a gradient of 15 PA to the adjoining area. The pressure gradients are monitored with 'U' tube manometers or magnahelic gauges. Alarm and warning systems may also be provided when the pressure gradients are disturbed.

All duct work must be non-flaking, corrosion resistant and thoroughly sealed. If any duct extension is provided from the final filter, it should be minimum in length and should preferably be of stainless steel upto the finishing suite. Remaining duct work could be of good quality galvanised steel. Round ducting is a natural choice, being self cleaning in shape, wherever space permits. All rotating equipments like the fans should be isolated from the rigid ducting. Ducts should be tested for leaks before the application of insulation. Volume control dampers must be provided appropriately for easy air balancing. Wherever possible, provision should be made for cleaning the duct work internally and externally. Grilles and diffusers should be flush mounted into ceiling, walls or duct work and all such grilles shall be fabricated from stainless steel in the clean areas.


A comprehensive documentation of HVAC systems is essential for a proper and complete evaluation. The documentation should cover design, operation and performance qualifications which should and the operation, maintenance and periodic testing of the system.
Design Qualification

The design qualification document should cover all the following issues:
Identification of various systems, their functions, schematics & flow diagrams, sensors, dampers valves etc., critical parameters & fail-safe positions.
Layout plans showing various rooms & spaces and the critical parameters like:
room temperature.
room humidity
room pressures and differential pressures between room and room and passages.
Process equipment locations and power inputs.
Critical instruments, recorders and alarms, if any.
Equipment performance and acceptance criteria for fans, filters, cooling coils, heating coils, motors & drives.
Duct & pipe layouts showing air inlets, outlets air quantities, water flows and pressures.
Control schematics and control procedures.
Operation Qualification

This is a commissioning documentation which shall provide all the details of equipment various points of performance, test readings, statement of compliance and noncompliance with the acceptance criteria. Broadly the features are as follows:
Installation date showing manufacturers, model no., ratings of all equipment such as fans, motors, cooling & reheat coils, filters, HEPA filters, controls etc.
As-built drawings showing equipment layouts, duct and pipe runs, control & fire dampers, settings of various sensors and controllers.
Contractor's rest readings covering rotation tests, megger readings, air quantities, temperatures and RH pressures of each space, dry & wet run of controls, air and water balance, HEPA filter integrity tests at final operating velocities testing of limits & alarms.
Identification of items spaces, parameters not meeting the acceptance criteria but cannot be corrected.
Performance Qualification

This is essentially for the system operating under full production conditions and covers among others:
Identification of agency for commissioning, for equipment and instruments and their calibration.
Test readings of all critical parameters under full operating conditions and full production, modification of readings in the contractors test results, acceptable and unacceptable departures from design qualification and acceptance criteria.


Manufacturing practices, air filtration techniques and air conditioning components are being constantly upgraded in order to improve the finished product and reduce energy consumption. Innovative system and product designers at different world locations constantly come up with better ideas and with globalization serious readers are advised to keep in touch with similar articles in other trade magazines.

Yet it is common to see in India today HVAC installations with world-class centrifugal, screw and reciprocating packaged chillers, double-skin AHUs and modern building automation systems coupled with poorly fabricated air duct distribution systems. Of course, it is only with the economy opening up in this decade that such advanced equipment and technologies have made their appearance in India and it takes time for components of any new system to find a balance.

Unfortunately, the reality of the weak link will not go away. If the end-user is to receive the benefits of present day HVAC technologies, we will have to stop relegating ductwork to the hidden spaces where it has long resided - out of sight and out of mind. The predominant practice in India is that of fabricating ducts at site using basic hand tools such as snips, mallets and channel sections functioning dually as anvils and straight edges. This has two consequences. First the fabrication process is naturally slow and is frequently the leading cause of delays at HVAC project sites. Second dependent as this practice is on the rudimentary tools used and individual skills of the sheet-metal workers, there is little scope for maintaining uniform quality of the ducted systems. Occasionally hand operated folders and electrical lock forming machines are brought to job sites-but these address only a narrow aspect of duct quality.

So why do we continue to rely predominantly on this practice of site fabrication in India especially when automated factories for sheet-metal ducting are now an established norm the world over? The answer appears to be rooted in the perception that this is the only economical way of fabricating ducts in this country.

The commonly held belief is that labour in India is plentiful and cheap, actual site conditions keep changing relative to plans ,and, if fabricated anywhere other than at site, additional expenses of transportation, duties and taxes would be incurred. But when the current practice of fabricating ductwork doesn't adequately satisfy the fundamental requirements of quality, reliability or speed of fabrication, it's clearly time to re-examine our economic assumptions and ways of doing things.

To begin with, one must keep in perspective that the total ducting cost is generally not much more than 10% of a typical AC project cost (somewhat greater for ventilation projects). The scope for controlling the overall project cost through ducting is therefore quite limited. It gets even more so as we focus selectively on isolated elements of ductwork cost such as fabrication labour without looking at the total economic picture.

For instance, we consistently tend to underestimate the high cost project sites. This is quite an oversight as raw material accounts for some 70% of overall ducting costs land is typically 3-4 times the unit labour cost for duct fabrication. Further, with site fabrication one also has to contend with poor storage conditions, difficulties of finding skilled manpower, absenteeism, temperamental job supervisers - all of which causes expensive delays in job completion and even more expensive call-backs. And yet the best-of-class technology and practices employed the world over for the fabrication and installation of sheet-metal and ducting is available in India as well and can readily be the norm.

To achieve this, four areas need to be addressed:
Duct Construction Norms

A review of existing industry standards is an obvious starting point.

Here, the Bureau of Indian Standards (BIS) specifications IS 655 governing metal air ducts is clearly out of date. Barring two very minor amendments in 1985 and 1991 these standards essentially remain as they were first drafted in 1963. By contrast, and DW 142/144 (UK) the corresponding international standards, have undergone several fundamental changes in the same period reflecting developments in both the demands of functional performance and sheet-metal fabrication technology.

One essential difference between IS 655 and the major international standards is in the manner they explicitly recognize the performance characteristics to be met. For instance we know that duct strength, deflection and leakage are more functions of pressure than velocity while noise, vibration and friction loss are more related to air velocity. Consequently, whereas duct design is primarily influenced by air velocity in order to meet the required flow requirements and to control parameters such as frictional losses, duct construction standards should be based primarily on static pressure considerations.

Hence, comparing the BIS and SMACNA standards with reference to the two key performance requirements:
Structural Rigidity: In 18-655, sheet-metal gauge is given only as a function of cross-sectional duct dimensions with no reference to reassure classification. Consequently, for a given duct size, the same gauge of material is prescribed regardless of whether the ducting is for a low pressure 1/2" w.g. Static pressure application or for a 6" high pressure one. Under SMACNA however, in addition of sheet metal gauge is also a function of pressure class, spacing between joints, reinforcements and type of joints. Further, SMACNA explicitly recognises that the shorter the section length, its inherent stiffness and rigidity will be greater and therefore a lower thickness of sheet metal is adequate In fact the tendency of SMACNA is to prescribe the least thickness of sheet-metal possible as an unduly heavier gauge will increase both the self weight of the assembly leading to greater deflection and leakage as well as resulting in greater noise and vibration.
Leakage: Except for mentioning that system should be "checked for air-tightness"' BIS is otherwise silent on the subject. It does not even specify the duct construction method to be followed to minimise air leakage.

Under SMACNA a leakage limit of 5% is deemed acceptable for most applications (less than this also achievable by greater use of sealants). Of course it is not practical to have every duct system tested for leakage and SMACNA explicitly recognises this. Having tested a range of duct systems of varying sizes and pressure classes, SMACNA confirms that by following their recommended construction standards for fabrication and assembly, leakage will be kept within limits.

Interestingly, while both leakage and structural rigidity characteristics inevitably deteriorate with an increase in the number of longitudinal seams or joints neither BIS nor SMACNA specify a limit on the number of such joints in a duct assembly.

In India, sheets of 2500 mm x 900 mm (or 1000 mm) are typically used with seams along the long lengths(Fig.1). With this method of construction, the number of seams will clearly increase with the cross-sectional dimensions of the duct section. Further, most seams will be in the side faces of the duct section and of the "Acme" lock type (Fig.2) which is relatively poor from both leakage and strength considerations. However, as the predominant form of fabrication in the West is with coils rather than sheets, seams are typically located on the edges (Fig.3) and commonly of the "Pittsburgh" or "Snap" lock type (Fig.4 & 5) both of which provide for lower leakage and higher reinforced structural strengths.

Selection of Suitable Raw Material

Raw material selection plays a vital row with three characteristics being particularly important for duct quality and economy.
Lock Forming Quality (LFQ) - i.e. the ability of the metal not to crack when folded, particularly relevant for GI material as the zinc coating will peel off at cracks or if folding is not uniform.
Zinc Coating : Above a threshold thickness of zinc coating, the uniformity of coating and adherence to substrate is more important than the amount of coating. Indeed extra heavy coatings may in fact be detrimental to long-term performance as the zinc may tend to flake.

BIS specifies 275 gsm (gsm/sq.m) zinc coating which appears to be over-specified. SMACNA uniformly recommends a G-60 grade (180 gsm) suitable for most applications using GI ducting. Coatings heavier than necessary (e.g. 350 gsm) have also led to a secondary problem - that of availability. Steel mills will generally produce these only against custom orders involving large quantities and long lead times.
Raw Material in Coil form as opposed to Sheets : The use of coils is vastly preferred to sheets from considerations of both quality and economy. As discussed earlier, the use of sheets (typically 2400 mm x 900 mm or 1000mm wide) will inevitably lead to an increase in the number of longitudinal seams as duct sizes get larger. By contrast, when coils are used in a "wrap-around" style (i.e. the length of the section equals the width of the coils), the longitudinal seams can be restricted to one (at corner) in the case of fully boxed sections or two (at diagonally opposite corners) in case of L-sections.

There is also considerably less wastage when the raw material is in coil form as opposed to fixed length sheets.

While raw material selection appears to be independent of the issue of whether the ducting should be factory - or hand-fabricated, it in fact has considerable bearing. Factory-made ducting will tend to use more uniform quality of material due to their bulk purchasing power. In any event, with GI coils sourced directly from the mills, available in coil weights of 4-5 MT or greater, these can only be handled at the factory rather than at job-site, for all practical purposes.
Sheet Metal Fabrication Technology

There is the perception in some quarters that the use of roll-forming equipment alone renders ductwork as machine-made. Roll-forming equipment such as a Pittsburgh lock-former are speciality equipment performing just one of the several manufacturing finishes required (in this case, end-finishing for longitudinal edge joints). Several other finishing equipment are typically required in today's sheet-metal shop - folders and flanging equipment for transverse connectors, edge-formers for contoured profiles and cleat-making equipment to name but a few. It is the collective use of such equipment which determines the extent to which ductwork is truly "machine-made".

In addition to the range of finishing machines a modern sheet-metal shop also has two other features - namely coil-handling capabilities (as discussed earlier) and computerization. Indeed the use of CAD/CAM and general computerization in today's sheet-metal fabrication shop has actually become a functional necessity. See photo 1.
Contrary to popular conceptions, speed of fabrication is most affected by the time taken for measurement of the customized sections (finished dimensions, location of folds and notches, development of contoured profiles, etc) rather than by the actual cutting and folding operations. Computer-aided measurements, an integral part of CAM, thus plays a vital role in minimizing job delays to an extent which is just not achievable with non-computerised factory fabrication, leave alone hand-fabrication.
A key determinant of duct quality the dimensional accuracy of each section of a duct assembly - also ensured by computer-aided manufacturing.
Given the large number of customized duct sections required on a job, computerisation also enables smooth job execution by permitting a detailed identification of parts to be fabricated, handled and installed through bills of materials, packing lists, labels, duct quantities/weights/surface area summaries required for billing, etc.

Logistics and Site Planning

Two arguments commonly used in favour of site-fabrication over factory-fabrication are:
Transportation costs involved in delivering ducts to job sites.
Changes in site conditions relative to original plans which make it easier plans which make it easier to fabricate ducts based on actual site measurements.

In actual practice, deliveries to sites over vast distances are eminently workable and even economical if ducts are provided in L-sections (in the case of rectangular ducting). Here, most of the sizing / folding / edge-preparation operations are done at the factory except for minimal assembly and actual installation at job site. See Photo 2

Also since raw material has to be supplied to the job sites in any event, the increase in transportation cost y supplying L-section is only marginal on a full truck - or tempo-load basis. As a rule of thumb, even half-load shipments of 500 - 600 sq.m. and over can be shipped quite economically (relative to the cost of ducting) almost anywhere in the country.

With reference to the second issue, while duct sizes are more definitively known when based on final site measurements, this aspect is nevertheless quit manageable even with factory-fabricated ducting. Factory fabrication does, however, impose a discipline on the planning process and there is ultimately no substitute planning. Even so, it is typical to find about 5% of the ductwork being done on a "suit-to-site" basis and in this, the situation in India is no different from elsewhere in the world. The important point is that factory fabrication is a proven, workable approach in the Indian context.
The Next Step

The process of change will have to begin with developing a new set of uniform specifications including changes to BIS. To this end we have outlined below the key elements of specifications which have their genesis in SMACNA and might be considered for inclusion in the relevant standards.

It's time to cross the next frontier.

Proposed Additions and changes to Specifications

Rather than re-invent the wheel, the sheet-metal industries of some countries have adopted a major international standard specification such as SMACNA in toto or with minor modifications. There is certainly merit in the Indian industry choosing to adopt the same approach - indeed even the UK standard of DW 142 /144 has its genesis in SMACNA.

While we will have to accept the fact that change in trade practice will not be sudden, we will have to ensure that our ductwork standards are set appropriate to the performance levels we wish to achieve. The objective after all is to have improved ducting - not merely accommodate the capabilities of the lowest common denominator.

We therefore list here the more important elements which we believe should be included in any set of specifications covering rectangular ductwork fabrication. These are drawn largely from SMACNA.
Raw Material
Coil stock preferred
Material should be of the minimum gauge necessary to resist both reflection caused by internal pressure and vibration due to turbulent air flow. For any given pressure class, alternative material gauges are permissible depending on the spacing between transverse joints/reinforcements. These shall conform to tables 1-3 to 1-9 of SMACNA 1995 Second Edition
GI material in particular must be of Lock Forming Quality (LFQ) conforming to the standards of ASTM A653 and A924 or conforming to grade D of IS 1079:1988 or IS 513: 1986 as specified in IS 277:1992.
Zinc coating should be of 180 gms./sq.m.
Yield strength for steel sheet and reinforcement should be 30,000 PSI (207 MPa). (Note: where no pressure classes are specified by the designer, the 1" W.G. (250 Pa) pressure class is the basis of compliance regardless of velocity in the duct. However, all variable volume duct upstream of VAV boxes should have a 2" W.G. (500 Pa) basis of compliance when the pressure class is not defined).

Longitudinal Joints (Seams)
Longitudinal joints shall be preferably restricted to two diagonally opposite edges.
These should be machine-formed of any of the following types:
a. Pittsburgh lock type.
b. Button Punch Snap lock type.
Joints and seams should be able to withstand 1.5 times maximum operating pressure without deformation or failure.
Table 1 Standard Duct Sealing Requirements
Seal Class Sealing Requirements Applicable Static Pressure Construction Class

All transverse joints, longitudinal seams and duct wall penetrations 4" (1000 Pa) w.g. and upwards

All transverse joints and longitudinal seams only 3" (750 Pa) w.g.
C Transverse joints only 2" (500 Pa) w.g.
In addition to the above, any variable air volume system duct of 1" (250 Pa) and ½" (125 Pa) w.g. construction class that is upstream of the VAV boxes shall be Seal Class C.

All Acme joints should be sealed.

Transverse Joints
Transverse joint must be able to withstand 1.5 times the maximum operating pressure without deformation or failure.
Where a transverse joint acts as a reinforcing member its maximum allowable deflection will be 0.25" (6.25 mm) for ducts upto 48" (1220 mm) width (W), and W/200 for greater widths.
For the spacing of transverse joints and type of reinforcement refer 'SMACNA' tables for rectangular ducting covering pressure class from 12" (125 Pa) W.G. to 10" (2500 Pa) W.G.
Crossbreaking or Beading

Crossbreaking or beading are effective ways in dealing with commercial tolerances on out-of-flatness, natural sag from dead weight and with the flexure reversals that may result when duct pressure is inadequate to stretch the sheet taut. Beading is preferred to crossbreaking (Fig.6 &7) applicable to 20g (1.00 mm0 or less and 3" W.G. (750 Pa) pressure or less. Ducts for 4" W.G. (1000 Pa) or more do not require beads or cross-breaks.

Leakage and Sealing ducts

Leakage is largely a function of static pressure and amount of leakage in a system is significantly related to system size. Economical and quiet performance of the ducting system can be ensured by making ducts reasonably airtight which can be achieved by a) selecting a static pressure construction class suitable for the operating condition, and b) properly scaling the duct work.

Transverse joints should be sealed with gaskets, and for ease of application, gaskets should preferably be self adhesive.

Heavy mastic sealants are more suitable as fillets in grooves of longitudinal seams. Mastics having excellent adhesion and elasticity are preferred.

Feb 10, 2009, 12:14 AM