Preamble
The layout of the plant and the air-conditioning system was designed with a specific brief to achieve the best cleanroom air quality, temperature, humidity and operator comfort with minimum energy consumption.
The conditioned space admeasures 12500 sq. ft. with a false ceiling at 12feet and houses 14 injection molding machines dissipating around 82kW heat.
A conventional air-conditioning system to achieve Class 1 lakh (ISO 14644 Grade 8) with supply air from the ceiling would require an aircon system having 3nos 18000 CFM (cubic feet per minute) AHUs (air handling units) with 99tons of refrigeration capacity and consume 162.72 kW peak energy, depending on the season.
Considering a different type of system called the displacement ventilation system with air being supplied from low level instead of from the false ceiling, it was possible to achieve the same parameters of Class 1 lakh using an aircon system with 3nos 7000 CFM AHUs with 80tons of refrigeration capacity and consume 91.3 kW peak energy, depending on the season.
This represents a 43% saving on peak loads!
In addition, as a feature, it was decided to go in for Class 10,000 (ISO Grade 7) cubicles, one for each injection molding room for conducting the packing activities. Each of the cubicles, when in use would consume 2.38Tons of refrigeration and 3.75kW of energy. The idea was to provide an even better environment with flexibility of using only when necessary.
The note below describes the design involved.
How the Conventional Air-conditioning Works
The conventional air-conditioning adopted for Class 1 lakh cleanrooms is based on mixing (dilution) ventilation systems concept in which air is supplied in a manner such that the entire room volume is fully mixed.
How the Displacement Ventilation System Works
Air Flow Penetration
Based on CFD analysis the following room temperature profile was expected in the production area:
Temperature Contour at 0.5M ( 20 inches) from Floor
Temperature Contour at 1.8M (6 feet) from Floor
Temperature Contour at False Ceiling Level, 3.6M (12 feet) from Floor
Legend for Temperature
Measures taken for Air Quality, Operator Comfort and Energy Savings
Air Quality
- HEPA filtered air supplied into the open areas at low level to achieve best air cleanliness class (Class 1 lakh) at the operating level.
- Each injection molding machine has a packing area adjacent to it. The packing area comprises of an air lock and packing room which are maintained at Class 10,000 cleanliness.
- Product exposure in the molding machine and the traverse by robot to the packing room conveyor are maintained at Class 100 cleanliness.
- The cleanliness of Packing Room, Airlock, Outside production areas are preserved by having a pressure gradient with clean to dirty axis.
Thus air quality is maintained best where it matters most. This makes the cleanliness levels affordable and easier to maintain.
Figure 7: Air Cleanliness and Temperature Scheme
Operator Comfort
- As described earlier, the displacement ventilation provides uniform comfort at 27 o C [ 80.6 o F ]in the occupied zone of the open production area. There are no air drafts.
Since the packing rooms are maintained at Class 10,000 require elaborate cleanroom gowning, these rooms are maintained at lower temperature of 73.4 o F [23 o C for operator comfort
- The high return air temperature at false ceiling level posed a peculiar problem of creating operator discomfort due to radiant heat from the false ceiling. The radiant heat can result in higher load on the air-conditioning system. Radiant heat was minimized by providing low emissivity anodized aluminum false ceiling material. Low emissivity results in transferring maximum heat through the false ceiling and into the attic space and reflecting back minimal heat into the conditioned space
- Due to maximum radiation getting into the attic space, heat would build up there and this would result in a hot floor in the floor above. This would make the operators in the
Figure 7: Air Cleanliness and Temperature Scheme
Energy Saving Measures
- The main production area is air-conditioned based on displacement ventilation. The supply air temperature into the area is around 65 o F as against 56 o F used in normal mixing ventilation air-conditioning system. The input energy required to obtain the higher temperature is one main source of energy savings.
- The air re-circulation required in normal mixing ventilation air-conditioning systems to obtain Class 1 lakh cleanliness class is around 20 air changes per hour. As against this, since the release of supply air in displacement ventilation system is at the operating level, around 10 air changes per hour air circulation has been used. This effectively has reduced the air handling unit fan energy requirement by half to achieve the same cleanliness.
- Further energy savings are obtained by affecting free cooling of return air. The air returning to the air-handling units is at 104 0 F. The ambient temperature is around 95 o F in summer & lower during night time and in mild seasons. By incorporating heat pipes, ambient air is used to cool the return air before feeding into the air-handling units. This way, around 10.2 Tons of air-conditioning is recovered,
Compiled by:
B Gautham Baliga
17th December 2015
Opal HVAC Engineers Pvt Ltd
Design & Build Contractors
gautham.baliga@opalhvac.com & gauthambaliga1@gmail.com
Acknowledgements
- Grateful thanks are due to Mr. Xerxes Rao, Managing Director, Jay Precision Pharmaceuticals Pvt Ltd for reposing confidence in us to do this path breaking project which came with its associated entrepreneurial risks. Thanks are also due for his guidance on the design aspects from his immense fund of knowledge, to look at things from the basics, which helped us in our own perspectives.
- Also acknowledge that the figures 1 to 6 are based on details of Price Industries, USA in their literature on displacement ventilation systems
