Design Concepts in Air Management Systems

Design Concepts in Air Management Systems


A comparison between biotech facilities and semiconductor facilities in HVAC system applications and components

Designers and owners of semiconductor facilities focus on air management issues and operational savings opportunities to stay competitive in a world economy.1 The same concerns apply to biotech facilities. While process equipment represents about half of total project cost, mechanical systems comprise the second biggest share, about 25 percent of the total project cost.2 At the same time, one needs to keep in mind that building utility and HVAC system operational expenditures have a great impact on the final product cost.

Getting the product out to market in time to catch the window of opportunity is the first priority for any advanced technology facility owner; therefore, the building operational costs are usually overlooked or given little attention. However, it is the engineer’s responsibility to evaluate the mechanical systems to provide reliable and cost-effective system alternatives and present them to facility owners during initial phases of the project. Yet, often the cGMP and FDA regulations, coupled with the risk of not receiving approval for the facility, prevents engineers from being innovative and looking into different design alternatives.

Both semiconductor and biotech facilities are designed from the inside out. The buildings are programmed and designed to answer process needs. Therefore, building systems are provided to support process requirements. For example, the HVAC systems are designed to meet space cleanliness, temperature, humidity, and noise level requirements for the process. Pressure hierarchy between spaces for contamination control and containment also plays an important role in HVAC system design decisions. The main goal of any good design, while maintaining the aforementioned parameters, is to provide reasonable installation and operational costs.

The major systems that make up the building mechanical systems are as follows:

Chilled water generation
Steam and heating water generation
Make-up air (outside air) moving and conditioning
Cleanroom air moving and conditioning This article will evaluate the mechanical systems, with a focus on air management variations. In general, cleanroom air systems consist of two major components:
Recirculation air to provide required cleanliness level and temperature control
Outside air to provide fresh air for indoor air quality, replenish the general and process exhaust, provide building pressurization, and establish cascading pressure control.
We recognize that the potential variations in design of different cleanroom HVAC systems are as many as there are cleanroom designers. To simplify the comparison of the air management systems for both semiconductor and biotech facilities, we will use two air management concepts that are common in both semiconductor and biotech facilities.

Case 1: This design utilizes one airflow loop with all make-up air and cleanroom return air mixture conditioned in one unit. This case results in a more complex air balancing process.

Case 2: This design utilizes 100 percent outside air combined with multiple cleanroom recirculation units (secondary air) to provide filtration and space air conditioning. This case is somewhat easier to provide differential pressure between spaces.

The air handling unit setup used in Case 1 is shown in Figure 1. The unit can incorporate a return fan or a relief fan depending on the return duct pressure drop. Wash down, or purge cycles can be provided as well. (The advantages and disadvantages of a return fan versus a relief fan require a separate study and are not in the scope of this article.) After the return fan section, a pre-filter section follows the relief, return, and outside air mixing section. Due to the outside air component, typically a 30 percent pre-filter, 85 or 95 percent after filter, and sometimes a carbon filter are provided with the air handling unit. Depending on the outside air quantity and humidification requirements, a preheating coil may be provided. The preheating coil can be eliminated if the outside air ratio is less than 15 percent of the recirculation air. However, if an outside air economizer is used, or the purge cycle is provided, a preheating coil needs to be provided.



There are debates among designers regarding the location of the humidifier. When there are high internal loads, the leaving air temperature can be as low as the dew point. Hence, it is not practical to add moisture into the air stream to maintain space dew point at these temperatures (the psychometric process is shown in Figure 2). Therefore, this article recommends using a humidifier upstream of the cooling coil. A good control system design will avoid the unnecessary humidifying and consequent dehumidifying the moist air in the cooling coil after the humidification process. One can also argue that the cooling coil will provide better dew point control by wringing out the excess moisture added by the humidifier. However, this is not an energy-efficient way to control the space conditions.

After the humidifier section, a cooling coil is provided. The cooling coil has to be sized to dehumidify the total recirculation air and maintain the supply dew point set point. It is a good practice to size the cooling coil carefully so that the air will be two or three degrees lower than the dew point temperature than the space requires. This design practice will provide the extra capacity that might be required for future flexibility or any moisture migration into the space. However, the designer should evaluate the higher and lower end operation points for controllability of the coil for the whole span.

When the supply fan is selected, the total system efficiency needs to be considered. Designers should strive for the highest fan efficiencies. The total static pressure for the supply fan can vary between 7 and 10 inches WC. The types of fans that can deliver high static pressure are limited to centrifugal or vane axial types. For cleanroom applications, direct drive fans are recommended due to less particulate generation from the vee belts. The direct drive application necessarily limits the designer’s choice to plug or vane axial fans. However, for high static pressure, the designer may have to use belt-driven centrifugal fans for the high system pressure applications. Belt-driven fans create a potential for prematurely loading up the final HEPA filters with particulates shedding from the belts. If plug fans are used, the total mechanical efficiency might be lower than for vane axial or centrifugal fans. However, the pressure drop coming from the air blenders and sound attenuators that might be required when using vane axial or centrifugal fans should be considered as part of the evaluation since the pressure drop introduced by these components might offset the energy benefits.4 It should be noted, however that several manufacturers are providing designs that include shaping of the fan entry and discharge using computational fluid dynamics. The result has been in some cases a very significant reduction in fan power energy. Finally, HEPA filters, depending on the area cleanliness classification, can be installed either inside the units or in the ceilings. If the area is ISO Class 7 or better, filters are installed in the ceiling; otherwise, they can be installed inside the air handling unit.

The air management system for Case 2 consists of two sets of air handling units as shown in Figure 2. A primary unit is used for preconditioning and introducing outside air into the cleanroom recirculation units. The air handling unit that is used as the make-up air unit consists of outside air intake louver or section, 30 percent efficient pre-filter, 85 or 95 percent after filter, preheating coil, humidifier, cooling and dehumidification coil, the fan of choice — either centrifugal, vane axial, or plug fan — and discharge HEPA filter at 99.97 percent efficiency. The secondary unit consists of a pre-filter section (designer’s choice) at 30 percent, sensible cooling coil using chilled water, makeup air mixing section, a direct drive fan (vane axial or plug fan), and discharge HEPA section. The primary unit construction can be a double wall, either custom built or industrial grade. Since the cleanroom air is not directly in touch with the make-up air unit, wipe down and cleanability requirements can be relaxed. The secondary units have to be constructed to allow cleaning with aggressive cleaning agents. They should be of double wall construction with aluminum or stainless steel inner walls. Coves (concave metal trim) should be used on a wholesale basis to make it much easier to maintain and clean the interior of the air handling units.

The concept described in Case 1 was used previously in semiconductor facilities; however, designers and owners have moved on to the system described in Case 2 to take advantage of the operational savings offered with this design. It is uncommon to see Case 1 systems in the latest state-of-the-art semiconductor facilities. Pharmaceutical facilities (smaller cleanrooms) use either 100 percent outside air units (once-through systems) or Case 1 systems, due to flexibility, simplicity, containment requirements, and the physical conditions of the facilities.

Biotech facilities, with respect to the layout of the buildings, are similar to semiconductor facilities. A bay and chase type of semiconductor facility greatly resembles a biotech facility with process and technical spaces housing support equipment.

The difference is that in semiconductor facilities there is less concern with bio burden than there is in biotech facilities. In semiconductor facilities, service chases are commonly used for return plenum. However, in biotech facilities, the ducted return air system is used for contamination control and for segregation purposes. Mechanical equipment rooms are usually unclassified spaces, and wash down or wipe down is not required. Using ducted return, in addition to using HEPA filters on returns (if required), increases the return system pressure drop.

Biotech facilities are designed to be mostly ISO Class- 8 and ISO Class-7 process areas and isolated ISO Class-5 filling areas. Cleanliness classifications are assigned to areas according to process needs; however, the recirculation airflow rate for the classification is determined for the activity level and recovery time desired within regulatory limits and client requirements. The airflow required to provide the space temperature is usually lower than the airflow required to maintain the cleanliness classification. In other words, the internal loads will not drive the recirculation airflow rate for cleanroom applications. Therefore, the supply air temperature is higher than the space dew point temperature.



Even though the cleanliness level for a biotech process area varies from ISO Class-7 to Class-8, the total recirculation airflow for the area may require a dedicated air handling unit due to the size of the process suite. On the other hand, the semiconductor process area is smaller in comparison to a biotech facility because of process tool requirements, and the cleanliness levels vary between ISO Class-3 and Class-5. Typically, each bay is serviced by dedicated air handling units. In biotech facilities, high ceiling heights require the airflow rate to be calculated by using air changes per hour (ACH) criteria. For semiconductor facilities, usually CFM per ft2 criteria is used. Refer to Table 1 for airflow criteria seen in both semiconductor and biotech facilities. ISO 14644-4, Annex–B was used to prepare this comparative table. The ratio of the outside air quantity for a biotech facility can vary from 10 to 30 percent of the recirculation air. A semiconductor facility might require 3.5 to 6 CFM per ft2 make-up air. Even though the outside air quantity is quite high for a semiconductor facility, the ratio of outside air to recirculation air is similar to that of a biotech facility.

ENERGY COMPARISON
A biotech facility of 200,000 ft2 and a semiconductor facility of 100,000 ft2 are used for comparison. A typical semiconductor process bay of 480 ft2 at ISO Class-4 requires 36,000 CFM. On the other hand, a biotech process suite of 3,200 ft2 with 16-ft ceiling height requires 43,000 CFM. If we compare the outside air quantities for both, a semiconductor facility would need 2,100 to 2,750 CFM outside air; the biotech suite would require 2,250 to 4,300 CFM outside air for pressure control and adequate air change to replace the entire air volume of the space to avoid any contamination buildup. The total outside air quantity for the aforementioned biotech facility would be around 50,000 to 75,000 CFM. The outside air to cleanroom air recirculation ratio is almost equal for both types of facilities.

The hypothetical facility is assumed to be located in Baltimore, MD. The design conditions for the outside and inside air are given in Table 2.



To maintain a process suite cleanliness level of ISO Class-7, a total of 43,000 CFM recirculation air is required. For space pressure and indoor air quality control, 4,300 CFM of outside air is required. For simplicity, one process bay is included in the energy calculations.

In Case 1, due to the outside air component of mixed air, the total recirculation air has to be cooled down to dehumidify the air to achieve the supply air dew point required for the space conditions given. At the design conditions, the supply air temperature will be 46.9°F (dew point temperature), which is colder than required to maintain space temperature. The reheat coil will raise the supply air temperature to maintain space temperature. In real life, the reheat coil control valves are usually at 50 to 70 percent open position depending on the internal load. This indicates that the recirculation air is cooled down for dehumidification purposes only. Most likely, a supply air temperature of 55 to 60°F would be adequate to maintain space temperature.

During winter, the return air is mixed with the outside air. Even during winter, the process suites require cooling due to high internal loads. The cold outside air provides free cooling of the return air and reduces the winter chilled water load. Mixed air is humidified to provide dew point control. The air brought to the space dew point line passes through the cooling coil to reject the internal heat. The reheat coil provided in the unit, or for each temperature control zone, maintains space temperature.

See Figure 4 for the psychometric process of the Case 1 air handling unit for winter and summer operations.



In Case 2, the outside air component is centrally conditioned to space dew point temperature in a primary air handling unit. The space air is not dehumidified and reheated; only outside air components are dehumidified and humidified to maintain space dew point. The moisture migration into the space can be offset by resetting the make-up air supply dew point up or down to compensate for the changes.

The conditioned outside air is introduced to the recirculation unit after the cooling coil. Since the space air is controlled to a constant dew point temperature, the secondary unit cooling coil performs only sensible cooling. The worst-case space temperature controls the supply air temperature; therefore, the reheat of the recirculation air can be minimized. The process is shown in Figure 5.



Energy calculations for both cases are performed to compare the energy use of each component. The bin data for Baltimore is used to calculate annual energy consumption. The energy usage summary is shown in Table 3.



Even though the Case 1 air handling unit uses outside air during winter months for free cooling, the energy use for humidification, dehumidification and reheat surpasses the savings. Also, the difficulty of space pressure control and filter loading should be considered if an outside air economizer is used.

For both cases, the humidification load does not change if an outside air economizer is not used. The Case 2 air handling system requires preheat for humidification; however, only the outside air component is conditioned to meet the space dew point conditions. That results in 396,000 MBH heating load and almost 228,000 ton-hr cooling load reductions. This equates to $23,000 annual savings for the system used for this analysis.

The calculations indicate that the total energy usage can be reduced by at least 34 percent with the primary/ secondary air handling scheme. Also, the separation of the dew point control from the space temperature control will simplify the control systems.

The other savings come from the fan horsepower. The Case 1 unit — as a result of high-pressure drops in the air stream, created by the elements such as after filters, dehumidification coils, and preheats coils — consumes higher energy to move the air in the system. The total pressure in the system can be around 8 to 10 inches WC. In Case 2, the higher-pressure drop components are located in the make-up air unit, which has lower airflow and, therefore, consumes less energy. The higher airflow that comes from the cleanroom air recirculation passes only through a sensible cooling coil and pre-filters. The total static pressure is typically around 2.5 to 4.5 inches WC; therefore, the fan energy is far less than in the Case 1 system. The Case 2 system requires 45 BHP vs. 85 BHP for Case 1.

The boiler and chiller sizes used for the Case 2 system will be smaller since the peak load for the heating and cooling is less than in the Case 1 system. This will reduce both capital expenditures and operational costs.

Advantages and Disadvantages

The recirculation units used for Case 2 are shorter in length than the Case 1 units.
The mechanical room space requirement for the Case 2 system is less than the Case 1 system, even though 100 percent outside air units need to be provided for the Case 2 system. Redundant outside air units are recommended for system reliability.
Centralized outside air units will enable the outside air intakes to be located to avoid exhaust air re-entrainment and will also provide the opportunity for heat recovery from the general space exhaust.
The louver sizes will be smaller in comparison to the Case 1 units.
The outside air will be ducted to each cleanroom recirculation unit. Since the outside air will be pressurized, the duct size will be smaller than the individual ducted outside air intakes.
• The total system initial cost is lower, since fewer unit components are purchased chased with the recirculation units.
In Case 1, the air handling unit is dedicated to a process suite or similar functional areas. The outside air is provided via this unit. If the unit fails, the space temperature and pressure control go outside the specifications. However, a centralized outside air unit with complete redundancy will provide the required outside air for space pressure control even if the cleanroom recirculation unit fails.
The space dew point and space pressure stay in the specified range. Separating dew point controls from space temperature controls will simplify the controls.
In Case 2, the engineer needs to design the 100 percent outside air units with extra capacity for future flexibility and unknowns. If the outside air units are undersized, increasing the pre-conditioned outside air will be costly. The Case 1 units have an advantage over the Case 2 system in this scenario, since it would be easier to increase the capacity of a single unit.
The Case 1 units have the capability to dehumidify all of the recirculation air. The coils are usually oversized; therefore, it will be easier to recover any temperature humidity upsets.
Smoke purge, wash down, or fumigation cycle control is easier with Case 1 systems. Also, the Case 1 system can recover the space faster than Case 2 during initial startup or after shutdowns. Since the Case 1 units are dedicated to a space, they can be isolated, or make-up airflow can be increased without impacting adjacent space pressure.
In Case 2 systems, the outside air ductwork is the short link between different spaces. In this case, the amount of outside air needs to be increased to achieve quicker recovery for the spaces going through one of the scenarios mentioned above.
The make-up air unit needs to be selected and designed to increase the amount of outside air to any space without impacting adjacent spaces. However, the space temperature and humidity recovery will be much slower.
CONCLUSION
Dedicated outside air units with cleanroom recirculation units using sensible cooling only are not the common system design for biotech facilities. However, this design concept has been widely used in semiconductor facilities for a long time due to its lower operating cost.

Similarities between biotech and semiconductor facilities and experience gained from other industries suggest that this concept should at least be carefully investigated and considered in biotech-pharmaceutical facility design. This simple comparison suggests that air management systems used for semiconductor facilities can be adapted to biotech facilities instead of carrying out traditional pharmaceutical facility HVAC design concepts. However, the designer should weigh total system advantages and disadvantages.

Bibliography

Naughton, P. “HVAC systems for Semiconductor Clean Rooms – Part 1 System Components,” pp 620-625, ASHRAE Transactions, SL-90-5-3
Naughton, P. “HVAC systems for Semiconductor Clean Rooms – Part 2 Total System Dynamics,” pp 626-633, ASHRAE Transactions, SL-90-5-4
Blanchard, Joseph A. “Pharmaceutical Facility Costs: Variance, Categories, and Causes,” pp 50-55, Pharmaceutical Engineering, May/June 2000
Naismith, R. “Clean Dry HVAC Air for GMP Laboratories,” pp 24-32, Pharmaceutical Engineering, January/ February 1999
Brown, W. K. “Make-up Air Systems Energy-Saving Opportunities,” pp 609-615, ASHRAE Transactions, SL 90-5-1
Taylor, Steven T. “Comparing Economizer Relief Systems,” pp 33-42, ASHRAE Journal, September 2000
Hunt, E., Benson, D.E., Hopkins, L.G. “Fan Efficiency Vs. Unit Efficiency for Clean Room Applications,” pp 616-619, ASHRAE Transaction SL-90-2
ISO Standard 14644-