Make-up air in the cleanroom
by Raymond K. Schneider P.E., Luwa Lepco
Depending on outdoor conditions as much as five to eight tons of air-conditioning is required to cool and dehumidify each 1,000 cfm of make-up air. Conditioning, controlling and delivering this air to the cleanroom can be costly. The substantial first costs and operating costs associated with treating make-up air for the cleanroom suggests an opportunity for value engineering.
Strategies that reduce the exhaust from the cleanroom or that capture energy otherwise exhausted can reduce first cost and/or operating cost of the facility. Such a reduction can result in a saving of as much as $20 per exhaust cfm the first year and as much as $5 per cfm thereafter for the life of the process housed in the facility.
The high volume of air being exhausted from many cleanrooms today coupled with the requirement for excess make-up air to maintain positive pressurization has resulted in the make-up air system being a high cost item in cleanroom construction. The make-up air is typically treated with high efficiency filtration, has its temperature and humidity controlled, and is delivered in sufficient quantity to maintain prescribed pressurization within the clean space.
The purpose of make-up air is to replace air exhausted from the cleanroom and to introduce sufficient excess air, commonly two air changes per hour, to provide proper cleanroom pressurization. The make-up air should be filtered to a level consistent with cleanroom cleanliness class and conditioned to a level that will maintain cleanroom temperature and humidity requirements. Providing highly filtered air with proper temperature and humidity characteristics is costly. First cost is affected by the size of chiller and heating plants as well as make-up air units. Operating cost is affected by the increased energy needed to properly condition the air.
Cleanroom designers should exercise judgment based on quality of the local outdoor air source. In some cases, they may want to specify a carbon bed filter in spite of the first and ongoing maintenance costs of this unit. The air then passes through a bag filter, commonly an 85 percent or 95 percent ASHRAE efficiency filter. For Class 1,000 or less stringent cleanroom applications, this may be the final level of filtration imposed on the make-up air before it is delivered to recirculation fans and final filtered by HEPA/ULPA filters.
In locations where winter outdoor design temperature is below freezing a preheat coil is commonly used. This coil is intended to increase the make-up air temperature to a value above the freezing point, 48 degrees Fahrenheit in this example.
While hot water or steam is frequently used for heating, some applications may use electric heat when precise control is required.
The cooling coil provides cooling and dehumidification. Chilled water is used in this example; however, a direct expansion refrigeration system could also be applied. The cooling coil is designed to provide a leaving air temperature of 48 degrees Fahrenheit dry bulb temperature and a 47 degrees Fahrenheit wet bulb temperature.
The reheat coil is intended to increase the make-up air temperature to the desired value, in this example 68 degrees Fahrenheit.
A final filtration stage may be included, particularly for Class 10 or cleaner cleanrooms. This stage may use HEPA or ULPA filtration.
Humidification is introduced downstream of the final filter. Some design engineers may incorporate the humidifier into the make-up air handler. A dedicated humidifier, often using DI water, is the preferred approach to providing cleanroom humidity control.
Psychrometrics
The outdoor conditions selected in this study are for Research Triangle Park, NC, and are illustrative of a locale requiring dehumidification in summer and humidification in winter.
In this example, the air is cooled and dehumidified and then reheated to a condition that will meet cleanroom cooling demand.
In the figure on page 30, values extracted from the chart are shown along with equations used and calculated results. The calculation is based on a weight flow of outside make-up air required to replace 30,000 cfm of air leaving the cleanroom through exhaust and exfiltration (i.e., leakage due to pressurization).
The make-up air is at 93 degrees Fahrenheit and 46 percent relative humidity (RH) and must be brought to 68 degrees Fahrenheit and 45 percent RH. This requires a substantial removal of moisture from the airstream, from a specific humidity of 109 grains per pound of air down to 47 grains per pound. To accomplish this, the air is cooled to 48 degrees Fahrenheit dry bulb and 47 degrees Fahrenheit wet bulb temperature leaving the cooling coil. The calculation shows that this will require 233 tons of chilled water cooling capacity, or almost 8 tons per 1,000 cfm of make-up air. The chilled water flow rate is 559 gpm based on a 10-degree temperature rise through the cooling coil. It is not at all unusual to see a 10- to 12-row cooling coil used in make-up air handlers to accomplish this task.
To heat the conditioned air to 68 degrees Fahrenheit, the reheat coil must have a capacity of 642,000 Btuh and will require a flow rate of 64 gpm of 180 degrees Fahrenheit hot water with a 20-degree temperature drop.
On a design day in winter the preheat coil must add over a million Btuh to the airstream to raise the temperature of 16 degrees Fahrenheit outside air to 48 degrees Fahrenheit. This will require a flow rate of 103 gpm of 180 degrees Fahrenheit hot water with a 20-degree temperature drop.
A conservative assumption of zero outdoor air specific humidity has been made for calculating the humidifier capacity. It is therefore determined that 899 pounds per hour will raise the specific humidity to the cleanroom requirement of 47 grains per pound that corresponds to 45 percent RH at 68 degrees Fahrenheit.
Cost factors
While each application must be examined in detail to come up with meaningful cost figures, it is safe to say that a first cost for make-up air beginning in the range of $10 to $15 per cfm can be “guesstimated.”
This cost consists of:
Make-up AHU incorporating prefiltration, final filtration, cooling, pre-heating, reheating, humidification, air moving equipment and cabinetry.
Ductwork including sheetmetal, insulation, intake louvers and dampers.
Chiller, cooling tower and boiler system augmentation, including circulation pumps and main piping header size increase, and specific make-up AHU piping runouts.
Control system including control of make-up airflow; temperature, humidity, and cleanroom pressure sensors; control wiring, as well as chilled water and hot water valving.
Power wiring including motor starters, circuit protection, as well as increased wire size for chillers and boilers.
Ongoing operating cost can equal first cost in 3 to 5 years. This will consist of power cost for chillers, boilers, pumps, humidifiers and fans, as well as preventive maintenance service cost including filter changes and carbon bed maintenance, if applicable.
Not considered here are such financial costs as interest on the added equipment and depreciation costs. Nor has the cost of the exhaust system been addressed.
Alternatives
The 30,000 cfm make-up air system could result in an initial capital cost of $600,000, and an ongoing operating cost of $200,000 per year. Alternative strategies that reduce the total exhaust from the cleanroom can have a significant effect on make-up air costs.
A careful study should be made initially to include only those high exhaust operations within the cleanroom that require a clean environment. Frequently the decision to place an exhausting work station in the cleanroom is guided more by work flow convenience or by a “we`ve always done it that way” rationale than by product quality issues.
Using reduced flow work stations and/or time of day (occupied/unoccupied) exhaust control can reduce operating cost even if first cost must be expended to insure that full make-up air is available when needed.
Using bulkhead installation of process equipment that permits heat exhaust using non-cleanroom, service chase air, can offer significant savings.
Air-to-air heat exchangers could be installed in the exhaust and make-up airstreams. Models designed for zero crossover flow are available for applications involving hazardous exhausts. They should be evaluated to determine if first cost and operating savings are sufficient to recoup investment in a reasonable time.
Raymond K. Schneider P.E. is vice president and manager of the Industrial Air Engineering Business Unit of Luwa Bahnson, Winston Salem, NC, where he is responsible for design and construction of cleanrooms throughout the U.S. and Canada, as well as for the marketing of the Luwa Bahnson line of cleanroom ceiling grid, mini-plug fan/filter units and air diffusion ceiling systems. Schneider has an aeronautical engineering degree from the Polytechnic University of New York and an MBA from Long Island University. He is a registered professional engineer in five states and the author of the book, Practical Cleanroom Design, as well as numerous technical articles and books on HVAC and cleanroom topics.
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The air is cooled and dehumidified and then reheated to a condition that will meet cleanroom cooling demand.
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Make-up air is drawn from outdoors. This air is pre-filtered by a 30 percent ASHRAE panel filter to remove large particles. The air may then pass through a carbon bed filter to remove gaseous contaminants.