Raising Air Conditioning Energy Efficiency to a Higher Level
The contamination control industry can adapt conventional building air-conditioning energy efficiency designs to a higher level of efficiency and cost-effectiveness.
By John Potter
Discussions always occur in the popular literature on ways to design and operate cleanrooms to be energy efficient. For example, minimizing duct static pressure to lower fan motor horsepower; controlling humidity via pretreatment of makeup air instead of treating an entire volume of mixed air; utilizing desiccant dehumidifying machines instead of brine-fed coils for high-end humidity control; setting back thermostats and fan-cycling time clocks; using variable frequency drive motors; using more accurate and interactive control algorithms and other means to properly size and operate cleanroom equipment have all been discussed. The value of preventive maintenance, including frequent prefilter changes, timely HEPA filter media replacement to save on fan operating costs, maintaining chiller plants, cooling towers, and other heat-rejecting devices in proper operating condition to enhance their factory efficiency have all been discussed.
All of these are valuable and straightforward tools for cleanroom users to save a significant amount of energy. Contamination control professionals must continue to deepen their understanding of these many methods. Of particular interest and value is further discussion on variable frequency drive fans and chilled-water pumps. In addition, variable frequency drives can be applied to exhaust fans and makeup air fans, controlled by appropriate transducers to maintain cleanroom positive pressures via variable inlet volume. These last two particular areas recall an enormous volume of popular literature from the commercial air-conditioning industry, where an enormous amount of information and expertise has been developed regarding energy efficiency in large-scale air conditioning design.
Many cleanrooms equal large commercial buildings in terms of volume of air and amount of heat rejected (one recent project was the functional equivalent of a 20-story office building compressed down to one story, 230,000 CFM of air circulation and 880 tons of cooling).
No experienced air-conditioning engineer could design a modern-day, large-scale office building without having to utilize this equipment in order to meet local energy codes and fulfill an industry-wide mandate to design for energy efficiency. These same techniques should be utilized in cleanroom design, whether they are scaled up for very large fab installations or scaled down for fairly small cleanrooms. The cleanroom industry must sell the difference in energy-efficient design to the customer. The cleanroom industry needs to calculate payback based on anticipated, realistic energy savings, as well as less easily established cost savings in preventive maintenance, etc.
Beyond adapting and adopting conventional building air-conditioning energy efficiency designs, cleanroom designers should consider how to “see” these designs and “raise” them to a new, and even more exciting level of efficiency and cost effectiveness.
This topic covers an area far broader than the space available in this article, but one particular area of interest is chiller plants. Virtually all larger ultra-clean projects (3,000 ft2 and larger) typically require a chiller plant in order to reject mechanical heat from fan horsepower, treat high volumes of makeup air, and reject substantial process loads, as more miniaturized and optimized equipment is installed in fairly small rooms.
In an engineering group, it is typical to look to one of the established manufacturers of large, packaged air-cooled chillers. Engineering groups shop in a marketplace that includes screw compressors with a remarkable number of stages–unloading, multiple-unloading reciprocating compressors, and multiple single-speed or two-speed hermetic compressors. Seldom do they see centrifugal equipment and, even less frequently, open-drive compressors installed in applied systems (“applied systems” is a somewhat archaic term in the air-conditioning industry used to describe refrigeration systems built up of discrete components, compressors, condensers, chillers, individual controls, etc.).
This happens for the following reasons: The first big determinant is cost. It is far less expensive to buy a packaged chiller with an electronic control package; simple-to-complex electronic diagnostics, configured to operate in varying climates; utilizing multiple condenser fans that can be cycled appropriately to maintain head pressure; and other very simple designs, including dual primary circuit and single secondary circuit barrels/heat exchangers.
In addition to cost, there has been, over time, a gradual loss of expertise among new entrants into the cleanroom engineering field. Of course, the “new” generation receives its tutelage from air-conditioning system vendors who themselves are of like age and experience. The veteran, older-generation knows that its knowledge cannot be easily transferred to younger colleagues, simply because it is extremely unfamiliar to them.
Alternatives to Chillers
Consider the alternatives to a packaged air-cooled chiller.
The first accessory option that makes very good sense would be a means to extract the super heat of compression from the discharge side of the compressors via a refrigerant-to-water heat exchanger. Then, this waste heat would be utilized for other purposes, including a separate recirculating hydronic loop for reheat downstream of chilled water dehumidification coils, preheat to meet complete domestic hot water heating requirements, and the possibility of recovering this heat to preheat feed water for boilers or other industrial heating requirements. Such a heat exchanger could be factory-installed as part of the design, and would utilize that enormous amount of thermal energy extracted from the room space, as well as the sensible heat generated by the compressor in operation. The condenser would be retained, with the hot gas discharge shunted through it or bypassing it via the new heat exchanger, operated by a simple automatically controlled valve. This device is quite simple to configure, and it only requires special attention to limit problems such as refrigerant oil return. The overall energy efficiency of an air-cooled chiller could be enhanced to a considerable degree.
The next logical step would be to consider another means to generate chilled water, namely absorption equipment. At the most recent ASHRAE (American Society of Heating, Refrigerating, and Air Conditioning Engineers) show, at least three gas-fired absorption chillers were on display. The smallest units were 40 tons. The largest absorption equipment available is, in fact, the largest size air-conditioning equipment that exists on the planet. Note that absorption gear can be gas-fired, oil-fired, integrated with a co-generation plant utilizing waste heat from a gas turbine or internal combustion engine, and can be steam-activated if a large enough boiler is available. The newer, packaged absorption chillers have a built-in water heating loop, capturing the waste heat off the gas-fired generator of the absorption cycle. These units thus operate as condensing furnaces, discharging fairly low NOX emissions. The absorption chillers are typically water-cooled rather than air-cooled, as an air-cooled condenser configuration is difficult to adapt to the heat absorption side of the water/bromide phase.
Utilizing natural gas fire lets the designer incorporate chillers of fairly large capacity into buildings whose electrical feeds are already operating near maximum. An absorption chiller has a very small electrical requirement, including the pump that circulates the water/bromide “liquor,” as well as chilled water and cooling water circulating pumps. If the natural gas reserves of the United States last into the 21st century, it may be a bad decision to utilize natural gas, forcing the customer to operate with an increasingly expensive utility. Note that absorption equipment can be configured for oil fire as well.
Installing an absorption air-conditioner in a cleanroom can be done. If an energy efficient system can be designed that, while higher at initial cost, is significantly more energy efficient, customers will receive a valuable service. A further attribute of this design: absorption equipment is non-CFC by nature, therefore, providing early compliance with the refrigerant phase-out mandate.
Absorption equipment is a likely, and viable, system for heat rejection in cleanrooms from moderate to large size. What are the other options? In terms of energy savings, another technology that can be borrowed from the commercial building air-conditioning industry is thermal storage.
Virtually every major manufacturer of heat exchange equipment has available thermal ice storage systems. Thermal storage utilizes the simple fact that if one can utilize a refrigeration cycle during off-peak hours (with significantly lower cost per kW of electricity), one can store a large mass of ice in appropriately designed insulated tanks. This storage system concentrates more energy than simply holding liquid water at a low temperature because of the property of water, which requires additional energy to freeze it (latent heat of crystallization), which is recovered as it melts (the phase-change from ice back to water). In operation, the system functions as a low-temperature chiller during the storage period. Then the compressors are shut off, the ice melts, and 32°F water can be circulated through the system.
Thermal ice storage not only saves electricity, but often allows installation of a down-sized mechanical system. If one stores 10 tons of ice (240,000 BTUs per hour) and melts it over an 8-hour period, one is able to obtain 720,000 BTUs of cooling effect before the ice is melted. In many designs, the compressor can be restarted to maintain chilled water temperature for the period in between full-melting and end-of-shift, onset of evening, or any other period when the sensible heat load is likely to drop. Proper design is crucial in order to avoid a crisis of lack of capacity.
Current ice storage systems utilize R-22 refrigerant because of its characteristics over the temperature range that the ice storage tank must operate. The system can be designed to operate utilizing non-CFC refrigerant, and this development work is rapidly under way in the research laboratories of major equipment manufacturers.
A Historical Perspective
If one reviews the history of the development of air-conditioning equipment, one sees that applied systems utilizing available technology were the dominant design for the first half-century of air-conditioning use. When post-war manufacturing processes provided the ability to build packaged equipment, much of this technology was allowed to wither. Cleanroom engineering firms, of course, typically are founded by capable individuals who represent the process side of cleanroom use, whether medical, biotechnical, semiconductor, or precision manufacturing. These organizations, then, are less familiar with, and have less access to, the “old” technology, which has been refined and vastly improved through work in the commercial air-conditioning and refrigeration field. n
John Potter of CRE, a division of CRP, is a 20-year veteran of the air conditioning and cleanroom industries. He has a B.A. and an M.A. from UCLA and is a frequent presenter at ASHRAE, IES and the CleanRooms East and West technical programs. In addition to his career designing and building cleanrooms and development new products, Potter also works as a technical consultant to the television and movie industry. His television credits include “MacGyver,” “Robocop” and the “Star Trek” series.