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How can we make clean rooms more energy efficient?
I. Summary: There are a lot of energy-saving places in clean rooms, such as heating, ventilation and air conditioning (HVAC), process cooling, compressed air, and some other facilities.
HVAC systems consume more than half of the electricity used by the entire chip manufacturing plant. The reason for the large amount of wasted electricity and excess capacity of HVAC is largely due to taking shortcuts when designing and constructing the factory to reduce the initial investment as much as possible, regardless of the later operating costs. High-efficiency design and high-efficiency equipment require a large up-front investment. The so-called shortcuts and cost reductions of "saving small and saving a lot of waste" will result in a reduction in the operating performance of the plant and an increase in energy consumption.
Alterations to established factories are often plunged into meaningless economic eddies. The return on investment for upgrades is usually much higher than buying new equipment directly. The investment recovery period for most factory equipment renovations does not exceed two years-that is, the investment recovery rate is usually at least 50%, compared with the investment of new fixed assets, only 10% to 15% (that is, the investment recovery period requires seven years time). These conditions have reduced the competitiveness of companies and the interest of investment shareholders. In today's highly developed industries, factory operations need to be reformed just like product design.
There are many examples of efficient use of energy. Mining wasted energy can increase profits more than selling products, because the saved money is immediately reflected in the final benefit. Although the cost of energy accounts for less than 2% of the cost of chip products, electricity is the largest expenditure of chip manufacturers in operation, and each factory consumes a total of millions of dollars per year. When building a new plant, energy saving measures can save capital and construction time. Although preliminary designs are expensive, the possibility of economic refurbishment remains. The investment payback period for equipment refurbishment does not exceed two years. Overall, the investment recovery rate is relatively high to a certain extent.
I. Low section wind speed design The section wind speed is the speed at which air in the air handling part passes through the filter or heating / cooling coil.
Most engineers design the air handler to 500 inches / minute based on "experience." Although this design saves time, it increases operating costs. In low-profile wind speed (LFV) designs, larger air handlers and smaller fans are used, which reduces the air flow rate, reduces energy consumption, and reduces lifetime costs.
The pressure drop determines the energy loss of the fan. It can be known from the "square rule" that the pressure drop is proportional to the square of the speed drop. If the cross section wind speed is reduced by 20%, then the pressure drop will decrease by 36%; if the cross section wind speed is reduced by 50%, the pressure drop will decrease by three-quarters. According to the "cubic rule", the change in the energy consumption of the fan is proportional to the cube of the change in flow. If the air flow is reduced by 50%, the energy consumption of the fan will be reduced by 88%.
Therefore, larger air handlers, larger filters, and coil areas consume less fan energy, and smaller fans and motors can be used. The small fan adds less heat to the air, reducing the difficulty of cooling. Coils with smaller thicknesses are easier to clean and work more efficiently, so the temperature of frozen water can be higher. The filter works better and has a longer life under the condition of low section wind speed.
The LFV design reduces air and water pressure drops and reduces the amount of water carried by the cooling coils. Streamlined design with almost no sharp corners, reducing pressure drop by 10% to 15%.
The LFV design can also reduce the pressure drop by a quarter. The goal is to reduce energy losses by at least 25% and reduce the size of variable speed fans. The optimal section wind speed range is 250-450 feet per minute, depending on usage and energy consumption.
Second, the number of air exchanges The clean room maintains a certain air flow to maintain cleanliness and particle count. The flow rate is determined by the number of air changes per hour, which also determines the fan size, building configuration and energy consumption. Under the premise of maintaining cleanliness, a reduction in air velocity can reduce construction and energy costs. Reducing the number of air changes by 20% can reduce the size of the fan by 50%. Air cleanliness is more important than energy saving, but the latest research results have been documented to reduce cleaning costs.
No consensus has been reached on the optimal number of ventilations. Many principles are outdated and are based on old ideas and use inefficient air filters. Surveys have shown that the recommended range of air change times for cleanrooms in accordance with ISO Level 5 standards ranges from 250 to over 700.
A national laboratory in the United States is setting standards for ISO Class 5 cleanrooms. Studies have shown that the actual number of air changes ranges from 90 to 250-much lower than the standard of operating procedures and does not affect production and cleanliness. Therefore, it is recommended that the number of air changes in the ISO Class 5 clean room is about 200, and the conservative upper limit is 300.
Third, the efficiency of the motor The motor consumes most of the electrical energy in the clean room. Continuously running motors consume a lot of electricity every month. Properly increasing efficiency and appropriately adjusting the size, after refurbishment, the economic effect is mostly good. A few percentage points increase in efficiency can increase profits.
Using a high-quality and efficient motor does not necessarily cost too much. High efficiency means minimal, minimizing the load before changing the size of the motor. When the output changes, the use of variable speed drive (VSD) can improve operating efficiency.
Fourth, the variable speed drive freezer The variable speed drive freezer can save a lot of energy and money. Many clean room designers and operators believe that it is not necessary to use variable speed drive freezers because the load is usually constant and multi-stage freezer units are usually controlled to operate at high loads. But a freezer with a constant load usually works below full load. Variable speed drive freezers usually work at 90% -95% of full load to save energy. A 1000-ton freezer works stably at 70% of full load. If a variable speed drive is used, it can save 20,000 to 30,000 US dollars per year. According to the manufacturer, the price of electricity is $ 0.05 / kWh, so the cost can be recovered in about a year.
Multi-stage chiller chillers rarely run at high loads. Under normal circumstances, the on-site load usually does not exactly match the energy level change of the unit. Many operators run additional freezers to be reliable. Once a freezer fails, other freezers can be replenished immediately to take over their full load. Therefore, chiller units often allow the freezer to have a cooling capacity of 60% to 80%. % Running.
When buying a new freezer, it's a good deal to designate a variable speed drive freezer. Variable speed drive freezers reduce energy consumption while allowing other freezers to operate reliably. There are many studies and experiments that prove that the effect of variable speed drive freezer is very good. For more than two decades, manufacturers of variable speed drive refrigerators have produced more reliable products for use in new and upgraded clean factories.
Fifth, the dual temperature refrigeration cycle refrigeration system is usually designed to withstand the maximum load, regardless of whether the maximum load occurs frequently. The temperature of the chilled water in the refrigeration cycle in the process is determined by the extreme thermal load, which only accounts for a small part of all loads, which is only one or two in many cases. This can result in excess refrigeration capacity and inefficiencies under load. When the temperature of the supplied chilled water is low, the working efficiency of the refrigerator is also low. On average, each increase in the temperature of the chilled water supply increases the freezer efficiency by more than one percentage point. If you divide the load and provide two chilled water at different temperatures, the work efficiency will be higher.
Designers can use parallel circulation pipelines to divide them into two subsystems, so that when the maximum cooling capacity is required, the freezer can work under relatively less severe conditions. Use a special freezer for medium temperature cycling (for example, 55oF to 65oF). Its operation is optimized for the temperature of the chilled water, which can meet most of the needs of the factory. Another smaller high-efficiency freezer provides a lower temperature cycle (for example: 39oF to 43oF) to meet the demanding part of the load.
This solution can quickly increase the efficiency of the entire chiller unit by 25 percent or more. For the same capacity freezer, high temperature work is much less expensive than low temperature work.
Sixth, the optimization of the cooling tower High-efficiency cooling tower improves the efficiency of the refrigerator by reducing the supply temperature of the condensed water.
For every ton of chilled water output from a refrigerator, a typical cooling tower requires 100 watts of energy. Efficiency can be increased up to ten times, such as closer to the inlet and outlet temperature difference, more efficient airflow design, high-quality and efficient fan with variable speed drive motor, reducing height to limit the pump lift, and increasing the filling area (select a large size Tower) and so on.
The temperature difference between the wet temperature of the outside air and the cooling water supply temperature is different and should be controlled between 3oF and 5oF.
All cooling towers should work in parallel, and evaporative cooling is optimal with increasing surface area.
Many mission plants use multi-stage towers, which use single or double speed fans and divide the tower into different stages. One tower runs at full speed until the load exceeds its capacity, then the other tower opens and it works at lower or higher power states. This solution can cause large and continuous changes in the cooling tower load, frequently lowering or exceeding the required rating, thereby causing a jagged energy consumption situation and reducing the efficiency of the refrigerator.
Therefore, all cooling towers should work in parallel, and evaporative cooling is optimal when the surface area is increased. If more towers work at low speed, use variable-speed drive to adjust the speed of the fan, which is adjusted with the load. According to the "cubic law", the fan can save energy at lower speeds.
Plants usually use a dedicated cooling tower to supply condensate to each refrigerator. This concept does not allow refrigerators to operate in parallel with cooling towers. Only adding ordinary headers to the condensate system allows the cooling towers to run in parallel, regardless of cooling requirements.
7. Free cooling It is economical to use outside air for cooling, and it is widely used in commercial buildings. Another "free cooling" solution is suitable for systems that require constant chilled water and fan coils, such as clean rooms.
Free cooling technology directly uses cooling towers in low-temperature or low-humidity environments to produce chilled water, reducing or replacing the use of freezers. Depending on the weather, using a free cooling system can reduce the energy consumption of the cooling system to one-tenth (from 0.5 kW / cold ton to 0.05 kW / cold ton).
Directly exchanging heat with the process load allows the free cooling technology to take advantage of the higher temperature outside the atmosphere, which takes longer than several hours for heat exchange in a secondary or tertiary heat exchange system. The temperature difference between the cooling water and the condensate separated by the plate heat exchanger is very close (for example, only 2oF). When the temperature and humidity are quite low, the cooling tower can operate independently without a fan. According to the temperature and humidity chart, many places can be freely cooled a lot of time each year.
Eight, heat recovery Many task factories consume a large amount of energy to heat, while consuming more energy to remove "waste" heat from the process, but they do not combine the two processes. The recovered heat can be used to pre-heat fresh air, supply air to reheat, and other uses. AHU preheating coils can be used to preheat external air with waste water (it can also be precooled in hot weather).
The reheating coil can recover waste heat from the return water of the air compressor or the condenser of the refrigerator, while saving energy of the refrigerator and boiler fuel. The heat exchanger can exchange heat between different media that cannot be mixed or directly contacted.
Nine, the frequency conversion pump equipment equipped with variable frequency drive in the past often fails, and the control is complex, so many engineers and managers are unwilling to use the variable frequency drive. Reliability is more important than energy saving, and old inverter drives have poor reliability. In the last ten years, the reliability of inverter drives has increased and the price has decreased. Many critical systems are now using variable frequency drives.
We consider it safe and cost-effective to use variable frequency drives on many systems and various types of pumps in clean rooms. In fact, it can prove irresponsible to consider the return on investment without using a variable frequency drive, because the payback period is less than one year.
The flow rate of the chilled water and condensate pump systems varies greatly. The minimum flow requirements for chilled water and condensed water systems are basically between 50% and 75%. According to the "cubic rule", a small reduction in flow will result in considerable energy savings. A 20% reduction in flow will result in a nearly 50% reduction in pump power.
Most known chilled water systems use a primary pump constant flow / secondary pump variable flow design, and the secondary pump is driven by variable frequency. When using variable frequency drive, all chilled water should use two-way valve, otherwise the meaning of using variable frequency pump will be lost.
When building a new plant, a variable flow primary pump system is used, eliminating the need for a secondary pump and saving engineering costs. Properly running, this simple and reliable system can save a lot of energy by changing the chilled water flow in the freezer. This technology is widely promoted by freezer suppliers and various professional associations, such as ASHRAE (American Society of Heating, Refrigerating & Air Conditioning Engineers).
10. Centrifugal compressor The improvement of air compressor has saved a lot of energy. Centrifugal compressors are oil-free and much more efficient than screw compressors. But centrifugal compressors cannot run dry, which makes them very inefficient under low load conditions. The most effective and economical approach is to use a combination of both centrifugal and screw compressors. Select the centrifugal unit to meet the basic load, and then use the smaller screw unit to meet the peak load. The compressor unit should be equipped with a heat recovery system.
Another solution is to use a high-efficiency centrifugal compressor as a large compressed air device, and an enlarged air storage tank and connecting pipes as a buffer. This can ensure that the entire plant maintains a constant load, reducing the loss of equipment during loading and unloading, and reducing energy waste.