Nine tips for clean room energy saving

There are plenty of energy-saving places in the clean room, such as heating, ventilation and air conditioning, process cooling, compressed air and some other facilities.

The HVAC system consumes more than half the electricity used by the entire wafer fabrication facility. The reason for the large amount of wasted power and HVAC capacity surplus is largely due to the shortcuts in the design and construction of the plant, as much as possible to compress the initial investment, regardless of the later operating costs. Efficient design and high efficiency equipment require a large upfront investment. The so-called shortcuts and cost reductions of "small places to save a lot of waste" will result in reduced plant performance and increased energy consumption.
Reconstruction of completed factories often falls into a meaningless economic vortex. Upgraded investment recovery rates are usually much higher than direct purchases of new equipment. Most of the plant equipment refurbishment has a payback period of no more than two years-that is, usually the investment recovery rate is at least 50%, compared to 10% to 15% compared to investing in new fixed assets. These conditions reduce the competitiveness of companies and the interests of investment shareholders. In today's highly developed industries, plant operations and product design require reform.
There are many examples of efficient use of energy. Excavating wasted energy can increase profits more than selling products, because the money saved can be reflected in the final benefits. Although energy costs account for less than 2% of the cost of wafer products, power is the biggest expense for wafer manufacturers in operation, and each plant consumes millions of dollars of electricity each year. Energy savings can save capital and build time when building a new plant. Although preliminary design quotations are expensive, the possibility of economic refurbishment still exists. The investment recovery period for equipment refurbishment is no more than two years. Overall, the investment recovery rate is higher to some extent.
Here are ten tips for new and existing plants to provide energy efficiency, providing reliable technology, minimal risk, low cost or no cost, and an attractive payback period.

I. Low profile wind speed design Section wind speed is the speed at which air passes through the filter or heating/cooling coil in the air treatment component. The Low Profile Wind Speed (LFV) design uses a larger air handler and a smaller fan to reduce air flow, reduce energy consumption and equipment life costs.

Most engineers designed the air handler to be 500 inches per minute based on "experience." This design saves time but increases operating costs. In low profile wind speed (LFV) designs, larger air handlers and smaller fans are used to reduce air flow rates, reduce energy consumption and set life costs.
The pressure drop determines the energy loss of the fan. From the "square rule", the pressure drop is proportional to the square of the speed drop. If the cross-section wind speed is reduced by 20%, the pressure drop will drop by 36%; if the section wind speed is reduced by 50%, the pressure drop will drop by three-quarters. According to the [Cubic Rule", the change in fan energy consumption is proportional to the cube of the flow change. If the air flow is reduced by 50%, the fan energy consumption will drop by 88%.
Therefore, larger air handlers, larger filters, and coil area consume less fan energy, and smaller fans and motors can be used. Small fans add less heat to the air, reducing the difficulty of cooling. The coil with a small thickness is easier to clean and work more efficiently, so the temperature of the chilled water can be higher. The filter works better and has a longer life at low cross-section wind speeds.
The LFV design reduces air and water pressure drop and reduces the amount of water in the cooling coil. Streamlined design with virtually no sharp corners, reducing pressure drop by 10% to 15%.
The LFV design also reduces the pressure drop by a quarter. The goal is to reduce energy losses by at least 25% and reduce the size of the variable speed fan. The optimal section wind speed range is 250-450 feet per minute, depending on usage and energy consumption.

Second, motor efficiency
The motor consumes most of the power in the clean room. A continuously operating motor consumes a lot of electricity every month. Properly improving efficiency and properly adjusting the size, after refurbishment, the economic effect is mostly good. When efficiency increases by a few percentage points, profits can increase.
Using a high-quality and efficient motor does not necessarily cost too much. High efficiency means minimal, and the load is minimized before changing the size of the motor. When the output changes, the variable speed drive (VSD) can improve the operating efficiency.

Third, the number of air changes
The clean room maintains a constant 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 size of the fan, the building configuration and the energy consumption. Under the premise of maintaining cleanliness, the reduction in air flow rate can reduce construction and energy costs. A 20% reduction in the number of air changes can reduce the size of the fan by 50%. Air cleanliness is more important than energy savings, but the latest research has documented the need to reduce cleanliness.
There is still no consensus on the optimal number of air changes. Many of the principles are outdated and built on the old concept of using inefficient air filters. According to the survey, the recommended number of air changes in the clean room of the ISO Level 5 standard ranges from 250 to over 700.
A national laboratory in the United States is determining the standard for ISO Class 5 clean rooms. Studies have shown that the actual number of air changes ranges from 90 to 250 – much lower than the operating standard and does not affect production and cleanliness. Therefore, it is recommended that the ISO Class 5 clean room has a gas exchange rate of approximately 200 and a conservative upper limit of 300.

Fourth, variable speed drive freezer
The variable speed drive freezer saves a lot of energy and money. Many cleanroom designers and operators believe that it is not necessary to use a variable speed drive freezer because the load is usually constant and the multistage freezer unit is typically controlled to operate at high loads. However, a refrigerator with a constant load usually operates below full load. Variable speed drive chillers typically operate at 90%-95% of full load to save energy. A 1000-ton chiller is stable at 70% of full load, and if a variable speed drive is used, it can save $20,000 to $30,000 per year. According to the manufacturer's data, the price of electricity is $0.05/kWh, so that the cost can be recovered in about a year.
Multi-stage freezer chilled water units operate at very low loads. Typically, field loads are usually not exactly matched to the unit's energy level changes. Many operators run additional chillers for reliability. Once a chiller fails, other chillers can be replenished immediately to take over the full load, so chilled units are often 60% to 80% of the chiller's cooling capacity. % running.
When purchasing a new freezer, it is cost-effective to specify a variable speed drive freezer. Variable speed drive chillers reduce energy consumption while allowing other chillers to operate reliably. There are many studies and experiments that prove that the effect of the variable speed drive freezer is very good. For more than two decades, variable speed drive chiller manufacturers have created more reliable products for use in new and upgraded cleanrooms.

Fifth, the optimization of the cooling tower
Efficient cooling towers increase the efficiency of the freezer by reducing the supply temperature of the condensate.

For every ton of chilled water that is exported from a freezer, a typical cooling tower requires 100 watts of energy. Efficiency can be increased by up to ten times, such as closer to the inlet, outlet temperature difference, more efficient airflow design, high-quality and efficient fan with variable speed drive motor, reduced height to limit pump head and increased fill area.
The temperature difference between the different outside air and the cooling water supply temperature is different and should be controlled between 3oF and 5oF.
All cooling towers should be operated in parallel to achieve optimum evaporative cooling with increased surface area.
Many mission plants use multi-stage towers that use single or dual speed fans and divide the tower into different stages. One tower runs at full speed until the load exceeds its capacity, then another tower opens and it operates at a lower or higher power state. This solution can result in large, ever-increasing changes in the cooling tower load, frequently below or exceeding the required rating, resulting in jagged energy consumption and reduced chiller efficiency.
Therefore all cooling towers should work in parallel and evaporative cooling is optimal with increased surface area. If more towers are operating at low speeds, the variable speed drive is used to adjust the speed of the fan, which is adjusted as the load changes. According to the "Cubic Law", the fan can save energy at lower speeds.
The factory usually uses a dedicated cooling tower to supply condensate to each freezer. This concept does not allow the freezer to operate in parallel with the cooling tower. Only the addition of a common header to the condensate system allows the cooling tower to operate in parallel, regardless of cooling requirements.

Sixth, heat recovery
Many mission plants consume a lot of energy to heat, while consuming more energy to remove "waste" heat from the process, but not combining the two processes. The recovered heat can be used to preheat fresh air, reheat the air, and other uses. The AHU preheating coil can preheat the outside air with wastewater (pre-cooled in hot weather).
The reheat coil can recover waste heat from the air compressor or the condenser backwater of the freezer, while saving the freezer energy and boiler fuel. The heat exchanger allows heat exchange of different media that cannot be mixed or directly contacted.

Seven, dual temperature refrigeration cycle Refrigeration systems are usually designed to withstand the maximum load, regardless of whether the maximum load occurs frequently. The chilled water temperature in the refrigeration cycle in the process is determined by the extreme heat load of only a small fraction of all loads, which is just one or two of many cases. This can result in excess refrigeration capacity and inefficiency in the event of insufficient load. When the temperature of the supplied chilled water is low, the operating efficiency of the freezer is also low. On average, the chiller efficiency increases by more than one percentage point for each additional 1/2 degree Fahrenheit supply temperature. If the load is divided and two different temperatures of chilled water are provided, the work efficiency will be higher.

8. Free Cooling It is economical to use outside air for cooling and is widely used in commercial buildings. Another [free cooling" solution is available for systems that require constant chilled water and fan coils, such as cleanrooms.
Free cooling technology directly uses chilled water in a cooling tower in a low temperature or low humidity environment to reduce or replace the use of the freezer. Depending on the weather, the free cooling system can reduce the energy consumption of the cooling system to one tenth (from 0.5 kW / ton to 0.05 kW / ton).
Direct heat exchange with the process load allows the free cooling technology to utilize the higher temperature outside the atmosphere for several hours longer than the heat exchange for the 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 map, many places can be freely cooled every year.

Nine, centrifugal compressor
Improvements in air compressors have saved a lot of energy. Centrifugal compressors are oil-free and much more efficient than screw compressors. However, centrifugal compressors cannot be idling, which makes them inefficient at low loads. The most efficient and economical solution is the combination of both centrifugal and screw compressors. The centrifugal unit is selected to meet the basic load, and the smaller screw unit is used 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 unit throughout the field, with an enlarged gas storage tank and connecting pipes as buffers. This ensures that the entire plant maintains a constant load, reduces the loss of loading and unloading equipment, and reduces energy waste.