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INTRODUCTION

The humidity of large-scale high-tech cleanrooms is often adjusted and maintained by the make-up air unit (MAU), and the equipment to maintain the humidity consists of the cooling coils and the humidifiers. These adjust the humidity in the external environment into a non-dust state, and then send it into the cleanroom to maintain the humidity. The major components of the MAU include: a fan, two stage cooling coils, a heating coil (or heater), filters and humidifier. Methods of humidifying include mist humidification and steam humidification. The steam humidification process is a quasi isothermal process, which needs energy to input to the water. The mist humidification process is an isenthalpic process, which takes evaporation energy from the air. Whether quasi isothermal humidification or isenthalpic humidification is applied, the heating system is indispensable. For mist-humidification, the outdoor air temperature should be pre-heated to a temperature with the same enthalpy of off-coil saturation condition.

Here, the heating system plays an important role. Whether using electro-heating or a boiler, it is a large burden in operation and maintenance costs. Even with a heat recovery chiller, it negatively affects the efficiency of the main cooling system. In addition, it consumes much of the capital cost of the heating system. Normally, MAU output air with the temperature controlled at 11 to 17°C (51.8 to 62.6°F), the humidity-ratio controlled at 9.65 × 10^-3 kg/kg (lb/lb) for TFT-LCD (thin-film transistor liquid-crystal displayer) fabrication plants. The make-up air (MA) mixes with return air (RA) to keep the cleanroom at 23 ± 1%°C and 40 ± 5% RH for most high-tech fabrication plants, including those in the semiconductor and TFT-LCD industries. However, certain area, such as cell area of TFT-LCD plant, requires higher humidity (normally 55 ± 5% RH). Additionally, the temperature in the cleanroom can be controlled by dry coil, but this does not regulate humidity, so the MAU output humidity control becomes very important, as it is the only mechanism to control the humidity in the clean room. Some cleanrooms have local steam generators to accomplish a double adjustment of humidity, but this only applies in certain portions of a cleanroom. Adiabatic humidification is an alternative method of controlling humidity. This method discharges water droplet into the cleanroom to enhance humidity ratio with spray nozzle either by using highpressure water atomization or by using compressed dry air atomization (so-called two-fluids system). Figure 1 shows the schematic diagram of adiabatic humidifying a high-tech cleanroom.

Many applications of adiabatic humidification in environment control such as greenhouse (Montero (1990), food process rooms (Abdalla (1991), poultry (Bottcher (1992), Ogura (1982), textile-spinning mills (Rajasekaran (2003) etc.,

However, no literature discusses the application of adiabatic humidification in large-scale cleanrooms. Figures 2a and 2b demonstrates the psychrometric processes of both humidifying a cleanroom by high-pressure water atomization adiabatic humidification and by MAU (i.e. by steam or mist humidification). In Figure 2a, evaporating cooling of the adiabatic humidification is noted.

On the other hand, the energy consumption for controlling humidity by MAU is huge. Generally, the power consumption of air-conditioning in a semiconductor cleanroom is about 40% of the total power consumption (Hu and Chuah (2003)). Within this percentage, the chiller creates around 50% of the load. Breaking the power burden down further, the MAU consumes nearly half of the power load of the chiller (Hu and Chuah (2003)). Therefore, it is very important to take energy consumption into consideration during selection of a MAU. Brown (1990) identified energy-saving opportunities within MAU systems for five climate regions in the US. Adiabatic humidification was mentioned, however, no field evaluation and further discussion on cost/energy-saving was presented. Most energy efficiency discussions on MAU focused on fan/cooling coil efficiencies such as Naughton (1990), Most recently, Tsao and Hu (2008) presented a comparative study for MAU system performance with eight different component combinations. The most energy efficient combination was highlighted. However, none of the above articles discussed the possibility of adiabatic humidification. Energy-saving effect by adiabatic humidification in cleanrooms was not discussed in previous papers either. This paper, therefore, aims to investigate the possibility of using adiabatic humidifying technology in high-tech cleanrooms and to evaluate its energy-saving effect.