1、2009 ASHRAE 299ABSTRACTThis study investigates the performance of adiabatic humidification in a large-scale cleanroom with various humid-ification methods. Theoretical analysis and experimental results show that adiabatic humidification is feasible. The rela-tive humidity (RH) distribution in the cl
2、eanroom was within 1% and the time variation of RH was within 1% also, even when humidity of make-up air varies drastically. We found that adiabatic humidification with spray nozzles using “high-pressure water atomization” method is favorable than the “two-fluids humidification” method. The effect o
3、n energy-saving of adiabatic humidification and operational/initial cost of various humidification methods are also discussed.INTRODUCTIONThe 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 consis
4、ts 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 hum
5、idifier. Methods of humidifying include mist humidification and steam humid-ification. The steam humidification process is a quasi isother-mal process, which needs energy to input to the water. The mist humidification process is an isenthalpic process, which takes evaporation energy from the air. Wh
6、ether quasi isother-mal humidification or isenthalpic humidification is applied, the heating system is indispensable. For mist-humidification, the outdoor air temperature should be pre-heated to a temper-ature with the same enthalpy of off-coil saturation condition. Here, the heating system plays an
7、 important role. Whether using electro-heating or a boiler, it is a large burden in opera-tion 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,
8、 MAU output air with the temperature controlled at 11 to 17C (51.8 to 62.6F), the humidity-ratio controlled at 9.65 103 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
9、 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 d
10、oes 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 clean
11、room. Adiabatic humidi-fication 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 high-pressure water atomization or by using compressed dry air atomization (so-called two-fluids syste
12、m). Figure 1 shows the schematic diagram of adiabatic humidifying a high-tech cleanroom.Many applications of adiabatic humidification in environ-ment control such as greenhouse (Montero (1990), food process rooms (Abdalla (1991), poultry (Bottcher (1992), Ogura (1982), textile-spinning mills (Rajase
13、karan (2003) etc., Humidification of Large-Scale Cleanrooms by Adiabatic Humidification Method in Subtropical Areas: An Industrial Case StudyJacky Chen, PE Shih-Cheng Hu, PhD Liang-Han Chien, PhDMember ASHRAEJames J.M. Tsao, PhD Tee LinJacky Chen is an assistant manager in the Facility Department, D
14、axin Materials Corporation, Hinchu, Taiwan. Shih-Cheng Hu is a professor, Liang-Han Chien is an associate professor, and Tee Lin is a PhD student in the Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, Taipei, Taiwan. James J. M. Tsao is
15、a research associate in the Department of Energy and Refrigerating Air-Conditioning Engineering, National Taipei University of Technology, and assistant manager at PECL Co. U.S.A.LO-09-026 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Publish
16、ed in ASHRAE Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.300 ASHRAE TransactionsHowever, no literature discusses the application of adiabat
17、ic humidification in large-scale cleanrooms. Figures 2a and 2b demonstrates the psychrometric processes of both humidify-ing a cleanroom by high-pressure water atomization adiabatic humidification and by MAU (i.e. by steam or mist humidifi-cation). In Figure 2a, evaporating cooling of the adiabatic
18、humidification is noted.On the other hand, the energy consumption for control-ling humidity by MAU is huge. Generally, the power consumption of air-conditioning in a semiconductor clean-room is about 40% of the total power consumption (Hu and Chuah (2003). Within this percentage, the chiller creates
19、 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 impor-tant to take energy consumption into consideration during selection of a MAU. Brown (1990) identified energy-saving opportu
20、nities 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), M
21、ost 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 effec
22、t by adiabatic humidification in clean-rooms 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 eval-uate its energy-saving effect.Figure 1 Schematic diagram of adiabatic hum
23、idification in the cleanroom. MP1 MP5 are the measuring points.Figure 2 The psychrometric process of humidification: (a) adiabatic humidification (1-2), (b) steam-humidification (1-2-3) and mist-humidification (1-2-3), where r is the room condition (23C, 55% RH).ASHRAE Transactions 301METHODSTheoret
24、ical AnalysisThe principle of adiabatic humidification is based on the use of the amount of recirculation air, which provides a large amount of transport medium. Although a unit volume of recir-culation air contains a very limited amount of water, the amount of recirculation air is far larger than t
25、he amount of make-up air. Therefore, recirculation air may play a role that carries the added humidity to the cleanroom. Assuming the droplet issued from the nozzle is completely evaporated, Table 1 shows the simple verifying calculation. The target increase of humidity ratio (w) and result relative
26、 humidity of recirculation air are depicted under different ratios of make-up air flow rate to recirculation air flow rate i.e. Qoa /Qra.In the situation of humidification by MAU, when Qoa /Qra= 7% and w = 7 103 kg/kg (lb/lb), the required mass flow rate of water vapor is 8,400 kg/hr (3,813lb/hr). W
27、ith the same volume of mv , in adiabatic humidification w = 0.49 103 kg/kg (lb/lb) and the RHra-aft = 69.3%. Therefore the recirculation air contains sufficient water to ensure humidity ratio value when mixed with outdoor air. Adiabatic humidification is theoretically feasible in a large cleanroom.W
28、hen adiabatically humidifying a cleanroom, evapora-tion of the fog should be considered. The appropriate location for installing the humidifiers in a cleanroom is on the return air duct route (return air shaft). Mist has to be completely evap-orated before entering the Fan Filter Unit. The size of t
29、he liquid droplets and the needed evaporation time are the key points to the success of this concept. The evaporation time t(sec) of a droplet with diameter dpto evaporate completely in environment Tland Plcan be evaluated by Langmuris equa-tion (Hinds 1999).t = for dp 1.0 m(1)where pis the droplet
30、density (kg/m3), R is the gas constant (J/K mol), Pd(Pa) is the vapor pressure at droplet temperature Td (K), Pl(Pa) is the vapor pressure at environment absolute temperature Tl(K), M is the water molecular weight (kg/mol), T is the ambient temperature (K), dpis the water droplet diam-eter ( m), and
31、 D is the diffusion coefficient of the vapor drop-let in the air (m2/sec). The diffusion coefficient of the water droplet in the air D can be estimated by the following equa-tion:(2)where D0(= 0.22 m2/sec) is the diffusion coefficient at 0C (32F) (To).Equation (1) exhibits that among the factors tha
32、t affect the evaporation time are size of liquid droplet and temperature. By Equations (1) and (2) the required time for different droplet sizes to evaporate in the return air-shaft can be evaluated. Once the evaporation time and the distance of return air in clean-room are calculated, it is possibl
33、e to estimate the size of liquid droplets from the humidifier production. This allows the selec-tion of the correct humidifier.Experimental Set-UpThe experimental equipment is a high-pressure micro mist system, and its function is to use the high-pressure pump to raise the water pressure to a suffic
34、ient high pressure. The high-pressure water goes through the micro mist sprinkler and forms water mist about 40 m in diameter. We used the DI (deionized) water, which was available in most of high-tech fabrication plants (fab). The major specifications of the DI pRdp28DM PdTd P1T1()-DD0TdTl-1.94Tabl
35、e 1. Theory Calculations: Humidification by MAU vs. Adiabatic Humidification (RHra-aftis calculated based on 21C (70F) and 62% RH of recirculation air before humidification)Humidification by MAU Adiabatic HumidificationQoa /QraOoaw mv(kg/hr), (lb/hr)Qra(m3/h),(ft3/min)w T/RHra-aft7%1 106m3/h(0.62 10
36、6ft3/min)7 103kg/kg(lb/lb)8,400 kg/hr(3,813 lb/hr)14.2 106m3/h (8.81 106ft3/min)0.49 103kg/kg (lb/lb)19.9C(68F)/69.3%8%1 106 m3/h(0.62 106ft3/min)7 103kg/kg(lb/lb)8,400 kg/hr(3,813 lb/hr)12.5 106m3/h (7.71 106ft3/min)0.56 103kg/kg (lb/lb)19.7C(67.5F)/70.8%9%1 106m3/h(0.62 106ft3/min)7 103kg/kg(lb/lb
37、)8,400 kg/hr(3,813 lb/hr)11.11 106m3/h (6.85 106ft3/min)0.63 103kg/kg (lb/lb)19.5C(67.1F)/72.3%10%1 106m3/h(0.62 106ft3/min)7 103kg/kg(lb/lb)8,400 kg/hr(3,813 lb/hr)10.0 106m3/h (6.17 106ft3/min)0.70 103kg/kg (lb/lb)19.3C(66.7F)/73.8%Qoa: outdoor airflow rate Qra: recirculation air flow ratew: incre
38、ase of humidity ratio mv: required mass flow rate of water vapor T/RHra-aft: dry-bulb temperature and relative humidity of recirculation air after humidification302 ASHRAE Transactionswater were: bacteria: less than 0.5 CFU/Liter (under 48 hour culture), particles greater than 0.5 m: less than 1 par
39、ticle/Liter, particles greater than 0.1m: less than 50 particle/Liter, sodium and chloride: less than 5 ppt, metals: less than 5 ppt. No problem due to water quality was reported. In the experi-ment, we used a pump with 70 bar of working pressure. As the length from the mist sprinkler to the inlet o
40、f the Fan Filter Unit (FFU) is over 6m, and the air velocity in this region is less than 3m/s, the selection of mist diameter of 40 m is small enough to ensure that all the mist is evaporated before entering the filter, according to Equation (1). The experiments were conducted under two different co
41、nditions, with the humidifi-ers installed in front of and after the cooling coil. The first condition is the observation of the evaporation of liquid drop-lets. The humidifier was installed in front of the cooling coil. This is to observe whether the rest of the liquid remains as droplets and whethe
42、r the fogging occurs on the cooling coil. The second condition, also shown in Figure 1, is to install the humidifier after the cooling coil and to install a dehumidified air-handling unit to simulate different make-up air conditions. This is to observe the distribution and the stability of RH and dr
43、y-bulb temperature. The location of measuring point and purpose of measurement are illustrated in Table 2.RESULTS AND DISCUSSIONField Humidification PerformanceIn the first part of the experiment, the chilled water valve of the cooling coil was fully opened. The objective was to test whether fogging
44、 occurs on the cooling coil under full loading conditions. Visual observation shows that there was no fog on the surface of the cooling coils. This was further confirmed by the measured RH data at MP2, which was about 63%. The second part of experiment was to observe the humidifying distribution and
45、 the stability. As shown in Figure 3, the average dry-bulb temperature at MP5 was about 21.8C (71.3F). The average relative humidity at MP5 was 59%, and the corre-sponding mist temperature was 13.3C (56F).These were similar to those controlled by MAU, indicat-ing that the mist had enough time and di
46、stance to be fully absorbed by the recirculating air. The result corresponded to that of the first part of the experiment. The measured data at MP1 represents the conditions in the cleanroom. As shown in Figure 4, the RH controlled by adiabatic humidification exhib-its a larger fluctuation (about 1%
47、) compared to that controlled by MAU, and the mean value within the target value (55 5%). The RH variation at MP3 is similar to that of MP4, indicating the mixing of make-up air with the recirculating air was far behind after the exit point of make-up air duct. When the RH of make-up air varies dram
48、atically (RH varies from 70 to 100% as the MP4 shown in Figure 5), the RH and dry-bulb temperature at MP1 was still very stable and can still be stabi-lized to within 1%, also shown in Figure 5.Energy-Saving Effect by High-Pressure Water Atomization Adiabatic HumidificationAs shown in Figures 2a and
49、 2b and Table 1, two impacts on energy-saving were observed. First, for humidifying by MAU, to increase the humidity in the cleanroom from 2.65 103kg/kg (lb/lb) to 9.65 103 kg/kg (lb/lb), the required mass of steam was 1,000,000 m3/hr 1.2 kg/m3 (9.65 2.65) g/kg= 8,400 kg/hr (3,813 lb/hr). By adiabatic humidification, the aforementioned mass of steam is not necessary. Therefore, the energy saved on
