Clean rooms have controlled environment, where low level of pollutants such as dust, airborne microbes, aerosol particles and chemical vapours are permitted to remain. The controlled level of contamination is specified by the number of particles per cubic meter at a specified particle size. Clean rooms find applications where small particles can adversely affect the manufacturing process. They vary in size and complexity and are used extensively in industries such as research organisations, semiconductor manufacturing, pharmaceuticals, medical, aerospace, optics, military, etc..
The key component of a clean room is the High Efficiency Particulate Air (HEPA) filter, used to trap particles that are 0.3 micron and larger in size. All of the air delivered to a clean room passes through HEPA filters, and in some cases where stringent cleanliness performance is necessary, Ultra Low Particulate Air (ULPA) filters are used. Persons working in cleanrooms are trained in controlling the contamination. They enter and exit the clean room through airlocks, air showers, etc. and they wear special clothing designed to trap contaminants, normally generated by human body. In clean rooms, technical arrangement is made to reduce the particulate contamination and also to control air parameters such as air temperature, velocity, humidity and pressure.
Air Conditioning
Schematic diagram of a typical air conditioning (AC) system of a clean room is shown in figure 1. Chilled water from a centralised AC plant is used to chill the air and in turn to condense the moisture present. Pan humidifier and electrical heaters are used to maintain the required relative humidity (RH). Fresh ambient air is also chilled and added to meet the ventilation air requirement and the excess air gets out through small openings in the clean room. The air conditioned space is provided with false ceiling and chilled air enters through supply grille and there is separate duct for return air. The temperature of room air is controlled by the actuation of modulating valve to regulate the chilled water flow. Whenever the required conditions (say 23 ± 1 0C temperature and 55 ± 5 % RH) are achieved, the modulating valve will close and heaters will become OFF. Whenever the modulated valve is actuated (to reduce the chilled water flow), the chilled water is by-passed to the return line. Proper selection of blower will ensure that required air velocity is maintained inside the clean room for air circulation. Timely maintenance of air filters will ensure that strict dust level is maintained.
Figure 1: Schematic of a typical clean room.
Energy Conservation Opportunities
The energy conservation opportunities in clean room applications are given below:
- The return air from clean room is hot and supply air is cold. Instead of using electrical heaters for RH control of chilled supply air, heat in return air can be transferred to supply air through a heat recovery wheel as shown in figure 2 and that much load on chilled water will also get reduced. The saving in energy will be enormous.
• Significant amount of power is being lost in transmission of power between motor and blowers. Now-a-days direct driven motor blowers are available with advanced controls for optimising the cooling load. Such systems need to be used.
• Conventionally, the air handling unit (AHU) filters are cleaned at some interval and some of the filters and chilled water coil fins are chocked. It is suggested to have 20-25 numbers of air filter panels of AHUs as stand by. The chocked filters need to be replaced (once in 2-3 weeks) with stand by ones. After washing, the clean filters may be used on next AHU and the process may be continued. This will improve the cooling performance of AHUs and avoid chocking of chilled water coils and to increase the life of AHUs.
• Removing the air filters, the chilled water coil fins (in front of the blower) need to be inspected. The dirt between the fins needs to be cleaned with either compressed air or water jet from inside the AHU (opposite to normal air flow) so that dust and gel like substance is removed. It may be ensured that torn / damaged air filters are not used. If filters are cleaned in time, these things do not happen and performance & life of AHU is increased.
• Unnecessary heat generating loads (like UPS, photocopy machine, etc.) inside clean room needs to be shifted out to non-AC rooms.
• The efficiency of blower needs to be estimated based on the air flow rate, pressure developed and power drawn and if the efficiency is below 40 %, it needs to be replaced with high efficiency one.
• Replacement of three way valve of modulating motor with two way valve will lead to energy saving if variable frequency drive (VFD) is used with chilled water pump.
Figure 2: Schematic of typical heat recovery based RH control system
Conclusions
Clean rooms find applications in many Industries and a detailed system study and analysis will lead to identifying all potential areas for energy conservation. Many such opportunities are described in this article.
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