A Case Study and Analysis of Cleanroom Energy Use

Abstract

A plant has a cleanroom HVAC system which is known to be a large energy user. This study found that the current cleanroom operation is indeed wasteful, and stands to benefit from improved controls and operation protocols. Proper data collection is instrumental in understanding how the system is operating. After analyzing data, the real cleanroom operation is thought to be different than the assumed cleanroom operation, a significant finding.

Data on cleanroom conditions, power draw for both dedicated cleanroom chillers, and power use of all five air handling units are collected. It is revealed that cleanroom chiller and reheat power cost is approximately $103,000/yr, much higher than necessary, but also much less than the original estimate of $385,000/yr. During the data collection process, changes to the cleanroom operation were made, and some of them are seen to have desirable results, but there are also very negative unintended consequences, seen in the data collected. To be more specific, the cleanroom reheats are now seen to be hyper-sensitive to times of high humidity, using an average of 200 kW of electric strip reheat when the dew point is above 50°F.

These consequences stem entirely from an inadequate control system. Whether it is the case that each of the five air handling units have no idea about what the other units are doing, or whether the control over incoming and outgoing air is not as complete as it was originally thought is unknown. The data collected from this project forces the plant to take a second look into how completeness and accuracy of its control setup.

Also notable is the finding that chiller power draw is heavily influenced (>100 kW) by daily fluctuations in outside temperatures, even though there is very little internal load placed on the chillers due to outside air infiltration (<14 kW), and no roof, wall or floor conduction load. This suggests that the condenser loop of the air-cooled chillers has a large pressure rise when temperatures outside are hot, decreasing efficiency dramatically. This helps set maximum summer demand, making its cost disproportionate to its energy waste. Also of note, the large chiller helical-screw compressors do not unload very well.