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Impact of pressurization on energy consumption for laboratories and cleanrooms.

Article Excerpt
INTRODUCTION

The objective of this study is to use simulation models to quantify the impact of pressure differential and air tightness on HVAC system energy consumption for a containment space. Containment spaces and facilities, such as hospital isolation rooms, cleanrooms, and chemical and biological laboratories, often utilize depressurization or pressurization as a secondary barrier to either prevent contaminated air with infectious agents in the laboratory space from escaping into nonprotected general areas or to prevent unclean air from general areas leaking into clean spaces.

When a room receives more supply air than exhaust air, the room is pressurized and the surplus air is pushed out to adjacent room(s). When a room receives less supply air than exhaust air, the room is depressurized and the deficit air is pulled in from adjacent room(s). Figure 1 demonstrates such a relationship. The net airflow rate between the supply and exhaust airflows is called the airflow differential or, as it is commonly referred, the offset. Based on mass conservation, the offset equals the summation of the leakage flow rates to/ from the adjacent room(s). The pressure difference(s) between a room and its adjacent room(s) is determined by the airflow offset and the room's air tightness, which further depends on the number and size of the holes and cracks on the room envelope (including walls, doors, and windows). For the same room (same air tightness), the higher the airflow offset, the higher the room pressure difference. Using the same airflow offset, the tighter the room, the higher the room pressure difference. That is to say, room air tightness plays a critical role in room pressure control.

[FIGURE 1 OMITTED]

Each room's air tightness level is unique and is unknown before the room is constructed. When creating rooms with certain positive or negative pressure, design engineers often do not know what will be a proper flow offset value for each controlled room. Due to the lack of engineering means to determine the "right" offset value, design engineers often intuitively select a pressure differential value or flow offset value. When this value is selected lower than necessary, room pressure control fails to meet the performance expectation. When this value is selected higher than necessary, then higher than the necessary airflow rate is provided to or exhausted from the room. If the space is under pressurization and is provided with higher than the necessary airflow rate, energy is wasted to condition the unnecessary additional airflow. If the space is under depressurization and is exhausted with higher than the necessary airflow rate, the increased pressure differential causes additional air to leak from the adjacent environment into the space. If the room is adjacent to environments that have a different temperature or humidity than the room itself, increased leakage airflow will increase the room heating or cooling load and, thus, increase energy consumption.

Several prior studies (Sun 2003, 2004; Siemens 2004) discussed the impact of improper design pressure differential value and flow offset value on pressure control. However, there is a lack of literature that quantitatively examines the impact of pressure differential and air tightness values on energy consumption for containment spaces. Engineers have traditionally ignored the possible large energy waste caused by improper pressure differential value and space air tightness. Since the containment spaces use far more energy per square foot than typical commercial buildings due to intensive ventilation requirements, even a small percentage of energy waste could lead to large overall energy waste.

As an initial study, only limited cases are considered in this project. The simulation model used for this study is developed based on an experimentally validated model reported by Ahmed et al. (1998).

SIMULATION MODEL DESCRIPTION

Simulated Laboratory Space

Laboratory spaces that are equipped with variable air volume (VAV) systems are simulated in this study (Figure 2). Three layouts are considered (Figure 3): Layout 1, a single pressurized laboratory space; Layout 2, a suite of pressurized laboratory...

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