Vapor Space Expansion Factor formula |
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| \( K_e \;=\; \dfrac{ \Delta T_v }{ T_{la} } + \dfrac{ \Delta P_v - \Delta P_b }{ P_a - P_{va} }\) | ||
| Symbol | English | Metric |
| \( K_e \) = Vapor Space Expansion Factor | \( dimensionless \) | \( dimensionless \) |
| \( \Delta T_v \) = Average Daily Vapor Temperature Range | \(K\) | \(R\) |
| \( \Delta T_{la} \) = Average Daily Liquid Surface Temperature | \(K\) | \(R\) |
| \( \Delta P_v \) = Average Daily Vapor Pressure Range (psi) | \(lbf \;/\; in^2\) | \(Pa\) |
| \( \Delta P_b \) = Breather Vent Pressure Setting Range (psi) | \(lbf \;/\; in^2\) | \(Pa\) |
| \( P_a \) = Atmospheric Pressure (psi) | \(lbf \;/\; in^2\) | \(Pa\) |
| \( P_{va} \) = Vapor Pressure at Average Daily Liquid Surface Temperature | \(K\) | \(R\) |
Vapor space expansion factor, abreviated as \(K_E\), a dimensionless number, is used in the estimation of evaporative emissions from fixed roof storage tanks. It quantifies the expansion of the vapor within the tank's headspace due to daily fluctuations in temperature and pressure. As the ambient temperature rises during the day, the vapor inside the tank heats up and expands. Similarly, changes in atmospheric pressure can also cause the vapor to expand or contract. This expansion forces some of the vapor-air mixture out of the tank's vents, leading to "breathing losses." The factor is a component in calculating the volume of these displaced vapors, which in turn helps determine the amount of volatile organic compounds (VOCs) emitted into the atmosphere.
The specific formula for calculating depends on various factors, including the daily average liquid surface temperature, daily vapor temperature and pressure ranges, the tank's breather vent settings, and atmospheric pressure. In other words, provides a measure of how much the vapor phase within the tank "breathes" in and out due to daily environmental changes.
