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This VRU graphic displays a DRYVac system using a Busch dry vacuum pump. This system was developed by SYMEX America’s, now SYMEX Technologies, to overcome the deficiencies of the older designs using liquid ring vacuum pumps with glycol. To answer your question the, the glycol solution is a mix of water and ethylene glycol. This mix, while providing cooling, is actually blended to maximize the liquid seal inside the liquid ring pump. This seal is critical in allowing the liquid ring pump to achieve the deep vacuum needed to design hydrocarbons from the activated carbon during carbon bed regeneration. In practice the water in the glycol-water mix evaporates due to the heat of compression. As evaporation occurs the vapor pressure of the mix changes. This change effects the ability of the pump to maintain the deep vacuum necessary to effectively regenerate the carbon beds. This results in less and less hydrocarbon desorption which in turn limits the adsorption rate of hydrocarbons in subsequent regen cycles. Eventually, hydrocarbon adsorption fails, and all inlet hydrocarbons exit to atmosphere. This signifies the complete failure of the VRU. As all of this transpires, the evaporation of the water component of the original glycol-water mix decreases reducing the volume of the glycol-water mix and prompting your question. The simple answer is evaporation. If you’d asked, What do I do about this loss?” I would have focused on the CRITICAL nature of the concentration of the glycol-water mix, necessary to maintain the deepest vacuum possible, which MUST be maintained for the VRU to function properly. Since 1980 over 1000 VRUs have been commissioned. Most of the early systems failed because of this issue, and today the DRYVac VRU has become the recognized “standard of the industry” since it eliminates the liquid ring pimp and its problematic glycol altogether. For more information you can call me at 918-231-9698, or reach out to William Lin in Taiwan through SYMEX Technologies or Quinella.