What can cause a decrease in removal rates when recovering vapor refrigerant?

The correct answer highlights the significance of vapor density in the recovery of refrigerants. When vapor density decreases, it indicates that the vapor is less concentrated, which can result in lower efficiency during the recovery process. A lower density means that there are fewer refrigerant molecules per unit volume, making it more challenging to effectively recover the vapor.

In contrast, increasing vapor pressure typically helps in recovery, as a higher pressure enhances the driving force for the vapor to move towards the recovery unit. Cooling the recovery unit, on the other hand, can lead to condensation, which might actually aid in the collection of the refrigerant and does not inherently cause a decrease in removal rates. System blockage would impede flow and could cause reduced recovery rates, but it is a different issue from decreasing vapor density affecting the recovery efficiency directly. Overall, understanding how vapor density impacts recovery is crucial for optimizing refrigerant retrieval processes.