Yes it is reasonable and possible for mineral oil, even at room temperature, to evaporate and condense on the duct work. Researchers working with high vacuum lines put low temperature traps on the low pressure side of the pump to prevent oil entering the system. While the vapor pressure of mineral oil is low it is significant that over time an oil film can develop on various surfaces.
This reference is not directly related to your observation but it does show that mineral oil does evaporate. https://www.cdc.gov/niosh/nioshtic-2/00229101.html
From: ACS Division of Chemical Health and Safety [mailto:DCHAS-L**At_Symbol_Here**PRINCETON.EDU] On Behalf Of Elizabeth Brubaker
Sent: Thursday, October 12, 2017 1:14 PM
To: DCHAS-L**At_Symbol_Here**PRINCETON.EDU
Subject: [DCHAS-L] Fume Hood Ducting
Has anyone experienced accumulation of oil (perhaps mineral oil from a heated oil bath?) in fume hood duct work? Is this reasonable or even possible? Could a sufficient amount remain in the system for it to condense out in the ducts when the exhaust fans were turned off for an extended period?
--
Beth Brubaker
Laboratory/Safety/Waste Coordinator
Murray State University Department of Chemistry
2226 Jesse D. Jones Hall
Murray, KY 42071-3300
(270) 809-6390
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