Ages. Best you look up the temperature gauge when 160c note the time and keep coming back till it is at your desired temperature.
What you mean by normal working temperature? Ambient?
The deaerator is an essential item for the Boiler Feed tank(if this is where it is) to safeguard the boiler form damage by oxygen as well as to safeguard on cost of oxygen scavenging chemicals. Why do you want to isolate it ?
Funny for now i am manufacturing Flash Condensing Deaerator heads and installing them on Boiler feed tanks where not avaialble.
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This isolation is required for a shut down in deareator so i want to know the approximate time required for complete cool of the temperature from 160degcel to ambient air temp.