[HopDen]

When nearing the end of fermentation I will remove the blow off hose and cap the fermenter with the gas manifold. I will usually try to do so with about 5 gravity points left. I have noticed that the temp will rise about 3-4*f after doing so. Fermenting a Bo Pils at 54* and it rose to 58* in 12 hours after capping.

Has anyone experienced this and what is your theory as to why this happens?

Edit: I should had added that the pressure in the FV rose to 10psi. Pressure=temperature at sea level?

[SCBeerDude7]

Gay-Lussac’s Law:  Temperature increases with pressure increase.  This is a possible root cause.

[Saccharomyces]

I think that it is more complex than that.  Latent heat is leaving a fermentation via evaporation with a blow-off tube affixed to a fermentation vessel (this phenomenon is easy to see because the vapor tends to condense in the blow-off tube).  It is not a huge amount of heat, but it is a heat loss.  Whenever a liquid makes the phase change to gas, it carries away heat.  That is why water cannot get any hotter than 212F at sea level.  The phase change from water to steam carries away the latent heat required to convert 212F water to 212F vapor.  If we want to delay the phase change and allow water to reach a temperature higher than 212F at sea level, we have to add pressure, which how a pressure cooker works.  A pressure cooker adds an additional 15psi of pressure (basically two atmospheres), which results in water not making the phase change to vapor until it hits 121C/250F.

[Saccharomyces]

The same kind of thing can be seen in your line of business.  At a high pressure, refrigerant is a liquid; however, when it goes through the expansion valve into the evaporator, the pressure drops and it makes the phase change from liquid to gas.  It just happens that refrigerant boils at low temperature. It cools the evaporator coils, which sink heat from the air.  The heat is then sent out to the high pressure side where the gas is condensed back into liquid and the heat that was amassed in the evaporator is expelled by the condenser because heat flows from hot to cold and the condenser coils are warmer than the ambient air.

[KellerBrauer]

+1 to the above.  Well said.