Having mentioned that it is difficult to keep fermenting wort at the correct temperature during hot weather, a local brewer said it was a shame I didn't have attemperation - the ability to warm & cool my wort under control. It got me thinking how I could improve on my system of putting the wort in the garage overnight & bringing it in the house in the day during hot weather.
During fermentation the action of the yeast metabolising - like all living things - generates heat, particularly in the most vigorous stage after the yeast have multiplied, but still has plenty of sugar available. Initially there isn't enough yeast to make much difference - unless you over pitch - & in the latter stages of fermentation the sugar concentration gets so low that fermentation slows exponentially.
Only having items found in the average household & not wanting to go to great lengths to find a cooling solution for the few weeks a year that the weather is hot, the answer would have to be simple.
A copper tube surrounding the fermentation vessel, lagged to the environment & with cold water running through would be too extravagant. A source of cold water, a pump & a drain would be needed. Sitting the whole FV in a large vessel of cold water & replenishing the cold water periodically would be simpler, but even this is impractical as I don't have such a vessel, the effort to lift such a weight & the probability of spilling water - or worse wort - everywhere is too great.
I settled for covering the whole FV in a bin liner, then making a jacket for the FV out of wet tea towels - using the evaporation to cool the FV. I chose to re-wet the towels twice a day manually rather than have the bottoms of them sit in a bowl of water & wicking the water up the towel as it evaporated. The bowl would mean that I wouldn't get a good seal around the FV with the tea towel jacket, the standing water may attract insects & bacteria & I'm not convinced the capillary action would raise water 50cm or so to the top of the FV.
I used a contact thermometer stuck to the FV surface around 10cm from the base to measure the most direct temp of the wort. I also used a data logging thermometer that sat on top of the FV, under the tea towels meauring the temperature of the air trapped there. The third set of temperature data came from the Met Office's files of historic data from local weather centres. The FV was in my garage for the whole of this period.
The conclusion I draw from this experiment is that the air above the FV was always warmer than the outside air temperature until I started using the wet tea towels on the Wednesday. Then the difference in temperature between these measurements narrowed as the outside air got warmer, but the air above the FV fell. The air temps rising & falling appeared to have little impact on the temp of the fermenting wort until both the outside air temp fell on the last day & the cooling was applied, then the wort temp dropped a little.
Although using a control would measure the effectiveness of the cooling effect accurately, it is enough for me with limited time & resources to say the cooling has worked.