This question as to why air conditioner capacity is in Tons had fascinated me for quite sometime... Tons is a unit of weight and what would it mean to have air conditioner capacity in tonne!! ?
Its basic purpose being to take out heat from the room , it made no sense.. shouldn't the capacity be something like in terms of heat extracted from the room per hour or per unit volume ... like thermal energy extracted in Btu / sec or kJ/sec... why would someone sell a cooling equipment with capacity in terms of "weight" ?
When I digged into it, I realised it has more to do with "History" rather than science or the equipment being sold itself.
It seems before AC conditioning as such what we see today was invented, people used to cool their homes by using the ice saved during wintertime.
We know from thermodynamics that amount of heat (thermal energy) required to melt ice into water is much more than amount of heat required to further heat water. In normal technical language this thermal heat required to melt ice would be called "latent heat of fusion".. Well enough of the boring technical terms...
Basically with the high thermal energy or heat required to melt ice, it would have been a useful tool to extract heat from the rooms , a large portion of which would be extracted for melting ice, which in turn would keep the room nice and cool.
It turns out the amount of heat required per hour to melt one tonne of ice in 24 hours (one day) was around 12,000 BTU/hr. In other terms, One tonne of ice was required to keep a room cool / pleasant for a day (24 hours).
(One BTU is approx amount of thermal energy released if you burn a single piece of matchstick)
So basically for today's AC's when we say they are One Tonne AC, we are basically saying they can remove 12,000 BTU/hr of thermal energy from the room, with history as explained above.
Hope this clarity was able to simplify this topic of 1 or 1.5 Tonne AC Jargon used in some simplistic terms.
