The cooling capacity of an evaporator is typically measured in British Thermal Units per hour (BTU/hr) or tons of refrigeration (TR). A 3-ton evaporator refers to its cooling capacity and indicates that it can remove 36,000 BTU/hr or 3 tons of heat from the air per hour.
To provide a comparison, 1 ton of cooling capacity is equal to 12,000 BTU/hr. Therefore, a 3-ton evaporator can remove 36,000 BTU/hr (3 tons x 12,000 BTU/hr per ton) from the air during the cooling process.