Firstly, a chiller is a refrigeration device that reduces the ambient temperature by circulating cold water or coolant. 

It consists of components such as a compressor, condenser, evaporator, and cooling tower. 

The working principle of a chiller involves compressing low-temperature, low-pressure refrigerant gas into high-temperature, high-pressure gas using a compressor. 

The heat is then released into the air or water through the condenser, causing the refrigerant gas to condense into liquid form. 

Finally, the refrigerant liquid evaporates into a gas and absorbs heat in the evaporator, thereby achieving the cooling effect. 

Chillers are commonly used in large buildings, industrial production lines, and refrigeration systems.

On the other hand, an air cooler is a ventilation device that lowers the ambient temperature by producing cold air. 

It typically consists of a fan and a circulating system for a cooling medium like water or ice. 

The working principle of an air cooler involves blowing air over the cooling medium, allowing the air to come into contact with the cooling medium and absorb its coldness. 

The cooled air is then delivered to the area that requires temperature reduction. Air coolers are commonly used in indoor air conditioning, heat dissipation for electronic devices, and cooling processes in industrial settings.

In summary, a chiller achieves cooling by circulating cold water or coolant, while an air cooler produces cold air to lower the temperature. 

They differ in terms of their applications and working principles. Chillers are mainly used in large buildings and industrial refrigeration systems, whereas air coolers are primarily used in indoor air conditioning and heat dissipation. 

The choice between the two depends on specific requirements and the intended application.