The rate at which water gains or loses heat depends on three specific variables. Temperature differences between the water and the heating source represent the first factor.
A second factor is the heat transfer coefficient, which measures the conduction and convection within a physical system. The third variable is the contact area between the water and the medium performing the heating or cooling.
Heating typically occurs via a flame or an electrical element, often where the source is hundreds of degrees hotter than the water. Cooling usually employs ambient air or a liquid refrigerant, which are generally less than a few tens of degrees cooler than the water.
Water density changes with temperature, creating currents that move heat away from the heating surface. Boiling water produces an extremely high heat transfer coefficient because turbulence carries more heat away and steam transfers large amounts of latent heat.
Faster heating results from a combination of a higher heat transfer coefficient and a greater temperature difference. In home evaporative coolers or industrial cooling towers, the surface area of water increases significantly by breaking it into small drops in contact with air.
These cooling systems maintain a high heat transfer coefficient because evaporated water carries away latent heat. Cooling is generally quicker than heating in these systems, though engineers can design systems to cool water as fast as they heat it.
Coolant is pumped into engine channels where the engine block rapidly heats it up. The fluid then enters a radiator containing multiple thin channels where air flow cools it.
The system reaches an equilibrium temperature governed by a thermostat once it hits its running temperature. Heating time is defined as the duration the coolant spends in the engine block channels, while cooling time is the duration in the radiator.
Engineers can match these heating and cooling times by resizing the radiator channels.
