A bubble pump is a device that is used to raise a fluid from one location to another, higher location without mechanical energy. Instead, it utilizes thermal energy to power this change in position by increasing the buoyancy of the fluid it is moving. It is sometimes referred to as an air-lift pump or a vapor-lift pump.
In its most basic form, a bubble pump consists of a tube with its inlet at a lower reservoir and its outlet at a higher reservoir. Its purpose is to raise a fluid from the lower reservoir to the higher one. Fluid in the lower reservoir is typically heated, causing some of the liquid to change phases to a vapor form. This vapor is more buoyant than the liquid form of the fluid and rises up through the tube, carrying some liquid upward with it. Generally, a one-way valve or other mechanism prevents fluid from flowing back down the tube as it cools.
A simple example of the use of a bubble pump is in a common drip coffeemaker. In many common coffeemakers, water is poured into a reservoir in the back and coffee grounds are placed in a filter at the top of the machine. To brew coffee, the water must be heated and then filter through the coffee grounds above it. For this to happen, the water must somehow be lifted up above the grounds so that it can drip down through it. A bubble pump with its inlet at the bottom of the water reservoir and its outlet above the grounds is used to accomplish this task.
While many pumps use mechanical means by which to move a fluid, the bubble pump is driven only by thermal energy. As a result, its construction is very simple with few moving components. This provides advantages in terms of reliability and cost in many applications. These pumps, however, are not typically suitable for use in high-pressure applications.
The force of gravity pulling downward acts to counter the upward buoyancy of the fluid being pumped. Friction between the fluid being pumped and the interior wall of the tube also resists its upward motion. Effective bubble pump design must take into account variables such as friction between the fluid and tube, the length and diameter of the tube, and so on. Proper design is necessary to ensure that forces acting against the upward motion of the fluid do not overcome it before the fluid reaches the upper reservoir.
Bubble pumps are used in a range of applications, from the very simple to the highly technical. Aquaculture facilities and hatcheries frequently make use of bubble pumps in their operations, for example. Some solar water heaters take advantage of the heated fluid they contain by using a bubble pump to circulate it through the system. A bubble pump is also a key part of a refrigeration cycle known as the Einstein cycle.