pump, device to lift, transfer, or increase the pressure of a fluid (gas or liquid) or to create a vacuum in an enclosed space by the removal of a gas (see vacuum pumps under vacuum). The centrifugal pump, the most common kind, consists basically of a rotating device, called an impeller, inside a casing. The fluid to be pumped enters the casing near the shaft of the impeller. Vanes attached to the spinning impeller give the fluid a high velocity so that it can move through an outlet. The reciprocating pump moves a fluid by using a piston that travels back and forth in a cylinder with valves to help control the flow direction. Examples are the lift pump and the force pump. In a lift pump the piston and cylinder are positioned vertically. When the piston moves upward, atmospheric pressure pushes water into the cylinder to fill the empty space beneath the piston. On the downward stroke, the water in the cylinder is forced to flow above the piston. Reversing direction, the piston moves up, allowing more water to come up under it into the cylinder and lifting the water held above it to an outlet pipe where the water flows out of the pump. Since atmospheric pressure will support a column of water no higher than about 33 ft (10 m), a lift pump can raise water no farther than this distance. The rotary pump is like the reciprocating pump in that it allows a fluid to fill a space that then decreases in volume, forcing the fluid out of the space. However, unlike a reciprocating pump, it has no valves and uses one or more rotating components in place of a piston. The jet pump has no moving parts; it uses a swiftly moving fluid to induce motion in another fluid. For example an atomizer, a type of jet pump, uses a high-speed stream of air to pump a liquid, such as a perfume. Compressors are used to pump air or other gases into a closed container. They range from hand pumps to large power-driven devices that furnish compressed air for operating pneumatic machinery and for various other purposes. In nuclear reactors that use liquid radioactive metal, the nonmechanical electromagnetic pump is employed. An electric current is either induced in the liquid metal or is passed through it by electrodes. A magnetic field surrounding the pipe then propels the current-carrying liquid forward.

heat pump: see air conditioning.

Machine that uses energy to raise, transport, or compress fluids. Pumps are classified by how they transfer energy to the fluid. The basic methods are volume displacement, addition of kinetic energy, and use of electromagnetic force. Pumps in which displacement is accomplished mechanically are called positive displacement pumps. Kinetic pumps pass kinetic energy to the fluid by means of a rapidly rotating impeller (blade). To use electromagnetic force, the fluid being pumped must be a good electrical conductor. Pumps used to transport or pressurize gases are called compressors, blowers, or fans.

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Device for transferring heat from a substance or space at one temperature to another at a higher temperature. It consists of a compressor, a condenser, a throttle or expansion valve, an evaporator, and a working fluid (refrigerant). The compressor delivers vapourized refrigerant to the condenser in the space to be heated. There, cooler air condenses the refrigerant and becomes heated during the process. The liquid refrigerant then enters the throttle valve and expands, coming out as a liquid-vapour mixture at a lower temperature and pressure. It then enters the evaporator, where the liquid is evaporated by contact with the warmer space. The vapour then passes to the compressor and the cycle is repeated. A heat pump is a reversible system and is commonly used both to heat and to cool buildings. It operates on the same thermodynamic principles as refrigeration.

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Machine for moving liquids and gases. Its two major parts are the impeller (a wheel with vanes) and the circular pump casing around it. In the most common type, called the volute centrifugal pump, fluid enters the pump at high speed near the centre of the rotating impeller and is thrown against the casing by the vanes. The centrifugal pressure forces the fluid through an opening in the casing; this outlet widens progressively in a spiral fashion, which reduces the speed of the fluid and thereby increases pressure. Centrifugal pumps are used for many purposes, such as pumping liquids for water supply, irrigation, and sewage disposal systems. They are also used as gas compressors.

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