Circulation Pumps

A circulation pump keeps a medium permanently moving in a closed circuit instead of transporting it from one place to another. In industrial plants it drives heat transfer circuits, forced-circulation boilers, refrigeration and air conditioning circuits as well as reactor circuits, in some cases at medium temperatures far above those found in building services. In buildings, circulation pumps keep heating circuits running and ensure that hot water is available instantly at every tap.

Technically, a circulation pump is a centrifugal pump that creates forced circulation in a closed system. Depending on the application, it is also known as a circulating pump or circulator, in heating systems as a heating pump or heating circulator, and in drinking water installations as a hot water circulation pump.

How does a Circulation Pump work?

Circulation pumps operate on the centrifugal pump principle. A rotating impeller transfers energy to the medium and accelerates it outwards, which creates a continuous flow within the circuit. Forced circulation is needed because natural circulation driven by temperature differences alone is no longer sufficient in modern process plants and heating systems to ensure the required flow rates.

Sizing a circulation pump differs considerably from sizing a transfer pump. Since the pump does not have to lift the medium against a static head, only the pressure losses of the system matter, that is pipe friction, valves and heat exchangers. The result is a comparatively low head combined with flow rates that can be considerable. Structurally, the pump is shaped by two other factors instead, namely the medium temperature and the axial thrust resulting from it, which is why the thrust bearings are reinforced accordingly.

Control is a further factor that decisively shapes operation. Modern circulation pumps adjust their speed continuously via the differential pressure and detect pressure changes in the pipe network automatically. Since the demand of a circuit changes considerably over the operating period, for instance when consumers are throttled back, these pumps run at part load for long stretches of their operating time. In domestic hot water systems, timer control or self-learning functions additionally limit operation to the periods in which hot water is actually needed.

What Types of Circulation Pumps are there?

Circulation pumps are distinguished primarily by their sealing concept, since this determines the permissible system pressures, the medium temperatures and the leakage safety. Further distinctions follow from the drive and control system and from the arrangement of several units.

• Dry-rotor pumps with shaft seal: Usually built in a horizontal design or as an inline pump, sealed by cooled packed glands or mechanical seals and driven by IEC standard motors. These pumps remain economical even at high system pressures and are the standard choice in larger industrial plants and heating systems.
• Vertical pumps without shaft seal: Designs such as condensate pumps or submerged-shaft pumps, in which the motor sits above the medium while the shaft extends into the tank or sump. As the pump does not have to be sealed against the pumped medium, a shaft seal is not required.
• Wet-rotor pumps with canned motor: The rotor runs in the pumped medium, separated from the motor winding by a can, so a shaft seal is dispensed with entirely. The medium simultaneously cools and lubricates the rotor and bearings, which makes the design low-maintenance, quiet and completely leak-free. In heating and domestic hot water systems, this design dominates the lower power range. In industry, canned motor pumps are used wherever hazardous media are circulated.
• High-efficiency pumps: Speed-controlled circulation pumps with a permanent magnet motor, available as wet-rotor or dry-rotor versions. The high-efficiency pump continuously matches its output to demand via the speed and is today the standard wherever pumps run in continuous operation.
• Double pumps / twin pumps: Two pump units in a shared casing, one of which serves as an operational standby. The design is used wherever a failure of the circuit cannot be tolerated.
• Special designs: Depending on requirements, with erosion-resistant protective linings, adapted impellers, special sealing systems or in an explosion-proof version.

The right design type is not decided by the pump type alone, but by the interplay of system pressure, medium temperature, required output and the demands placed on the tightness of the circuit.

What are the Benefits of a Circulation Pump?

Compared with pure natural circulation and with uncontrolled designs, circulation pumps offer a number of benefits that pay off above all in continuous operation.

• Reliable flow rate independent of temperature differences: Forced circulation makes the flow independent of natural buoyancy and ensures the required throughflow everywhere, even in branched networks.
• Slimmer pipework and lower installation costs: Since the flow does not have to arise from density differences alone, smaller cross-sections can be specified than with natural circulation systems.
• High medium temperatures manageable: Reinforced thrust bearings and sealing concepts matched to the temperature allow continuous operation with hot water that is kept liquid above boiling point by the system pressure.
• Freedom from leakage with critical media: Canned motor designs rule out any escape of the pumped medium by construction, which qualifies them for hazardous or high-purity circuits.
• Demand-based control at part load: Speed-controlled pumps detect pressure changes in the network automatically and adjust their speed instead of generating excess differential pressure. Since circulation circuits run at part load for most of their operating time, this range determines the costs over the year.
• Low power consumption in continuous operation: Because the pump only draws the output actually needed, energy consumption falls considerably compared with uncontrolled designs, which run at full output regardless of demand.
• Availability through double pumps: In systems with no tolerable downtime, the standby pump takes over in the event of a fault without interrupting the circuit.
• Instant hot water and better hygiene: In domestic hot water systems, hot water reaches every tap without waiting and without wasting water. Keeping the water moving also reduces stagnation and with it the risk of Legionella growth, provided the required water temperatures are maintained.

For glandless circulators in heating and cooling circuits, the EU Ecodesign Regulation sets minimum requirements based on the Energy Efficiency Index (EEI). Modern high-efficiency pumps undercut these requirements considerably and, compared with older uncontrolled models, reduce power consumption over the year many times over. Dry-rotor pumps with shaft seal and drinking water circulators fall outside these requirements, but the principle of demand-based control holds there just as well.

Where are Circulation Pumps used?

Circulation pumps are used wherever a medium is kept moving permanently within a closed circuit, whether for heat transfer, for cooling or for treatment.

• Heat transfer systems: In industrial heat transfer circuits, the circulation pump keeps the medium moving between the heat source and the consumers, frequently with hot water that can only be kept liquid under system pressure. Temperature-resistant sealing concepts and appropriately dimensioned thrust bearings are required here.
• Reactor circuits: In process plants and reactors, the circulation pump keeps the process medium in constant motion. Where the medium is hazardous, the choice falls on glandless designs, as they operate entirely leak-free.
• Forced-circulation boilers: Whereas in a natural-circulation boiler the circulation arises solely from the density differences between heated tubes and downcomers, in a forced-circulation boiler a circulation pump drives the circuit. As the plant depends on the pump, standby units must be provided in order to remove residual heat in the event of a pump failure.
• Refrigeration and air conditioning systems: Circulation pumps carry chilled water or brine in a circuit between the chiller and the consumers. Solar thermal and geothermal systems work on the same principle.
• Heating systems: The heating pump carries heated water to the radiators and the cooled return flow back to the heat generator, almost continuously throughout the heating season. Smaller installations rely on wet-rotor pumps with a canned motor, larger ones on dry-rotor inline pumps with a mechanical seal.
• Swimming pool technology: In filter systems, the pump circulates the pool water continuously through filters and treatment stages and thereby keeps the water quality constant. A special form are filter pumps, that is centrifugal pumps with an integrated filter, which remove foreign matter and lost objects directly from the pool water.
• Domestic hot water systems: In residential, commercial and public buildings, a hot water circulation pump moves the water through a loop of supply and return line between the water heater and the farthest tap. Hotels, office buildings and public facilities with many fixtures and fluctuating demand rely on it for a consistent supply throughout the day.

What all these applications have in common is continuous operation, all year round in industrial circuits and throughout the heating season in building services. Service life, tightness and behaviour at part load therefore usually weigh more heavily over the years than the purchase price alone. While standard pumps are usually sufficient for residential use, industrial and commercial applications call for pumps built for professional-grade performance in terms of materials, service life and certification.

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