Open-Loop vs. Closed-Loop Circulation

By Aimee O'Driscoll, 24 March 2019

External circulation can be closed-loop or open-loop. We explain the differences between the two and the implications when choosing a chiller or circulating bath.

Closed-loop circulation involves fluid being pumped from the circulator through an external device, such as a water jacket or condenser, and then returning directly to the circulating bath.

Open-loop circulation involves liquid being pumped to an open tank, and liquid from that tank being returned to the bath.

When you’re shopping for equipment, the main difference between a unit suitable for open vs. closed loop circulation will be the type of pump it utilizes:

  • Closed-loop circulation only involves pressure so can usually be achieved using a pressure-only (simplex) pump, also known as a push pump. This type of pump has just one piston. If additional pressure is needed, you may require the use of a duplex pump.
  • Open-loop circulation uses pressure to push liquid out and suction to pull it back. This means a pump capable of both pressure and suction (known as a duplex pump or push-pull pump) is required. These pumps have two pistons that work simultaneously in opposite directions.

Manufacturer specifications will often explicitly point out whether or not a unit is suitable for open-loop or closed-loop circulation. For example, below are the specifications for the PolyScience Polycarbonate Open Bath System 17 Liter. The Economy MX model is capable of closed-loop circulation only, but the Advanced Programmable model is capable of both open- and closed-loop external circulation.


PolyScience Polycarbonate Open Bath System specifications.


However, it might not always be quite so obvious. For example, below are some of the specifications for Julabo 26 Liter Heating Circulators. Although they don’t specify open-loop or closed-loop capability for each model series, you can see that some models have suction capability while others don’t. The latter will only be suitable for closed-loop circulation.


Suction specifications.