Temperature Control For Distinguished Lab Managers, Pt. II: Open Bath Circulators Vs. Closed Systems

In our last post, we started discussing how to think about precision temperature control for lab managers by highlighting four major factors to consider when choosing a system that will not only meet your current applications’ requirements, but also keep your laboratory operations robust & adaptable, safe and streamlined for the long run.

Let’s continue this theme by looking at the differences between open bath units (open bath circulators) and closed bath systems (highly dynamic temperature control).

Open Bath Units

An open bath unit (also called an open bath circulator, heated circulating bath, heating circulator or integrated heated bath) is a temperature control unit with a lid or cap that opens directly into the bath tank, which houses the bath heat exchanger. This includes the heating coils of the heating circuit, cooling coils of the refrigeration circuit (on cooling systems) and circulation pump.

Note: An immersion circulator can be attached to a bath tank to create an (integrated) open bath unit.

Working Temperature Range

An open bath circulator’s working temperature range is determined by the unit’s heating/cooling capacity and the properties of your heat-transfer fluid (bath fluid).

• The unit’s low-temperature range is based on:
  • Cooling capacity (refrigeration system capability)
  • Fluid viscosity (how easily it circulates)
• The unit’s high-temperature range is based on:
  • Heating capacity (heating system capability), plus its effect on the refrigeration system
  • Fluid flashpoint (as a safety precaution, upper limit ~10°C lower than fluid flashpoint)

Possible Constraints

Keep in mind that the tank lid is the only barrier between the bath fluid and atmospheric exposure. It is not sealed, and this can deplete efficiency for certain applications.

• For low-temperature applications (down to -90 °C):
  • Obvious issue: When using nonaqueous bath fluids (e.g., silicone oil) in a humid environment, moisture can condense and accumulate inside the tank. This ice buildup will hinder pump capability and cooling capacity.
    • NOTE: JULABO condensation traps are available on select models to minimize this effect.
  • Not-so-obvious issue: Even at low temperatures, volatile fluids (e.g., hydrofluoroether [HFE]) may still evaporate, and would be better suited in a sealed or semi-sealed system.
• For high-temperature applications (>150°C):
  • Obvious issue: Volatile bath fluids (e.g., HFE) will inherently evaporate.
  • Not-so-obvious issue: Even high-heat fluids (e.g., silicone oil rated for use up to 250°C) may start to evaporatesmoke/outgas and should be used in a fume hood.

Check out JULABO’s open-system heating circulators here.

Closed Bath Systems (Highly Dynamic Temperature Control)

A closed bath system (highly dynamic temperature control) incorporates an expansion vessel with additional bath fluid to form a hydraulic seal over the heating/cooling components. This hydraulic barrier protects the bath fluid from exposure to the atmosphere, mitigating the condensation/evaporation issues mentioned above.

Fluid Hydraulic Seal

Active heating/cooling zones are never exposed to the atmosphere.

• For low-temperature applications:
  • Eliminates humidity condensation/ice buildup
  • Eliminates evaporation of volatile bath fluids
  • Eliminates unpleasant vapors/odors
• For high-temperature applications:
  • Operating temperature range extended above bath fluid flashpoint
  • Reduces bath fluid evaporation
  • Eliminates unpleasant vapors/odors
  • Fluid oxidation at high temperature operation greatly reduced

Highly Dynamic Temperature Control

Closed bath systems also have more advanced control technology, a smaller bath fluid reservoir and a more robust design than open bath units. This opens up a whole slew of advantages:

• Highly dynamic ICC temperature control (stability to ±0.01°C)
• Rapid temperature changes (faster time-to-temperature)
• Quick and accurate response times to demanding applications and difficult temperature challenges (exothermic and endothermic reactions)
• Ideal for precise control of external applications (including jacketed reaction vessels)
• Powerful, magnetically coupled circulating pumps with multiple pump speeds and ability to regulate flow by stage or specific pressure
• Segregated three-chamber design (pump, heater and refrigeration components separated) for a wider working temperature range
• Ability to use a single bath fluid with an extended operating temperature range
• Extended operating life of bath fluid

So, which one is right for you?

When it comes to deciding between an open and closed system, it all depends on your application type and how much control you require. While open bath units can be used for both internal and external applications, closed systems with highly dynamic control are ideal for those external applications with more demanding temperature requirements and challenges.

But wait, there’s still more to consider! In part III of our “distinguished lab managers” series, we’ll dive into four distinctions between polycarbonate bath tanks and stainless-steel bath tanks.

Still have questions? Ready for decisions? Learn more about JULABO USA by visiting our site at julabo.us.