Temperature Control: Why Power
is Not the Same as Efficiency
Many research tasks in the laboratory require exact tem- perature control. However, choosing the right circulator is not always straightforward. Advertising claims such as
“extremely fast” or “high cooling capacity” are not very meaningful and do not permit any objective assessment. Also difficult is the
comparison of manufacturer information since the measurement
methods used to determine it often differ.
It is essential for the user to be able to make meaningful
comparisons. DIN 12876 defines various characteristics and
measurement methods for this purpose. These should be used as
an orientation for heating circulators and cryostats. Characteristics recorded in this manner facilitate a reliable comparison of the
There are two types of temperature control devices on the
market. Along with open bath circulators, closed temperature control
systems are also available, sometimes referred to as process control
circulators. Closed temperature control systems are circulators/ther-mostats without an internal bath. An expansion tank replaces the
conventional bath to accommodate thermal expansion and this is
where the volume change takes place. The mass to be thermoregulated is reduced, which can increase the speed of temperature change.
Thermodynamics– how fast is a system?
When considering the question of the dynamics of a tem-
perature control device, the heating or cooling capacity (k W) is
normally used as a comparative value. The power generated in the
thermostats, however, is not sufficient for making a meaningful
by Michael Sauer,
assessment. The system mass—which has to be thermoregulated—
must be considered. That is why, to make a meaningful compari-
son, the cooling power density (watts/liters) as per DIN 12876 is
most suitable. Always applicable: the greater the cooling power
density, the more dynamic (faster) the thermostat can react to a
temperature change requirement.
Example: Consider two temperature control devices from dif-
ferent suppliers. The cooling capacity of both devices is the same;
likewise the pump flow rate (L/min) and temperature control
devices are connected to identical applications (e.g. glass reactors).
To be able to make a statement on the dynamics (cool down/heat
up time), we can use the following formula:
P = m c *d T/dt
(P = power; m = total mass; c = spec. heating capacity; d T =
temperature difference; dt = time)
Rearranged for the cooling time dt: dt = m c d T/P. The expression c d T/P is the same for both applications. However, it’s
worth taking a closer look at the mass.
Temperature control device 1 has a mass of 5 kg (fill volume,
not the inherent weight of the device). Temperature control device
2 has a mass of 10 kg. The mass of the external applications
amounts to 5 kg. The first case results in a total mass of 10 kg (
internal filling volume plus the external application); in the second
case 15 kg have to be cooled (or heated). The ratio is 2: 3, or in
other words: temperature control device 1 is only needed 2/3 of
the time. That means the time savings is 33 percent.
This example demonstrates that cooling capacity is certainly an
important factor. However, this should also be applied to the temperature control fluid volume being used. The result is the cooling
capacity density and this can be used to make a meaningful
comparison (see DIN). Additionally, temperature control device 1
saves 1/3 of the temperature control fluid and energy.
Pressure or flow rate?
The flow rate of the circulation pump is another important criterion. It has a strong influence on the heat transfer between a circulator and the temperature control fluid. According to DIN, the
cooling capacity should be measured at full pump output. When
the pump output is reduced, the heat transfer is lower. This leads
to more net cooling capacity and facilitates lower temperatures.
Important for most applications is not the pressure output (bar)
but the highest possible flow rate (L/min). The point is ultimately
not how much power a thermostat generates; rather it is the efficiency of the heat transfer to or from the process. Output is useless