If applications may only be subjected to a maximum pressure (system pres-
sure) of less than 10 bar, this can be set via the pressure limitation in the
pump settings. This pressure limitation means that the pump performance
is automatically limited by the static pressure of the pressure overlay on a
priority basis.
Fig. 81: Graph of system pressure
If the pressure overlay is set too high, the remaining pressure of the pump
may not be sufficient for the desired volume flow of the heat transfer liquid.
It may therefore be necessary to reduce the pressure overlay, in order to
obtain a higher volume flow.
A reliable temperature control process is only provided if the heat transfer
liquid does not boil. Boiling processes occur when the steam pressure of
the heat transfer liquid falls below the static pressure (pressure overlay). The
temperature of the heat transfer liquid must be at least 5 K below the steam
pressure curve to prevent this.
n
If you wish to maintain the temperature of a water-glycol mixture (in a
ratio of 40:60, equivalent to LAUDA Kryo 30) at 130 °C, read off the
steam pressure of the heat transfer liquid at 135 °C. This is stated as
2.0 bar absolute (see steam pressure table).
n
Set the pressure overlay (= set pressure Pset) to 1.5 bar.
Calculation: Steam pressure - ambient pr 0.5 bar safety
margin
Note: The set pressure (Pset) is stated as a relative pressure in
relation to the ambient pressure.
n
Set TiH 2 K above the set point (in this example 132 °C); this means that
the constant temperature equipment will not heat any higher.
n
If you wish to dissipate heat, the reverse flow temperature is higher
than the outflow temperature. Use the highest system temperature to
determine the pressure overlay.
Example:
Fig. 82: Table for steam pressure of water-
glycol mixture
V6
Integral Process Thermostats and High-Temperature Thermostats
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