Cooling with hysteresis

Three computers run in the server room, with an arc furnace next door. It gets warm. A wall cooler should fix that – but it has no set point you could adjust. It cools as long as it is on. When it is on is decided by your chip.

The wall cooler#

• It pulls heat out of the room air and passes it to the gas in its pipe network. So the pipe has to be filled with gas, and the heat has to move on from there, for example through a radiator.
• The larger the temperature difference between pipe gas and room, the worse it works. An ice-cold cooling loop is, of all things, the worst operating point.
• It only works when there is enough pressure in the room – above 6.3 kPa, the Armstrong limit.
• Switched on and off through On, like a heater.

Hysteresis the other way round#

For heating it was: on below LOWER, off from UPPER. For cooling everything flips:

• above the upper limit: switch on
• below the lower limit: switch off
• in between: keep the state

sgt tmp temp OFF       # still above the off limit?
and cool cool tmp
sgt tmp temp ON        # above the on limit?
or cool cool tmp

slt becomes sgt, and the limits swap roles: the and checks against the off limit, the or against the on limit.

Typical mix-ups#

Sensor gone?#

If the sensor fails, both comparisons are false – the cooler switches off. Whether that is safe depends on the room. In the server room a switched-off cooler is a problem, so you report the failure with snan on a warning light.