For
training at PB level, this section and the others will not be
studied.
Reminder:
The
pressure switch controls suction rate at the fan air inlet, to ensure
correct operating, and in the case of abnormally low vacuum pressure,
shuts off the motor.
The
pressure switch should be chosen with a regulating scale compatible
with the vacuum pressures to be controlled.
Question
300 [Pa] (or +40 to +300 [Pa])
800 [Pa] (or +100 to +800 [Pa])
300 to –1,500 [Pa] (or +300 to +1,500 [Pa]) Which model seems to be the most appropriate for controlling the fan below with a normal scale from vacuum to suction of –350 [Pa]? Justify your choice.
The pressure switch’s sole function is to control the operating of the fan. It should activate if the registered vacuum pressure is significantly lower than it should be (or almost no pressure at all with a variable speed fan able to operate at low speeds).
For a normal vacuum pressure of –350 [Pa], the switch can be programmed to activate if, for example, a pressure of –200 [Pa] is registered.
We could use, as a basic rule for regulating the pressure switches, a setting of half of the value of normal vacuum pressure (hence a setting of –350/2 = –175 [Pa]) (and even less for variable speed fans able to operate at low speeds).Under these conditions we note that the pressure switch model n°1 is perfectly suitable.
Certainly model n°2 could be used, but would be effective at the bottom end of the regulating scale, which is not desirable.
As a general rule, when one has to select a device where the monitoring or detection is able to be regulated, it is better to choose a model that can operate midway on the measuring scale.
Finally we can consider that model n°3 is totally unsuited to our requirements.
Question
What would be the risk of using, to control the operating of the fan above with a normal vacuum to suction pressure range of –350 [Pa], the pressure switch model n°3 (this would be regulated between –300 [Pa] and –350 [Pa]).
– Model n°1 : –40 to –300 Pa (or +40 to +300 [Pa])
– Model n°2 : –100 to –800 Pa (or +100 to +800 [Pa])
– Model n°3 : –300 to –1,500 Pa (or + 300 to +1,500 [Pa])
In purely theoretical terms model n°3 is valid since –300 [Pa] is a vacuum pressure considered to be abnormally low.
However, this selection runs the risk of provoking untimely cut-offs of the fan due to a slightly inaccurate “reading” of the pressure switch (imprecisions at the extreme ends of the regulating scale) or to possible fluctuations of the average normal vacuum pressure of – 350 [Pa].
Question
Amongst the models below, select that which seems to be the most suitable.
Justify your choice. The supplier’s catalogue offers 3 models of pressure switch capable of controlling, respectively, the following vacuum pressures (or excess pressure):
40 to –300 [Pa] (or +40 to +300 [Pa])
100 to –800 [Pa] (or +100 to +800 [Pa])
300 to –1,500 [Pa] (or +300 to +1,500 [Pa])
The pressure switch’s sole function is to control the operating of the fan. It should activate if the registered vacuum pressure is significantly lower than it should be (or no pressure at all).
For a normal vacuum pressure of –1,400 [Pa], the switch can be programmed to activate if for example a pressure of:
–1,400/2 = –700 [Pa] is registered.
Under these conditions we note that the pressure switch model n°2 could be suitable, but would be used at the upper end of the regulating scale, which is not desirable.
It would be better to use model n°3.