37
On-off control
On-off control is the simplest control scheme, and
does exactly what its name implies: the output device
turns fully on, then fully off.
The on-off humidistat differential is designed into the
control action between the on and off switching
points. The differential is established at a range
sufficient to prevent output short-cycling. In other
words, the humidity level will have to fall a little
below set point before the humidistat will close and
energize the humidifier. Once the humidifier is
energized, the humidistat will stay closed until the
humidity is a little above set point. This creates an
operating range that prevents the humidifier from
running for very short periods of time.
This type of operation is applicable to all
DRI-STEEM humidification systems controlled by
VAPOR-LOGIC
®
3
. In applications where there are
multiple contactor outputs, such as a
VAPORSTREAM
®
, the contactors for the different
heat stages are pulled in one at a time with a two
second interval between them. In applications with a
variable output stage, such as an STS
®
or GTS
®
, the
outputs are ramped up until they reach 100%.
Demand signal control
With demand signal control, the VAPOR-LOGIC
3
controller provides the output level that a master
control signal calls for. This signal can either be
generated by a humidistat or by a building energy
management system. The signal that is sent to the
VAPOR-LOGIC
3
board is a modulating signal
(typically 4-20 mA or 0-10 VDC). The board
responds to this signal by producing a directly
proportional output. In other words, with a
4-20 mA signal, a command of 4 mA would produce
no output. A command of 12 mA would have the
humidifier running at 50% capacity, and a command
of 20 mA would have the humidifier running at 100%
capacity. With a humidistat provided by
DRI-STEEM providing this signal, the humidity set
point is set at the humidistat. The keypad/display is
then used for maintaining and troubleshooting the
humidification system, with humidifier control
stemming from the humidistat itself. With an energy
management system (EMS) providing the signal, the
humidity set point is established somewhere on the
EMS and the humidifier responds to the EMS
commands.
Transmitter control
With transmitter control, the VAPOR-LOGIC
3
board
receives a linear signal that corresponds to the actual
humidity level being measured. (With a
DRI-STEEM-provided transmitter, the signal is
4-20 mA, which corresponds with 0-100% RH.) The
VAPOR-LOGIC
3
controller then uses this humidity
measurement, along with a user defined desired
humidity set point, and calculates a demand level.
This demand level is the level at which the humidifier
will run.
Control modes.
Operation
Operation:
Control modes
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