Application

Sequence of Operation
Optional Rate Limiting of Actuators
9
Siemens Industry, Inc.
Application Note, App 2928
140-1302
2015-07-07
To deal with the possibility of unequal flow rate changes, the application includes two
new points which allow field adjustment to slow down actuators.
SUP MAX RATE
effectively limits the speed of the supply actuator;
GEX MAX RATE effectively limits the
speed of the exhaust actuator.
SUP MAX RATE and GEX MAX RATE should be changed to values other than 0 only
after a thorough analysis has been made of the job specific scenarios. For example,
slowing down a given supply actuator to match a general exhaust actuator may
actually be detrimental overall if the installation includes faster acting fume hoods.
To limit the rate of the actuator (supply or exhaust depending on your setup) that
controls the larger and faster changing flow rate, do the following: Take the max flow
cfm value of the box you want to slow down and divide by its actuator runtime in
seconds, then divide the result by 4 and round to the nearest integer. Enter this final
value in either SUP MAX RATE or GEX MAX RATE depending on which box you want
to slow down.
Here is the formula in its simplest terms, where “
max cfm ” represents the maximum
flow value of the box that you want to slow down:
(max cfm ÷ runtime) ÷ 4 = SUP (or GEX) MAX RATE
NOTE:
This value is a good starting point. If necessary, you can raise it (must be an integer)
to speed up the rate of flow change, or lower it to further slow down the rate of flow
change, while observing the overall stability of your setup.
To accommodate the largest variety of room configurations and pressurization needs,
the rate limit will ONLY be applied when the actuator is moving in the direction that, if
left uncontrolled, could cause pressurization reversal. See the following table:
SUPPLY
GENERAL EXHAUST
OCC DIF STPT
(or
UOC DIF STPT) <0
Maintain
positive
pressurization
The SUP MAX RATE is
applied to supply closing, but
not to supply opening.
The GEX MAX RATE is
applied to general exhaust
opening, but not to general
exhaust closing.
OCC DIF STPT
(or
UOC DIF STPT) >=0
Maintain
negative
pressurization
The SUP MAX RATE is
applied to supply opening, but
not to supply closing.
The GEX MAX RATE is
applied to general exhaust
closing, but not to general
exhaust opening.