Pure Humidifier INTAC, Инструкция по установке и руководство по обслуживанию

Страница: 12 / 32

10
PID CONTROL LOOP TUNING
For most humidification applications, the parameters in a humidifier control loop that are based off of a
room humidity sensor or return air humidity sensor should be set so that the % demand signal changes
relatively slowly. This is due to the high thermal inertia involved with boiling water. There is an
appreciable lag between the moment heat is applied to water to the moment it reaches a steady state of
steam production. Additionally, it takes typically 10
- 30 seconds for an increase or decrease of the
steam injected in the duct to be sensed by the space or return air humidity sensor. In most applications
the amount of outside air is not dramatically changed from one minute to the next. The ambient
moisture of the outside air does not change radically either. All of these reasons lend to a relatively slow
room control response.
Conversely, a supply air modulating high - limit sensor control loop should be set to react quickly to
prevent the supply air from being saturated while still allowing reduced modulation of the humidifier. The
supply air humidity sensor’s purpose is to prevent wetting. The supply control loop is there to override
the room/return air control until typical operating conditions return. This can happen on VAV systems
where the amount of total airflow is changed abruptly. It can also happen when units are first turned on
or when rotating units in redundant setups.
The PID ( P roportion, I ntegral and D erivative) terms all affect the way a control signal will react to a
change in the process variable, which in this case is the relative humidity. The actual analog (0 - 10 VDC
or 4 - 20 mA) signal from a typical humidity sensor will vary at the 0.1% level even when the room
temperature and relative humidity are held constant. An example of this is that if one looked at a
reading from a humidity sensor on a second
- by - second basis in a stable environment, the readings
would be 50.0, 50.1, 49.8, 50.1, etc. Unless the raw value of this signal is filtered (by time averaging
with menu 310 RH Input Filter) the derivative term (MENU 304) should not be used because it will add
instability to the control loop. Similarly the proportion band (MENU 301) should be wide enough (small
proportion gain) so that the control signal does not bounce up and down due to this small fluctuation of
the raw humidity reading. The proportion term will generally be between 10% to 25%. With the
proportion band set the speed of the control loop can now be adjusted by increasing or decreasing the
integral term (MENU 303). Increasing the integral term will slow down the rate the power increases
when the humidity is below set point.
One final note about the room control loop and the supply air high - limit control loop is that the two loops
have different functions and therefore should behave differently. The supply air high - limit is exactly as
its name suggests, a limit type of control. It is trying to keep the relative humidity below a particular
value. Conversely, the room/return control loop’s goal is to maintain around a particular value.