About PID

TECH’s controllers may be divided into three groups due to the type of the output signal controlling the flow:

• Regulators with a non-continuous output signal (two-state – on / off) – the blow-in is switched off after the set temperature on the boiler is reached, the drop of temperature below the set value plus hysteresis, the controller switches on the fan again. It is possible to adjust the blow-in force in the regulator. The disadvantage of this type of adjustment is the temperature oscillation around the set value. In practice, this does not have a great significance in the case of high thermal inertia of the facility (heated building).

• Regulators with a non-continuous output signal (two-state-on / off) plus “sigma” adjustment. When the temperature on the boiler is reaching the set value, the controller gradually reduces the fan’s rotations, and the blow-in is switched off after the set temperature is reached. The drop of temperature below the set value activates the blow-in, initially at a lower gear. If the temperature is still dropping, the regulator gradually increases the fan’s rotations. The oscillation around the set value takes place in a milder manner. Overregulation is smaller. Of course, this also depends on the thermal dynamics of the adjustment’s object.

• Regulators with a continuous output signal zPID (using the PID adjustment algorithm). The blow-in power in these controllers is calculated on the basis of the measurement of the boiler’s temperature and the temperature of flue gas measured on the boiler’s outlet. The fan operates in a continuous manner in time and the blow-in power depends directly on the measured boiler temperature, the temperature of flue gas and the difference of these parameters and their set values. Stable maintenance of the set temperature without any unnecessary over regulations and oscillations are the advantages of the zPID regulator. Heat is not blown out unnecessarily through the chimney. Savings in fuel combustion may range from several to more than ten percent. It is important to install the appropriate fan.

The results of tests conducted on a charging boiler with the use of a controller with a non-continuous output signal (two-state-on/off) as well as with a zPID controller are presented below.

Diagram Presenting the Operation of a Boiler with a ST-81 Controller

An ST-81 controller with hysteresis of 2°C. Oscillations around the set value typical of two-state adjustment. If the boiler is reacting to temperature changes too slowly, the deviation (up or down) as compared to the set value may be too high. The negation of the application of this control in CH boilers would be an overstatement. In most cases, these regulators can handle the heating of buildings well and the accuracy of maintaining the set temperature is satisfactory.

Diagram Presenting the Operation of a Boiler Equipped with an ST-81 zPID Controller

The ST-81 zPID controller. The first overregulation taking place after firing up in the boiler may be seen in the diagram. Then, our regulator handles the maintenance of set temperature on the boiler very well. The adjustment is smooth, the measured temperature remains at the set level despite the instability of the adjustment’s object, namely the CH boiler and the heated building. The curve below the supply and return temperature presents the temperature of flue gas measured on the boiler’s outlet. This is the second basic parameter used for adjustment, apart from the set temperature.