[SP_FIELD] Determination of room setpoints via setpoint field

The following functions are integrated in the function block:

• Conversion of relative humidity values into absolute humidity values.
• The limits for the setpoint field can be set.
• Summer/winter compensation is possible.

The function block calculates the variable setpoints for sequence cascade control from the present room air state in accordance with the parameterized limit values (setpoint field). The parameterized relative humidity limit values are then converted to absolute humidity values.

Example: Energy-free zone

Example: Cooling/humidifying

Example: Heating/dehumidifying

Parameterizing the setpoint field

The humidity setpoints [SpRHu] and [SpRDHu] are parameterized in % of relative humidity.
The absolute setpoint for humidity [SpRHuAbs] is parameterized in g/kg of absolute humidity.

The limit values for the setpoint field are preset according to DIN 1946.

Basic design

SP_FIELD typically serves as a setpoint transmitter for sequence cascade control. SP_FIELD supplies the setpoints [SpHiTR], [SpLoTR] to temperature control, and the setpoints [SpHiHuR], [SpLoHuR] to humidity control.

Summer/winter compensation

The room setpoints for heating [SpRH] and cooling [SpRC] can be changed via pins [DSpH] and [DSpC], e.g., by interconnecting to an outside temperature-controlled setpoint compensation (characteristic curve of CHAR) or to a room unit.

Example: Outside temperature-controlled summer compensation

CHAR (CONTROL) supplies an outside temperature-controlled correction value for cooling based on the following sample points:

The set cooling setpoint [SpRC] is corrected by factor [DSpC] in dependence of the outside air temperature TOa. Thus, the effective cooling setpoint is corrected to 27 °C for a cooling setpoint [SpRC] of e.g. 25 °C at an outside air temperature of 32 °C.

Example: Outside temperature-controlled winter compensation

CHAR (CONTROL) supplies an outside temperature-controlled correction value for heating based on the following sample points:

The set heating setpoint [SpRH] is corrected by factor [DSpH] in dependence of the outside air temperature TOa. Thus, the effective heating setpoint is corrected to 24 °C for a heating setpoint [SpRH] of e.g. 20 °C at an outside air temperature of -10 °C.

Parameterizing air pressure [AP]

The air pressure influences the conversion of relative humidity to absolute humidity. If the air pressure is not available as a measured variable, it can be defined as a constant.

Air pressure decreases at increasing altitude above sea level; the constant thus needs to be defined in dependence of the altitude above sea level:

Setpoint field

The function block allows the user to define the setpoints of the air conditioning plant according to a Comfort field. Humidity and temperature setpoints are defined in a physical unit [% rel. humidity] in a function block and are transmitted as absolute humidity for the humidity control loop.
SP_FIELD allows for parameterizing room setpoints, which include temperature comfort in a room (setpoint field). Thermic comfort exists when human beings are content with temperature and humidity of the surrounding environment. The range of the comfort room temperature and humidity is described in DIN 1946 Part 2 and is considered state-of-the-art technology.

There are two reasons why a field is used as a setpoint:

• Comfort sensation:
  Optimal working environment conditions exist not just at a specific setpoint for room temperature and room humidity, but within a particular range. Additionally, it is meaningful to limit the absolute humidity at high temperatures; else, the air is too muggy. This limit value typically is 11 g/kg.
• Energy savings:
  The greater the setpoint field area, the greater the energy saving potential.

Summer/winter compensation

Today, summer/winter compensation represents a standard solution for ventilating and air conditioning plants. In this respect, the setpoint of the room temperature is shifted in dependence of the outside air temperature, e.g. with the characteristic curve of CHAR (CONTROL).
In summer operation, increasing the cooling setpoint prevents excessive temperature differences between the outside air temperature and the room temperature. Additionally, cooling energy is saved.
In winter operation, the influence of colder surrounding surfaces is balanced out by increasing the heating setpoint.

If the heating setpoint [SpRH] is greater than the cooling setpoint [SpRC], the cooling setpoint is corrected:
[SpRH] = [SpRC] - 0.5 K.

If the humidification setpoint [SpRHu] is greater than the dehumidification setpoints [SpRDhu], the dehumidification value is corrected:
[SpRHu] = [SpRDhu] - 5.0 %.

In these cases, the outputs are calculated according to the corrections.

The setpoint low: Room humidity [SpLoHuR] is also corrected if SpHiHuR - SpLoHuR is less than 0.5 g/kg.
SpLoHuR = SpHiHuR – 0.5 g/kg.