FB_BA_RcvMonit

The function block FB_BA_RcvMonit is used to calculate the efficiency of an energy recovery system.
The function block requires the following measured temperature values to calculate the efficiency (the so-called heat recovery coefficient):

• Outside air temperature fTOts
• Exhaust air temperature fTExh
• Air temperature of the energy recovery system in the supply air duct (alternatively: in the extract air duct) fTAftRcv

The function block logs the temperature values every 10 seconds and forms minutely averages from 6 consecutive values. The results are used to check whether the plant has reached a "stable" state.

• This is the case when the recorded temperatures of outside air, exhaust air and air after energy recovery are almost constant, i.e. none the 6 individual values deviate by more than 0.5 K from the respective average value.
• The temperature difference between outside air and exhaust air is at least 5 K.

If this is the case, this measuring cycle is acknowledged with a TRUE signal at output bStblOp, and the calculated efficiency is output at fEffc. If the state is not "stable", a FALSE signal appears at output bStblOp, and fEffc is set to 0.

In any case, each measuring and analysis cycle is marked as completed with a trigger (a TRUE signal lasting one PLC cycle) at bNewVal.

Enable (bEn) and Reset (bRst)

The function block is only active if a TRUE signal is present at bEn. Otherwise its execution stops, and all outputs are set to FALSE or 0.0.

An active measuring and evaluation cycle can be terminated at any time by a TRUE signal at bRst. All outputs are set to FALSE or 0.0, and the measuring cycle restarts automatically.

Selection of the temperature value "after recovery" (bSnsRcvTExh)

A FALSE entry at bSnsRcvTExh means that the temperature measurement after the heat recovery in the supply air duct is used for calculating the efficiency.
To use the temperature measurement after the heat recovery in the exhaust air duct, TRUE must be applied at bSnsRcvTExh.

Limit value exceeded (fContrVar, fLmtEffc, bLmtRchd)

A limit violation has occurred, if the calculated efficiency is less than the specified limit value rLmtEffc, and at the same time the control value for the heat recovery is at 100%. For this purpose, the manipulated variable must be connected to the input fContrVar.
The limit violation message can be delayed by an entry at nLmtVioDly_sec [s]: If the two criteria, violation and override, are present for longer than nLmtVioDly _sec [s], this is indicated by a TRUE signal at bLmtRchd.
A warning message that has occurred disappears if a complete measuring cycle provides "good" values or if there is a rising edge at bRst or if the function block is deactivated.

This warning message only occurs if the plant is in a stable operation mode (bStblOp=TRUE).

Taking into account the temperature increase of the exhaust air due to the fan motor (fTIncFan)

It is possible that the outlet air is warmed by a fan motor, resulting in distortion of the measurement. This temperature increase can be specified through fTIncFan. Internally, the measured outlet air temperature is then reduced by this value.

Inputs

VAR_INPUT
  bEn               : BOOL;
  bRst              : BOOL;
  fContrVar         : REAL;
  fTOts             : REAL;
  fTExh             : REAL;
  fTAftRcv          : REAL;
  bSnsRcvTExh       : BOOL;
  fTIncFan          : REAL;
  fLmtEffc          : REAL;
  nLmtVioDly        : UDINT;
END_VAR

Outputs

VAR_OUTPUT
  bNewVal      : BOOL;
  fEffc        : REAL;
  bLmtRchd     : BOOL;
  bStblOp      : BOOL;
END_VAR

Requirements