FB_BA_RcvMonit

Functional description

The function block requires the following measured temperature values for calculating the efficiency (heat recovery rate):

• Outside air temperature rTOts
• Exhaust air temperature rTExh
• Air temperature of the energy recovery system in the inlet air duct (alternatively: in the outlet air duct) rTAftRcv

The function block logs the temperature values every 10 seconds and forms minutely averages from six consecutive values. The results are used to assessed 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 rEffc. If the state is not "stable", a FALSE signal appears at output rEffc, and rEffc 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 only operates 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 heat recovery in the supply air duct is used to calculate the efficiency.
If, on the other hand, the temperature measurement after heat recovery in the exhaust air duct is to be used, a TRUE must be applied to bSnsRcvTExh.

Limit violation (rContrVar, rLmtEffc, 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%. To this end the control value must be linked to the input rContrVar.
The limit violation message can be delayed by an entry at udiLmtVioDly: if the two criteria, violation and override, are met for longer than udiLmtVioDly [s], this is indicated with a TRUE signal at bLmtRchd. The occurrence can be tracked at the countdown output udiRemTiLmtRchd.
A warning message, which may have occurred, is canceled if a complete measuring cycle provides "good" values, or with a rising edge at bRst or deactivation of the function block.

Such a warning message can only occur, if the plant is in stable operation (bStblOp=TRUE).

Taking into account the temperature increase of the outlet air due to the fan motor (rTIncFan)

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

VAR_INPUT

bEn          : BOOL;
bRst         : BOOL;
rContrVar    : REAL;
rTOts        : REAL;
rTExh        : REAL;
rTAftRcv     : REAL;
bSnsRcvTExh  : BOOL;
rTIncFan     : REAL;
rLmtEffc     : REAL;
udiLmtVioDly : UDINT;

bEn: enable of the function block

bRst: reset - all determined values are deleted

rContrVar: control value for the heat recovery, i.e. the actual value

rTOts: outside temperature

rTExh: exhaust air temperature

rTAftRcv: temperature after energy recovery

bSnsRcvTExh: temperature at the measuring point after energy recovery: FALSE -> in the supply air duct (SupplyAir) - TRUE -> in the exhaust air duct (ExhaustAir).

rTIncFan: temperature increase due to fan

rLmtEffc: limit value efficiency

udiLmtVioDly: limit violation delay [s]

VAR_OUTPUT

rEffc            : REAL;
bStblOp          : BOOL;
bNewVal          : BOOL;
bLmtRchd         : BOOL;
udiRemTiLmtRchd  : UDINT;

rEffc: efficiency

bStblOp: stable operation

bNewVal: output trigger new value rEffc

bLmtRchd: limit value reached

udiRemTiLmtRchd: countdown timer limit value reached

Requirements