FB_BA_FrstPrtc

The function block is used for frost monitoring of a heating coil in an air conditioning system.

A frost risk is present, if the input bFrst is TRUE. The frost alarm must be linked in the plant program such that the plant is switched off immediately, the heater valve opens, and the heater pump is switched on.

If there is risk of frost, the output bOn is set, and udiT1_sec (seconds) is started. If the frost risk remains (bFrst=TRUE) after udiT1_sec has elapsed, bOn remains set. It can only be reset at input bRst.

If the frost alarm ceases due to activation of the heating coil within the time udiT1_sec (bFrst=FALSE), the plant automatically restarts. For the plant restart bOn becomes FALSE, and at output bHWRst a pulse for acknowledgement of a latching circuit in the control cabinet is issued. With the restart a second monitoring period udiT2_sec (seconds) is initiated. If another frost alarm occurs within this period, the plant is permanently locked. bOn remains set until the frost alarm has been eliminated and bRst has been acknowledged.

In a scenario where frost alarms recur with time offsets that are greater than udiT2_sec, theoretically the plant would keep restarting automatically. In order to avoid this, the restarts within the function block are counted. The parameter udiAlmCnt can be used to set the number of possible automatic restart between 0 and 4.

An acknowledgement at input bRst resets the alarm memory within the function block to zero.

Sample:

t0 = frost alarm at input bFrst, alarm message at output bOn, start of timer T1 (udiT1_sec [s])
t1 = frost alarm off, resetting of bOn, output of hardware pulse, start of timer T2 (udiT2_sec [s]), plant restart
t2 = further frost alarm within T2, alarm message at bOn, start of timer T1, locking of the frost alarm
t3 = frost alarm off.
t4 = acknowledgement of the alarm at bRst, resetting of bOn.

VAR_INPUT

bFrst      : BOOL;
udiT1_sec  : UDINT;
udiT2_sec  : UDINT;
udiAlmCnt  : UDINT;
bRst       : BOOL; 

bFrst: Connection for frost events on the air and water side.

udiT1_sec: Timer for restart delays [s]. Internally limited to a minimum value of 0.

udiT2_sec: Timer monitoring time [s]. Internally limited to a minimum value of 0.

udiAlmCnt: Maximum number of automatic plant restarts without reset. Internally limited to values between 0 and 4.

bRst: Resetting and acknowledgement of the frost alarm.

VAR_OUTPUT

bOn           : BOOL;
bHWRst        : BOOL;
udiRemTi1_sec : UDINT;
udiRemTi2_sec : UDINT;
bAlmLck       : BOOL;
udiStaCnt     : UDINT;

bOn: Frost alarm active.

bHWRst: Output of a pulse for acknowledgement of the frost protection hardware.

udiRemTi1_sec: Time remaining to plant restart after frost alarm.

udiRemTi2_sec: Remaining monitoring time.

bAlmLck: Alarm lock - stored alarm.

udiStaCnt: Status counter – current number of unacknowledged false starts.

Requirements