FB_BA_FrstPrtc

Functional description

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 a frost risk is detected, the output bOn is set and the timer T1 is started. If the frost risk persists (bFrst=TRUE) once the time udiT1 (seconds) 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 (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. Timer T2 is started with the plant restart. If another frost alarm occurs within a period of udiT2 (seconds), 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, 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 [s])
t1 = frost alarm off, resetting of bOn, output of hardware pulse, start of timer T2 (udiT2 [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     : UDINT;
udiT2     : UDINT;
udiAlmCnt : UDINT;
bRst      : BOOL; 

bFrst: connection for frost events on the air and water side

udiT1: timer for restart delays [s]

udiT2: timer monitoring time [s]

udiAlmCnt: maximum number of automatic plant restarts without reset. The maximum possible entry is 4; it is internal limited.

bRst: resetting and acknowledgement of the frost alarm

VAR_OUTPUT

bOn       : BOOL;
bHWRst    : BOOL;
udiRemTi1 : UDINT;
udiRemTi2 : UDINT;
bAlmLck   : BOOL;
udiStaCnt : UDINT;

bOn: frost alarm active

bHWRst: output of a pulse for acknowledgement of the frost protection hardware

udiRemTi1: time remaining to plant restart after frost alarm

udiRemTi2: remaining monitoring time

bAlmLck: alarm lock - stored alarm

udiStaCnt: status counter – current number of unacknowledged false starts

Requirements