[PURGE] Purge optimization

Start purge optimization

Purge optimization starts, when the following conditions are met:

• The plant is started ([EnFnct] = 1).
• The plant was switched off for at least the minimum switch-off time [TiOffMin] ([EnFnct] = 0 for at least [TiOffMin]).
• The outside temperature [TOa] is lower than the low limit for outside temperature [TOaLoLm].

Calculate purge time

The estimated purge time [EstmPgti] is calculated based on the outside temperature [TOa] and the characteristic as per the following setting values:

• Maximum purge time [TiPrgMax]
• Minimum purge time [TiPrgMin]
• Design temperature low [TDsgnLo]
• Lower limit for outside air temperature [TOaLoLm]

Calculate valve position

The setpoints for the maximum valve position [VlvMaxLm] and minimum valve position [VlvMinLm] during the purge are calculated based on the outside temperature [TOa] and the characteristic as per the following setting values:

• Setpoint for maximum valve position [SpVlvMax]
• Setpoint for minimum valve position [SpVlvMin]
• Design temperature low [TDsgnLo]
• Lower limit for outside air temperature [TOaLoLm]

During purge, the maximum valve position [VlvMaxLm] and the minimum position [VlvMinLm] are set to the same value.

The maximum valve position [VlvMaxLm] and the minimum valve position [VlvMinLm] differ prior to and after purge.

Example for [TOa] = [TDsgnLo]:

Vlv = Valve position

Example for [TOa] = [TOaLoLm]:

Vlv = Valve position

During active purge optimization, the maximum valve position is sent to output [VlvMaxLm] and the minimum valve position to output [VlvMinLm].

Purge function process

1. The purge function starts by switching on the plant, i.e. [EnFnct] = 1. An active purge function is displayed at output [Prg] with value 1.
2. During purge, the outputs for the maximum valve position [VlvMaxLm] and the minimum position [VlvMinLm] are set to the calculated value. The valve moves to the corresponding position and the heating coil starts to heat. Output [VlvLmd] indicates that the damper position is limited by the purge function.
3. In addition, output [DmpMaxLm] is set to 0 [%] and the air damper is thus closed. Output [DmpClsd] indicates that the damper position is maintained in the closed position by the purge function.
4. After the calculated purge time expires, the maximum permissible valve position [VlvMaxLm] is set to 100 [%]. The minimum valve position [VlvMinLm] is set to the setpoint at the end of the purge time [SpVlvPrg], unless the present value from [VlvMinLm] is less than [SpVlvPrg].
5. If external logic determines that the heating coil is sufficiently warm prior to the end of the purge time, the purge time can be concluded early over input [CnclPrg].
6. After the calculated purge time expires, the valve is deployed to the minimum valve position [VlvMinLm]. The time for closing is based on the valve close time set on input [TiClsVlv]. While closing, the output [VlvLmd] indicates that the valve position is limited by the purge function.
7. The delay [DlyOpnDmp] starts as soon as the minimum valve position [VlvMinLm] reaches 0 [%].
8. The air damper may only start to open after the delay expires. The time to open the air damper is based on the opening time for the air damper set at input [TiOpnDmp]. While opening, output [DmpLmd] indicates that the damper position is limited by the purge function.
9. The minimum switch-off time off [TiOffMin] restarts if the plant is switched off ([EnFnct] = 0). If the plant is switched on again within in this time, the purge function does not start, since the heating coil is sufficiently warm and therefore protected against freezing.

Vlv = Valve position

Dmp = Damper position

Cancel purge function

Purge function is canceled in the following cases:

• The plant is switched off ([EnFnct] = 0).
• If the outside temperature [TOa] exceeds the low limit for outside temperature [TOaLoLm].

After cancelation, the purge function is only switched on, if the plant is restarted ([EnFnct] = 1).