Carel k.Air, User Manual, Page: 88 / 192

88
Functions
k.air +030220981 rel. 1.2 - 01.12.2020
End defrost conditions
The unit ends the defrosting procedure if one of these conditions occurs:
1. condensing temperature > end defrost threshold
2. “defrost step 3: maximum time” has elapsed since the cycle was reversed
Step 1 - Action on bypass damper
When the three conditions are met, the unit activates the defrost procedure and a timer starts counting (“Defrost step 1: start delay”).
Actions:
1. The bypass damper opens according to the evaporation temperature, to prevent cold air from entering the heat recovery unit;
this continues to be crossed by the fl ow of warm return air (~ 20°C), which assists defrosting.
30220980_209_R01
100%
0%
Def_start_thr
Evap_temp
Bypass_dmp_op (%)
diff
Key
Def_start_thr Start defrost threshold
Diff Evaporation temperature delta
Bypass_dmp_op Bypass damper opening
Evap_Temp Evaporation temperature
Fig. 9.au
2. If within the Start delay time the evaporator temperature does exceed the “Start defrost thr 1°C”, the unit goes to step 2.
Step 2 - Action on the compressor
For the “Delay before reversing cycle” time, depending on the type of compressor, the action is as follows:
• On-Off : switches off
• BLDC: goes to minimum speed
In this way, the refrigerant temperature increases and the evaporator is cooled less.
If within the “Compressor OFF time” and after the “Delay before reversing cycle” time has elapsed, the evaporator temperature
does not exceed the “Defrost start threshold plus 1°C”, the unit goes to step 3.
Step 3 - Reverse cycle
When the refrigeration cycle is reversed, the roles of the evaporator and condenser are exchanged.
1. the 4-way valve reverses the cycle
2. a timer starts, used to calculate the “Defrost step 3: minimum and maximum time”
3. the
compressor:
– On-Off : starts and remains on for a time between “Min time - step 3” and “Max time - step 3”;
– BLDC: goes to maximum speed for a time between “Min time - step 3” and “Max time - step 3”
4. the unit ends the defrosting procedure when the end defrost conditions occur.
The minimum defrost time is used to guarantee a minimum defrost duration. The maximum defrost time is a safety feature that
avoids any abnormal conditions (end defrost threshold not reached). The time between defrosts is needed to prevent the unit
from defrosting too frequently and thus only partly meeting demand.
The actions in this step are as follows:
• Decreased compressor capacity to end defrost (B): the procedure reduces compressor capacity to the minimum and reverses
the cycle. The fan operates at the defrost speed and the reversing valve is activated in the heat pump position, controlled
based on the diff erence between discharge and suction pressure. As soon as this diff erence drops below the minimum valve
actuation delta (reversing valve pressure delta), or at most after 60 s, the cycle is reversed.
• Dripping (C), phase in which the compressor is stopped (On/Off ) or goes to minimum speed (BLDC) and the fans go to the
speed set for “Fan request during dripping”, awaiting the coil to complete defrosting by thermal inertia and fi nish dripping; the
duration can be set for “Defrosting step3: dripping time”;
• Post-dripping (D), phase in which the fans go to the speed set for “Fan request after post-dripping” (Ua07) so as to completely
expel any water that is still on the coil. The post-dripping duration can be set, and once it ends the compressor starts again
in normal heat pump operation.