General Expansion Valve Control - Daikin SkyAir Service Manual

SiBE28-804
5.2

General Expansion Valve Control

Outline After the start up control function has been terminated the general expansion valve control
function will regulate the expansion valve opening in function of the target suction SH value.
The discharge SH value will be used to set the target SH value.
The measured suction SH value will be used to control the opening of the expansion to the
target SH value.
Details When the unit is in cooling or heating operation the opening of the expansion valve will be
controlled in order to keep the amount of superheat at the evaporator outlet constant. This way
the evaporator can be used at maximum efficiency under all conditions. The initial target heat
exchanger outlet superheat value = 5°C.
The target heat exchanger outlet superheat value can be increased in case the discharge
superheat value decreases.
The target heat exchanger outlet superheat value can be decreased in case the discharge
superheat value increases.
Control During normal control 2 situations can decide on the expansion valve opening degree:
1. Target superheat amount:
2. Frequency change: At the time of compressor frequency change, the expansion valve
Calculations The heat exchanger outlet superheat value is calculated from the saturated suction temperature
Te(using LP sensor) and the suction pipe temperature R4T : SH = R4T-Te
RZQ71,
RZQS71·100
The discharge superheat value is calculated from the saturated discharge temperature Td (HP
value calculated out of PI, frequency and LP) and the discharge pipe temperature R3T : SH =
R3T-Td
Calculations The heat exchanger outlet superheat value is calculated from the saturated suction temperature
Te(using indoor coil sensor in cooling, outdoor coil sensor in heating) and the suction pipe
RZQ100~140,
temperature R4T : SH = R4T-Te
RZQS125·140
The discharge superheat value is calculated from the saturated discharge temperature Td (HP
value calculated out of PI, frequency and Te) or Tc and the discharge pipe temperature R3T :
SH = R3T-Td or R3T or SH = R3T-Tc (whichever is the lowest)
Functional Description
When the target heat exchanger outlet superheat > actual heat exchanger
outlet superheat --> the expansion valve will close.
When the target heat exchanger outlet superheat < actual heat exchanger
outlet superheat --> the expansion valve will open.
The superheat amount is checked every 10 seconds.
opening will be changed with a fixed value. This value will be in function of the amount of
compressor frequency change.
Expansion Valve Regulating Functions
79