Outdoor metering and measured saturation temps

[mburns]

I have quick question which has been bugging me, asked around, researched a bit all to no avail.

Concerning some smaller Daikin equipment in particular smaller sized residential ducted units. Basically they are metered outdoors, so esentially that smaller pipe is part of your evaporator. Now measuring the pressure from this port on many different units i have found the saturated pressure of this line to be between 15-20c with the suction to
be within the 4-6c mark quite commonly. This is not all units but mainly 4-6hp side discharge Daikin units, whereas some other Daikin units i've tested this line saturation temp measured is around the SET.

Now my main question is where does the extra pressure loss come from in order to reduce to the lower SET? Could it just be part of the design process of piping within the indoor unit? And if so what are the advantages of a process such as this?

Originally i concluded the my gauge readings we're wrong, as there is a triple state flow of liquid, saturated liquid/vapour and vapour and thought gauge reading were only accurate reading a full state of liquid or vapour. But as soon as the line temp is measured this corresponds to the equivalent saturation temp.

Thanks in advance!

[Brian_UK]

The line from the outdoor unit is indeed used as a slow expansion extension to the primary device.

They use it to reduce the operationg pressures within the pipeline system. When you consider the potentially higher pressure that would be in the 'liquid' line when using gases like R410a it makes sense.

[mburns]

Actually had the opportunity today to research a bit more in front of a unit.

What i did find is that between the outdoor EEV and just before the distributor in the evap there was around a 4-5K difference in line temp (approx 8-10m long line), and after the distributor there was once again quite some noticeable temperature difference.

What i believe is happening is that with the presence of vapour within this line, creating more volume, thus higher velocity therefore higher pressure drop within this line, compared to a same sized line with a full liquid column. So as the pressure drops liquid is flashing off, absorbing the heat from the remaining refrigerant to cool it down.

Not sure if double metering would be the correct term to use?