Expansion device placement

[narkom]

Hi everyone!
In almost all the split systems an expansion device (cap. tube or EEV) is installed in outdoor unit.
To my shame I cant understand the reason for this, because in this case the refrigerant starts to boil long before the evaporator.
Can anyone give an explanation?
________
FERRARI 195 S HISTORY (http://www.ferrari-wiki.com/wiki/Ferrari_195_S)

[nike123]

Hi
everyone!
In almost all the split systems an expansion device (cap. tube or EEV) is installed in outdoor unit.
To my shame I cant understand the reason for this, because in this case the refrigerant starts to boil long before the evaporator.
Can anyone give an explanation?When system is in heating mode evaporator is in outdoor unit and expansion device is also in outdoor unit.

Also, refrigerant cannot boil without heat. It needs heat to change state. What is inside pipe between expansion device and indoor heat exchanger in cooling mode is flash gas and liquid refrigerant.


Flash gas in a TEV is a different thing altogether and a little more difficult to explain. It's a normal occurrence and is part of the function of the TEV. Liquid refrigerant enters the TEV at a high pressure and compared to the evaporation condition, high temperature.
We all understand the pressure loss through a TEV but typically, as in an A/C application, the temperature difference can be 30 deg C or more. Whatever the application, there will be a TD in the refrigerant across the valve. At the point of exit from the TEV orifice we have to drop this temperature at the same time as the pressure drops, to the equivalent of the evaporation temperature. This is done through the spontaneous 'sacrifice' of a small portion of the mass of liquid that evaporates (or "Flashes"). To do this it takes heat from the surrounding body of liquid and drops its temperature. These two things happen simulataneously and together, they cannot be separated.http://www.refrigeration-engineer.com/forums/showpost.php?p=6009&postcount=2

Look at that portion of piping as part of expansion device without much pressure drop. Only in heat exchanger with addition of heat from air we have refrigerant boiling. In that pipe, only available heat is from liquid refrigerant, who entered in expansion device at higher temperature, and in moment at what that liquid cools to saturation temperature corresponding to pressure in that part of pipe, it stops boiling and producing flash gas.

For example, in typical AC unit only 15% of weight of liquid refrigerant turns in to flash gas, and that cools down liquid refrigerant who entered at 40°C to 5°C but that 15% of flash gas takes 90% of volume in pipe after expansion device but rest of 85% weight of liquid refrigerant is still liquid and takes only 10 % of pipe volume.

So, generaly, it doesnt matter in what part of split system is expansion device. Grizzly has explained why is better to be in outdoor unit.

[Grizzly]

Hi. Narkom.
If it is a Heat pump then outside makes more sense.
Also it allows more different style evaporators to be sized to the outdoor condensing unit.
Also from a maintenance point of view a indoors mounted expansion valve would be a nightmare sometimes.
I suspect there are more reasons than I have stated.
So over to whoever?
Grizzly

[narkom]

Ok, gentleman! Thanks for answers.
Let us look at the problem from another side. I mean maximum height difference between outdoor and indoor units. At what case it may be bigger?
a) at the exit of outdoor unit we have good sub cooled 100% liquid refrigerant (exp. devise is in indoor unit)
b) at the exit of outdoor unit we have flash gas 90% of pipe volume and liquid refrigerant 10% of pipe volume (exp. devise is in outdoor unit)
________
WEED (http://www.youtube.com/dispensaries)