Evaporator

[skyboy6142]

Dear Friends,

Would you please tell me why in DX evaporator we fed evaporator form top and suction is from bottom and for example in water chiller evaporator we fed evaporator form bottom and suction is from top ?

If problem is oil return why in water chiller evaporator we fed from bottom of evaporator ?

Thanks for sharing your opinion with me.

[Rob White]

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Oil is a factor but it's to do with pressure drop and liquid vaporising.

If a DX coil had a large pressure drop then a distributor would be used to share the flow through many passes on the coil to share the heat exchange out.

In smaller DX systems there is just one pipe that goes back and forward 10, 15 or 20 times.
If you fill the evaporator pipe up at the bottom the liquid will sit in the bottom third of the coil
and evaporate there so reducing the surface area of the heat exchange.
By allowing it to go in from the top gravity will play it's part and distribute the liquid throughout the whole coil.

Remember that DX coils have expansion devises that measure the superheat out of the evaporator
so the valve will open and close dependant on the temperature of the gas out of the coil.

As for the oil, yes it does make it easier for oil to return but that can also be resolved using
the correct pipe sizing and oil traps.

In a water chiller because of the nature for water to be a better heat exchanger than air the evaporator
coil will be filled two thirds full of liquid unlike the forced air evaporators one third.

There will be a lot more complicated reasons but that is what I was always told :)

Rob

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