Sub cooling / SH quick question?

[Daz man]

Very simple super heat & sub cooling question for you all if that's ok? Just to make sure I'm right




The scenario first R407c refrigerant on a basic air/water chiller with small plate heat exchanger.

Ambient air today 11 degrees.


Super Heat - Suction gauge line connected to the Schrader connection just before the screw compressor. Pressure 3 bar (converted I make that -4 degrees)
My Fluke thermometer is reading a suction line pipe temperature of 9.5 degrees (its not an infrared, K couple)




Sum = 9.5 - -4 is 13.5 degrees of super heat.




Sub Cooling - High side gauge line connected to the Schrader connection just before the expansion valve after the liquid inline core drier. Pressure 16 bar (converted I make that 40 degrees)
thermometer is reading a liquid line pipe temperature of 26 degrees (measured just before where my line is connected)




Sum = 40 - 26 is 14 degrees of sub cooling.




Seems to me I have high sub cooling and low super heat?




R407c refrigerant charge has been charged to data plate (72kg) so can't be low on charge, can it?




Hope this makes sense, is this the correct way to work out the equation?

thanks

[Rob White]

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Your maths is more or less correct ( I work out your liquid temp to be slightly different but not so much to worry)

Super heat needs to be slightly lower than 10 rather than above.
13 - 14 degs of Superheat could indicate undercharged or just too much
load over the evaporator. (true superheat can only be measured when the
system is nearly down to temp.
Although 13 - 14 degs of superheat seems a bit high don't worry about it because
407c is a zeotropic with a wide glide and superheat measurements always need
to be taken from the Dew (vapour) part of the comparitor.

On normal working systems the sub cooling is only about 4 - 6 degs so subcooling of
14 degs is good and the main reason is the low ambient temp.

Most air condensers run about 15 degs above ambient temperatures.
11 degs ambient means your condensing temperature will be in the region of
26 degs so if you measure it on your gauges at 30 it's not that much off
the rule of thumb and if the pipe run is uninsulated with a good clean condenser
all that has happened is your liquid has cooled down a little and under normal
conditions is nothing to worry about. Good subcooling is desirable actually because
the cooler your liquid is before the expansion valve means you will have more liquid
in the evaporator, which means higher efficiency and better results.

So it could be ok and the high-ish superheat only indicates the liquid is boiling off
too early in the evaporator, which could just be because it is flogging it's bits
off doing all the work.

Rob

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[Daz man]

Appreciate that Rob, cheers. And I understood it too!

[Rob White]

Appreciate that Rob, cheers. And I understood it too!

Phew....... It took some time saying.....

Superheat this - subcool that.............. I forget if I'm upside down or right way up sometimes :D


Rob

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