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Working with a 10 K temperature difference over the evap coil
and an evaporating temperature approximately 10 k bellow air off,
if average room temperature is 20 deg C then air on evap is about
25 deg c air off about 15 ish?

If air off evap is about 15 ish then evaporating temperature would
be about 5 ish deg c. If air on evap is 25 ish c then superheated suction
vapour would be 4 k to 7 k above saturation temp and obviously no
higher than ambient.

5 dag c evaporating temp and 4 to 7 k superheat would leave the suction
return temp at between 9 deg c and 12 deg c.

Assuming the saturated liquid line is in contact with the suction for half the
distance then the two pipes will have a temperature difference of at least
the superheated vapour value of between 4 and 7 k.

But inside the insulation the two pipes will exchange heat to the air space
around the pipe and inside the insulation so heat exchange will still take place
but at a reduced level.

Assuming that level is half the level of the contacting pipes, you could imagine
the difference to be 2 to 3 k instead of the 4 to 7 k.

That means over the length of the 18 mtrs there is a temperature difference of
on average maybe somewhere about 4 ish k????

If the evaporating temperature is increased that means that the suction temp
will be decreased, the return temperature to the compressor will be reduced by
a few degrees as the evaporating temperature is increased by a few degrees??

That means the evaporating temperature is not plus 5ish it is plus 7ish so the air
off is going to respectively higher and so is the room temperature?????

For the system to deliver the same duty as the double insulated system the compressor
it will have to work a little longer or harder??? If the suction return is cooler
the condensing temperature would be lower ( by how much I do not know) so
that would affect the condensing temperature and that in turn would affect
the overall performance of the system?????

Guesswork and assumptions I know, I have no proof to back this up but
City & guilds have a figure for AC systems that claim every degree reduced
between the evaporating temperature and the condensing temperature
increases efficiency by 3% and reduces costs by 3%, if that is true the extra
workload of the two pipes touching could effect the whole system by at least
1 deg c and maybe as much as 4 deg c????

That could be an increase in costs of at least 3% and maybe as much as 12%???

But if the discharge is reduced as a result, maybe that would have an effect on
the subcooling. Subcooling with air cooled condensers can only achieve about 4 k to
7 k at best, but if the subcooling is reduced then there would be more physical
liquid present after the metering device and not as much flash gas??

When I started this I thought I would prove that the system is bound to be less
efficient, but after thinking about it and writing it out long hand, I can say I
honestly don't know. My gut reaction is to say it is wrong but if I'm honest it
would take a lot better person than me to prove this one way or the other???

No help at all, Rob

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