Sub cooling from a condenser

[Drew]

Hi all,

how does a condenser design influence it's sub cooling?
The way I understand it is that the bigger the condenser the lower the head pressure .
the smaller the condenser the higher the hp.
Does it stand to reason that the bigger the condenser the more sub cooling and the smaller the condenser the less sub cooling we achieve? We can further increase the sub cooling by further cooling the liquid line but I am wondering just about condenser sub cooling, people sometimes talk about the last few rows of the condenser being designed to achieve better sub cooling?
any ideas?

[RANGER1]

I might be wrong but does not give more subcooling, instead just a lower condensing pressure or closer to ambient , which makes more efficient.
In some cases it causes problems with Tx valve due to smaller pressure drop, even if all fans turned off.

[Rob White]

.

Subcooling is liquid cooled below the Saturation temperature (condensing)
and is very dependant on what you are transferring the heat into.

Air is a poor heat transfer so the temps are always higher, water is better
but Evaporative condensers are the best, they spray water and then draw
air over the coil to increase the cooling effect by evaporation.

In air, you can only cool down (theoretically) to the ambient air temperature,
but in reality you will not cool as low as ambient and there will always be a
difference. If an air cooled condenser is designed and sized well it will only
give about 4 degC maybe 6 degC at a push, subcooling. you can never cool
lower than ambient (in air).

Oversizing the condenser will give greater heat exchange but if the ambient is high
the condensing temp will be high and subcooling will only be a few degrees lower.

The trouble with oversizing the condenser is in the cooler months of the year, if the
condenser is oversized, in the winter the condensing saturation temp will reduce too
much and then you have problems with liquid logging in the condenser and receiver.

Subcooling, when done correctly does increase efficiency but you only get a limited
amount of subcooling for free. If you want increased subcooling then you have to
design and pay for the privilege.

Some AC systems do not have a receiver so they have an oversized condenser.
The last few passes of the condenser act as a liquid receiver and also as a liquid
subcooler.

Most proper subcooling is done by subcoolers designed into the liquid line to cool
the liquid down below ambient temps, but this does come at a cost and can cause
it's own problems.

Rob

.

[BradC]

If I look at the serpentine condenser that came out of my car as an example, there are three condensation circuits in parallel which then manifold down into a single circuit for about 8 passes as a sub-cooler. The idea is that the condensing happens in the parallel circuit, and once it's a liquid a single circuit has enough flow capacity to sub-cool the liquid.

The parallel flow condenser that replaced it has two circuits. The top 80 off % of the condenser is the condensing portion, the bottom remainder is the subcooler. It works *much* better than the condenser it replaced.

[Segei]

Subcooling means temperature of the liquid refrigerant below condensing temperature. Small condenser can give condensing temperature 95F and liquid cooled down to 75F has 20F subcooling. Big condenser can give condensing temperature 80F and liquid subcooled to 60F has 20F subcooling as well.

[RANGER1]

Drew do you have something in mind, like a system you want to improve, or just for knowledge.

[mbc]

in theory end of bigger condenser you can reach to near ambient temp. not ambient temp.

what we do in sub-cooling we go (in temp.) under ambient temp. and our pressure is a ambient.
what happen in this case . we have very small babels in inside our liquid when we cool it they became to liquid
(they change phase from gas to liquid) and when we expand it in expansion valve we get more efficiency .

[Peter_1]

In many condensors, the last rows are going up again so that the liquid collects first in a lower part of the condensor and then this collected pure liquid is flowing upwards again in some rows creating the subcool. Installing a bigger condensor will result in a lower HP, not a larger SC.

[mbc]

our liquid temp in condenser is going to reach to ambient temp.
( ambient temp +10 up 15 degree ) is a our liquid temp in normal air cooled condenser and
in water cold , temp of our water cooling in condenser + 5 degree also in vapor condenser is a same
bigger condenser only give us to use less cooling fan .
why we get more efficiency when we have sub cooling system. (for sub cooling might we use suction gas our we use economizer cooling system our some times in 2 stage system we use separator for cooling our inlet temps or other ways).
because we cool our inlet liquid (in expanding of liquid we do not use energy to cool it self and we save energy and also we remove small babels in liquid as I told before .

[mikeref]

Reading above comments has answered OP's question.

The theory is to maintain a balance of head pressure and liquid line temperature to avoid efficiency loss at the Expansion valve.....and in turn, a loss of efficiency in the Evaporator, and the ultimate rise in superheat. A vicious circle.

Subcooling can have the same effect if head pressure is compromised.
Excess subcooling reduces head pressure and liquid temperature. This reduces the TX valve's ability to feed the Evap. On a long liquid line run, insufficient pressure will cause a mixture of liquid and vapour to enter the TX.

[mbc]

Hi mikerek

in subcooling we just cool liquid and it does not effect and reduce our head pressure.

subcooling does not add any thing to our condenser it is after changing phase (our gas vapour to liquid) .

[mikeref]

Well...As we know, Condensing and sub cooling are carried out in one of the many variations of condensers on the market. Maximum and Minimum temperatures of ambient's around the world have to be considered in the design.

Variable speed condenser fans are necessary to maintain a constant head pressure in some of the 3 Phase kits in Australia.
Some single phase units designed for 40+ Degrees C have a HP switch to drop the condenser fan in say a 15 Degrees C Ambient as there is way too much Sub Cooling and pressure drop.

[Drew]

Thanks all for your replies. I've been offline so have now only now checked the forum.

[sterl]

Actually: An oversize condenser with a small amount of its height devoted to subcooling(that is trapped....) will operate at LESS subcooling than smaller one....If the Big One handles the corresponding compressors at 15 degrees over ambient as a saturated condensing temperature, the best it can possibly do is 15-deg of subcooling. If a smaller condenser is selected for 25-deg over ambient, than the best it can do is 25-deg of subcooling. This becomes useful should the rest of the circuit be such that subcooling of this extent (not 60 or 70 Deg worth) is necessary to maintain TXV flow as commonly occurs if the liquid line has a couple of verticals built in.....

[Rob White]

Actually: An oversize condenser with a small amount of its height devoted to subcooling(that is trapped....) will operate at LESS subcooling than smaller one....If the Big One handles the corresponding compressors at 15 degrees over ambient as a saturated condensing temperature, the best it can possibly do is 15-deg of subcooling. If a smaller condenser is selected for 25-deg over ambient, than the best it can do is 25-deg of subcooling. This becomes useful should the rest of the circuit be such that subcooling of this extent (not 60 or 70 Deg worth) is necessary to maintain TXV flow as commonly occurs if the liquid line has a couple of verticals built in.....

If the condensing temp is 15 degs higher than ambient (which is normal in most air cooled condensers)
the amount of energy used to lift the pressure / temperature will be a fraction of that used to lift
the condensing pressure / temperature up by 25 degs.

I can't see any advantage of lifting the condensing temp that high just to gain more subcooling
and bear in mind the subcooling will still be above ambient.

The only true advantage is when the liquid is cooled way below ambient and that can't be done
for free so a well designed system with extra / additional subcooling will improve the efficiency
of the system but the subcooling is additional to the free subcooling gained in the condenser.

Rob

.

[Grizzly]

Wise Words Rob!