What is overcondensing

[hammy]

Hey folks. About a year ago a did a 2 week refrigeration and airconditioning course. It gave me an understanding of refrigeration but when i read some of these posts I realise how little I actually know :)

When I did my course we worked on the simplest of set ups and since then ive realised this was woefully inadequate to give me any kind of real knowledge

Ive bought loads of books and browse this forum but one thing I have heard mentioned is the term overcondensing. I did a little reasearch and came across this on a website.

Often in the winter
months you hear the term
‘over condensing’. This is a
myth, as you cannot over
condense a refrigerant.

Can a system over condense and how would i know this (by measuring subcooling?). Anybody know of any decent links to articles etc....Many thanks

[The Viking]

The term "over condensing" might be miss leading or even incorrect, as from a chemical point of view, you can not "over condense".

BUT
It is an accepted, widely used, term for a very real problem in our trade.
The Condensor gets too cold, the head-pressure drops and too much liquid refrigerant gets "stuck" in the condensor. This in turn will cause the rest of the system to behave like if it were short of refrigerant (low suction pressure and high superheat, maybe even a frosted up evaporator).

You will manly experience this on cheap, comfort cooling units without head-pressure control.

[Lowrider]

The term "over condensing" might be miss leading or even incorrect, as from a chemical point of view, you can not "over condense".

BUT
It is an accepted, widely used, term for a very real problem in our trade.
The Condensor gets too cold, the head-pressure drops and too much liquid refrigerant gets "stuck" in the condensor. This in turn will cause the rest of the system to behave like if it were short of refrigerant (low suction pressure and high superheat, maybe even a frosted up evaporator).

You will manly experience this on cheap, comfort cooling units without head-pressure control.

It's actually physical, not chemical. The material just changes it state from gas to liquid and not becoming an other material!

The term to be used is excessif subcooling.

[Grizzly]

Viking has got it "spot on"!
What He describes is a real problem.
Especially on a system with a large sized condensor (air or water cooled.)
Where the system is at low load or start up.
Because of unusual circumstances,
be it colder than usual Ambient temps or High winds flowing through the condesor doing the same as if a fan or fans are running.

So you get the situation where the refrigerant gasess are being cooled before the Discharge pressure has had chance to reach the desired working discharge pressure.

The resultant lower gas pressures then have less heat in them. The high pressure side of the system is in a state of being "over-condenced"
Which causes the formentioned faults mentioned by Viking.
Grizzly

Credit to lowrider to as whilst typing this he has corrected us with the correct terminology!

[PaulZ]

Hi Hammy
To add to the previous posts the reason the system will show symptoms of being short of refrigerant is that a TX valve is sized for a set pressure drop across the valve to get the capacity (tonnage) out of the valve (this also applies to a capillary system). This is the difference between suction and discharge.
If for arguments sake the nominal capacity of a TX at a pressure difference of 1200 kpa is 10TR if you reduce the pressure difference then the capacity of the valve will be reduced. If the evap is rated at 10 TR and the capacity of the valve drops to say 6TR then the evap will be starved of refrigerant and the compressor will be oversized. This will show as very low suction, similar to short of refrigerant.
On some refrigerants this can cause oil logging in the evap.
Hope this helps you understand "over-condensing".
Paul

[hammy]

wow thanks for all the swift responses :)

The pressures and temperatures in an evapourator indicate what our space temperature should be and vice versa...but how do we know what the condensor pressure should be for different refrigerants

sorry if these are silly and basic questions. I started my working life as an electrician but fridge is always something ive wanted to learn :)

[hammy]

sorry that last post was reffering to the condesnsor pressure in an optimised state and not over condensing etc :)

[Lowrider]

wow thanks for all the swift responses :)

The pressures and temperatures in an evapourator indicate what our space temperature should be and vice versa...but how do we know what the condensor pressure should be for different refrigerants

sorry if these are silly and basic questions. I started my working life as an electrician but fridge is always something ive wanted to learn :)

Forget about pressure! Use temperature!
Then it depends on what it is designed at. For instance 42 degrees saturated liquid temperature at 32 degrees out door air temperature (dgr C). When no way of pressure regulation is used and the fan(s) is/are running and the oat is lower then designed the condensation temperature will get lower and you'll get a larger or even too large subcooling.
untill a point this will be beneficitial, depending on pressure drop in the liquid line, untill difference over the exv becomes too little.
If oat is higher then design, condensation temperature will get higher until no more subcooling can be achieved and the capacity off the evaporator will drop.

Hope it makes sence!

[Lowrider]

To make it more clear I've added a h lg p diagram.

following points 1-3-6-9 will be the ideal situation with 1 being entering compressor, 2 compressor discharge, 6 leaving condensor and 9 just after the expansion valve.

When there is no condensation pressure ragulation and oat is low there will be more subcooling (point 7).

When aot is higher the refrigirant will leave the condensor at point 5.

Without a sensing bulb, in the "over-condensation situation" the refrigirant would leave the exv at point 8 as liquid. Because of the bulp the exv would close.

If "over-condensation" would increase even more the pressure would drop so much, the exv has no way of expanding the liquid to gas!

Hopethis helps!

[Pooh]

PaulZ
is the nearest as far as I can see, basically if the pressure in the condenser drops below a certain pressure above the evaporating pressure required the refrigerant stops flowing around the system. If you are using a TEV you require a minimum of 3.5bar deltaP across the valve for it to work if you do not have this the system simply pumps down.

Grizzly
Surely if liquid backs up in the condenser would the discharge pressure not rise due to the reduction in effective condenser size?

Ian

[Lowrider]

PaulZ
is the nearest as far as I can see, basically if the pressure in the condenser drops below a certain pressure above the evaporating pressure required the refrigerant stops flowing around the system. If you are using a TEV you require a minimum of 3.5bar deltaP across the valve for it to work if you do not have this the system simply pumps down.

Grizzly
Surely if liquid backs up in the condenser would the discharge pressure not rise due to the reduction in effective condenser size?

Ian

When skipping some steps the system will stop flowing, yes, but not right away. If the pressure difference is too low, at first it will run. With a cap-tube it will keep flowing and flooding the compressor. On a exv it will go out on low pressure because the exv will close because it doesn't measure any superheat.
If the difference is too low it will flow, but there is no expansion!