A few questions regarding temps / pressures

[Devilman]

My main question here is, does lowering the condensing temp have any real effect on system performance or capacity.

As far as I have grasped it, Lowering the condensing temps will lower the pressure required to condense the refrigerant. What does that do to system capcity or what not?

From what I know, a refrigerant boils off at whatever temp its boil point is, regardless, so what are the real benifits of lowering condensing pressure? Does it allow for more charge to be put into the system, to bring that pressure back up again to a more "normal" pressure for that refrigerant? Does it simply mean the system is "safer" becuase there is a lower high-side pressure, or am I missing something completely here?

Am I mistaken in believing you can have a refrigerant boil off at a lower temp than its normal boiling temp by running it in a vacuum (I seem to recall seeing that boiling temps are qouted at 1bar, so I asume if you run the suction side in a vacuum the boil point is lower?)

Forgive my n00bness, But I am still unsure as to what effect lowering the condensing temp (and therefore pressure) has to the overall effect of the system.

Cheers all

[Gary Lloyd]

Boiling temp = evaporating temp = condensing temp = saturation temp = etc. These are all ways of describing the same thing, i.e. saturation temperature.

This is the temperature at which a given substance will change from a liquid to a vapor if heat is added or from a vapor to a liquid if heat is removed.

And yes, the saturation temperature changes as you raise or lower the pressure.

The compressor pulls the pressure way down in the evaporator, causing the refrigerant to evaporate at a low temperature, absorbing heat in the process. The compressor then pushes the condenser up to a high pressure, causing the refrigerant to condense at a high temperature, rejecting heat in the process.

[Devilman]

Thanks gary, but something I am still unclear on, is what affect does it have on the system as a whole when you lower the condensing temp?

Could you (or anyone else for that matter) be good enough to list the advantages and disadvantages (if any) of lowering the condensing temps, with regards to what it actually does to the overall system.

Thanks

[Redwolf]

A lower Condensing P/T will give more capacity than a higher one. Too low of a Diff between the High and Low and you'll end up with to much flash gas coming out of the metering device. Which brings us to Suction pressure. Droping 10psia in the suction has more of an effect on compression ratio than raising the Head up 20psia.
Examples....
PSIA = current psi + 14.7
High side PSIA
Compression Ratio = ---------------------
Low side PSIA

Head 209.7psia/Low 23.7psia = 8.5:1 (R-22 @ 195high/9low)
Head 229.7psia/Low 23.7psia = 9.7:1 (+20psia in the High side)
Head 209.7psia/Low 13.7psia = 15.3:1 (-10psia on the Low side)

[Devilman]

I see.

So ideally you want to achieve a lower condensing temp as possible, while still maintaining enough pressure drop over the metering device to get a good liquid seal and push the liquid through the metering device.

I asume that the less flash you have the better, since more of the refrigerant goes into actually cooling the load, instead of just flashing off in the evap.

[Redwolf]

Right you are
Sub-cooling comes to play with the % of saturated liquid vs % of saturated vapor as the refrigerent exits the metering device.

Gary.. Mr old and wise man For the life of me I can not remember why to much subcooling is bad. Something to do with pure liquid hammering. In other words.. sub-cooled liquid exiting the metering device.

And something else for you to think about.

[Devilman]

Hmm, interesting.

so if increasing the lowside pressure actually makes it easier on the compressor, why do people try to run the suction side in a vacuum? to reduce the boiling point of the refrigerant?

I guess its all a balance, you find what the compressor will do at "normal" setup, then lower the highside temp to make the compressor not work as hard, then put the lowside into a vacuum to reduce the boil point, thus putting the compressor load back up again, hopefully to where it was before.

Am I right?

[Redwolf]

For these applications.. Yes, they run them in a vaccum to get a lower boiling point.

Yes in theory.. you just have to remember that with cooling the condensor with air. You can't go below the tempature of the air. Same with water.

Not to say it doesn't work.. you just end up lowering the overall effeciency and longevity of the system.

[TheDude]

Great thread! Excellent question Devilman!
Redwolf, your pressure vs. HP chart really helped me to grasp the overall balance and interplay involved.

[Devilman]

gotta love those tripple posts for boosting the thread interest :P

Anyway, aside from that, Wouldnt running the suction side in a vacuum have a tendency to try and suck some liquid refrigerant back into the compressor and slugging it to death?

Can it be avoided by say, wrapping the last few feet of the suction line around the compressor to not only help cool the compressor (minimal at best) but more importantly boil off the last of the refrigerant before it enters the compressor?

[berkut]

Originally posted by Redwolf
Same with water.

I think you can acctually go below... With a bong cooler...

[TheDude]

gotta love those tripple posts for boosting the thread interest :P

LOL...sorry about that...pc hung when I hit submit.
Being a noob at DIY this thread is a real help to me...I have to understand these interactions if I ever hope to build one myself.

[Devilman]

Im exactly the same TheDude. Im slowly gathering the parts to build one, but I need to know how systems react to what changes so I can understand why Im doing something, not just because "doing it is good"

[Redwolf]

Berkut..
Due to the evaporation.. hence refigeration effect (Cooling Towers and Steam Jet systems)
Sorry was talking about what we call dry coolers.

That table doesn't take into acount Superheat or Subcooling.. It assumes they really didn't change. Their more like, if you will, safty guide lines for the design process.
Superheat and Subcooling come more into the tweeking of the system.

Liquid flood back is only due to improper superheat.. alot of things can cause that.

[Gary Lloyd]

On the high side, less pressure means the compressor has an easier time pumping against it. Lower pressure means lower temperature, therefore the liquid being delivered to the evaporator is cooler which helps the evaporator. On the other hand, less pressure means less liquid is being pushed through the cap tube.

Subcooling is the cooling of the liquid after it has condensed. One degree below saturation is one degree of subcooling. Maximum subcooling at the evaporator entrance is a very good thing. Too much subcooling at the outlet of the condenser is a very bad thing. It means that liquid is backing up into the condenser, reducing its ability to reject heat, raising the high side pressure, and giving the compressor a hernia.

On the low side, less pressure means less vapor is being forced into the cylinder on each downstroke of the piston. The compressor is pumping less refrigerant, therefore its capacity is decreased. On the other hand, lower pressure means lower temperature of the refrigerant in the evaporator and therefore lower evaporator temperature IF the evaporator is completely flooded with this lower temperature refrigerant. That is a very large IF.

Superheat is the heating of the vapor after it has evaporated. One degree above saturation is one degree of superheat. Too much superheat at the entrance to the compressor means the evaporator is not completely flooded. Too little means the compressor is being flooded with liquid, washing out the oil and breaking things. Compressors are designed to pump vapor, not liquid. Usually the valves are the first to go, but liquid can even break crankshafts.

And to complicate things even more, both subcooling and superheat are subject to carryover, which is why we need more than one degree of each.

[Devilman]

Thanks gary, that helped a lot.

So what can you actually do to a system to influence subcooling and superheat? I asume my earlier comment about wrapping the last few feet of suction line round the compressor would increase superheat, but is this a geniune accepted way of doing things, or is it just a "bodge" which getting more superheat is the side effect of?

Thanks

[Gary Lloyd]

Hmmmmmm... Transferring heat from the compressor to the refrigerant in the suction line, which is then carried into the compressor. What's wrong with this picture?

[Devilman]

Ok yes, that was bad I guess lol.

So, lets take an example. Say (for arguements sake) you wanted to provent liquid slugging of the compressor, obviously the way to do this would be by having sufficiant superheat so that all refrigerant had boiled off before reaching the compressor, but how?

The main problem as I see it (and no doubt everyone else) is that we are esentially tuning these systems for the lowest temp possible while at a given load (which would be full load), so ideally we want the system to be "just right" when loaded.

But when the system becomes idle, load decreases and your gonna start having liquid refrigerant returning to the compressor, since there isnt a large enough heatload to boil it all off. Whats the solution? is there one? Or do we simply have to make a compromise that we must get the superheat right at idle, and accept the fact that at full load, the system isnt going to perform as well as it could?

[Gary Lloyd]

More often than not, this is not the case. If we charge the system under minimum load, with just enough superheat to prevent flooding, then we usually need to remove a little under full load in order to achieve lowest evaporator temperature. For lowest evap temp, we flood the evaporator, but not the suction line. At idle, we flood the evaporator and part of the suction line, but not the compressor. This probably means that the cap tube should be just a little bit longer, and/or the suction line could use a little more insulation.

[Devilman]

Heh, I guess it really is like you say Gary "everything is a trade-off"

Your constantly having to balance 1 scenario against another to not only achieve optimum results, but safeguard from trashing the system should the headload drop.

Ok so, with that said, how come we need to remove some refrigerant from the system once its at full load, if charged at idle?

I would have expected the system to require more refrigerant when under load than when under minimal - no load conditions, could you explain plz?

Thanks :E

[Gary Lloyd]

As you correctly pointed out, under minimum load there is less heat to boil off the refrigerant, but there is also less heat to be rejected by the condenser, thus lowering the high side pressure and pushing less refrigerant through to be boiled off. The difference between full load and minimum load in its effects on the superheat are there, but are not as great as you might think.

Going for lowest temp in the evaporator in fact may not provide cool enough vapor back to the compressor under full load to ensure long compressor life, hence the discussion between Redwolf and I about charging for minimum superheat at minimum load versus charging for maximum superheat at full load.

Yep, everything is balances and trade-offs. Whatsmore, the discussion thus far is highly oversimplified. You ain't seen nothin yet.

[Devilman]

Ahh I see what your saying now.

If you were to tune the system so it was achieving minimum evap temps then your likely to have it completely boiling off in the evap at full load and returning "excessively superheated" vapour back to the compressor, which will not only affect high side temps (and thus pressures) but also make the compressor run hot, shortening its lifespan.

[Gary Lloyd]

Exactly so. A trade-off.

[Devilman]

Hmm. perhaps you would be able to answer this one Gary.

My current compressor is rated at 160w @ -35c evap temp. Now, after doing a few calculations, My CPU (at desired oc, which I know is obtainable) puts out approx 110w. Would it be a little to hopefull to asume that with a "fairly well tuned" system, one could achieve a loaded evap temp of somewhere in the region of -35c? Im guessing -40c may be a bit too hopefull.

[Gary Lloyd]

No two compressors are exactly alike, but it is reasonable to assume that almost all within a given model will be capable of achieving their rated capacity or better. Keep in mind that the heat load is not just from the CPU. Insulation is our friend.