Printable View
When we increase the charge we get an increase in condensor pressure correct? As we add more refrigerant, there is more liquid at the cap tube entrance due to a larger degree of subcooling, and as the condensing pressure is rising we get more mass flow hence more capacity. But if the compressor cant shift the extra volume created by the rise in mass flow then the evaporator pressure/temperature must rise to compensate. This sounds about right
I don't know quite where to start on all that. Mix in a lot of words like 'usually', 'sometimes', 'depends', and the ever popular 'there is more to it than this', and we can make this a true and accurate statement.Quote:
When we increase the charge we get an increase in condensor pressure correct? As we add more refrigerant, there is more liquid at the cap tube entrance due to a larger degree of subcooling, and as the condensing pressure is rising we get more mass flow hence more capacity. But if the compressor cant shift the extra volume created by the rise in mass flow then the evaporator pressure/temperature must rise to compensate.
As we add refrigerant, the pressure rises until there is enough pressure to start condensing (SCT above ambient).Quote:
When we increase the charge we get an increase in condensor pressure correct?
From this point the pressure is determined by the rate of heat removal, and the pressure remains relatively constant as refrigerant is added. The rate of heat removal depends upon the capacity of the condenser/fan combination and the ambient temperature.
If we continue to add refrigerant, we reach a point where we are backing up liquid in the condenser, in effect reducing the condenser size. When the condenser is severely undersized by liquid, the pressure rises rapidly.
And as always, there is more to it than this. :D
Let's make that "resulting in a larger degree of subcooling".Quote:
As we add more refrigerant, there is more liquid at the cap tube entrance due to a larger degree of subcooling...
Yep.Quote:
and as the condensing pressure is rising we get more mass flow hence more capacity.
In a stable closed loop system, the rate of flow is equal at all points (conservation of mass flow).Quote:
But if the compressor cant shift the extra volume created by the rise in mass flow then the evaporator pressure/temperature must rise to compensate.
The point of highest resistance to flow dictates the rate. That would normally be the cap tube.
The compressor will pump whatever flow is dictated by the cap tube, first by raising low side pressure (hence evap temp), and then at the extreme by trying to pump liquid. Then things break, and the compressor suddenly and violently becomes the dictator of flow rate (or lack thereof).
And of course, there is more to it than this. :D
Exactly, but the compressor can only shift so much volume in a given time. Therefore its the rate of volume removal by the compressor that will dictate the evaporator pressure/temperature for a given mass flowrate (defined by capillary tube length).Quote:
In a stable closed loop system, the rate of flow is equal at all points (conservation of mass flow).
Do we know how to calculate the volumetric flowrate of a compressor? I can find out the displacement but I need to know the number of times the given displacement it shifted in a given time period, eg. rpm. I cannot find this data so easily, am I missing something or is the rpm of the compressor dynamic and hence why it is not showen explicitly?
You also need to know the low side vapour density. As you increase the vapour density volumetric efficiency will also increase, and therefore capacity increases.
How do you go about increasing the vapor density?
By raising low side pressure, by making the cap tube less resistive or increasing the refrigerant charge. This will also increase saturation temp, which is a bad idea for this application.Quote:
How do you go about increasing the vapor density?
I think that's right anyway :D
At the end of the day your absolute minimum evaporator temperature is dictated by your compressor size in this application with all other system components sized correctly. With a larger volumetric flowrate (bigger compressor) you can lower your evaporating temperatures.
Mass flowrate in the system is primarily dictated by the capillary tube length.
Volumetric flowrate in the system is primarily dictated by the size of the compressor.
For a given heat load at a given evaporator temperature we can calculate the required mass flowrate.
As we drop evaporating temperatures, the specific heat that can be removed reduces (assuming condensing pressure and degree of subcooling are kept constant) therefore mass flowrate must increase. Also when we reduce evaporator temperatures our specific volume of the gas increase. This has a 2-fold effect on increasing volumetric flowrate, therefore since the compressor dictates volumetric flowrate in a system we need a larger compressor to get lowest temperatures. Obviously there is a limit to how much lower you can get with a bigger compressor, although that will take some facts, figures and some maths to get an idea of the limit.
I understand the desire to bring compressor size into the selection process, and rightly so. A larger compressor drops the low side pressure and therefore increases the flowrate, but the effect is relatively small and IMO unnecessarily complicates the selection process.
It does however bring up a valid point concerning the model. The model uses atmospheric pressure, which is proper for most applications, as it seeks to optimize compression ratios for capacity. But we want lowest temperature, so it might make more sense for us to use something like .7 bara/9" hg for our model.
Russell, let's try the model with 30C condensing, 25K subcooling, 10K superheat, and .7 bara, and see what effect that has.
Unfortunatly the low side pressure has no effect on flowrate in the current models I have been using. The only purpose it serves is to calculate how much heat can be removed by the system by using enthalpies at the evaporator pressure. In fact even the most complicated models I have studied do not include the low pressure as a basis of calculation. The low side pressure is a result from the analysis of the capillary tube length and high side pressure.
When we want to get lower temperatures from the systems, compressor size seems to be the limiting factor. When we reach the current temperature limit with correctly sized capillary tube and condensor then its time to get a bigger compressor. With a bigger compressor we need to resize the capillary tube and check that the condensor can meet the extra duty. Although as we drop below 1bara, compression ratios start to rise quite quickly which will quickly reduce the efficiency of a compressor putting a bigger strain on the condensor and we end up with the law of diminishing returns which is frequently observed in the overclocking commnunity :D
The models are on my computer at home, I will have a look at them tonight. I can tell you that dropping the LP will reduce the capacity, allthough you probably allready knew that. ;)
I disagree. A bigger compressor does not change the flowrate needed to handle the heat load. It's only effect is to drop the low side pressure, which will cause some minimal increase in flow, and therefore capacity.Quote:
With a bigger compressor we need to resize the capillary tube and check that the condensor can meet the extra duty.
On the contrary, it will increase the capacity.Quote:
The models are on my computer at home, I will have a look at them tonight. I can tell you that dropping the LP will reduce the capacity, allthough you probably allready knew that.
I agree.Quote:
Originally posted by Gary Lloyd
I disagree. A bigger compressor does not change the flowrate needed to handle the heat load. It's only effect is to drop the low side pressure, which will cause some minimal increase in flow, and therefore capacity.
On the contrary, it will increase the capacity.
If you maintain the same system charge and metering device and then increase displacment in the system you will move more refrigerant. If you keep the same displacment and remove refrigerant you will decrease your suction density and decrease capacity because the mass flow rate will also decrease.
I think the relevant question here is whether the increased flow from a larger compressor warrants going to a more restrictive cap tube. I don't think that it does.
And the next question is whether the increased capacity warrants increased condenser size. I would say no to that one, too.
For the systems we are dealing with here, I believe we can properly size both the cap tube and the condenser without regard to compressor size.
I have a 1/12 hp compressor Embarco em40her ?
Using it for a paperweight?
At 53W at -35 probably...
I do have seven EM60 HER compressors,, 157 W @ -25C with R134A, Probably a good canadate for a cascade GPU I could fit two of these in a Prometeia case with heatexchanger...
Even the little one could do some cascading. :D
But it would not do enough the lower limit is about 1/8 hp
I may use four of these em60her embarco in tandem cascade This should give me about 300W at -100C with R1150.. These have plastic pistons.. don't know how they will hold up...
I beg to differ. Let's imagine for example, that we want to mod a prommie for R410A. That little 1/12 would be just about right for the high stage with R134a, the R410A prommie being the low stage. That would be a serious machine.Quote:
But it would not do enough the lower limit is about 1/8 hp
Ill try it when I get the current projects done..
I am currently using 6 (!) m of 0,78mm cap tube with r22 and it is not enough.
i put a sight glash 1 cm before the cap tube just to verify this, if i charge for full liquid in the cap tube entrance i get way too much cooling, cpu output power is measured at 110Watt.
That is about .031 inch id way too big, you don't want to use a sight glass on a captube system it acts as a small reciever and will reduce effeciency. Can you get .028 or .026. and what size compressor do you have?