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hi all
I am trying to understand what causes the compressor limits / envelope , or to be more precise the maximum evaporating temperature SST given by the manufacturer.
In that is it the temperature of the return gas that would give insufficent cooling to the suction cooled windings or is it the pressure relevant to the SST that would overload the compressor due to a high suction pressure ?
If anyone could help me it would be great;y appreciated
Thanks
Limits,
Understanding the compressor first.
If it is a fixed speed, the it can move a certain volume. How dense the inlet refrigerant you are moving and how far you are moving it determines how much power you require(suction to discharge) "Like driving a car up a hill."
Then you need to know the efficiency of the compressor, both mechanically and electrically (for semi and hermetic)
This gives you your upper limit.
When you have low(er) suction pressure, (you have less refrigerant mass) you increase your compression ratio, which increase the discharge temperature (which if it gets to hot can cause problems with oil and some parts of the machinery), If in some cases the motor inside the compressor is gas cooled (often known as suction cooled compressor) then a certain amount of mass of refrigerant is required to remove the heat from the motor. The refrigerant internally becomes greatly superheated.
These combinations determine the lower limits.
You also have limits on minimum compression ratios, valves can chatter if ratios are to low
Plus the manufactures are sometimes a little conservative especially around maximum SST.
Hope this is enough
Exactly so. I especially like the car-up-the-hill analogy.Quote:
Originally Posted by mad fridgie
Can this motor drive the car up the hill without overloading?
That depends:
a. How heavy is the car (mass refrigerant flow)?
and
b. How steep is the hill (compression ratio)?
Cheers Gary, I think that explains it very well:DQuote:
Originally Posted by Gary
Thanks Guys
that helps me
great analogy Gary!
So I guess I am trying to say if i can keep the suction pressure down with a CPR that will stop the compressor overloading , but i think the return gas could be above the maximun SST given as the evaporatoe pressure temp could be high . ?
May be your are confusing SST (saturated suction pressure, or in other words a temperature and pressure at which both liquid and vapour exist with no further changes happening like in a closed refrigerant bottle) and superheat. You have a maximum SST (maximum weight of the car) and controllerd by the CPR and maximum superheat (the difference between the SST and the actual temperature) if the superheat is to high, then discharge temp can be to high.Quote:
Originally Posted by kfjoe
The CPR should not be used as a main control but for short term protection (after defrost, or short bursts of high load)
How the superheat is affected by the CPR is dependent on the system. (this is where you will get different points of view) A CPR causes a pressure drop thus in therory increase superheat, but A CPR resticts the flow out of the evaporator thus allowing the evap to have more liquid in than normal, possibly causing a slight wet suction, so when this wet suction goes through the CPR at little bit of phase change can happen and no excessive superheat occurs (a bit hard to write in a few words)
This is an area where mad fridgie and I have a technical disagreement. :)Quote:
Originally Posted by kfjoe
I say that the CPR, (installed near the compressor inlet) will do the trick no matter what the evap pressure/temp does.
Try it out and let us know if the discharge temp rises.
I agree a CPR installed by the compressor will surfice.Quote:
Originally Posted by Gary
But should be used as a mehtod of protection not the main method of refrigerant control.
The internal compression ratio is constant due to constant internal volume ratio for Piston/Srew/Scroll compressors. Evap Temp(ET), Cond Temp (CT) and presssure drop of suction line and discharge line will also affect the compressor operation.Quote:
Originally Posted by mad fridgie
ET -pressure drop of suction line , decide SST, which plus SH will decide the compressor inlet condition, including mass flow.
and then, internal pressure of compressors can be gotten due to fixed compressor ( constant internal volume ratio), discharge pressure, is NOT equal to the internal pressure of compressors, and affect by the pressure drop and actual condensing pressure which is up to the heat exchanger performance.
So DST maybe increase , or also Not.
I agree that low mass flow will affect the compressor's low limit.
Thanks Joey, I think what Joey is saying, is that if you had you discharge pipe (outlet) is open (no restrictions) then changing your inlet pressure/mass flow will not change the internal compression ratio, because we have a fixed swept volume. How ever in practice the discharge is not open it is restricted (condensor & TEV) then changes to the inlet to the compressor change compression ratios, because the outlet remains more constant in comparission.:eek:Quote:
Originally Posted by Joey.zhang
kfjoe, confused yet?
A very good question, very hard to explain in a few words
I think that SST will decrease if CPR is used, comparing to the chiller without CPR, and mass flow will reduce, so cooling capacity also decrease expect that "stronge" Evap will be applied. The CT will increase due to lower mass flow and slight increasing heat rejection, so DST will also increase, and theoretical discharge temp will increase too if you don't improve condenser performance.Quote:
Originally Posted by Gary
However, it reduces greatly the preformance of the chiller with CPR.
Thank Mad for the explanation. Yes, you are correct. Compression ratio will change in pratice as SST and SH varies. Compression ratio may either incease or decrease if the SST is lower, because compressor will over or off compress in pratice due to effect of the actual condensing compressor.Quote:
Originally Posted by mad fridgie
Piston compressors are NOT fixed Vi machines providing the valves are pressure activated....If they seated and unseated by mechanical means the compressor makes a Vi; which may be near or far from the system's operating requirements.
High suction pressures make high inlet side velocities and high top of piston pressures. The high velocities largely effect suction filters and similar but can lead to oil lift....The high pressures lead to both high motor loads and high stresses on pistons, rods, crank and so on.
The system balance, with CPR in place, will depend on the following:Quote:
Originally Posted by Joey.zhang
1. Evaporator pushes compressor;
or,
2. Compressor pulls evaporator.
One mode is unstable, for heat-pumps, for instance, resulting in Te,sat drift towards Ta,in. It doesn't seem to overly matter in a refrigeration, or aircon situation, where the evap sits in a controlled space & Ta,in lowers with space pull-down.
The 'dither mode' of evaporators, where the evap runs at it bifurcation point e.g. dTlm=0/0, seems to often occur on oversize evaps, under (1) above.
So, where are we now in terms of exactly why compressor manufacturers place their envelope limits, as they do?
Compressor limits are based upon inlet and outlet conditions only, and have no relation to the actual application.Quote:
Originally Posted by desA
I must admit I do not understand your point about pushing and pulling.
If we look at the second law of thermodynamics
Energy travels from positive to negative, so it could be said that the evap is always pushing, but on the other hand the driving force for flow is the compressor, so the compressor is pushing the evap.
As a closed circuit one has to effect the other.
If stall was to occur in the evap it would only be momentry and have little effect the compressor.
Does anyone have any designer-based info from the 'inner sanctum' of the compressor design world, from which we could better understand how & why they publish their compressor operational windows, as they do?Quote:
Originally Posted by mad fridgie
I'll not muddy this thread, but perhaps we could re-continue this discussion on the evap thread, where the concept of evap bifurcation was discussed? :)Quote:
I must admit I do not understand your point about pushing and pulling.
If we look at the second law of thermodynamics
Energy travels from positive to negative, so it could be said that the evap is always pushing, but on the other hand the driving force for flow is the compressor, so the compressor is pushing the evap.
As a closed circuit one has to effect the other.
If stall was to occur in the evap it would only be momentry and have little effect the compressor.
Quote:
Originally Posted by mad fridgie
I'm trying to design a aicon system with dry evap coil even in evaporator entering air (30C 60%RH). Similar question, I try to push the SST to 25C while at same time keeps the SDT below 50C with entering air 35C.
I find myself out of the envelope. To try to keep the small car carres heavier things, can I choose a bigger motor(for example, choose a same displacement R407c compressor with refrigerant R134a)? Does there other things should be taken care except for the motor overload? I ever heard of that the discharge valve inside the compressor will fatigue handling bigger volum of refrigerant mass.
Using a bigger motor on the same displacement will surfice as far as overloading.Quote:
Originally Posted by isobutane
Regarding other issues, this is dependent on the type of compressor, your compressions ratios are much lower, which in itself is good, but how the discharge valves handle the small ratio, may cause the valves to bounce (extra fatigue), this is to do with spring strength.
Without testing (which you should do) my gut feeling would be at you descibed conditions that it would be OK.
I assume that compressor is a fool and It didn't know what it's compressing.:confused: but the motor is quit clever for the load and temp. When I tested it, it strikes when the discharge is about 105 c as well as suction temp is 48 C. What confused me is that the current draw is only 6.4A(whose rated current is 6A for R407c). the Pd is 24 barg and Ps is 5 barg for R134a(I use R134a in this R407c compressor).Quote:
Originally Posted by mad fridgie
I guess it's because the R134a's flow is smaller than that of R407c so the motor's heat can't be controlled to a safe level before it triggered the motor's temperature switch.
Is it true? or somethings else missed?
Sounds like hot water heat pump to me!Quote:
Originally Posted by isobutane
24barg R134a is not under SCT 50C, so running under these conditions is a different kettle of fish.
Many other factors have to be taken into consideration. There are particular methods of achieving best results in this area, this is not the place to descuse further.
I and Others have spent a lot of time resolving this problems, and have had ideas stolen.
I agree with these sentiments, MF... it's unfortunate that some folks prefer to copy, instead of innovate - or even, fairly trade expertise.Quote:
Originally Posted by mad fridgie
Oh well, I guess that is one deficiency of the internet medium... some things are better discussed behind closed doors... :(
I agree. A CPR valve is traditionally only used to prevent motor or condenser overloads due to short duration conditions (in normal conditions) or transients that occur frequently.Quote:
Originally Posted by mad fridgie
:off topic: actually, what I'm confused is that when the condition is out of compressor's limit, what happened to the compressor. The detailed example is only used to explain the question. There are only a motor protect switch for a rotary comp(like most compressor in aircon field). Only temp and current can trigger this. In this case, the current is normal, but what makes the motor's temp going so high?Quote:
Originally Posted by mad fridgie
B.T.W it's not a hot water heat pump, but a limit test of aic cooled DX system. I put the aicon in 60 c ambient and indoor inlet of 55 c. Just to find out what failes first.
Ideas is produced to be shared, not to be hidden. That's may be the soul of the internet.
:off topic:Quote:
Originally Posted by isobutane
Yes, they are... I'm going to be a little blunt here - following on from M_F's emotion...
The problem lies in the fact that few contribute, many share, few give back credit, or reward(s), where they/it are/is due.
Asia has, in general, a reputation of sucking in technology, without paying for it. Herein lies the real danger of the internet, unfortunately.
The western model is to establish technology agreements (contracts), & so, to reward the information source.
Hi Iso, I aplogise if I insulted you, but you did change the working conditions. This does make me a little suspicious?
DesA clearly states my (our) reasons for being cautious.
Like many on here, we are willing to assist if we can (for the good of the industry), but we are not here to give away our Intelectual Property!
Pay the cash , then not a problem!
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
Where there is giving without taking there is no incentive.
Where there is taking without giving there is no justice.
Where there is give and take in both directions there is balance and everyone benefits.
What goes around should come around.
^ A sage in our midst... indeed. Excellent. :)
Quote:
Originally Posted by Gary
Agree!
That makes the forum more reasonable
few....!
and relax!