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Thread: AWHP superheat & sub-cooling
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11-09-2009, 06:58 AM #801
Re: AWHP superheat & sub-cooling
Last edited by Gary; 11-09-2009 at 07:05 AM.
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11-09-2009, 07:20 AM #802
Re: AWHP superheat & sub-cooling
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11-09-2009, 07:43 AM #803
Re: AWHP superheat & sub-cooling
Where I see it improving capacity is on the superheat side of the high pressure side of the system.
With raised Tcomp,suct will come a pretty high gas discharge temperature. The Alfa-Laval example gives 130.4'C for example. This is going to require some careful management, in order to not over-stress the compressor.
The additional superheat can then be taken care of with a dedicated desuperheater, so that the condenser receives a superheat it can manage.
The scroll compressor in the lab rig, has a line on the envelope at 25'C suction gas return temperature. They must be considering this to be a safe upper limit - I'd imagine.
So using VIC with a scroll compressor, could possibly bring some issues, which an alternative compressor could manage better.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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11-09-2009, 03:31 PM #804
Re: AWHP superheat & sub-cooling
Feedback on the Te,sat drift simulation:
I built an equation (to back-check Chef's findings) which uses the evap heat-transfer coeff, including superheat etc, then subjected it to various sensitivity trials.
The upshot seems to be as follows:
1. Major player is x (vapour fraction) - seems to concur with your simulation, in principle;
2. Adjusting air mass flow (fan speed) offers a solution;
3. Increasing sub-cooling - hence decreasing x, also offers a solution.
There is still a 'closing correlation' to go into it - eg. relationship between SH & TXV m'g (mass delivery) - for now, it was per observation.
I also need to model a correlation between RH% & effect on (UA) - where U=heat-trfr coeff & A=reference area.
This drift is an issue, & the parametric variations hold promise, for fine-tuning. I'm still convinced, however, that a CPR-style guarantee for the maximum suction pressure into the compressor needs to be guaranteed.
So, thus far, the combination of CPR (SV), fan speed tuning, & sub-cooling control, seem to offer the promise of protecting the compressor under severe local temperature conditions.Last edited by desA; 11-09-2009 at 03:55 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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11-09-2009, 05:08 PM #805
Re: AWHP superheat & sub-cooling
The Alfa-laval example has the bulb on the inlet, not the outlet. That changes everything.
With the bulb on the outlet:
There is no raised Tcomp,suct. There is no additional superheat. There is no overstressing the compressor.
The Alfa-laval example is irrelevant, all of the above concerns do not apply... and the capacity improvement will be on the low side, not the high side.
In one post you are concerned about flooding the compressor. In the next post you are concerned about starving the compressor. Which is it? These are total opposites. You can't have it both ways. And Alfa-laval can't have it both ways.Last edited by Gary; 11-09-2009 at 06:26 PM.
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12-09-2009, 02:39 AM #806
Re: AWHP superheat & sub-cooling
As I read the Alfa-Laval position, their recommendation to best manage the instability (overfeed/underfeed) from the VIC on the vapour side, is to re-position the TXV SH sensor to after the VIC - as we have been working with.
This signal should operate in a reverse sense to what a normal bulb would do & actually shut back on high SH, rather than increase flow - hence their suggestion for an electronic controller.
With surging, unstable flows, you could very well have both flooding & starving within short durations of time, if the system dynamics are not carefully managed.
Personally, I like simple TXV bulbs - they are slow acting & don't introduce unnecessary system disturbances of their own.
Is there a way to have a 'reverse acting' TXV bulb i.e. one that shuts back flow slightly on rising SH - within a control band?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 02:43 AM #807
Re: AWHP superheat & sub-cooling
Try an AEV and see if your simulator likes that better.
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12-09-2009, 02:53 AM #808
Re: AWHP superheat & sub-cooling
Actually, the simulator, as it's currently set up for the VIC has a simple valve type - so there is no additional feedback dynamics, yet, to further complicate the picture. So, basically, it's like a fixed orifice, controlling mass flow to be a fixed value at constant reduced pressure.
The system instability comes from liquid line, to vapour time-lags & huge mismatch between latent heat of vapourisation & gas specific heat capacity.
---------------
Let's talk through the AEV concept a little more. What mass-flow target would you set it to, in the first place & how would you go about setting it up on a new system?
Could we emulate an AEV, using a TXV, with the bulb placed in something, to basically de-activate the bulb-temperature effect? I have a few spare TXV's on hand. Is there a way to trick the TXV bulb to go into a 'reverse mode'?
I can size an Alfa-Laval unit suitable for the VIC & have it brought up, to be put into the test rig - no problem. We can just confirm the duty requirements. What to do about the excessive Tcomp,discharge?Last edited by desA; 12-09-2009 at 03:02 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 03:11 AM #809
Re: AWHP superheat & sub-cooling
In the Alfa-laval example, they are showing 5K superheat at the evap outlet and 30K superheat at the VIC outlet. If the bulb were mounted at the VIC outlet there would be 5K superheat at that point. There would be no excessive Tcomp,disch. The numbers in the example are all wrong. The temps would be different throughout the system.
And the equalizer line needs to be moved along with the bulb.Last edited by Gary; 12-09-2009 at 03:34 AM.
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12-09-2009, 03:26 AM #810
Re: AWHP superheat & sub-cooling
Last edited by Gary; 12-09-2009 at 04:26 AM.
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12-09-2009, 03:31 AM #811
Re: AWHP superheat & sub-cooling
Ok, fair-enough. Point taken.
What is a typical maximum SH range that can be taken up on a TXV?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 03:51 AM #812
Re: AWHP superheat & sub-cooling
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12-09-2009, 04:06 AM #813
Re: AWHP superheat & sub-cooling
The AEV holds the evap pressure, and therefore the mass flow, constant. The superheat decreases as load decreases and increases as load increases.
The charge must be limited to avoid excessively low superheat at low loads (below setpoint).
The fan speed needs to be regulated in order to avoid excessively high superheat at high loads (above setpoint).Last edited by Gary; 12-09-2009 at 04:16 AM.
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12-09-2009, 04:46 AM #814
Re: AWHP superheat & sub-cooling
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12-09-2009, 05:53 AM #815
Re: AWHP superheat & sub-cooling
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12-09-2009, 07:41 AM #816
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12-09-2009, 09:35 AM #817
Re: AWHP superheat & sub-cooling
I have the VIC now running stably, but, there is a twist in the tail...
I'll work it up a little more & see where the sweet-spots lie.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 01:12 PM #818
Re: AWHP superheat & sub-cooling
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 01:59 PM #819
Re: AWHP superheat & sub-cooling
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12-09-2009, 02:33 PM #820
Re: AWHP superheat & sub-cooling
Last edited by Gary; 12-09-2009 at 02:35 PM.
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12-09-2009, 03:03 PM #821
Re: AWHP superheat & sub-cooling
This is in the simulator. Now please tell me if this makes sense.
What I found stabilises the system, under certain states is as follows:
1. Flash flow - say at 0.6MPa(a) - in liquid line, prior to entry into the VIC. The fluid is now 2-phase & holds temperature across the VIC.
2. The 2-phase fluid now moves onto the TXV/orifice/what you will, prior to entry into the evap.
I'll snapshot the flow diagram & let you make sense of it.
Under this scenario, I can get manageable stable spots in the simulation. The reason for all this malarky has to do with the difference in refrigerant latent heat & its gas specific heat. The flow tries to jump into all sorts of quasi-stable states, that probably would not exist for long, in a real machine - where damping occurs. In numerics, there is no real damping to speak of, unless I build it in...Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 03:11 PM #822
Re: AWHP superheat & sub-cooling
Circuit diagram. (Ignore the inter-cooler, 2nd compressor & desuperheater - they are not active in this simulation)
VIC evaporation duty, under liquid line - 1st flashEngineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 03:15 PM #823
Re: AWHP superheat & sub-cooling
Last edited by Gary; 12-09-2009 at 03:23 PM.
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12-09-2009, 03:21 PM #824
Re: AWHP superheat & sub-cooling
^ It's a simulator, so think of line (8) as having zero length...
Ok, so on the real VIC, we hard-mount the valve directly onto the unit - or provide a small impact plate to protect the HE.
Next question - from the VIC, the feed is 2-phase to the evap TXV... Is this doable, or should a different valve/orifice be used here, instead? I'm thinking from a control perspective.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 03:27 PM #825
Re: AWHP superheat & sub-cooling
Last edited by Gary; 12-09-2009 at 03:29 PM.
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12-09-2009, 03:33 PM #826
Re: AWHP superheat & sub-cooling
Ok, let me give that a whirl...
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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12-09-2009, 04:28 PM #827
Re: AWHP superheat & sub-cooling
To get a temp differential across the VIC, an orifice, or 2nd valve will be required as follows:
Liquid line>TXV>VIC>valve>evap>VIC>bulb>compressor
The TXV will also have to open at higher line pressure. I'll simulate some more & then try to make sense of the findings. I'm looking for stability as much a possible.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 02:47 AM #828
Re: AWHP superheat & sub-cooling
Now you've got me doing it. I'm trying to make the simulator work instead of trying to make the VIC work.
Let's go back to the original configuration, i.e. liquid line>VIC>TXV>evap>VIC>bulb>equalizer>suction line.
It doesn't need 2-phase flow. It doesn't need that extra valve before the VIC. It will work. Let's build it and then deal with the issues, if there are any.
Tell Alfa-laval to move the bulb AND the equalizer line to the VIC outlet and come up with some real numbers. Changing the equalizer line location along with the bulb location will minimize or eliminate the hunting, TXV's do not act in reverse... and the VIC is NOT going to cause high superheat.
I don't know what's wrong with your simulator. Is it too late to get your money back?
As to sizing, you are sizing a DX/liquid subcooler coil to chill the liquid flowing through the liquid line from 60C to 20C. On the DX side the Te,sat=15C.Last edited by Gary; 13-09-2009 at 03:39 AM.
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13-09-2009, 03:46 AM #829
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13-09-2009, 05:10 AM #830
Re: AWHP superheat & sub-cooling
I'll do the re-routing - it's been done umpteen times in the last weeks.
I'll predict again, ahead of time, that as long as the evaporator remains an evaporator, with full evaporation service & the VIC acts as a sub-cooler/super-heater, then the system will remain smooth. The instant that the main evaporator is called on to have a wet exit, & the VIC to take on a partial evaporation load, is where all hell breaks loose.
The reason for this effect lies in the huge difference numerically between the latent heat of evaporation of the refrigerant while evaporating & its specific heat, as a gas.
This swing over function for the VIC from a super-heater, to partial evaporator seems to cause system instabilities. The next stable point, under this scenario is a liquid temp of something like -60'C - to get the correct heat balance across the VIC.
To my mind, it looks like there is a mismatch of heat & an attempt to defeat the laws of thermodynamics - the simulator will not allow this to occur.
I will lay my hat on the table that, when we've built this animal, that it will hunt & that we'll have to damp it down somehow.
I don't know what's wrong with your simulator. Is it too late to get your money back?Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 05:44 AM #831
Re: AWHP superheat & sub-cooling
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13-09-2009, 05:50 AM #832
Re: AWHP superheat & sub-cooling
This is the wise way to go. Keep the filter-drier vertical & set up for tight charge control.
How effective would a suction-line filter-drier be at catching small liquid fling-overs? I wouldn't want to go as far as a suction accumulator as this would just defeat the whole charge-conservation principle.
Another question of importance:
When I spec the VIC, do I spec it as a sub-cooler/super-heater (sensible heat transfer), or partial evaporator (evap on LP side)? I'm thinking that the likes of Alfa-Laval & SWEP would like to know this.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 05:54 AM #833
Re: AWHP superheat & sub-cooling
As the charge approaches its limit there would be 2-phase liquid entering the VIC.
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13-09-2009, 05:55 AM #834
Re: AWHP superheat & sub-cooling
I'm working on a table that shows the relationships between latent heat & specific heats for the liquid line & LP side. The ratio has units of Kelvin i.e. a temp difference.
The size of these ratios is what causes huge, huge numerical instability in the simulator & will cause some level of boiling instability in the VIC - this is a fact of physics.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 05:58 AM #835
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13-09-2009, 05:59 AM #836
Re: AWHP superheat & sub-cooling
One option that may help stability is a "Henry Superheat Sensor" I have never used one, Maybe gary has expericience with these (US made I believe)
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13-09-2009, 06:08 AM #837
Re: AWHP superheat & sub-cooling
Carefully limiting the orifice size can help, too.
I think the important point here is that we can work out any issues we run into.
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13-09-2009, 06:10 AM #838
Re: AWHP superheat & sub-cooling
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13-09-2009, 03:22 PM #839
Re: AWHP superheat & sub-cooling
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13-09-2009, 03:44 PM #840
Re: AWHP superheat & sub-cooling
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13-09-2009, 04:05 PM #841
Re: AWHP superheat & sub-cooling
Hmmm... You have a 4 row coil. What if the first 3 rows were the evap and the 4th row were the superheater, with the VIC in between?
liquid line>VIC>TXV>evap123>VIC>evap4>bulb>equalizer>suction line
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13-09-2009, 04:15 PM #842
Re: AWHP superheat & sub-cooling
Originally Posted by desA
I wouldn't want to go as far as a suction accumulator as this would just defeat the whole charge-conservation principle.
I don't understand this. Can you elaborate?
This adds both cost (the accumulator) & additional refrigerant mass.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 04:19 PM #843
Re: AWHP superheat & sub-cooling
Originally Posted by Gary
Unless there is not enough refrigerant in the system to allow hell to break loose... limited charge.
Can your simulator handle this? Can it reduce the load until the TXV starts hunting and then reduce the charge until it stops hunting?
That's why I use the lab machine... a lot less simulation headachesEngineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 04:28 PM #844
Re: AWHP superheat & sub-cooling
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13-09-2009, 04:37 PM #845
Re: AWHP superheat & sub-cooling
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 04:42 PM #846
Re: AWHP superheat & sub-cooling
Originally Posted by desA
As I understand the accumulator principle, a certain amount of liquid will reside in it during operation - at very best, it would only have vapour in it, during operation.
This adds both cost (the accumulator) & additional refrigerant mass.
Rotary compressors all have small accumulators mounted on the side of the shell (and for good reason). I'm betting these are very inexpensive.
There are most probably other reasons, of course.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 04:49 PM #847
Re: AWHP superheat & sub-cooling
There we go. Evap>VIC>s/h
I'll run up some tests tomorrow to see what comes out...Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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13-09-2009, 04:58 PM #848
Re: AWHP superheat & sub-cooling
As I understand it, on a rotary compressor the suction gas feeds directly to the compressor section and thus has near-zero tolerance for liquid, hence the accumulator.
The discharge from the compressor section dumps into the shell. IOW, the entire compressor is on the high side of the system. The shell runs very hot.Last edited by Gary; 13-09-2009 at 05:03 PM.
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13-09-2009, 05:09 PM #849
Re: AWHP superheat & sub-cooling
Last edited by Gary; 13-09-2009 at 06:47 PM.
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14-09-2009, 01:37 AM #850
Re: AWHP superheat & sub-cooling
^ The super-heater coil row is generally placed in the path of hottest air stream - ie. at air inlet. The air leaving super-heater is then used to evaporate the lower temp liquid.
Another thing to notice is that, in general, the cold air outlet temp is rarely colder than the superheated refrigerant outlet temp. In other words, very little 'temperature cross' occurs.
My commercial evaps do actually have around 1-1.5'C temp cross, but the lab machine can't get there - inefficient coil design.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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