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  1. #1
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    Variable RPM compressor/cap tube issue



    Why does a BD35F work at all RPM settings yet has a fixed length cap tube.

    After installing a new fridge system the option was available to run the compressor at 2000 RPM up 3500 RPM but what revs should be chosen to match the cap tube. After reading Henry Ehrens excellent cap tube article it would appear that only one setting would work but they all work, this should not be possible.

    So noting the discharge and suction pressure, amps draw and cycle time for various RPM settings with no other changes, the system was run at many RPMs and the following table is a synopsis of many tests and it does appear that 2500 revs is the most thermally efficient. Of course pull down from warm is quicker at higher revs but so is the power consumption!

    Revs
    On time
    cycle time
    amps
    A/hr
    3500
    10
    13
    6.2
    4.77
    3000
    6
    9
    5.9
    3.93
    2500
    5.5
    8.5
    4.6
    2.98
    2000
    8.5
    11.5
    3.7
    2.73


    The discharge pressure changed by only 2 PSI between any test and the suction stayed about the same. It certainly does not fit with any cap tube sizing data one can get so anyone with an idea why this setup is so forgiving?

    Bestest Chef



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    Re: Variable RPM compressor/cap tube issue

    Sorry - teh table was a mess

    Revs On time Cycle time Amps Amp/Hr
    3500 10 13 6.2 4.77
    3000 6 9 5.9 3.93
    2500 5.5 8.5 4.6 2.98
    2000 8.5 11.5 3.7 2.73

    Hope this is better

    Chef

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    Re: Variable RPM compressor/cap tube issue

    A capillary system is considered a critical charge system. It uses the condenser as a buffer for differing loads - if lo load it will be cooler and use more volume of condenser as reciever (filling with liquid). then at hi load uses more of condenser for heat tansfer (via hi temp = hi press = increased condensing temp) hence more space for condensing. By varying the volumetric flow rate of refrigerant within the design operating paremeters would only improve load control and efficiancy. This is why the system is forgiving

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    Re: Variable RPM compressor/cap tube issue

    Thanks Tesla and I beleived I had "the backing up into the condenser theory" sorted but maybe not.
    This unit is rated at 320 Btu/hr at 2000 revs and a nice 540 Btu/hr at 3500 revs so one would expect to see a rise in the discharge pressure and even a fall in suction pressure as the unit is switched to 3500 revs but the Dx is just 2 PSI higher.

    The condenser feels the same temperature and the frost line on the evaporator is in the same place. Non of this seems to line up with published data and so it seems cap tubes are way more forgiving.

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    Re: Variable RPM compressor/cap tube issue

    Yo Chef,
    if you want some more data you can go to the Danfoss site and download the free program "Dancap" .
    there you can give the lengt and diam of the existing tube and find out on wich presure it works best.

    just you get an idea

    Ice

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    Re: Variable RPM compressor/cap tube issue

    Thanks for the link Ice - I tried it.

    The Dancap program gives me 11" for 540Btu?hr

    I also looked at the excellent ACC tables and that gives me 41.7" for the same conditions.

    My system is running fine at 94" - again all conditions are the same.

    And JB sizing chart gives 144". All based on TC-26 tube.

    So with all this great variation in recommended lengths how does one make an intelligent guess let alone a design without some trial and error experimentation.
    For my system I already have the tube and so I just need to select the best RPM to run for max efficiency but for guys actually 'cutting a length' to solder in it must be a gamble.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    cut messure sh cut messure Sh, check load handing, check sh, got it! write down length and cut every tube you need for that bit of kit @ that length, thats why I love TXVs!

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    Re: Variable RPM compressor/cap tube issue

    check temperatures in differend places, just behind the capilair,middle of evap/condenser and at the end of evap/condenser. normal optimum temp between end of capilair and outlet evap/condensor is between 4C° and 8C°.if not, look the amound of refrig,take out or fill in.also check the rpm of motor at the high and low, wat the best result is.


    Ice

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    Re: Variable RPM compressor/cap tube issue

    Chef,

    It's been a long time since you posted this - I don't know if you'll see my follow up. I'm curious what ambient air and interior compartment temperatures you were using with your data tables? I may be in dire straits as the cap tube length used in my design comes from Dancap. And my system performance isn't what I need it to be. Throw in the variable speed capability and I am really unsure how to decide what is the most energy efficient way to run the system. Did you pursue this further?

    I've been reading all your posts here, looking for more clues.

    Thanks for your time,

    Aaron_K

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by aaron_k View Post
    Chef,

    It's been a long time since you posted this - I don't know if you'll see my follow up. I'm curious what ambient air and interior compartment temperatures you were using with your data tables? I may be in dire straits as the cap tube length used in my design comes from Dancap. And my system performance isn't what I need it to be. Throw in the variable speed capability and I am really unsure how to decide what is the most energy efficient way to run the system. Did you pursue this further?

    Aaron_K
    Just spotted your reply. The condenser is water cooled and is at 30C so discharge pressure is about 125PSI on R134a. The evaporator is set to -10C on the plate.

    What is it that you are trying to achieve and some details may help to give a clue how to recommend some RPM to run at and the tube size.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Chef,

    Thanks for your reply. We have a passively cooled condenser system with a fin and tube evap and forced air inside our refrigerator. The sticking point is that the ambient temp is 52C while the inside temp is 5C. As currently designed (1.7 m cap tube, 0.71mm ID) the refrigeration system will only barely maintain the required temp, and does so with a lot of energy usage.

    Here's the latest running data:

    Mean evaporator temperature (at steady state operation, 52C (126F) ambient, 5C (41F) compartment temperature): 35.1ºF
    Compressor speed: 3000 rpm
    High side pressure : 206 PSIg (as condensing temp 132.2ºF)
    Suction pressure: 29.6 Psig
    Superheat and subcooling values : 1ºF superheat & 6.6ºF subcooling.
    Estimated refrigeration capacity (W, kcal/h, or btu/h): 133 W (per Danfoss).
    180 g of R134a charge.

    Below 2800 rpm the internal temperature can't be held. The compressor almost never cycles off. I need it to run for only 30% of any given hour to reach our goal. Charge migration is adding 15C to the air temperature in the box if the compressor does shut off. It's obvious that the evap needs to be colder.

    Insulation is no problem (we're using a very high insulation value, equivalent to 6 inches of urethane foam).

    I need to run the system for 24 hours in a 60 A-hr battery power envelope. We're no where close. Tecumseh and ACC (and you) report much longer cap tubes than the DanCap derived value (what we used in our design). Circumstantial data suggests we're way off. I have no way to know for sure, other than guessing. Worst of all for us, we don't build the system (it's contracted out), and don't have any tools for "tweaking". I do though want to know what we can suggest for changes, then get them put in place and tested. Needless to say the design isn't finished!

    What are your initial thoughts on my predicament?

    You're great knowledge and expertise is obvious Chef, reading these forums; thank you in advance if you have some ideas for us!

    Regards,

    Aaron_K

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by aaron_k View Post
    Chef,

    Thanks for your reply. We have a passively cooled condenser system with a fin and tube evap and forced air inside our refrigerator. The sticking point is that the ambient temp is 52C while the inside temp is 5C. As currently designed (1.7 m cap tube, 0.71mm ID) the refrigeration system will only barely maintain the required temp, and does so with a lot of energy usage.

    First question is what compressor is it - a BD35 or a BD50 - it will make a differance at high Rpm's? The
    52C ambient is confusing - is it inside a 'hot' compartment and can you get cool air to it with a duct system? It may be the best option to improve performance dramatically.

    Here's the latest running data:

    Mean evaporator temperature (at steady state operation, 52C (126F) ambient, 5C (41F) compartment temperature): 35.1ºF
    Compressor speed: 3000 rpm
    High side pressure : 206 PSIg (as condensing temp 132.2ºF)
    Suction pressure: 29.6 Psig
    Superheat and subcooling values : 1ºF superheat & 6.6ºF subcooling.
    Estimated refrigeration capacity (W, kcal/h, or btu/h): 133 W (per Danfoss).
    180 g of R134a charge.

    Very nice to see some good data to work on. How did you get the 133W of refrigeration capacity at your conditions? What is the volume of your box, L*H*W would be best info here?
    Are you sure your getting 6.6F subcooling as the other data would suggest that your well into the 2 phase zone at the entrance to your tube.

    Below 2800 rpm the internal temperature can't be held. The compressor almost never cycles off. I need it to run for only 30% of any given hour to reach our goal. Charge migration is adding 15C to the air temperature in the box if the compressor does shut off. It's obvious that the evap needs to be colder.

    This is a good indicator of how much cold your loosing from the box but without the the unit running at max performance it wont help us yet to decide on how much performance you will get when it is properly tuned, but it will later so it is an important measurement.

    Insulation is no problem (we're using a very high insulation value, equivalent to 6 inches of urethane foam).

    I need to run the system for 24 hours in a 60 A-hr battery power envelope. We're no where close.

    Wow - I am not sure you will get to 30% as an ON time in any hour but we shall try.

    Tecumseh and ACC (and you) report much longer cap tubes than the DanCap derived value (what we used in our design). Circumstantial data suggests we're way off. I have no way to know for sure, other than guessing. Worst of all for us, we don't build the system (it's contracted out), and don't have any tools for "tweaking". I do though want to know what we can suggest for changes, then get them put in place and tested. Needless to say the design isn't finished!

    It all depends on whether it is a BD35 or a BD50 but as an initial input the tube length is way too short.I cannot estimate a better length without the other information.

    What are your initial thoughts on my predicament?

    You're great knowledge and expertise is obvious Chef, reading these forums; thank you in advance if you have some ideas for us!

    Regards,

    Aaron_K
    Are you OK with PH diagrams? because it is easier to explain what is going on in your system with them.

    If your tube is too short then both gas and liquid enter it at the entrance, the gas makes up for the tube being too short as it creates faster flow rates and more friction. The problem here is you are not sending down the tube as much liquid as you could and therefore your getting less cooling. Basically your just pumping gas around your circuit and that costs amp/hrs.

    I am currently working on a unit similar to yours but it has a sight glass and you can see the outlet port of the sight glass is covered to a max of 5% with liquid and remainder of the oulet lets gas get entrained. Very illuminating indeed.

    Initial (and I mean initial) it looks like you may need around 2 metres of .63mm tube but that depends on your BD model. Your current 1.7m of 0.71mm is pretty radical!

    Any info on size of the condenser and evap in terms of line sizes and lengths would also help.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Chef,

    Many, many thanks! It's a BD35F, and the ambient is equal to desert conditions (where this will be used, outdoors). It's a 5 cu. ft. (rectangular box about 1.5X taller than deep or wide) refrigerated compartment. Capacity was read from Danfoss' RS+3 program. The insulation system performance is such that at the stated conditions only 135 btu/hr of heat lead enters the compartment. I'm not sure (completely) about SC, as the data is second hand from our contractor. This is the data they report.

    I agree with you about the system just pumping gas around. There seems to be just enough phase change in the evap to overcome the heat lead (above 2800 rpm) and to cool the gas / liquid with the rest of the energy wasted. I tried Tecumseh's cap tube software and come up with around 4.5m of the 0.71mm ID cap tube... so yes, I agree that the cap tube is a big contributor to our problem.

    With my additional data what are your thoughts? I think I've piqued your interest!

    I am so grateful for your ideas. Thank you!

    Aaron_K

    PS - PH diagrams are fine.

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    Re: Variable RPM compressor/cap tube issue

    Internal condenser tube volume is around 40 cu. in. (3/8" tube ID, one row of tubes). Evap is around 9 cu. in. (same construction).

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    Re: Variable RPM compressor/cap tube issue

    Hi Chef,
    by varying speed of compressor, this will lower compression ratio, and vary flow rate through fixed length capilary tube.

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    Re: Variable RPM compressor/cap tube issue

    Hi Magoo

    Thanks for that but Aaron has posted a new series of questions about his system and so the thread has changed. Now its how to get a BD35 running in a desert at 52C ambient.

    Any ideas

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Aaron_K

    First try seaching google for
    DEHC.PK.100.C8.02/
    520N0436

    It is the BD manual and has all the details you will need to see the performance and operating conditions.

    The max temp of the discharge is quoted as 60C continuous and 70C intermittent - as you are running around 68 to 70C you're right on the edge!! They also say a fan is necessary (compulsory) blowing over the compressor above 55C so you may like to consider that but at 52C ambient its a bit of a question if it will be of much use? Also they strap on an oil cooler to the side of the pot - a sort of aluminium finned heat exchanger. You may need that as well.

    The 133W is based on ambients of 32C and 55C condensing - that is not what you have so the cooling will be less. It shows the test conditions on page 12 of the manual

    I have done some quick runs on your conditions and these results are for various lengths of 0.71 tube.
    4.5m x=0
    3.1m x=0.1
    2.5m x=0.2
    2.0m x=0.3
    1.7m x=0.4

    This shows what the entry conditions are of the refrigerant as it enters the tube. With a 1.7m tube the quality is 0.4 and h4=h3=338, so you are operating in the middle of the bell. If the tube is too short it can only meter the refrigerant by having excess uncondensed gas flowing through the tube to give the needed pressure drop.

    A tube of about 4.5m should have a quality of about 0.0 at the tube entrance and so h4=h3=280Kj/Kg
    You gain 58Kj/Kg of cooling and the system should perform way better.

    There seems to be no jargon for x=0.4 at the entrance like 'negative subcooling' - probably because it cant be measured! Do you have a good jargon word to describe it?

    I'll have a look at the 0.63 diameter tube later to see if it more suitable with more reasonable lengths.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by aaron_k View Post
    Here's the latest running data:

    Mean evaporator temperature (at steady state operation, 52C (126F) ambient, 5C (41F) compartment temperature): 35.1ºF
    Compressor speed: 3000 rpm
    High side pressure : 206 PSIg (as condensing temp 132.2ºF)
    Suction pressure: 29.6 Psig
    Superheat and subcooling values : 1ºF superheat & 6.6ºF subcooling.
    Estimated refrigeration capacity (W, kcal/h, or btu/h): 133 W (per Danfoss).
    180 g of R134a charge.

    Below 2800 rpm the internal temperature can't be held. The compressor almost never cycles off. I need it to run for only 30% of any given hour to reach our goal. Charge migration is adding 15C to the air temperature in the box if the compressor does shut off. It's obvious that the evap needs to be colder.
    Sounds like an inefficient compressor to me... which is to say that the compressor does not have enough cacacity for the evaporator... nor does it have enough capacity for the condenser.
    Last edited by Gary; 28-11-2009 at 04:12 AM.

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    Re: Variable RPM compressor/cap tube issue

    But then this assumes standard design and I take it this is a system you are designing?
    Last edited by Gary; 28-11-2009 at 05:57 AM.

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    Re: Variable RPM compressor/cap tube issue

    aaron, I will leave the cap sizeing to chef (more is skill), I do not believe you are getting any sub-cooling.
    So what can we do?
    Do you have a drain off your evaporator, if so trap the water and run the liquid line (between the condersor and the capillary) in this trapped water (preferably a long high tube). The over flow dripple over the condensor (this may give the effect of dropping the air temp) Do not change your speed, over shoot your set point (large differential) Install a direct acting solenoid valve (no pressure drop) in the liquid line and Non return valve or another direct acting valve in the suction line ( as low a pressure drop as possible) These measures should stop refrigerant migration.

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by aaron_k View Post
    Internal condenser tube volume is around 40 cu. in. (3/8" tube ID, one row of tubes). Evap is around 9 cu. in. (same construction).
    The smallest size normally used with a BD35 evaporator is 10 cu in and they can drive up to 25 cu in quite well especially at 3500 revs and get it down to -15C or so. Plate sizes and volumes can be found on various web sites including Weaco and Frigoboat. So your evap may be a little on the small size!

    You can also increase the charge to 300g max so a larger evap may make sense.

    The condenser seems to be a good size for your conditions and should help to keep the head pressures down a little.

    I am not sure why Gary thinks they are way too large for your compressor, I am sure he will explain later.

    Mad - good idea with the condensate, sort of a loose it on the roundabouts but get it back on the swings approach.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    If the system has a hand valve in the liquid line, it should be fairly easy to pinpoint the problem.

    Close off the valve (liquid flow) until the evap outlet superheat is 10-15F/5.5-8.5K, then take a full set of measurements.

    If everything comes together, then we know that the cap tube is not restrictive enough.
    Last edited by Gary; 28-11-2009 at 09:17 AM.

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    Re: Variable RPM compressor/cap tube issue

    Studied the above a bit more, superheat is very low, what is your discharge temp, You may find that a lot of the latent exchanger (cooling) is actually occuring in the compressor.
    Stating obvious!
    So you are overfeeding.
    Run your capillay through (around the suction) reduce the vapour contet entering the evap (coil becomes more flooded) reduces the evap pressure drop, increasing LMTD.
    With suction pressure and superheat as is the evap does not look undersize. (not knowing your paticular evap) unless the evap internal pressure drop is high, masking the true performance (check the pressure at evap inlet)

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Chef View Post
    I am not sure why Gary thinks they are way too large for your compressor, I am sure he will explain later.
    The system has low TD's coupled with low superheat. If this were a system that had worked well in the past, I would be looking long and hard at that compressor.

    An inefficient compressor has the same symptoms as a compressor that is undersized for the evap and cond... which is the same thing as the evap and cond being oversized for the compressor.

    But being that this is a work in progress, maybe its the cap tube.
    Last edited by Gary; 28-11-2009 at 09:35 AM.

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by mad fridgie View Post
    Studied the above a bit more, superheat is very low, what is your discharge temp, You may find that a lot of the latent exchanger (cooling) is actually occuring in the compressor.
    Stating obvious!
    So you are overfeeding.
    Run your capillay through (around the suction) reduce the vapour contet entering the evap (coil becomes more flooded) reduces the evap pressure drop, increasing LMTD.
    With suction pressure and superheat as is the evap does not look undersize. (not knowing your paticular evap) unless the evap internal pressure drop is high, masking the true performance (check the pressure at evap inlet)
    Mad are you mad,the capillary is overfeeding, thats why the evap is to small, but Mad if there is no liquid sub-cooling the liquid could whipping the evap and only flashing of with the pressure drop.
    Either way measure the evap inlet pressure
    cheers Mad and Mad

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Gary View Post
    The system has low TD's coupled with low superheat. If this were a system that had worked well in the past, I would be looking long and hard at that compressor.

    An inefficient compressor has the same symptoms as a compressor that is undersized for the evap and cond... which is the same thing as the evap and cond being oversized for the compressor.

    But being that this is a work in progress, maybe its the cap tube.
    These may be fine conclusions for a TXV system but you need to consider that in the current operating condition the cap tube is fed with an x=0.4 so the condenser and evaporator are not operating as they would in a normal TXV system. It is classic to have very low TD's in this circumstance.

    The normal rules you usually apply do not count in this case and so you need to think 60% liquid and 40% gas flowing through the condenser and into the tube and then 30% liquid with 70% gas entering the evapoator - this is well off the usual situation. Plot it on a PH diagram and you will see the problem.

    Aaron also mentions the data about SC and maybe SH came from the contractor so it is suspect.

    The SC is obviously wrong as it never passes the saturation line in the PH diagram (h2 - h3) in the condenser.

    Your thoughts?

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by mad fridgie View Post
    Mad are you mad,the capillary is overfeeding, thats why the evap is to small, but Mad if there is no liquid sub-cooling the liquid could whipping the evap and only flashing of with the pressure drop.
    Either way measure the evap inlet pressure
    cheers Mad and Mad
    Your Madness - No there is no subcooling and most of it flashes down the tube so its just passing gas (no pun intended) thats why the evap looks small? Seems your alter ego sees it correct.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Chef View Post
    These may be fine conclusions for a TXV system but you need to consider that in the current operating condition the cap tube is fed with an x=0.4 so the condenser and evaporator are not operating as they would in a normal TXV system. It is classic to have very low TD's in this circumstance.

    The normal rules you usually apply do not count in this case and so you need to think 60% liquid and 40% gas flowing through the condenser and into the tube and then 30% liquid with 70% gas entering the evapoator - this is well off the usual situation. Plot it on a PH diagram and you will see the problem.

    Aaron also mentions the data about SC and maybe SH came from the contractor so it is suspect.

    The SC is obviously wrong as it never passes the saturation line in the PH diagram (h2 - h3) in the condenser.

    Your thoughts?

    Chef
    The rules apply for both TXV and cap tube systems.

    Although the mixture is 60/40 at the cap tube inlet, there is no way it is 60/40 throughout the condenser.

    The low TD's are not unlike those of an inefficient/undersized compressor.

    All of the data comes through both the contractor and Aaron which makes it doubly suspect... but we work with the numbers we are given.

    Try plotting an inefficient/undersized compressor on your PH diagram.

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    Re: Variable RPM compressor/cap tube issue

    Being a hands-on kinda guy, I would restrict the liquid line flow as outlined earlier and see what the numbers tell me then. A measurement beats a calculation every time.

    I am not saying the compressor is inefficient/undersized. There is little doubt the cap tube is not restrictive enough for the conditions... the jury is still out on the compressor.
    Last edited by Gary; 28-11-2009 at 05:01 PM.

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    Re: Variable RPM compressor/cap tube issue

    Has a set of charge-determination trials been done Aaron?

    The charge will affect your system dramatically.

    I'd suggest taking your current setup, beginning with a minimal charge, run the system. Record the results, add in additional charge (say 2.5-5%) test again. Continue the tests, recording as many parameters as you can. Plot the results & see if the system sweetens at some point.
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by desA View Post
    Has a set of charge-determination trials been done Aaron?

    The charge will affect your system dramatically.

    I'd suggest taking your current setup, beginning with a minimal charge, run the system. Record the results, add in additional charge (say 2.5-5%) test again. Continue the tests, recording as many parameters as you can. Plot the results & see if the system sweetens at some point.
    If we are to believe the SC and SH measurements, then we are already flooding the compressor. Adding charge would make this worse. Removing charge would give the compressor a break, but would eliminate what little SC we have.

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    Re: Variable RPM compressor/cap tube issue

    I've not had much experience with tuning cap tubes, but, with the low SH, I'd expect to want to increase the cap pressure drop (restriction) & permit less fluid to flow.

    This would probably back the condenser up a little further.

    Just how much SC would you be looking for?
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    Re: Variable RPM compressor/cap tube issue

    Gentlemen,

    Thank you all for your discussion about the system. As it's a holiday weekend here in the US I'm not online as much, so I apologize if I don't respond quickly.

    I don't have a hand valve in the system, and I think everyone agrees that the cap tube is not restrictive enough. I'm going to try to start with that, re-test and will report back. I'm also going to add a blower to the condenser to help the SC a bit. I like the ideas about using solenoid valves to eliminate charge migration; I looked into that earlier and didn't find a latching solenoid that seemed to be small enough, or that had solder connections. The fridge will see rough service and any threaded connection is more prone to leakage.

    Many many thanks to you all for your thoughts and suggestions. I'll update the group after we've actually made a change and can re-measure the results.

    Aaron_K

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by desA View Post
    I've not had much experience with tuning cap tubes, but, with the low SH, I'd expect to want to increase the cap pressure drop (restriction) & permit less fluid to flow.

    This would probably back the condenser up a little further.

    Just how much SC would you be looking for?
    Ideally, I would like to see 10-15F/5.5-8.5K for both SC and SH, although the question then arises: What happens when the ambient is far less than 126F/52C? I might expect both SC and SH to be higher at lower ambient and lower at higher ambient. A happy medium is needed, thus I would be aiming for about 10F/5.5K for both SC and SH at the higher ambient and then see how that works out for lower ambient.

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by aaron_k View Post
    Gentlemen,

    Thank you all for your discussion about the system. As it's a holiday weekend here in the US I'm not online as much, so I apologize if I don't respond quickly.

    I don't have a hand valve in the system, and I think everyone agrees that the cap tube is not restrictive enough. I'm going to try to start with that, re-test and will report back. I'm also going to add a blower to the condenser to help the SC a bit. I like the ideas about using solenoid valves to eliminate charge migration; I looked into that earlier and didn't find a latching solenoid that seemed to be small enough, or that had solder connections. The fridge will see rough service and any threaded connection is more prone to leakage.

    Many many thanks to you all for your thoughts and suggestions. I'll update the group after we've actually made a change and can re-measure the results.

    Aaron_K
    During the off cycle refrigerant migrates to the coldest part of the system, which generally means the evaporator. If the suction line exits the evap at the bottom and goes downhill to the compressor, then gravity carries the liquid to the compressor and you need to worry about migration. This can usually be resolved by running the suction line up to the top of the evap and then down to the compressor, trapping the liquid in the evap.

    If the compressor is above the evap or the evap is fed from the bottom up, then migration is not something you would normally need to worry about.

    At this point I don't really see a need for a condenser fan given the very low cond TD, although the need may reveal itself as the other problems are resolved. On the other hand, a fan wouldn't hurt.

    On an experimental/prototype system it is a good idea to have valves, access ports and sensors everywhere.
    Last edited by Gary; 28-11-2009 at 06:59 PM.

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    Re: Variable RPM compressor/cap tube issue

    I think we are going to get very lttle natural sub-cooling (additional can be added as stated above), so it is important that we have a liquid seal to the cap.
    I would suggest, that you install a "P" trap on the outlet of the condensor, then take the pipe up, to the height of the condensor, have "u' bend, then directly in the drier (vertical position) then into the cap. by nature we would have a little liquid head sub-cooling.
    I agree with gary when doing R&D more data reading pionts, the easier it is to make descisions.
    (Its been many moons since I have delt with caps)

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by mad fridgie View Post
    I think we are going to get very lttle natural sub-cooling (additional can be added as stated above), so it is important that we have a liquid seal to the cap.)
    I agree. The SC can be no more than the cond TD, and in fact must be less... and we have very low cond TD.

    Similarly, the SH must be less than the evap TD... and we have very low evap TD.

    We may need something like the Magoo rule for both SC and SH.
    Last edited by Gary; 29-11-2009 at 08:58 AM.

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Gary View Post
    The rules apply for both TXV and cap tube systems.

    Although the mixture is 60/40 at the cap tube inlet, there is no way it is 60/40 throughout the condenser.

    The low TD's are not unlike those of an inefficient/undersized compressor.

    Try plotting an inefficient/undersized compressor on your PH diagram.
    Here are 3 sets of readings from a system.

    Dx=10.6 Sx=2.52 SC=6.1 SH=1.8
    Dx=9.7 Sx=2.32 SC=1.0 SH=0.6
    Dx=9.22 Sx=2.05 SC=0.0 SH=0.0

    Dx and Sx are bara and SC and SH are in K

    They are taken for preset temperatures of 0C, -3C and -7C, its just a simple fridge system.

    So what your saying is these obey the same rules as a TXV and you can determine if all is well or not from them. That I doubt very much.

    Its a precharged system from a most reputable manufacturer.

    Cap tube systems do not obey the same SH and SC rules as TXV systems and trying to use those rules will be a big problem. Henry Ehrens puts it very nicely in his paper.

    Oh and last-

    Please enlighten us on how to plot an inefficient compressor on a PH diagram. An example would be great.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Here are 3 sets of readings from a system.

    0'C : Dx=10.6 Sx=2.52 SC=6.1 SH=1.8
    -3'C : Dx=9.7 Sx=2.32 SC=1.0 SH=0.6
    -7'C : Dx=9.22 Sx=2.05 SC=0.0 SH=0.0

    Dx and Sx are bara and SC and SH are in K
    Is this correct ie. taken at those temps (was a little confused by your reference).

    Thoughts:
    If that is the case, then I feel that the condenser is running out of capacity (large SC), with the evap slightly over-capacity (large SH), at the 0'C operating condition. It seems to have a system imbalance.

    I'd not be at all happy with SH=0K heading towards a compressor, unless the line on the way gets hot & evaporates the droplets.
    Last edited by desA; 29-11-2009 at 12:37 PM.
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Chef View Post
    Here are 3 sets of readings from a system.

    Dx=10.6 Sx=2.52 SC=6.1 SH=1.8
    Dx=9.7 Sx=2.32 SC=1.0 SH=0.6
    Dx=9.22 Sx=2.05 SC=0.0 SH=0.0

    Dx and Sx are bara and SC and SH are in K

    They are taken for preset temperatures of 0C, -3C and -7C, its just a simple fridge system.

    So what your saying is these obey the same rules as a TXV and you can determine if all is well or not from them. That I doubt very much.

    Its a precharged system from a most reputable manufacturer.

    Cap tube systems do not obey the same SH and SC rules as TXV systems and trying to use those rules will be a big problem. Henry Ehrens puts it very nicely in his paper.

    Oh and last-

    Please enlighten us on how to plot an inefficient compressor on a PH diagram. An example would be great.

    Chef
    I have no idea what your point is. Where did I say I could tell if all is well or not from those few numbers? I didn't. Nor did I say that all rules apply the same for TXV and cap tubes. The specific rules we were discussing were low TD's coupled with low superheat being an indicator of inefficient/undersized compressor.

    Oh and last:

    I was challenging you to plot an inefficient compressor on a PH diagram thereby enlightening us. If you don't want to do this just say so.
    Last edited by Gary; 29-11-2009 at 06:25 PM.

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by desA View Post
    Is this correct ie. taken at those temps (was a little confused by your reference).

    Thoughts:
    If that is the case, then I feel that the condenser is running out of capacity (large SC), with the evap slightly over-capacity (large SH), at the 0'C operating condition. It seems to have a system imbalance.

    I'd not be at all happy with SH=0K heading towards a compressor, unless the line on the way gets hot & evaporates the droplets.
    Very likely Chef's system has a suction/liquid HX... which changes everything by adding both SC and SH.

    As far as we know Aaron's system does not have a suction/liquid HX.

    Why he (or his contractor) would charge the system to the point where it is flooding the compressor (1'F SH) is anybody's guess.
    Last edited by Gary; 29-11-2009 at 08:13 PM.

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Gary View Post

    The rules apply for both TXV and cap tube systems.

    .
    I thought you did mention that the rules apply for both TXV and cap tube systems?

    The point of posting the 3 conditions of a system (not Aaron's) was to demonstrate that a cap tube system operates with varying SH and SC and that it cannot be used to diagnose the systems condition directly.

    I dont think its useful for the forum to use TXV based analysis on a cap tube system - much better to use cap tube based analysis. Some of it has been documented but much of it has not. On the basis that they are very very different this may be an opportunity to move forward and start to develop some rules and guidelines.

    DesA kindly demonstrated this with a possible analysis of a condenser running out of capacity, a slightly oversize evap and system imbalance. Well this is a production unit with thousands of units worldwide and the system functions perfectly, the results shown are exactly correct for a cap tube system.

    Also as it is critically charged running with very low SH is tyipical when the tube inlet is right on the saturation line, no SC and X=0.

    Control and balance are more determined from the excess pressure in the condenser which could be caused, for one, by a high heat load. This may show a SC value of between 5.5K to 8.5K which Gary suggests would be his aim point, when in fact it represents a cap tube that is too long - it would force excessive head pressure, refrigerant backed into the condenser, starve the evap and have a lower than optimum COP.

    Another example may be low TD's, here balance is achieved by suction throttling the compressor, lowering the flow rate, allowing gas and liquid to pass through the tube (with an x=0.4 perhaps), less gas is condensed and so the TD shows up as low. This indicates a short cap tube. By TXV rules it seems to indicate an inefficient compressor.

    The thread should really concentrate on the cap tube issues.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Think of a TXV as a variable capacity cap tube - the variation driven from a temp signal. If you took the bulb off the evap line & stuck it in a fluid at preset reference temp, you could tune the 'variable cap tube'.

    The rules of RHVAC then should apply. If not, then why not?

    Actually, you could perform the same function with an AEV, orifice or even fine-tuned hand valve. They are all restriction devices. There is no black art in this.

    Why did the 'large manufacturer' get it wrong? Simple. Their systems were designed in a country far from the current environment in which they currently operate, or, ... they messed up...
    Last edited by desA; 30-11-2009 at 04:30 AM.
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Gary View Post
    I have no idea what your point is. Where did I say I could tell if all is well or not from those few numbers? I didn't. Nor did I say that all rules apply the same for TXV and cap tubes. The specific rules we were discussing were low TD's coupled with low superheat being an indicator of inefficient/undersized compressor.

    Oh and last:

    I was challenging you to plot an inefficient compressor on a PH diagram thereby enlightening us. If you don't want to do this just say so.
    Nicely sidestepped. Not sure if it is rhetoric or semantics though.

    The specific rule we are discussing then is low TD's and low SH. Well as mentioned in the above post it is TXV rules that suggest an ineffcient compressor and cap tube rules that suggest it is a very short tube. My point again is we should be using and building on the right set of rules.

    Oh and last.
    I did indeed suggest you plot an x=0.4 on a PH diagram and was also just challenging you, so here is the plot I was alluding to.
    PH diagram.GIF

    Hopefully you will see the h2-h3 is greatly reduced and hence the TD's will fall.

    Presumably you'll do the gentlemanly thing and post your inefficient compressor PH diagram.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by desA View Post
    Why did the 'large manufacturer' get it wrong? Simple. Their systems were designed in a country far from the current environment in which they currently operate, or, ... they messed up...
    Quote Originally Posted by desA View Post
    Why don't you test it for yourself, instead of slagging off a perfectly sound company?

    Build a small prototype & test if your theory is correct, or not.

    Then tell us the results of your experiment.
    So which one of these 2 quotes do you stand by?

    Chef

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    Re: Variable RPM compressor/cap tube issue

    Both, actually.

    The first quote is a typical occurrence for foreign companies manufacturing for a market with a local climate different to their own. It happens in the automotive heat-transfer industry all the time. It is the principal reason for CKD pack substitutions with local parts.

    The second is that you punted the 'name-dropping' scenario of 'big company is perfect'. I don't know their name, nor do I wish to. No big company is perfect - they are as good as their weakest designer. Your results prove exactly this.

    Better to keep the discussion technical - not personal, eh...
    Last edited by desA; 30-11-2009 at 05:07 AM.
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by Chef View Post
    The specific rule we are discussing then is low TD's and low SH. Well as mentioned in the above post it is TXV rules that suggest an ineffcient compressor and cap tube rules that suggest it is a very short tube.
    TXV rules would suggest an inefficient compressor or TXV stuck wide open (overfeed).

    Cap tube rules would suggest the same, the equivalent for TXV stuck wide open being the short cap tube (overfeed).

    IOW, inefficient compressor or overfeed, be it TXV or cap tube.


    Quote Originally Posted by Chef View Post
    Oh and last.
    I did indeed suggest you plot an x=0.4 on a PH diagram and was also just challenging you, so here is the plot I was alluding to.
    PH diagram.GIF

    Hopefully you will see the h2-h3 is greatly reduced and hence the TD's will fall.

    Presumably you'll do the gentlemanly thing and post your inefficient compressor PH diagram.

    Chef
    PH diagrams are your thing, not mine. I don't use them.

    I declined your challenge and apparently you have declined mine. Fair enough.
    Last edited by Gary; 30-11-2009 at 06:00 AM.

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    Re: Variable RPM compressor/cap tube issue



    I don't want to be rude, but this diagram is plainly incorrect.

    x=0 must, by definition, sit on the liquid saturation line - if you are referring to the LP line. If you are referring x=0 to the HP line, then your condenser is critically sized & any x>0 implies an undersized condenser...
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    Re: Variable RPM compressor/cap tube issue

    Quote Originally Posted by desA View Post


    I don't want to be rude, but this diagram is plainly incorrect.

    x=0 must, by definition, sit on the liquid saturation line - if you are referring to the LP line. If you are referring x=0 to the HP line, then your condenser is critically sized & any x>0 implies an undersized condenser...
    Plainly incorrect or misread and misunderstood?

    Previously we have been talking about the conditions at the entrance to the cap tube ie at point h(3) on the PH diagram. The arrows point to 3 cycles on the PH diagram for different X values at the h(3) point.

    As you can see the red line touches the saturation curve in the top left hand side. Maybe I should have been totally correct and said "This curve shown in red is the PH cycle for a cap tube system with an entry to the tube of x=0 ie at the h(3) point" but it did not fit on the diagram. I just figured you had been following the debate more closely, sorry for that.

    Quote Originally Posted by desA View Post
    If you are referring x=0 to the HP line, then your condenser is critically sized & any x>0 implies an undersized condenser...
    Again your quoting some dogma from TXV conditions and this just does not apply to cap tube systems - and the condenser is not critically sized nor is it undersized its just perfect. Whats imperfect is the cap tube length for the system and the PH diagram shows how it chooses to balance itself out even when the tube is wrong. By having entrained gas in the flow to make up more pressure drop. It is not an ideal situation nor one that should be designed for but it happens and it is used as a tool to evalute the correctness of the tube, not the condenser.

    Chef

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    Re: Variable RPM compressor/cap tube issue

    I think that you are confused on the technical definitions of x (vapour fraction).

    If you do want to refer to 'x' then it may be wiser to keep this to the LP portion of the graph - this is the traditional place in which it is used in relation to the vapour compression cycle. The reason for this is that the cycle is defined on full saturated liquid at point 3.

    There should be no vapour fraction going into a cap tube, or TXV... If there is, then the design needs to be altered - the cap tube is not meant as a total system imbalance corrector.

    If you do want to refer to the vapour fraction in some other place, then it may be wiser to state clearly 'x=0 @ HP' or something similar, to avoid confusion.
    Last edited by desA; 30-11-2009 at 10:31 AM.
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