Page 4 of 6 FirstFirst 123456 LastLast
Results 40 to 52 of 67
  1. #40
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    Quote Originally Posted by Andy Schoen View Post
    Yes.


    That's part of the problem.


    We have 5 tons of air flow across the evaporator and about 75 percent of it is missing the mark... It is flowing across tubes with no refrigerant in them.
    So the 25% airflow that is hitting the mark is overloading the evaporator with air to refrigerant heat transfer in that section.

    Quote Originally Posted by Andy Schoen View Post
    That is correct. But even with my analogy, a 1.25 ton evaporator that is attempting to cool the 5 tons of air properly (say 2000 CFM), you would have very high TDs resulting in very low suction pressures. Compressor capacity would drop from 5 tons to perhaps around 3 tons.
    Effective capacity would drop to 3 tons, but the compressor is still a 5 ton compressor that is trying to pump 5 tons.
    What TD's are you talking about?
    The low suction is being caused by too little evaporator and too much compressor.

    Quote Originally Posted by Andy Schoen View Post
    The TEV is way oversized in either case. Blocked distributor tubes will not cause the TEV to control at high superheat, unless all of the distributor tubes are blocked.
    The TEV is oversized, BUT the feeder tubes are undersized, that's the whole problem.
    If all tubes are blocked, there is no refrigerant being pumped.
    BTW, I really enjoy these discussions.
    jogas

  2. #41
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    Quote Originally Posted by jogas View Post
    So the 25% airflow that is hitting the mark is overloading the evaporator with air to refrigerant heat transfer in that section.
    Not really. It is more or less doing normal heat transfer in the case of 3 out of 4 blocked distributor tubes. The other 75 percent of the airflow/heat load is cruising across the evaporator not being cooled.

    Quote Originally Posted by jogas View Post
    Effective capacity would drop to 3 tons, but the compressor is still a 5 ton compressor that is trying to pump 5 tons.
    Yes, the 5 ton compressor becomes a 3 ton compressor operating at these conditions.

    Quote Originally Posted by jogas View Post
    What TD's are you talking about?
    The low suction is being caused by too little evaporator and too much compressor.
    You seem to have a basic understanding of TD (temperature difference). Condenser and evaporator coil sizing is done based on TD. Read up on this subject if you're going to size condenser and evaporator coils.
    Quote Originally Posted by jogas View Post
    The TEV is oversized, BUT the feeder tubes are undersized, that's the whole problem.
    In the case of blocked distributor tubes, the unblocked tube(s) will in fact be more or less correctly sized for the load the active portion of the evaporator is seeing. In the case of an undersized evaporator coil that is properly working, the distributor tubes will generate greater than designed pressure drop, but this would be compensated by an oversized TEV. If the undersized evaporator has a properly sized TEV, then I would expect a higher than normal superheat condition, and least until the coil iced up.

  3. #42
    Join Date
    May 2008
    Location
    Southern California
    Posts
    55
    I think what they are saying Jogas is that only 1/5 of the air is in contact with the active part of the evap. the other 80% is passing thru the parts of the evap that have no refrigerant because of the clogged tubes. Those tubes and that air are out of the calculation, only the remaining 20% have air and freon. The suction gas is overcold because of extra compressor capacity causing low suction and the small amout of freon in contact with the air.
    I hope I am making myself clear. I was out in 100+ degree temps changing out a 400 ton condensor for the last two days and my brain is baked.

    Have fun, be safe,
    Mike

  4. #43
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    Quote Originally Posted by Andy Schoen View Post
    Yes, the 5 ton compressor becomes a 3 ton compressor operating at these conditions.
    It doesn't magically reduce it's displacement from 3 to 5 tons. The proof is the low suction pressure. It is in fact effective tonnage given the present condition.

    Quote Originally Posted by Andy Schoen View Post
    You seem to have a basic understanding of TD (temperature difference). Condenser and evaporator coil sizing is done based on TD. Read up on this subject if you're going to size condenser and evaporator coils.
    Nice try, but your answer has nothing to do with my question.
    Please send me any material you have on sizing an evaporator given the conditions we are discussing now.
    AGAIN, what TD?
    An evaporator with only 25% active heat transfer surface is going to give high TD's? 1.25 ton of cooling is going to cause hi TD to 5 tons of air?
    Please enlighten me.

    Quote Originally Posted by Andy Schoen View Post
    In the case of blocked distributor tubes, the unblocked tube(s) will in fact be more or less correctly sized for the load the active portion of the evaporator is seeing. In the case of an undersized evaporator coil that is properly working, the distributor tubes will generate greater than designed pressure drop, but this would be compensated by an oversized TEV. If the undersized evaporator has a properly sized TEV, then I would expect a higher than normal superheat condition, and least until the coil iced up.

    How can an oversized TEV that is upstream of a restriction (only one feeder tube) compensate for anything downstream? In this case, the feeder tube is acting almost like a cap tube. It is sized only to handle it's circuit (unless it has HGBP). Re-reading sizing evaporator feeder tubes seems appropriate here.
    The net effect will be high superheat or starving. Then, and only then, because of the restriction causing that circuit's low evaporator pressure, will that circuit ice up reducing it's heat transfer, further reducing "TD".
    Often, DX systems ice up in the same manor when low on refrigerant.
    It is a side effect caused by low evap temp/suction pressure caused by the feeder tube acting as a restriction.
    jogas

  5. #44

    restriction vs. load, sunbaked not half-baked

    Andy said:

    Sorry, when I'm balancing a long thread and several quotes I'm not sure how you get the effect that names the source of the quote. If I wasn't baked from several days in the sun swaping a 7.5 ton compressor (you got me beat by a few tons there Quixote, and you beat me to a decent screen name although in these contexts I can't help but pay homage to De Niro in Brazil - get's me thown off the prissy boards sometimes). So, anyway, I'll do it the old way

    Andy said:

    The blocked distributors [sic] tubes do not represent a meter restriction.

    The best analogy for 3 blocked distrbutor tubes out of 4 on a 5 ton evaporator is you've replaced a properly working 5 ton evaporator with matched TEV with a 1.25 ton evaporator controlled by a 5 ton TEV.
    I'm buying this if the size of the distributor tubes is large relative to the meter opening which, upon reflection seems likely although I was wondering how large they could be to get blocked by solder. still no word from HVAC689 on this. To rephrase this for the picayune amongst us, there is a theoretical metering effect, i.e. pressure drop, of trying to put the same compressor capacity down 1 tube rather than 4 but it is marginal in terms of the pressure regulation in the system compared to the lack of load from the other three tubes. The TEV would presumably close as far as its range allows. I assume the normal range never extends to fully closed although I haven't had the sporlan class that greenears has so I'm just guessing that would be bad -- although when the charge is gone they close all the way so there doesn't seem to be any positive stop that holds them marginally open.

    which leads to the next thing

    Andy said:

    The TEV is way oversized in either case. Blocked distributor tubes will not cause the TEV to control at high superheat, unless all of the distributor tubes are blocked.
    What about low superheat? It seems to me, as mentioned above that there might be some lower limit to the closure of the TEV so that you would loose ability to throttle below a certain point - maybe there is not a physical limit that prevents full closure but various heads list various operating temps through which they can control. I'm pretty sure the temps we're talking about 15 to 25 deg. F (I also don't know how to make the fancy deg. symbol) are within the range, but the sizing of the TEV apparently affects its ability to throttle subtlely at low loads. I didn't get wide variations at low load, but I got low superheat (2 or 3 degrees) - now this is a new XGA head on a sporlan SVE style valve that called for an XCP-100 but they didn't have it and the XGA is supposed to be pretty close in temp. and superheat range. And, I have never tested the valve under full load conditions so I can't compare its performance, but all of the "put down the wrench and get away from the valve stuff" says to me that the TEV doesn't perform adequately way below, and obviously way above its target capacity.

    Quixote said:

    I think what they are saying Jogas is that only 1/5 of the air is in contact with the active part of the evap. the other 80% is passing thru the parts of the evap that have no refrigerant because of the clogged tubes. Those tubes and that air are out of the calculation, only the remaining 20% have air and freon. The suction gas is overcold because of extra compressor capacity causing low suction and the small amout of freon in contact with the air.
    Aside from the minor point that we're talking 75% and 25% vs. 80% and 20% (not to quibble, just to clarify) I want to confirm my reading of such a circumstance which is that if there is no significant additional metering effect then this duplicates the circumstance where you pass 25% of your load across and entirely working evap. with no blocked tubes. In other words it duplicates a low load condition in terms of symptoms.

    This thread is going places, I wouldn't call this hijacking exactly, but it has touched a nerve given its lifespan. So I hope it doesn't seem like I'm beating it to death- a far worse crime than hijacking IMO.

    Brian

    There is no excuse for me and...
    ...There is no such thing as an EXTRA beer.

  6. #45
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    Quote Originally Posted by archibaldtuttle View Post
    Andy said:

    This thread is going places, I wouldn't call this hijacking exactly, but it has touched a nerve given its lifespan. So I hope it doesn't seem like I'm beating it to death- a far worse crime than hijacking IMO.
    Brian
    IMHO,
    If I'm a hijacker, I apologize.
    This thread has members who are experienced and passionate about our trade and this forum. We are discussing and debating an unusual system condition that causes everyone to think beyond the routine.
    Conflicting theories should be used as thought-provoking and not in any way personal.
    jogas

  7. #46
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    FWIW,
    TEV capacity with all other factors the same, is directly related to the condensing temperature and the amount of sub-cooling at the valve's inlet.
    I've always used the Sorlan "E" valve as it would throttle down (under ideal conditions) to 20% of rated capacity. Best valve I've ever used in 30 years.
    Sporlan now has an "E" replacement, but I cannot remember the number. I only used them once when I replaced 8 worn out Alco Take-Aparts on a 100 ton AHU. These worked well also.
    jogas

  8. #47

    no relevant capillary action

    Jogas, sorry we were posting around the same time so I would have brought in your comments as well.

    You said:
    How can an oversized TEV that is upstream of a restriction (only one feeder tube) compensate for anything downstream? In this case, the feeder tube is acting almost like a cap tube. It is sized only to handle it's circuit (unless it has HGBP).
    This was my intuition, generally speaking, that aside from lower load because only a quarter of the air is actually passing over this area (although presumably air temps would not drop throughout the system such that the load posed by air temps and humidity vs. just by airflow would probably be more than a 25% load upon consideration but I think still a relevantly 'low load'.

    However, Andy suggests, and I am beginning to come around that the amount of restriction posed by the bloackage of 3 distributor tubes is simply not a relevant pressure drop insofar as system metering. The lower suction pressures aren't so much related to blocked tubes but to the lower vapor pressure at the lower evap exit, i.e. suction, temp resulting from lower load.

    next Jogas said:

    It doesn't magically reduce it's displacement from 3 to 5 tons. The proof is the low suction pressure. It is in fact effective tonnage given the present condition.
    But to put this a different way. If HVAC689 were to replace the 5 ton compressor with a 1.25 ton instead of clearing the blocked tubes, the pressures would norm out -- caveat being the 'fine motor skills' of the oversized TEV.

    finally (well not exactly, I'm taking these out of order but they were all in his most previous post) Jogas said:

    An evaporator with only 25% active heat transfer surface is going to give high TD's? 1.25 ton of cooling is going to cause hi TD to 5 tons of air?
    Please enlighten me.
    yes and no in my thinking. Assuming the condenser is running full bore and higher than normal head -- by comparison to tail, i.e. suction, not necessarily in absolute terms but maybe a little higher absolutely as well, but mostly a wider pressure spread -- you should present more subcooling in theory (right? -- anyone), and a lower than normal refrigerant temp on avg. throughout the working tube area on the evap given suction pressures anyway as reported (right? -- anyone). Since the building air is going to remain warmer since 75% of it is not getting cooled on each pass the resulting coil vs. airflow temp in the working coil would increase it's effective load but nowhere near enough to compensate for the blocked tubes.

    It would be helpful Andy if you could clarify, I'm assuming your talking refrigerant TD's but maybe it is assumed your talking building air,

    Back to getting baked in the sun. Well my mendacious side is getting the better of me cloudy, a little too humid to make the tenants at the job we're in the middle of happy with the promise that AC will be working Tuesday, and maybe a little rain. Real crap of this is that the sunny days are when the synoptic winds get an adiabatic boost and I should have been kiteboarding. I'll try to confirm that when I figure out how you post an avatar.

    Brian

  9. #48
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    Quote Originally Posted by archibaldtuttle View Post
    Jogas, sorry we were posting around the same time so I would have brought in your comments as well.

    You said:


    This was my intuition, generally speaking, that aside from lower load because only a quarter of the air is actually passing over this area (although presumably air temps would not drop throughout the system such that the load posed by air temps and humidity vs. just by airflow would probably be more than a 25% load upon consideration but I think still a relevantly 'low load'.

    However, Andy suggests, and I am beginning to come around that the amount of restriction posed by the bloackage of 3 distributor tubes is simply not a relevant pressure drop insofar as system metering. The lower suction pressures aren't so much related to blocked tubes but to the lower vapor pressure at the lower evap exit, i.e. suction, temp resulting from lower load.

    next Jogas said:



    But to put this a different way. If HVAC689 were to replace the 5 ton compressor with a 1.25 ton instead of clearing the blocked tubes, the pressures would norm out -- caveat being the 'fine motor skills' of the oversized TEV.

    finally (well not exactly, I'm taking these out of order but they were all in his most previous post) Jogas said:



    yes and no in my thinking. Assuming the condenser is running full bore and higher than normal head -- by comparison to tail, i.e. suction, not necessarily in absolute terms but maybe a little higher absolutely as well, but mostly a wider pressure spread -- you should present more subcooling in theory (right? -- anyone), and a lower than normal refrigerant temp on avg. throughout the working tube area on the evap given suction pressures anyway as reported (right? -- anyone). Since the building air is going to remain warmer since 75% of it is not getting cooled on each pass the resulting coil vs. airflow temp in the working coil would increase it's effective load but nowhere near enough to compensate for the blocked tubes.

    It would be helpful Andy if you could clarify, I'm assuming your talking refrigerant TD's but maybe it is assumed your talking building air,

    Back to getting baked in the sun. Well my mendacious side is getting the better of me cloudy, a little too humid to make the tenants at the job we're in the middle of happy with the promise that AC will be working Tuesday, and maybe a little rain. Real crap of this is that the sunny days are when the synoptic winds get an adiabatic boost and I should have been kiteboarding. I'll try to confirm that when I figure out how you post an avatar.

    Brian
    Now that was a thought-provoking post
    Synoptic winds...adiabatic boost....kiteboarding explains all

    One point:
    What is causing the lower vapor pressure at the lower evaporator exit i.e. suction, temp resulting from lower load?
    It has to be the missing refrigerant contribution of the plugged feeder tubes.
    Each feeder tube is designed to provide an equal amount that totals the evaporator load. I've had our Sporlan Rep out several times on problem jobs and reviewed existing evaporator feeders.
    BTW, he also told me a long time ago if you add hot gas bypass (at the TEV outlet) to a system, the feeders should be increased one size. It's hardly ever done in the field, or ordered that way from the factory.
    I did do it a couple of times with great control results.
    jogas

  10. #49
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    Quote Originally Posted by jogas View Post
    It doesn't magically reduce it's displacement from 3 to 5 tons. The proof is the low suction pressure. It is in fact effective tonnage given the present condition.
    I'm not sure what your point is here. A compressor's displacement is a volumetric calculation and it is, of course, fixed. A compressor's rating is its ability to pump refrigerant mass flow, and this figure is influenced by the refrigerant type and the operating conditions.

    Quote Originally Posted by jogas View Post
    Please send me any material you have on sizing an evaporator given the conditions we are discussing now.
    AGAIN, what TD?
    icemeister had a nice post on this subject in this very forum: http://www.hvac-talk.com/vbb/showthread.php?t=101124 I sure there are others. You may also want to download a product catalog from one of the coil manufacturers.

    Quote Originally Posted by jogas View Post
    How can an oversized TEV that is upstream of a restriction (only one feeder tube) compensate for anything downstream? In this case, the feeder tube is acting almost like a cap tube. It is sized only to handle it's circuit (unless it has HGBP).
    You're under the mistaken belief that somehow the entire 5 tons of refrigerant mass flow is being pushed thru this one circuit. It is not. The TEV will not allow it. The TEV will only allow as much refrigerant thru this one circuit as it can handle, which will be about 25 percent of the total 5 ton rating of the evaporator.

    Remember, the TEV only controls superheat. It does not control nor compensate for low suction pressure.

    Quote Originally Posted by jogas View Post
    Re-reading sizing evaporator feeder tubes seems appropriate here.
    I carry that stuff in my head. Some say I wrote the book on this subject.

    Quote Originally Posted by jogas View Post
    The net effect will be high superheat or starving.
    Nope. Low superheat and an evaporator that is function at 25 percent of its capacity. "Starving" is a bad term here as it implies high superheat with low suction pressure.

    Quote Originally Posted by jogas View Post
    Then, and only then, because of the restriction causing that circuit's low evaporator pressure, will that circuit ice up reducing it's heat transfer, further reducing "TD".
    Actually, coil icing increases TD, but you understand the relationship of low suction pressure and coil icing.

    Quote Originally Posted by jogas View Post
    Often, DX systems ice up in the same manor when low on refrigerant.
    It is a side effect caused by low evap temp/suction pressure caused by the feeder tube acting as a restriction.
    It's normally cause by a low refrigerant charge causing flash gas in the liquid line causing the TEV to act like a restriction. Properly sized distributor tubes can rarely act like restrictions themselves unless, of course, they have been crimped or have solder in them.

  11. #50

    I say yes

    Quote Originally Posted by jogas View Post

    What is causing the lower vapor pressure at the lower evaporator exit i.e. suction, temp resulting from lower load?
    Yes that is my understanding.


    Quote Originally Posted by jogas View Post
    our Sporlan Rep ....
    BTW, he also told me a long time ago if you add hot gas bypass (at the TEV outlet) to a system, the feeders should be increased one size. It's hardly ever done in the field, or ordered that way from the factory.
    I did do it a couple of times with great control results.
    jogas
    I haven't worked on hot gas bypass system. Is this a way of keeping the head from getting too high when the TEV is restricting flow?

    Brian

    PS -tried to fix the avatar, hope it worked.

    PPS - I was alleging any hijacking earlier, I think that concept is thrown around much too readily. Any thread that hasn't hijacked itself by the 4th page of posts could likely be has and rehash.

  12. #51
    Join Date
    Jun 2005
    Location
    Ohio
    Posts
    1,720
    Quote Originally Posted by Andy Schoen View Post
    I'm not sure what your point is here. A compressor's displacement is a volumetric calculation and it is, of course, fixed. A compressor's rating is its ability to pump refrigerant mass flow, and this figure is influenced by the refrigerant type and the operating conditions.
    The compressor is a positive displacement, and in this example, a constant volume pump. This one is sized to pump 5 tons of superheated refrigerant. It's only seeing 25% of that.
    Hence the low evaporator/suction pressure.
    In it's present state, the whole system is trying to operate at 5 tons EXCEPT the feeder tubes. Hence the low evaporator/suction pressure.
    ANY temperatures are a byproduct of this condition.

    Quote Originally Posted by Andy Schoen View Post
    icemeister had a nice post on this subject in this very forum: http://www.hvac-talk.com/vbb/showthread.php?t=101124 I sure there are others. You may also want to download a product catalog from one of the coil manufacturers.
    I will read it when I get the time, thanks.

    Quote Originally Posted by Andy Schoen View Post
    You're under the mistaken belief that somehow the entire 5 tons of refrigerant mass flow is being pushed thru this one circuit. It is not. The TEV will not allow it. The TEV will only allow as much refrigerant thru this one circuit as it can handle, which will be about 25 percent of the total 5 ton rating of the evaporator.
    Nope. If the whole 5 tons could be pushed thru the one feeder tube, the feeder tube was not designed properly. It should handle 25% (if there are 4 tubes) of the evap design load.
    It can't, hence the low evaporator/suction pressure.

    Quote Originally Posted by Andy Schoen View Post
    Remember, the TEV only controls superheat. It does not control nor compensate for low suction pressure.
    I carry that stuff in my head. Some say I wrote the book on this subject.
    I don't know about any books, but I'm relying on 30 years of service technician experience at and including residential/commercial/industrial systems.
    To predict what a TEV will do in this out of design condition is impossible. System configuration, load balance, TEV type, TEV bulb location make it a guess at best.

    Quote Originally Posted by Andy Schoen View Post
    Nope. Low superheat and an evaporator that is function at 25 percent of its capacity. "Starving" is a bad term here as it implies high superheat with low suction pressure.
    The evaporator is starving if it's seeing only 25% of it's intended refrigerant.

    Quote Originally Posted by Andy Schoen View Post
    Actually, coil icing increases TD, but you understand the relationship of low suction pressure and coil icing.
    Actually, no, otherwise we would not have finned coils, but "slabbed" evaporators making ice.
    As I'm sure you know, the reason we use "evaporators" is to take advantage of the latent heat change of state capacity of the refrigerant, not ice melting from air moving over a slab.

    Quote Originally Posted by Andy Schoen View Post
    It's normally cause by a low refrigerant charge causing flash gas in the liquid line causing the TEV to act like a restriction.
    Properly sized distributor tubes can rarely act like restrictions themselves unless, of course, they have been crimped or have solder in them.
    Or you have only one feeder tube open because the others have been filled with solder. Thank you.
    jogas

  13. #52
    Join Date
    Apr 2002
    Location
    Dallas, TX
    Posts
    2,987
    Quote Originally Posted by jogas View Post
    The compressor is a positive displacement, and in this example, a constant volume pump. This one is sized to pump 5 tons of superheated refrigerant. It's only seeing 25% of that.
    Hence the low evaporator/suction pressure.
    In it's present state, the whole system is trying to operate at 5 tons EXCEPT the feeder tubes. Hence the low evaporator/suction pressure.
    ANY temperatures are a byproduct of this condition.
    You are correct up to this point. Where you are messing up is thinking this will cause a high superheat condition with the TEV controlling refrigerant flow thru an active circuit. I don't think you fully grasp how the TEV functions.

    Quote Originally Posted by jogas View Post
    Nope. If the whole 5 tons could be pushed thru the one feeder tube, the feeder tube was not designed properly. It should handle 25% (if there are 4 tubes) of the evap design load.
    It can't, hence the low evaporator/suction pressure.
    Nope. The TEV will control the flow thru the one distributor tube. Hence the low evaporator/suction pressure. And operate at a low to normal superheat. That is what the TEV is designed to do. Control superheat. Repeat. Control superheat. It doesn't care about an evaporator with 75 percent inactive circuits operating at low suction pressure. It will control the superheat coming out of the active circuit.

    Quote Originally Posted by jogas View Post
    I don't know about any books, but I'm relying on 30 years of service technician experience at and including residential/commercial/industrial systems.
    To predict what a TEV will do in this out of design condition is impossible. System configuration, load balance, TEV type, TEV bulb location make it a guess at best.
    My 30 years in this industry primarily as an engineer at Sporlan, and having done service work myself suggests you might want to rethink this.

    Quote Originally Posted by jogas View Post
    Actually, no, otherwise we would not have finned coils, but "slabbed" evaporators making ice.
    As I'm sure you know, the reason we use "evaporators" is to take advantage of the latent heat change of state capacity of the refrigerant, not ice melting from air moving over a slab.
    Trust me on the TD thing. Reduced heat transfer equals greater TD.

Page 4 of 6 FirstFirst 123456 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event