Page 1 of 2 12 LastLast
Results 1 to 13 of 23
  1. #1
    Join Date
    Jul 2016
    Location
    New Jersey
    Posts
    3,116
    Post Likes

    Theoretical exercise in the sizing of HVAC system equipment

    *Disclaimer: This is just an exercise in theory. I have no particular installation in mind, and my general angle for the question is residential cooling, where installation of equipment is determined by the manufacturer, not custom-sized and built.

    Which components of an HVAC system are most critical when considering sizing and capacity, or are all components equally critical? Example, if the condensing unit was properly sized, but an undersized evaporator coil was put in, could you compensate for that by over-sizing the blower and ducts (ie. adding more CFM to grab more BTUs without screwing up static pressure)? Optimal efficiency wouldn't be reached, of course, but could a smart guy tweaking the system make up for one under-sized component (or an over-sized one, for that matter) with some creativity?

    Has anyone ever been forced to do something like this in the field, that is, try to make mismatched equipment work somehow?

    I'm not looking for a referral to Manuals J, S, T, D...etc. I just want to pick the brains of people who know more than me. Thanks.

  2. #2
    Join Date
    Jul 2014
    Location
    Apache Junction, Arizona
    Posts
    1,132
    Post Likes
    I do run the numbers if i have time through quickloads for good estimate what i gonna need ..

    Sometimes I have to replace lets say a condenser unit with a new one and has a refrigerant change like from R22 to 410A.

    Mostly I only have to change the metering device if is piston the recommended size or the TXV for that size tons and refrigerant type.

    I did suspect I had a mismatched coil once since the outside unit was stolen and when I replaced it I simply experimented a little with different orfice sizes on the piston..

    system ran well even mismatched.

    I pay close attention to the subcooling and superheat and airflow and condition of the ductwork and its integrity.

    I may not know more than you but I thought I would contribute to your thread..
    "I never lie because I don't fear anyone. You only lie when you're afraid." - John Gotti

    “Always shoot first . . . that way they know you’re armed!” - Orrin Porter Rockwell

    "Individuals and entities performing contracting work illegally and without a license place the public at risk and effectively steal millions from Arizona's hardworking, law-abiding contractors and their employees," - Jeff Fleetham, director of the Arizona Registrar of Contractors.

  3. #3
    Join Date
    Mar 2013
    Location
    Billington Heights, NY
    Posts
    21,628
    Post Likes
    PHM does this all the time. He's the Franken-King.

  4. Likes HellGato, MadMasticator liked this post
  5. #4
    Join Date
    Jul 2016
    Location
    New Jersey
    Posts
    3,116
    Post Likes
    Thread Starter
    Quote Originally Posted by HellGato View Post
    I do run the numbers if i have time through quickloads for good estimate what i gonna need ..

    Sometimes I have to replace lets say a condenser unit with a new one and has a refrigerant change like from R22 to 410A.

    Mostly I only have to change the metering device if is piston the recommended size or the TXV for that size tons and refrigerant type.

    I did suspect I had a mismatched coil once since the outside unit was stolen and when I replaced it I simply experimented a little with different orfice sizes on the piston..

    system ran well even mismatched.

    I pay close attention to the subcooling and superheat and airflow and condition of the ductwork and its integrity.

    I may not know more than you but I thought I would contribute to your thread..
    Haha...trust me HellGato, I guarantee you know more than me about this stuff. Heck, I didn't even think to mention orifice sizes, but now that you did, that's one more factor for the equation. That broadens (and complicates) the scope of my question. Cheers.

    In the case of the stolen unit, how did you come up with the size of the outdoor unit, if you didn't know the tonnage of the indoor coil? Just a good guess?

  6. Likes HellGato liked this post
  7. #5
    Join Date
    Jul 2016
    Location
    New Jersey
    Posts
    3,116
    Post Likes
    Thread Starter
    Quote Originally Posted by HVAC_Marc View Post
    PHM does this all the time. He's the Franken-King.
    Then I'll have to ask His Highness the next time I'm in the king's court...haha.

  8. Likes HellGato liked this post
  9. #6
    Join Date
    Jul 2014
    Location
    Apache Junction, Arizona
    Posts
    1,132
    Post Likes
    Quote Originally Posted by MadMasticator View Post
    Haha...trust me HellGato, I guarantee you know more than me about this stuff. Heck, I didn't even think to mention orifice sizes, but now that you did, that's one more factor for the equation. That broadens (and complicates) the scope of my question. Cheers.

    In the case of the stolen unit, how did you come up with the size of the outdoor unit, if you didn't know the tonnage of the indoor coil? Just a good guess?
    I ran a Manual J type heat load with something like O'Brians quick-loads pro it calculates heat load, the branch duct sizing and heat, cooling operation costs and payback analysis. Gives me the needed tons for the structure.. fairly close and can estimate my duct plan with it.

    This particular job came out to needing a 4 ton straight cool, I used York, and was a aftermarket R22 coil in a case on a old Lennox gas furnace.

    I and the customer were pleased.

    The job came out well even though I was not happy the condition of the ductwork they were not prepared to replace that.
    "I never lie because I don't fear anyone. You only lie when you're afraid." - John Gotti

    “Always shoot first . . . that way they know you’re armed!” - Orrin Porter Rockwell

    "Individuals and entities performing contracting work illegally and without a license place the public at risk and effectively steal millions from Arizona's hardworking, law-abiding contractors and their employees," - Jeff Fleetham, director of the Arizona Registrar of Contractors.

  10. #7
    Join Date
    Jan 2015
    Location
    East Tenn
    Posts
    194
    Post Likes
    Quote Originally Posted by HellGato View Post
    I ran a Manual J type heat load with something like O'Brians quick-loads pro it calculates heat load, the branch duct sizing and heat, cooling operation costs and payback analysis. Gives me the needed tons for the structure.. fairly close and can estimate my duct plan with it.

    This particular job came out to needing a 4 ton straight cool, I used York, and was a aftermarket R22 coil in a case on a old Lennox gas furnace.

    I and the customer were pleased.

    The job came out well even though I was not happy the condition of the ductwork they were not prepared to replace that.
    Yes, Custom Tweaks or Perfection of the Match is more like - A Real Installer - Not - just a - Plug & Play - Guy ..
    2 Thumbs Up to Real Installers that Care ..

  11. Dislikes kdean1 disliked this post
  12. #8
    Join Date
    Sep 2002
    Posts
    26,908
    Post Likes
    To some extent; sure. Although "there is no good substitute for surface area" should be kept in mind. <g>

    Refrigeration is a kind of a ballet - you are balancing a group of variables to achieve a good working result.

    In the A/C temp ranges you are also limited by the need to avoid frosting.

    Almost anything can be compensated for - to some extent. For example: you can undersize a liquid line and compensate for the pressure drop with greater condenser subcooling. But . . . you have to pay for that (using up condensing surface area to devote to subcooling) by pumping up higher pressures in the condenser. That costs you in operating energy for the compressor.

    In your example question the short answer is yes - but the long answer gets more involved. You can increase the heat loading of the evaporator by increasing air flow - but fairly soon you will be limited by the fluid dynamics of the air itself. The basic laws of physics always ultimately stand in your way. <g> At first more air flow increases the heat being presented to the evaporator - but fairly soon you will have been thwarted because the air moves both too fast and too inefficiently to effectively transfer the heat. I think this might be less true with a micro channel coil than with a tube & fin coil - because the round tubes represent a greater 'blunt object' for the air to effectively move around. And the faster the air moves - the greater the 'eddy current' / stagnant air accumulated behind each tube. The dead-air on the back side substantially reduces the effective surface area of the tube. That is why increasing water flow through the shell of a DX chiller barrel soon Decreases the performance. More and more water 'hides' behind each tube. I don't know how much theory you are interested in but Lord Kelvin's wave theories are fascinating reading. And apply to all kinds of various design. They help explain why ducks and battleships both produce the same wave structures. <g>

    But anyway: let's say the evap coil is undersized. It is not very easy to add heat that it was not designed to absorb - but the compressor automatically adapts to any Lower heat loading by decreasing it's own capacity. BTW: a compressor's capacity always equals the heat loading of it's connected evaporator - well; so long as the evaporator is smaller anyway. <g> This is because lower heat loading of the evaporator decreases the suction pressure. Lower pressure vapor is less dense. So each piston-stroke has fewer BTU's in it's volume. So the compressor only pumps the BTU's capacity of the connected evaporator.

    With an A/C application you can use an EPR valve to unevenly distribute the low suction pressure. Keeping it at (say) 33º inside the evaporator but at very low levels at the compressor. The net refrigerating effect is the same as the pressures net out - but the coil cannot frost.

    Adding relative heat exchanger surface area is always for the best. A big evaporator and a big condenser with a smaller compressor is exactly what manufacturers do to produce "high efficiency" equipment.

    As an example; I have a little rotary compressor out of a window box (I think it's 15K BTU's) in my 60K BTU condenser coil condensing unit. And it's pumping through a 36K BTU evaporator coil. R-407C and it runs suction pressure in the low 50's. It's inexpensive to run and cools well but now all my air ducts are too large and the ends of the house suffer for distribution. <g>

    I have been trying to answer your question in-between fielding phones calls about a half dozen far away crisises. So I feel like I'm rambling - but just ask a direct question and I see if I can answer it for you. <g>

    PHM
    -------



    Quote Originally Posted by MadMasticator View Post
    *Disclaimer: This is just an exercise in theory. I have no particular installation in mind, and my general angle for the question is residential cooling, where installation of equipment is determined by the manufacturer, not custom-sized and built.

    Which components of an HVAC system are most critical when considering sizing and capacity, or are all components equally critical? Example, if the condensing unit was properly sized, but an undersized evaporator coil was put in, could you compensate for that by over-sizing the blower and ducts (ie. adding more CFM to grab more BTUs without screwing up static pressure)? Optimal efficiency wouldn't be reached, of course, but could a smart guy tweaking the system make up for one under-sized component (or an over-sized one, for that matter) with some creativity?

    Has anyone ever been forced to do something like this in the field, that is, try to make mismatched equipment work somehow?

    I'm not looking for a referral to Manuals J, S, T, D...etc. I just want to pick the brains of people who know more than me. Thanks.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  13. #9
    Join Date
    Jul 2016
    Location
    New Jersey
    Posts
    3,116
    Post Likes
    Thread Starter
    Quote Originally Posted by Poodle Head Mikey View Post
    To some extent; sure. Although "there is no good substitute for surface area" should be kept in mind. <g>

    Refrigeration is a kind of a ballet - you are balancing a group of variables to achieve a good working result.

    In the A/C temp ranges you are also limited by the need to avoid frosting.

    Almost anything can be compensated for - to some extent. For example: you can undersize a liquid line and compensate for the pressure drop with greater condenser subcooling. But . . . you have to pay for that (using up condensing surface area to devote to subcooling) by pumping up higher pressures in the condenser. That costs you in operating energy for the compressor.

    In your example question the short answer is yes - but the long answer gets more involved. You can increase the heat loading of the evaporator by increasing air flow - but fairly soon you will be limited by the fluid dynamics of the air itself. The basic laws of physics always ultimately stand in your way. <g> At first more air flow increases the heat being presented to the evaporator - but fairly soon you will have been thwarted because the air moves both too fast and too inefficiently to effectively transfer the heat. I think this might be less true with a micro channel coil than with a tube & fin coil - because the round tubes represent a greater 'blunt object' for the air to effectively move around. And the faster the air moves - the greater the 'eddy current' / stagnant air accumulated behind each tube. The dead-air on the back side substantially reduces the effective surface area of the tube. That is why increasing water flow through the shell of a DX chiller barrel soon Decreases the performance. More and more water 'hides' behind each tube. I don't know how much theory you are interested in but Lord Kelvin's wave theories are fascinating reading. And apply to all kinds of various design. They help explain why ducks and battleships both produce the same wave structures. <g>

    But anyway: let's say the evap coil is undersized. It is not very easy to add heat that it was not designed to absorb - but the compressor automatically adapts to any Lower heat loading by decreasing it's own capacity. BTW: a compressor's capacity always equals the heat loading of it's connected evaporator - well; so long as the evaporator is smaller anyway. <g> This is because lower heat loading of the evaporator decreases the suction pressure. Lower pressure vapor is less dense. So each piston-stroke has fewer BTU's in it's volume. So the compressor only pumps the BTU's capacity of the connected evaporator.

    With an A/C application you can use an EPR valve to unevenly distribute the low suction pressure. Keeping it at (say) 33º inside the evaporator but at very low levels at the compressor. The net refrigerating effect is the same as the pressures net out - but the coil cannot frost.

    Adding relative heat exchanger surface area is always for the best. A big evaporator and a big condenser with a smaller compressor is exactly what manufacturers do to produce "high efficiency" equipment.

    As an example; I have a little rotary compressor out of a window box (I think it's 15K BTU's) in my 60K BTU condenser coil condensing unit. And it's pumping through a 36K BTU evaporator coil. R-407C and it runs suction pressure in the low 50's. It's inexpensive to run and cools well but now all my air ducts are too large and the ends of the house suffer for distribution. <g>

    I have been trying to answer your question in-between fielding phones calls about a half dozen far away crisises. So I feel like I'm rambling - but just ask a direct question and I see if I can answer it for you. <g>

    PHM
    -------
    I will certainly ask another question...actually, it's more like asking a favor:
    Would you kindly dumb down your answers to my questions in the future? I feel like a vastly undersized mortal coil that is struggling to convert the heavy load of your knowledge into sensible understanding.


    Hahaha...just kidding. I do appreciate the weightiness of your answer, and the generosity with which it was given...but honestly, I have to re-read your post a couple times before I can begin to formulate a follow-up question.

    Cheers!

  14. #10
    Join Date
    Sep 2002
    Posts
    26,908
    Post Likes
    You're right - I'm sorry. Yesterday I was handling all kinds of telephone and text nonsense in regard to far away incompetence.

    You are correct - always think in the most basic terms. Yes; you can manipulate the heat transferring processes - but always look to make it easier and not harder to do. Or rather; for the machines to do. <g>

    The equipment you can typically buy is a huge compromise of factors. Costs of all kinds are always considered, manufacturing ease, warehousing, materials handling - all that has to be factored in in ways that do not impact what you and I can do.

    Someone on here has a great signature line: "If you cannot explain what you are doing, as a process, then you don't know you are doing."

    In your refrigeration work start thinking less about What happens and focus more on Why it happens - AND how each thing which happens impacts every Other thing that happens.

    Mere memorizing is not real learning. Understanding is learning.

    PHM
    -------



    Quote Originally Posted by MadMasticator View Post
    I will certainly ask another question...actually, it's more like asking a favor:
    Would you kindly dumb down your answers to my questions in the future? I feel like a vastly undersized mortal coil that is struggling to convert the heavy load of your knowledge into sensible understanding.


    Hahaha...just kidding. I do appreciate the weightiness of your answer, and the generosity with which it was given...but honestly, I have to re-read your post a couple times before I can begin to formulate a follow-up question.

    Cheers!
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  15. #11
    Join Date
    Jul 2016
    Location
    New Jersey
    Posts
    3,116
    Post Likes
    Thread Starter
    Quote Originally Posted by Poodle Head Mikey View Post
    You're right - I'm sorry. Yesterday I was handling all kinds of telephone and text nonsense in regard to far away incompetence.

    You are correct - always think in the most basic terms. Yes; you can manipulate the heat transferring processes - but always look to make it easier and not harder to do. Or rather; for the machines to do. <g>

    The equipment you can typically buy is a huge compromise of factors. Costs of all kinds are always considered, manufacturing ease, warehousing, materials handling - all that has to be factored in in ways that do not impact what you and I can do.

    Someone on here has a great signature line: "If you cannot explain what you are doing, as a process, then you don't know you are doing."

    In your refrigeration work start thinking less about What happens and focus more on Why it happens - AND how each thing which happens impacts every Other thing that happens.

    Mere memorizing is not real learning. Understanding is learning.

    PHM
    -------
    Please, Mikey, no apology necessary. On the contrary, I really appreciated your initial response and the wealth of information in it...I just needed a little time to ruminate on its various points. And you're absolutely right, every mechanical system is a kind of ballet in which the designer directs an interplay of natural forces and man-made components to some particularly narrow spectrum of usefulness. Cool stuff.

    When I first pondered your description of air flow around the evaporator coil tubes, I wondered about thermal conductivity as a factor. Copper and aluminum have very good thermal conductivity, but not the best, correct? So then, have you ever heard of a heat exchanger coil that was coated with some kind of superior thermal conductor (graphite or graphene perhaps)? No idea if it could help significantly (due to the limits of the underlying metal, and perhaps to increased air turbulence if coating was not smooth enough), but it seemed like a question worth asking.

    How about a situation in which the evaporator is grossly over-sized in relation to the compressor and condenser? If I understand correctly, the suction pressure and condenser pressure would go higher, which would in turn inhibit subcooling, so how could you try to compensate for this imbalance? A CPR of some sort? Slow down air flow across evaporator?

    (Bear with me, if I'm off-base with any of this...I'm trying to keep up with you.)

    Your custom A/C unit sounds neat, but I can't figure out why air distribution is bad and the ducts are over-sized. I'm really curious to know because this is the first time I've heard of a scenario with over-sized ducts, and I've always wondered where and how the line of duct over-sizing could be crossed.

    Thanks again for sharing your expertise.

  16. #12
    Join Date
    Sep 2002
    Posts
    26,908
    Post Likes
    Picture just a single tube. Air flows toward and then around the tube - half above and half below. On the far side of the tube the divided air flows come back together again. As a result the entire tube is in contact with the air flow and heat is exchanged all the way around the tube.

    But, as you increase the air flow / velocity / FPM the upper and lower flows around the tube no longer come together around on the far side of the tube - the air flows past and comes together Away from the tube: past it. This leaves a gap - which is filled with non-flowing air. This 'dead' air acts as an insulator and so that portion of the tube no longer cools the air flow.

    PHM
    -----------


    Quote Originally Posted by MadMasticator View Post
    . . . . When I first pondered your description of air flow around the evaporator coil tubes . . . .
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  17. #13
    Join Date
    Sep 2002
    Posts
    26,908
    Post Likes
    New Jersey eh?

    Do you really chew very very fast?
    Last edited by Poodle Head Mikey; 09-22-2016 at 10:29 AM.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

Page 1 of 2 12 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
  •