Theoretical exercise in the sizing of HVAC system equipment

[MadMasticator]

*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.

[HellGato]

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..

[HVAC_Marc]

PHM does this all the time. He's the Franken-King.

[MadMasticator]

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?

[MadMasticator]

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.

[HellGato]

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.

[QualityAssure]

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 ..

[Poodle Head Mikey]

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
-------

*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.

[MadMasticator]

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!

[Poodle Head Mikey]

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
-------

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!

[MadMasticator]

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.

[Poodle Head Mikey]

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
-----------

. . . . When I first pondered your description of air flow around the evaporator coil tubes . . . .

[Poodle Head Mikey]

New Jersey eh?

Do you really chew very very fast?

[Poodle Head Mikey]

I do not know of, and find it hard to picture, a use-able coating which would improve the thermal performance of commonly available materials. It is pretty hard to beat copper, and aluminum is right up there too. Some of the heavy salts are better; lithium and maybe some others, but copper is ever better than silver and gold (I am pretty sure <g>) and making heat exchangers is a cost-factor kind of thing. <g>

Steel is about 1/4 as good of the conductor that aluminum is - and copper is about twice as good as aluminum. It is somewhat interesting that aluminum's performance Increases with temperature. Although I don't know why. It is always odd to me the weird useless details that stick in my head. <g>

PHM
-------

. . . . 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)? . . . .

[Poodle Head Mikey]

The evaporator size is somewhat less important to match to the compressor capacity than it is match the air flow across it.

Picture a matched-component 3 ton A/C system. Now we take out the 3T evap and install a 5T evap in it's place. What happens? Sure the HX surface area is greater but the air flow and it's heat content are the same. So the suction pressure would likely increase slightly - but we could trim it back by reducing the air flow slightly to compensate. The head pressure would not really change as the amount of heat being exchanged would be just about the same. The added surface area of the evap would increase the moisture removal though.

Why does increased head pressure inhibit subcooling? What exact process causes this result?

PHM
-------

. . . . 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? . . . . .

[Poodle Head Mikey]

My original system was heating-only and heated both floors of the house. When I changed from oil to natural gas I added 3 tons of A/C and disconnected all the ducts to the 2nd floor of the house. The 2nd floor got a Unico air handler and ducts with a 250,000 BTU water coil in the return.

I gradually improved the envelope. Added foam board to the exterior and then foam filled siding over that. So the walls are about R-20 now. Some new thermal windows - with storm windows over them. <g> Eventually I installed a white solar reflective roof (Sheffield's Cool/R product in Solar White) Now on a sunny 100º*afternoon my attic temp is about 80º.

The heating system went through various configurations; wood furnace, boiler w/air coil, down flow furnace, upflow furnace, horizontal furnace. Eventually ending up with a 15K (it could be 18K) compressor in a 5T condensing unit. 3T evap but to hold the suction down in the low 30 degree range I reduced the air flow. I'm guessing but I'm sure I have maybe 600-700 CFM at this point. Which makes the existing air ducts (relatively) much too large. <g> The system is centered and the house is somewhat long so the ends suffer for effective air flow.

I have Zero desire to change out the entire duct system but I have been pondering installing two motorized dampers where the main trunk tees. Put them on an either/or control relay and then heat and cool the two halves of the house that way: one at a time. Open and close the damper alternately at maybe 2 minute intervals. Say; an interval timing relay which cycles back and forth so long as the stat is calling. Or maybe so long as the blower motor is pulling amps.

It makes me laugh, and somewhat sad too, to think that if I ever hit by a careening bus whoever gets called in to fix the HVAC at mu house will either stare, go crazy, and run away screaming. Or more likely; sternly announce that the whole system is rubbish and who ever built this madness was a complete idiot and the only remedy is a new off-the-shelf system.

Hell; just looking at the solar system piping and heat exchangers and controls makes my helper cringe. <g>

And really; maybe they are all right. Other than me enjoying doing it all - is it really better?

PHM
-------

. . . . 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.

[MadMasticator]

New Jersey eh?

Do you really chew very very fast?

Jersey born and raised, but I spent most of my adult life in Western states. Came back a few years ago to look after my ailing parents.

Are Jerseyans now characterized as fast chewers, or is that code for some kind of stereotypical speech pattern?

[Poodle Head Mikey]

What does 'Mad Masticator' mean then ? <g>

PHM
--------

Jersey born and raised, but I spent most of my adult life in Western states. Came back a few years ago to look after my ailing parents.

Are Jerseyans now characterized as fast chewers, or is that code for some kind of stereotypical speech pattern?

[MadMasticator]

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
-----------

I get it. In any mechanical process, we eventually hit a limit of what is possible within the laws of physics. At some point, the sweet spot of the designed "cooperation" in an evaporator (in this case, cooperation between the functions of fluid dynamics and thermal conductivity) turns sour, and we must concede to rules of proportionality in Mother Nature.

[MadMasticator]

What does 'Mad Masticator' mean then ? <g>

PHM
--------

Ohhhhh...pun-stunned by my own creation! Hahahaha...

In truth, my preferred style (whether slinging mastic, or chewing intellectual cud) is slow and deliberate. Don't get me wrong, I'm no sloth...it's just that I think that speed is vastly overrated, in work, in music, in food...in pretty much everything. The 'mad' in MadMasticator was meant to convey enthusiasm, intensity and unpredictability (and perhaps a tad of irreverence), but definitely not speed.