Doubling the air flow-rate across an evaporator coil

[HighPotential]

Years ago I masqueraded as a mechanical engineer and designed my own home HVAC dual zone duct and register system. One my buddies who is a mech, loaned me his "ductillator", and with the help of these forums, I proceeded to oversize my ducts and registers to about 3x the system size as I wanted it to be silent. Well I succeeded but learned an important lesson that you do need a minimum amount of velocity as part of the cooling effect. I had a lot of fun doing it and the system does keep the house conditioned. Anyway, flash forward to today. The Mac Guyver side of me is scheming to custom fit a larger blower (double the flowrate) in the heater/fan unit. This would not quite double the airflow across the evaporator coil and heater element. I do not need more cooling/heating capacity, just more airflow.

My question is a fluid dynamics/heat transfer one- would this double velocity ruin the efficiency of my HVAC system or only affect it minimally while providing the greater velocity I am seeking?

Thanks!

 

[Dougt115]

The temperature drop from input to output of the heater/cooler would be less. Just because the heater/cooler has less time to interact with the air. However twice the air would pass through

Your current system may drop the temperature by 2 degrees at a rate of 600 cfm. Your new system may drop the temperature 1 degree at a rate of 1200 cfm.

Efficiency of the system change...??? Not sure. I think it would be nearly unchanged.

May want to check your system first. My system can be hard wired to one of four speeds. From one being a gentle breath blown at you from across the room to four where the papers whirl around the room and dust bunnies out run the dog. Manufacture claims same efficiency for all speeds. (I wanted an electrician to install a switch so that I could change the speeds but all the licensed electricians say that it is not allowed for some reason I am not clear about.)

 

[HighPotential]

Thanks Doug115. That's what I am thinking (half the cooling with twice the airflow equals the same heat exchange). I have examined the schematics and the current blower is already at maximum in cooling mode.

 

[EdStainless]

You also know that you fan runs slower when heating than when cooling right?
Our new system is nearly continuously variable. We run the blower all of the time, and if no heat or cool is required it runs slow (prob like yours now) to maintain uniformity. Then as the system demand heat, cooling, or dehumidification it adjusts the fan speed.

Does you blower run all of the time? You might try that and see if it helps.
The other option is put booster fans in just some of the ducts.
I have a room that I wanted cooler, so in installed a fan (actually two very quiet high CFM computer case fans) in the return line from that room. It worked wonders.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[willard3]

If you don't know the pressure drop/system curve of the existing system, how will you choose the fan?

 

[HighPotential]

Thanks Ed. The system is undersized to begin with. 2.5 Tons at SEER 12 for 3,480 sq ft. I focused on efficiency in the register layout and in the insulation of the system. But the blower does run at max all the time in cooling mode.

willard,

The bigger blower can only increase airflow so I am not worried about losing efficiency. You are correct though that I may actually not increase my airflow all that much even with adding a bigger blower due to restrictions in the evaporator and heater. My bet is that I will get at least 50% flow. But it's just a bet!

 

[dbill74]

Sorry to say, doesn't quite work that way. The big problems with doubling your airflow without changing anything else are 1) water blow off and 2) damaged filters.
"Water blow off" is just what it sounds like, if the velocity of the air across your coils is too large, water that condenses on the coil as part of the natural latent cooling the coil does, will be blown back into the airstream. This is not good as you will end up with water lined ductwork which could come through your diffusers, or worse you will get mold growing in your ductwork (even if the ductwork is metal).

With your filters, the increased air flow will deform or break your filters. Deformed or broken filters will not remove particulates as efficiently as unbroken filters resulting in more particulates getting to your coil and back into your house. Dirty cooling coils will not provide as much cooling to the air, and in a worst case scenario freeze preventing the coil from doing any cooling at all.

Your best bet is to reduce the size of your ductwork to proper sizes. In a properly sized duct system, the only noise you will hear is the air coming through the diffusers.

One more thing, doubling the air flow will require about 8x (2[sup]3[/sup]) the amount of power as your current fan. Can the current wiring and power circuit handle the increased demand?

 

[HighPotential]

Great post dbill, thanks very much.

I had not thought of the condensation issue. Certainly could be a problem and I wouldn't know how to manage it. The filters I am not concerned about as I have oversized everything, including the filters.

I am curious about your airflow equation. The manufacturer makes a blower with double the CFM that will fit and work within the limits of my current unit. I understand that I will not get double the flow due to an exponential resistance (pressure drop) to flow through my existing evap and heater rated for the original CFM. Where are you getting your power equation from?

 

[Compositepro]

To add to Dbill74's comments, humidity control will be a problem at higher air flow, and not just because you may be blowing condensate off the evaporator. The amount of condensation on the coil (dehumidification) depends on how cool the evaporator coil gets. A a very high air flow you will reach a point where you get no dehumidification at all. You will have a cool and clammy house with mold problems.

 

[HighPotential]

Another useful bit, Compositepro, thanks. This is looking less and less like a clever idea!

I live in SoCal where humidity is an infrequent problem in the hot months (at most 5-6 days a year) do you think this would affect my "net" comfort as much in SoCal?

 

[Dougt115]

These theoretical discussions are great but So Cal does not have a humidity issue unless you are on the coast or don't vent your shower and often it is too dry. Also if the air flow is too low now (sub-optimal for the ducting) then doubling it wouldn't likely cause any of these issues mentioned. Finally, look at the actual flow rate increase not the relative increase. What I mean is increasing from 50 to 100 CFM in a 6 inch duct is not the same as increasing from 50 to 100 in an 18 inch duct. Air velocities are very different. My intake filter is 16x22 my duct at the blower is 22 inch diameter on full speed I can blow a sheet of newspaper off the table from 20 away from the nearest duct. I have no mold, filter or other issues and except for the sound of the wind pretty quiet.