"Off Label" Heat Exchanger Sizing

[Hunderliggur]

Now that my heating season is over, I want to work on a cooling & dehumidification project. We use a lot of well water on our farm (about 500 gallons/day, maybe up to 1000 gallons/day). All that water is going though a 1" line in my basement before going to where it is used (mostly outside). I would like to run that 55F water through a water to air heat exchanger to cool and dehumidify. Pressure drop through the HX is not a problem, I can pump 30GPM at 70PSI with my Grundfos constant pressure pumps. We typically do not run more than 10GPM. I was thinking of a simple HX with a fan to blow air through and a drip pan for the condensation. Air temp going in would be about 70 to 75. Any ideas on an appropriate size for the HX? If it is too small/too big would I get the temp drop to dehumidify? Anyone know where to get a reasonably priced HX?

 

[Frozen Canuck]

I think the biggest problem you will run into is having the water moving when the house t-stat calls for cold. Unless you have a cistern or a large tank where you can hold water waiting to be used for cooling. Or perhaps the abilty to flow through the HX & then back to the well. Another area where "storage" would be a benefit. Excellent idea as well as a real cost saver vs AC. Hopefully you can find a solution that lets you use that cold well water to cool your home. Thought about the same on our farm but could not fit the storage into the mech room. So we went with the pricey AC. Cringe everytime that thing cuts in, $$$$$$ "poof" me sad but wife happy. ;-)

 

[ewdudley]

cooling & dehumidification project

500 gallons/day, maybe up to 1000 gallons/day

55F water

Air temp going in would be about 70 to 75.

Any ideas on an appropriate size for the HX?

Let's assume an immense water to air HX returning 65F air to the conditioned space. DeltaT of 10F at 1000 gallon per day gives us 83268 btu day, or 0.289 tons of refrigeration. With a COP of 3.0 and electricity at 0.12 dollar per kWh we're looking at saving 98 cents a day, which is something at least, but Bart Simpson saves five times that amount just by not starting smoking.

But we're still going to need some other type of air conditioning to supply another couple two three tons of refrigeration, not to mention the dehumidification, which 65F cooling won't accomplish.

Assuming a dollar a day is worth going after, and since we still need a conventional AC unit, it might be simpler to sweat in a plate HX on the high side between the compressor and the conventional condensing coil. Then use a stainless circulator to run cool water from the bottom of a 30 gallon or so buffer tank through the HX and back to the top of the tank, while the compressor is running. Supply water to the farm runs into the bottom of the buffer and out the top. This would lower your head pressures and would save electricity.

 

[heaterman]

You can get water coils made specifically for cooling purposes and a system like that can work very nicely if,..... and only if, you have a couple things working for you. Those being very good airflow and cold water. I'd say you have a major strike against you given the 55* ground water temp. On the airflow side once you get above a 45* coil you basically have to double your airflow per ton of cooling to have any good results. This usually means far more blower and duct system capacity than one would find in a typical residential duct system.

We did one for a guy with a small commercial machine shop that had a flowing well putting out water in the 42* range and it works very well. I ordered the coil from an HVAC supply house......can't remember the name of the manufacturer but their catalog showed hundreds of water coil configs. The one we used was an A configuration measuring 20x20 on the base and was 26" tall. Seems like it was a four row coil and we got a 7* temp rise on approximately 12 gpm flow which translates to about 3.5Tons per hour. It does a fantastic job of humidification in his machine shop which is the main thing he was after. He runs the interior temp at 76 which gave us a nice delta T to work with from entering air temp to coil temp. The waste water was dumped back into his trout pond.

 

[ewdudley]

You can get water coils made specifically for cooling purposes and a system like that can work very nicely if,..... and only if, you have a couple things working for you. Those being very good airflow and cold water. ...

Three things, actually. Air flow, cold water, and enough cold water. According to his numbers, Hunderliggur could have air flow but he doesn't have cold water or enough of it.

--ewd

 

[Hunderliggur]

Thanks for the great advice and calculations. I know it takes a lot of BTUs to make it really cold, and there are only 8 BTU/Gallon/F. I was considering the HX because I was planning a forced air duct from the basement to the 2nd floor anyway to aid in balancing the temps in the house (the second floor is warmer in the summer). If I add the well water HX to the airflow, I am only out the cost of the HX since I will have the blower and duct anyway. It looks like, best case, I might save $1/day. It would take a couple of years, maybe 3, to pay for the cost of a HX (16x20). I think it would last for a long time anyway. We do not have forced AC in the house. We may install a split system in the future with an wall air handler on each floor to reduce the humidity and add a little cooling. We were reasonably comfortable last year with 3 smallish window units (2nd floor, living area, and master BR). If we can get the humidity down a little and some cooling it will help.