Car Radiator as Air-to-Water Heat Exchanger

[Gusto]

TL;DR

I have a steady source of 110 degree air. Would flowing my pool water through a couple of truck radiators situated in the heat flow in an insulated chamber be an effective way to heat my pool?

I am looking for a passive way to convert a supply of hot air into warm pool water. My pool can be in the low 70s without heat. Looking to get a 10 degree lift in pool temp. Pool is in-ground and relatively small at 8,500 gal. Water flow rate through the radiators can be regulated by valves. The airflow at the delivery point can range from 100-120F consistently for about 8 hours a day (daylight) depending on the source intensity. Airflow is currently delivered by three 8" flexible ducts each with a 240 CFM in-line duct fan used for proof of concept.

I am looking for: 1. Confirmation that this is viable; 2. the most efficient passive heat exchange method to convert the air to warmer water; and 3. Assessment of whether solar powered fans can be used to deliver totally self-sustained operations.

I have scoured online forums and cannot seem to find a definitive answer. I know that there are a lot of broadly expressed variables, but hopefully someone can point me in the right direction. Many thanks. Gusto

 

[Lnewqban]

Welcome, Gusto!

For an estimate, I would start by investigating the heat-exchange performance, as well as resistance to different flow rates, of those radiators, for both, water and air side.

The airflow rate will be more or less constant, but certain percentage less that 240x3 cfm’s due to the restriction rate of both radiators.
The water flow rate will depend on how much your pump can reasonably due (especially if driven by wind, which tend to be best when Sun is high and heating less necessary).

Your water will gain some energy from the radiators and will lose some while flowing through pipes; therefore, the shorter and more insulated pipes the better.

 

[Gusto]

Welcome, Gusto!

For an estimate, I would start by investigating the heat-exchange performance, as well as resistance to different flow rates, of those radiators, for both, water and air side.

The airflow rate will be more or less constant, but certain percentage less that 240x3 cfm’s due to the restriction rate of both radiators.
The water flow rate will depend on how much your pump can reasonably due (especially if driven by wind, which tend to be best when Sun is high and heating less necessary).

Your water will gain some energy from the radiators and will lose some while flowing through pipes; therefore, the shorter and more insulated pipes the better.

Thanks Lnewqban! So the concept of the radiator working in reverse sounds viable? Radiators normally dissipate heat. Will the thermal transfer properties be the same in reverse (meaning if a radiator can cool fluid at a certain rate of air and fluid flow, will the same rates apply to heating fluid)? Sorry if that is a basic question. Best. Gusto

 

[Lnewqban]

Thanks Lnewqban! So the concept of the radiator working in reverse sounds viable? Radiators normally dissipate heat. Will the thermal transfer properties be the same in reverse (meaning if a radiator can cool fluid at a certain rate of air and fluid flow, will the same rates apply to heating fluid)? Sorry if that is a basic question. Best. Gusto

I am not sure, but I believe that heat transfer mainly depends on the temperature difference and the area of the interchage surface.
If any, the reduced rate of exchange should not be dramatic.
Perhaps other members of this site have a more accurate idea.

 

[jrmichler]

A car radiator will transfer heat from air to water exactly as well as it transfers heat from water to air. Car radiators are optimized for large air flow and large water flow. They work even better at low air and water flows.
If you are interested in calculating the effect of changing air and/or water flows on heat exchanger performance, a good book is Compact Heat Exchangers by Kays and London: https://www.amazon.com/dp/1575240602/?tag=pfamazon01-20. The book has numerous graphs that show the effect of changing flow, saving the need for tedious calculations.

Some ball park calculations to get started:
You mention 240 CFM duct fans in 8" flex duct. Flex duct has extremely high friction loss. If those duct fans are muffins that have low static pressure, the air flow will be lot less than 240 CFM per fan and duct. I will assume a total of 500 CFM at 100 deg F.

Assume the air enters the heat exchanger at 100 F, and leaves at 85 F. The heat will be:

$500 ft^3/min * 0.018 BTU/ft^3-deg F * (100 - 85) deg F * 60 min/hr = 8100 BTU/hr$

If we further assume that the water enters at 75 F and leaves at 80 F, the required flow rate will be:

$8100 BTU/hr / (80 - 75) deg F * 1 lb-deg F/ BTU * 1 gal / 8.34 lbs * 1 hr/60 min = 3.2 GPM$

Increasing the water flow rate will result in less temperature rise across the heat exchanger, and slightly increase the total heat transferred. The above temperatures are rough guesses, but should be good enough to to give you an idea of the amount of heat you can expect to put into the water with an automotive heat exchanger. They also show that the water pump needs to move at least 3 GPM of water.

 

[256bits]

Assume the air enters the heat exchanger at 100 F, and leaves at 85 F. The heat will be:

500ft3/min∗0.018BTU/ft3−degF∗(100−85)degF∗60min/hr=8100BTU/hr

So 8100 pounds of water ( 810 gallons ) increases in temp 1 degree F in 1 hour.
His pool is 8500 gallons - 10 hours for a one degree rise in temperature.

Off the cuff, I honestly thought it would be more than that.

If he leaves it running, he should notice a difference.

 

[rbelli1]

One thing to keep in mind is that once you get this all built you will probably not feel the temperature difference in the outlet flow. It will still be doing its job but the small fraction of a degree increase is too small for the human body to differentiate.

BoB

 

[Tom.G]

IIRC, the main heat loss from swimming pools is evaporation. You might want to check that effect and compare it to the BTU gain of the proposed solution.

 

[Rive]

...the most efficient passive heat exchange method to convert the air to warmer water...

Those radiators are not really optimal. They are meant to change the temperature of the air flow by a few degrees only (so the temperature of the air flow on the output will be just a few degrees apart from the input temperature), but an ideal heat exchanger would change the temperature of the outgoing air to the temperature of the water in the pool.
Check 'counter flow heat exchanger'.

You can make it work though: just use more than one radiator in series with the first one, attached to the air output of the first one, to serve as the first one for the water flow.
... and if the output airflow is still warm, then just add one more, depending on your supply of truck radiators

 

[jrmichler]

A radiator for, say, a 300 hp engine is operating in a different regime than the same radiator in the OP's situation. For example, assume a radiator for a 300 hp engine. That engine burns about 25 gallons of gasoline per hour at maximum power: 300 hp * 0.5 lb/hp-hr / 6 lb/gallon = 25 gal/hr.

Gasoline has about 120,000 BTU per gallon, and about 1/3 of that goes to the cooling system. The heat to the radiator is then 25 gal/hr * 120,000 BTU/gal / 3 = 1,000,000 BTU/hr. That's more than 100 times as much heat as the OP wants to transfer.

The book referenced in Post #5 has the following performance chart for a cross flow heat exchanger with fluids unmixed:

In an automotive application where large amounts of heat are transferred from large amounts of fluid, the operating point is somewhere around 1 NTU. With the much smaller flow rate in the swimming pool application, the operating point is off the chart to the right. With the flow rates in Post #5, the $C_{min}/C_{max}$ is 0.33, so the effectiveness approaches that of a counterflow heat exchanger.