Thoughts on counterflow air-to-liquid heat exchanger?

DanHolohan

but perhaps someone else has.

Dominic Larkin

looking for air-to-liquid heat exchanger

Hi,
We have a building in cold climate, with a make-up air system of 2600 cfm in the basement, and 2 exhaust air system on the roof, 1000 cfm and 1600 cfm. We are trowing the energy out with no recovery.

The distance between the in and the out is about 200 ft.
We cannot install an HR wheel or HR core.
I'm looking to install a counterflow air-to-liquid heat exchanger (plates or pipes), one at the basement duct and one at the roof, with a pump to circulate the liquid.
Since the heat transfer is passive, a counterflow system would be the most efficient, but I don't know if it exists on the market.
Any recommandation would be appreciated.

Charlie from wmass

perhaps airhandler coils could be used for this purpose? just a thought.

Dekstrous

heat exchanger for heat recovery

I sent you an email. let me know if you have questions.

Aidan (UK)

air-to-liquid heat exchanger

Yes.

They're called run-around coils (over here they are anyway). Have a search on Google.

They're usually 6 or 8-row coils in the supply and exhaust air streams, with a pump circulating water between the two. The water usually has anti-freeze in it. Besides that, they need an expansion vessel/tank, pressure gauge and a pressure relief valve. You need a pump to inject the glycol mixture; topping up can be from the mains via a check valve, they shouldn't lose much water in a year.

The resistance imposed by the coil on the air, and so on the fan, is significant and the extra energy needed could make nonsense of your energy savings. The maximum amount of heat energy you can save is 50%. They are very simple and very reliable.

I have refurbished a 25 year old run-around system, that had never worked since it was installed, 12 cubic metres per second, 100% fresh air. When completed, the air and water temperatures were monitored on a BMS/DDC system and it delivered a consistent 50% heat recovery.

They are less common than they used to be, mainly I think because of the price of the copper in the coils, but also because cross-flow plate heat exchangers have become available. There are several in a new building project I'm involved in at present; also thermal wheels and cross-flow heat exchangers.

There's probably some data in the ASHRAE handbooks. I think there was some in the similar UK CIBSE guides. I believe the main equation was that the mass flow rate x the specific heat capacity was equal for the air and water flows (in metric units). Besides that, it is fairly basic pipe and pump sizing.

Dominic Larkin

Counter flow liquid to water heat recovery

Thanks for all the information.

I was looking for counterflow because cross flow has a maximum efficiency of 75%. With a round around passive system (like in our case, because we cannot install wheel or plate air-to-air HR)), and all the points you mentionned, I understand that it is difficult to obtain 50% for the whole system.

As a few ones have suggested, maybe a heat pump, would be a good alternative. A glycol heat exchanger does already heat the incoming air at the building basement.

So an heat maybe the heat pump can extract the heat from the warm exhaust air at the roof an inject in the glycol loop of in the incoming air.

Brad White_185

I agree with Aidan

I too call them run-around coils and like Aidan's descriptions.

I find the optimum to be about four, sometimes six rows, balancing recovery with a constant pressure drop, but each application has to be vetted economically.

Counterflow is absolutely essential to extract those BTU's, especially given the relatively narrow delta-T's.

That said, I have use run-around HRCs with an in-line booster to pre-heat 100% OA units. Even used first-in-line chilled water coils to act as a pre-heater for 100% OA units while providing free chilled water to interior fan-coil units. You can see how entering water at 50 degrees can heat incoming zero degree air to the high 30's or low 40's, leaving less work for the downstream heating coil to do. At the same time, that entering 50F water leaves at 44 degrees, perfect for year-round fan-coil loads in deep buildings. Such a deal.

Just to name a couple of possible applications.